Effects of mesoscale convective organization and vertical wind shear on the cumulus-environment interaction.

摘要

This study is made to understand the thermodynamic and dynamic aspects of cumulus-environment interaction. Specifically, we examine (1) the similarities and differences of cumulus-environment interactions in the tropical and midlatitude convective systems, (2) the impact of the presence of mesoscale circulations on the interpretation of cumulus-environment interaction, and (3) the effects of vertical wind shear on the dynamic interaction of cumulus convection with the large-scale motion.; Analyses of PRE-STORM and GATE data show larger moist convective instability, large-scale forcing and vertical wind shear in the midlatitude MCCs and squall lines than in the tropical non-squall clusters. It is found that the interaction mechanism based on the cumulus-induced subsidence and detrainment is capable of explaining most of the observed heating and drying under widely different environment conditions. The Arakawa-Schubert (A-S) "quasi-equilibrium" assumption is valid and holds better in the midlatitudes since the large-scale forcing is much stronger. Both the cumulus and stratiform cloud effects are stronger in midlatitude convective systems than in tropical systems.; The heat and moisture budget results using the fine resolution SESAME data show pronounced "dipole" patterns in the horizontal distributions of vertically integrated heat source and moisture sink. Further analyses, using the "large-scale" and "mesoscale" data which are obtained by applying low-pass and band-pass spatial filters to the original SESAME data, show that the dipole pattern is closely related to the horizontal fluxes of heat and moisture due to mesoscale circulations.; Tests of the A-S parameterization scheme with the "mesoscale" and "large-scale" data show that the quasi-equilibrium assumption becomes more accurate for the data resolving mesoscale circulations. The inclusion of downdrafts is required to accurately predict the cumulus heating and drying.; We find the significant difference in vertical transports of horizontal momentum between the MCC and squall line. The downgradient transport of momentum is dominant in the MCC. On the other hand, the vertical transport of momentum normal to the squall line is upgradient, while the transport of momentum parallel to the line is downgradient.; A new cloud momentum model which includes the convective-scale horizontal pressure gradient force has been developed. The pressure gradient force is related to the vertical wind shear, convective mass flux and orientation of organized convection. The application of the new cloud momentum model to the MCCs and squall lines observed during SESAME and PRE-STORM shows that the new model can simulate both the upgradient and downgradient transports of cloud momentum.