Hook Echoes and Rear-Flank Downdrafts: a Review

Home , Hook echo, Paul Markowski, Rear flank downdraft

852 MONTHLY WEATHER REVIEW VOLUME 130

Hook Echoes and Rear-Flank Downdrafts: A Review

PAUL M. MARKOWSKI Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania

(Manuscript received 19 March 2001, in ®nal form 24 September 2001)

ABSTRACT Nearly 50 years of observations of hook echoes and their associated rear-¯ank downdrafts (RFDs) are reviewed. Relevant theoretical and numerical simulation results also are discussed. For over 20 years, the hook echo and RFD have been hypothesized to be critical in the tornadogenesis process. Yet direct observations within hook echoes and RFDs have been relatively scarce. Furthermore, the role of the hook echo and RFD in tornadogenesis remains poorly understood. Despite many strong similarities between simulated and observed storms, some possibly important observations within hook echoes and RFDs have not been reproduced in three-dimensional numerical models.

1. Introduction 9 April 1953, although van Tassell (1955) is given credit Perhaps the best-recognized radar feature in a horizontal for coining the term. The re¯ectivity appendage usually depiction associated with supercells is the extension of is oriented roughly perpendicular to storm motion. Hook low-level echo on the right-rear ¯ank of these storms, echoes are typically downward extensions of the rear called the ``hook echo.'' Hook echoes are known to be side of an elevated re¯ectivity region (Forbes 1981) associated with a commonly observed region of subsiding called the echo overhang (Browning 1964; Marwitz air in supercells, called the ``rear-¯ank downdraft.'' Rear- 1972a; Lemon 1982), with the region beneath the echo ¯ank downdrafts have been long surmised to be critical overhang termed a weak echo region (Chisholm 1973; in the genesis of signi®cant tornadoes within supercell Lemon 1977) or vault (Browning and Donaldson 1963; thunderstorms. In this paper, observations of hook echoes Browning 1964, 1965a). Browning and Donaldson are reviewed ®rst, beginning with the ®rst documentations (1963) and Browning (1965b) noted that the southern of the 1950s and 1960s. Rear-¯ank downdrafts are re- edge of the hook formed a wall of echo ``which was viewed next, including discussion of pertinent Doppler often very sharp and sometimes rather upright.'' Hook radar observations and theoretical and numerical simula- echoes are typically several kilometers in length and tion studies. Finally, relatively recent observations made several hundred meters in width, at least as viewed by during the Veri®cation of the Origins of Rotation in Tor- operational radars (e.g., Garrett and Rockney 1962). A nadoes Experiment (VORTEX; Rasmussen et al. 1994) variety of shapes that the hook echo may take were and smaller subsequent ®eld operations are reviewed. De- presented by Fujita (1973; Fig. 2). spite the well-known association among hook echoes, rear- Fujita (1958a) documented hook echoes associated ¯ank downdrafts, and tornadogenesis, their dynamical re- with other supercells on the same day Stout and Huff lationship remains poorly understood. Our current con- made their observations. He inferred the concept of fusion serves as a motivation for the collection of new, in thunderstorm rotation from viewing the evolution of the situ observations having unprecedented spatial and tem- hook echoes, which he studied in unprecedented (and poral detail within hook echoes and rear-¯ank downdrafts. since unparalleled) detail (Fig. 3). Brooks (1949) earlier These data are the foundation of a companion paper (Mar- had referred to these circulations, having radii of ap- kowski et al. 2002). proximately 8±16 km, as tornado cyclones.1 Wind velocity data obtained following the installation of Dopp- 2. Hook echoes ler radars in central Oklahoma in the late 1960s con- a. Characteristics ®rmed an association between hook echoes and strong The hook echo ®rst was documented by Stout and horizontal shear zones associated with storm rotation Huff (1953; Fig. 1) in an Illinois tornado outbreak on and tornadoes (e.g., Donaldson 1970; Brown et al. 1973;

Corresponding author address: Dr. Paul Markowski, 503 Walker 1 The tornado cyclone terminology now typically refers to distinct Building, University Park, PA 16802. circulations having a scale larger than a tornado but smaller than a E-mail: [email protected] mesocyclone (e.g., Agee 1976). APRIL 2002 MARKOWSKI 853

Fujita (1981) proclaimed ``Mesoscale modelers should be attracted by such a pair of cyclonic and anticyclonic torandoes which were evidenced in the Grand Island storm on 3 June 1980 and in the central Iowa storm on 13 June 1977.'' However, it was unclear why such attention should be given to the vortex couplet, and the origin of the couplet was not well understood.

b. Formation Fujita (1958a) originally attributed hook echo formation to the advection of precipitation from the rear of the main echo around the region of rotation associated with the tornado cyclone and updraft. Browning (1964, 1965b) also documented hook echoes and attributed their evolution (Fig. 6) to essentially the same process described by Fujita (1958a). Fujita (1965) later attri- FIG. 1. Radar image from the ®rst documentation of a hook echo. buted hook echo formation to the Magnus force. He The hook echo was associated with a tornadic supercell near Cham- paign, IL, on 9 Apr 1953. [From Stout and Huff (1953).] explained that this force pulled the spiraling updraft out of the main echo, resulting in the hook-shaped re¯ectivity appendage commonly observed on radar displays Lemon et al. 1975; Ray et al. 1975; Ray 1976; Brandes (Fig. 7). 1977a; Burgess et al. 1977; Lemon 1977; Barnes Fulks (1962) hypothesized that hook echo formation 1978a,b). was due to a large convective tower extending into the Garrett and Rockney (1962) were the ®rst to relate a levels of strong vertical wind shear, which produced circular echo on the tip of a hook echo to the tornado cyclonic and anticyclonic ¯ows at opposite ends of the or tornado cyclone. They called this ball-shaped echo towerÐthe cyclonic ¯ow to the southwest gave rise to an ``asc'' (annular section of the cylinder of the vortex), hook echo development. No mention was made of the but the authors did not offer an explanation for the exact possibility of an anticyclonic hook echo forming on the cause of the appearance of the asc. Stout and Huff north side of the tower from the same mechanism. (1953) also observed a similar feature, but little was Probably no one presented as many detailed Doppler discussed of it. Donaldson (1970) noted an echo hole radar analyses of supercells as Brandes [1977a,b, 1978, in the tornado he studied, and found that it was collo- 1981, 1984a,b; Brandes et al. (1988)]. Brandes (1977a) cated with a tornado vortex. Forbes (1981) also ob- looked at a nontornadic supercell on 6 June 1974. Hook served similar re¯ectivity features during the tornado echo formation was attributed to the ``horizontal ac- outbreak of 3±4 April 1974, as did Fujita and Wakimoto celeration of (low-level) droplet-laden air'' as the down- (1982) in their study of the Grand Island, Nebraska, drafts intensi®ed and the out¯ow interacted with the tornadoes of 3 June 1980. inward-spiraling updraft air. Apparently this hypothesis Van Tassell's (1955) images of a hook echo near was essentially that precipitation advection was respon- Scottsbluff, Nebraska, on 27 June 1955 (it moved di- sible for hook echo formation, similar to the Fujita rectly over the radar) suggested the presence of a faint (1958a) and Browning (1964, 1965b) hypotheses. A anticyclonic protrusion from the tip of the hook, ex- three-dimensional numerical simulation by Klemp et al. tending outward in a direction opposite that of the cy- (1981) of the Del City, Oklahoma (20 May 1977) su- clonic protrusion. An anticyclonic re¯ectivity ¯are also percell also suggested that the horizontal advection of has been documented by Brandes (1981), Fujita (1981), precipitation was important for hook echo development. and Fujita and Wakimoto (1982). Multiple Doppler ra- In some observations of hook echoes associated with dar wind syntheses almost invariably have revealed a tornadoes in nonsupercell storms, the hook echoes also region of anticyclonic vorticity on the opposite side of have appeared to result largely from the horizontal ad- the hook echo as the more prominent (cyclonic) vorticity vection of hydrometeors (e.g., Carbone 1983; Roberts region (Brandes 1977b, 1978, 1981, 1984a; Ray 1976; and Wilson 1995). Ray et al. 1975, 1981; Heyms®eld 1978; Klemp et al. Other reliable radar observations have been made that 1981; Fig. 4). It is perhaps surprising that the ubiquity suggest that hook echo formation, in at least some cases, of the vorticity couplet straddling the hook echo largely appears to result from the descent of a rain curtain in has been ignored, with the exception of Fujita and his the rear-¯ank downdraft (e.g., Forbes 1981; E. N. Ras- collaborators. Fujita and Wakimoto (1982) documented mussen 2000, personal communication;2 L. Lemon an anticyclonic tornado within the region of anticyclonic vertical vorticity (Fig. 5). The cyclonic member of the 2 This was an oral presentation at the VORTEX Symposium in vorticity couplet also was associated with a tornado. Long Beach, California. 854 MONTHLY WEATHER REVIEW VOLUME 130

TABLE 1. Summary of ®ndings pertaining to hook echoes and RFDs. Observation/Conclusion References RFD originated at or above 7 km Nelson (1977), Lemon et al. (1978), Barnes (1978a), Lemon and Doswell (1979) RFD originated below 7 km Klemp et al. (1981)

Low ␪w at surface in RFD van Tassell (1955), Beebe (1959), Ward (1961), Browning and Ludlam (1962), Browning and Donaldson (1963), Charba and Sasaki (1971), Lemon (1976a), Nelson (1977), Brandes (1977a), Barnes (1978a,b), Klemp et al. (1981), Klemp and Rotunno (1983), Rotunno and Klemp (1985), Wicker and Wilhelmson (1995), Dowell and Bluestein (1997), Adlerman et al. (1999)

