Blocking by California's coastal mountains in winter as revealed using island and coastal boundary-layer wind profilers

Numerous studies have explored the blocking effects of coastal mountains. Several mechanisms have been described that can lead to blocking, from upstream propagating internal bores, to localized regions of gap flow, to large-scale effects associated with the Rossby radius of deformation. This study uses a unique pair of boundary layer wind profilers to document the blocking effects of coastal mountains in California that were part of the California Land-falling Jets experiment (CALJET) conducted during the winter of 1997/98. CALJET deployed numerous wind profilers along the coast that continuously measured the vertical profile of horizontal wind every few minutes (usually hourly average values are used). The lowest altitude profiler data are at 100 m above mean sea level (MSL), and then extend upward every 100 m up to 2-4 km MSL. An especially unique part of the array was sited north and west of the San Francisco Bay area in a region of 500-m-tall coastal mountains. In addition to a profiler at sea level at the coast, and one on the mountain top at 500 m elevation and 35 km inland, a third profiler was sited on the Farallon Islands, a set of very small islands roughly 60 km offshore. The islands are small enough to only slightly disrupt the ambient airflow, and thus provided an ideal location for measuring offshore conditions just south of the coastal sites. Although CALJET included research aircraft flights in the region, the profiler on the Farallon Islands malfunctioned on 22 January, just as the P-3 research aircraft became available. Because of the difficulty of reaching the site in general and the bad weather that characterized the rest of the experiment, the site remained inoperable for the remainder of the experiment. Nonetheless, two months of good measurements were made before then, while all three profiler sites were operating. This presentation will describe the time evolution and structure of blocking as revealed through analysis of a several-day-long period in early January 1998. These data, combined with surface measurements at each site, as well as at buoys in the vicinity, illustrate the onset of the blocked flow and its persistence for 3-4 days. Direct comparison of profiler winds at the island and coastal sites revealed that the blocked flow (roughly easterly) at the coast extended to about mountain top (600-800 m MSL), and that the barrier jet had a 7 m/s amplitude at 400 m altitude MSL at the coast. Evidence that gap flow played a role in this event will be presented.