Mechanisms for precipitation enhancement in a North American monsoon upper-tropospheric trough

Tropical upper-tropospheric troughs (TUTTs), transient summertime disturbances over the Pacific and Atlantic Oceans, are also observed to frequently occur in the North American monsoon (NAM) region. However, unlike TUTTs over the Pacific and Atlantic, which feature a predominance of precipitation on the eastern flank of the disturbances, TUTTs in the NAM region have been shown to enhance precipitation on their western flank as they pass over the mountains of northern Mexico. To investigate this phenomenon, convection-permitting simulations are performed over the core NAM region for the 12–14 July 2004 TUTT event that occurred during the North American Monsoon Experiment (NAME). The effects of the TUTT are isolated using an approach that removes the vorticity anomaly associated with it. Six simulations, three with the TUTT and three without it, are then executed. It is found that the mean of the TUTT simulations has increased surface–500-hPa and 700–400-hPa shear and that there is incrementally more convective available potential energy (CAPE) along the Sierra Madre Occidental (SMO). These differences lead to convective changes, with the TUTT simulations having more convection, larger maximum updraft velocities, and more precipitation in lower elevations. Overall, the TUTT simulation mean has about 15% more precipitation during the primary period of TUTT interaction with the northern SMO. There are also slight simulated microphysical differences that agree with nearby polarimetric radar observations. Finally, TUTT removal impacts the gulf surge event on 13 July via the convective modifications.