Mesoscale Convective Patterns of the Southern High Plains

Home , Cold front, Line echo wave pattern, Mesocyclone, Mesohigh, Mesoscale meteorology, Squall

David Blanchard Mesoscale Convective Patterns NOAA/NSSL/Mesoscale Researc° h Division of the Southern High Plains Boulder, Colorado 80303

Abstract (Cunning 1986) indicate that there is, in fact, a broad spectrum of internal structural characteristics associ- Mesoscale convective systems observed in the southern High ated with MCSs. As part of the program, high-density Plains during the Oklahoma-Kansas Preliminary Regional Experi- upper-air soundings, satellite imagery, aircraft obser- ment for STORM-central (PRE-STORM) field program were ana- vations, high-density automated surface observa- lyzed using radar and rawinsonde data. Although radar data indicate that no two systems are identical, basic recurring meso- tions, and both conventional National Weather scale structures are evident. Based on these recurrent features, the Service (NWS) WSR-57 radar data and special Dop- systems have been classified into three types of mesoscale con- pler-radar data were collected (figure 1). Specifically, vective patterns: linear mesoscale systems, occluding mesoscale radar data indicate that several mesoscale patterns of systems, and chaotic mesoscale systems. Examples of all three types are discussed. High-density rawinsonde data collected in the convection are associated with MCSs. These include regions ahead of the mesoscale systems have been averaged to squall lines with leading or trailing stratiform precip- produce composite soundings; the composites exhibit differences itation, chaotic convective systems, mesoscale occlu- in both thermodynamic and wind structure between types. sions, and frontal bands. Digital and photographic data from the NWS WSR-57 radars were analyzed, and radar-observed convection within MCSs that 1. Introduction were within or near the PRE-STORM area was classified according to these mesoscale patterns, to be The development of nocturnal mesoscale convective described in detail here. From this initial classifica- systems (MCSs) has been shown to be a function of tion, the relative frequencies of each pattern of meso- both large-scale synoptic patterns and terrain-in- scale convection were determined. duced features, such as elevated heat sources (Mad- Other classification schemes have been presented dox 1980; Cotton et al. 1983; Tripoli and Cotton in the literature. Bluestein and Jain (1985) and Blue- 1989). Using objective analysis and compositing stein et al. (1987) classified radar-observed types of techniques for ten mesoscale convective complexes severe and nonsevere squall-line formation that occur (MCCs), a subset of MCSs, Maddox (1983) identified in Oklahoma during the spring, and related each type several distinctive features at the surface and in the of squall line to a composite environmental sounding lower, middle, and upper troposphere during the for- and to the synoptic environment. Houze et al. (1990) mation, maturation, and dissipation stages of MCCs. presented a classification of mesoscale convection Classification as an MCC was based on characteris- associated with springtime rainstorms (i.e., at least tics observable in satellite imagery because of the 25 mm of rain in 24 h over an area exceeding 12 500 wide range of atmospheric scales that could be mon- km2) in Oklahoma. These classifications are derived itored, but did not address internal structural char- from a single radar and focus on Oklahoma. Houze acteristics. More recently, Cotton et al. (1989), using et al. (1989) discussed the interpretation of radar data compositing techniques that permit greater temporal from single- and multiple-Doppler radars during PRE- resolution, examined 134 MCC events. Their results STORM in Kansas and included a conceptual model are similar to those of Maddox, but are more detailed of the observed variations of linear mesoscale con- regarding the temporal evolution of the system. Like vection. The classification system presented here is Maddox, however, Cotton et al. did not directly ad- more general and includes all types of MCSs over dress the question of variable structures within the both Oklahoma and Kansas during their entire life- MCCs. times. It addresses the problem of identifying radar- MCCs. observed mesoscale patterns of convection on a Data collected in the southern High Plains during larger temporal and spatial scale than that of Blue- the Oklahoma-Kansas Preliminary Regional Experi- stein and Jain (1985), Bluestein et al. (1987), and ment for STORM-central (PRE-STORM) field program Houze et al. (1989, 1990), and smaller-scale internal structures than those identified by Maddox (1983)

