A Numerical Study of the Low-Level Jet during TAMEX IOP 5

Abstract

The Pennsylvania State University?National Center for Atmospheric Research Mesoscale Model Version 4 is used to simulate the cyclogenesis and the development of a low-level jet (LLJ) that occurred during the Taiwan Area Mesoscale Experiment intensive observing period 5 over southern China. Evaluation of the model results during a 36-h period indicates that the model successfully reproduces most principal features of this event, including cyclone path, intensification of the LLJ, distribution of precipitation, and the secondary circulation across the jet?front system. Sensitivity tests show that latent heat release is important for the deepening of the cyclone and the development of the LLJ, whereas the model results are not sensitive to boundary layer physics. The lee trough east of the Tibetan Plateau provides the initial low-level vorticity. The initial deepening of the lee cyclone and the development of the low-level southwesterly flow are caused by the vertical motion associated with the upper-level short-wave trough. The potential vorticity and tropopause folding associated with the upper-level front are present in the model simulations even without latent heating. The low-level southwesterly flow transports warm, moist air from the south as the moisture source for condensation. Latent heating results in an increase in the thickness ahead of the short-wave trough in the upper levels, further deepening of the lee cyclone, and a stronger secondary circulation. The LLJ develops through the Coriolis force acting on the cross-contour ageostrophic winds in response to the increased pressure gradients related to the development of the cyclone and is enhanced by latent heating. The dynamic forcing aloft is also enhanced. Condensation heating and warm advection (evaporative cooling and cold advection) exceed adiabatic cooling (warming) ahead of (behind) the cyclone in the lower troposphere, therefore enhancing the low-level baroclinity. These processes interact nonlinearly leading to the further intensification of the LLJ.