Dry-to-Wet Soil Gradients Enhance Convection and Rainfall over Subtropical South America:
A Land-Atmosphere Interactions Perspective
Soil moisture-precipitation (SM-PPT) feedbacks at the mesoscale represent a major challenge for numerical weather prediction, especially for the prediction of storms over transition zones between dry and wet climatic regions where soil moisture variations account for the bulk of the surface variability. How does surface heterogeneity, specifically mesoscale gradients in SM, affect convective initiation (CI) and PPT over subtropical South America? Using satellite data from multiple infrared and microwave radiometers, we track nascent, daytime convective clouds over subtropical South America (12 S-40 S, 65 W-40 S) and quantify the underlying, antecedent (morning), SM heterogeneity. We find that convection initiates preferentially on the dry side of strong dry-wet SM boundaries with spatial scales of the order of tens of kilometers. Over surface gradients of 30 km length scale, CI maximizes during weak background low-level wind (<2.5m/s), high convective available potential energy (>1500J/kg), low convective inhibition (<250J/kg) and low vegetation density (enhanced vegetation index<0.4). Surface gradients of 100 km length scale are associated with afternoon convection during convectively unfavorable synoptic conditions and strong background flow. The location of the precipitation maxima following CI onset is most sensitive to the low-level background flow at the time of CI. The low-level wind during weak background flow does not support propagation of convective features away from the dry regions leading to a negative SM-PPT feedback whereas strong background flow leads to storm motion and rainfall hundreds of kilometers away from the CI location, thus, confounding the sign of SM-PPT feedbacks.
Analysis of convection-permitting climate simulations using the Weather Research and Forecasting (WRF) model performed by the South America Affinity Group (NCAR) over 2001-2020 at 4-km horizontal resolution provides strong support that explicitly resolving convection captures the preferred location of daytime CI close to strong dry-wet morning SM boundaries. Wavelet scale decomposition of SM anomalies underlying CI locations indicates a spectrum of length scales between 16-150 km that can provide the necessary convective forcing depending on the environmental conditions.
This work presents the first observational evidence that convective initiation over subtropical South America is associated with dry soil patches on the order of tens of kilometers. We highlight that the sign of SM-PPT feedbacks at the mesoscale is dependent on the strength of the background flow. High-resolution numerical weather prediction models need to be further examined for accurately capturing the effect of SM heterogeneity in the initiation and the diurnal cycle of convection over semi-arid regions such as subtropical South America.
