PNNL

Abstract

Coastally Trapped Disturbances (CTDs) are shifts in wind direction from the predominant equatorward to poleward for a period of time. These CTDs occur during the warm season off the California coast and impact coastal weather conditions. This study assesses the characteristics of CTD events as observed by lidar and other offshore buoys, then evaluates the ability of modeling systems to capture the correct characteristics, leveraging model output from the High-Resolution Rapid Refresh (HRRR) operational modeling system and the NOW-23 model dataset. CTDs were analyzed for October 2020 and May through October of 2021, identifying 18 unique CTD events, confirmed by a nearby National Data Buoy Center (NDBC) buoy. The HRRR model captured most of these events, but the NOW-23 (National Offshore Wind) model output contained only 12 events. Composites of the wind, temperature and pressure perturbations pre-, during, and post-event demonstrated the diminishment in wind speed, particularly for the along-shore component. Although the NOW-23 model captured the along-shore wind component, temperature, and pressure well, the cross-shore wind component varied substantially. When turbulent kinetic energy (TKE) and wind shear was positive across all levels pre-event, the NOW-23 modeling system is less likely to capture the CTD event. In contrast, the events that were captured by the model tended to have negative wind shear aloft pre-event.