Analysis of an upper-level inverted trough during the 2004 North American Monsoon Experiment

An upper-level inverted trough (IV) is a subtropical upper-tropospheric low that progresses westward over the North American Monsoon (NAM) region on the south side of the subtropical ridge. While IVs are important contributors to the variability of rainfall and convection during the NAM, little attention has been given to the dynamics of these systems owing to the sparse observational network over the NAM region. High temporal and spatial observations taken during the 2004 North American Monsoon Experiment (NAME) are utilized to analyze a significant IV (IV4) that passed over northwestern Mexico from 10-13 July 2004. The Colorado State University gridded dataset is the primary data source used in the study.

Results show that the vertical temperature structure of IV4 is characterized by a warm anomaly around 100 hPa and a cold anomaly that extends from 200 hPa to the surface. The strongest circulation of IV4 is in the upper-levels around 200 hPa. The quasi-geostrophic (QG) vertical motion pattern indicates that IV4 forces weak subsidence (weak rising motion) to the west (east) of the upper-level low center. In particular, the Laplacian of thermal advection overwhelms the vorticity advection forcing term in the QG omega equation. There is a midlevel cyclonic circulation associated with IV4, with northeasterly (southeasterly) midlevel flow to the west (east) of the low. Analysis of individual synoptic times reveals that significant mesoscale convective system activity along the Sierra Madre Occidental (SMO) foothills is collocated with regions of anomalous northeasterly midlevel flow and increased northeasterly shear on the western (leading) flank of IV4. It is proposed that IV4 induces northeasterly midlevel flow on its leading flank, thereby creating an environment that is favorable for convective storms to grow upscale as they move off the high terrain of the SMO.