1106 WEATHER AND FORECASTING VOLUME 13

The Initiation of Moist Convection at the Dryline: Forecasting Issues from a Case Study Perspective

CONRAD L. ZIEGLER AND ERIK N. RASMUSSEN NOAA/National Severe Storms Laboratory, Norman, Oklahoma

(Manuscript received 21 November 1997, in ®nal form 30 July 1998)

ABSTRACT The processes that force the initiation of deep convection along the dryline are inferred from special mesoscale observations obtained during the 1991 Central Oklahoma Pro®ler Studies project, the Veri®cation of the Origins of Rotation in Tornadoes Experiment 1994 (VORTEX-94), and the VORTEX-95 ®eld projects. Observations from aircraft, mobile CLASS soundings, and mobile mesonets de®ne the ®elds of air¯ow, absolute humidity, and virtual temperature in the boundary layer across the dryline on the 15 May 1991, 7 June 1994, and 6 May 1995 case days. Film and video cloud images obtained by time-lapse cameras on the NOAA P-3 are used to reconstruct the mesoscale distribution of cumulus clouds by photogrammetric methods, permitting inferences concerning the environmental conditions accompanying cloud formation or suppression. The results of the present study con®rm the classical notion that the dryline is a favored zone for cumulus cloud formation. The combined cloud distributions for the three cases are approximately Gaussian, suggesting a peak expected cloud frequency 15 km east of the dryline. Deep mesoscale moisture convergence is inferred in cloudy regions, with either subsidence or a lack of deep convergence in cloud-free regions. The results document the modulating effect of vertical wind shear and elevated dry layers in combination with the depth and strength of mesoscale updrafts on convective initiation, supporting the notion that moist boundary layer air parcels must be lifted to their lifted condensation level and level of free convection prior to leaving the mesoscale updraft to form deep convection. By relaxing the overly restrictive assumptions of parcel theory, it is suggested that a modi®cation of proximity soundings to account for mesoscale lift and westerly wind shear effects can improve the diagnosis of the mesoscale dryline environment and the prediction of convective initiation at the dryline. Conversely, proximity environmental soundings, taken by themselves with consideration of CAPE and convective inhibition values according to parcel theory but neglecting vertical boundary layer circulations, are found to have less prognostic value than is conventionally assumed.

1. Introduction induced by the failure to anticipate the development of Improved knowledge of how deep, moist convection large, rain-cooled airmasses and cloudiness. On some is initiated is of fundamental importance to the U.S. occasions during the spring, large regions are forecast National Weather Service (NWS) and other operational to have a moderate or high risk of severe thunderstorms, forecasting groups. Improved nowcasts (i.e., 0±3-h fore- but the skies remain clear. Though statistical techniques casts) and short-term forecasts (6±12 h) of convective may ultimately provide reliable forecasts of precipita- initiation and the mode of early convection could prove tion amounts and coverage, successful mesoscale pre- invaluable to severe local storm forecasters to help focus diction with numerical atmospheric forecast models will their attention on rapidly evolving mesoscale weather require improved understanding and an accurate explicit scenarios. or parameterized representation of convective initiation. It is our impression that dramatic warm season weath- This forecast challenge stems from a fundamental lack er forecast failures are often the result of an inability of knowledge regarding the processes that allow or pre- to anticipate the initiation of convection, or forecasting vent the initiation of deep, moist convection. convection that fails to develop. Incorrect convective In spite of the dif®culties of collecting representative initiation forecasts cause quantitative precipitation fore- in situ measurements on the scales of individual incip- cast errors, and large errors in temperature forecasts are ient clouds, special mesoscale observations collected by a variety of ®xed and mobile instrumented platforms have provided important insights regarding the envi- ronments of developing convection. The capability to Corresponding author address: Dr. Conrad L. Ziegler, Mesoscale detect clear air boundary layer structures near devel- Research and Applications Division, National Severe Storms Labo- ratory, 1313 Halley Circle, Norman, OK 73069. oping moist convection is offered by single or multiple E-mail: [email protected] ground-based Doppler radars (Eymard 1984; Parsons et

Unauthenticated | Downloaded 09/29/21 11:18 PM UTC DECEMBER 1998 ZIEGLER AND RASMUSSEN 1107 al. 1991; Wilson et al. 1994) and airborne Doppler ra- in the boundary layer raises the question of whether dars (Wakimoto et al. 1996). Research aircraft have pro- near-surface air feeds developing boundary layer cu- vided in situ observations of air¯ow and thermal prop- mulus clouds (Renno and Williams 1995). erties near boundaries that are believed to play a central Thunderstorms often form near drylines of the south- role in the initiation of convection (e.g., Ziegler and ern U.S. plains (Rhea 1966; Bluestein and Parker 1993), Hane 1993). The subjective interpretation of geosta- but their initiation is dif®cult to forecast. According to tionary satellite data provides fundamentally important an ``ingredients based'' approach to thunderstorm fore- guidance on the initiation of convection along bound- casting (Johns and Doswell 1992; McNulty 1995), the aries (Purdom 1982), while cloud-mesoscale models coincidence of low convective inhibition (CIN) with have played an increasingly important role by providing high convective available potential energy (CAPE) and complete and internally consistent datasets with which deep tropospheric wind shear along a well-de®ned me- to test various hypotheses. This combination of obser- soscale boundary with strong low-level convergence vations and models has led previous investigators to strongly suggest a high likelihood for the initiation of conclude that storm initiation is closely linked to bound- severe convection. During the 1994 and 1995 ®eld ary layer convergence lines. phases of the Veri®cation of the Origins of Rotation in The primary effect of a convergence line is to deepen Tornadoes Experiment (VORTEX) (Rasmussen et al. the moist layer locally and provide a region potentially 1994), deep moist dryline convection did not develop favorable for deep convection. The initiation of moist in several cases despite very favorable ingredients. In convection has been investigated along a variety of each of these cases where deep convection did not de- boundaries in differing geographical regions, including velop, small values of CIN at the dryline suggested that Florida sea breezes (Wakimoto and Atkins 1994; Fank- rising air parcels could easily attain the LCL and LFC hauser et al. 1995); mountain-induced ridge-top and lee and grow into deep convection (Colby 1984). Since se- convergence zones in New Mexico (Raymond and Wil- vere storms developed along several drylines observed kening 1982) and Colorado (Banta 1984), respectively; during the 1991 Central Oklahoma Pro®ler Studies pro- Colorado Front Range convergence zones (Wilson and ject (COPS-91) (e.g., Hane et al. 1993), a comparison Schreiber 1986); and drylines (Hane et al. 1993; Ziegler of the COPS and VORTEX cases affords the opportu- et al. 1997; Hane et al. 1997; Atkins et al. 1998) as well nity to begin exploring the necessary requirements for as the intersection of an east±west baroclinic zone with initiating deep dryline convection. a dryline (Bluestein et al. 1990) on the southern U.S. In accord with a key U.S. Weather Research Program plains. Koch and Ray (1997) use Weather Surveillance (USWRP) objective of re®ning quantitative precipita- Radar-1988 Doppler (WSR-88D) data to document the tion forecasts via improved observations and knowledge initiation of thunderstorms along sea breezes, the Pied- of boundary layer processes (Emanuel et al. 1995), the mont front, and other boundaries in North Carolina. present study focuses on the impact of boundary layer Moist boundary layer air may be elevated to its lifted evolution on the initiation or inhibition of deep moist condensation level (LCL), forming a ``forced'' cumulus, convection along the dryline. Employing selected cases while additional forced lifting may bring enough air to from the 1991, 1994, and 1995 ®eld projects, we ex- the level of free convection (LFC) to form an ``active'' amine the connections between the development of shal- deep, moist convective cloud (Stull 1985). Deep con- low and deep moist convection, the intensity of the hor- vection may form at the intersection of a convergence izontal thermal gradients and the vertical circulations line with horizontal convective rolls where enhanced that accompany cloud initiation, and the realization of updrafts are present (Wilson et al. 1992; Atkins et al. convective instability. Section 2 presents the data 1995) as well as at collision points of thunderstorm sources and analysis techniques, while section 3 pre- out¯ows with other out¯ows or sea breezes (Kingsmill sents the case studies. Discussion of the results in sec- 1995). tion 4 is followed by conclusions in section 5. The sensitivity of boundary layer cumulus convection to wind shear and thermal strati®cation effects and their 2. Mesoscale data analysis mesoscale variability is receiving increased attention by researchers. Storm initiation, organization, and lifetime In several cases during COPS and VORTEX, in situ may be enhanced when the convective clouds move at aircraft observations were concentrated along dryline a velocity similar to that of the convergence line (Wilson segments to document the potential impact of prestorm and Megenhardt 1997). The horizontal mesoscale vari- environmental conditions on convective initiation and ation of moisture may dictate which of several simul- storm development. The National Oceanic and Atmo- taneously preexisting mesoscale boundaries initiate spheric Administration (NOAA) P-3 aircraft measured deep convection in a given case (e.g., Weaver et al. environmental variables across the dryline during the 1994). The initiation of convection is very sensitive to daytime during each experiment, concentrating on small-scale variability in the boundary layer thermo- stepped traverses but including horizontal box survey dynamics (Mueller et al. 1993; Weckwerth et al. 1996; patterns in the dryline environment. The National Cen- Crook 1996). The existence of adiabatic updraft plumes ter for Atmospheric Research (NCAR) Electra joined

