Mechanisms for Precipitation Enhancement in a North American Monsoon Upper-Tropospheric Trough

Tropical upper-tropospheric troughs (TUTTs), transient summer-time 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. Multiple simulations, with and without the TUTT, are then executed with several microphysical parameterizations. It is found that the TUTT simulations do show an increase in precipitation along the SMO. The mechanisms for convective enhancement along the Sierra Madre Occidental (SMO) are slightly increased Sfc-500 and 700-400 hPa shear and incrementally more convective available potential energy (CAPE). These differences lead to important changes in convective updrafts and microphysical evolution, which will be discussed in detail.