WRF Hindcast Sensitivity for Delhi's 28 June 2024 Extreme Rainfall: Role of Boundary Conditions, Microphysics, and Initial Time

The extreme rainfall event of 27–28 June 2024 resulted in 228.1 mm of rain in Delhi, the heaviest in nearly eight decades, causing severe damage across the city and its surrounding regions. To understand the evolution of the spatio-temporal variability of this event and to quantify uncertainties in short-range forecasts of such urban extreme precipitation, we employ an ensemble of high-resolution, urban-aware WRF simulations. This study examines the sensitivity of WRF hindcasts to boundary condition datasets by comparing simulations forced with ERA5 and NCEP-FNL reanalyses. We also assess the model's sensitivity to microphysics parameterizations, as an accurate representation of cloud microphysical processes is crucial for forecasting extreme rainfall. Two widely used schemes, Thompson and WSM6, are evaluated. In addition to boundary conditions and microphysics schemes, we test the model performance for four different initialization times, starting from 1200 UTC on 25 June at 12-hour intervals, using both ERA5 and NCEP-FNL forcing. Preliminary results (from NCEP-FNL-driven runs) show that both microphysics schemes underestimate total rainfall. The Thompson scheme, when initialized at 0000 UTC on 26 June 2024, effectively captures the spatial structure of intense rainfall. The WSM6 scheme better reproduces the overall magnitude of the extreme rainfall but exhibits a spatial displacement bias. Among all initialization times, simulations starting at 0000 UTC on 26 June 2024 perform the best. The same experimental setup will be applied using ERA5 boundary conditions, and the outcomes will be compared against the NCEP-FNL-driven simulations to determine which boundary condition better represents the observed extreme rainfall event.