The Temporal Climatology, Teleconnective Associations, and Climatic Impacts of Regional-Scale Troughing in the Southwestern United States

Although not common, long wave troughs over the southwestern United States produce strong temperature and moisture gradients and often favor the development of intense surface storms. The analyses outlined in this dissertation focused on the climatological characteristics of southwestern troughs and were driven by three key objectives. These objectives were: (1) to determine the temporal frequency of these features; (2) to detail their middle tropospheric wave associations; and (3) to examine the contribution of these systems on precipitation and energy transfer processes. Data for these analyses included daily northern hemispheric 500 mb geopotential height and temperature values and daily precipitation totals for fifty-four stations throughout the central and western United States. All southwestern events during the data record were objectively identified and a series of seasonal composite 500 mb geopotential height, geostrophic wind, and twenty-four hour height change maps constructed for the period before and after the troughing period. Precipitation influences were evaluated in terms of specific precipitation density and percent of total precipitation. In addition, daily mean transient eddy sensible heat transfers were calculated for troughing periods and compared to similar values calculated for non-trough transient eddies and quasi-stationary eddies. Results of these procedures indicated that: (1) southwestern troughs occurred most frequently during spring and fall, with somewhat fewer events during winter and very few events in summer; (2) trough development followed the same developmental sequence each season and involved the formation and eastward movement of a triggering short wave over the western Pacific, amplified ridge development over the Gulf of Alaska, and downstream teleconnection; (3) these events provided the west-central United States with a large proportion of the monthly precipitation total during the data record; and (4) the southwestern trough daily mean transient eddy sensible heat flux was quite large compared to non-trough transient and quasi-stationary eddy flux.