1.2.4.2.3. Evaporative cooling, virga and downbursts

  • 8 months ago
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  • Author: SW101

[ADVANCED] X.2 Dry downburst

  • DRY DOWNBURST: are produced by evaporative cooling of the air through evaporation of rain in very dry air. As the rain falls through the dry air, it evaporates. Evaporation is an endothermic process, it absorbs heat from the surrounding air, cooling it. The cooled air is denser than the surrounding air and rapidly sinks towards the surface. This process is called dry air entrainment. Dry downbursts occur in environments with very dry air in the lowest several kilometers (lower troposphere). Thunderstorms that produce dry downbursts are high-based, due to low moisture near the ground, and produce little or no rainfall. Virtually all from the thunderstorm rainfall evaporates in the dry air, forming virga. As there is little or no rainfall associated, a dry downburst is difficult to spot prior to ground contact. Severe winds kick up dust on the surface and can produce a distinctive swirl along the gust front, providing visual cues of the downburst. Thunderstorms that have the potential to produce a dry downburst will have high bases and little or no rainfall. They often produce intense lightning – as there is little rainfall they are sometimes called dry thunderstorms.
    • Little or no rain reaches the surface, often associated with virga shafts.
    • Negative buoyancy produced by evaporation of precipitation below the cloud base.
    • Downdraft entrainment minimal, due to near or complete absence of precipitation.
    • Very dry air below cloud base (dry or nearly dry adiabatic cloud base).
    • Convective updraft can be weak.
    • High cloud base.
    • Function of solar heating, most frequent around mid-afternoon local time.
    • Small drop in surface temperature during the downburst.