1.2.4.2.3. Evaporative cooling, virga and downbursts

  • 8 months ago
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  • Author: SW101

[ADVANCED] X.3 Wet downburst

  • WET DOWNBURST: form in environments with very dry air aloft (mid troposphere), but very moist air in the lowest several kilometers (lower troposphere). In wet downbursts evaporative cooling is important, but a significant part of the downward air motion is produced by the weight of the rain dragging the air downward (water loading). A wet downburst is accompanied by heavy precipitation at the surface. As seen from a distance a wet dowburst in the contact page looks like a water baloon hanging from below the cloud base. In outburst stage it may resemble an upside down mushroom, with a visually distinct rain shaft and a more or less distinct rain foot.
    • Heavy rainfall and precipitation core is mainly in the form of ice (e.g. melting hail).
    • Surface winds caused by precipitation loading and negative buoyancy. Downwind momentum transfer and/or dynamically induced pressure gradient may also contribute, particularly in strong events (copy).
    • Downdraft entrainment of environmental air important.
    • Moist air below cloud base, dry at mid level.
    • Strong convection / updraft.
    • Low cloud base.
    • Function of solar heating, most frequent around mid-afternoon local time.
    • Relatively large lowering of surface temperature during the downburst.

    Wet downbursts can produce intense rainfall, with rates >200 mm/h over several minutes. Downbursts are frequently asymmetric: stronger and more spread out in certain directions than others.