Unauthenticated | Downloaded 09/29/21 11:18 PM UTC 1108 WEATHER AND FORECASTING VOLUME 13 the P-3 to probe drylines during VORTEX-95, adding positioning of clouds than possible from satellite im- a clear air remote sensing capability. Both the P-3 and agery, 2) measurements of cloud base and top heights, the Electra were based at Will Rogers World Airport in 3) detailed imaging of overall cumulus cloud morphol- Oklahoma City, Oklahoma, during the COPS and VOR- ogy. Although image data were gathered through most TEX projects. of a given mission, the photogrammetric analyses were Proceeding from objective techniques described in restricted to the images recorded on stepped traverse Ziegler and Hane (1993) and Hane et al. (1993), the P-3 legs. Presented in detail in appendix A, the photogram- stepped traverse data were analyzed to document the metric technique is based on known information of the properties of the boundary layer across the dryline. In position and absolute east±west speed of the P-3, the the ®rst analysis step, the P-3 horizontal wind mea- measured extent of cloud boundaries in the horizontal surements were ®ltered to provide a smooth input di- and vertical image directions, and several known or es- vergence pro®le for kinematically calculating vertical timated properties of the camera lens and recording sys- motion (Mohr et al. 1986). Pre®ltering of the input east± tems (Holle 1988). The output of the photogrammetric west (u) wind component1 time series was accomplished analysis are the positions and ranges of bounding boxes with a four-pass application of a triangular weighting that just enclose individual clouds or cloud clusters. In function varying linearly from a value of one at the cases of complex cloud ®elds, only the closest clouds current scan point to zero over a lag time of Ϯ12 s,2 (i.e., relatively unmasked by intervening cumuli) are while boundary points of each leg were smoothed with recorded. Due to a gap in the image record between a simple elliptic ®lter. A one-pass application of the 2220 and 2300 UTC (all times are UTC) on 15 May Barnes (1964) scheme was used to spatially interpolate 1991 (about 12% of the combined image record), the all data, including the pre®ltered u component, to a reg- properties of the cloud ®eld were approximated from a ular Cartesian grid that was oriented in the east±west detailed weather event log during that time interval. direction and centered on the sets of ¯ight legs in a Two additional sources of in situ pressure, tempera- given stepped traverse pattern. The analysis grid had a ture, dewpoint, and horizontal wind measurements were horizontal grid spacing of 0.2 km and a vertical grid available. Atmospheric soundings were obtained from spacing of 0.1 km. Vertical velocities were derived from surface vehicles employing the Mobile Cross-chain Lo- the interpolated u-component ®eld by upward integra- ran Atmospheric Sounding System (M-CLASS), based tion of the Boussinesq continuity equation from w ϭ 0 on CLASS technology developed by NCAR (Lauritzen at ground level. et al. 1987) and converted by the National Severe The Barnes (1964) weighting function takes the form Storms Laboratory (NSSL) for mobile operation in vans w ϭ exp(Ϫr2/␬), where r is the distance in the x±z plane (e.g., Rust et al. 1990). Values of CAPE and CIN were separating a datum and a grid point and ␬ is a smoothing computed for each sounding based on the virtual tem- parameter. Since ␬ is constant, the ®ltering is uniform perature buoyancy (Doswell and Rasmussen 1994) and or isotropic in the plane. The frequency response Do of the lowest 50-mb average parcel. Doswell and Ras- the Barnes ®lter (0 Յ Do Յ 1) may be expressed as Do mussen (1994) point out that the virtual warming effect ϭ exp[Ϫ␬*(␲/␭*)2] (Koch et al. 1983), where ␬* ϭ of water vapor may be suf®cient to substantially reduce ␬/L 2, L ϭ 2⌬, and ␭* ϭ ␭/L. Picking ⌬ϭ0.3 km to (or even eliminate) CIN and signi®cantly augment small represent the coarsest (vertical) data spacing in the values of CAPE. The difference between the balloon stepped traverses, and selecting ␬* ϭ 0.439, we obtain rise rate and its rise rate in still air, roughly5msϪ1, a theoretical ®ltering response (i.e., 100 ϫ Do) of about was used to estimate mesoscale vertical motion to within 50% at ␭ ϭ 1.5 km and ϳ1% at the Nyquist wavelength approximately Ϯ0.5 m sϪ1. A total of two M-CLASS ␭ ϭ L. vans were used in COPS-91, while as many as four M- Photogrammetric cloud analyses were performed us- CLASS vans were deployed during VORTEX. A ``mo- ing the time-lapse 16-mm color ®lm or video camera bile mesonet'' of as many as 15 vehicles (including M- systems ¯own on the P-3 during the COPS and VOR- CLASS vans) were equipped with instruments to obtain TEX projects, respectively. Advantages of the photo- pro®les of thermodynamic variables across the dryline grammetric cloud analysis over visible satellite imagery, during VORTEX (Straka et al. 1996). At a nominal which was also used, include the following: 1) improved traverse speed of 25 m sϪ1, the 6-s sampling rate of a resolution of smaller cumuli and more precise east±west mobile mesonet corresponded to a spatial sampling in- terval of 150 m. The information from photogrammetry was depicted on individual soundings to estimate the typical envi- 1 The u component is approximately the dryline-normal wind com- ronments of observed cumulus clouds. Linear interpo- ponent, since long (ϳ100 km) dryline segments are often oriented lation of the observed cloud base and top heights to the on average approximately north±south. The normal wind component observed pressure±height pro®le of the sounding was in cases where undulations or bulges occur along dryline segments would have both zonal and meridional components. used to estimate the corresponding pressure values. 2 The 12-s smoothing period corresponds to a horizontal distance Equating cloud-base pressure to the LCL pressure or of roughly 1.4 km at a nominal ground speed of 120 m sϪ1. condensation pressure p* and following Betts (1984),

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the corresponding LCL temperature TLCL was inferred based (i.e., much higher than the moist boundary layer from a piecewise-linear regression of measured TLCL ver- depth) cumulus west of the dryline. By the 1649±1804 sus p* along a single mid±boundary layer aircraft tra- traverse (Fig. 2b), the convergence, lifting, vertical wind verse through the dryline for each case.3 After plotting shear, and horizontal gradients of the nearly stationary a given cloud-base point, a moist-adiabatic pro®le was dryline had become much better de®ned, while the stra- constructed and plotted upward from the cloud base to tocumulus just east of the dryline was in the process of the estimated cloud-top pressure. Although moist-adi- dissipating. By the 1807±1935 traverse (Fig. 2c), gra- abatic conditions may rarely be achieved in cumuli due dients had weakened as convective boundary layer to mixing of cloudy and environmental air (e.g., Prup- (CBL) growth east of the quasi-stationary dryline trans- pacher and Klett 1978), the most buoyantly unstable ported low-level moist, (virtually) potentially cool air pro®le could be visualized in this manner. upward. Mainly high-based, small cumulus±fractus and cumulus had developed around the surface dryline lo- cation. By the 2014±2208 traverse (Fig. 2g), the dryline 3. Results had moved eastward 5 km to x ϭϪ55 km. Hence, the a. 15 May 1991 dryline (COPS-91) original dryline had nearly dissipated by midafternoon due to deep CBL growth and vertical mixing to the east Special mesoscale ®eld observations for the 15 May of the dryline. Widespread, high-based cumulus and 1991 dryline case during the COPS-91 experiment are towering cumulus clouds had developed from the sur- described by Hane et al. (1993). These observations face dryline location eastward for a distance of over 55 represent the most extensive data collection ever ob- km. tained for a single dryline, with aircraft traverses and By the 2220±2305 traverse (Fig. 2h), the dryline had mobile soundings being obtained at approximately 1- reformed at x ϭϪ22 km, with additional concentrated to 2-h intervals from midmorning through early evening zones of moisture convergence and lifting at x ϭϪ14 in the eastern Texas panhandle region. Mesoscale mod- km, x ϭϪ3 km, and x ϭ 11 km [i.e., ``multiple dry- eling studies of this case have investigated the mech- lines,'' as in Hane et al. (1993) and Crawford and Blue- anisms of deep convective initiation (Ziegler et al. stein (1997)]. A deep surge of easterly winds and mois- 1997), the sensitivity of dryline formation to the spatial ture at low levels (i.e., ``moisture surge'' or simply soil moisture distribution (Shaw et al. 1997), and the ``surge'') was present east of the mesoscale boundary impact of land use patterns on moist convection (Pielke at x ϭϪ3 km. Since the surge boundary is embedded et al. 1997). in easterly ¯ow while the ¯ow veers and becomes west- The dryline sharpened just east of Amarillo, Texas, erly around the dryline, westward advection would during midmorning and moved slowly eastward, be- cause the surge boundary to approach the dryline. Tow- coming nearly stationary during the late afternoon just ering cumuli were concentrated over an interval of 30- west of McLean, Texas, in the eastern Texas panhandle. km width to the east of the surface dryline location, and By midafternoon the dryline was oriented approxi- a towering cumulus with a lowered base (i.e., lower than mately north±south around 100.5ЊW longitude (the east± adjacent and previous bases) had developed directly west analysis origin) (Fig. 1a), and convection had be- above the boundary at x ϭ 11 km. By the 2307±0006 gun to form along its extent from the northern Texas traverse (Fig. 2i), the westward moisture surge and east- panhandle into western Kansas and northeastern Col- ward-propagating dryline had collided, dramatically in- orado (Fig. 1b). A photogrammetric cloud analysis tensifying horizontal gradients, moisture convergence, based on all east±west traverses (Fig. 1c) reveals a high and mesoscale lift at the dryline and assisting the de- spatial density of cumulus clouds through the afternoon velopment of towering cumulus east of the dryline. A along and just east of the dryline. small cumulus above the analysis domain top of 3 km The dryline was probed by east±west stepped tra- at x ϭϪ3 km was embedded in the intense mesoscale verses of the P-3 aircraft east of Amarillo, Texas. During updraft, which lifted mostly hot, dry air from west of the traverse from 1532 to 1646 (Fig. 2a), the dryline the dryline, explaining the rather high cloud base. Deep, was already marked by pronounced east±west gradients swelling cumuli had developed east of the dryline, with of water vapor mixing ratio (qv ) and virtual potential a pronounced eastward tilt in response to the prevailing temperature (␪v ); the deceleration, stagnation, and lift- vertical wind shear in the cloud layer. Due to steadily ing of westerly ¯ow; and a weak moisture bulge. Pho- decreasing ␪␷ east of the dryline and surge, the collision togrammetric data indicated the presence of a shallow of the two boundaries is an occlusion process. stratocumulus layer east of the dryline, and a small high- During the P-3 maneuvers, M-CLASS soundings from NSSL-1 and NSSL-2 were obtained roughly 30 km west and east of the dryline, respectively. The 2311 NSSL-1 sounding (Fig. 3a) reveals a hot, dry, deeply 3 Betts (1984) showed that an aircraft traverse or a balloon or aircraft sounding through the convective boundary layer is suf®cient well-mixed layer to about 600 mb west of the dryline, to characterize the thermal mixed layer properties in the vicinity of while the 2307 NSSL-2 sounding (Fig. 3b) shows the the in situ measurements. relatively cool, moist conditions east of the dryline. The

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FIG. 1. Surface and boundary layer observations and cloudiness on 15 May 1991. (a) Surface state at 2100 UTC, with solid-contoured above mean sea level (MSL) pressure (mb) and dashed-contoured dewpoint temperature (ЊC); (b) visible satellite imagery at 2100 UTC; (c) map representation of photogrammetric estimates of north±south position (i.e., range from a P-3 leg) and east±west extent (i.e., relative to P-3 location) of all clouds or cloud areas (gray line segments), with 2307±2315 UTC P-3 traverse across dryline at ϳ150 m above ground level (AGL). Surface station model in (a) includes winds (half barb ϭ 2.5msϪ1, full barb ϭ 5msϪ1), temperature (ЊC) at upper left, dewpoint temperature (ЊC) at lower left, and MSL pressure (mb) at upper right. Selected NCAR PAM mesonet sites are denoted by ®lled squares. The symbols N1, N2, and A in (c) locate the 2311 NSSL-1 sounding, and the NSSL-2 soundings at Shamrock, TX, and Alanreed, TX (the westernmost NSSL-2 sounding site), respectively, while the plotted traverse was chosen to depict conditions between the soundings presented in Fig. 3. The P-3 temperature is about 1.5ЊC cooler than the local surface temperature assuming a dry-adiabatic lapse rate. The heavy dashed line in (a) denotes a convergence line. Gray ®lled and open white rectangles in (a) and (b), respectively, depict the location of east±west traverses of the NOAA P-3 aircraft as discussed in the text.