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Unauthenticated | Downloaded 10/10/21 10:14 PM UTC Bulletin American Meteorological Society 995 2. Data sources a. Radar data The first step in viewing and analyzing the radar data involved using standard 16-mm film of the radarscope. Radar-observed mesoscale patterns of convection and motions were documented for each day when MCSs were present within the range of the radar. All 61 days of May and June, for both the Okla- homa City, Oklahoma (OKC), and Wichita, Kansas (ICT), radar sites were examined. Nearby sites (Amar- illo, Texas [AMA], Garden City, Kansas [GCK], and Kansas City, Missouri [MCI]) were used when the OKC or ICT radars were not available, or when the MCS was partially beyond the range of the two sites. Detailed examination and comparison of the mesoscale echo patterns were accomplished by compositing digital-radar data. The composite radar data provided a large-scale overview of the mesoscale convection that could not be obtained from a single radar. NWS radar-data processor (RADAP II) (Greene FIG. 1. Location of the PRE-STORM field program and et al. 1983) data were available from several NWS placement of the observational network. WSR-57 radar sites within and adjacent to the PRE- STORM experimental network and consisted of dig- included plotting thermodynamic diagrams and time- itized radar reflectivity from volume scans taken height cross sections of potential temperature and every 10-20 min. These digitized data have a spatial winds, and the objective and subjective analyses of resolution of 2° azimuthally and 1.85 km (1 n mi) numerous parameters on constant pressure levels. radially. Digitized radar data were also recorded by With these analyses, bad data were detected and cor- the NOAA/Hurricane Research Division (HRD) at the rected or deleted when not correctable. ICT radar site; these data had a resolution of 2° azi- The soundings were divided into groups corre- muthally and —0.9 km (0.5 n mi) radially. These sponding to the various types of mesoscale organi- higher-resolution data were used when available, zation of convection within the MCS. Soundings that and the ICT RADAP II data were used at other times. were not representative of the ambient conditions The NWS operated another radar digitizer at MCI ahead of the MCS were not used. The soundings were with resolution similar to the HRD digitizer. The data interpolated to levels spaced 25 mb apart from the from both digitizers had more quantization intervals surface to 100 mb; surface data were not changed. and finer resolution than the RADAP II data. There were occasional instances when the reflectivity val- Finally, soundings were averaged to determine mean ues at the boundary between adjacent radars did not thermodynamic and wind properties for each meso- match, owing to attenuation, distance, different cal- scale convective pattern type. ibrations, or nonstandard beam propagation; but in most cases the boundaries matched satisfactorily. De- spite these occasional problems, no reflectivity data 3. Classification of mesoscale were rejected because the data were used in a qual- convective patterns itative instead of quantitative manner (i.e., the pri- mary goal was to ascertain the radar-echo pattern). During the PRE-STORM field program, MCSs oc- b. Rawinsonde data curred within the region on 21 days. These MCSs To address the environmental conditions supporting exhibited a variety of different spatial characteristics, the different mesoscale patterns of convection, spe- but can be broadly classified into three basic patterns cial high-density PRE-STORM and NWS soundings of mesoscale convection illustrated schematically in were examined. PRE-STORM soundings were taken figure 2. Table 1 shows the number of events for each every 3 h, and occasionally as often as every 1.5 h. pattern of mesoscale convection observed during the Locations of the sounding sites are shown in figure 1; PRE-STORM field program. Some days experienced station spacing averaged approximately 200 km. more than one MCS, so there was a total of 25 MCS Data checks were performed on all soundings and events. Table 2 lists all the days during PRE-STORM

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TABLE 1: Frequency of occurrence of the different convective patterns observed during the PRE-STORM field program.

Convective pattern Cases Percent

Linear 17 68 Occluding 2 8 Chaotic 6 24

convection occurred with approximately equal frequencies; there were ten meso-a- and seven meso- p-scale events. Figures 2a-c show a typical development of echoes into a linear structure and, finally, into a mature system having a region of trailing stratiform precipitation and a "reflectivity trough" or "transition" region located between the stratiform precipitation and the leading line of convection (Sommeria and Testud 1984; Smull and Houze 1985). The degree of organization and symmetry can vary significantly with this class; these variations, however, were not considered in the present classification. Houze et al. (1990), on the other hand, clas- sify mesoscale precipitation systems into several subcategories using these criteria. The second pattern of mesoscale convection is the occluding mesoscale system1; an occlusion is defined

FIG. 2. Schematic of the evolution of the three convective in its most general form by the Glossary of Meteo- modes observed during the PRE-STORM program: (a-c) linear con- rology (Huschke 1959) as a composite of two fronts, vective systems, (d-f) occluding squall-line convective systems, formed as a cold front overtakes a warm front or a and (g-i) chaotic convective systems. Contours and shading indicate increasing reflectivity roughly corresponding to 20, 40, and quasi-stationary front. The occluding mesoscale sys- 50 dBZ. tem qualitatively bears resemblance to the supercell occlusion (Lemon and Doswell 1979) and the synoptic-scale frontal occlusion, but at an intermediate scale. Similar to the supercell occlusion, the occlud- that had MCSs, the pattern of mesoscale convection, ing mesoscale system exhibits boundaries separating and remarks concerning the convection. A day is de- air with differing temperature and moisture charac- fined as the 24-h period from 1800 UTC (1300 cen- teristics that are largely a result of the precipitation tral daylight time) to 1800 UTC the following day. processes taking place. Only two cases (8%) were Most MCSs develop during the afternoon, mature observed during PRE-STORM. Although the small during the evening hours, and dissipate by morning numbers indicate that this is an infrequent event, (Maddox 1980, 1983; Cotton et al. 1989); thus, any mesoscale systems of this type have been observed day having an MCS that approximately followed this at other times and locations (Leary and Rappaport scenario was considered to be a single MCS-day and [1987] and Joe et al. [1988]). The development of is noted as, for example, "6-7 May/' Some (subjec- this type of system (figures 2d—f) starts with a con- tive) tolerance was permitted for both the start and vective line oriented east-west and situated near a end times. Occasionally, MCSs were observed to quasi-stationary frontal boundary; a second convec- have lifetimes and life cycles notably different from tive line is organized north-south along an advancing the 1800 to 1800 UTC cycle. These cases are logged, pressure trough. New cells move northward along the for example, as "12-14 May." Multiple MCSs oc- north-south convective line toward the intersection curred during the two multiday events and, additionally, on 6-7 May and 3-4 June. Table 1 indicates that the linear structure was the most common pattern (68%) of mesoscale convection observed during the PRE- STORM field program. 1 Other convective patterns that resemble the occluding meso- The linear structure discussed here includes both scale system are the "LEWP" (Line Echo Wave Pattern [Nolen 1959]), the "Bow Echo" and "Comma Echo" (Fujita 1978). Each meso-a-scale and meso-p-scale (Orlanski 1975) lin- of these usually operates on a smaller space and time scale than ear convective systems. These two scales of linear the occluding mesoscale system described here.

