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David W. Reynolds, Thomas B. McKee, and Kelvin S. Danielson

Abstract

The relationship between a cumulus clouds' brightness, horizontal dimension and internal microphysical structure are investigated. Cumulus clouds located over the South Park region of Colorado are observed by the SMS-2 satellite and their brightness and size are determined. Aircraft observations were made in-cloud to obtain the drop size distributions and liquid water content (LWC) of the cloud. A Monte Carlo cloud model is used to imitate the sun-satellite-cloud geometry in an effort to understand the role of cloud size and microphysical structure in affecting cloud brightness.

Results show that for clouds of optical thickness between 20 and 60 (i.e., LWC of 0.037 gm−3 and 0.11 gm−3 for a 2 km deep cloud), information about a cloud's LWC may be obtained through monitoring cloud brightness for clouds of uniform depth and variable width. Theoretical results using this Monte Carlo method approximate very closely the relative brightness changes of clouds of the size and depth monitored by the SMS-2 satellite for these few days. Theory and observation both conclude that a cloud having a width to depth ratio of approximately 10:1 (and constant optical thickness) is nearly reaching its maximum brightness. Theory predicts that geometric factors affect cloud brightness more than microphysical changes.

It is also discussed that the previously reported work on the cloud height-cloud brightness relationship may indeed be seeing increasing brightness with increasing horizontal size changes (size being related to height) with finite small perturbations on top of the growing cloud slowing its approach to maximum brightness.

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