NSSL-2 sounding displays a pronounced temperature Wheeler County, Texas (Fig. 4b). The Shamrock storm inversion around 500 mb with nearly dry-adiabatic lapse was in its early development phase at 0101 on 16 May rates above, suggesting that the western boundary layer (Fig. 4b), had intensi®ed and subsequently propagated top has been lifted about 100 mb while deeper tropo- southwestward4 toward the dryline by 0136 on 16 May spheric subsidence has capped the elevated residual lay- (Fig. 4c), and had produced an F3 tornado beginning 8 er to the east of the dryline. The inferred moist adiabats miles south of Shamrock from 0217 to 0310. The Lav- of unmixed cumuli and towering cumulus clouds are erne tornado (0135±0211) had just formed by 0135 (Fig. generally cooler than the environmental temperature 4c). The Laverne, Wheeler County, and Shamrock pro®le, with deeper tops being restricted by the elevated storms all initiated to the east of the dryline (e.g., note inversion layer. The boundary layer moisture deepens moist southeasterly ¯ow in Figs. 4b,c to the west of the and increases and the low-level winds back slightly at storms), subsequently organizing and ultimately moving the NSSL-2 sounding site by 0057 (Fig. 3c). northeastward away from the dryline in accord with the Severe thunderstorms developed during the early eve- scenario described by Rhea (1966) and Bluestein and ning along the dryline, producing small tornadoes near Parker (1993). Garden City, Kansas, and strong tornadoes and large hail near Laverne, Oklahoma, and Shamrock, Texas (USDOC 1991). Small cumulonimbi developed just east 4 The term ``propagation'' refers to the storm movement due to the of the dryline during late afternoon and early evening combined effects of northeastward advection and rapid redevelop- (e.g., Fig. 4a), preceding the storm over northeastern ment on the southwest storm ¯ank.

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An analysis of the daytime evolution of CAPE and VORTEX observation in extreme northwestern Okla- CIN reveals a dif®cult short-term forecasting scenario homa (Fig. 6a). Spatially isolated, high-based cumuli in which there is actually a slight, gradual reduction of developed near the dryline (Fig. 6b) and were detected instability from morning through midday east of the from the P-3 by photogrammetric analysis5 (Fig. 6c). dryline as heating erodes the inhibition to convection The dryline was probed by east±west stepped traverses (Fig. 5). The P-3 observations suggest that vertical mix- of the P-3 aircraft near Buffalo, Oklahoma. The ®rst ing forces a drying trend in the lowest 50-mb layer two stepped traverses of the P-3, performed in the pe- during midday east of the dryline (Figs. 2c and 2g), riods 1934±2031 and 2030±2103 (not shown), revealed accounting for the decreasing CAPE in spite of increas- weak westerly wind shear and diffuse horizontal gra- ing surface temperatures. From around 2000 to 2307 at dients of q␷ and ␪␷ across the dryline. High-based, weak the Shamrock, Texas, sounding site, CAPE dramatically cumulus convection had developed just west of the dry- increases from under 1000 J kgϪ1 to over 2500 J kgϪ1. line within a 20-km-wide zone containing several lo- The westward-propagating moist and unstable air mass calized 1 m sϪ1 updrafts in the boundary layer. documented in Figs. 2h,i replenishes the supply of un- During the subsequent traverse from 2215 to 2245 stable air and increases the potential for severe con- (Fig. 7a), the dryline was marked by pronounced east± vection despite slowly increasing CIN values. Our me- west gradients of moisture and ␪␷ and the deceleration, soanalysis suggests that a trough in the southeastern stagnation, and lifting of westerly ¯ow. A few scattered, Texas Panhandle (Fig. 1a) delineated the western edge high-based cumuli and towering cumuli had developed of the moisture surge observed in the P-3 data. The over and to the west of the dryline. By the 2248±2328 increased instability between the 2307 and 0057 traverses (Fig. 7b), the dryline had moved eastward by NSSL-2 soundings (Figs. 3b,c) supports the hypothesis 3 km and had become more sharply de®ned as cumulus that strong, deep, localized moisture convergence and coverage decreased. By the 2332±0001 traverses (Fig. mesoscale lifting developed near the dryline during the 7c), the dryline had begun retreating to the west as the early evening, the former possibly assisting the increase vertical circulation decreased in depth and cumuli dis- of CAPE and the latter helping to release the growing sipated. convective instability and initiate the intense storms. In Soundings from four M-CLASS systems were ob- a later section we will present further analyses of the tained in a quasi-linear west±east array across the dry- P-3 data, which support the hypothesis of strengthening line between 2100 and 2200. The NSSL-2 sounding convergence near the dryline. (Fig. 8a) reveals a hot, dry, almost homogeneously well- A study by Bluestein et al. (1990) provides added mixed CBL air mass with westerly winds to the west support to the notion that surges of moist, low ␪␷ air of the dryline, while warm, moist air and southerly low- near drylines may aid in initiating deep convection. level winds are in place east of the dryline as docu- Bluestein et al. (1990) performed an west-to-east tra- mented in the soundings from NSSL-4 (Fig. 8b), verse through a surge boundary (as depicted in their NSSL-3 (Fig. 8c), and NCAR (Fig. 8d). With the ex- Fig. 8) on 28 May 1985, measuring decreasing tem- ception of two rather deep towering cumuli, other cu- perature, increasing absolute humidity, and backing mulus clouds are largely restricted by the environmental winds. We computed ␪␷ from their data assuming a sur- temperature pro®le (Figs. 8b,c). These intense spatial face pressure (estimated from their soundings) of 950 gradients of stability parameters across the dryline (Fig. mb, noting a sensitivity of 0.1 K mbϪ1 of error in as- 9) have not previously been observed to the authors' sumed surface pressure (negligibly dependent on dew- knowledge, although they have been successfully sim- point temperature). Since pressure perturbations across ulated by Ziegler et. al. (1997) and Shaw et al. (1997) such boundaries are only of order 0.1 mb, the implied using a research mesoscale model employing ®ne spatial uncertainty of the ␪␷ gradient from the surface pressure resolution. Although CIN decreased to zero at the dry- uncertainty is negligible. A ␪␷ drop of 1.5 K was com- line in the presence of strong convergence and large puted over a 6-km west-to-east traverse of the surge CAPE, yet deep cumuli did not develop, we note the based on their ``mobile'' measurements during their nearly 200-mb-deep layer of warm, dry air and sub-

2221±2241 traverse, yielding a ␪␷ gradient comparable stantial wind shear through which a boundary layer par- to values reported for an Oklahoma dryline by Ziegler cel would have to rise undiluted to initiate a storm. and Hane (1989) and determined from the near-surface Bluestein et al. (1987) present another example of a P-3 pass for the 15 May surge in the present study (Fig. dryline sounding (their Fig. 2f) in which CAPE and CIN 2h). b. 7 June 1994 dryline (VORTEX-94) 5 Photogrammetric analysis was used to estimate the heights of The 7 June 1994 dryline was nearly stationary during cloud bases when visible in images. Additionally, cloud shadows visible in Fig. 6b correspond to a cloud height of 3.7 km AGL given the late afternoon and was oriented approximately a computed local sun angle of ϳ37Њ and a roughly estimated distance north±south around 99.7ЊW longitude (the assumed of 5 km from the tip of a cloud to the edge of the corresponding east±west analysis origin) during the period of intensive cloud shadow.

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FIG. 2. Objective analyses of P-3 stepped traverse measurements with cloud observations on 15 May 1991. Analyses (left column): (a) 1532±1646; (b) 1649±1804; (c) 1807±1935; (g) 2014±2208; (h) 2220±2305; (i) 2307±0006. Contours are water vapor mixing ratio (g kgϪ1), while vector velocity in the cross section is scaled at upper left. Cloud images in the right column are either VIS imagery (time in parentheses) or views from P-3 side-looking cameras obtained at the location indicated by a heavy black dot in the indicated analysis (viewing direction in parentheses): (d) VIS image (1600); (e) VIS image (1800); (f) VIS image (1900); (j) 2122 (south); (k) 2153 (south); (l) 2327 (north);

(m) 2348 (north). The margin between white and light gray ®ll in (a), (b), (c), and (g)±(i) is ␪v ϭ 308, 309, 311, and 312 K, respectively, while gray ®ll increases darkness in 1 K increments. The x-distance scale of an unlabeled panel is the scale of the next lower labeled panel. values appeared to be favorable for convective initia- weaker at the surface than aloft, suggesting the existence tion, yet where the LCL was within an elevated dry of a superadiabatic layer near the surface east of the 6 layer and storms did not develop. Although deep con- dryline. The ␪␷ gradient at the location of the moisture vection in such an environment is unlikely, storms could increase marking the dryline (i.e., the3gkgϪ1 increase conceivably be initiated given localized mesoscale con- of q␷ from x ϭϪ20 to x ϭϪ15 km) is rather weak at vergence extreme enough to dominate small-scale tur- bulent mixing and force large volumes of moist bound- ary layer air up to the LFC. The dryline was probed by east±west traverses of the 6 There must be a superadiabatic layer near the surface as long as the turbulent convective boundary layer is active, since it is the un- NSSL-2 mobile mesonet during midafternoon about 15 stable strati®cation that drives the turbulent vertical ¯uxes. Ziegler km south of a low-level traverse of the P-3. The hori- et al. (1997) present additional examples of superadiabatic layers in zontal pro®les of absolute humidity from NSSL-2 and observed and modeled soundings during afternoon east of or along the P-3 are very similar despite slight differences of the 15 May, 16 May, and 26 May 1991 drylines (Figs. 8, 16, and 18), and explain on pp. 1015±1016 that a mesoscale horizontal in¯ux north±south position and timing of the two traverses of potentially cool air toward the boundary offsets the vertical ¯uxes (Fig. 10). The corresponding ␪␷ pro®les have different and helps maintain a minimum virtual potential temperature above amplitudes, and surface gradients at the dryline are the surface layer.