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TABLE 2: Classification of mesoscale convective patterns observed during the PRE-STORM field project.

Date Convective pattern Remarks

5-6 May Linear meso-p-scale; weak system 6-7 May Chaotic First MCS Occlusion Second MCS 10-11 May Linear meso-a-scale 12-14 May Linear meso-p-scale; various orientations of lines Linear meso-a-scale Linear meso-a-scale 19-21 May Chaotic weak system Linear meso-p-scale; various orientations of lines Linear meso-a-scale 26-27 May Linear meso-a-scale 27-28 May Linear meso-p-scale 28-29 May Linear meso-p-scale 29-30 May Linear meso-p-scale; weak 3-4 June Chaotic First MCS Occlusion Second MCS; started as chaotic convective system Chaotic Third MCS; started to organize, but dissipated 9-10 June Chaotic 10-11 June Linear meso-a-scale; dissipated while occluding 14-15 June Linear meso-p-scale 16-17 June Chaotic largest MCC (cloud shield) in PRE-STORM 17-18 June Linear meso-a-scale 21-22 June Linear meso-a-scale 23-24 June Linear meso-a-scale early; then meso-p-scale 26-27 June Linear meso-a-scale point of the two convective lines and then dissipate, rated downdrafts, and moves southward behind the leaving a stratiform precipitation area. If a circulation cold gust front. The evidence strongly suggests that is present, it is located near the intersection point, the mesoscale dynamics of occluding mesoscale sys- and the stratiform precipitation region may wrap cy- tems are significantly different from the 2-D squall clonically around the backside of the circulation cen- lines and should be classified as a different mesoscale ter. This pattern of mesoscale convection has been convective pattern. recognized as a distinct type by Blanchard and Wat- The third pattern observed, and the second most son (1986); it has also been classified as a "weakly frequent (24%), was that of chaotic mesoscale con- classifiable/asymmetric" squall line by Houze et al. vection. This pattern is characterized by numerous (1990), and as a "squall line with a well-developed convective echoes developing in an apparently ran- mesovortex" by Houze et al. (1989). dom or chaotic manner over a short time. This cha- There are some fundamental differences between otic development of convection occurred both with quasi-two-dimensional (2-D) squall lines and three- and without preexisting light rain or stratiform pre- dimensional (3-D) occluding mesoscale systems. cipitation areas; these two subtypes occurred with Smull and Houze (1985, 1987) have shown that the approximately equal frequencies. At each stage of de- essential flows include both front-to-rear (FTR) and velopment, the pattern resembles the "broken areal" rear-to-front (RTF) flows across the 2-D squall-line and "embedded areal" formations of nonsevere system. Fortune (1989) developed a conceptual squall lines (Bluestein et al. 1987), but the chaotic model for the 6-7 May and 3-4 June MCSs (termed system never organizes into a linear structure. Figures "rotating MCC" by Fortune). The model shows that 2g-i show the typical evolution of a chaotic convec- there are warm "conveyer belts" ahead of the MCS tive system. An analysis of the thermodynamic struc- and cold "conveyer belts" behind the system in ad- ture associated with these systems (discussed later) dition to the FTR and RTF flows across the system. shows that the atmosphere might support ducted Fortune notes that the warm conveyer belt has its gravity waves (Lindzen and Tung 1976). It is possible origin as a low-level jet located approximately 0.5- that these gravity waves play some role in initiating 2 km above the surface. Portions of this jet ascend convection, as well as potentially disrupting any or- vertically in convective cells along the cold gust ganizing tendencies, but this topic is beyond the front; other portions ascend in the apex region (i.e., scope of this paper. the occlusion point) and turn cyclonically to flow to- It should be pointed out that several MCSs exam- wards the rear of the MCS. The cold conveyer belt ined in this study, including some linear and chaotic enters beneath the stratiform cloud from the north at systems, as well as the occluding systems, developed midlevels, descends in both saturated and unsatu- a vortex—often located in the stratiform precipitation