Warm air (but not necessarily high ␪w) at surface in RFD Tepper and Eggert (1956), Garrett and Rockney (1962), Williams (1963), Fujita et al. (1977), Brown and Knupp (1980), Bluestein (1983), Brandes (1984a), Johnson et al. (1987), Rasmussen and Straka (1996) Hypothesized that the occlusion downdraft is driven by a down- Klemp and Rotunno (1983), Brandes (1984a,b), Hane and Ray ward-directed, nonhydrostatic pressure gradient arising from the (1985), Rotunno (1986), Brandes et al. (1988), Trapp and Fied- intensi®cation of low-level vertical vorticity ler (1995), Wicker and Wilhelmson (1995), Wakimoto et al. (1998), Adlerman et al. (1999), Wakimoto and Cai (2000) Hypothesized the RFD is forced mainly thermodynamically from Browning and Ludlam (1962), Browning and Donaldson (1963), aloft (which may result from stagnation) Browning (1964), Nelson (1977), Barnes (1978a), Brandes (1981), Klemp et al. (1981) Hypothesized the RFD is initiated by dynamic pressure excess Bonesteele and Lin (1978), Lemon and Doswell (1979) aloft but maintained thermodynamically Re¯ectivity gradients found on the upshear side of storms Nelson (1977), Bonesteele and Lin (1978), Barnes (1978a), Forbes (1981) Tornadogenesis observed before hook formation Garrett and Rockney (1962), Sadowski (1969), Forbes (1975) Tornadogenesis observed at the time of overshooting top collapse Fujita (1973), Lemon and Burgess (1976), Burgess et al. (1977) Visual observations of clear slots accompanying tornadoes Beebe (1959), Garrett and Rockney (1962), Moller et al. (1974), Peterson (1976), Stanford (1977), Burgess et al. (1977), Lemon and Doswell (1979), Marshall and Rasmussen (1982), Rasmus- sen et al. (1982), Jensen et al. (1983), Wakimoto and Liu (1998) Vertical vorticity couplets associated with hook echoes Ray (1976), Ray et al. (1975, 1981), Brandes (1977b, 1978, 1981, 1984a), Heyms®eld (1978), Klemp et al. (1981), Fujita (1981), Fujita and Wakimoto (1982), Klemp and Rotunno (1983), Bran- des et al. (1988), Wicker and Wilhelmson (1995), Wurman et al. (1996), Straka et al. (1996), Bluestein et al. (1997), Dowell and Bluestein (1997), Blanchard and Straka (1998), Gaddy and Blue- stein (1998), Wakimoto and Liu (1998), Wakimoto et al. (1998), Wakimoto and Cai (2000), Wurman and Gill (2000), Ziegler et al. (2001), Bluestein and Gaddy (2001) Anticyclonic re¯ectivity ¯ares on hook echoes van Tassell (1955), Brandes (1981), Fujita (1981), Fujita and Wak- imoto (1982), Wurman et al. (1996), Blanchard and Straka (1998), Wurman and Gill (2000), Ziegler et al. (2001) Hook echo formation attributed to rotation (but not necessarily the Fujita (1958a, 1965), Fulks (1962), Browning (1964, 1965b), Bran- same mechanisms) des (1977a) Hook echoes associated with strong horizontal shears or tornadoes Stout and Huff (1953), van Tassell (1955), Fujita (1958a, 1965), (prior to 1980, in the interest of brevity) Garrett and Rockney (1962), Browning (1964, 1965b), Freund (1966), Sadowski (1958, 1969), Donaldson (1970), Forbes (1975), Ray et al. (1975), Lemon et al. (1975), Ray (1976), Brown et al. (1978), Lemon (1977), Burgess et al. (1977), Bran- des (1977a), Barnes (1978a,b) Hook echoes associated with downdrafts (prior to 1980, in the in- Browning and Donaldson (1963), Haglund (1969), Fujita (1973, terest of brevity) 1975b, 1979), Lemon et al. (1975), Lemon (1977), Brandes (1977a), Burgess et al. (1977) Hook echoes located in strong vertical velocity and temperature Marwitz (1972a,b), Burgess et al. (1977), Lemon and Doswell gradients, somewhat behind the surface windshift associated (1979), Brandes (1981) with the RFD Air parcels that enter the tornado pass through the RFD Brandes (1978), Davies-Jones and Brooks (1993), Wicker and Wil- helmson (1995), Dowell and Bluestein (1997), Adlerman et al. (1999) [and implied by visual observations of Lemon and Do- swell (1979), Rasmussen et al. (1982), Jensen et al. (1983)] Hypothesized that the RFD is important for tornadogenesis Ludlam (1963), Fujita (1975b), Burgess et al. (1977), Barnes (1978a), Lemon and Doswell (1979), Brandes (1981), Davies- Jones (1982a,b) APRIL 2002 MARKOWSKI 855

FIG. 2. Fujita introduced ®ve variations on the shapes of hook echoes. [From Fujita (1973).]

FIG. 3. Development of the hook echo associated with the Cham- 2000, personal communication). It may be worth the paign, IL, tornado from 1724±1738 CST 9 Apr 1953, as analyzed by effort in future ®eld experiments to use temporally and Fujita. [From Fujita (1958a).] spatially high-resolution mobile radar data to systematically investigate the extent to which the hook echo forms from the above process, versus being due to a developed before or after tornadogenesis. The tornado streamer of precipitation that is extruded from the main studied by Garrett and Rockney (1962) apparently echo core by horizontal advection. The relative contri- formed before the hook echo became prominent, unless bution to hook echo formation from these two processes a narrow hook echo went undetected by the Weather is not yet known. It is entirely possible that one might Surveillance Radar-3 (WSR-3) (4Њ beamwidth) prior to dominate in one case, while the other dominates in a tornadogenesis. The tornado dissipated when the hook different case. Or perhaps the relative contribution from ``closed off'' or merged with the forward-¯ank echo. the two processes could be a function of time within a Sadowski (1969) later documented a large amount of single case. success using hook echoes to detect tornadoes within For completeness, it is noted that at least one docu- thunderstorms. In a 1953±66 study, he computed an mentation has been made of hook echoes not associated average time of 15 min between hook echo appearance with updraft rotation. Houze et al. (1993) showed ex- and tornadogenesis in a sample of 13 cases in which amples of hook-shaped (in a cyclonic sense, with the hook echoes appeared before tornadoes were reported. hooks pointing toward the right with respect to storm Sadowski reported a false alarm rate of only 12%. On motion) re¯ectivity structures in left-moving severe the other hand, Freund (1966) found that only 6 of 13 storms in Switzerland. These features, termed false tornadic storms near the National Severe Storms Lab- hooks by the authors, apparently were associated with oratory in 1964 were associated with hook echoes, and the cyclonic downdraft regions on the right (southern) Golden (1974) found that only 10% of waterspouts were ¯anks of the anticyclonically rotating storms, in which associated with hook echoes. the updrafts would have been on the left (northern) The so-called Super Outbreak of tornadoes on 3±4 ¯anks (Klemp and Wilhelmson 1978a; Wilhelmson and April 1974 (Fujita 1975a,b) provided a large sample of Klemp 1978; Rotunno and Klemp 1982). a variety of ``distinctive echoes'' that were studied by Forbes (1975, 1981). Forbes (1975) found that 1) a ma- jority of hook echoes were associated with tornadoes, c. Tornado forecasting based on hook echo detection 2) hook echoes often were associated with tornado fam- The forecasting potential of hook echo detection be- ilies, and 3) tornadoes associated with hook echoes tend- gan to be explored in the mid-1960s. Sadowski (1958) ed to be stronger than those from other echoes. Forbes documented a tornado that occurred after the hook echo (1975) also found that, on average, hook echoes ap- became visible (in fact, the hook echo was becoming peared 25 min prior to tornadogenesis; however, much less discernible on radar at the time of the reported variance was present in his sampleÐ10 of 27 (37%) of tornado formation). Sadowski might have been the ®rst the hook echoes associated with the ®rst tornado pro- to speculate that if hook echoes generally preceded tor- duced by a supercell were detected after the reported3 nadogenesis, then it might be possible to issue tornado tornado formation times. Forbes (1981) found a false warnings in advance. alarm rate of just 16% when using hook echoes to detect In Stout and Huff's (1953) report, the evidence had tornadoes. But because hook echoes were relatively rare been inconclusive as to whether the hook echo preceded tornadogenesis, or vice versa. In van Tassell's (1955) 3 The accuracy of the reported tornado times may be questionable summary, it was not mentioned whether the hook echo for some of the tornadoes studied. 856 MONTHLY WEATHER REVIEW VOLUME 130

FIG. 4. Three-dimensional wind ®eld relative to the Del City, OK, supercell 1845 CST 20 May 1977 at 400 m. Left panel shows horizontal wind vectors with radar re¯ectivity superposed. The right panel depicts vertical velocity (m s Ϫ1), with the 30-dBZ contour accentuated on both panels. The tornado path is stippled. [From Brandes (1981).]

(as he de®ned them), a less restrictive shape (a ``dis- (1975), Burgess et al. (1977), Brandes (1977a), Lemon tinctive echo'', e.g., appendages, line-echo wave pat- (1977), and Forbes (1981) also documented an associ- terns, etc.) also was considered. Distinctive echoes were ation between hook echoes and downdrafts. According associated with a probability of detection of tornadoes to Forbes (1981), ``the hook represents a band of pre- of 65%. Forbes (1975, 1981) did raise concern about cipitation accompanied by downdraft and out¯ow, sur- the generality of his ®ndings, since his statistics were rounding a weak echo region (a region of in¯ow and based on the events of a single day. Also, the statististics updraft).'' Brandes (1977a) tentatively concluded that were based on Weather Surveillance Radar-1957 (WSR- the hook echo re¯ected downdraft intensi®cation. Hag- 57) data, which may not have adequately resolved ®- lund (1969) concluded that the hook echo slightly trails nescale echo structures. the surface wind shift associated with the out¯ow of the RFD, and that the hook echo is located near the boundary between updraft and downdraft. Surface analyses 3. Rear-¯ank downdrafts and aircraft penetrations have revealed that the hook a. Association with hook echoes echo is located in a region of large vertical velocity and temperature gradients (Burgess et al. 1977; Marwitz Rear-¯ank downdrafts (RFDs) are regions of subsid- 1972a,b). ing air that develop on the rear side of the main updraft of supercell storms, and these regions of descent have a well-established association with hook echoes. The b. Visual characteristics ®rst documentation of an RFD, although not recognized as such, probably was by van Tassell (1955). In that The number of visual and surface observations of case study and in another by Beebe (1959) on the same supercells increased during the 1970s, largely because storm complex, three ``reliable'' reports of severe down- of organized storm intercept programs at the National drafts on the south side of the Scottsbluff tornado (27 Severe Storms Laboratory (Golden and Morgan 1972; June 1955) were made. Davies-Jones 1986; Bluestein and Golden 1993). Many Browning and Ludlam (1962) and Browning and of these observations have advanced our understanding Donaldson (1963) also were among the ®rst to mention of the basic structures associated with tornadoes and the presence of a downdraft in the vicinity of the stron- their parent storms. gest low-level rotation, behind the main storm updraft. Golden and Purcell (1978) photogrammetrically Browning and Donaldson (1963) noted that the hook documented subsiding air on the south side of the echo itself may be associated with this downdraft region. Union City, Oklahoma, tornado (24 May 1973), ap- Haglund (1969), Fujita (1973, 1979), Lemon et al. parently a visual manifestation of the RFD and also APRIL 2002 MARKOWSKI 857

