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Melting and freezing in a mesoscale convective system

Microphysical and thermodynamic fields retrieved from dual-Doppler synthesized winds are used to examine the distribution of melting and freezing 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 region, where the layer is typically marked by a radar bright band. The retrieved fields reveal the melting and freezing in the convective as well as stratiform region. Two-dimensional retrieved fields indicate that 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, in 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 convective updraughts, with peak heating rates of 8 K h−1 just above the 0 °C level. For the mature stage of the storm, the convective 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 one-dimensional retrieval model is applied to mean vertical-velocity profiles to retrieve area-mean profiles of the melting and freezing rates. In the convective region, average cooling rates of −10 K h−1 are found. The strong cooling by melting is expected to exert a strong influence on the development of convective downdraughts, the cold pool, and 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 °C level. In the stratiform precipitation region, a melting rate of about −2 K h−1 produced a near 0 °C isothermal layer approximately 500 m deep, similar to isothermal layers observed in stratiform precipitation areas associated with fronts.