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Stephen K. Cox
and
Keith T. Griffith

Abstract

The methodology used to generate the GATE A/B-scale radiative divergence budget estimates is described. The technique consists of compositing radiative divergence profiles on the basis of synoptic observations and satellite and radar inferred cloud structure. Cloud-top distributions were generated from SMS infrared brightness data observed hourly over the entire GATE A/B array. Cloud-clear thresholds were determined using concurrent visible brightness and infrared brightness observations. Corrections to cloud-top height were made for water vapor extinction and for non-blackness of clouds. Cloud-base distributions were inferred statistically from a limited sample of concurrent radar and satellite data. Temperature and moisture distributions from 6 h synoptic analyses of rawinsonde data were used in the radiative divergence computations.

The computational algorithms used to generate the shortwave and longwave radiation divergence estimates were compared with aircraft observations; average cloud properties of upper level clouds inferred from the observations were used in the computational algorithms. The area-average radiative divergence profiles were then generated by computational algorithms.

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Stephen K. Cox
and
Keith T. Griffith

Abstract

The GATE Phase III radiative divergence profiles generally show less upper tropospheric radiative divergence and more middle level divergence than previous cilimatological estimates suggest. These differences are due primarily to the extensive middle and upper tropospheric cloudiness in the GATE area, the large mean values of total precipitable water vapor (∼5.1 cm), and the inclusion in the present study of the effects of the water vapor pressure broadened continuum.

Averages for the 6 h local time periods 0000–0600, 0600–1200, 120–1800 and 1800–2400 show all layers of the GATE Phase III B-scale atmosphere experience a net radiative loss of energy. However, actual radiative heating of some layers is evident near midday. For a convectively suppressed composite case all levels above 700 mb show heating for the 1000–1400 LST period. The total troposphere shows a net radiative gain for the same 6 h interval (0900–1500 LST). For the enhanced convection case absolute warming is generally confined to the 100–400 mb layer and the 0800–1600 LST time interval with no net heating of the entire troposphere occurring during the day. The diurnal variability of the horizontal gradients in the radiative divergence fields appears adequate to explain at least some of the diurnal variations in cloud cover and precipitation suggested by other authors.

The daytime tropospheric total radiative divergence is remarkably stable for all observed cloud-top distributions during Phase III over the A/B- and B-scale arrays. This characteristic constancy of the daytime total tropospheric divergence (TTD) values is a potentially useful tool in the inference of maritime tropical surface energy budgets from satellite data.

Average TTD values computed over various tune and space scales are examined. It is shown that for an area the size of the B-scale array the 6 day averages do not vary more than 5 W m−2 (912 mb)−1

Cross sections of the Phase III mean and the disturbed composite radiative divergence values for the A/B-scale array suggest a north–south radiative forcing caused by east–west oriented cloud bands centered around 8–9°N latitude. Coupled with the analysis of the diurnal radiative effect of clouds and adjacent clear areas, this suggests the possibility of a diurnal radiative forcing on the basic Hadley circulation.

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Keith T. Griffith
,
Stephen K. Cox
, and
Robert G. Knollenberg

Abstract

Longwave emissivities and the vertical profile of cooling rates of tropical cirrus clouds are determined using broadband hemispheric irradiance data. Additionally, a broadband mass absorption coefficient is defined and used to relate emissivity to water content. The data used were collected by the National Center for Atmospheric Research (NCAR) Sabreliner during the GARP Atlantic Tropical Experiment (GATE) in the summer of 1974.

Three case studies are analyzed showing that these tropical cirrus clouds approached an emissivity of 1.0 within a vertical distance of 1.0 km. Broadband mass absorption coefficients ranging from 0.076 to 0.096 m2 g−1 are derived. A comparison of these results with other work suggests that tropical cirrus cloud emissivities may be significantly larger than heretofore believed.

Ice water content of the clouds was deduced from data collected by a one-dimensional particle spectrometer. Analyses of the ice water content and the observed particle size distributions are presented.

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