High-resolution seasonal simulations of Northeast Pacific atmospheric rivers and comparison to in-situ observations

Atmospheric rivers (ARs) are narrow bands of enhanced atmospheric water vapor that provide an important atmospheric linkage between the subtropics and the mid-latitudes, facilitating over 90% of the meridional water vapor flux between these two regions (Zhu and Newell 1998). ARs are a crucial ingredient in a large fraction of cool-season extreme precipitation events along the Pacific coast of North America and have been identified as useful short-term predictors of significant flood events (Ralph et al. 2006). Various numerical models are capable of capturing the general spatial characteristics of ARs, but the scarcity of observations over the Pacific Ocean and the narrowness of the water vapor plumes associated with ARs make difficult any direct evaluation of model-simulated fine-scale structure. The present study seeks to address this gap using dropsonde observations from the CALJET and PACJET experiments of 1998 and 2001, respectively, which were originally reported by Ralph and Neiman (2005). These in-situ observations spanning multiple AR events in each year are compared to high-resolution seasonal simulations using the Weather Research and Forecast model (WRF, v.3.2.1). A single domain with 4 km horizontal resolution and 30 vertical levels is used for this study, and WRF is configured using a nonhydrostatic dynamical core (NMM) and the WRF Single-Moment 5-Class Microphysics Scheme (WSM5). In general, WRF is observed to accurately reproduce the broad spatial characteristics of ARs during the study period. Smaller-scale differences between observed and modeled AR structure, with a particular focus on water vapor fluxes and stability profiles, are discussed.