Dynamics of Convective Clouds and 'CISK' in Vertical Shear Flow-with its Application to Easterly Waves and Squall-Line Systems

Abstract

The properties of convective clouds and CISK (instability of the large-scale disturbance interacting with the clouds) under vertical shear flow are studied by numerical experiments as an extension of previous studies (Yamasaki, 1975, 79, 83), which have used a fine resolution model with explicit calculations of convective clouds. The CISK treated in this study is such that the downdraft and cooling due to the evaporation of falling rain play an essential role. The primary objectives are to understand how this type of CISK is modified by vertical shear flow and to show how it can account for some observed phenomena in the tropical easterlies.
According to numerical experiments, when the intensity of an easterly flow decreases with increasing height (quasi-linear flow with westerly shear), convection in the eastern portion of the convective area is more preferred than that in the western portion. When the easterly flow is strongest at some height such as 3km (jet-type flow), convection in the western portion is preferred. These results indicate that vertical shear in the lowest few kilometers is important and that convection is more enhanced when low-level inflow into the convection is augmented by vertical shear flow.
Individual convective clouds in the absence of vertical shear are organized into a larger convective system (referred to as 'mesoscale convection') with a time scale of about 3 hours (Yamasaki, 1983). It is shown in this study that this time scale is not much modified by the quasi-linear flow (L) and that it is modified to about 8 hours for the jet-type flow (J). In the cases of no shear and (J), convective clouds grow successively from a low-level cloud
which persists continuously, whereas in the case of (L), new clouds tend to form at some distance from existing clouds, constituting mesoscale convection in all cases.
The properties of a large-scale disturbance obtained for the flow (L) are similar to those of the easterly wave of the type studied by Riehl (1954) and Yanai (1961) ; that is, a coldcored structure at low levels, an eastward tilt of a trough, major convective activity located to the east of the trough and an upper-level ridge located further to the east are well simulated. It is also shown that vertical shear is not indispensable to the growth of the disturbance, thought it is very important in explaining observed features of the structure.
The western convection which is preferred in the flow (J) exhibits several important features in observed squall-lines studied by Zipser (1977) and others. The simulated structure is also similar to that obtained in numerical experiments for an isolated cloud (e. g., Moncrieff and Miller, 1976). However, in the present model, convective clouds form successively from persistent low-level clouds. Such successively formed clouds constitute mesoscale convection, the lifetime of which is roughly in agreement with observations.