Development and evaluation of a convection scheme for use in climate models

Cumulus convection is a key process in controlling the water vapor content of the atmosphere, which is in turn the largest feedback mechanism for clim ate change in global climate models. Yet scant attention has been paid to d esigning convective representations that attempt to handle water vapor with fidelity, and even less to evaluating their performance. Here the authors attempt to address this deficiency by designing a representation of cumulus convection with close attention paid to convective water fluxes and by sub jecting the scheme to rigorous tests using sounding array data. The authors maintain that such tests, in which a single-column model is forced by larg e-scale processes measured by or inferred from the sounding data, must be c arried out over a period at least as long as the radiative-subsidence times cale-about 30 days-governing the water vapor adjustment time. The authors a lso argue that the observed forcing must be preconditioned to guarantee int egral enthalpy conservation, else errors in the single-column prediction ma y be falsely attributed to convective schemes. Optimization of the new scheme's parameters is performed using one month of data from the intensive flux array operating during the Tropical Ocean Glo bal Atmosphere Coupled Ocean-Atmosphere Response Experiment, with the aid o f the adjoint of the linear tangent of the single-column model. Residual ro ot-mean-square errors, after optimization, are about 15% in relative humidi ty and 1.8 K in temperature. It is difficult to reject the hypothesis that the residual errors are due to noise in the forcing. Evaluation of the conv ective scheme is performed using Global Atmospheric Researh Program Atlanti c Tropical Experiment data. The performance of the scheme is compared to th at of a few other schemes used in current climate models. It is also shown that a vertical resolution better than 50 mb in pressure is necessary for a ccurate prediction of atmospheric water vapor.