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FIG.2.(Continued) An open rectangle in (d), (e), and (f) locates the adjacent stepped traverse. The ``cloud box'' boundaries are thick if imaged, and thin either if the cloud extended outside the image area or if the cloud base was located by its shadow. Cloud boundaries are black if horizontal range Rh is less than or equal to 10 km and gray if Rh Ͼ 10 km. A cloud base above the analysis domain top ztop is denoted by a line segment just above ztop. Thin quasi-horizontal curves are the individual ¯ight legs.

the surface, consistent with surface observations of dry- line is con®rmed by the M-CLASS soundings (Figs. line passage by Crawford and Bluestein (1997), but has 8b,c). The horizontal ␪␷ and q␷ gradients in the boundary a value of 1ЊC per 8 km east of the dryline. However, layer at distances greater than 5 km east of the dryline Ϫ1 the ␪␷ gradient as low as 150 m above ground level (i.e., ϳ1Њ±2ЊCand4gkg per 30 km, respectively) (AGL) (value of 1.5ЊC per 4 km) is considerable. A are remarkably constant in time in the presence of af- superadiabatic layer with peak amplitude near the dry- ternoon heating, based on comparisons with other tra-

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FIG. 3. Skew T±logp diagrams of soundings on either side of the dryline on 15±16 May 1991: (a) NSSL-1 (2311); (b) NSSL-2 (2307); (c) NSSL-2 (0057). Soundings indicated at upper right are denoted by solid, black curves, with sounding parameters indicated at lower left. Dashed gray curves in (a) and (c) are the (reference) NSSL-2 (2307) sounding. The (x, y) sounding coordinates (km) relative to the analysis origin are indicated at upper left. Open triangles and horizontal segments joined by moist-adiabatic curves (gray) denote the bases and tops, re- spectively, of individual, photogrammetrically measured cumuli. The parameters CBP (cloud base pressure) and CBZ (cloud base height) have units of mb and km MSL, respectively. Height scale has units of km MSL. Due to noisy Loran data, NSSL-1 winds in (a) are not present at heights between 750 and 500 mb while NSSL-2 winds in (c) are not plotted at pressures less than 800 mb. verses (not shown), suggesting a nearly constant thermal increases of moisture observed by NSSL-2 and the P-3 solenoid intensity and a degree of horizontal uniformity (not shown). In addition, some north±south variation of of vertical mixing. moisture is suggested during traverses in the interval An additional three mobile mesonets (i.e., Probe-1, between 2030 and 2130 (not shown). Convergence and Cam-2, Cam-3) performed traverses in concert with veering of the surface wind from a southerly to westerly NSSL-2, each con®rming the existence of the steplike direction was noted to coincide with the moisture de-

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FIG. 4. Deep dryline convection on the evening of 16 May 1991. (a) Photo looking north at 0040, with NSSL-2 in foreground and developing Wheeler County, TX, storm in background (courtesy C. Hane, NSSL); (b)±(c) WSR-88D radar re¯ectivity from KTLX (Oklahoma City) at 0101 and 0136, respectively. The location and viewing direction of photo (``P'') in (a) are indicated in (b), while times and locations of tornado touchdowns (``T'') are indicated in (c). Selected 1-min PAM mesonet observations are depicted in (b) and (c) as in Fig. 1. creases on all traverses. On many of these traverses, analysis at close range to the east±west stepped traverses decreases of virtual temperature occurred as moisture ¯own by the P-3 (Fig. 11c). Atkins et al. (1998) cor- content increased. related satellite-derived cloud locations with data from the Amarillo WSR-88D radar and the sensitive EL- DORA Doppler radar ¯own on the NCAR Electra, c. 6 May 1995 dryline (VORTEX-95) showing that large isolated cumulus clouds developed The 6 May 1995 dryline sharpened and remained along the dryline in regions of enhanced convergent nearly stationary during the midafternoon over the east- air¯ow and mesoscale rising motion (e.g., Fig. 11c). ern Texas panhandle (Fig. 11a), just east of the east± The dryline was probed by east±west stepped tra- west analysis origin, Amarillo, Texas. Isolated cumuli verses of the P-3 aircraft southeast of Amarillo, Texas. and cumulus bands developed in the vicinity of the dry- During the traverse from 2145 to 2213 (Fig. 12a), the line by midafternoon (Fig. 11b), and numerous small, dryline was marked by pronounced east±west gradients shallow cumulus were documented by photogrammetric of moisture and ␪␷ and the deceleration, stagnation, and lifting of westerly ¯ow. Photogrammetric data indicated the presence of shallow cumuli both along and east of the dryline. By the 2216±2246 traverses (Fig. 12b), the surface dryline had moved westward about 5 km as localized upward motion, abruptly deeper moisture, and cumulus activity around x ϭ 55 suggest the presence of a persisting horizontal boundary at the top of the moist layer (i.e., ``elevated dryline''). By the 2248±2319 traverse (Fig. 12c), the surface dryline had sharpened and propagated eastward, while cumulus developed within regions of rising motion around x ϭ 40. By the 2322±2351 traverse (Fig. 12d), the surface dryline had moved 3 km westward as four areas of cumuli developed within mesoscale updrafts. By the 2354±0046 traverse FIG. 5. Evolution of CAPE and CIN as derived from NSSL-2 M- (Fig. 12i), the dryline had continued moving westward CLASS soundings on 15±16 May 1991. The labels A and S denote the NSSL-2 sounding locations in the east±west (W) direction relative while cumulus activity had largely dissipated. to the towns of Alanreed and Shamrock, TX, respectively, located Soundings from Amarillo, Texas (AMA), and by the symbols A and N2 in Fig. 1c. NSSL-4 were obtained roughly 30 km west and east of

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FIG. 6. Same as in Fig. 1 but for 7 June 1994 case. (a) Surface state at 2200 UTC, with ®lled black squares locating Oklahoma mesonet observations; (b) visible GOES-8 satellite image at 2231 UTC; (c) photogrammetric cloud observations and P-3 measurements along ϳ150 m AGL legs at 2114±2125 (western S±N leg), 2137±2148 (eastern N±S leg), and 2215±2223 (east±west leg). Westernmost three soundings in Fig. 8 are located in (c). The circled cross in (a) locates the NCAR M-CLASS sounding taken near Alva, OK. the dryline, respectively. The 2300 AMA sounding (Fig. An east±west traverse of the dryline was performed 13a) reveals a warm, dry, deeply well-mixed boundary by the Probe-3 mobile mesonet during midafternoon layer west of the dryline, while the 2250 NSSL-4 sound- about 15 km north of a low-level traverse ¯own by the ing (Fig. 13b) shows the relatively cool, moist condi- P-3. The shapes of the horizontal pro®les of absolute tions east of the dryline. The NSSL-4 sounding displays humidity from Probe-3 [5 g kgϪ1 (4 km)Ϫ1] and the P-3 a very strong temperature inversion at p ϳ 590 mb, and are very similar despite slight differences of north±south a balloon rise rate of ϳ3±4msϪ1 in the elevated in- position and timing of the traverses and a known hys- version layer (not shown) implies mesoscale downdrafts teresis of the chilled mirror dewpoint sensor on the P- stronger than Ϫ1msϪ1 (refer to discussion in section 37 (Fig. 14). Horizontal moisture gradients of such ex- 2). The thermal lapse rates from the NSSL-4 sounding treme magnitudes have been reported in classic studies also suggest pronounced subsidence in the free atmo- (e.g., NSSP Staff 1963; Schaefer 1973). The corre- sphere from about 550 to 640 mb. The strong subsidence sponding ␪␷ pro®les suggest the existence of a local revealed by the P-3 analyses after 2322 (Figs. 12d,i) is temperature maximum around the dryline, with cooler consistent with the descending motions inferred from temperatures to the east of the dryline. As in the case the suppressed balloon rise rate in the elevated inversion of the 7 June 1994 traverses (Fig. 10), the surface hor- layer. The inferred moist adiabat of unmixed cumulus izontal ␪␷ difference at the location of the peak moisture and towering cumulus clouds are generally cooler than gradient on 6 May is less than half the value of the the environmental temperature pro®le, with deeper tops corresponding gradient aloft at ϳ150 m AGL, empha- being restricted by the elevated inversion layer (Fig. sizing that surface observations may not be very rep- 13b). Bluestein et al. (1987) present a dryline sounding (their Fig. 2b) that is similar in many respects to our NSSL-4 sounding, including a shallow CAPE-bearing 7 The chilled mirror dewpointer system ¯own on the P-3 during layer above the CBL that was capped by an elevated COPS and VORTEX could not respond to dewpoint temperature layer of high CIN and inferred subsidence. drops exceeding roughly 5ЊCkmϪ1 (Ziegler and Hane 1993).

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FIG. 7. Same as in Fig. 2, but for 7 June 1994 case. Analyses (left column): (a) 2215±2245; (b) 2248±2328; (c) 2332±0001. Cloud images (right column): (d) 2225:40 (east); (e) 2236:52 (west); (f) 2242:02 (north); (g) 2310:03 (east). The margin between white and light gray ®ll is ␪␷ ϭ 321 K, while gray ®ll increases darkness in 1 K increments.

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FIG. 8. Same as in Fig. 3 but for 7 June 1994 case: (a) NSSL-2 at 2131; (b) NSSL-4 at 2143; (c) NSSL-3 at 2131; (d) NCAR at 2118. Soundings are located (in order of presentation) in Figs. 6c and 6a. Dashed gray curve in (a) is NSSL-4 sounding.

