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G. L. Potter
,
R. D. Cess
,
P. Minnis
,
E. F. Harrison
, and
V. Ramanathan

Abstract

This study addresses two aspects of the planetary albedo's diurnal cycle, the first of which refers to directional models of the planetary albodo. It is found that even for clear regions there appear to be deficiencies in our knowledge of how to model this quantity. Over land surfaces, for example, Nimbus-7 data for the directional planetary albedo compare best with model calculations for which a Lambertian surface is assumed, despite ample evidence that the albedo of land surfaces is dependent upon solar zenith angle. Similarly, over ocean surfaces both GOES and Nimbus-7 data produce a weaker dependence of the planetary albedo upon solar zenith angle than would be suggested by model calculations.

The second aspect of the study concerns a comparison of the diurnal amplitude factor, defined as the ratio of the diurnally averaged planetary albedo to that at noon, between two general circulation models (GCMs) and measurements made from a geostationary satellite (GOES). While these comparisons indicate reasonable consistency between the GCMs and the satellite measurements, this is due in part to compensating differences, such as an underestimate in cloud amount by a GCM being compensated for by a corresponding underestimate of the diurnal amplitude factor for overcast regions. The comparisons further underscore difficulties associated with converting local-time albedo measurements, as made from sun-synchronous satellites, to diurnally averaged albedos.

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Patrick Minnis
,
J. Kirk Ayers
,
Michele L. Nordeen
, and
Steven P. Weaver

Abstract

Contrails have the potential for affecting climate because they impact the radiation budget and the vertical distribution of moisture. Estimating the effect requires additional knowledge about the temporal and spatial variations of contrails. The mean hourly, monthly, and annual frequencies of daytime contrail occurrence are estimated using 2 yr of observations from surface observers at military installations scattered over the continental United States. During both years, persistent contrails are most prevalent in the winter and early spring and are seen least often during the summer. They co-occur with cirrus clouds 85% of the time. The annual mean persistent contrail frequencies in unobscured skies dropped from 0.152 during 1993–94 to 0.124 in 1998–99 despite a rise in air traffic. Mean hourly contrail frequencies reflect the pattern of commercial air traffic, with a rapid increase from sunrise to midmorning followed by a very gradual decrease during the remaining daylight hours. Although highly correlated with air traffic fuel use, contrail occurrence is governed by meteorological conditions. It is negatively and positively correlated with the monthly mean 300-hPa temperature and 300-hPa relative humidity, respectively, from the National Centers for Environmental Prediction (NCEP) reanalyses. A simple empirical model employing the fuel use and the monthly mean 300-hPa temperatures and relative humidities yields a reasonable representation of the seasonal variation in contrail frequency. The interannual drop in contrail frequency coincides with a decrease in mean 300-hPa relative humidities from 45.8% during the first period to 38.2% in 1998–99, one of the driest periods in the NCEP record.

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