Unauthenticated | Downloaded 10/10/21 10:14 PM UTC 998 Vol. 71, No. 7, July 7 990 region—at the mature or dissipating stage of their ev- fit Bluestein and Jain's (1985) "back-building" line olution. Usually, the vortex was not the major feature classification; the east-west convection is either the of the system, as determined from the radar data, and "broken areal" or the "embedded areal" type (Blue- the system was classified based on other predominant stein et al. 1987). After the north-south line formed mesoscale patterns. on the cold front, a stratiform precipitation region From the preceding discussion, it might be inferred developed and began to grow at the intersection of that these mesoscale convective patterns are clearly the two lines. At this mature stage (figure 2f), it re- distinct from one another and that each MCS assumes sembled the squall line with a well-defined meso- the characteristics only of the particular mesoscale scale vortex described by Houze et al. (1989). Time- pattern. This is not entirely correct, because each lapse photography of the radarscope indicated a well- convective system progresses through an evolution- defined circulation developing in this stratiform pre- ary process. Thus, the genesis stage, mature stage, cipitation. Figure 4c shows the system in the dissi- and dissipation stage may or may not assume the pation stage, in which the squall line had outrun the same pattern. It is clearly evident from the analysis circulation and was weakening. The circulation was of the radar data that some systems do assume more evident in time-lapse photography until all precipi- than one mesoscale convective pattern during their tation echoes dissipated and no tracers remained. lifetime; however, for this classification, a MCS is Analyses of the surface mesonet data and PRE- assigned to the type that dominates its life cycle. STORM supplemental rawinsonde data demonstrate clearly that both cases in this category (6-7 May, 3-4 June) were occlusions. Figures 5a-c depict the surface evolution of the 6-7 May occluding mesoscale system. Boundaries separating warm moist air, 4. Examples of mesoscale cool moist air, and cool dry air are evident, and the convective patterns cool dry air behind the north-south convective line overtook the warm moist air ahead of the line. In Figure 3 depicts the life cycle of a linear convective figure 5a, the synoptic-scale quasi-stationary front system that occurred on 10-11 June 1985 (see Smull was oriented east-west along the Oklahoma-Kansas and Houze [1987]; Johnson and Hamilton [1988]; border, and a meso-a-scale cold gust front was ori- and Rutledge et al. [1988] for further discussion of ented north-south in western Oklahoma. These fronts this mesoscale system). The system started as individ- are shown using symbols discussed by Young and ual convective elements (not shown) that grew and Fritsch (1989). Using this symbolism, synoptic-scale filled in the gaps to form a near-solid line of convec- fronts assume the standard convention, and meso- tion. This convective system fits the "broken line" scale boundaries resulting from adiabatic and dia- classification for severe squall-line formation given batic processes have a double crossbar between by Bluestein and Jain (1985). As the squall line ma- barbs. Note that along the quasi-stationary front there tured, a large stratiform precipitation region devel- were meso-p-scale outflow boundaries associated oped to the rear of the convection. Later, the with localized intense convection. These features are stratiform precipitation region experienced an in- shown for completeness but they are not important crease in reflectivity over a moderately large area, to the development and evolution of the occlusion. similar to the case discussed by Smull and Houze Contours of potential virtual temperature (6V) are (1985). The maximum extent of the convective line drawn at 1°C intervals and clearly define the large was over 800 km in the northeast-southwest direc- cold pool behind the gust front. The intersection of tion. A "reflectivity trough" or "transition zone" sep- the quasi-stationary front and the cold gust front de- arated the convection from the stratiform fines the point of occlusion, and is seen to progress precipitation region. Most linear convective systems from west to east in figures 5a-c. The squall-line por- observed during the PRE-STORM field effort exhib- tion of the occluding mesoscale system lies along the ited the trailing stratiform precipitation characteristics cold gust front and south of the quasi-stationary front. shown here. The stratiform region and circulation are north The second pattern of mesoscale convection is il- through west of the occlusion point. lustrated by the occluding mesoscale system (figure The third pattern is illustrated (figure 6) by the cha- 4) that occurred on 6-7 May 1985 (see Fortune otic convection that occurred on 3-4 June 19852 (see [1989] and Brandes [1990] for further discussion of this mesoscale system). An east-west line of convection along the quasi-stationary front was present before the development of the north-south line of 2 This was the third MCS of the day. The previous system was convection. The north-south convection appears to an occluding system and has been mentioned above.