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FIG. 9. East±west pro®les of CAPE and CIN across the dryline as FIG. 10. Intercomparison of east±west pro®les of q␷ (solid) and ␪␷ derived from M-CLASS soundings on 7 June 1994. (dashed) across the dryline on 7 June 1994. The 2149±2159 P-3 (black) and 2136±2158 NSSL-2 mobile mesonet (gray) traverses are depicted. The P-3 traverse is at ϳ150 m AGL. resentative of the boundary layer state. The surface ␪␷ difference across the dryline, 0.5ЊC(2km)Ϫ1, is con- sistent with the largest surface ␪␷ difference value re- 1975), grouped samples taken from a population (e.g., ported by Crawford and Bluestein (1997). all cumulus clouds along drylines) will converge toward a Gaussian distribution whose peak approximates the 4. Discussion population mean increasingly well as the total sample size increases. With cautious interpretation due to the a. Cumulus and storm development relative to rather small cloud sample size, our combined cloud fre- boundary layer circulations quency data suggest that the peak frequency of the cloud The dryline is collocated with a mesoscale updraft in population should be located around approximately x ϭ the (western) rising branch of a secondary circulation, 15 km east of the dryline. This ®nding is consistent with while additional roll-like secondary circulations are usu- Rhea (1966), who documented the strong tendency of ally present on either side of the dryline (e.g., Atkins storms to initiate very close to the dryline. et al. 1998). Most clouds were detected within north± The presence of strong mesoscale moisture conver- south distances of 10 km of the P-3 traverses on both gence at the surface is an important factor for increasing 15 May 1991 (Fig. 1c) and 6 May 1995 (Fig. 11c), with the likelihood of convective initiation. Indeed, pro®les somewhat greater spacings in the north±south direction of horizontally averaged moisture convergence and ver- on 7 June 1994 (Fig. 6c). Since the inferred shapes of tical motion over the analysis cross sections reveal high both the 8 June 1974 dryline (Koch and McCarthy 1982) values of surface moisture convergence and large near- and the 6 May 1995 dryline exhibited a wavelike char- surface vertical updraft gradients (Fig. 16). On 15 May acter with a wavelength of about 30 km and an ampli- 1991, deep layer maximum updrafts exceeding 0.3 m tude of less than 5 km, and since most of the observed sϪ1 are 50% more intense than on 7 June 1994 and triple clouds were concentrated over distances less than this the peak magnitudes on 6 May 1995. Moisture conver- wavelength, the east±west positioning errors of individ- gence displays very large vertical variations character- ual clouds relative to circulation bands is probably on ized by an exponentially decreasing magnitude above a the order of a few kilometers. Assuming two-dimen- nearly constant boundary layer of varying depth, with sional boundary layer structures in the east±west direc- the deepest and most intense moisture convergence on tion (i.e., no meridional variation), the cross-sectional 15 May 1991. The presence of the relatively deep and analyses suggest a close correspondence between in- intense boundary layer circulations on 15 May 1991 dividual clouds and regions of mesoscale lifting (Figs. produced the most effective forcing of deep convection 2, 7, and 12). of the three case days. It is the presence of strong deep The photogrammetrically measured cumulus clouds layer convergence in the present cases, rather than were counted within 10-km-wide bins relative to the strong surface convergence alone, that appears to be surface dryline location for each of the case days, with the more effective predictor of deep convective initia- results summarized in Fig. 15. On each of the three days, tion. Our data are consistent with the notion that or- cumulus cloud frequency achieves a maximum value ganized mesoscale updrafts are required to lift parcels within 25 km of the dryline, while clouds tend to con- through their LCL and LFC to initiate deep, moist dry- centrate in the range Ϫ10 Յ x Յ 40 km. Although line convection. Since Rhea (1966) documented that samples on the individual case days do not readily con- storms also organize and intensify close to the dryline, form, the distribution composed of the sum of the three it is reasonable to speculate that mesoscale lifting along case days is approximately Gaussian in shape. Follow- the dryline is important for both the initiation and the ing the Central Limit theorem of statistics (e.g., Lapin growth of storms.

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FIG. 11. Same as in Fig. 1 but for 6 May 1995 case: (a) Surface state at 2200; (b) visible GOES-8 satellite image at 2202; (c) photogrammetric cloud observations and P-3 measurements at ϳ150 m AGL (2216±2223 leg). In (c), the heavy curve denotes dryline location as inferred from clear air convergence and local maximum radar re¯ectivity; contours are 0.5 and 1.0 m sϪ1 vertical velocity values at 0.7 km AGL using the 2219±2229 NCAR Electra ELDORA radar analysis leg [adapted from Fig. 12a of Atkins et al. (1998)]. The AMA and NSSL-4 (N4) soundings in Fig. 13 are located in (c). b. Cumulus and storm development relative to parcel infers that measured CIN values must decrease sub- stability stantially below 16 J kgϪ1 by afternoon heating for ini- tiation of deep convection in boundary layer updrafts It is usually assumed that deep convection will de- of several meters per second magnitude. On the other velop either along or in the vicinity of strong mesoscale hand, Rhea (1966) quali®es the prediction of convective boundaries in unstable environments if CIN decreases initiation using CIN, noting that, ``no existing stable to zero. Another common assumption, a variant on the layer was found to be suppressing thunderstorm devel- ®rst, is that convection will be initiated if CIN becomes opment . . . in the ®rst 50 mi to the east of the . . . dryline smaller than the kinetic energy of the mesoscale updraft location,'' despite the overwhelming tendency of deep that provides the forcing. From the forecasting per- convection to develop along the dryline. Conversely, spective, it is problematic that the ``proximity'' sound- the 7 June 1994 and 6 May 1995 NSSL-4 soundings ings used to gauge the level of instability are typically (Figs. 8b and 13b, respectively) possess zero CIN,8 yet no closer than a few tens to a few hundreds of kilometers only very shallow convection develops along those dry- from the area of boundary layer convergence, thus far lines where the secondary circulations increase the enough removed from the area of highest convective strength and depth of boundary layer lifting during the potential to be unrepresentative. Moreover, the bound- late afternoon. Furthermore, the CIN on 15 May 1991 ary layer updraft is in large measure an externally forced is ϳ100JkgϪ1 around 1900 when cumuli begin to form, secondary mesoscale circulation, rather than being a is as low as ϳ19 J kgϪ1 during the late afternoon as turbulent thermal ``bubble'' of air possessing local buoyancy or high initial vertical momentum that may be invoked to locally ``break the capping inversion'' or exceed the residual CIN. 8 The virtual temperature correction decreases the computed CIN To illustrate the notion that vanishing CIN increases value over that value de®ned by the area enclosed by the union of the likelihood of convective inititation, Colby (1984) the environmental and lifted parcel temperature pro®les.

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FIG. 12. Same as in Fig 2, but for 6 May 1995 case. Analyses (left column): (a) 2145±2213; (b) 2216±2246; (c) 2248±2319; (d) 2322± 2351; (i) 2354±0046. Cloud images (right column): (e) 2209:49 (south); (f) 2240:10 (north); (g) 2304:03 (north); (h) 2325:12 (north); (j)

0001:06 (south). The margin between white and light gray ®ll is ␪␷ ϭ 309 K, while gray ®ll increases darkness in 1 K increments.

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FIG. 13. Same as in Fig. 3 but for the 6 May 1995 case: (a) Amarillo (2300); (b) NSSL-4 (2250). Soundings are located in Fig. 11c. Dashed gray curve in (a) is NSSL-4 sounding. swelling and towering cumuli become more widespread, vanishing CIN is not a suf®cient condition for initiating and more than doubles to over 40 J kgϪ1 by 0100 before storms. the development of tornadic deep convection (Figs. 3b, Ziegler et al. (1997) conducted mesoscale model sim- 4, and 5). Note that although CIN should be zero in ulations of drylines and dryline convection, including areas experiencing deep convection (implying a nec- the 15 May 1991 case, showing that intense moisture essary condition for the occurrence of deep convection), convergence could locally reduce CIN to zero as me- our results are consistent with the ®ndings of Bluestein et al. (1987) and Mueller et al. (1993) in suggesting that

FIG. 15. Histogram of cloud frequencies in 10-km-wide intervals FIG. 14. Same as in Fig. 10 but for the 6 May 1995 case. The across the dryline for all case days. One cloud event means that one 2322±2331 P-3 (black) and 2323±2330 Probe-3 mobile mesonet cloud area was contained within or overlapped into the speci®ed (gray) traverses are depicted. distance interval (i.e., one or more clouds per interval).

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FIG. 16. Vertical pro®les of horizontally averaged moisture ¯ux convergence [(a), (c), (e)] and vertical velocity [(b), (d), (f)] over the cross-sectional analysis domains in the 15 May 1991 (row 1), 7 June 1994 (row 2), and 6 May 1995 (row 3) cases. Moisture ¯ux convergence is the east±west component. soscale updrafts lifted moist air through the LCL and over the surface dryline location from the west. Since LFC and initiated deep convection. Crook and Mon- the depth of the thermal solenoid ®eld is around the top crieff (1988) had previously shown that deep moist con- of the moist layer, as judged by the extent of the low vection was initiated along convergence lines as CIN ␪␷ air east of the dryline [Figs. 2, 7, 10, 12, and 14; see was reduced to zero. The secondary dryline circulation also Ziegler and Hane (1993) and Hane et al. (1997)], is solenoidally forced, with an active frontogenesis pro- the height and intensity of the maximum updraft scales cess leading to enhanced thermal and air¯ow gradients with the moist boundary layer depth. and a well-de®ned mesoscale updraft along the dryline The simulated soundings where deep convection (Ziegler and Hane 1993; Ziegler et al. 1995). The me- forms feature a nearly constant absolute humidity below soscale updraft along the dryline achieves a peak value cloud base and a saturated, absolutely unstable layer around the top of the moist layer, and the updraft van- between the base and top of a developing cumulus (Zie- ishes around the level of the deep CBL, which projects gler et al. 1997). While the 7 June 1994 and 6 May

