A Numerical Study of Lapse-Rate Adjustments in the Tropical Atmosphere

Abstract

Observations in the tropics have shown that the lapse rate of virtual Potential temperature ?v normally resembles that of a reversible moist adiabat. In the present study, a mesoscale numerical model with parameterized convection is used to examine the adjustment to equilibrium of the tropical atmosphere in the presence of large-scale forcing. The physical processes that enable the model to produce an equilibrium lapse rate similar to what is observed are examined. The results indicate that a spectrum of cloud sizes, which is produced by a variable entrainment rate, is essential to enable the atmosphere to approach an equilibrium state of small conditional instability in the presence of large-scale destabilization. Lateral detrainment is not essential for modeling the large-scale stability of the atmosphere, despite its significance for modeling individual mesoscale convective systems. When downdrafts are removed from the parameterized convection, the model only temporarily produces a realistic large-scale lapse rate. Other results suggest that the departure of observed ?? soundings from adiabats around the melting level is due to ice processes. The feedbacks between convective-scale process and the environmental stability that allow the atmosphere to maintain a state of small conditional instability are discussed.