Diabatic Initialization of the Canadian Regional Finite-Element (RFE) Model Using Satellite Data. Part II: Sensitivity to Humidity Enhancement, Latent-Heating Profile and Rain Rates

Abstract

A frontal development observed during the CASP (Canadian Atlantic Storm Project) project is used to investigate the dependence of the spinup time of the divergent circulation on (i) the initial humidity held, (ii) the vertical structure of the latent-heating profile, and (iii) the precipitation rates used in the diabatic initialization. Five 12-hour simulations using the Canadian regional finite-element (RFE) model with diabatic initialization are carried out: one control and four test simulation with varying initial conditions. One experiment uses the analyzed relative humidity in lieu of the satellite-inferred humidity enhancement, two simulations use rain rates reduced to half and zero, and in a fourth experiment the condensation scheme of the model is replaced by the moist adiabatic approach (generation friction) to determine the initial rain rates. The results indicate that in spite of the diabatic initialization, the spinup times remain long (6?9 hours) if no humidity enhancement is applied where latent heat is released. The accuracy of the specified rain rates is not as crucial as the use of the satellite-inferred humidity: a reduction of the rain rates by a factor of 2 only results in a modest increase of the spinup time. The identification of rain areas is, however, essential, since the total absence of precipitation (i.e., an adiabatic initialization with a humidity enhancement) leads to a 9-hour long spinup. The condensation scheme used in the initialization does not seem to be of prime importance, as the shortening of the spinup process remains practically unchanged regardless of whether the model condensation scheme or the generation function is used.