ESRL/PSD Seminar Series
Winter storms over the US west coast: Precipitation characteristics and implications for prediction
Department of Marine, Earth, and Atmospheric Sciences North Carolina State University
During the winter months, extra-tropical cyclones develop over the Pacific Ocean and move across the U.S. west coast. The coastal and interior mountain ranges modify the precipitation patterns of these storms leading to enhanced precipitation. This study focuses on mesoscale observations obtained over five cool seasons, including operational radar and upper air soundings, of storms affecting the Oregon Cascades. The conditional probability of flooding is examined based upon the values of single parameters and the combination of multiple parameters. All flooding storms were associated with land-falling bands of enhanced vertically-integrated water vapor (atmospheric rivers), high 0 °C levels, strong cross-barrier flow, and long durations of precipitation. However, only of 56% of storms with these conditions produce floods near Portland, OR. Compared to non-flooding storms, flooding storms had higher precipitation persistence and intensity in Willamette Valley and higher precipitation intensity over the upper portion of the Cascade windward slope. Precipitation enhancement over the valley was related to two-layer flow where the lower, blocked layer acts to increase the effective cross-barrier width. Additionally, low-level flow convergence enhances precipitation within a concave section of the mountain barrier near the Lewis River Valley. Simulation of a flooding storm in northern California indicates that another factor contributing to flooding potential is the relative timing of the short-lived water vapor source from the atmospheric river and the peak strength of the barrier jet. For the storm studied, coincident timing of the maximum water vapor flux though the gap in the coastal mountains and into the central valley with the period when the barrier jet was well developed yielded northward along-barrier water vapor fluxes that replenished about a quarter of the water vapor lost over the coastal mountains. These results point to the importance of the representation of lower layer blocked flows and small scale terrain gaps in predicting flooding storms.
Thursday, October 28
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