FIG. 5. Radar image and analysis from Grand Island, NE, 0208 UTC 4 Jun 1980 showing a vortex couplet straddling a hook echo, with tornadic circulations associated with both members of the vortex couplet. [From Fujita (1981) and Fujita and Wakimoto (1982).] evidence that the tornado occurred in a strong vertical to systematically analyze traces of thermodynamic data velocity gradient. Moreover, a clear slot was seen to near tornadoes, and consequently, within some RFDs. wrap itself at least two-thirds of the way around the Data were obtained within 25 km of tornadoes in more tornado. Other observations of clear slots, which are than 50 cases. Many of the thermograph traces measured probably always visual manifestations of subsiding air only minor ¯uctuations during the passage of the tor- in an RFD,4 have been presented by Beebe (1959; this nadoes and associated RFDs, and other traces revealed was probably the ®rst documentation), Moller et al. cooling and moistening near the tornadoes. Only a few (1974), Peterson (1976), Stanford (1977), Burgess et observations were available within 5 km of the torna- al. (1977), Lemon and Doswell (1979), Marshall and does, however. Rasmussen (1982), Rasmussen et al. (1982), and Jen- Fujita (1958b) inferred the presence of a surface high sen et al. (1983) (Fig. 8). pressure annulus encircling the Fargo, North Dakota, Burgess et al. (1977) found that the clear slot could tornado cyclone (20 June 1957) from pressure traces in be associated with a hook echo: ``Perhaps large droplets the vicinity of the tornadoes (Fig. 9). Although Fujita are present in the downdraft and are brought down from speculated that the high pressure was associated with a the echo overhang, even though the air contains only ring of subsiding air around the tornado, he was unable ragged clouds or is visibly cloudless at low levels. If to verify this speculation. [Ward (1964, 1972) and Snow so, since radar re¯ectivity is more strongly dependent et al. (1980) found high pressure rings surrounding lab- on the size rather than on the number of droplets, radar oratory and numerically simulated vortices, but it is not may show substantial echo in the `clear' slot.'' Analysis clear whether these are the same phenomena inferred of the 2 June 1995 Dimmitt, Texas, tornadic supercell by Fujita, which appeared to be of a slightly larger also indicated an association between the hook echo and scale.] Surface pressure excesses within RFDs of up to clear slot, based on photogrammetrically determined a few millibars also have been documented by subse- cloud positions (E. N. Rasmussen 2000, personal com- quent investigators (e.g., Charba and Sasaki 1971; Lem- munication).5 on 1976a; Bluestein 1983), although no one else has documented a high pressure ring as Fujita did. c. Surface characteristics The studies of the Scottsbluff tornado by van Tassell Direct observations within RFDs have been scarce. (1955) and Beebe (1959) contain some of the ®rst de- Tepper and Eggert (1956) appear to have been the ®rst scriptions, albeit qualitative, of surface temperature within an RFD at close range from a tornado. At least a couple of observers, located a few hundred meters 4 The absence of a clear slot does not necessarily indicate an absence of subsiding air. south of the tornado, reported that the downdrafts felt 5 This was an oral presentation at the VORTEX Symposium in ``cold.'' Browning and Ludlam (1962) and Browning Long Beach, California. and Donaldson (1963) also reported cold temperature 858 MONTHLY WEATHER REVIEW VOLUME 130

FIG. 6. Evolution of the hook echo in an Oklahoma supercell on FIG. 8. Photograph of a typical clear slot associated with an RFD 26 May 1963 studied by Browning. [From Browning (1965b).] (2 Jun 1995 at Dimmitt, TX; photograph by the author). and equivalent potential temperature (␪ ) measurements e gust front and Ն5 K farther behind the gust front), Lem- (with respect to the in¯ow) in the wakes of the Wok- on (1976a; supercell on 25 June 1969; ϳ6K␪ de- ingham, England (9 July 1959) and Geary, Oklahoma w crease), and Charba and Sasaki (1971; supercell on 3 (4 May 1961) supercells, presumably within the RFDs. April 1964; ϳ7K␪ decrease). The relatively low ␪ Ward (1961) observed ``cooler northwest winds a couple e e and ␪ values have been shown to be compatible with miles southwest of the (Geary) tornado.'' w environmental air anywhere from 1±5 km above the In a supercell (25 May 1974) investigated by Nelson ground (e.g., Lemon 1976a; Charba and Sasaki 1971). (1977), the lowest wet-bulb potential temperature (␪w) values observed at the surface were within the RFD, In contrast to the ®ndings summarized above, Garrett and Rockney (1962) reported that a warm downdraft where they were ϳ6 K lower than the ambient ␪w values. Complete separation of the forward-¯ank downdraft was observed about 12±15 km south of a tornado near (FFD) and RFD was evidenced by separate temperature Topeka, Kansas, on 19 May 1960. The observer de- minima, as Lemon (1974) earlier had found in another scribed the air as ``suddenly becoming noticeably hot, similar to a blast of heat from a stove.'' Williams (1963) case. Additional observations of relatively low-␪ and e showed that RFD air can arrive at the surface warmer low-␪w air at the surface within RFDs have been presented by Brandes (1977a; supercell on 6 June 1974; than the surrounding air. He noted that when such an event occurs, it may be south of the hook echo or wher- Ͼ3K␪ decrease), Barnes (1978a,b; supercell on 29 w ever forced descent is less likely to encounter suf®cient April 1970; 2±3 K ␪ decrease directly behind the RFD w liquid water to maintain negative buoyancy. Fujita et al. (1977) documented a warm RFD (only temperature data were available) near an F4 tornado in the Chicago, Illinois, area on 13 June 1976. On the same

day, Brown and Knupp (1980) found nearly constant ␪w 3 km east of the Jordan, Iowa, F3 and F5 tornado pair, and those observations probably were in the RFD air mass, based on the pressure trace, which measured a pressure excess of a few millibars. In his summary of Totable Tornado Observatory (TOTO) observations, Bluestein (1983) documented a relatively warm RFD and pressure rise of Ͼ2mbina nontornadic supercell on 17 May 1981. Bluestein also presented evidence of a 1.5-K temperature rise in an RFD approximately 1.3 km south of the Cordell, Oklahoma, tornado on 22 May 1981.6 Similar to what Fujita (1958b) ®rst inferred, Bluestein also showed data that suggested high pressure at least partly encircling a tornado (his Fig. 7). In the violent Binger, Oklahoma, tornado on 22 May 1981, only small temperature ¯uc-

FIG. 7. Fujita once hypothesized that the Magnus force led to the 6 The reported pressure and temperature ¯uctuations were with re- formation of hook echoes. [From Fujita (1965).] spect to the storm in¯ow environment. APRIL 2002 MARKOWSKI 859

FIG. 9. Pressure ®eld near the center of the Fargo tornado cyclone (20 Jun 1957); A and B represent barograph stations. [Adapted from Fujita (1958b).] tuations (Ͻ1 K) were observed as the tornado passed hydrometeors are distributed over a larger horizontal within a few hundred meters north of TOTO (Fig. 10). region, and the intensity of out¯ow in close proximity Klemp et al. (1981) referred to ``cold downdraft to the updraft is reduced (Gilmore and Wicker 1998; (RFD) out¯ow'' in the Del City supercell, but no evi- Rasmussen and Straka 1998). In some recent, unpub- dence was presented demonstrating that this air actually lished simulations, relatively warm downdrafts have was coldÐretrieved temperatures by Brandes (1984a) been produced at the surface when ice physics and a and observations by Johnson et al. (1987) suggested that relatively ®ne spatial resolution (Ͻ250 m in the hori- at least parts of the Del City storm's rear-¯ank out¯ow zontal directions) were used (M. Gilmore 2000, personal were warm, although ␪w values may not have been as communication). large as in the in¯ow. Although there have been surface observations of d. Characteristics above the surface warm, high-␪e air within RFDs, three-dimensional numerical simulations of supercells almost invariably have Johnson et al. (1987) presented observations of the produced cold, low-␪e RFDs at the surface (e.g., Klemp RFD and FFD of the Del City storm collected by a 444- et al. 1981; Rotunno and Klemp 1985; Fig. 11). The m tower as the storm passed overhead. The RFD was pioneering numerical modeling studies of supercells associated with ␪e values approximately 4 K lower than conducted in the 1970s (e.g., Schlesinger 1975; Klemp the ambient conditions; however, the temperature in- and Wilhelmson 1978a,b; Wilhelmson and Klemp 1978) creased 1.5 K and the dewpoint temperature decreased and the parameter space studies of the 1980s (e.g., Weis- 2.5 KÐif ␪e was nearly conserved, then air had subsided man and Klemp 1982, 1984), with no known exceptions, from approximately 1 km (all heights are above ground used warm rain microphysics. The relatively simple pa- level). Although the RFD was not sampled well by the rameterization was not computationally demanding; tower, the data that were available suggested the pres- thus, experiments requiring large numbers of simula- ence of a downward-directed perturbation pressure gra- tions were feasible. However, the exclusion of ice may dient within the lowest half kilometer. have promoted unrealistically excessive amounts of la- Dowell and Bluestein (1997) documented the passage tent cooling near updrafts. When ice physics is included, of another tornadic supercell over the same instrumented 860 MONTHLY WEATHER REVIEW VOLUME 130

peratures were retrieved within the RFDÐradar re¯ectivity was a minimum here, possibly implying that evaporation was occurring. Behind the rear-¯ank gust front in the eastern mesocyclone quadrants,7 evidence was found of warm temperatures at low levels during the tornadic stage. The relatively warm conditions were attributed to subsidence within the RFD. In the Harrah storm, Brandes (1984a) also retrieved negative buoyancy in the RFD aloft, but no mention was made of the low-level buoyancy immediately behind the gust front in the eastern quadrants of the mesocyclone. Hane and Ray (1985) also completed a thermodynamic retrieval for the Del City storm. In the pretornadic stage, the pressure distribution included at each level a high±low couplet across the updraft with the maximum horizontal pressure gradient generally oriented along the environmental shear vector at that altitude, in agreement with linear theory predictions (Rotunno and Klemp 1982). Although the orientation of the horizontal pressure gradient agreed relatively well with linear theory, its magnitude did not agree as well. The authors stated that possibly the orientation and magnitude of the environmental shear vector were not known exactly (also, calculations of the horizontal vertical velocity gradient may have had signi®cant errors). In the tornadic stage, the pressure ®eld contained a pronounced minimum at low levels coincident with the mesocyclone, probably due to strong low-level vertical vorticity. Hane and Ray found weak high perturbation pressure (ϳ1 mb) in the RFD at low levels behind the gust front during the time FIG. 10. Pressure, wind speed, wind direction, and temperature traces from TOTO near the Binger, OK, tornado on 22 May 1981 of the tornado. Vertical gradients of nonhydrostatic pres- from approximately 1922±1931 CST. Only relative wind direction is sure perturbations may be relevant to the formation and known; veering (backing) is indicated by a downward (upward) evolution of the RFD, as will be discussed in the next change. The tornado passed approximately 600 m north of TOTO. subsection. The RFD contained signi®cant negative Note the small (less than ϳ1 K) temperature ¯uctuations recorded. It is inferred that the hook echo and RFD associated with this violent buoyancy at low levels in Hane and Ray's analysis (tem- tornado were relatively warm. [From Bluestein (1983).] perature de®cits as low as Ϫ4.5 K; Fig. 12). The retrieval results of Brandes (1984a) and Hane and Ray (1985) in the Del City storm were in relatively tower on 17 May 1981. They found large ␪e and ␪␷ close agreement with the direct measurements within (virtual potential temperature) de®cits in the portion of the in¯ow and FFD regions reported by Johnson et al. the RFD sampled by the tower (Ͼ12 and Ͼ5 K, re- (1987). But Johnson et al. cautioned that ``noticeable spectively), which was within a region of fairly high differences in the RFD region suggested that there was (Ͼ40 dBZ) radar re¯ectivity west of the circulation cen- room for improvement in the retrieval methods.'' Bran- ter. The de®cits were prominent over the entire height des and Hane and Ray had used considerable smoothing of the tower. on the buoyancy ®eld to eliminate noise; the details in With the exception of the direct observations pre- the retrieved low-level buoyancy ®elds may have been sented by Johnson et al. (1987) and Dowell and Blue- suspect. stein (1997), thermodynamic quantities above the ground within RFDs have been obtained only by indirect means. Brandes (1984a) and Hane and Ray (1985) were e. Origins and formation among the ®rst to use the methods proposed by Gal- In reviewing previous studies and hypotheses per- Chen (1978) and Hane et al. (1981) to retrieve ther- taining to RFD formation, it may be helpful to begin modynamic ®elds in supercells from three-dimensional wind ®elds synthesized from multiple Doppler radars. Brandes (1984a) retrieved the pressure and buoyancy 7 The RFD had wrapped cyclonically around the updraft, so that ®elds in the Del City and Harrah, Oklahoma (8 June downdraft air also was found in the eastern (forward) quadrants of 1974) tornadic storms. At 3.3 km on the rear side of the mesocyclone. This ``occlusion process'' will be discussed in sec- the updraft of the Del City storm, relatively cold tem- tion 4. APRIL 2002 MARKOWSKI 861

