Convection Embedded in an Atmospheric River: Exploring Precipitation Sensitivity to Convective Parameterizations

Atmospheric rivers (ARs) are major drivers of orographic precipitation along the mountainous west coasts and often involve embedded convection delivering extreme local bursts of rainfall associated with flash floods. Understanding the convection within ARs is critical for assessing flood risks and improving precipitation forecasts and numerical weather models. This study evaluates the sensitivity of the 2019 Valentine’s Day atmospheric river precipitation in California to Grell–Freitas and Tiedtke convective schemes using the Weather Research and Forecasting (WRF) Model. Ensemble simulations at a 9-km horizontal resolution}a scale within the gray zone where some convection is resolved but parameterization remains necessary}are compared with seven observational datasets, including ground- and satellite-based products. Over the ocean, WRF successfully captures key synoptic features and the overall distribution of total precipitation due to the dominance of stratiform precipitation. Convective precipitation, however, is simulated differently: Grell–Freitas produces it within the AR, whereas Tiedtke simulates it west of the AR. Given the challenges in diagnosing AR-embedded convection at 9 km, an alternative method based on outgoing longwave radiation and precipitation rate thresholds is tested. The alternative convective precipitation aligns with observations, occurring within the AR over the ocean, coastal mountains, and Sierra Nevada. Using the alternative convective method, the overall pattern of convective precipitation is generally consistent between the Grell–Freitas and Tiedtke schemes. This agreement contrasts with the patterns of parameterized convective precipitation. These results are relevant for global models or regional large ensembles approaching gray-zone resolution, guiding diagnostics of embedded convection in ARs.