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Lawrence Cheng
and
Marianne English

Abstract

Hailstone size distributions have been determined from 41 time-resolved hailstone samples collected at the ground from seven storms that occurred in Alberta in the summer of 1980. Most size distributions were found to quite closely fit an exponential function of the form n(D) = n 0 e −λD . In studying variations in n 0 and λ, it was found that a relationship exists between the two. In particular, correlation coefficients of ∼−0.9 were found when least-square linear regressions were fitted to the values of logn 0 versus logλ. For Alberta storms, therefore, n 0 can be expressed in terms of λ as n 0 = 115λ3.63, and hail size distributions can be expressed in terms of the single parameter λ as n(D) = 115λ3.63 e −λD . From an examination of hail size distributions from one storm that occurred in Switzerland, it appears likely that similar relationships can be determined for hailstorms from other regions.

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Han-Ru Cho
and
Lawrence Cheng

Abstract

Dynamic processes of cumulus clouds which may produce significant horizontal eddy transport of vertical vorticity in the tropical atmosphere are discussed. It is shown in this paper that horizontal eddy transport of vorticity by cumulus convection, if it exists, must be due entirely to the irrotational component of the horizontal wind produced by clouds. The ability of clouds to produce this eddy transport depends critically on the presence of cyclonic-anticyclonic vortex couplets in the cloud circulations. The generation processes of these vortex couplets are also discussed.

A formula is derived to parameterize this eddy transport process in the large-scale mean vorticity equation. The formulation is tested using. GATE data. GATE-A/B-scale mean vorticity budgets are analyzed for two 1-day periods during Phase III of the experiment. The agreements between the theoretically predicted and the observed apparent vorticity sources are found to be much improved by including the effects of cloud horizontal eddy transport of vertical vorticity.

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Lawrence Cheng
and
David C. Rogers

Abstract

Observational evidence from an Alberta hailstorm was examined in an attempt to demonstrate the link between feeder clouds and hailfalls. Radar data, time resolved surface collections of hail, and cloud photographs from a storm were analyzed. It was found that the streak events in the surface hailfall can he linked to small-scale radar reflectivity maxima in the new growth region of the storm. The results suggest that the hail growth process began with packets of hail embryos in distinct feeder clouds, and that the separation between feeder clouds was eventually manifested as distinct hail streak events at the surface. The feeder clouds formed approximately in a line parallel to the vertical ambient wind shear near the cloud base level. The spacings between feeder clouds were almost equal and estimated to he 3 km. Theoretical predictions indicate that convective spacing in a horizontally uniform atmosphere is determined by environmental wind shear, stability, and depth of the shear layer. The results of this and other observational studies lead to the speculation that the spacing between distinct hail streak events may be controlled by the same factors in the vicinity of the new growth zone of hailstorms.

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Lawrence Cheng
,
Tsoi-Ching Yip
, and
Han-Ru Cho

Abstract

The effects of cumulus clouds on the large-scale potential vorticity field are investigated using GATE data. Clouds are found to modify the mean potential vorticity field not only through vertical mixing but also through the generation of potential vorticity by the release of latent beat. Overall, the dynamic effect and the thermodynamic effect of clouds are found to contribute about equally to the large-scale potential vorticity budget.

A diagnostic method is also developed to determine mean cloud vertical vorticity profiles from observed large-scale potential vorticity sources. The method is applied to GATE AIB-scale potential vorticity budgets. The results show that 1) the mean cloud vorticity is of the same order of magnitude as the large-scale mean vorticity, despite the smallness of the horizontal scales of cumulus clouds, and 2) the mean cloud vorticity is smaller than the large-scale mean vorticity in the mean detrainment layer of the cloud population, and larger than the large-scale mean vorticity in the mean cloud entrainment layer. These properties are in agreement with the theoretical analysis presented in Choet al. (1979a).

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Han-Ru Cho
,
Lawrence Cheng
, and
Robert M. Bloxam

Abstract

A formulation is proposed to represent the effects of cumulus clouds in the large-scale vorticity equation. Cumulus clouds are found to modify the large-scale vorticity field through two processes: 1) the vertical advection of mean vorticity by the cumulus vertical cloud mass flux and 2) the twisting of horizontal vorticity into the vertical direction due to the inhomogeneous spatial distribution of the cloud population. Under the assumption that the divergence of mean horizontal eddy flux of vorticity can be neglected, the net effect of cumulus clouds can be completely specified in terms of the total cloud mass flux alone. No detailed knowledge of the dynamic fields of cumulus clouds is needed.

GATE A/B-scale heat, moisture and vorticity budgets have been analyzed for a 1-day period (0000–2400 GMT 9 September) during Phase III to verify the validity of the theoretical formulation. The agreement between the theoretically predicted and the observed cloud effects appears very encouraging.

Another important conclusion of this study is that the cloud vorticity when averaged over a cloud cross-sectional area is of the same order of magnitude as the large-scale mean vorticity, despite the smallness of the horizontal scale of cumulus clouds. This has some important implications on the analysis of mesoscale dynamic fields using observational data.

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