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FIG. 3. Radar composite of the linear convective system ob- FIG. 4. Radar composite of the occluding mesoscale system observed on 11 June 1985, at (a) 0103 UTC, (b) 0151 UTC, and (c) served on 7 May 1985, at (a) 0800 UTC, (b) 1054 UTC, and (c) 0351 UTC. Gray scale (medium gray, dark gray, light gray, and 1154 UTC. Gray scale (medium gray, dark gray, light gray, and black) corresponds to 18, 30, 40, and 50 dBZ. black) corresponds to 18, 30, 40, and 50 dBZ.

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FIG. 5. Analysis of surface mesonet data depicting the evolution of the occluding mesoscale system that occurred on 7 May 1985 at (a) 0700 UTC, (b) 0800 UTC, and (c) 0900 UTC. The station plot consists of the temperature (upper left), dewpoint (lower left), pressure (upper right) and 0V, potential virtual temperature (lower right). Pressure has been reduced to the mean elevation of the network (500 m) and is written with the leading digit and decimal point omitted (i.e., 534 is 953.4 mb). 0V is written without a decimal point (i.e., 181 is 18.1°C). Contours for 0V are drawn every degree. Symbols for frontal analysis are discussed in the text.

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Fortune [1989] for a discussion of this mesoscale system). Isolated convective cells were randomly scat- tered, and a few meso-p-scale clusters of convective cells were interspersed. The convection remained in the chaotic pattern for most of the lifetime of the MCS; only near the time of dissipation did any organization occur, and then only into short meso-p- scale lines with varying orientations. This pattern, then, was composed of several meso-p-scale "building blocks" (McAnelly and Cotton 1986) that together produced the MCS.

5. Mean soundings and hodographs

Mean soundings and hodographs were constructed for the three types of mesoscale convective patterns by selecting all soundings characteristic of the MCS inflow. As discussed in section 2, data were interpolated from the original mandatory and significant levels to 25-mb intervals. The hodographs were computed by subtracting the mesoscale-system motion; the results are system relative, with the storm motion directed along the positive x-axis and linear systems oriented normal to the x-axis. The motion for linear systems was computed from the movement of the leading edge of the convective line; for chaotic systems it was computed from the movement of the cells; and for the occlusions it was computed from the movement of the leading edge of the line for the southern (linear) portion, and from the movement of the cells from the northern portion. The averaged soundings and hodographs are shown in figure 7. The linear mesoscale systems were divided into two categories, meso-a-scale lines (271 soundings) and meso-p-scale lines (149 soundings), shown in figures 7a and 7b. Both composite soundings exhibited conditional instability through a significant portion of the troposphere; however, the meso-a-scale mean sounding was slightly more moist through a greater depth than the meso-P-scale mean sounding. The wind structures exhibited several differences. The system-relative hodograph indicates stronger inflow and low-level veering of the wind (i.e., a larger "loop") in the meso-a-scale systems than in the meso-p-scale systems. The low-level winds of the meso-a-scale systems have the characteristics associated with a low-level jet, (i.e., strong veering in the lowest layers and weak wind speeds above as indicated by the speed minimum at approximately 700- 500 mb). Also, there is no RTF flow that exceeds the system motion at any level for the meso-a-scale systems. This presumably is related to the observation FIG. 6. Radar composite of the chaotic mesoscale system observed on 4 June 1985, at a) 0700 UTC, b) 0800 UTC, and c) that many of these systems exhibit a trailing stratiform 0900 UTC. Gray scale (medium gray, dark gray, light gray, and precipitation region. In contrast, the meso-p-scale black) corresponds to 18, 30, 40, and 50 dBZ.

Unauthenticated | Downloaded 10/10/21 10:14 PM UTC FIG. 7. Mean soundings and hodographs for (a) meso-a-scale linear systems, (b) meso-p-scale linear systems, (c) occluding mesoscale systems (northern portion), (d) occluding mesoscale systems (southern portion), and (e) chaotic mesoscale systems. Tem- perature and dew-point curves (°C) are shown in heavy lines. Ground-relative winds are plotted on the right with the standard convention: half barb, 2.5 m s~1; full barb, 5 m s~1; flag, 25 m s_1. Hodographs in the upper-left corner depict system-relative winds (defined in text). Speed rings are at 5 m s_1 intervals; azimuth marks are every 30°. Points along the heavy curve labeled 'S/ '8/ '7/ '5/ and '2' denote the surface, 850-mb, 700-mb, 500-mb, and 200-mb levels, respectively.

