Mesoscale Gradients in Soil Moisture over South America Lead to Enhanced Convection

This work presents the first observationally-based study over subtropical South America linking the spatial location of convection and drier soil patches of the order of tens of kilometers, as well as observational evidence of the control of background flow on the sign of SM-PPT feedbacks at convective scales. Using satellite data from multiple infrared and microwave radiometers, we track nascent, daytime convective clouds over subtropical South America and quantify the underlying, antecedent (morning), SM heterogeneity. We find that convection initiates preferentially on the dry side of strong dry-wet SM boundaries that are associated with spatially drier and warmer patches of tens of kilometers scale consistent with findings in other parts of the world. This preference maximizes during weak background low-level wind, high convective available potential energy, low convective inhibition and low vegetation density when analyzing surface gradients of 30 km length scale. On the other hand, surface gradients of 100 km length scale are significantly associated with afternoon convection during convectively unfavorable synoptic conditions and strong background flow, unlike previous studies. The location of the precipitation maxima following CI onset is most sensitive to the lower tropospheric background flow at the time of CI. The wind profile during weak background flow does not support propagation of convective features away from the dry regions and rainfall accumulates over the dry patch. Convection during strong background flow leads to greater rainfall hundreds of kilometers away from the CI location.