Heating, Moisture, and Water Budgets of Tropical and Midlatitude Squall Lines: Comparisons and Sensitivity to Longwave Radiation

Abstract

A two-dimensional, time-dependent, and nonhydrostatic numerical cloud model is used to estimate the heating (Q1, moisture (Q2), and water budgets in the convective and stratiform regions for a tropical and a midlatitude squall line (EMEX and PRE-STORM). The model is anelastic and includes a parameterized three-class ice-phase microphysical scheme and longwave radiative transfer processes. A quantitative estimate of the impact of the longwave radiative cooling on the total surface precipitation as well as on the development and structure of these two squall lines is presented. It was found that the vertical eddy moisture fluxes are a major contribution to the model-derived Q2 budgets in both squall cases. A distinct midlevel minimum in the Q2 profile for the EMEX case is due to vertical eddy transport in the convective region. On the other hand, the contribution to the Q1 budget by the cloud-scale fluxes is minor for the EMEX case. In contrast, the vertical eddy heat flux is relatively important for the PRE-STORM case due to the stronger vertical velocities present in the PRE-STORM convective cells. It was found that the convective region plays an important role in the generation of stratiform rainfall for both cases. Although the EMEX case has more stratiform rainfall than its PRE-STORM counterpart, the relative contribution to the stratiform water budget made by the horizontal transfer of hydrometeors from the convective region is less. But the transfer of condensate from the convective region became relatively less important with time in the stratiform water budget of the PRE-STORM system as it developed from its initial stage, such that the relative contribution to the stratiform water budget made by the horizontal transfer of hydrometeors from the convective region is similar at the mature stages of both systems. Longwave radiative cooling enhanced the total surface precipitation about 14% and 31% over a 16-h simulation time for the PRE-STORM and EMEX cases, respectively. The relative contribution to the stratiform water budget from the convective region is, however, more sensitive to the longwave radiative cooling for the PRE-STORM case than for the EMEX case. These results are due to the relatively moist environment and comparatively earlier development of the stratiform cloud in the EMEX squall system. Nevertheless, the effect of radiative cooling is shown to increase as systems age in both cases. It was also determined that the Q1 and Q2 budgets in the convective and stratiform regions are only quantitatively, not qualitatively, altered by the inclusion or exclusion of longwave radiative transfer processes.