Numerical Simulation of a Tropical Mesoscale Convective System Using the Arakawa-Schubert Cumulus Parameterization

Abstract

As an extension of the work presented in an accompanying paper of Kao and Ogura, the Arakawa-Schubert cumulus parameterization scheme is examined prognostically in a modeling study of the evolution of a convectively driven tropical mesoscale rainband that developed on 12 August 1974 over the eastern Atlantic. A two-dimensional hydrostatic model is used with a mixed layer parameterization. Observed soundings were used as initial conditions and 24-h integrations of the model are made. In the control experiment, a prescribed time-independent large scale forcing is imposed in a limited area at low levels. Many aspects of the observed evolution of the rainband are well simulated by the model, including the shift of height of the area-averaged maximum upward motion from low levels to upper levels and the development of downward motion at low levels while upward motion of a significant magnitude is still present at upper levels. The predicted rainfall rate, in the mature stage, is also found to agree with the observation. A secondary development of deep convection away from the primary convective area is generated at later times due to the adiabatic subsidence warming at upper levels. The predicted cloud population is bimodal at the developing stage of the mesoscale circulation, whereas it is dominated by deep convection at the mature stage, consistent with earlier diagnostic studies. Quantitatively, however, some discrepancies are observed between the predicted and observed evolution of the mesoscale circulation. Several sensitivity tests of the model are made. Removing the large-scale forcing at the time when the convective system reaches its peak intensity immediately results in the decay of the system. Including the vertical wind shear does not have much effect on the life cycle of the system except that the convective activities shift slightly to the downwind area. Permitting cumulus downdraft effects in the Arakawa-Schubert scheme reduces the intensity of circulation due to the cooling of subcloud layer introduced by the detrainment of the downdrafts.