Daytime Cycle of Low-Level Clouds and the Tropical Convective Boundary Layer in Southwestern Amazonia

Abstract

During the wet season in the southwestern Amazon region, daytime water transport out of the atmospheric mixed layer into the deeper atmosphere is shown to depend upon cloud amounts and types and synoptic-scale velocity fields. Interactions among clouds, convective conditions, and subcloud-layer properties were estimated for two dominant flow regimes observed during the 1999 Tropical Rainfall Measuring Mission component of the Brazilian Large-Scale Biosphere–Atmosphere (TRMM-LBA) field campaign. During daytime the cloud and subcloud layers were coupled by radiative, convective, and precipitation processes. The properties of cloud and subcloud layers varied according to the different convective influences of easterly versus westerly lower-tropospheric flows. The most pronounced flow-regime effects on composite cloud cycles occurred under persistent lower-tropospheric flows, which produced strong convective cloud growth with a near absence of low-level stratiform clouds, minimal cumulative attenuation of incoming solar irradiance (25%), rapid daytime mixed-layer growth ( 100 mh 1 ), and boundary layer drying (0.22 g kg 1 h 1 ), high convective velocities (1.5 m s 1 ), high surface buoyancy flux ( 200 Wm 2 ), and high latent heat flux (600 W m 2 ) into cloud layer. In contrast, persistent westerly flows were less convective, showing a strong morning presence of low-level stratiform genera (0.9 cloud amount), greater cumulative attenuation of incoming solar irradiance (47%), slower mixed-layer growth ( 50 mh 1 ) with a slight tendency for mixed-layer moistening, and a delayed peak in the low-level cumuliform cloud cycle (2000 versus 1700 UTC). The results reported in this article indicate that numerical models need to account for cloud amounts and types when estimating water vapor transport to the cloud layer.