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r ,I L , Reprinted from JOURNAL OF ApPLIED METEOROLOGY, Vol. 20, No.9, September 1981 American Meteorological Society Printed in U, S. A, Natural Variability of Thermodynamic Features Affecting Convective Cloud Growth and Dynamic Seeding: A Comparative Summary of Three High Plain Sites from 1975 to 1977 DAVID A. MATTHEWS Office of Almospheric Resources Research, Bureau of Reclamalion, Denver, CO 80225 (Manuscript received 11 September 1979, in final form 22 May 1981) , ABSTRACT A statistical summary of the thermodynamic features generally thought to be relevant to convective cloud development and the dynamic seeding hypothesis, as diagnosed by a one-dimensional steady-state model, has been compiled for selected locations on the High Plains. The summary is based primarily on rawinsonde data collected at Miles City, Montana, Goodland, Kansas, and Big Spring, Texas during the summer months from 1975 to 1977, as part of the High Plains Cooperative Program (HIPLEX). Data were stratified according to the occurrence or non-occurrence of convective clouds and, when clouds occurred;}:, according to cloud type as seen in geosynchronous satellite imagery. Geographic comparisons are presented showing significant variations from north to south over the High Plains, the most intense convection occurring in the central and southern plains and the least intense in the north. Large annual variations were noted in mean values of thermodynamic features at each site between the three summers. Analyses of mesoscale variability of thermodynamic features were made using observations from a mesoscale rawinsonde network. Although large spatial variations in thermodynamic variables on a given day were found, seasonal mean values were similar, suggesting that the variations are related to small- scale dynamics and appear statistically random but that the aVl:rage values at anyone site are representa- tive of a given region. While no cloud seeding was performed to test the dynamic seeding hypothesis in field experiments, the cloud model indicated that soundings on the High Plains had potential for additional dynamic growth. The dynamic modification potential (DMP) is defined as the post-seeding enhancement of growth that is predicted by the cumulus model to result from heat released by converting supercooled water to ice. This is equivalent to Simpson's (1976) "seedability," and is similar to model-predicted changes in cloud depth used by other authors. While supercooled water is presumed to exist in sufficient quantity for a long enough time to produce changes in cloud dynamics, no in-cloud measurements of the presence and duration of supercooled water were made in this study. However, the model indicated that DMP was found to exist in about 50% of the soundings analyzed in this study. The maximum frequency of occurrence of DMP was associated>with mesoscale convective systems in Montana and Texas; whereas in Kansas, it occurred on days with smaller air mass convective clouds. Large variations in the frequency of modeled DMP occurred as a function of observed cloud types and mesoscale types. I I I ". 1. Introduction Experiments to develop the technology for scientific precipitation management of summer con- vective clouds on the High Plains require back- ground information which describes the natural variability of convective clouds and the thermo- dynamic controls of natural moist convection. Conditions favorable for convective cloud develop- ment have been summarized in the Thunderstorm Project (Byers and Braham, 1949) and by Newton (1963). These convective controls include condi- tional and convective instability, moisture in the lower troposphere, surface heating to the condensa- tion level and mesosynoptic forcing functions. House (1963) demonstrated the importance of dynamic mechanisms which lift the atmosphere, resulting in release of instability. Mesoscale trig- gering effects generated by dissipating thunder- n <" .i:ci:c";; storm outflow which initiate new cells have been identifIed by Fujita (1963). These local mesocold fronts and convergence lines associated with the moist downdraft of thunderstorms act as efficient convective triggering mechanisms .when sufficient moisture and instability exist. In the Plains, the convergence associated with the dry line is also an important mesoscale triggering mechanism (Rhea, 1966; Schaefer, 1974). Favorable conditions for mountain orographic convective cloud develop- ment have been studied by Booker (1963). He showed that the elevated heat source and the flow 'of air over mountain ranges may initiate and inten- sify convective development in mountainous terrain when sufficient moisture is present. This paper investigates some of the thermody- namic features thought to be relevant to convective cloud development by examining the variability in the thermodynamic structure of rawinsonde data 971 I I ---J