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1995 NSSL-4 soundings possess zero CIN due to high ence or absence, respectively, of thermally buoyant boundary layer absolute humidity, as the simulated upward accelerations. ``cloud initiation'' soundings, they also possess rather warm, dry, and stable layers above the moist boundary c. Nowcasting the destabilization of proximity dryline layer that inhibit the formation and deepening of cu- soundings mulus clouds. We will explore the apparent role of in- suf®ciently deep and strong mesoscale lifting on con- The application of parcel theory to determine con- vective inhibition in section 4c. vective instability of a sounding adopts the simplest The photogrammetric analyses suggest that a system- possible Lagrangian approach in which the naturally atic lowering of the cloud-base height occurs from west occurring parcel displacements are neglected, and it is to east across the dryline. Cumuli are typically high implicitly assumed a priori that the air¯ow needed to based to the west of the dryline due to lifting by thermals force air parcels through the LCL and LFC exists. A or organized secondary circulations and the nearly ho- common ®nding of dryline analyses to date, including mogeneously mixed conditions, while cloud bases are the present study, is that the secondary circulation at progressively lower with increasing distance east of the the dryline may be shallow and therefore incapable of dryline (Figs. 2, 7, and 12). The gradual lowering of lifting boundary layer air through the LCL and LFC. A observed cloud bases are consistent with increasing val- similar restriction on convective initiation has been ues of condensation pressure p* (Betts and Ball 1995) demonstrated from ®eld observations of boundary layer in response to the progressively more moist and virtually air¯ow in cloud-prone regions in studies of Florida sea cooler conditions east of the dryline. breezes (Atkins et al. 1995; Fankhauser et al. 1995; A rising air parcel in the convective boundary layer Kingsmill 1995; Weckwerth et al. 1996; Wilson and would need to remain unmixed with drier environ- Megenhardt 1997) and convergence boundaries in mental air to achieve its LCL and LFC as predicted by northeast Colorado (Wilson et al. 1992; Crook 1996). parcel theory. Such a restriction on mixing would limit Recent mesoscale model simulations that explicitly re- warming and drying of the parcel, which in turn would solved large cumulus clouds and storms demonstrate the limit increases of the LCL and LFC values for the need for the joint occurrence of deep vertical displace- parcel following the motion.9 In the event that mesos- ments and suf®ciently large p* (e.g., high values of low- cale lifting was suf®cient to achieve parcel saturation, level absolute humidity) for convective initiation along thus forming a cumulus cloud, but insuf®ciently deep the dryline (Ziegler et al. 1997). to lift the air parcel through its LFC, mixing and its Mesoscale lifting along the dryline produces mois- effect on parcel buoyancy forcing would become im- tening and virtual cooling at and above the top of the portant. Inspection of the individual adiabatic cumulus moist boundary layer, thereby enhancing the horizontal cloud pro®les (Figs. 3b, 8b,c, and 13b) suggests the gradients of virtual temperature and absolute humidity. following: 1) The lower levels of all cumuli are con- The intensi®cation of thermal gradients increases the vectively stable and hence ``forced,'' while the tops of rate of eastward advection of the hot, dry boundary many of the cumuli are convectively unstable (``ac- layer, which in turn opposes the vertical advective cool- tive'') as based herein on the assumption of moist- ing and moistening caused by the mesoscale updraft. adiabatic ascent to graphically depict individual in- Typical widths of drylines and their updraft regions are cloud temperature pro®les; 2) mixing between cumuli in the 1±10-km range, while the updraft depth is on the and their environment, which internally cools the cloud order of the deep, often well-mixed convective bound- by evaporation and forces the within-cloud lapse rates ary layer to the west of the dryline [Fig. 17; see also substantially to the left of the moist parcel adiabat, is Fig. 14 of Ziegler and Hane (1993)]. Since westerly suppressing deep growth by forcing cloudy air parcels vertical wind shear is commonly present, owing mainly toward an equilibrium state with the local environment. to larger-scale baroclinicity or local solenoidal effects The side-looking camera images (Figs. 2, 7, and 12) or some combination, the lower levels of the mesoscale appear to con®rm these sounding-based inferences of updraft ingest and lift moist boundary layer air from the cumulus dynamics, in that they reveal convective turret east as a combination of drier air from the west and structures at the tops of a few cumuli with other cumuli moist air from below are processed through the upper having smoother cloud-top shapes, suggesting the pres- levels (see also Fig. 9 of Ziegler et al. 1995). Since the u component achieves a magnitude of U ϳ 10 m sϪ1 while the maximum updraft is of order 1 m sϪ1 at the 9 Our present dataset is regrettably too limited to allow an assess- dryline location, individual air trajectories are rather ment of mixing following the motion of boundary layer and cumulus steeply inclined toward the east in upper levels. From air parcels. Enhanced mesoscale observations that de®ne the 4D ®elds around the top of the moist layer down toward the sur- of small mesoscale boundary layer air¯ow and absolute humidity, face, the horizontal wind typically becomes southerly together with in situ thermodynamic parcel samples targeted to spe- ci®c volumes of rising air, would provide a means to evaluate the or southeasterly as the u component changes sign and bulk impact of mixing, if any. The concluding section of this paper U becomes small compared to values either west of the offers remarks about preliminary plans to obtain such measurements. dryline or aloft. Air parcels have residence times in the

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modi®ed using our simple conceptual model of the dry- line environment (Fig. 17) and a prognostic kinematic numerical approach described in appendix B. The kin- ematic model is based on conservation equations of po- tential temperature and water vapor, considering hori- zontal and vertical advection by the mesoscale ¯ow while neglecting turbulent vertical mixing due to un- resolved scales of motion (i.e., length scales less than 600 m). It is hypothesized that the vertical ¯uxes due to the mesoscale updraft (i.e., ``mesoscale ¯uxes'') dom- inate over the turbulent ¯uxes associated with smaller scales of motion (Raymond and Wilkening 1980; Pielke et al. 1991), under which circumstances the small-scale chaotic, random parcel displacements from turbulence may be neglected compared to the systematic mesoscale lifting. In addition, the kinematic model is initialized FIG. 17. Conceptual model of dryline environment during afternoon and early evening, showing dryline position in relation to cumulus by observed soundings, which implicitly carry the full clouds and air¯ow streamlines. The lower heavy dashed curve denotes effects of turbulent vertical mixing in the CBL. By con- the extent of the moist convective boundary layer, while the upper sidering both the thermal gradients and the relative heavy dashed curve locates the deep, dry convective boundary layer strengths of the horizontal wind and the mesoscale up- (west of dryline), and the elevated residual layer (east of dryline and above moist layer). The gray dashed curve locates the surface of zero draft through the boundary layer, the ability of an air westerly wind component. The vertical gray lines locate proximity parcel to become water saturated before exiting the east and dryline soundings described in the text. The heavy dashed stream- side of the mesoscale updraft can be assessed. line denotes a buoyantly accelerated cloudy air parcel trajectory. Results of the sounding modi®cations with the simple kinematic model, along with key kinematic parameter mesoscale updraft on the order of 10 min, imposing the values for each case day, are presented in Fig. 18. In strong temporal constraint that the LCL and LFC need the following discussion we de®ne hLCL as the height of to be achieved before the air parcel exits the zone of the LCL (referred to as CBZ in Fig. 18), while hwmax is lifting. de®ned in appendix B. On 15 May 1991 the dimen-

To assess the relative roles of the horizontal and ver- sionless ratio Hlcl ϵ hwmax/hLCL ϭ 2.2 (km AGL)/[2.0 tical transports of boundary layer air for cloud forma- (km MSL) Ϫ 0.7] ഠ 1.7 (Fig. 18a), while Hlcl ϭ 1.0/ tion, the proximity soundings in the various cases were 0.8 ഠ 1.3 on 6 May 1995 (Figs. 18c). Since Hlcl Ͼ 1

FIG. 18. M-CLASS soundings (black) modi®ed by a combination of horizontal and vertical transport using the simpli®ed model described in the text: (a) 0000 UTC 16 May 1991; (b) 2112 UTC 7 June 1994 (uwest Ͼ 0); (c); c) 2250 UTC 6 May 1995; (d) 2112 UTC 7 June 1994 (uwest ϭ 0). In (b) and (d), the modi®ed sounding is compared to the 2143 UTC sounding (gray) released at the dryline location by NSSL-4. CAPE and CIN have units of J kgϪ1, while CBP and CBZ have units of mb and km (MSL), respectively. The parameter hmax (the height of the maximum updraft) has units of km (AGL); the de®nitions of all listed model parameters are given in the text. Parameter subscripting is omitted for clarity.

Unauthenticated | Downloaded 09/29/21 11:18 PM UTC 1126 WEATHER AND FORECASTING VOLUME 13 in both the 15 May 1991 and 6 May 1995 cases, hor- d. Nowcasting the development of deep convergence izontal advection does not signi®cantly warm and dry along drylines the mesoscale updraft until lifted air has attained its LCL. In other words, air parcels are likely to achieve To apply the sounding modi®cation technique de- scribed in section 4c, a knowledge of the depth and the LCL provided that Hlcl signi®cantly exceeds unity. The modi®ed 7 June 1994 sounding (Fig. 18b), which intensity of boundary layer convergence near the dryline shows excellent agreement with the 2143 NSSL-4 is required. Although the suite of mesoscale observa- sounding released at the dryline, does not achieve sat- tions available to forecasters does not resolve small- uration due to the drying effects of the relatively deep scale features such as drylines explicitly, it is possible to infer a range of likely divergence pro®les at the dry- westerly wind-bearing layer in that case. Since Hlcl ϭ 1.5/2.8 ഠ 0.54 on 7 June 1994, the LCL is not attained line from wind pro®le measurements representing the before a lifted air parcel detrains from the mesoscale ``eastern'' and ``western'' soundings in Fig. 17. Recall- updraft. ing the simple sounding modi®cation technique in ap- If lifting through a layer deeper than the LCL con- pendix B and neglecting variations along the dryline tinued beyond the onset of saturation, deep convection compared to across-dryline gradients, key environmen- would be triggered by the formation of an absolutely tal parameters include the depths of the western and convectively unstable layer with its base around the eastern boundary layers (AGL), the pro®le of the dif- LCL (Crook and Moncrieff 1988; Ziegler et al. 1997). ference of the normal component of the low-level hor- Following the same reasoning as used to develop and izontal wind across the dryline (i.e., must consider local dryline orientation), and the width of the dryline. apply the quantity Hlcl, a second useful dimensionless ratio is H ϵ h /h , where h is the height of the One method of estimating local conditions on either lfc wmax LFC LFC side of the dryline is to modify appropriate regional LFC. For example, the estimated values of Hlfc based on the modi®ed soundings in Fig. 18 are as follows: 15 morning soundings to re¯ect afternoon surface condi- May 1991 (2.2/1.3 ഠ 1.7 from Fig. 18a), 7 June 1994 tions (e.g., McGinley 1986). One example of this sound- (1.5/3.4 ഠ 0.44), and 6 May 1995 (1.0/3.5 ഠ 0.29). ing modi®cation technique for the dryline environment Considering the magnitude of the lapse rate in the layer is the classic study of Rhea (1966). A second useful overlying the predicted LCL and the likelihood that cu- information source for estimating boundary layer pro- mulus circulations are mixed with environmental air, the ®les is from output of the National Centers for Envi- modi®ed 15 May 1991 sounding with both H Ͼ 1 and ronmental Prediction's (NCEP) Rapid Update Cycle lcl model (Benjamin et al. 1998). Regional boundary layer Hlfc Ͼ 1 would have the highest probability of initiating deep convection of the three cases considered. wind measurements from WSR-88D velocity azimuth Local circulations could support cumulus formation display (VAD) scans and the NOAA Wind Pro®ler Dem- even though the larger mesoscale environmental ¯ow onstration Network would be useful in gauging the evo- does not favor the attainment of saturation. An example lution of the depth and intensity of secondary circula- is the clear air Doppler radar analysis of the 6 May 1995 tions that produce convergence at the dryline. Since dryline by Atkins et al. (1998), who found that localized neither mesoscale observations nor operational model updrafts having deeper vertical circulations than along forecasts can resolve the dryline itself (Ziegler et al. the segments of dryline between updraft cores some- 1997), it is necessary to consider varying dryline widths times initiated cumulus clouds. If the local circulation in the range of order 1±10 km. Applying the estimated strengthened and became more erect, thereby decreasing normal velocity component pro®les over an assumed the westerly horizontal in¯ux of dry air, a lifted parcel dryline width and vertically integrating the resulting would spend more time in the mesoscale updraft and pro®le of the normal divergence component, the vertical its peak value of relative humidity prior to detrainment motion pro®le at the dryline can then be estimated. In would increase. With weakening of the horizontal in¯ow principle the forecaster can then take the following ac- below some threshold value, saturation could be tions: 1) modify the eastern proximity sounding follow- achieved. For example, setting U to zero in the 7 June ing the method outlined in appendix B; 2) compute Hlcl 1994 case (Fig. 18d) yields a predicted LCL that is and Hlfc (as described in section 4c). This approach consistent with the observed cloud base of 3.7 km (i.e., should be suf®cient to estimate the likelihood of shallow cloud above ``subgrid-scale moist plume'' in Fig. 7a). or deep convective initiation, subject to the represen- Note that setting U to zero in the kinematic model is tativeness of the estimated eastern and western sound- not equivalent to parcel theory, since a layer of air of ings and the assumed dryline width. ®nite depth is being lifted for a ®nite time period through an updraft pro®le. It is concluded that initiation of forced 5. Conclusions or active cumulus convection requires that the magni- tude of the horizontal ¯ux of dry air from the west be The processes that force the initiation of deep con- locally negligible in relation to the vertical ¯ux of moist vection along the dryline have been inferred from spe- air in the mesoscale updraft below the LCL or LFC, cial mesoscale observations obtained during the COPS- respectively. 91, VORTEX-94, and VORTEX-95 ®eld projects. Ob-