FIG. 11. Contours at z ϭ 250 m of (a) potential temperature ¯uctuation (␪ ϭ 303 K), in intervals of 1 K; and (b)

equivalent potential temperature ¯uctuation (␪ e ϭ 340 K), in intervals of 2 K, in the simulation conducted by Rotunno and Klemp (1985). The updraft is represented by the shaded region. The thick circular line encloses the region where the cloud water is greater than 0.1 g kgϪ1 (wall cloud). The thick dashed line encloses the region where the cloud water is greater than 0.1 g kgϪ1 at z ϭ 500 m. [Adapted from Rotunno and Klemp (1985).]

with an inspection of the inviscid vertical momentum equation written as

␲ץ wץ dw (١w ϭϪc ␪ ϩ B, (1 ´ ϭϩv zץ t pץ dt

where dw/dt is the vertical acceleration following a par- (t is the local vertical acceleration, v ϭ (u, ␷,wץ/wץ ,cel ١w is the ´ is the three-dimensional velocity vector, Ϫv

advection of vertical velocity, cp is the heat capacity, ␪ is the mean potential temperature, ␲ is the perturbation Exner function, and B is the buoyancy which can be written as

␪Ј B ϭ g ϩ 0.61qЈϪq Ϫ q , (2) ΂΃␪ ␷ li

where ␪Ј andqЈ␷ are potential temperature and water vapor mixing ratio ¯uctuations from the base state, re-

spectively, and ql and qi are liquid water (includes cloud FIG. 12. Horizontal distribution of buoyancy and vector horizontal water and rain water) and ice mixing ratios, respectively. wind at 1 km at 1847 CST 20 May 1977 in the Del City, OK, storm. By taking the divergence of (1) and assuming that Buoyancy (potential temperature ¯uctuation) has been ®ltered to re- -١␲ ϳϪ␲ (a reasonable assumption for ``well-be move noise, and is contoured at 1.5ЊC intervals (negative values dashed). Updraft maxima are denoted by ࠘ and vorticity maxima haved'' ®elds away from the boundaries), it can be are denoted by ᮍ. [From Hane and Ray (1985).] shown (Rotunno and Klemp 1982) that 862 MONTHLY WEATHER REVIEW VOLUME 130

22 2 ␲ lץ␲nlץ wץ w 1ץ ␷ץ uץ 22 ١w ϭϪcpp␪ Ϫ c ␪ ´ ␲ ϰϩϩϩ[|D| Ϫ |␩|] ϩ v zץ zץt ΂΃ץ z 2ץy ΂΃ץx ΂΃ץ΂΃[] ␲ץ Bץ ϩ , (3) ϩϪc ␪ b ϩ B (10) zץ z ΂΃pץ

␲ bץ␲dnץ -where | D | and | ␩ | are the magnitudes of the total de ϭϪcpp␪ ϩϪc ␪ ϩ B , (11) zץz ΂΃ץ /␷ץ) x)2 ϩץ/uץ)] formation and vorticity, respectively. The -z)2] terms are referred to as the ¯uid extenץ/wץ) y)2 ϩץ -z sometimes is referred to as the dyץ/␲dnץwhere Ϫcp␪ sion terms. If (3) is linearized about a base state con- -z ϩ B) sometimes is reץ/␲bץ namic forcing and (Ϫcp␪ taining vertical wind shear (primed velocity components ferred to as the buoyancy forcing. If the vertical gra- represent ¯uctuations from the base state, which is given dients of the ¯uid extension terms are neglected, along byv (z) ϭ [(u z),␷ (z), 0]), it may be rewritten as with the vertical gradients of the deformation and horizontal vorticity, then it can be shown that

␨2ץ ␲ץ 2 22 (wЈ nl ϰϪ (12ץ␷ЈץuЈץ ␲ ϰϩϩ z z ץ ץ zץy ΂΃ץx ΂΃ץ΂΃[] vץץ ␲ץ (١w (13 ´ ␷Ј l ϰץwЈץuЈץwЈץuЈץ␷Јץ ϩ 2 ϩϩ z z z ץ΂΃ ץ ץ zץ yץ zץ xץ yץ xץ΂΃ 2Bץ ␲ץ (B b ϰϪ , (14ץ vץ ١wЈϪ (4) z z2´ϩ 2 ץ ץ zץ zץ where ␨ is the vertical vorticity. ϭ ␲ ϩ ␲ ϩ ␲ (5) nllb From (11) it is evident that descent can arise owing to negative buoyancy, which can be generated according ϭ ␲dnϩ ␲ b, (6) to (2) by cold anomalies produced by evaporative cooling or hail melting, or by precipitation loading, and by where ␲nl, ␲l, and ␲b are the contributions to ␲ from vertical perturbation pressure gradients that can arise nonlinear, linear, and buoyancy effects, respectively, from, according to (12)±(14), vertical gradients of vertical vorticity, ``stagnation'' of environmental ¯ow at an updraft,8 and pressure perturbations due to vertical wЈ 2ץ22␷ЈץuЈץ ␲ ϰϩϩ buoyancy variations (which are partially due to hydro- z static effects), respectively.9 Research presented in theץy ΂΃ץx ΂΃ץnl []΂΃ past 40 years has found that all of the terms in (11) can .␷Ј be signi®cantץwЈץuЈץwЈץuЈץ␷Јץ ϩ 2 ϩϩ (7) z Browning and Ludlam (1962) and Browning andץ yץ zץ xץ yץ xץ΂΃ Donaldson (1963) suggested that the RFDs in the Wok- vץ ١wЈ (8) ingham and Geary supercells might have been driven´␲ ϰ 2 ''z by negative buoyancy (i.e., ``thermodynamicallyץ l B forced) due to evaporation. Browning (1964) surmisedץ ␲ ϰϪ , (9) that the rightward propagation of supercells increased zץ b

