A Climatological Parameterization for Cumulus Clouds

A. M. Blyth Physics Department, NMIMT, Socorro, New Mexico

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J. Latham Physics Department, UMIST, Manchester, United Kingdom

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Abstract

A research airplane was used to study the microphysical characteristics of ice-free, nonprecipitating summertime cumulus clouds in Montana. Each cloud was penetrated at a multiplicity of levels encompassing, in general, a large fraction of the cloud depth. Similar studies covering a more limited altitude range were made in New Mexico.

The clouds were substantially diluted by entrainment of environmental air, which produced great variability—at all levels and on all scales of measurement—in the liquid water content, L, and droplet number concentration, N.

The effective radius, reff, at any particular level was found to be essentially independent of L or N. Consideration of this result leads to the prediction that the parameter A = reff/rad ≈ 1 everywhere within these clouds, where rad = (¾πρw) (Lad/N), Nad and Lad are the “adiabatic” values of N and L, and ρw is the density of water.

Analysis of the airborne data (35 cloud penetrations) for the Montana cumuli reveals that A = 0.83 ± 0.07, while for the New Mexico study (25 penetrations) A = 0.93 ± 0.05. Thus, the foregoing prediction is confirmed to a reasonable degree of accuracy. Model calculations for both Montana and Hawaii cumulus consistently yield values of A close to 1.0.

It is considered that the parameter A should be useful in climate modeling.

Abstract

A research airplane was used to study the microphysical characteristics of ice-free, nonprecipitating summertime cumulus clouds in Montana. Each cloud was penetrated at a multiplicity of levels encompassing, in general, a large fraction of the cloud depth. Similar studies covering a more limited altitude range were made in New Mexico.

The clouds were substantially diluted by entrainment of environmental air, which produced great variability—at all levels and on all scales of measurement—in the liquid water content, L, and droplet number concentration, N.

The effective radius, reff, at any particular level was found to be essentially independent of L or N. Consideration of this result leads to the prediction that the parameter A = reff/rad ≈ 1 everywhere within these clouds, where rad = (¾πρw) (Lad/N), Nad and Lad are the “adiabatic” values of N and L, and ρw is the density of water.

Analysis of the airborne data (35 cloud penetrations) for the Montana cumuli reveals that A = 0.83 ± 0.07, while for the New Mexico study (25 penetrations) A = 0.93 ± 0.05. Thus, the foregoing prediction is confirmed to a reasonable degree of accuracy. Model calculations for both Montana and Hawaii cumulus consistently yield values of A close to 1.0.

It is considered that the parameter A should be useful in climate modeling.

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