Sensitivity analysis applied to two extreme rainfall events over Kerala using TIGGE ensembles

The uncertainties associated with synoptic-scale atmospheric conditions of two catastrophic heavy rainfall events over Kerala, the southwestern coastal state of the Indian subcontinent, are quantified using Ensemble-based Sensitivity Analysis (ESA). Kerala witnessed two extreme rainfall events and floods during August 2018 (KF18) and August 2019 (KF19) causing large-scale destruction to life and properties. The synoptic features that are in common for the two events include the formation of a depression over the Bay of Bengal, a stronger low-level jet stream over the Arabian Sea, an off-shore trough, and circulations over the Western North Pacific (WNP). The present study attempts to identify the atmospheric flow features that are important to the predictability of these two heavy rainfall events using ESA. Further, the possible similarities and differences in the dynamics of the two events have been explored using an ensemble framework. Sensitivity analysis indicates that greater height fall needs to occur over the WNP to increase the KF18 precipitation, and any shift in the location of these features may affect the precipitation patterns over Kerala. Additionally, the results indicate that the circulations positioned farther east of its mean position are related to stronger precipitation over the response function region. However, the absence of sensitivity dipole suggests that the circulations over WNP have not impacted the KF19 event. The magnitude of positive sensitivity for low-level moisture-laden flow is stronger in KF19 than in KF18. The enhanced low-level flow in KF19 could be one potential reason for the development of deep convective clouds.