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The Southwestern Monsoon

Streamlines and Fronts

Val Mitchell traced the flood of air across the complex landscape of western U.S. from “resultant mean winds” calculated from the observed winds. The x and y components of the observed winds are calculated and the average x and the average y is calculated. From these averages, the climatological average winds or the resultant wind speed and direction is obtained. The resultant mean winds at each station are then mapped and the streamlines analyzed. Streamlines are parallel to the local wind at each station. In the eastern U.S., we are done and the map products used. In the west decisions about the resultant wind are at times problematic due the complex terrain.

Additional information was needed. At each of the station locations Mitchell calculated potential equivalent temperatures or the temperature of the air would be if taken down to the surface (1000 mb) and the latent energy of water vapor converted to sensible heat. With this thermodynamic state variable, frontal air mass boundaries can be determined and mapped and resolve difficult streamline trajectories decisions.

Mitchell map November - February November, December, and January

This was the core of Val L. Mitchell’s dissertation in 1974. Multiple sclerosis eventually resulted in his retirement in 1982. His dissertation can be found in University Microfilms or a copy dissertation may be found at the University of Wisconsin Library. The 12-panel, mini-atlas of Val’s work is displayed here.

Streamlines are represented by solid lines with an arrowhead indicating the direction of air flows. Dashed lines show the boundaries between air masses of different origin, that is frontal boundaries.

November, December, and January – An anticyclonic gyre is centered over southern Navada. Mitchell referred to the mass of air leaving the gyre as basin air.

February, March, April, May, and June air enters the Southwest from the Pacific Ocean, flowing around the southern end of the Sierra Nevada mountains and Baja California and floods the Great Basin with air from the Pacific.

Beginning in June and lasting through October, maritime tropical air rises from the Gulf of Mexico. The boundary between the Pacific air from the west and tropical air from the east Mitchell called the west Texas dew-point front. There is little temperature difference across this frontal boundary. However, the air from the Gulf of Mexico has a higher vapor pressure. The average position west Texas dew-point front migrates westward from Texas to Arizona as the summer progresses.

Other aspects of Mitchell’s maps are left to the beholder.



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