Unauthenticated | Downloaded 10/10/21 10:14 PM UTC Bulletin American Meteorological Society 1003 systems do have a RTF component that exceeds the to the lowest 2.5 km above ground level. Although system speed. Radar data indicated that the meso-p- the system-relative hodograph for the chaotic con- scale systems exhibited leading stratiform precipita- vective systems indicates that the low-level shear ap- tion more often than the meso-a-scale systems. proaches this value, no linear organization takes These hodographs do not precisely match those place. It is likely that the stability of the layer from presented by Bluestein and Jain (1985) or Bluestein the surface up to approximately 850 mb may be pre- et al. (1987). This is not surprising as both earlier venting downdrafts from reaching the surface. With- analyses examined the formation of linear systems, out the penetrative downdraft and low-level wind- whereas the current results include formation and shear interaction, organization along the gust front mature stages of the linear convective systems. Ad- on a scale larger than the convective cell is not sup- ditionally, the former studies included data from ported and the MCS remains chaotic. April, which generally has larger shear values than To a lesser degree, this argument can be applied either May or June. The largest difference between to the northern portion of the occluding mesoscale these results is that the previous studies show less systems. In contrast to the chaotic systems, however, curvature in the lowest levels of the hodograph. It there is a "conveyer belt" (Fortune 1989) of warm, cannot be determined whether this is statistically sig- moist air that glides up the frontal surface. The lo- nificant because of different procedures for selecting cation north of the surface front at which the parcels cases. Despite these differences, all the results con- reach the level of free convection is approximately tain the same basic features of the hodograph. parallel to the quasi-stationary front. The radar data The occluding mesoscale systems were divided indicate that this convective line does not have the into two regions: 1) north of the quasi-stationary front same degree of organization or longevity as the ad- (27 soundings), and 2) south of the quasi-stationary vancing convection that is oriented north-south. front (55 soundings). The east-west band of convection and the stratiform region are located in the area north of the quasi-stationary front. The mean sounding from this region (figure 7c) indicates a stable layer 6. Summary and discussion (probably associated with the front) near 850 mb. The low-level winds (ground-relative) are typical of those Data collected in the southern High Plains as part of found in the cool air north of a quasi-stationary front the PRE-STORM field program show that there is a (i.e., easterly surface winds). The system-relative ho- broad spectrum of structural characteristics associ- dograph shows only a shallow layer of veering, then ated with MCSs. Despite this, MCSs can be broadly a deep layer of winds backing with height. By com- classified into three patterns of mesocale convection. parison, the region south of the front (figure 7d) is These patterns are linear convective systems, chaotic markedly different, with steep lapse rates in the low- convective systems, and occluding mesoscale sys- est layers and some drying aloft compared to the tems. Twenty-five MCSs were catalogued on 21 days northern region. The system-relative hodograph is during the PRE-STORM field program. Frequency dis- more typical of the linear systems (figures 7a and 7b). tributions show that two-thirds (17 events, 68%) of The chaotic convective systems, like the northern the systems were linear; occluding mesoscale sys- portion of the occluding mesoscale systems, have a tems accounted for two events (8%), and chaotic stable region up to approximately 850 mb (figure 7e); convection accounted for six events (24%). however, the mean sounding (47 soundings) is In a study of MCSs, Kane et al. (1987) indicated slightly more unstable and drier. Owing to the effects four preferred regions relative to fronts and cyclones of averaging, the stable layer has been smoothed ap- for the development of MCSs. They defined these re- preciably; nonetheless, its retention in the mean is gions as "synoptic," "mesohigh," and two variations an indication of the frequent occurrence and strength of "frontal" (see their figure 5). Surface synoptic of this feature. The system-relative hodograph ex- weather maps indicate that the linear convective sys- hibits a boundary layer with veering winds topped by tems observed in PRE-STORM are primarily of the a deeper layer of backing winds. The layer of veering "synoptic" type (i.e., ahead of the cold front and winds is more pronounced than that of the occluding south of the cyclone). The "mesohigh" configuration mesoscale systems (northern portion), and the is also responsible for some of the meso-p-scale lin- ground-relative surface winds are more southerly. ear systems. The occluding mesoscale systems and Weisman and Klemp (1986) have shown that there chaotic systems tend to occur in or adjacent to the is a tendency for an isolated cell to evolve into a cool air and are classified as "frontal." quasi-steady arc of cells, oriented along a line per- Rawinsonde data have been interpolated to 25-mb pendicular to the low-level wind-shear vector when intervals and composited for each of the types of strong unidirectional shear (>15 m s~1) is confined mesoscale convection. Additionally, the linear sys-

