The impacts of atmospheric river flow through the Carquinez Strait and blocked flow along the Sierra Nevada on precipitation characteristics observed during HMT and CalWater

NOAA's Hydrometeorology Testbed (hmt.noaa.gov) conducts research on the meteorological and microphysical processes contributing to orographically enhanced precipitation in the coastal and inland mountain ranges of California, Oregon, and Washington. A better understanding of these processes eventually should lead to improvements in quantitative precipitation estimation and forecasting. Some of HMT's precipitation research has been focused on a shallow rainfall process driven by collision-coalescence that often is undetected by the National Weather Service's operational scanning radar network (NEXRAD) but that can produce rain rates that are capable of creating floods. Originally it was believed that this shallow rainfall process would occur more prevalently over California's coastal ranges than over the Sierra Nevada, since the higher mountains of the Sierra Nevada would force deeper atmospheric ascent and produce deeper precipitating cloud systems that would extend well above the melting level. This notion was disproved when it was recently discovered that a site in the Sierra Nevada had as large of a contribution to seasonal rainfall from this shallow rainfall process as a site in the coast range. This presentation examines this apparent paradox using vertically pointing precipitation profiler (S-PROF) observations collected during the HMT-West and CalWater field campaigns. We show that one S-PROF observing site in the northern Sierra Nevada observes more of this shallow rainfall process than an S-RPOF observing site further south because it is downstream from the gap in coastal terrain created by the Carquinez Strait. This gap allows flow in a landfalling atmospheric river (AR) to be unperturbed by an upstream barrier that would otherwise result in at least partial rain out of the moisture in the AR. In addition, the northward-directed blocked flow induced by the Sierra Nevada (i.e., the Sierra Barrier Jet) during storm conditions (irrespective of whether an AR is present) preferentially allows minimally modified low-level Pacific moisture to reach the northern S-PROF observing site from the Carquinez Strait, thus resulting in comparatively frequent occurrence of the shallow rainfall process. A case study is presented to highlight these meteorological processes and to examine the air mass arriving at each S-PROF observing site with respect to the presence and concentration of ambient aerosols, using collocated air chemistry data collected during CalWater. The aerosols may provide further insight into the origin of the air mass and whether it has been processed by rain out.