8 Note that the use of the term stagnation here does not imply that and ␲dn collectively refers to the nonlinear and linear supercell updraft are solid obstacles, as as been suggested by several effects as ``dynamic'' effects on ␲. Equation (7) indi- investigators in the past (e.g., Newton and Newton 1959; Fujita 1965; Fujita and Grandoso 1968; Alberty 1969; Fankhauser 1971; Charba cates that nonlinear dynamic high (low) pressure per- and Sasaki 1971; Brown 1992). Theoretical studies have exposed turbations are associated with convergence and diver- serious weaknesses in the obstacle analogy (e.g., Rotunno 1981; Ro- gence and deformation (rotation). Equation (8) indicates tunno and Klemp 1982; Davies-Jones et al. 1994), and these studies that linear dynamic high (low) pressure perturbations also have shown that the pressure distribution around an updraft is not what would be expected if the updraft was behaving as an ob- are located upshear (downshear) of an updraft. Equation stacle, except at the storm top (Davies-Jones 1985). Some studies (9) indicates that high (low) pressure perturbations due have shown that updrafts occasionally can display behavior that ap- to buoyancy are located above (below) the level of max- pears similar to how a solid obstacle might be expected to behave imum buoyancy. (e.g., Lemon 1976b; Klemp et al. 1981). 9 For additional discussion of downdraft forcings in terms of the Using (5) and (6), the vertical momentum equation vertical momentum equation, the reader is referred to the review by can be written as Knupp and Cotton (1985). APRIL 2002 MARKOWSKI 863 their midlevel storm-relative ¯ow so as to increase evap- (upshear) of the mesocyclone that they believed was orative cooling, and ultimately aid in the genesis of associated with a downdraft. Barnes' conclusion that downdrafts (both on the rear and forward storm ¯anks). the RFD formed between 6.0±7.5 km was based on his These hypotheses were proposed at least partly because study of tornadic storms in Oklahoma on 29±30 April of ®ndings by Browning and Ludlam (1962) and Brown- 1970. He surmised that the storm-relative midlevel ¯ow Ϫ1 ing and Donaldson (1963) of low ␪w air in the wakes (20±25 m s ) approaching the cyclonically rotating of the Wokingham and Geary storms, which apparently updraft was decelerated and de¯ected on the upwind had midlevel origins. (south) side while the relative upwind stagnation point Brandes (1981) also concluded that RFDs are initiated shifted to the left of the intercepting wind vector; that by the production of negative buoyancy aloft: ``presum- is, toward the southwest ¯ank. Here ``stagnating'' air ably the initiating downdraft (associated with the rear- experienced the longest contact with the adjacent up- ¯ank gust front) is formed by precipitation falling from draft while mixing only slightly with itÐboth cloud and the sloping updraft . . . we suppose the intruding ¯ow small precipitation drops chilled this air by evaporation has low ␪w, and when chilled by evaporation, becomes and began its downward acceleration before saturation negatively buoyant . . . because the entrained air pen- could occur. Barnes added ``We emphasize that the high etrates well into the storm, evaporative cooling rather horizontal momentum and proximity to the updraft than perturbation pressure forces may initiate the down- make the RFD a potentially important interactant with draft.'' Brandes (1984a) made a similar claim, based on the gust front and updraft's surface roots . . . We also retrieved buoyancy: ``at 3.3 km, cool temperatures on note that the location and extent of such a downdraft the southern fringe of the storm were suggestive of evap- probably depends upon the ambient ¯ow relative to the orative cooling as environmental air mixed with storm storm, which very likely requires a speci®c vertical air.'' shear pro®le to place it on the rear ¯ank of a storm Klemp et al. (1981) attributed the RFD in the Del where it attains an in¯uential position.'' Barnes inter- City storm to water loading and evaporation based on preted the large re¯ectivity gradient on the midlevel precipitation trajectories crudely approximated using es- upwind (southwest) ¯ank as indicating dry ambient air timated terminal fall speeds. Moreover, midlevel ¯ow adjacent to a precipitation-laden updraft. Bonesteele and approaching the storm from the east ¯owed through the Lin (1978) made a similar inference. FFDÐnot through the RFD as Browning (1964) had Lemon and Doswell (1979) developed a conceptual conceptualized. RFD air at the surface appeared to have model of a supercell from an extensive compilation of come from 1±2 km above the ground, directly behind surface, visual, and radar observations (Fig. 13). This the gust front, based on trajectory analyses in their nu- model included an FFD and RFD, a surface gust front merical simulation and observations of the storm. Air structure resembling a midlatitude cyclone, a hook- from higher levels reached the surface further behind shaped re¯ectivity region surrounding a cyclonically ro- the storm. tating updraft, and a tornado, if present, that resided Nelson (1977) found an erosion of the hydrometeor within the vertical velocity gradient between the updraft ®eld at and below 7 km, as well as a sharp re¯ectivity and RFD. This model has undergone little modi®cation gradient on the west ¯ank of an Oklahoma multicell since its presentation over 20 years ago. Based largely storm that evolved into a supercell on 25 May 1974Ð on the work by Barnes (1978a,b), Lemon and Doswell these radar observations were believed to have been a inferred that the RFD typically originated between 7± manifestation of RFD formation that apparently oc- 10 km on the relative upwind side of the updraft [note curred at the start of the transition from multicell to that they did not say upshear side; refer to (8); Rotunno supercell. Nelson noted two mechanisms suggestive of and Klemp (1982) showed that the linear forcing for RFD formationÐevaporative cooling and/or dynamic pressure ¯uctuations depends on the vertical shear, and pressure perturbations (presumably he was referring to numerical results con®rmed this theoretical prediction, those related to linear effects, i.e., stagnation). Nelson as did some later dual-Doppler radar ®ndings (e.g., Hane believed that the evaporation-driven effect was more and Ray 1985)]. The authors cited the observation of likely because of the echo erosion aloft; he also cited an echo-free hole at 7.5 km, directly above a notch strong storm-relative winds (ϳ16msϪ1 in the 7±9 km behind the low-level hook echoÐthey believed this to layer) and a large dewpoint depression (ϳ21 K) at the be the signature of the RFD. Lemon and Doswell pro- level of apparent RFD formation. Forbes (1981) found posed that storm-relative in¯ow impingement was the similar radar signatures suggesting echo erosion and RFD source, because Darkow and McCann (1977) RFD formation during the 3±4 April 1974 tornado out- showed that the relative ¯ow at these levels is much break. stronger than the storm-relative ¯ow minimum they Lemon et al. (1978) and Barnes (1978a) also con- found at 4 km, and because of the Barnes (1978a,b) and cluded that the RFD forms at middle to upper levels. Nelson (1977) observations. Lemon and Doswell also Lemon et al. based their ®ndings on an analysis of the hypothesized that the RFD initially is dynamically Union City tornadic supercell. They analyzed a persis- forced, and then enhanced and maintained by precipi- tent di¯uent ¯ow region in the 7±10 km layer northwest tation drag and evaporative cooling. 864 MONTHLY WEATHER REVIEW VOLUME 130

behind the gust front, air from higher levels reached the surface. For the sake of completeness, it might be worth men- tioning that Shapiro and Markowski (1999) recently investigated the formation of downdrafts in simple two- layer vortices using an analytic model.10 The applica- bility of the idealized model to real atmospheric vortices, in which buoyancy, buoyancy gradients, precipitation, and asymmetries probably are important, is questionable. Their results demonstrated how the ``vortex valve'' effect (Lemon et al. 1975; Davies-Jones 1986) can transport vorticity from the top of a homo- geneous, axisymmetric, rotating ¯uid to low levels via an annular downdraft and secondary circulation, when the top layer of ¯uid rotates with an angular velocity larger than that of the bottom layer of ¯uid. Prior to 1983, investigators sought forcing for the RFD from middle and upper levels, as has been reviewed in this section. But downdraft forcing also can arise at low levels. This is the subject of the next section.

4. Occlusion downdrafts FIG. 13. Conceptual model of a tornadic supercell at the surface based on observations and radar studies. Thick line encompasses a. Evolution as simulated in numerical models radar echo. The thunderstorm gust front structure and occluded wave also are depicted using a solid line and frontal symbols. Surface Klemp and Rotunno (1983) investigated the transition positions of the updraft (UD) are ®nely stippled, forward-¯ank downdraft (FFD) and rear-¯ank downdraft (RFD) are coarsely stippled. of a supercell into its tornadic phase through use of a Associated streamlines are also shown. Tornado location is shown by high-resolution (250-m horizontal grid spacing) model an encircled T. [From Lemon and Doswell (1979).] initiated within the interior of the domain of the Del City supercell simulation performed by Klemp et al. Brandes (1984a) retrieved excess pressure aloft on (1981). With the enhanced resolution, Klemp and Ro- the rear of the Del City storm, which may have sug- tunno found that the low-level cyclonic vorticity in- gested that the RFD was partly forced by a downward- creased dramatically and the gust front rapidly occluded directed dynamic pressure gradient, as Lemon and Do- as small-scale downdrafts developed in the vicinity of swell had proposed, but the vertical pressure gradients the low-level circulation center. They concluded that the in the stagnation region could not be examined due to intensi®cation of the RFD during the occlusion process a paucity of scatterers and corresponding lack of radar was dynamically driven by the strong low-level circu- velocity data. (A lack of data due to the pristine air lation, that is, by way of a dynamic perturbation pressure common in RFD regions probably still is one of the gradient such as that given by (12). This was the ®rst biggest obstacles in understanding the formation mech- study to propose such a mechanism for downdraft gen- anisms and the role of the RFD today.) esis and intensi®cation. Later, Brandes (1984a,b), Hane Klemp et al. (1981) simulated a supercell with a com- and Ray (1985), and Brandes et al. (1988; see section posite sounding derived from three ``proximity'' sound- 3) made the same conclusion based on Doppler radar ings on 20 May 1977, and compared the simulated storm analyses of tornadic storms, as did Trapp and Fiedler characteristics to those observed in the Del City storm. (1995), Wicker and Wilhelmson (1995), and Adlerman ``Trajectory'' analysis (these were not true trajectories, et al. (1999) based on numerical simulations. but rather streamlinesÐif the storm was assumed to be Klemp and Rotunno (1983) de®ned the RFD as the quasi-steady, then the streamlines would be similar to downdraft ``which supports the storm out¯ow behind trajectories) in the simulated supercell showed obstacle- the convergence line on the right ¯ank.'' They stated like ¯ow at 7±10 km. Parcels at 7 km that impinged that since nontornadic storms often were observed to upon the upshear side of the updraft did not appear to persist for long periods of time with a well-de®ned gust sink [in contrast to observations made in different cases front, these storm-scale downdrafts were not uniquely by Barnes (1978a), Nelson (1977), and Lemon and Do- linked to tornadogenesis within a storm. On the other swell (1979)], but those at 4 km did; that is, the RFD hand, noted Klemp and Rotunno, if a storm progressed apparently was 4±7 km deep (parcels from 4±7 km did into a tornadic phase, the gust front became occluded not reach the surface, but negative vertical velocities extended to 4±7 km). Directly behind the gust front, 10 Idealized three-layer vortices also were investigated using a nu- RFD air appeared to come from 1±2 km aloft; farther merical model. APRIL 2002 MARKOWSKI 865

The ®nding of Klemp and Rotunno that the occlusion downdraft is driven by low-level rotation sometimes has been implied as being in con¯ict (e.g., Carbone 1983; Brandes 1984a; Klemp 1987) with their predecessors' early hypotheses that the RFD is driven from aloft thermodynamically or dynamically and is responsible for increasing low-level rotation (e.g., Fujita 1975b; Bur- gess et al. 1977; Barnes 1978a; Lemon and Doswell 1979). However, it is the author's opinion that the apparent con¯ict may be one of semantics. If the occlusion downdraft and RFD are to be viewed as two distinct entities, as proposed by Klemp and Rotunno, then the formation mechanisms and roles of the occlusion downdraft and RFD should not be anticipated to be necessarily identical. Observations of RFD formation preceding the increase of vorticity near the ground are plen- tiful (e.g., Barnes 1978a; Lemon and Doswell 1979; Brandes 1984a,b). Once a downdraft forms, the distribution of vortex lines invariably must be affected according to Helmholtz's theorem, and feedbacks on the

