The role of the Topographic Amplification Factor in the breakup of the nocturnal inversions in Yosemite Valley, Sierra Nevada

This paper illustrates the use of ArcGIS software for computing the Topographic Amplification Factor (TAF). The Topographic Amplification Factor, which is defined as the ratio of the area/volume of the valley to the area/volume over a plain, is a useful indication of how much the incoming energy from the valley top is amplified by the shape of the valley topography and is used in various analytical models that describe the inversion break up in the mountain valleys. Traditionally, TAF is estimated using valley cross section by assuming a uniform cross section area along the valley axis, which produces poor estimations of TAF for valleys that have complex shapes of cross sections along their axis. The ArcGIS software allows the TAF to be computed accurately and over a large area with many different terrain types with relative ease. Here, we used ArcGIS to compute TAF for the Yosemite Valley in the Sierra Nevada. TAFs were computed for both individual sections of the valley and over the entire valley area. The average TAF for the valley length was found to be 1.78, a value that is consistent with the near vertical sidewalls resembling a U-shaped valley. The TAF value is then applied to an analytical model to estimate the time required to break up temperature inversion developed at night in the Yosemite Valley. The analytical solution is validated against tethersonde measurements of inversion breakup in the Yosemite Valley during 15-16 March 1998. The inversion breakup in the Yosemite Valley is compared to the breakup in a small basin of the Peter Sinks in Utah where a series of tethersonde measurements were made during a field study.