Coupling of Moist-Convective and Stratiform Precipitation Processes for Variational Data Assimilation

Abstract

Some problems posed by the coupling of moist-convective and stratiform precipitation processes for variational assimilation of precipitation-rate data are examined in a 1D-Var framework. Background-error statistics and vertical resolution are chosen to be representative of current operational practice. Three advanced parameterization schemes for moist-convection are studied: the relaxed Arakawa?Schubert (RAS) scheme, Tiedtke?s mass-flux scheme (operational at the European Centre for Medium-Range Weather Forecasts), and the Betts?Miller scheme. Both fractional-stepping and process-splitting approaches for combining physical processes are examined. The behavior of the variational adjustment for background profiles of temperature and specific humidity in the neighborhood of saturation is of particular interest. In the 1D-Var context examined here, it is demonstrated that the introduction of the stratiform precipitation process can have a negative impact on the minimization in the sense that, even when only slight supersaturation occurs, the minimization is controlled by the stratiform precipitation process at the expense of convective precipitation. This is generally true in process-splitting mode and conditionally true in fractional-stepping mode. The net result in such cases is an adjustment to the wrong type of precipitation over convective regions. In some of the cases examined (1D-Var with the RAS scheme, for instance), it is preferable to deactivate the stratiform precipitation process and to explicitly control the degree of supersaturation during the adjustment of convection. Evaporation of precipitation in subsaturated layers also appears as an important factor influencing the partition of precipitation. The method of fractional stepping appears less problematical compared to the process-splitting approach. These results also indicate the need for a detailed examination of the partition of precipitation between convective and stratiform type in more sophisticated 3D/4D-Var data assimilation systems, and for a better combined parameterization of the two physical processes.