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

Abstract

An adjoint model that includes precipitation physics is applied to four synoptic cases in order tocompare the sensitivities of both convective and non-convective precipitation rates with respect to initialperturbations of temperature, water vapor mixing ratio, vorticity or divergence. These are contrastedwith sensitivities of barotropic vorticity. Forecast periods between 1 and 24 h are investigated. Rootmean-square values of the sensitivities as a function of vertical coordinate and field are presented aswell as time series of impacts of optimal perturbations weighted by initial variances of uncertaintiesin the fields. For all the forecast aspects and cases considered, the greatest sensitivity and impacts arewith regard to the temperature field. Precipitation is equally sensitive to vorticity and divergence, butwhen 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. Thesensitivities of precipitation were not found to increase with forecast periods as much as the sensitivitiesto vorticity, suggesting that even within the same synoptic feature, different processes are responsiblefor the development of these distinct characteristics. Although comparison with corresponding impactsin the nonlinear model suggests that a 24 h period is too long for the adjoint-estimated precipitationimpacts to be good approximations to the nonlinear results in some of the cases, 6-h adjoint-estimatedresults appear useful.