Synoptic-scale forcing of the low-level jet in developing baroclinic waves

Abstract

A series of numerical simulations of low-level jet (LLJ) formation in a developing baroclinic wave is presented. The simulations were performed using the Limited Area Mesoscale Prediction System (LAMPS) model. The LAMPS model is a 15 layer primitive equation model with a terrain following vertical coordinate. The numerical simulations were initiated with a schematic baroclinic wave which includes a sloping polar front and a realistically shaped tropopause. The location of the tropopause was diagnosed from derived equations which constrain the geostrophic wind at the top of the model atmosphere to vanish. The initial conditions were altered selectively to study the effect of low-level stable layers on the subsequent development of the LLJ. In each simulation a similar synoptically-forced LLJ develops in the absence of surface terrain. A low-level stable layer is not necessary to confine the LLJ to low-levels. Each LLJ develops with a highly ageostrophic component which in turn has a large tendency wind component as described by Brill et al. (1985). The horizontal configuration of the LLJ is similar to the warm-conveyor belt described by Carlson (1980) and the schematic LLJ model of Djuric and Ladwig (1983). Areas of mass convergence and mass divergence at low-levels are not strongly related to the position and strength of the upper-level jet streak or the location and magnitude of the upper-level mass divergence pattern. A two-layer mass coupling between the lower and upper troposphere is difficult to establish in these simulations due to the curvature of the upper-level jet and the non-linearity of the baroclinic development. The LLJ develops in a region where weak or negative vertical geostrophic speed shear and pronounced low-level ageostrophic parcel accelerations coexist. The ageostrophic wind shows strong vertical shear above the level of the LLJ. The geostrophic forcing is due to the thermal structure of the baroclinic wave. The low-level ageostrophic wind is not directly forced by the upper-level mass-divergence pattern. In the simulations presented geostrophic forcing as described by Browning and Pardoe (1973) and ageostrophic forcing similar to that described by Uccellini and Johnson (1979) play a dual role in the formation of the LLJ. Because each of these forcing mechanisms may vary in strength, the description of synoptically-forced LLJs varies widely in the literature.