The Supercooled Warm Rain Process and the Specification of Freezing Precipitation

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  • 1 Department of Meteorology, University of Maryland, College Park, Maryland
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Abstract

About 30% of freezing precipitation cases are observed to occur in a subfreezing atmosphere (contrary to the classical melting ice model). We explain these cases with the concept of the “supercolled warm rain process” (SWRP): the warm rain process can yield liquid hydrometeors at subfreezing temperatures whenever too few ice nuclei are available to create solid hydrometeors. We find that all of the freezing precipitation cases in a subfreezing atmosphere show a rapid decrease of moisture content in the zone above the inferred cloud top (decreasing from liquid to ice saturation in lm than 20 mb), at temperatures ranging from 0° to −10°C. Additionally, this structure prevails among freezing cases (43%) much more than among solid or liquid cases (10% and 15%, respectivelly).

Regression experiments demonstrated that freezing precipitation was best described when (discretized) predictors were combined to describe particular physical processes, such as the SWRP. Besides the SWRP, the usual melting ice process variables, such as cold-layer size, and surface temperature are important for specifying freezing precipitation. The system handles liquid and solid precipitation adequately, but is deficient in specifying freezing cases. This last result is in agreement with previous studies, and reflects the small sample size due to the rarity of freezing cases.

Abstract

About 30% of freezing precipitation cases are observed to occur in a subfreezing atmosphere (contrary to the classical melting ice model). We explain these cases with the concept of the “supercolled warm rain process” (SWRP): the warm rain process can yield liquid hydrometeors at subfreezing temperatures whenever too few ice nuclei are available to create solid hydrometeors. We find that all of the freezing precipitation cases in a subfreezing atmosphere show a rapid decrease of moisture content in the zone above the inferred cloud top (decreasing from liquid to ice saturation in lm than 20 mb), at temperatures ranging from 0° to −10°C. Additionally, this structure prevails among freezing cases (43%) much more than among solid or liquid cases (10% and 15%, respectivelly).

Regression experiments demonstrated that freezing precipitation was best described when (discretized) predictors were combined to describe particular physical processes, such as the SWRP. Besides the SWRP, the usual melting ice process variables, such as cold-layer size, and surface temperature are important for specifying freezing precipitation. The system handles liquid and solid precipitation adequately, but is deficient in specifying freezing cases. This last result is in agreement with previous studies, and reflects the small sample size due to the rarity of freezing cases.

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