Background
Atmospheric rivers (ARs) are narrow corridors of enhanced water vapor transport within extratropical cyclones. When they arrive in California, ARs contribute significantly to the water supply and flood generation in the State. Although focused research during the last few years has yielded quantitative linkages between ARs and both regional water supply and extreme precipitation events, questions remain regarding the modification and redistribution of water vapor and precipitation in ARs by California's coastal mountains and Sierra Nevada. Previous work indicates that all recent flooding events on the US west coast were associated with an AR but not all ARs yielded flooding. Several factors can potentially turn an AR event into a flooding event. There is limited understanding of the relative roles of atmospheric stability, barrier jets, and small scale ridges along the windward slope on watershed precipitation totals.
Frequency of precipitation ≥ 0.2 mm/hr (colors) during a heavy rainfall event on 31 Dec 2005 in Northern California overlaid by topography (black lines). A frequency value of 70% indicates that it was raining in that location for 70% of the duration of the storm. Rainfall on the western slopes of the coastal mountains is underestimated since there are no low level unobstructed radar data for those regions.
Work Plan
A key missing piece on the role of ARs in flooding events is knowledge of the detailed spatial distribution of precipitation over the windward slopes of the Sierra Nevada for each AR event and for groupings of AR events with similar environmental variables. The proposed work will utilize operational radar data from several National Weather Service WSR-88D radars in Northern California and Nevada to construct a radar echo precipitation climatology of all AR events for a 10 year period. A long-term radar echo climatology is needed since existing rain gauges provide only incomplete information on precipitation in this region, particularly over rugged mountainous terrain. We will build on methodology developed to obtain a similar precipitation climatology for the windward slope of the Cascade Mountains near Portland, OR region.