Unauthenticated | Downloaded 09/29/21 11:18 PM UTC DECEMBER 1998 ZIEGLER AND RASMUSSEN 1127 servations from aircraft, mobile CLASS soundings, and To assess the relative roles of the horizontal and ver- mobile mesonets de®ne the ®elds of air¯ow, absolute tical transports of boundary layer air for cloud forma- humidity, and virtual temperature in the boundary layer tion, the proximity soundings in the various cases were across the dryline on the 15 May 1991, 7 June 1994, modi®ed using a simple conceptual model of the dryline and 6 May 1995 case days. Film and video cloud images environment and a prognostic kinematic numerical obtained by time-lapse cameras on the P-3 are used to scheme. Using a simpli®ed set of conservation equations reconstruct the mesoscale distribution of cumulus by for heat and water vapor composed of horizontal and photogrammetric methods, allowing inferences con- vertical advection terms, the proximity soundings were cerning the environmental conditions accompanying modi®ed according to the measured mean mesoscale cloud formation or suppression. updraft and horizontal wind pro®les at the dryline. The The results of the present study are consistent with technique was directly veri®ed by successfully approx- the classic notion that the dryline is a favored zone for imating the observed dryline sounding in the 7 June cumulus cloud formation. The combined cloud distri- 1994 case. Cloud formation is predicted when the ver- butions for the three cases examined approximate a tical mesoscale moisture ¯ux predominates below the Gaussian shape, suggesting a peak cloud frequency 15 LCL, and deep convection is predicted if strong me- km east of the dryline based on the Central Limit the- soscale lifting is deeper than the LFC. Equivalently, orem. Cumuli were concentrated within the interval cloud or storm formation is predicted if the dimension- from 10 km west to 40 km east of the dryline. Intense, less ratio of the maximum updraft height to the LCL or deep mesoscale moisture convergence is inferred in LFC height is greater than unity, respectively. It is sug- cloudy regions, with mesoscale subsidence or a lack of gested that deep convection is likely to follow the joint deep vertical motion in cloud-free regions. Our results attainment of the LCL and LFC if the lapse rate of the document the modulating effect of westerly wind shear top of the lifted portion of the sounding is signi®cantly on convective initiation in the mesoscale updrafts at the larger than the predicted moist adiabatic value. Results dryline, suggesting that moist boundary layer air parcels generally suggest that a modi®cation of proximity must be lifted to their LCL and LFC prior to leaving soundings to account for mesoscale lift, the across-dry- the mesoscale updraft to form deep convection. Clouds line differences of environmental thermal strati®cation, are high based to the west of the dryline, and bases and westerly wind shear effects can improve the diag- progressively lower with distance east of the dryline as nosis of the mesoscale dryline environment and the pre- higher humidities and cooler temperatures promote low- diction of convective initiation at the dryline. er LCLs in the boundary layer. Our conclusions about the nature of the convective Nowcasting convective initiation from proximity en- initiation process are necessarily limited by the lack of vironmental soundings according to conventional prac- spatially detailed measurements of boundary layer air- tice must be strongly quali®ed due to the assumptions ¯ow, moisture, and temperature around the time and in of deep lifting and lack of mixing invoked by parcel the location where storms ultimately form. For example, theory. On two of the case days, 7 June 1994 and 6 we have speculated that locally deep, intense mesoscale May 1995, deep moist convection did not develop de- convergence could conceivably initiate storms at isolated spite a proximity sounding with a zero value of CIN locations along the dryline in a given case, even though (note: stability parameter value as computed using vir- larger-scale conditions are hostile to more widespread tual temperature and lifting the lowest 50-mb average convective initiation. The depth and amount of precipi- parcel). Since vertical circulations must be deep and table boundary layer water vapor is another critically strong enough to initiate clouds, the presence of strong important, but poorly resolved, quantity required to fore- surface convergence is also not an unambiguous pre- cast convective initiation. To address such observational dictor for convective initiation. For example, surface de®ciencies near a variety of (slowly moving) boundaries convergence values during the late afternoon on 6 May including stationary fronts, warm fronts, decayed thun- 1995 were comparably strong relative to the other cases, derstorm out¯ow boundaries, and drylines, the authors yet weak, shallow mesoscale updrafts resulted from a have recently joined with NSSL colleague J. Schneider very shallow convergence layer just above the surface and other scientists to propose a ``Thunderstorm Initiation capped by divergence in higher levels. Though meso- Mobile Experiment'' (TIMEx). Community discussions scale lifting was occasionally just deep enough to attain of the proposed ®eld study are taking place via planning the LCL in each of the cases, the mesoscale updrafts meetings and an interactive World Wide Web site (http: were not deep enough to lift moist boundary layer air //www.nssl.noaa.gov/srad/timex). To assist the NWS, we through the LFC and thereby initiate storms. In all cases, intend for TIMEx to provide a unique set of observations it was inferred that mixing between the cumulus cloud that provide the basis for conceptual models of the con- and its environment was diluting the buoyancy of the vective initiation process and help improve the accuracy cloudy updraft and suppressing deep growth. The pres- and speci®city of storm forecasts. ence of intense, deep mesoscale lifting was inferred to be necessary to overcome the retarding effects of mixing Acknowledgments. The staff of the NOAA Aircraft on storm initiation. Operations Center capably operated the P-3 aircraft and

Unauthenticated | Downloaded 09/29/21 11:18 PM UTC 1128 WEATHER AND FORECASTING VOLUME 13 its data systems during COPS-91 and VORTEX. We are the closest clouds relatively unmasked by intervening indebted to the numerous COPS and VORTEX scientists cumuli are analyzed. and volunteers whose dedication and skill in planning Interpretation of the ®lm and video camera images and executing the scienti®c aspects of the P-3 missions required a knowledge of the time of each frame. For and operating the M-CLASS sounding systems and mo- the video camera system ¯own during VORTEX-94 and bile mesonets produced a very useful dataset. Discus- -95, time (UTC) was manually preset into the P-3 data sions with Ronald Holle were critical to the successful system to within Ϯ1 s of the WWV10 standard and dig- application of photogrammetric analysis in this study. itally recorded as a time stamp on each video frame. Reviews of early versions of the manuscript by Robert Since a time stamp was not included on the ®lm from Maddox and David Schultz and the comments offered the COPS-91 case, it was necessary to determine be- by three anonymous reviewers were very helpful. Dis- ginning and ending times corresponding to reference cussions with Robert Maddox and Jeff Trapp motivated frames near the beginning and end of the time-lapse the application of the Barnes scheme to the objective movie segment. The determination of Ts and Te, the analysis of P-3 data. Daniel Geiszler analyzed the mo- beginning and ending reference times, respectively, re- bile mesonet data for the 7 June 1994 case under the quired careful matching of the time of onset of recorded mentorship of the lead author (CLZ) for his special pro- banking maneuvers to visible rapid appearances or dis- ject in the 1995 Research Experience for Undergradu- appearances of ground. For Nmax equal to the number of ates (REU) program at the Oklahoma Weather Center. frames from Ts to Te and for time (s), the time between We gratefully acknowledge the skillful and tireless ef- frames R ϭ (Te Ϫ Ts)/Nmax ϭ 4.97 s. The time (s) of forts of John Cortinas, William Beasley, Jeanne Schnei- an arbitrary 16-mm movie frame was computed from T der, Jerry Straka, and Cindy Machacek for their work ϭ To ϩ NR, where N is the frame number from the to coordinate the REU program, Sonia Lasher-Trapp beginning reference time To, followed by conversion to offered assistance with access to the 7 June 1994 mobile UTC. The Ts and Te values are accurate to within ϮR/2 mesonet data. Funding and support for VORTEX ®eld or approximately Ϯ2.5 s, while the error in R is neg- operations were provided by NOAA and the National ligible due to the length of the footage and the large Severe Storms Laboratory, the Center for the Analysis number of frames. and Prediction of Storms at the University of Oklahoma A cloud's horizontal (along track) and vertical posi- (OU) under Grant NSF ATM 912-0009, and the OU tions and the transit time across the image frame are Graduate College. The National Science Foundation fundamental photogrammetric measurables. The method also provided major funding for the REU program. of crossed diagonals is used to locate the center of a movable measurement grid at the center of the projected or displayed image viewed either on paper, if from ®lm, APPENDIX A or through transparency, if viewed via video monitor. Photogrammetric Cloud Analysis Technique After the measuring grid is rotated about the image cen- ter to align the horizontal centerline with the horizon, Photogrammetric cloud analyses were performed fol- values of the vertical distance from the horizontal line lowing Holle (1988) using the right-side-mounted time- that divides the image in half to the cloud feature, y, lapse 16-mm color ®lm or video camera systems ¯own can be determined. The time (or frame number) Tc at on the P-3 during the COPS and VORTEX projects, which a cloud edge crosses the vertical centerline ef- respectively. Left-side camera images were inspected fectively locates the vertical cloud edges according to but were not utilized to maximize the statistical inde- the known aircraft location. Measurements are obtained pendence of the individual sampled clouds. Examples only during periods of rather smooth, straight, and level of the images used are presented in Figs. 2, 7, and 12 ¯ight, easily detected as a straight and level path of a in the text. Nose camera images such as shown in Fig. cloud tag across the grid, and possible deviations of the 7 were also not analyzed. The overall quality of both camera orientation due to minor roll, pitch, or yaw ma- the ®lm and videotape footage is considerably better neuvers of the P-3 are neglected. than the sample images, which are obtained from Following Holle (1988) the full horizontal or vertical scanned video frames (i.e., requiring a ®lm-to-video angle of the lens ␣ (i.e., the angular ®eld of view in transfer in the COPS case). The photogrammetric tech- radians of a camera) is nique is based on known information of the position d and absolute east±west speed of the P-3, the measured ␣ ϭ 2 tanϪ1 , (A.1) position of cloud boundaries in the horizontal and ver- 2 f tical image directions, and several known or estimated where d (mm) is the horizontal or vertical dimension of properties of the camera lens and recording systems. The output of the photogrammetric analysis is the po- sition and range of a vertically and east±west-oriented bounding box that just encloses individual clouds or 10 Call letters of government-operated radio station broadcasting cloud clusters. In cases of complex cloud ®elds, only time standard.

