Analysis and Classification of Mesoscale Cloud and Precipitation Systems

Four distinctly different meso-(beta) scale cloud systems have been objectively defined and analyzed using satellite imagery and digital radar data to produce more homogeneous classes of precipitation events in the west Texas High Plains region. The four classes were (1) isolated convective cells, (2) convective clusters, (3) cloud lines, and (4) mesosynoptic systems. A nearly logarithmic increase in total rain volume and mesoscale precipitation efficiency was observed from isolated cells to mesosynoptic systems. Automated band-pass objective analyses of seven rawinsondes separated by 80 to 100 km in a meso-(beta) (25 to 250 km) scale network with observations every 3 h were used to compute the kinematic controls of mesoscale forcing within the 1979 Texas HIPLEX field program. Vertical motion and vertical moisture flux were highly correlated in time and space with the intensity and location of clouds and precipitation. A conceptual model of jet streak triggering of different types of systems was developed that showed jet entrance quadrant 4 and exit quadrant 2 having 200 mb divergence of 20 to 40 x 10('-5)/s with strong lifting of 20 to 40 (mu)bar/s while entrance quadrant 1 and exit quadrant 3 had convergence of -5 to -40 x 10('-5)/s with moderate sinking of -5 to -20 (mu)bar/s. The most intense and organized precipitation systems (lines and mesosynoptic events) occurred with the most intense lifting, while upper level subsidence was predominant in isolated cell and cluster events. From 96 to 98% of the total rain volume estimated by radar fell from echoes with maximum rain rates (GREATERTHEQ) 30 mm/h and echoes with duration (GREATERTHEQ) 1 h provided 95% of the total rain volume observed in the 1979 field season. Line and mesosynoptic classes accounted for 91% of the total rain volume observed, while clusters provided (TURN) 5% and isolated cells (TURN) 4%. Mesoscale precipitation efficiency ranged from 2 to 80%, increasing from isolated cells to mesosynoptic systems respectively. A single radar echo, which represents from 2 to 3% of the echoes typically observed during a case day, provided from 34 to 88% of the total rainfall observed during the day. This result indicates the critical need to analyze and predict these larger echoes in order to (1) determine the most appropriate clouds for study in weather modification programs and later (2) determine the most productive targets for seeding in operational programs.