Unauthenticated | Downloaded 10/10/21 10:14 PM UTC 1004 Vol. 71, No. 7, July 7 990 terns were further stratified into meso-a-scale systems the spring. ). Atmos. Sci. 42: 1711-1732. and meso-p-scale systems; the occluding mesoscale , G. T. Marx, and M. H. Jain. 1987. Formation of mesoscale lines of precipitation: Nonsevere squall lines in Oklahoma dur- systems were stratified into regions north and south ing the spring. Mon. Wea. Rev. 115: 2719-2727. of the quasi-stationary front. The mean soundings Brandes, E. A. 1990. Evolution and structure of the 6-7 May 1985 show there are differences in both the temperature mesoscale convective system and associated vortex. Mon. Wea. and the moisture structure with resulting differences Rev. 118: 109-127. in conditional instability. The mean system-relative Cotton, W. R., R. L. George, P. J. Wetzel and R. L. McAnelly. 1983. A long-lived mesoscale convective complex. Part I: The hodographs indicate that different wind structures are mountain-generated components. Mon. Wea. Rev. 111: 1893- present for the different mesoscale convective types. 1918. It has been shown that although some of these wind , M. Lin, R. L. McAnelly and C. J. Tremback. 1989. A com- structures have low-level shears that approach the posite model of mesoscale convective complexes. Mon. Wea. values specified by Weisman and Klemp (1986) for Rev. 117: 765-783. Cunning, J. B. 1986. The Oklahoma-Kansas Preliminary Regional upscale organization of an isolated cell into an arc Experiment for STORM-Central. Bull. Amer. Meteor. Soc. 67: of cells, linear structures do not develop and may be 1478-1486. a result of downdrafts not reaching the surface be- Fortune, M. A. 1989. The evolution of vortical patterns and vor- cause of the stable boundary layer. tices in mesoscale convective complexes. Ph.D. dissertation, These results should not be extrapolated beyond Colorado State University, 183 pp. Fujita, T. T. 1978. Manual of downburst identification for Project this dataset, which is temporally brief and cannot be NIMROD. SMRP Res. Paper 756, University of Chicago. shown to be statistically significant. Use of RADAP II Greene, D. R., J. D. Nelson, R. E. Saffle, D. W. Holmes, M. D. data from other years has been considered as a means Hudlow and P. R. Ahnert. 1983. RADAP II: An interim radar of enlarging the sample size and increasing the sta- data processor. Preprints, 21st Conference on Radar Meteorol- tistical significance. The RADAP II program, how- ogy, American Meteorological Society, Boston, Mass., 404- 408. ever, was only able to collect digitized radar data Houze, R. A., S. A. Rutledge, M. I. Biggerstaff and B. F. Smull. from a minimum of four of the six High Plains stations 1989. Interpretation of Doppler weather radar displays of mid- on a total of 61 days during the warm season (May, latitude mesoscale convective systems. Bull. Amer. Meteor. Soc. June, and July) during the period 1985-87. With 70: 608-619. fewer than four stations, it is often difficult to generate , B. F. Smull and P. Dodge. 1990: Mesoscale organization of springtime rainstorms in Oklahoma. Mon. Wea. Rev. 118: 613- composites for examining the evolution of mesoscale 654. convective patterns. Other data sources are necessary Huschke, R. E. 1959. Glossary of Meteorology, American Mete- to expand this part of the study. orological Society, Boston, Mass., 638 pp. An important future goal will be to analyze case Joe, P. I., T. R. Nichols and C. L. Crozier. 1988. The three-di- studies from the PRE-STORM data, using the synoptic mensional nature of an occluded mesoscale convective system. Extended abstracts, Lower tropospheric profiling: Needs and and mesoscale surface and rawinsonde data, profiler technologies. American Meteorological Society, Boston, Mass. data, satellite imagery, and Doppler-radar data to de- 151-152. termine the relationship between the radar-observed Johnson, R. H., and P. J. Hamilton. 1988. The relationship of mesoscale convective patterns and the forcing at mul- surface pressure features to the precipitation and airflow struc- tiple scales. tures of an intense midlatitide squall line. Mon. Wea. Rev. 116: 1444_1472. Kane, R. J., C. R. Chelius and J. M. Fritsch. 1987. Precipitation Acknowledgments. The author thanks all the participants of the characteristics of mesoscale convective weather systems. J. Cli- PRE-STORM field program for their outstanding contributions in mate Appl. Meteor. 26: 1345-1357. the collection of the data. Special thanks are extended to L. Show- Leary, C. A., and E. M. Rappaport. 1987. The life cycle and in- ell (NOAA/NSSL) for coordinating the rawinsonde program and to ternal structure of a mesoscale convective complex. Mon. Wea. the National Weather Service and the Oklahoma Climatological Rev. 115: 1503-1527. Survey for the RADAP II data. Appreciation is extended to J. Brown Lemon, L. R., and C. A. Doswell III. 1979. Severe thunderstorm (NOAA/FSL) and D. Burgess (NOAA/NSSL) for their reviews of the evolution and mesocyclone structure as related to tornadoge- manuscript and to M. Weisman (NCAR) for his help in the inter- nesis. Mon. Wea. Rev. 107: 1184-1197. pretation of the hodographs. Lindzen, and Tung. 1976. Banded convective activity and ducted gravity waves. Mon. Wea. Rev. 104: 1602-1617. Maddox, R. A. 1980. Mesoscale convective complexes. Bull. Amer. Meteor. Soc. 61: 1374-1387. References , 1983: Large-scale meteorological conditions associated with mid-latitude, mesoscale convective complexes. Mon. Wea. Rev. Blanchard, D. O., and A. I. Watson. 1986. Modes of mesoscale 111: 1475-1493. convection observed during the PRE-STORM program. Preprints, McAnelly, R. L., and W. R. Cotton. 1986. Meso-(3-scale charac- 23rd Conference on Radar Meteorology, American Meteorolog- teristics of an episode of meso-a-scale convective complexes. ical Society, Boston, Mass., J155—J158. Mon. Wea. Rev. 114: 1740-1770. Bluestein, H. R., and M. H. Jain. 1985. Formation of mesoscale Nolen, R. H. 1959. A radar pattern associated with tornadoes. lines of precipitation: Severe squall lines in Oklahoma during Bull. Amer. Meteor. Soc. 40: 277-279.

