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 clas- sified 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) © 1990 American Meteorological Society and Cotton et al. (1989). 994 Vol. 71, No. 7, July 1990 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 radar- scope. Radar-observed mesoscale patterns of con- vection and motions were documented for each day when MCSs were present within the range of the ra- dar. 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 meso- scale echo patterns were accomplished by compos- iting 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 Unauthenticated | Downloaded 10/10/21 10:14 PM UTC 996 Vol. 71, No. 7, July 7 990 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 fre- quencies; there were ten meso-a- and seven meso- p-scale events. Figures 2a-c show a typical devel- opment of echoes into a linear structure and, finally, into a mature system having a region of trailing stra- tiform precipitation and a "reflectivity trough" or "transition" region located between the stratiform precipitation and the leading line of convection (Sommeria and Testud 1984; Smull