Observed Soil Moisture Impact on Strong Convection over Mountainous Tibetan Plateau

Abstract:

The Tibetan Plateau (TP) is the highest and most extensive plateau in the world, profoundly affecting climate and weather in the region. Due to its average elevation of more than 4000 m, it provides strong thermal and dynamical forcing in the mid-troposphere during summer, fostering the frequent development of intense storms. Mesoscale convective systems (MCSs) are known to be associated with extreme rainfall events and contribute up to ~60% of rainfall over the TP and adjacent areas. In particular, MCSs that form on the TP may move off and bring heavy rain and flooding to downstream parts of China, affecting millions of people. A better understanding of the processes that impact MCS genesis over the TP could contribute to improved forecasting of these events. TP scale (~1000 km) soil moisture (SM) gradients have been shown to influence formation of MCSs over the eastern TP. The importance of smaller scale (~10 km) variability has been identified in other regions (including the Sahel and Mongolia) but has yet to be investigated for the TP. In addition, compared to studies over flat terrain, much less is known about SM-convection feedbacks above complex topography. In this study we use satellite observations of cold cloud, land surface temperature (LST) and SM to analyze the effect of mesoscale SM heterogeneity on the initiation of strong convection in the complex TP environment. We find strong convection is favored over negative (positive) LST (SM) gradients. The signal is strongest for low topographic complexity, though still significant up to a local standard deviation of 300m in elevation, accounting for ~70% of cases. In addition, strong convective initiation is only sensitive to local (~10s km) SM heterogeneity for light wind conditions, though large scale (~100s km) gradients may still be important for strong wind conditions. Our results demonstrate that, even in the presence of complex topography, local SM variability plays an important role in storm development.