Characteristics of Cumulus Band Clouds off the Coast of Hawaii

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
  • | 2 National Center for Atmospheric Research, Boulder, Colorado
  • | 3 Geophysics Program, University of Washington, Seattle, Washington
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Abstract

We have analyzed aircraft observations from seventeen cumulus cells within cloud bands observed off the east coast of Hawaii during the Joint Hawaii Warm Rain Project (JHWRP) of 1985.

Low level convergence generated by the encounter of the trade winds and the island determines the location of the initial convection. However, the upward momentum below cloud base seems to be less important in the subsequent evolution of the bands than buoyancy associated with latent heat release.

Entrainment into the clouds occurs at all levels, and almost all cloudy parcels below the trade inversion are moving upwards. While evaporative cooling does not seem to enhance entrainment below the inversion, it does play a role in the descent of cloud top air to the bottom of the inversion. Despite the existence of undiluted cores, the average thermodynamic characteristics of the clouds below the inversion appear well described by a very simple, constant lateral entrainment rate parcel model. We have used in-cloud and near environment vertical fluxes of mass, water, heat, and horizontal momentum to estimate the band cloud impacts on vertical profiles of these tracers in the cloud and above cloud layers.

Abstract

We have analyzed aircraft observations from seventeen cumulus cells within cloud bands observed off the east coast of Hawaii during the Joint Hawaii Warm Rain Project (JHWRP) of 1985.

Low level convergence generated by the encounter of the trade winds and the island determines the location of the initial convection. However, the upward momentum below cloud base seems to be less important in the subsequent evolution of the bands than buoyancy associated with latent heat release.

Entrainment into the clouds occurs at all levels, and almost all cloudy parcels below the trade inversion are moving upwards. While evaporative cooling does not seem to enhance entrainment below the inversion, it does play a role in the descent of cloud top air to the bottom of the inversion. Despite the existence of undiluted cores, the average thermodynamic characteristics of the clouds below the inversion appear well described by a very simple, constant lateral entrainment rate parcel model. We have used in-cloud and near environment vertical fluxes of mass, water, heat, and horizontal momentum to estimate the band cloud impacts on vertical profiles of these tracers in the cloud and above cloud layers.

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