FIG. 14. Low-level ¯ow ®eld (z ϭ 1 km) in Klemp and Rotunno's downdraft by the new vorticity distribution would be (1983) nested, 250-m horizontal resolution simulation of the Del City, probable (e.g., when rotation near the ground becomes OK, supercell (20 May 1977) at the time that an occlusion downdraft substantial, a downward-directed vertical pressure gra- was observed. The region in which the cloud water exceeds 0.4 g kgϪ1 dient could become established, accelerating air toward is shaded. The heavy black line encloses the region where the rain water mixing ratio exceeds 0.5 g kgϪ1. The circulation center is the ground). Early hypotheses that the RFD is respon- indicated with the black dot. Vertical velocity is contoured at 5 m sϪ1 sible for bringing rotation to low levels never asserted intervals. What has been referred to as the occlusion downdraft is that once low-level rotation began to intensify that dy- located within a broader region of negative vertical velocities, which namic effects could not feed back on the downdraft. has been referred to as the rear-¯ank downdraft. [Adapted from Klemp Therefore, it is believed that the occlusion downdraft and Rotunno (1983).] can be viewed as a rapid, small-scale intensi®cation of the RFD that occurs after the RFD transports larger and a strong downdraft formed directly behind the gust angular momentum air toward the ground. Stated an- front at low levels and also might divide the updraft at other way, the occlusion downdraft may be viewed as midlevels. Klemp and Rotunno referred to this smaller- a by-product of the near-ground vorticity increase, and scale downdraft as the occlusion downdraft. The occlu- vorticity cannot become large next to the ground without sion downdraft, which was associated with a local ver- the RFD (Davies-Jones 1982a,b; Davies-Jones and tical velocity minimum, was situated within a broader Brooks 1993; Brooks et al. 1993, 1994; Wicker and region of negative vertical velocity, which was referred Wilhelmson 1995); that is, the RFD is ultimately nec- to as the RFD; that is, the downdraft region on the rear essary for occlusion downdraft formation. Perhaps not side of the storm, which partially wrapped around the surprisingly, observations have not been made in su- circulation center at the time when low-level vertical percell storms of an occlusion downdraft preceding or vorticity reached its peak, was a single, contiguous en- occurring in the absence of an RFD, or even at a location tity (Fig. 14). The occlusion downdraft also was located not within an RFD.11 within a larger, contiguous downdraft region in the high- It might be tempting to argue that the RFD and oc- resolution simulations of Wicker and Wilhelmson clusion downdraft should be considered separate down- (1995; their Figs. 5±9). Klemp and Rotunno proposed that the occlusion pro- 11 In some nonsupercell tornadoes in which preexisting vertical cess and its associated occlusion downdraft were dy- vorticity is present at the surface, an RFD is not needed to transport namically induced by the strong near-ground rotation circulation to low levels in order for tornadogenesis to occur. It might that evolved along the convergence line. The rotation be possible for the low-level vorticity ampli®cation associated with induced low pressure coincident with the center of cir- this tornadogenesis process to dynamically induce a downdraft similar to that which Klemp and Rotunno (1983) called an occlusion down- culation and dynamically forced air down from aboveÐ draft in a supercell storm; thus, it might be possible for an occlusion the downdraft formed ®rst at low levels and then ex- downdraft to occur in the absence of an RFD in such a case. Carbone tended upward as the ¯ow adjusted to the nonhydrostatic (1983) compared a downdraft he observed in a nonsupercell tornado vertical pressure gradient force. Rotunno (1986) also event to the occlusion downdraft de®ned by Klemp and Rotunno. However, the dominant forcing for the downdraft was uncertain; thus, hypothesized that the occlusion downdraft was initiated it is not known whether the downdraft documented by Carbone (1983) by the explosive growth of vertical vorticity at low lev- should be regarded as an occlusion downdraft, at least as de®ned by els. Klemp and Rotunno. 866 MONTHLY WEATHER REVIEW VOLUME 130 drafts because the dominant forcings are different. It is to be ``driven'' by low-level rotation even if the occlu- the author's opinion that a contiguity criterion should sion downdraft is not collocated with the low-level ro- be considered when debating whether two phenomena tation.12 having different forcings are regarded as ``separate.'' Otherwise, the updraft of a supercell should be viewed b. Observations as two separate updrafts, with one updraft being driven by nonhydrostatic pressure gradient forces below the Brandes (1978; the Harrah storm) appears to have level of free convection, and another updraft being driv- made observations prior to the Klemp and Rotunno en largely by buoyancy forces above the level of free (1983) simulation of a downdraft not becoming prom- convection. The evolution put forth in the paragraph inent until after low-level rotation became substantial. above is not at odds with early proposals that the RFD Brandes (1981) also stated, following his analysis of the is initiated at middle to upper levels and is responsible Del City storm, that ``the sudden appearance of strong for initiating rotation near the ground, nor is it in con¯ict rear downdrafts in storms persisting for hours may also with contentions that strong subsidence develops near relate to the intensity and distribution of updrafts and the tornado during or after its formation. vorticity.'' Brandes (1984a) attributed sudden occlusion It is speculated that the clear slot may be a visual downdraft formation in the Del City and Harrah storms manifestation of an intensifying RFD or occlusion to the vertical pressure gradient owing to the explosive downdraft. Updrafts also have been shown to weaken growth of low-level vorticity as Klemp and Rotunno during the stage when low-level rotation rapidly in- (1983) found. Furthermore, Brandes's data also showed creases, probably also due to the formation of a down- that the occlusion downdraft did not descend along the ward-directed dynamic pressure gradient induced by the axis of the strong low-level vorticity. Brandes (1984b) rotation (e.g., Brandes 1984a,b). Fujita (1973), Lemon claimed that the occlusion downdraft formed after the and Burgess (1976), and Burgess et al. (1977) have incipient tornado had been detected, and roughly co- documented the collapse of overshooting storm tops incided with tornado formation. Hane and Ray (1985) near the time of tornadogenesis, which presumably is a also documented occlusion downdraft formation in the manifestation of updraft weakening. Del City storm. It also should be noted that although the occlusion Based on their analyses of the Lahoma and Orienta, downdraft in the Klemp and Rotunno (1983) simulation Oklahoma, supercells (2 May 1979), Brandes et al. was found to be driven by low-level vertical vorticity (1988) hypothesized that because RFDs possess weak ampli®cation, the occlusion downdraft did not descend positive or negative helicity (because couplets of ver- along the axis of low-level rotation. An explanation was tical vorticity straddle RFDs), the decline of storm cir- not offered. One might expect that the vertical pressure culation might be hastened by turbulent dissipation gradient associated with the vertical gradient of vertical when the downdraft air eventually mixes into supercell vorticity would lead to a maximum acceleration along updrafts. As did Brandes (1984a,b) and Klemp and Ro- the rotation axis. Two reasons might account for the tunno (1983), Brandes et al. claimed that ``the updraft asymmetry: 1) the dynamic vertical perturbation pres- minimum in the Lahoma storm and RFD in the Orienta sure gradient associated with the vertical gradient of storm apparently owed their existence to the build-up z) does not contribute to of low-level vorticity and related downward verticalץ/␨2ץ) vertical vorticity squared vertical velocity directly, but rather to vertical accel- pressure gradients.'' Large downward pressure forces erationsÐthus, dw/dt might be a minimum in the vor- existed within the RFD and left-hand portions of the ticity maximum center, but if this occurs within the up- persistent updraft region in the Orienta storm, and to draft (where w k 0), then a downdraft (w Ͻ 0) may the rear of the persistent updraft in the Lahoma storm. ®rst appear on the periphery of the updraft, away from Brandes et al. (1988) probably presented the most com- the center of rotation, where w is less positive; and 2) prehensive analyses, discussion, and insight into the other terms in the vertical momentum equation, when pressure distribution in supercells to date. combined with the dynamic vertical perturbation pressure gradient force, may force the strongest downward acceleration away from the axis of largest vertical vor- 5. Role of RFDs in tornadogenesis ticityÐfor example, the buoyancy forcing may favor a. Observations-based hypotheses ascent in the updraft center, so that the net effect of the buoyancy forcing and dynamic vertical perturbation Ludlam (1963) was one of the ®rst to write that down- pressure gradient may lead to the strongest downward drafts, especially those located on the rear ¯ank of su- acceleration on the updraft periphery. Superposition of percells, actually may be important to tornadogenesis: the ®elds of the vertical momentum equation forcing ``It is tempting to look for the spin of the tornado in terms in Klemp and Rotunno's (1983) simulation leads to the strongest downward acceleration being to the 12 Wakimoto and Cai (2000) proposed that it also may be possible southeast of the maximum low-level rotation (Fig. 15). for an occlusion downdraft to reach the surface away from the center Thus, it is entirely possible for an occlusion downdraft of strongest near-ground vorticity if a mesocyclone is vertically tilted. APRIL 2002 MARKOWSKI 867

FIG. 15. (a)±(c) Cross sections of the forcing terms in the vertical momentum equation at t ϭ 2 min and z ϭ 250 m in the nested, 250-m horizontal resolution simulation by Klemp and Rotunno (1983). Contours are drawn at 10Ϫ2 msϪ2 intervals: (a) dynamically induced pressure gradient, (b) advection terms, and (c) buoyancy forcing. The black dot indicates the location of the circulation center. [From Klemp and Rotunno (1983).] (d) A composite of the forcing terms in the vertical momentum equation. The hook echo and regions where w exceeds 0 and3msϪ1 are shaded according to the legend. The region where downward local t) are largest also is shaded. This region is located where the superpositionץ/wץ) vertical velocity accelerations of the buoyancy forcing, advection, and dynamic pressure forcing leads to the strongest downward local t K 0). The regions where the buoyancy forcing and advection of verticalץ/wץ) vertical velocity changes velocity are positive, and where the dynamic vertical pressure gradient is most negative (directed downward), also are indicated in the legend. Note that the largest downward acceleration occurs southeast of the low- level circulation center; a downdraft driven by increasing low-level rotation need not be collocated with the axis of strongest rotation. [Adapted from Klemp and Rotunno (1983).] the vorticity present in the general air stream as shear nado, 2) this process results in an appreciable conver- and tilted appropriately in the vicinity of the interface gence on the back side of the (developing) tornado, and between the up- and down-motions.'' Fujita (1975b) 3) the downward transport of the angular momentum also proposed that the downdrafts associated with hook by precipitation and the recycling of air into the tornado echoes may be fundamentally critical to tornado for- will create a tangential acceleration required for the in- mation, in terms of his ``recycling hypothesis'': 1) tensi®cation of the tornado. Research conducted with downdraft air is recirculated into the (developing) tor- the aid of coherent radars in the ensuing years led others 868 MONTHLY WEATHER REVIEW VOLUME 130

