Isidora Jankov
Cooperative Institute for Research in the Atmosphere, Colorado State University
Affiliated with NOAA/ESRL/Global Systems Division
Boulder, CO
08 September 2009, 3:30 PM
National Weather Center, Room 1313
120 David L. Boren Blvd.
University of Oklahoma
Norman, OK
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Significant precipitation events in California during the winter season are often caused by land-falling “atmospheric rivers” associated with extratropical cyclones from the Pacific Ocean. Atmospheric rivers are narrow, elongated plumes of enhanced water vapor transport over the Pacific and Atlantic oceans that extend from the tropics and subtropics to the extratropics. Large values of integrated water vapor are advected within the warm sector of extratropical cyclones immediately ahead of polar cold fronts, although the source of these vapor plumes can originate in the tropics beyond the cyclone warm sector. When an atmospheric river makes landfall on the coast of California, the northwest to southeast orientation of the Sierra Mountain chain exerts orographic forcing on the southwesterly low-level flow in the warm sector of approaching extratropical cyclones. As a result, sustained precipitation is typically enhanced and modified by the complex terrain. The importance of gaining understanding of these events via numerical simulations is amplified by the fact that land-falling atmospheric rivers represent a major water source for California’s large population.
The National Oceanic and Atmospheric Administration (NOAA) established the Hydrometeorological Testbed (HMT) to design and support a series of field experiments to better understand and forecast precipitation in the Central Valley. Researchers of two ESRL divisions (Forecast Applications Branch and Physical Sciences Division) have been collaborating on several studies involving mesoscale modeling and ensemble simulations related to the HMT project. All of these studies focused on atmospheric river events. One study’s objective was to improve Quantitative Precipitation Forecast (QPF) by estimating the impact that various microphysical schemes, Planetary Boundary Layer (PBL) schemes, and initialization methods have on cold season, primarily orographically-induced precipitation. Other research highlighted the evaluation of various microphysical schemes’ performance (e.g. Lin, WSM6, Ferrier, Thompson and Morrison) by using available observational data sets. Relevant mesoscale attributes for simulated storms were evaluated based on observations from 915-MHz wind profilers, vertically pointing S-band radars and collocated GPS water-vapor sensors, and surface meteorological instrumentation. The value of using synthetic satellite imagery to evaluate the performance of various microphysical schemes was assessed. For this purpose, synthetic GOES-10 imagery at 10.7 μm was produced. Finally, a newly developed theoretical method for evaluating possible precursors (derived diagnostic variables) and their employment in an optimization of ensemble precipitation forecasting was tested.
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