Advances in Forecasting Monsoonal Upper Tropospheric Lows: Potential Vorticity Disturbances

During the North American Monsoon Experiment (NAME) in 2004, quasi-stationary or westward-moving upper tropospheric lows were recognized as a key modulator of monsoon-related convection. Pytlak, et al. (2005) proposed a conceptual model for these features in a differential vorticity and mass conservation framework. While it adequately described some of the underlying atmospheric processes, it had several shortcomings. The hypothesis assumed the systems were quasi-barotropic, thus it did not include possible baroclinic evolution or attending vertical shear enhancement. Finch and Johnson (2010), though, show convincingly that these upper low not only induce many of the atmospheric responses predicted by the original hypothesis, but they also induce mid level shear on the west flank of these disturbances. This paper proposes an update to the conceptual model using a potential vorticity and dynamic tropopause (DT) framework proposed by Sukup (2010), Melino (2010), and other works, and suggests tools to better predict the likelihood, coverage and location of convective outbreaks associated with these upper lows as they interact with the monsoon regime. Figure: Cross section analysis of potential temperature (°K), potential vorticity (PVU), wind (m s ^-1) and upward vertical motion (hPa s ^-1 ) of an upper level low over Arizona. Data from Rapid Update Cycle (RUC) initialization 1800 UTC 28 July 2006. From Sukup, S. M.S. thesis presentation (2010).