MELTING AND FREEZING IN A MESOSCALE CONVECTIVE SYSTEM

Microphysical and thermodynamic fields retrieved from dual-Doppler syn thesized winds are used to examine the distribution of melting and fre ezing rates within a midlatitude squall line with trailing stratiform precipitation. Previously, the effect of the heating/cooling processes associated with the release/consumption of the latent heat of fusion have emphasized the melting layer in the stratiform precipitation regi on, where the layer is typically marked by a radar bright band. The re trieved fields reveal the melting and freezing in the convective as we ll as stratiform region. Two-dimensional retrieved fields indicate tha t the strong melting effect is not confined to the stratiform region, but extends across the entire breadth of the storm. The peak rates of cooling in the stratiform precipitation region are -2 to -3 K h(-1), i n general agreement with previous studies. In contrast, cooling rates in the convective precipitation region reach -14 K h(-1). The heating by freezing is concentrated in a vertical column associated with the c onvective updraughts, with peak heating rates of 8 K h(-1) just above the 0 degrees C level. For the mature stage of the storm, the convecti ve region is found to account for 56% of the total cooling by melting, and 87% of the total heating by freezing within the squall line. A on e-dimensional retrieval model is applied to mean vertical-velocity pro files to retrieve area-mean profiles of the melting and freezing rates . In the convective region, average cooling rates of -10 K h(-1) are f ound. The strong cooling by melting is expected to exert a strong infl uence on the development of convective downdraughts, the cold pool, an d the gust front. The heating by freezing occurs through a deep layer, with a peak average heating rate of about 4 K h(-1) just above the 0 degrees C level. In the stratiform precipitation region, a melting rat e of about -2 K h(-1) produced a near 0 degrees C isothermal layer app roximately 500 m deep, similar to isothermal layers observed in strati form precipitation areas associated with fronts.