Observation of the Diurnal Cycles of Shallow, Congestus, and Deep Convective Days in the Amazonian Wet Season

Abstract

Studying convection in the Amazon region is important for several reasons: its water budget contributes significantly to the global hydrological cycle on both seasonal and annual time scales, the intense convection affects the global tropical circulation, and the expansive deep cloud cover impacts the global radiation budget. In this context, one scientific challenge is to understand and correctly model the shallow-to-deep (STD) convective transition in this region. The current climate models fail to reproduce the diurnal cycle of convection and precipitation in the Amazon, showing a precipitation peak almost in phase with the surface fluxes, about 2-4 hours earlier than the observational time. These deficiencies are associated with the model difficulties to simulate a smooth STD transition in tropical continental regions.Here we evaluated the mechanisms responsible for the STD transition through observations in the Amazon rainforest, focusing on isolated convection formed as a response to the diurnal cycle during the wet season (October to May). For this purpose, we used data collected during the GoAmazon (2014-2015) experiment, including datasets from the W-band ARM cloud radar, radar wind profiler, micropulse lidar, ceilometer, band-S radar, and balloon-borne sounding system. We classified each day into either a shallow, congestus and deep convective regimes, and computed the mean diurnal cycle for cloud, atmospheric, surface, and boundary layer properties. Here we analyze the differences between the convective regimes as an indicator of which variables may play an important role on the STD transition.Our results indicate that deep convective days are slightly moistier than congestus days and with more significant differences only with respect to shallow cumulus days, especially in the low and middle troposphere up to 5 km. The STD transition is usually preceded by shallow precipitating clouds, which modify the convective environment above the boundary layer, favoring the vertical cloud development later in the afternoon. By analyzing the water vapor profile of the shallow cumulus days during the wet season and the deep days during the dry season, we noted that the shallow days is usually drier only below 3.5 km, indicating that the availability of free tropospheric moisture in the wet season does not guarantee that the STD transition will take place. By contrasting the diurnal (10-18 LT) precipitation with the column water vapor (CWV), lifting condensation level (LCL), level of free convection (LFC) and vertical wind shear at 14 LT, we observed that precipitation is still associated with CWV, where the maximum accumulation occurs for CWV about 5.5-6.0 cm. Precipitation is also associated with lower values of LCL, because of the moistier boundary layer, and much lower values of LFC (< 1.5 km), from the moistening of the upper levels by the shallow cumulus. We found that precipitation occurs in the afternoon, after the LFC reaches the level of the LCL, the moment of maximum instability. Regarding the vertical wind shear, we found a random behavior between the accumulated precipitation and shear intensity, indicating that it probably does not play an important role to the STD transition during the wet season.