Understanding the effects of complex terrain on Intermountain cyclogenesis (Formerly Paper 9.8)

The major lee cyclogenesis event of 15 April 2002 over the Intermountain region of the Western United States is examined and compared with studies of lee cyclogenesis in other parts of the world. The 15 April cyclone deepened 14 hPa over 9 h as the associated surface cold front underwent intense frontogenesis over northern Utah and Nevada. Part of the Intermountain cyclogenesis process follows traditional thinking, in that a strong upper-level system moves overhead of a baroclinic zone. However, the mountains play a crucial role in the development of Intermountain cyclones. This poster will show that cyclogenesis is initiated by lee troughing under strong cross-barrier flow and is enhanced by diabatic processes such as evaporative cooling and differential sensible heating, where dry prefrontal air enhanced by downslope flow to the lee of the Sierra Nevada increases cross-frontal baroclinicity. The most damaging aspects of Intermountain cyclogenesis are the strong winds, which in the 15 April case exceeded 30 m/s over most of Utah, causing over 4 million dollars of damage. This poster will investigate the processes important to lee cyclogenesis and present an analysis of the 15 April event.