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Last modified
7/28/2009 2:38:56 PM
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4/18/2008 9:58:57 AM
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Weather Modification
Title
Sensitivity of Convective Cloud Growth to Mesoscale Lifting: A Numerical Analysis of Mesoscale Convective Triggering
Date
7/7/1980
Weather Modification - Doc Type
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<br />JULY 1980 <br /> <br />NOTES AND CORRESPONDENCE <br /> <br />1063 <br /> <br />conditions when organized convection or random <br />isolated convection were observed in satellite <br />imagery, the convective instability was sensitive <br />to lifting (Table 4). Considerable deep convec- <br />tion was diagnosed in both conditions when lifting <br />was applied, and relatively little development was <br />diagnosed when surface heating and surface eddy <br />mixing were the only sources of destabilization. <br />When surface heating and surface eddy mixing were <br />the only sources of destabilization, the air mass <br />sample had a larger number (13 cases) of convective <br />events than the organized mesoscale sample (3 <br />cases), suggesting that the days with air. mass con- <br />vection were relatively unstable without mesoscale <br />lifting while days having organized convection were <br />more stable and required mesoscale lifting to initiate <br />convection. However, once destabilized by lifting, <br />those days having organization had a high potential <br />for convective development. These modeling results <br />indicate that mesoscale dynamics appear to de- <br />termine when and where organized convection <br />will occur. <br /> <br />5. Discussion and conclusions <br /> <br />For an atmospheric sounding, the thermodynamic <br />characteristics such as stable layer height, strength, <br />number and shape determine the potential for meso- <br />scale triggering. Strong stable layers which often <br />cap a moist boundary layer have a large potential <br />for deep moist convection when lifting locally de- <br />stroys the capping inversion. In many cases bound- <br />ary-layer energy appears to be constrained by this <br />inversion and is released when the inversion is <br />destroyed. Severe convective storms often form in <br />this type of environment when mesoscale triggers <br />are present (House, 1963). <br />The model simulations showed that the potential <br />for deep moist convection on the High Plains is <br />very sensitive to mesoscale lifting. Both organized <br />convective days and isolated convective days were <br />sensitive to lifting. This result suggests that the <br />three-dimensional atmospheric dynamics may be the <br />controlling factor which determines whether the <br />mesoscale convection is organized or random and its <br />relative intensity. MESOCU is a simple tool for <br />diagnosing the potential for convective triggering <br />by lifting. This may be a useful forecasting index; <br />however, the large sensitivity for mesoscale lifting <br />in nearly all cases indicates that the CPI will over- <br />predict or will have a large false alarm rate if used <br />without observations or reliable forecasts of lifting. <br />It will also be necessary to initialize the model with <br />a representative atmospheric sounding. Preliminary <br />analyses of simultaneous soundings from a network <br />of four stations located in an approximately square <br />grid 80 km on a side showed large variations in <br />model predictions. Analysis of the difficult problein <br />of sounding representativeness is continuing. <br /> <br />" J <br /> <br />TABLE 4. Comparison of the effect of lifting on the convective <br />potential index for cases with sateIlite observed mesoscale or- <br />ganized and random convection at Big Spring, Texas, in 1976 <br />and 1977, <br /> <br />Satellite stratification <br /> <br />Organized <br />convection <br /> <br />Random or isolated <br />convection <br /> <br />Model analysis of CPI <br /> <br />No lifting <br />10 cm S-I lifting <br />20 cm S-1 lifting <br /> <br />70,8 <br />576;2 <br />991.5 <br /> <br />39.9 <br />379,7 <br />762.8 <br /> <br />Comparisons of convective cloud response due <br />to lifting and those due to ice phase seeding show <br />that lifting often has a larger effect than seeding. <br />Previous studies by Simpson and Wiggert (1971) <br />showed that dynamic seeding responses to seeding <br />may produce increases in cloud depth of 1-4 km. <br />These increases often occurred when natural cloud <br />growth was inhibited by stable layers at heights <br />having temperatures between -5 and -1OoC. The <br />MESOCU model simulations indicate that lifting <br />may account for similar changes in cloud growth. <br />For example, on 12 July 1976, the Big Spring <br />1500 GMT sounding analysis by MESOCU indicated <br />a dynamic seeding response of 2.8 km when no <br />lifting was simulated. The natural CPI in this case <br />was 8.5 km and the seeded CPI was 11.0 km. When <br />20 cm S-1 lifting was simulated the natural CPI was <br />.32 km, a threefold increase in total cloud depth. <br />Similar increases due to natural lifting occurred in <br />all of the samples shown in Table 3. These natural <br />variations in cloud development due to lifting may <br />create serious sampling and bias problems in <br />weather modification experiments which have <br />heterogeneous samples of lifting. <br />These model results and rawinsonde stratifica- <br />tions by satellite observed mesoscale structure <br />suggest the following generalizations: <br /> <br />1) Natural variability of convective develop- <br />ment may largely be the result of mesoscale lift- <br />ing which increases the amount of PBE resulting in <br />greater available buoyant energy for deep convec- <br />tive cloud development. <br />2) Days with air mass convection were relatively <br />unstable without mesoscale lifting while days hav- <br />ing organized convection were more stable and <br />required mesoscale lifting to initiate convection. <br />3) The magnitude of the mesoscale variability of <br />convective cloud development due to lifting may ex- <br />ceed the effect of seeding, thereby masking or lead- <br />ing to misinterpretation of seeding effects in samples <br />with heterogeneous mesoscale lifting. <br />4) Mesoscale covariates and stratification criteria <br />derived from MESOCU analyses based on observed <br /> <br />j' <br />
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