Terrain-blocked airflow and orographic precipitation along the coast of northern California

Several past studies have addressed the role of terrain-blocked airflow on orographic precipitation (OP) over relatively large-scale mountain ranges, such as the Cascade Mountains of Oregon, the Sierra Nevada of California, and the European Alps, to name a few. In a broad sense, terrain-blocked flow acts as a virtual barrier, lifting an incoming airflow and initiating OP upstream of the orographic barrier's windward slope, thus complicating classical mechanisms like upslope ascent and seeder-feeder.

An accurate understanding of OP forcing is needed where complex terrain is directly exposed to strong upslope winds carrying large amounts of water vapor because the hydrological consequences can be devastating for nearby populated areas. For example, past studies have shown large hydrological impacts from atmospheric rivers (ARs) impacting the mountainous west coast of the United States. Similarly, previous studies performed along the coast of northern California have revealed modulation of OP on the coastal range's windward slope due to terrain-blocked airflows below 1 km MSL. Nevertheless, while previous studies exploring OP and terrain-blocked airflows in the relatively small-scale coastal range offer unique insights, they rely primarily on one-dimensional profile information or suffer from a lack of low-level coverage (e.g. observations below 1 km MSL). As a result, they are not able to provide three-dimensional kinematic context for the ARs, the terrain-blocked flow offshore, and their interaction near surface.

In the present study we address these limitations by documenting the kinematic and precipitation structure of an orographic precipitation event along and upstream of the relatively small-scale terrain (~1 km MSL) of coastal northern California. Some of the questions we explore are: do terrain-blocked flows along the relatively small-scale coastal range manifest similarly to the larger-scale mountains? If terrain-blocked flow is able to force or modify precipitation along and offshore of the northern California coast, what OP mechanisms are in operation? What change if any can be expected in precipitation upstream of the coastal range?

The primary observing asset employed in this investigation is a ground-based scanning X-band Doppler radar that provides three-dimensional airflow and precipitation structure information along and up to 50 km offshore of the coastline and from near sea surface up to 4 km MSL. Additional observational context is provided by a 915 MHz wind profiler, a radio acoustic sounding system (RASS), surface observations, rawinsondes, and a GPS receiver for retrieval of integrated water vapor. Major results from our study include the three-dimensional documentation of a terrain-blocked flow along the coast and offshore, identification of a terrain-blocked interface moving toward the coast, and evidence suggesting a seeder-feeder mechanism associated with orographic precipitation forcing near the coast and offshore.