Impacts of Hydrometeor Drift on Orographic Precipitation: Two Case Studies of Landfalling Atmospheric Rivers in British Columbia, Canada

Abstract

Two severe winter storms in 2016 and 2017 caused by landfalling atmospheric rivers over British Columbia (BC) are investigated in this study. Our main concern is the impact of hydrometeor drift on the orographic precipitation. It is shown that the dominant contribution to the windward orographic precipitation was from the horizontal moisture convergence. The precipitation distributions across southern BC were also influenced by the convergence/divergence of condensed water due to the wind-driven effect on hydrometeors. Observed hourly and daily precipitation amounts are used to verify the performances of three Canadian numerical weather prediction systems. Our results indicate that these operational systems were capable of predicting the general features of orographic precipitation in BC. However, the two coarse-resolution systems used a diagnostic precipitation scheme that does not fully simulate the hydrometeor drift process. The High-Resolution Deterministic Prediction System (HRDPS) with a prognostic precipitation scheme was substantially more accurate and skillful in predicting the upwind precipitation as well as the spillover of precipitation on the leeward slopes for these two storms. There was evidence suggesting that the spillover effect was overpredicted by the HRDPS due to a systematic bias originating in the model microphysics. This problem has been improved in the current HRDPS with a new microphysics scheme. Based on our atmospheric water balance analysis, we also proposed two postprocessing schemes that could be applied to improve the quantitative precipitation forecasts of the diagnostic precipitation schemes.