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<br />1062
<br />
<br />MONTHLY WEATHER REVIEW
<br />
<br />TABLE 2, Summary. of mesoscale lifting effects on the
<br /> potential for convective development.
<br /> Mesoscale
<br /> No lifting
<br /> lifting
<br /> 0.0 10 20
<br /> Sample cm S-1 cm S-1 cm S-1
<br />Montana-1975
<br />number of raobs yielding clouds 7,0 16,0 31.0
<br />number of clouds simulated 20,0 36,0 67,0
<br />CPI (km) 46,0 127,5 249.5
<br />Kansas-1976
<br />number of raobs yielding clouds 2,0 6,0 17,0
<br />number of clouds simulated 3,0 19,0 69,0
<br />CPI (km) 12,4 95,9 407.2
<br />. Texas-1976
<br />number of raobs yielding clouds 4,0 18,0 22,0
<br />number of clouds simulated 7,0 61.0 122,0
<br />CPI (km) 39.9 379,7 762,8
<br />Kansas-I977
<br />number of raobs yielding clouds 13,0 23,0 26,0
<br />number of clouds simulated 23,0 101,0 174.0
<br />CPI (km) 116.9 655,9 1203,5
<br />Texas,-1977
<br />number of raobs yielding clouds 7.0 21.0 27.0
<br />number of clouds simulated 12,0 83.0 142,0
<br />CPI (km) 70,8 576,2 991.5
<br />
<br />duced large changes in environment relative hu-
<br />midity, which permitted stronger cloud growth due
<br />to less entrainment erosion of cloud buoyancy.
<br />
<br />b. Effects of lifting on convective potential
<br />
<br />Lifting produced large changes in the convective
<br />potential for cloud growth in most soundings exam-
<br />ined by the model. Table 2 summarizes the effect
<br />of lifting in all cases examined. The number of
<br />soundings able to support convective development,
<br />the number of clouds diagnosed, and the total CPI
<br />increased as lifting was increased in the model
<br />simulations for all High Plains sites. The samples
<br />of CPI for each lifting rate at each site were com-
<br />pared by testing a null hypothesis that the CPI with
<br />one lifting rate equaled that of another, i.e., there
<br />was no significant effect of lifting on CPI. Table 3
<br />summanzes the results of these statistical tests
<br />showing that lifting had a highly significant positive
<br />effect on convective development diagnosed by the
<br />model. Results were significant at better than the 1 %
<br />level in all samples.
<br />It can be seen from Tables 2 and 3 that the effect
<br />of lifting varied geographically from north to south
<br />over the High Plains. In general, the Montana sam-
<br />ple was least affected, whereas the Texas sample
<br />was most affected by lifting. However, conditions
<br />can apparently be quite variable in Kansas which
<br />had the second lowest CPI in 1976 and the highest
<br />in 1977. The more consistently favorable conditions
<br />in Texas may be due to the mid-tropospheric stable
<br />
<br />VOLUME 108
<br />
<br />layer which often persists over the southern plains
<br />due to the Bermuda High (Carlson and Ludlam,
<br />1968). The subsidence associated with the western
<br />. edge of the Bermuda High produces stable layers
<br />which mesoscale lifting can destabilize, thereby
<br />increasing the potential for convective development.
<br />Planetary boundary-layer moisture (surface to 1.5
<br />km AGL) also increases from north to south over
<br />the High Plains. This distribution of moisture re-
<br />sults in a larger potential for deep moist convec-
<br />tion in Texas when stable layers are removed by
<br />lifting. The higher tropopause height in the Southern
<br />Plains also accounts for the increased potential of
<br />convective development in this region.
<br />
<br />c. Discrimination between mesoscale-triggered
<br />and isolated convection
<br />
<br />Although MESOCU showed a large effect of lift-
<br />ing on the release of available potential instability,
<br />it was less successful at distinguishing between
<br />thermodynamic conditions characteristic of or-
<br />ganized mesoscale convection and random isolated
<br />convection. Model results indicated that under
<br />
<br />TABLE 3, Statistical significance of lifting on total convective
<br />development (CPI), Null hypothesis: There is no significant
<br />effect of lifting,
<br />
<br />Paired (-test results
<br />
<br />Site/year
<br />
<br />Mean
<br />difference
<br />~
<br />
<br />Stand-
<br />ard
<br />devi-
<br />ation
<br />a
<br />
<br />Corre-
<br />lation
<br />r P-values
<br />
<br />MLS-1975
<br />10 vs 0
<br />20 vs 0
<br />20 vs 10
<br />GLD-1976
<br />10 vs 0
<br />20 vs 0
<br />20 vs '10
<br />GLD-1977
<br />10 vs 0
<br />20 vs 0
<br />20 vs 10
<br />BGS-1976
<br />10 vs 0
<br />20 vs 0
<br />20 vs 10
<br />BGS-1977
<br />10 vs 0
<br />20 vs 0
<br />20 vs 10
<br />
<br />2,2
<br />5,3
<br />3,1
<br />
<br />3,3 4.0 0,78
<br />5,3 6,0 0,67
<br />3,6 5,2 0,87
<br />
<br />7.6
<br />16,2
<br />9,7
<br />
<br />10,3 3,7 0.45
<br />19.0 4,5 0,39
<br />10.4 4,6 0,93
<br />
<br />17.4
<br />36.1
<br />18.7
<br />
<br />18,0 22.0 0.,70
<br />61.0 122,0 0.45
<br />3.4 5,6 0,89
<br />
<br />14,2
<br />31.0
<br />16,8
<br />
<br />9,9 7,3 0,72
<br />10,8 19.6 0,74
<br />7,2 11.8 0,88
<br />
<br />17,1
<br />32,1
<br />15.0
<br />
<br />14,0 6.3 0.67
<br />13,2 12.6 0.67
<br />6,2 12.6 0.94
<br />
<br />0,001
<br />0.001
<br />0.001
<br />
<br />0,001
<br />0,001
<br />0,001
<br />
<br />0,001
<br />0,01
<br />0,001
<br />
<br />0,001
<br />0.001
<br />0,001
<br />
<br />0,001
<br />0,001
<br />0,001
<br />
<br />~ = mean difference total natural depth (CPI) between paired
<br />cases of one lifting rate vs another.
<br />a = standard deviation of mean difference,
<br />( = (-value from (-test.
<br />r = correlation between cloud depth at different lifting rates.
<br />P = probability that the difference could have occurred by
<br />chance-all cases"" 0,001.
<br />
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