(e.g., Burgess et al. 1977; Barnes 1978a; Lemon and Doswell 1979; Brandes 1981) to make the same general speculation. Burgess et al. (1977) believed that the RFD, hook echo, and tornadogenesis were intimately con- nected: ``The formation and evolution of the RFD is judged extremely important to tornado formation. . .the severe tornado [the Oklahoma City tornado of 8 June 1974] appears related to the increased vorticity source provided by presumed downdraft intensi®cation and gust front acceleration along the right ¯ank.'' Forbes (1981) also discovered signatures (e.g., a sharp re¯ectivity gradient along the upshear side of the updraft, and occasionally a small echo mass several kilometers to the right of the right-rear edge of the main echo) suggesting RFD formation (1±10 min) prior to tornadogenesis. Lemon and Doswell (1979) noted that just before tornadogenesis, the mesocyclone center shifted from FIG. 16. Brandes's (1978) conceptual model of low-level meso- near the updraft center to the zone of high vertical ve- cyclone characteristics during the tornadic phase included an oc- locity gradient. The early mesocyclone apparently was cluded mesocyclone with air parcels from the RFD feeding the tor- a rotating updraft, whereas the transformed mesocy- nado. [Adapted from Brandes (1978).] clone had a divided structure, with strong cyclonically curved updrafts to the east in the ``warm in¯ow sector'' and strong cyclonically curved downdrafts to the west clones being nearly totally occluded by the RFD, as in the ``cold out¯ow sector.'' And while the tornado was evidenced by observations of the clear slot during and apparently found in a strong vertical velocity gradient, just prior to the tornadic stage, such as those made by Lemon and Doswell noted that it probably was located Lemon and Doswell (1979), Rasmussen et al. (1982), on the updraft side of that gradient. and Jensen et al. (1983), also may imply that the air Lemon and Doswell explained the evolution of the entering the tornado comes from the RFD. The possible RFD and tornadogenesis as follows: 1) air decelerates importance of the clear slot also did not escape the at the upwind stagnation point, is forced downward, and attention of Ludlam (1963): ``. . . often the funnel is mixes with air below, which then reaches the surface photographed spectacularly against a segment of bright through evaporative cooling and precipitation drag; 2) and practically cloud-free sky beyond the edge of the the initially rotating updraft is then transformed into a arch cloud.''14 In addition to the observational evidence, new mesocyclone with a divided structure, in which the simulations also have indicated that air parcel trajec- circulation center lies along the zone separating the RFD tories pass through the RFD en route to intensifying from the updraft (this process appears to result, in part, near-ground circulations (Davies-Jones and Brooks from tilting of horizontal vorticity); and 3) ``descent of 1993; Wicker and Wilhelmson 1995; Adlerman et al. the mesocyclone circulation occurs simultaneously 1999). (within the limits of temporal resolution) with the de- Davies-Jones (1998) recently has questioned whether scent of the RFD.'' the hook echo is really a passive indicator of a tornado, Observations of low-level vorticity couplets within since close-range airborne and mobile radar observa- RFDs that seem to straddle the hook echoes (introduced tions during recent ®eld experiments have revealed hook in section 2a) may be indications that tilting of vorticity echo formation prior to tornadogenesis in every case.15 by the RFD is important in the formation of tornadoes Davies-Jones hypothesized that the hook echo may ac- within supercell storms, as hypothesized by Davies- tually instigate tornadogenesis, either baroclinically, by Jones (1982a,b) and Davies-Jones and Brooks (1993). way of buoyancy gradients within the hook echo (Da- Furthermore, it may be worth noting that during the vies-Jones 2000a), or barotropically, by redistributing tornadogenesis phase in supercells, the parcels of air angular momentum (Davies-Jones 2000b). that enter the tornado or incipient tornado regularly seem to pass through the hook echo and RFD (Brandes 14 The `àrch cloud'' Ludlam refers to was that `òn the forward 1978; Klemp et al. 1981; Dowell and Bluestein 1997; right ¯ank of severe storms, outside of the precipitation region;'' that J. Wurman et al. 2000, personal communication;13 Fig. is, that which is on the leading edge of the trailing gust front. 16), which may have been the basis for Fujita's (1975b) 15 Previous studies documenting cases in which tornadoes were recycling hypothesis. Visual observations of mesocy- reported prior to hook echo detection (e.g., Garrett and Rockney 1962; Forbes 1975) relied on temporally and spatially coarser radar data compared to what was available in ®eld experiments such as VOR- 13 This was a poster presented at the 20th Severe Local Storms TEX, and reported tornado times may not always have been accurate Conference in Orlando, Florida. to within a few minutes. APRIL 2002 MARKOWSKI 869 b. Theoretical considerations Most of the theoretical and numerical modeling studies pertaining to supercell storms have investigated the development of midlevel and low-level rotation by way of tilting of horizontal vorticity (either associated with the large-scale mean vertical wind shear or generated solenoidally by a baroclinic zone) by an updraft (e.g., Rotunno 1981; Rotunno and Klemp 1982, 1985; Lilly 1982, 1986a,b; Davies-Jones 1984). However, Davies- Jones (1982a,b) noted that in order to obtain large vertical vorticity at the ground in an environment in which vortex lines are initially quasi-horizontal, a downdraft would be necessary. Tornadoes may arise in the absence of a downdraft in environments containing preexisting vertical vorticity at the surface, such as in some cases FIG. 17. Schematic diagram showing how cyclonic vorticity may of ``nonsupercell tornadogenesis'' (e.g., Wilson 1986; be generated from tilting of baroclinic horizontal vorticity in a down- Wakimoto and Wilson 1989; Roberts and Wilson 1995; draft. In the case of streamwise vorticity with ¯ow to the right of the horizontal buoyancy gradient and a southerly shear component, a Lee and Wilhelmson 1997a,b, 2000). combination of tilting and baroclinic generation causes the vorticity Davies-Jones (1982a,b) concluded that in a sheared of parcels to change from anticyclonic (denoted by a) to cyclonic environment with negligible background vertical vor- (denoted by c) while still descending. [Adapted from Davies-Jones ticity, an `ìn, up, and out'' circulation driven by forces and Brooks (1993).] primarily aloft would fail to produce vertical vorticity close to the ground [this conclusion depends on eddies with less inclination than the trajectories because hor- being too weak to transport vertical vorticity downward izontal southward vorticity was being generated contin- against the ¯ow; this was veri®ed by Rotunno and uously by baroclinity within the hook echo. Because of Klemp (1985) and Walko (1993)]. If a Beltrami model the geometry, vortex lines crossed the streamlines from is crudely assumed to represent the ¯ow in a supercell lower to higher ones (with respect to the ground), and (Davies-Jones and Brooks 1993), then vortex lines are the barotropic effect served to turn the vortex lines up- coincident with streamlines and parcels ¯owing into the ward even during descent. The baroclinic effect acted updraft at very low levels do not have signi®cant vertical to increase horizontal vorticity further but did not con- vorticity until they have ascended a few kilometers. Oth- trol the sign of the vertical vorticity; thus, air with cy- erwise, argued Davies-Jones, abrupt upward turning of clonic vertical vorticity appeared close to the ground. streamlines, strong pressure gradients, and large vertical As this air passed from the downdraft into the updraft, velocities would be required next to the ground. its cyclonic spin was ampli®ed substantially by vertical Davies-Jones (1982a,b) neglected baroclinic vorticity stretching (Fig. 17). and suggested that the downdraft had the following roles Brooks et al. (1993, 1994) found that the formation in near-ground mesocyclogenesis: 1) tilting of horizon- of persistent near-ground rotation was sensitive to the tal vorticity by a downdraft produces vertical vorticity, strength of the storm-relative midlevel winds. When 2) subsidence transports air containing vertical vorticity storm-relative midlevel ¯ow was weak, RFD out¯ow closer to the surface, 3) this air ¯ows out from the undercut the updrafts and associated mesocyclones. downdraft and enters the updraft where it is stretched When storm-relative midlevel ¯ow was too strong, the vertically, and 4) convergence beneath the updraft is cold pool was not oriented suitably for vorticity gen- enhanced by the out¯ow. Davies-Jones also showed ki- eration within the baroclinic zone immediately behind nematically that the ¯ow responsible for tilting and con- the updraft, which was found to be needed for the de- centrating vortex lines also tilts and packs isentropic velopment of near-ground rotation in their simulations. surfaces, thus explaining observations of strong entropy Wicker and Wilhelmson (1995) used a two-way in- gradients across mesocyclones near the ground. teractive grid to study tornadogenesis. During a 40-min period, two tornadoes grew and decayed within the me- c. Potentially relevant simulation results socyclone. Wicker and Wilhelmson's Fig. 9 depicted a spiraling, asymmetric RFD associated with tornadoge- Davies-Jones and Brooks (1993) showed that the ver- nesis. Their ®gure also indicated anticyclonic vertical tical vorticity of air parcels descending in an RFD can vorticity on the opposite side of the RFD as the cyclonic be reversed during descent, from anticyclonic initially, vertical vorticity. Furthermore, the RFD contained low to less anticyclonic, then to cyclonic in the lowest 50± ␪e values (␪Јee was as small as Ϫ15 K;␪Ј was approxi- 125 m of their descent. As air subsided in the downdraft, mately Ϫ5toϪ8 K in the hook echo). vortex lines turned downward due to the barotropic Wicker and Wilhelmson found that parcels entered ``frozen ¯uid lines'' effect (Helmholtz's theorem), but the mesocyclone from the RFD (Fig. 18) and descended 870 MONTHLY WEATHER REVIEW VOLUME 130

they did not specify whether the downdraft also could be cold, nor what the advantages of a warm downdraft over a cold downdraft were. Davies-Jones (2000b) recently has shown that an annular rain curtain can transport suf®cient angular momentum from aloft to the ground to result in tornadogenesis in an idealized axisymmetric numerical model.16 No evaporation was per- mitted in the model; thus, no cold downdraft air was present. Furthermore, Leslie and Smith (1978) found that some vortices could not establish contact with the ground when low-level stable air was present, even if very shallow. Remarkably, Ludlam (1963) many years earlier had argued that ``at least a proportion of the air that ascends in the tornado must be derived from the cold out¯ow; if this contains the potentially cold air from middle levels its ascent might be expected soon to impede if not destroy the tornado . . . it may be par- ticularly important for the intensi®cation and persistence of a tornado that some of the downdraft air may be derived from potentially warm air which enters the left ¯ank of the storm at low-levels.'' FIG. 18. Vertical velocity and trajectories at 100 min (tornadolike Signi®cant advances in our understanding of super- vortex present in simulated storm at this time) for z ϭ 100 m in the cells no doubt have been made by numerical models. It 120-m resolution simulation by Wicker and Wilhelmson (1995). The trajectories entering the vortex have come from the hook echo and is probable that some conclusions drawn from simula- RFD region. [Adapted from Wicker and Wilhelmson (1995).] tion results never could have been made from observations or theory alone. However, the author shares the view expressed by Doswell (1985): ``The RFD's role from ϳ500 m, as Davies-Jones and Brooks (1993) had remains confusing with respect to tornadogenesis. Truly found. Furthermore, these parcels initially contained con®rming evidence about the various aspects of nu- negative vertical vorticity; however, vertical vorticity merical simulations awaits better observations, despite increased to only weakly negative values, not to large the compelling similarities between simulations and real positive values as in the simulations of Davies-Jones storms.'' and Brooks. Trajectories into the tornado from the RFD revealed that positive vertical vorticity was acquired only after parcels began to ascend, not while they were 6. Recent observations from VORTEX still descending (Davies-Jones and Brooks 1993; Trapp New radar and surface observations having unprec- and Fiedler 1995; Adlerman et al. 1999). It is believed edented spatial and temporal resolution have been ac- that these differences should be regarded as minor, for quired by VORTEX and smaller, post-VORTEX ®eld there is a time tendency for increasing positive vertical experiments. The ®ndings of these recent operations that vorticity in the parcels passing through the downdraft are pertinent to this review are highlighted below, with in all of the studies. the exception of analyses of new surface data within Though numerical models have been instrumental in hook echoes and RFDs, which will be presented by advancing our understanding of supercell dynamics, Markowski et al. (2002). models may have limited utility in exploring the ther- Recent radar observations, which include data ob- modynamic characteristics of RFDs, and the potential tained from both airborne and ground-based mobile sensitivity of low-level vorticity intensi®cation to these Doppler radars (Jorgensen et al. 1983; Ray et al. 1985; characteristics, owing to the unavoidable parameteri- Bluestein and Unruh 1989; Bluestein et al. 1995; Daugh- zation of microphysical processes. For example, three- erty et al. 1996; Wurman et al. 1997), have resolved dimensional simulations have not been able to produce structures within hook echoes that were barely resolv- the warm and moist RFDs that often have been observed able or unresolvable in the early radar studies of su- near some strong tornadoesÐthe RFDs of simulated percell storms. For example, in high-resolution (Ͻ100 supercells almost invariably have large potential tem- m spatially) data of tornadoes presented by Wurman et perature and equivalent potential temperature de®cits, as discussed in section 3c. However, some idealized simulations have suggested that tornadogenesis may be 16 This simulation had some similarities with those conducted by Das (1983, unpublished manuscript), in which a precipitation-driven favorable if downdrafts are not too cold. Eskridge and downdraft was imposed upon a wind ®eld in which angular momen- Das (1976) proposed that a warm, unsaturated down- tum increased with height. The downdraft was found to be able to draft could be important for tornadogenesis; however, transport suf®cient vorticity from aloft to result in tornadogenesis. APRIL 2002 MARKOWSKI 871

