Coupling of moist-convective and stratiform precipitation processes for variational data assimilation

Some problems posed by the coupling of moist-convective and stratiform prec ipitation processes for variational assimilation of precipitation-rate data are examined in a 1D-Var framework. Background-error statistics and vertic al resolution are chosen to be representative of current operational practi ce. Three advanced parameterization schemes for moist-convection are studie d: the relaxed Arakawa-Schubert (RAS) scheme, Tiedtke's mass-flux scheme (o perational at the European Centre for Medium-Range Weather Forecasts), and the Betts-Miller scheme. Both fractional-stepping and process-splitting app roaches 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 ID-Var context examined here, it is demonstrated that the introducti on of the stratiform precipitation process can have a negative impact on th e minimization in the sense that, even when only slight supersaturation occ urs, the minimization is controlled by the stratiform precipitation process at the expense of convective precipitation, This is generally true in proc ess-splitting mode and conditionally true in fractional-stepping mode. The net result in such cases is an adjustment to the wrong type of precipitatio n over convective regions. In some of the cases examined (1D-Var with the R AS scheme, for instance), it is preferable to deactivate the stratiform pre cipitation process and to explicitly control the degree of supersaturation during the adjustment of convection. Evaporation of precipitation in subsat urated layers also appears as an important factor influencing the partition of precipitation. The method of fractional stepping appears less problemat ical compared to the process-splitting approach. These results also indicate the need for a detailed examination of the part ition of precipitation between convective and stratiform type in more sophi sticated 3D/4D-Var data assimilation systems, and for a better combined par ameterization of the two physical processes.