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.

Modernization of Atmospheric Physics Parameterization in Canadian NWP

“…Impact of subgrid‐scale orographic fields on precipitation during an atmospheric river event (Mo et al., ). Accumulated precipitation between 12 and 36 hr is shown (color shaded in millimeters as indicated on the color bar) for integrations initialized at 0000 UTC 16 January 2017 with the revised Regional Deterministic Prediction System physics configuration using the updated (a) and operational (b; “old”) estimates of subgrid‐scale orography.…”

“…() to generate reliable simulations of clouds and precipitation, replaced the Milbrandt and Yau () scheme in the HRDPS in late 2018. This upgrade improved the spatial distribution of precipitation guidance in mountainous areas because of the ability of the P3 scheme to represent gradual increases to particle fall speeds during riming (Mo et al., ; Morrison et al., ). The use of two or more ice categories in P3 will be considered in a future update to the physics suite, once the implementation of an optimized advection scheme is complete.…”

Column Relative Humidity and Primary Condensation Rate as Two Useful Supplements to Atmospheric River Analysis

“…This is usually the case when an AR is blocked by a large mountain range. A fraction of the condensation over the windward slope will be carried by strong winds to the leeward side of the mountain, leading to the spillover phenomenon (e.g., Mo et al., 2019). To deal with this issue, one needs to estimate the vertical distribution of the specific condensed water .…”

An anomalous atmospheric river linked to the late June 2021 western North America heatwave

“…Landfalling ARs are capable of producing heavy precipitation in mountainous areas, especially in the cold season with water vapour supplied by tropical moisture sources 8,11,23,[32][33][34][35][36][37] . ARs in the warm season should produce less orographic precipitation because the warmer atmosphere can contain more water vapour before condensation begins than can a cooler atmosphere.…”