A NUMERICAL STUDY OF THE LOW-LEVEL JET DURING TAMEX-IOP-5

The Pennsylvania Stale University-National Center for Atmospheric Rese arch Mesoscale Model Version 4 is used to simulate the cyclogenesis an d the development of a low-level jet (LLJ) that occurred during the Ta iwan Area Mesoscale Experiment intensive observing period 5 over south ern China. Evaluation of the model results during a 3-h period indicat es that the model successfully reproduces most principal features of t his event, including cyclone path, intensification of the LLJ, distrib ution of precipitation, and the secondary circulation across the jet-f ront system. Sensitivity tests show that latent heat release is import ant for the deepening of the cyclone and the development of the LLJ, w hereas the model results are not sensitive to boundary layer physics. The lee trough east of the Tibetan Plateau provides the initial low-le vel vorticity. The initial deepening of the lee cyclone and the develo pment of the low-level southwesterly flow are caused by the vertical m otion associated with the upper-level short-wave trough. The potential vorticity and tropopause folding associated with the upper-level fron t are present in the model simulations even without latent heating. Th e low-level southwesterly flow transports warm, moist air from the sou th as the moisture source for condensation. Latent heating results in an increase in the thickness ahead of the short-wave trough in the upp er levels, further deepening of the lee cyclone, and a stronger second ary circulation. The LLJ develops through the Coriolis force acting on the cross-contour ageostrophic winds in response to the increased pre ssure gradients related to the development of the cyclone and is enhan ced by latent heating. The dynamic forcing aloft is also enhanced. Con densation healing and warm advection (evaporative cooling and cold adv ection) exceed adiabatic cooling (warming) ahead of (behind) the cyclo ne in the lower troposphere, therefore enhancing the low-level barocli nity. These processes interact nonlinearly leading to the further inte nsification of the LLJ.