Deep convection modifications for gustiness, timescale and entrainment

Abstract

Tropical deep convection is well know to be poorly represented in courser grid climate models. The focus has thus far been on improving convective precipitation and its vertical profile of heating. However, convection by it's very nature also has sub-grid scale variability in surface wind structure that would not be resolved on the model grid scale. Redelsperger et al., (2000) shows that gustiness has a strong definable relationship to convective precipitation in the TOGA-COARE region with the greatest. This presentation shows early result from the implementation of this parameterization in the ACME model where we apply the implied rainfall-based gustiness as a additional wind speed for the purposes of calculating surface latent heat fluxes. In AMIP simulations at low resolutions the impact is greatest in the deep tropics where rainfall is highest and surface flux biases the greatest (too low). The scheme acts to remedy some of the surface latent heat biases and as a related consequence enhances rainfall in the same region where low biases exists. During northern summer in particular, the response involves a shift in the center of action for the Monsoon from Arabia toward South East Asia, which is a general improvement in the simulation. Very preliminary simulations at high resolution (ne120/0.25 deg) demonstrate similar, but somewhat weaker impacts during northern summer.

Work has also been undertaken to implement modifications to deep convection parameterizations as summarized in Bechtold et al., (2008). Modifications provide new sensitivity in the calculations of convective timescale and entrainment rate These are not intended to be major modification to the parameterization of deep convection in the ACME model, but it is hoped that they may provided incremental improvements in the simulation. Early results from this work at low resolution and future plans will be presented.