Unauthenticated | Downloaded 09/29/21 11:18 PM UTC DECEMBER 1998 ZIEGLER AND RASMUSSEN 1129 the original ®lm frame or video image and f (mm) is Inserting this viewing angle estimate into Eq. (A.1) and the focal length of the lens. Assuming an imaged ho- assuming f ϭ 10 mm, the corresponding horizontal im- rizon and an untilted camera, the vertical angle ␴ (rad) age dimension is d ϭ 7.5 mm. of a cloud feature to the horizon is Since the parameters Tc, ␣, and y either require hand± eye measurement or are based on manufacturer speci- y ␴ ϭ tanϪ1 , (A.2) ®cations of uncertain precision, and since these param- Mf eters are subject to error, a sensitivity check was per- where y has dimensions of millimeters and the magni- formed on the photogrammetric calculations. The error ®cation factor M is the ratio of the projected ®lm or of the east±west cloud position is ϮVR/2 (120 m sϪ1 monitored video image size to the actual image size on ϫ 2.5 s) or roughly Ϯ 300 m in the COPS-91 case and the recording medium. The slant range r (km) to a cloud Ϯ120 m in the VORTEX cases. Two error levels were feature is considered for ␣ (Ϯ0.2Њ, Ϯ1Њ) and y (Ϯ1 mm, Ϯ2.5 mm) to allow for error sources beyond those due to V␶ r ϭ , (A.3) precision of the measuring tools alone. The ␣ error level s ␣ for the 16-mm ®lm system should be small compared 2 tan 2 to that for the video system that is considered in these calculations. For ®xed y ϭ 0, rh Ͻ 30 km, and ␣ within Ϫ1 where V is the aircraft ground speed (km s ) and ␶ (s) Ϯ1Њ, the range error of less than about Ϯ0.8 km is is the time required for a cloud feature to cross the ®eld negligible. For ®xed ␣ ϭ 40.9Њ and y within Ϯ2.5 mm, of view of a side camera. The crossing time ␶ can be the height error is less than about Ϯ100 m within 15 determined directly as the difference between the dis- km and less than Ϯ300 m within 30 km. As an inde- appearance and appearance times of a cloud feature from pendent check, the heights of clearly discernible ground the image frame based on the digitally encoded time targets (m MSL) within 5-km range (i.e., y Ͻ 0) were stamp on the video from the VORTEX cases. Given the computed by the photogrammetric technique and number of crossing frames F in the COPS-91 case, ␶ showed excellent agreement with the known altitudes ϭ FR. The height h (km) of a cloud feature is h ϭ r s of the target locations (i.e., errors of order Ϯ10 m). tan␴, while the horizontal range rh (km) is computed from rh ϭ rs cos(␴). Finally, height hMSL (m MSL) ϭ hac ϩ h, where hac is the aircraft pressure altitude (m APPENDIX B MSL). The P-3 employed a 10-mm focal length lens on all Simple Numerical Transport Model for Modifying ¯ights, allowing the determination of other camera pa- Input Soundings rameters using Eq. (A.1). By direct measurement of in- dividual frames, the following 16-mm movie parameters To represent the evolution of the ␪ and q␷ pro®les at were obtained: 1) dimensions of 10 mm horizontally ϫ the dryline, we employed a set of one-dimensional, ®- 7.7 mm vertically (Ϯ0.1 mm); 2) ␣ ϭ 53.1Њ from Eq. nite-difference kinematic equations of the form (A.1). In comparison, the Sony DXC151A video camera ␪␪Ϫ ␪␦␪westץ has a speci®ed imaging element size of 2/3 in. (16.93 ϭϪU Ϫ W (B.1) t ␦x ␦zץ -mm), implying an 11-mm imaging circle and a hori zontal image dimension of 8.8 mm due to the 4/3 aspect ratio of television. and Corrections were determined for the video system to qq␷␷␷Ϫ q west ␦q␷ץ -account for apparent differences between the manufac ϭϪU Ϫ W , (B.2) t ␦x ␦zץ turer-speci®ed and actual (i.e., apparent) video image sizes. Using a 400Ј:1Љ aerial photographic survey of the northwest quadrant of Will Rogers World Airport and where U is the prescribed zonal wind component, W is selected video frames obtained while either on the taxi- the prescribed vertical velocity component, and a way or crossing the end of the runway before landing, ``west'' subscript denotes a ®xed value representing the individual landmarks on both sides of the video frame deep mixed layer west of the dryline. The horizontal

(e.g., hanger edges, water towers) were located on the grid increment ␦x ϭ Lx/2 corresponds to the assumed aerial survey. After plotting rays corresponding to the half-width of the dryline gradient zone (Lx ϭ 10 km), viewing angle that emanated from the known camera while the vertical grid increment ␦z equals the spacing location, constructing a line normal to one ray that in- between successive observations in the sounding (typ- tersected the other ray, and measuring the three sides ically about 50 m). Hence, the vertical grid mesh co- of the right triangle thus formed, the horizontal effective incides with the individual sounding levels and is there- viewing angle of the video system was computed using fore slightly irregular. The air¯ow components are par- trigonometry. Averaging the ␣ values obtained from the ameterized based on the P-3 stepped traverse analyses sine and cosine relations yielded ␣ ϭ 40.9ЊϮ0.2Њ. and have the form

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Betts, A. K., 1984: Boundary layer thermodynamics of a High Plains z Ϫ hu0 U ϭ U(z) ϭ Uwest , hu0 Ͻ z Ͻ hwmax, severe storm. Mon. Wea. Rev., 112, 2199±2211. hwmax Ϫ hu0 , and J. H. Ball, 1995: The FIFE surface diurnal cycle climate. (B.3) J. Geophys. Res., 100, 25 679±25 693. Bluestein, H. B., and S. S. Parker, 1993: Modes of isolated, severe 2h Ϫ z convective storm formation along the dryline. Mon. Wea. Rev., W ϭ W(z) ϭ Wzwmax , z Ͻ h , (B.4) max h2 wmax 121, 1354±1372. wmax , E. W. McCaul Jr., G. P. Byrd, R. L. Walko, and G. R. Woodall and 1987: Forecasting and nowcasting cumulus convection with soundings released from a storm-intercept vehicle. Proc. Symp.

W ϭ Wmax(2hwmaxϪ h top Ϫ z) on Mesoscale Analysis and Forecasting, Vancouver, BC, Can- ada, International Association of Meteorology and Atmospheric

htop Ϫ z Physics, ESA SP-282, 135±139. ϫ , , , and , 1990: An observational study of splitting con- (hwmaxϪ h top)(h top Ϫ hwmax) vective clouds. Mon. Wea. Rev., 118, 1359±1370. Colby, F. P., Jr., 1984: Convective inhibition as a predictor of con- hwmaxϽ z Ͻ h top, (B.5) vection during AVE-SESAME II. Mon. Wea. Rev., 112, 2239± where W is the maximum W value, h is the height 2252. max wmax Crawford, T. M., and H. B. Bluestein, 1997: Characteristics of dryline of Wmax, and hu0 is the height below which U is zero. passage during COPS-91. Mon. Wea. Rev., 125, 463±477. The vertical velocity equations (B.4) and (B.5) are qua- Crook, N. A., 1996: Sensitivity of moist convection forced by bound- dratic pro®les matched through ®rst order (w ϭ Wmax, ary layer processes to low-level thermodynamic ®elds. Mon. Wea. Rev., 124, 1767±1785. z ϭ 0) at z ϭ hwmax. Here U ϭ 0 below z ϭ hu0ץ/wץ (ϭ0.85h ), U ϭ U above z ϭ h , while W ϭ 0 , and M. W. Moncrieff, 1988: The effect of large-scale conver- max max max gence on the generation and maintenance of deep moist con- and horizontal gradients vanish above z ϭ htop. vection. J. Atmos. Sci., 45, 3606±3624. Neglecting horizontal gradients of all variables on Doswell, C. A., III, and E. N. Rasmussen, 1994: The effect of ne- either side of the dryline, the proximity soundings are glecting the virtual temperature correction in CAPE calculations. used as proxy for conditions along the lateral boundaries Wea. Forecasting, 9, 625±629. of the updraft in the dryline zone (Fig. 17). Either the Emanuel, K., and Coauthors, 1995: Report of the First Prospectus Development Team of the U.S. Weather Research Program to P-3 measurements or a proximity sounding on the west NOAA and the NSF. Bull. Amer. Meteor. Soc., 76, 1194±1208. side of the dryline is assumed to characterize the deep, Eymard, L., 1984: Radar analysis of a tropical convective boundary dry boundary layer. 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