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ESRL/PSD Seminar Series

PSD Flash Seminars: Moisture pathways associated with Heavy Winter Precipitation in the Western US

Michael Alexander
NOAA/ESRL PSD Climate Analysis Branch


It is not obvious how the large volume of water necessary to sustain intense precipitation events in the intermountain west reach their destination given the distance from the moisture source in the Pacific and the complex topography that impedes the flow of moisture to that region. Since flow over mountain causes air to cool and thus hold less moisture, air parcels may take unique pathways and/or have multiple moisture sources, to retain enough water to have intense precipitation events in states such as Arizona, Colorado, Idaho and Utah. In general, it would be useful to both scientists and water mangers, to better understand the synoptic and climatic processes that influence heavy precipitation events in the US intermountain west.

We investigate extreme precipitation events during winter and their relation to atmospheric rivers, long narrow bands of enhanced water vapor transport, in the US intermountain west (taken here to be between the Sierra Nevada/Cascade Mountains and the Continental Divide). We employ air-parcel trajectory analysis, empirical orthogonal function (EOFs) of integrated water vapor transport (IVT) and time-averaged climate diagnostics to determine the moisture pathways and the broader set of processes that result in these intense precipitation events. These analyses will be performed using Climate Forecast System Reanalysis (CFSR) from NCEP, which has a resolution of ~40 km.

To identify the frequency at which the trajectories pass through certain regions, count maps have been generated, where the count indicates the number of times a trajectory passed through a given CFSR grid square. The greatest number of trajectories traverse northern California, through the Burney Gap, just to the north of the highest portion of the Sierra’s. Additional pathways identified by both the EOF and trajectory analyses includes a southern route over southern Baja California and into Arizona, Utah and Southern Colorado and a northern route through the Columbia River valley and portions of the Cascades, across Northern Washington, Idaho and Montana.

Wednesday, Apr 30
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