FIG. 19. Radar (left) re¯ectivity (dBZ) and (right) velocity ®elds associated with a hook echo and tornado on 2 Jun 1995 near Dimmitt, TX. In the left image, the arrow labeled N points toward the north. In the right image, the arrow labeled R points toward the radar position. [From Wurman and Gill (2000).] al. (1996), Wurman and Gill (2000), and Bluestein and to the surface during tornadogenesis, as many previous Pazmany (2000), the echo-free holes that were only mar- investigators (e.g., Ludlam, Fujita) have conjectured. ginally resolved by Garrett and Rockney (1962) were Using surface observations obtained from automo- well resolved (Figs. 19 and 20). The images presented bile-borne sensors (Straka et al. 1996), Rasmussen and by Bluestein and Pazmany (2000) even begin to mar- Straka (1996) documented a relatively warm RFD south ginally resolve structures, possibly subvortices, within of the Dimmitt, Texas, tornado. In the same case, which the echo-free hole itself. Moreover, the radar re¯ectivity was during VORTEX, the hook echo was collocated depictions ``looked like a tropical cyclone, with con- with the surface divergence maximum, implying an as- centric inner bands and outer spiral bands'' (Bluestein sociation between the hook echo and (at least) a low- and Pazmany 2000). Hook echoes as narrow as 100 m level downdraft, as also had been suggested by numer- or less have been detected (Wurman et al. 1996; Blue- ous predecessors. stein and Pazmany 2000), perhaps implying that pre- In two other VORTEX storms, Wakimoto et al. (1998) cipitation loading and evaporative cooling within the and Wakimoto and Cai (2000) concluded that the oc- hook echoes of some storms may not be the most sig- clusion downdraft was driven largely by the reversal of ni®cant effects in driving the associated RFDs, at least the vertical gradient of dynamic pressure, owing to in- at low levels. creasing vorticity at low levels. In the supercell docu- Just as the early multiple-Doppler radar analyses of mented by Wakimoto et al. (1998; the 16 May 1995 the 1970s and 1980s revealed, radar observations ob- VORTEX storm), it was found that the precipitation- tained in the last decade also have detected vorticity loading forcing of the occlusion downdraft was an order couplets straddling the hook echoes of tornadic storms of magnitude less than the forcing provided by the non- (Rasmussen and Straka 1996; Wurman et al. 1996; hydrostatic vertical pressure gradient. [In a different Straka et al. 1996; Bluestein et al. 1997; Dowell and case, Carbone (1983) previously had suggested that pre- Bluestein 1997; Dowell et al. 1997; Wakimoto and Liu cipitation loading may contribute to occlusion down- 1998; Wakimoto et al. 1998; Wurman and Gill 2000; draft genesis.] In the 12 May 1995 VORTEX storm Ziegler et al. 2001) and nontornadic storms (Gaddy and studied by Wakimoto and Cai (2000), a thermodynamic Bluestein 1998; Blanchard and Straka 1998; Wakimoto retrieval indicated that the occlusion downdraft was as- and Cai 2000; Bluestein and Gaddy 2001). These vor- sociated with a warm core. ticity doublets could be evidence that RFDs are involved Perhaps the most remarkable observational ®nding in a downward displacement of initially quasi-horizontal during the last 10 years is that the differences between vortex lines, perhaps necessarily transporting rotation tornadic and nontornadic supercells may be subtle, if 872 MONTHLY WEATHER REVIEW VOLUME 130

FIG. 20. Radar re¯ectivity ®elds (dBZ) associated with a hook echo on 15 May 1999 (left) early in the life of a tornado, (center) during the mature stage, and (right) during the dissipation stage. [From Bluestein and Pazmany (2000).] even distinguishable, even in dual-Doppler radar anal- years; however, the precise dynamical relationship still yses of the wind ®elds just prior to tornadogenesis. Blan- is not known today. The analysis of the three-dimen- chard and Straka (1998) documented a mobile radar sional wind structure of supercells afforded by Doppler signature of a spiraling hook echo in a nontornadic su- radar, along with speedy increases in the feasibility of percell having an appearance similar to those that have numerical cloud modeling, led to relatively rapid gains been associated with tornadic supercells (e.g., similar in knowledge of the recurrent storm structures and evo- to the radar image in Fig. 20). Perhaps such near-ground lution associated with supercells. Within 30 years of the circulations are more common in nontornadic supercells ®rst radar image of a hook echo, we knew that the most than previously believed. Trapp (1999) and Wakimoto damaging tornadoes were associated with supercells, we and Cai (2000) also documented circulations in non- knew about the existence of the parent circulations of tornadic supercells at levels close to the ground. Trapp tornadoes (mesocyclones), we developed an understand- found that the low-level mesocyclones associated with ing of the dynamics of midlevel storm rotation and storm tornadogenesis had smaller core radii and were asso- propagation, radar and visual features common to su- ciated with more substantial vorticity stretching than percells were well documented, and downdrafts were those associated with tornadogenesis ``failure.'' Based recognized as being important in tornadogenesis. Yet in on a comparison of pseudo-dual-Doppler analyses, Wak- the decades that followed the period of rapid advances, imoto and Cai concluded that the ``only difference be- no breakthroughs emerged with respect to the role of tween the Garden City storm and Hays storm (the 16 the RFD in tornadogenesis. In fact, it is debatable wheth- May 1995 and 12 May 1995 VORTEX storms) was the er we can better anticipate tornadogenesis within su- more extensive precipitation echoes behind the rear- percell storms today than we could 20 years ago. ¯ank gust front for the Hays storm.'' If the hook echo and its associated RFD truly are critical to tornadogenesis, as hypothesized for many years, then perhaps signi®cant gains in understanding 7. Concluding remarks will not be possible until more spatially and temporally The association between hook echoes, RFDs, and tor- detailed observations of this region can be made, in nadogenesis has been well documented for nearly 50 addition to numerical simulations with more realistic APRIL 2002 MARKOWSKI 873 representations of entrainment and microphysical pro- Blanchard, D. O., and J. M. Straka, 1998: Some possible mechanisms cesses. It is believed that some of the important out- for tornadogenesis failure in a supercell. Preprints, 19th Conf. on Severe Local Storms, Minneapolis, MN, Amer. Meteor. Soc., standing questions include 116±119. R What are the dominant forcings for RFDs, as a func- Bluestein, H. B., 1983: Surface meteorological observations in severe thunderstorms. Part II: Field experiments with TOTO. J. Climate tion of location within the RFD and stage in storm Appl. Meteor., 22, 919±930. evolution? ÐÐ, and W. P. Unruh, 1989: Observations of the wind ®eld in tor- R How do the dominant RFD forcings vary across the nadoes, funnel clouds, and wall clouds with a portable Doppler spectrum of supercell types (e.g., nontornadic vs tor- radar. Bull. Amer. Meteor. Soc., 70, 1514±1525. nadic, low-precipation vs heavy-precipitation ÐÐ, and J. H. Golden, 1993: A review of tornado observations. The Tornado: Its Structure, Dynamics, Prediction, and Hazards, storms)? Geophys. Monogr., No. 79, Amer. Geophys. Union, 319±352. R How do the thermodynamic and microphysical char- ÐÐ, and A. L. Pazmany, 2000: Observations of tornadoes and other acteristics of hook echoes and RFDs vary across the convective phenomena with a mobile, 3-mm wavelength, Dopp- supercell spectrum, and why? ler radar. Bull. Amer. Meteor. Soc., 81, 2939±2952. R How does the large-scale environment affect RFD ÐÐ, and S. G. Gaddy, 2001: Airborne pseudo-dual-Doppler analysis of a rear-in¯ow jet and deep convergence zone within a supercell. characteristics? Mon. Wea. Rev., 129, 2270±2289. R Is the tornadogenesis process sensitive to the ther- ÐÐ, A. L. Pazmany, J. C. Galloway, and R. E. McIntosh, 1995: modynamic and microphysical properties of RFDs? Studies of the substructure of severe convective storms using a R What is the role of the RFD in tornadogenesis, and mobile 3-mm wavelength Doppler radar. Bull. Amer. Meteor. does the hook echo have an active role? Soc., 76, 2155±2169. ÐÐ, S. G. Gaddy, D. C. Dowell, A. L. Pazmany, J. C. Galloway, A number of direct observations have been reviewed R. E. McIntosh, and H. Stein, 1997: Doppler radar observations herein; however, these observations have been relatively of substorm-scale vortices in a supercell. Mon. Wea. Rev., 125, scarce and often have been simply fortuitous. A new 1046±1059. Bonesteele, R. G., and Y. J. Lin, 1978: A study of updraft±downdraft mobile surface observing system (Straka et al. 1996), interaction based on perturbation pressure and single-Doppler introduced in 1994 for VORTEX, recently has collected radar data. Mon. Wea. Rev., 106, 113±120. the largest number of in situ measurements within su- Brandes, E. A., 1977a: Flow in a severe thunderstorm observed by percell storms to date. Analyses of these spatially and dual-Doppler radar. Mon. Wea. Rev., 105, 113±120. temporally dense ``mobile mesonet'' observations with- ÐÐ, 1977b: Gust front evolution and tornado genesis as viewed by Doppler radar. J. Appl. Meteor., 16, 333±338. in hook echoes and RFDs will be presented in a com- ÐÐ, 1978: Mesocyclone evolution and tornadogenesis: Some ob- panion paper (Markowski et al. 2002), and these data servations. Mon. Wea. Rev., 106, 995±1011. may begin to shed some light on at least a couple of ÐÐ, 1981: Finestructure of the Del City±Edmond tornadic meso- the questions posed above. circulation. Mon. Wea. Rev., 109, 635±647. ÐÐ, 1984a: Relationships between radar-derived thermodynamic Acknowledgments. I am indebted to Drs. Jerry Straka variables and tornadogenesis. Mon. Wea. Rev., 112, 1033±1052. ÐÐ, 1984b: Vertical vorticity generation and mesocyclone suste- and Erik Rasmussen for their encouragement and per- nance in tornadic thunderstorms: The observational evidence. spectives during the course of this work. I also extend Mon. Wea. Rev., 112, 2253±2269. thanks to Dr. Chuck Doswell and two anonymous re- ÐÐ, R. P. Davies-Jones, and B. C. Johnson, 1988: Streamwise vor- viewers, who noticeably improved the clarity and or- ticity effects on supercell morphology and persistence. J. Atmos. ganization of the paper. Finally, I also am grateful to Sci., 45, 947±963. Brooks, E. M., 1949: The tornado cyclone. 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