The Nature of the Meteorological Fluctuations in Clouds

Bernice Ackerman The University of Chicago, Ill.

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Abstract

As in-cloud measurements have accumulated it has become increasingly apparent that clouds are markedly non-uniform in structure. This is demonstrated by sizable and rapid variations in the horizontal for almost all meteorological variables. A systematic study was made of the fluctuations in measurements obtained in hurricane clouds to establish whether they could be ascribed to a random process or whether they exhibited some spatial order.

Variance spectrum analysis of continuous measurements of temperature, liquid-water content and air-speed for 49 cloud areas indicated that, in most cases, the variations were not random and that, in addition to the cloud scale, there were non-random components of smaller size. With considerable consistency, particularly in turbulent convection, the spectra indicated the existence of two frequency “domains” within which one or more dominant modes of activity occurred. These domains extended a) from 1.8–4 cycle mile−1 with peaks most frequent around 2.5 cycle mile−1, and b) from 4.5–9 cycle mile−1 with peaks occurring most often between 5 and 6 cycle mile−1.

Abstract

As in-cloud measurements have accumulated it has become increasingly apparent that clouds are markedly non-uniform in structure. This is demonstrated by sizable and rapid variations in the horizontal for almost all meteorological variables. A systematic study was made of the fluctuations in measurements obtained in hurricane clouds to establish whether they could be ascribed to a random process or whether they exhibited some spatial order.

Variance spectrum analysis of continuous measurements of temperature, liquid-water content and air-speed for 49 cloud areas indicated that, in most cases, the variations were not random and that, in addition to the cloud scale, there were non-random components of smaller size. With considerable consistency, particularly in turbulent convection, the spectra indicated the existence of two frequency “domains” within which one or more dominant modes of activity occurred. These domains extended a) from 1.8–4 cycle mile−1 with peaks most frequent around 2.5 cycle mile−1, and b) from 4.5–9 cycle mile−1 with peaks occurring most often between 5 and 6 cycle mile−1.

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