Large-Scale Effects of Deep Convection on the GATE Tropical Boundary Layer

Abstract

The large-scale response of the atmospheric boundary layer to the passage of tropical wave disturbances is investigated. Observations from GATE indicate that during periods of extensive deep convective activity rather shallow mixed layers frequently are found, primarily in association with mesoscale precipitation systems. Convective-scale precipitation downdrafts accompanying such systems contribute to the formation of large trailing “wakes” wherein there exist considerably enhanced fluxes of sensible heat from the mean surface. Observations in the convectively active trough portion of tropical waves indicate that a considerable fraction of the total area is covered by wakes. However, there remain regions between rain systems in the wave trough covering ∼15–30% of the total area, depending on how these regions are defined, which have mixed layers more characteristic of those observed in the undisturbed ridge portion of the wave than those in wakes. A simple model is developed to study the maintenance of the mixed layer in regions between wakes. In the model, horizontal and vertical boundaries of the well-mixed layer are treated as zero-order discontinuities. The model takes into account, in a large-scale sense, the convergence of relatively cool downdraft air into the region between convective systems or wakes. Variables at the top of the mixed layer are specified using large-scale rawinsonde observations and a diagnostic model for the cloud layer above. Application of the model to a composite of a number of GATE wave disturbances has shown that during periods of abundant deep convective activity, environmental subsidence (subsidence away from cumulus and mesoscale downdraft systems) is weak and yet the mixed layer there does not grow without bound because of the inversion-strengthening effect of cool downdraft air outflow into the between-cloud region. Although the model is not fully closed, in the sense that a budget is not carried out to determine thermodynamic properties of downdraft air near the ground, it does suggest that large-scale numerical prediction models may be able to carry the mixed layer height as a predicted variable even during convectively disturbed situations.