The Effect of Melting Processes on the Development of a Tropical and a Midlatitude Squall Line

Abstract

Several sensitivity tests are performed to assess the effect of melting processes on the development of a midlatitude continental squall line and a tropical oceanic squall line. It is found that melting processes play an important role in the structure of a midlatitude continental squall system. For the maritime tropical case, squall development is not as sensitive to the presence of melting, due to the dominance of warm rain processes. Melting processes exert an influence on midlatitude cloud system development through the conversion of ice particles to rain. The simulated convective system was found to be much weaker in the absence of evaporative cooling by rain. For a given vertical shear of horizontal wind, cooling by evaporation in the convective region was found to be essential for maintaining a long-lived cloud system. Diabatic cooling by melting played only a secondary role in this respect. In the absence of melting processes, the simulated mildlatitude squall system acquired the characteristics of unicell-type (erect and steady) convection rather than the observed multicellular (upsher tilt) structure. This suggests that the diabatic cooling by melting can have significant impact on the structure (dynamics) of a simulated midlatitude squall system. In addition, results from air parcel trajectory analyses indicate that jump-type downdrafts that originate either from the convective region or from above the melting level in the stratiform region are not simulated for convection that develops in the absence of melting. The horizontal momentum transport associated with the midlatitude squall system simulation were quite different in the presence and absence of melting. Significant horizontal momentum transport by convection was not observed in the absence of melting. However, an upper-level jet was simulated in the case where melting processes were active. It is also found that the horizontal perturbed pressure gradient force is comparable in magnitude yet almost always opposite in sign to the vertical transport effect by clouds.