Satellite-Inferred Morning-to-Evening Cloudiness Changes

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Abstract
Outgoing infrared radiation (IR) values inferred from radiance measurements in the water vapor window (10.5–12.5 μm) taken at approximately 0900 and 2100 LT by scanning radiometers aboard the polar orbiting NOAA satellites are compared in order to determine whether a significant diurnal variation exists. Data with a resolution of 2.5° in latitude and longitude are mapped for latitudes between 50°N and 50°S for two seasons. June–August 1975 and December 1975-February 1976. On the basis of these maps it is possible to identify broad categories of geographical regions which display similar patterns of 12 h differences in outgoing IR. For several of these categories composite histograms are constructed in order to investigate the morning to evening difference in outgoing IR for ranges of values corresponding to high clouds, middle clouds and low clouds. Over land regions and coastal waters it is found that the 12 h differences are dominated by middle and high clouds, likely of convective origin. The difference patterns are consistent with widely held views concerning the modulation of convection by diurnally oscillating, thermally driven boundary-layer circulations. Over the tropical oceans the pattern of morning to evening differences in outgoing IR shows remarkable geographical consistency. There is evidence of a morning bias in high cloudiness (possibly of convective origin) and low cloudiness (stratus, stratocumulus and trade wind cumulus), and an evening bias in middle cloudiness. Because of the compensation between these three cloud regimes the net morning to evening differences in outgoing IR are rather small except over the eastern oceans where the low cloud regime predominates and tends to produce a weak but statistically significant evening maximum.

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