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Evolution of the Vertical Mass Flux and Diagnosed Net Lateral Mixing in Isolated Convective Clouds

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  • 1 Department of Meteorology, University of Hawaii at Manoa, Honolulu, Hawaii
  • | 2 National Center for Atmospheric Research, Boulder, Colorado
  • | 3 National Weather Service, Milwaukee, Wisconsin
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Abstract

The evolution of the vertical mass flux in isolated cumulus and cumulus congestus clouds is documented using two King Airs during the Convection and Precipitation/Electrification Experiment (CaPE,), conducted in east-central Florida during the summer of 1991. These clouds develop over the sea-breeze convergence in an environment characterized by low shear and moderate convective available potential energy. The aircraft, separated vertically by 600–1000 m, commence flying near-simultaneous penetrations as the cloud top passes through the altitude of the upper aircraft and continue sampling until the cloud disappears. The net vertical mass flux for each level is estimated; the difference in the mass flux between the two levels leads to a diagnosis of the net entrainment or detrainment that occurs laterally in the intervening cloud layer. This kinematic technique relies on determination of cloud edge based on liquid water measurements, a cloud shape factor, and the vertical velocity. The technique is not limited to the period prior to precipitation fall out like most conserved variable techniques nor does it require accurate measurement of in-cloud total water and temperature.

Results from 12 isolated clouds with radii of 0.5–1.5 km and lifetimes less than 25 min demonstrate that the vertical mass flux evolves in a well-behaved manner with clear growth and decay phases. Significant net lateral entrainment or detrainment is diagnosed, which complements the top entrainment that has been inferred from conserved thermodynamic variable techniques. Net entrainment dominates the growth stage, whereas net detrainment is most often seen during the decay stage of the cloud. An approximate entrainment rate averaged over cloud life is 1 km−1. The continuous nature of the updrafts demonstrates that these small clouds are best described as a single shedding thermal, not a series of bubbles ascending in the wake of each preceding bubble.

Abstract

The evolution of the vertical mass flux in isolated cumulus and cumulus congestus clouds is documented using two King Airs during the Convection and Precipitation/Electrification Experiment (CaPE,), conducted in east-central Florida during the summer of 1991. These clouds develop over the sea-breeze convergence in an environment characterized by low shear and moderate convective available potential energy. The aircraft, separated vertically by 600–1000 m, commence flying near-simultaneous penetrations as the cloud top passes through the altitude of the upper aircraft and continue sampling until the cloud disappears. The net vertical mass flux for each level is estimated; the difference in the mass flux between the two levels leads to a diagnosis of the net entrainment or detrainment that occurs laterally in the intervening cloud layer. This kinematic technique relies on determination of cloud edge based on liquid water measurements, a cloud shape factor, and the vertical velocity. The technique is not limited to the period prior to precipitation fall out like most conserved variable techniques nor does it require accurate measurement of in-cloud total water and temperature.

Results from 12 isolated clouds with radii of 0.5–1.5 km and lifetimes less than 25 min demonstrate that the vertical mass flux evolves in a well-behaved manner with clear growth and decay phases. Significant net lateral entrainment or detrainment is diagnosed, which complements the top entrainment that has been inferred from conserved thermodynamic variable techniques. Net entrainment dominates the growth stage, whereas net detrainment is most often seen during the decay stage of the cloud. An approximate entrainment rate averaged over cloud life is 1 km−1. The continuous nature of the updrafts demonstrates that these small clouds are best described as a single shedding thermal, not a series of bubbles ascending in the wake of each preceding bubble.

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