Radiative and Convective Driving of Tropical High Clouds

Abstract

Using satellite cloud data from the Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) and collocated precipitation rates from the Advanced Microwave Scanning Radiometer (AMSR), it is shown that rain rate is closely related to the amount of very thick high cloud, which is a better proxy for precipitation than outgoing longwave radiation (OLR). It is also shown that thin high cloud, which has a positive net radiative effect on the top-of-atmosphere (TOA) energy balance, is nearly twice as abundant in the west Pacific compared to the east Pacific. For a given rain rate, anvil cloud is also more abundant in the west Pacific. The ensemble of all high clouds in the east Pacific induces considerably more TOA radiative cooling compared to the west Pacific, primarily because of more high, thin cloud in the west Pacific. High clouds are also systematically colder in the west Pacific by about 5 K. The authors examine whether the anvil cloud temperature is better predicted by low-level equivalent potential temperature (ΘE), or by the peak in upper-level convergence associated with radiative cooling in clear skies. The temperature in the upper troposphere where ΘE is the same as that at the lifting condensation level (LCL) seems to influence the temperatures of the coldest, thickest clouds, but has no simple relation to anvil cloud. It is shown instead that a linear relationship exists between the median anvil cloud-top temperature and the temperature at the peak in clear-sky convergence. The radiatively driven clear-sky convergence profiles are thus consistent with the warmer anvil clouds in the EP versus the WP.