Estimates of Radiative Divergence during Phase III of the GARP Atlantic Tropical Experiment: Part II. Analysis of Phase III Results

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.