Sensitivity of simulated convection to soil moisture in a region in central Amazon

Abstract

We study the sensitivity of convective precipitation to soil moisture in a continental region in Central Amazon based on Atmospheric General Circulation Model (AGCM) simulations and further analyze the physical processes involved, in particular those related to the diurnal cycle. The region was selected on the analysis of perpetual January simulations with the University of California at Los Angeles AGCM. It shows a uniform and differentiated behavior in the diurnal cycle of convection, which is strongly associated to surface forcing and in opposite phase to large scale ascending motion. Sensitivity to ground wetness was explored based on a control parameter (which varies between 0 and 1) that multiplies evaporation. Results show that the largest sensitivity appears for relatively dry conditions, confirming previous studies. Although evaporation decreases monotonically as the control parameter is reduced, a slight increase in moisture convergence maintains precipitation almost unchanged up to a value of 0.3 of the parameter. The daily maxima in moisture convergence (excluding the extreme case with no evaporation within the region), convective precipitation, and large scale ascending motion at 500 mb are reached in the simulation with a value of 0.4 of the control parameter. Soil and surface temperature increase with decreasing sensitivity parameter, and show retarded daily maxima and larger diurnal amplitude. The same behavior is observed in the sensible heat flux and planetary boundary layer (PBL) height. These effects combine with the variations in PBL water vapor mixing ratio to produce a gradual sensitivity in PBL top relative humidity, in particularly during the morning hours when the humidification observed in the control simulation is gradually reverted, eventually inhibiting early afternoon convection. However, sensitivity of moist convection to ground wetness can not be fully explained without also considering large scale circulation feedbacks. In the more humid cases, deep convection is responsible for mid tropospheric warming. In the drier cases, convective induced warming is insufficient and a differential large scale subsiding motion is induced in the mid troposphere. In the extreme case, with the control parameter equal to zero, the large scale circulation changes drastically with the development of a shallow direct convergent cell that explains the notorious increase in large scale precipitation.