Vertical Mass Flux Calculations in Hawaiian Trade Cumulus Clouds from Dual-Doppler Radar

Scott A. Grinnell Geophysics Program, University of Washington, Seattle, Washington

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Christopher S. Bretherton Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Alistair M. Fraser Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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David E. Stevens Courant Institute of Mathematical Sciences, New York, New York

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Abstract

Two ground-based Doppler radars and an instrumented aircraft provided a means for computing the vertical mass flux in trade wind cumulus clouds that formed east of the island of Hawaii dining the Hawaiian Rainband Project of 1990. This study compares the mass fluxes of small isolated cells with larger groups of clouds and rainbands. Because of excellent sensitivity, the 5.5-cm wavelength radars were capable of detecting Bragg backscatter, which extended the measurements to include some precipitation-free air within and surrounding the clouds.

The shape of the vertical profile of vertical mass flux within shallow cumulus clouds and cloud groups varied considerably over the cloud's life cycle but was comparatively independent of cloud size. The early stages of convection displayed a mass flux profile that resembled those produced by buoyancy sorting and entraining plume models, but the mature and later stages were considerably more affected by precipitation-driven down-drafts and included mean downward mass fluxes. The vertical mass flux profiles predicted by a three-dimensional LES model of an isolated cumulus cloud showed the same evolutionary phases as the observations.

Abstract

Two ground-based Doppler radars and an instrumented aircraft provided a means for computing the vertical mass flux in trade wind cumulus clouds that formed east of the island of Hawaii dining the Hawaiian Rainband Project of 1990. This study compares the mass fluxes of small isolated cells with larger groups of clouds and rainbands. Because of excellent sensitivity, the 5.5-cm wavelength radars were capable of detecting Bragg backscatter, which extended the measurements to include some precipitation-free air within and surrounding the clouds.

The shape of the vertical profile of vertical mass flux within shallow cumulus clouds and cloud groups varied considerably over the cloud's life cycle but was comparatively independent of cloud size. The early stages of convection displayed a mass flux profile that resembled those produced by buoyancy sorting and entraining plume models, but the mature and later stages were considerably more affected by precipitation-driven down-drafts and included mean downward mass fluxes. The vertical mass flux profiles predicted by a three-dimensional LES model of an isolated cumulus cloud showed the same evolutionary phases as the observations.

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