Examination of the sensitivity of forecast precipitation rates to possible perturbations of initial conditions

An adjoint model that includes precipitation physics is applied to four synoptic cases in order to compare the sensitivities of both convective and non-convective precipitation rates with respect to initial perturbations of temperature, water vapor mixing ratio, vorticity or divergence. These are contrasted with sensitivities of barotropic vorticity. Forecast periods between 1 and 24 h are investigated. Root-mean-square values of the sensitivities as a function of vertical coordinate and field are presented as well as time series of impacts of optimal perturbations weighted by initial variances of uncertainties in the fields. For all the forecast aspects and cases considered, the greatest sensitivity and impacts are with regard to the temperature field. Precipitation is equally sensitive to vorticity and divergence, but when their relative uncertainties are considered, impacts of the vorticity dominate those of divergence. Precipitation is sensitive to initial specific humidity, but so is barotropic vorticity within a cyclone. The sensitivities of precipitation were not found to increase with forecast periods as much as the sensitivities to vorticity, suggesting that even within the same synoptic feature, different processes are responsible for the development of these distinct characteristics. Although comparison with corresponding impacts in the nonlinear model suggests that a 24 h period is too long for the adjoint-estimated precipitation impacts to be good approximations to the nonlinear results in some of the cases, 6-h adjoint-estimated results appear useful.