Unauthenticated | Downloaded 10/10/21 10:14 PM UTC Bulletin American Meteorological Society 1005

Orlanski, I. 1975. A rational subdivision of scales for atmospheric convection in continental tropical regions. Bull Amer. Meteor. processes. Bull. Amer. Meteor. Soc. 56: 529-530. Soc. 65: 4-10. Rutledge, S. A., R. A. Houze, M. I. Biggerstaff and T. Matejka. Tripoli, G. J., and W. R. Cotton. 1989. Numerical study of an 1988. The Oklahoma-Kansas mesoscale convective system of observed orogenic mesoscale convective system. Part 1: Simu- 10-11 June 1985: Precipitation structure and single-Doppler ra- lated genesis and comparison with observations. Mon. Wea. dar analysis.Mon. Wea. Rev. 116: 1409-1430. Rev. 117: 273-304. Smull, B. F., and R. A. Houze. 1985: A midlatitude squall line Weisman, M. L., and J. B. Klemp. 1986. Characteristics of isolated with a trailing region of stratiform rain: Radar and satellite ob- convective storms. Mesoscale Meteorology and Forecasting. servations. Mon. Wea. Rev. 113: 117-133. Amer. Meteor. Soc., 331-358. , and , 1987: Rear inflow in squall lines with trailing stratiform precipitation. Mon. Wea. Rev. 115: 2869-2889. Young, G. S., and J. M. Fritsch. 1989. A proposal for general Sommeria, G., and J. Testud. 1984. COPT 81: A field experiment conventions in analyses of mesoscale boundaries. Bull. Amer. designed for the study of dynamics and electrical activity of deep Meteor. Soc. 70: 1414-1421. • announcements

International Technical Conference on Long- junction with the NCAR Atmospheric Technology Division range Weather Forecasting Research Users' Conference, 2 October 1990, at the same location. The ICTP/WMO International Technical Conference on For further information, please contact Shelley Zucker, Long-range Weather Forecasting Research will be held (303) 497-8833. 8-12 April 1991 in Trieste, Italy. The conference will provide an international forum for NCAR Atmospheric Technology Division experts to review the major achievements made during the Users' Conference past five years. The program consists of theoretical basis of The first ATD User's Conference, sponsored by the NCAR LRF, monthly and seasonal dynamical forecasts and inter- Atmospheric Technology Division (ATD), will be held at annual forecasts, statistical methods, applications, and the Westin Resort in Vail, Colorado, 2 October 1990. The economic aspects of LRF. objectives of the meeting are to obtain the views of the People wishing to submit a paper should forward their users of ATDs existing observational platforms and instru- camera-ready extended abstracts before 1 October 1990 mentation systems about the capabilities and performance to the Secretary-General, Attention PTR Division, WMO, of these facilities; to review current progress and plans Case Postale No. 2300, 1211 Geneva 2, Switzerland. regarding the development of new ATD facilities; and to Please send a second copy to K. Miyakoda, GFDL, P.O. obtain advice from users on these on these and other new Box 308, Princeton, NJ 08542. Additional information developments. about the conference can be obtained from either of these This meeting will be held in conjunction with the addresses. Workshop on Atmospheric Research Measurements, 3-4 October 1990 at the same location. For further information, Workshop on Atmospheric please contact Shelley Zucker at (303) 497-8833. Research Measurements A workshop on atmospheric research measurements, jointly Fourth Annual International Winter Weather hosted by the National Center for Atmospheric Research Workshop (NCAR) Atmospheric Technology Division and the National This workshop focused on improving ice and snow control Oceanic and Atmospheric Administration (NOAA) Aeron- procedures will be held in St. Louis, Missouri, 10-12 omy Laboratory, will be held at the Westin Resort in Vail, October 1990. Surface Systems, Inc. will sponsor and host Colorado, on 3-4 October 1990. this event. For registration or more information, please call The objectives of the meeting are to assess the scientific Ann Buchmann, 1 (800) 325-SCAN. needs for measurements in selected atmospheric research areas; to identify gaps in the measurement systems currently available for filling these needs; and to recommend the Notice of registration deadlines for meetings, workshops, and development of needed new systems. seminars, deadlines for submission of abstracts or papers to be presented at meetings, and deadlines for grants,'proposals, All interested atmospheric scientists, instrumentation de- awards, nominations, and fellowships must be received at least velopers, and research managers are invited to attend this three months before deadline dates.—News Editor • important meeting. This workshop will be held in con-

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