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Extratropical Control of Monsoonal Surges in the Northern Great Basin
AuthorJoros, Andrew Nicholas
AdvisorKaplan, Michael L.
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The northwestern fringe of the North American Monsoon (NAM) circulation covering the Great Basin of the United States is characterized by highly variable intraseasonal convective precipitation activity. Although typically dry during the summer months, sporadic moisture pulses associated with transient monsoonal surges interacting with mid-latitude disturbances yield low-frequency high-impact events including lightning-ignited wildfires and flash floods. Mountainous terrain in the Northern Great Basin (NGB) is extremely vulnerable to the dangers of monsoonal convection as the initial development of convective showers typically occurs over high-altitude terrain due to differential heating between the terrain and surrounding atmosphere. We have incorporated expert knowledge from four National Weather Service Offices in the NGB along with 30 years (1980-2009) of data from the North American Regional Reanalysis (NARR) to answer the following question: What mechanisms fuel monsoonal moisture surges into the NGB? In order to better understand the nature of these surges, an objective means of identifying surge events was done using precipitable water and vertically-integrated moisture flux during the months of July-September. Preliminary results indicate that moisture surges are associated with either a progressive mid-latitude trough that infiltrates the West Coast, or a strong four-corners ridge. Trough event signals reveal statistically significant moisture flux extending from the northern Gulf of California northward into southeast Idaho with positive flux anomalies on the order of 70-100 kg/m2/s. With ridge events, we see a similar trend but with a greater northward extension of moisture flux into northern Idaho with similar anomaly values as that of trough events. Additionally, 330K isentropic potential vorticity (IPV) was plotted for various case studies in order to better understand if there exists a Rossby Wave Break signal (e.g., Postel and Hitchman 1999; Abatzoglou and Magnusdottir 2006) prior to the monsoonal outbreaks in the NGB. There is a strong possibility that these extratropical wave breaks play a significant role in adjusting the subtropical dynamics which lead to enhanced moisture surges northward. Our results also suggest large interannual variability with a significant number of these events occurring during 82/83 and 97/98 coincident with strong El Nino events. Ultimately, we would like the results of our work to be readily translatable to forecasters in order to improve the predictability of these events.