Scanning Doppler radar observations of a coastal orographic precipitation event in northern California during PACJET

A significant fraction of the world's population lives in or near coastal zones. These regions often contain complex topography, such as along the west coast of North America, where precipitation can be intense and lead to severe flooding. Thus, improving fundamental understanding of the factors controlling orographic precipitation development in these areas is essential for producing better hydrologic forecasts.

Past studies of coastal orographic precipitation have documented several important structures and relationships. For example, there is a strong correlation between horizontal water vapor flux observed as atmospheric rivers (ARs) impacting the coast and rain intensity over the adjacent mountains. This correlation is modulated by the presence of terrain-parallel blocked flow associated with stable air advected from the continental interior through gaps in the coastal terrain. In unblocked conditions, rain rates over the mountains are about double those along the coast. However, in blocked conditions, rain rates in the mountains and along the coast are about the same.

While these studies offered unique insights, they relied primarily on one-dimensional profile information. As a result, they could not provide three-dimensional kinematic context for the ARs, blocked flows and their interaction. This study addresses these limitations by documenting the kinematic and thermodynamic structure of an orographic precipitation event along the coast of northern California during 16-18 February 2004. The primary observing asset employed in this investigation is a scanning X-band Doppler radar that provides three-dimensional airflow and precipitation structure information along and up to 50 km offshore of the coastline. Additional observational context is provided by a 915 MHz wind profiler, nine rawinsondes, and a GPS receiver for retrieval of integrated water vapor. The analysis of this storm was divided into two distinct episodes of precipitation: episode1: 16 Feb (06 to 00 UTC) and episode 2: 17 Feb (06 to 00 UTC). Major findings from our study include:

-During episode 1, blocked flow of ~0.6 km depth initially extended offshore about 30 km, but gradually retreated toward the coast over a ~5 h period. During episode 2, blocked flow was not evident.

-Blocked flow is characterized by an area of relatively low standard deviation in the radial velocity field, whereas the retreatment area shows larger values related to changes in wind speed and direction.

-The maximum horizontal gradient of reflectivity is collocated with the offshore edge of the blocked flow.

-Low-level static stability along the coast was stronger during episode 1 compared to episode 2.