The dependence of clouds and their radiative impacts on the large-scale vertical velocity

Abstract

Middle-tropospheric vertical velocity(o500) is used to isolate the effect of large-scale dynamics on the observed radiation budget and cloud properties in the Tropics. The ratio of shortwave to longwave cloud forcing (N=-SWCF/LWCF) is approximately 1.2 and independent of the magnitude of w5oo when it is upward over the warmer water. Increasingly negative net cloud forcing (NetCRF) with increasing upward motion is mostly related to an increasing abundance of high-thick clouds. Although a consistent dynamical effect on the annual cycle can be identified, the effect of the PDF of o 500 on long-term variations in the tropical mean radiation budget is generally small compared with observed variations.The East Pacific in 1987 and 1998 showed large reductions of N in association with an increase in the fraction of the area with upward motions, and concomitant increases in high clouds. For the West Pacific in 1998 a large increase/decrease in N/NetCRF was caused not by a change in o500, but rather by a shift of the vertical structure of vertical velocity.The structure of tropical large-scale vertical velocity from ERA-40 is compared with satellite measurements. The first two EOFs of the vertical velocity profile represent the traditional deep circulation (PC1) and a middle level divergence (PC2). Together they explain 90% of total variance and can distinguish the "top-heavy" and "bottom-heavy" vertical motions. Cloud and radiation budget quantities have coherent relationships to PC1 and PC2 at all time scales. The relative importance of PC2 is greater on short temporal scales."Top-heavy" ascent is associated with deep cloud systems. SWCF depends primarily on PC1, while NetCRF depends more on PC2. High-thin clouds are less sensitive to short-term variations of the vertical velocity. Shallow precipitation measured by TRMM-PR is associated with "bottom-heavy" upward motions. During the evolution of intense precipitating systems, upward motion profiles evolve to a more "top-heavy" shape over time, which is consistent with previous radar and sounding measurements. Associated high-thick clouds develop first and extensive anvil clouds later. These results suggest that the elevated latent heating (stratiform type) is responsible for the development of "top-heavy" upward motion profiles in the Tropics.