A multi-season statistical analysis of orographic rainfall in California's coastal mountains using a GPS sensor, 915-MHz wind profiler, and rain gauges

The pre-cold-frontal low-level jet within landfalling extratropical cyclones approaching the West Coast of the United States resides at ~1 km MSL and represents the lower-tropospheric component of a deeper corridor of concentrated water vapor transport in the cyclone warm sector. These corridors are referred to as atmospheric rivers because they tend to be quite narrow relative to their length scale and are responsible for almost all of the meridional water vapor transport at midlatitudes. Most of the water vapor transport within these rivers occur within the lowest 2.5 km of the atmosphere. Atmospheric rivers play a critical role in transporting water vapor inland from the eastern Pacific Ocean, resulting in significant orographically enhanced precipitation that replenish reservoirs across the semi-arid West but can also generate devastating flooding and debris flows. Our study will take advantage of four winters of data collection from a 915-MHz wind profiler, a GPS integrated water vapor (IWV) sensor, and a pair of rain gauges in coastal northern California to document the impacts of landfalling storms on rainfall and orographic rainfall enhancement, including within atmospheric rivers. Specifically, we will explore the statistical relationship between GPS IWV measurements at the coast and orographic rainfall characteristics in the downstream coastal mountains. By combining the GPS IWV measurements with collocated wind profiler observations, we will also investigate orographic linkages between vertically integrated moisture flux and rainfall behavior. Initial findings show a direct relationship between IWV and rain intensity, and between landfalling water vapor flux directed orthogonal to the coastal mountain ranges and rain intensity. In addition, the greatest values of IWV occur during southerly-component flow at ~1 km MSL (i.e., the controlling level for generating orographic precipitation locally), which is a flow direction generally associated with pre-cold-frontal conditions where atmospheric rivers typically reside. Composite wind profiles composed of measurements taken during periods of large IWV possess low-level jet attributes, whereas this is not the case for composite wind profiles composed of measurements taken during drier conditions. This result lends further support to the notion that periods of large IWV, southerly flow, and enhanced rainrate are tied to atmospheric river conditions.