UPDRAFT DYNAMICS WITHIN A NUMERICALLY SIMULATED SUBTROPICAL RAINBAND

In this study a three-dimensional numerical cloud model is used to exa mine the early evolution bf deep convective rainbands that occur in an environment of weak to moderate buoyancy and directionally varying lo wer-tropospheric vertical wind,shear. A simulation based on a case obs erved on 8 June 1987 during the Taiwan Area Mesoscale Experiment produ ced a narrow bow-shaped rainband that comprised 1) short-lived updraft s along the downshear portion of the weak rain-induced cold pool and 2 ) more persistent updrafts along its southern flank, which were highly correlated with vertical vorticity. Trajectory calculations and an an alysis of the dynamic portion of the perturbation pressure field are p resented to illustrate the hybrid dynamical character of the simulated rainband. The shorter-lived updrafts were associated with weak upward -directed pressure gradient forces at the leading edge of the surface- based cold pool. The more persistent updrafts exhibited much stronger upward-directed pressure gradient forces, which have previously been n oted to play an important role in the longevity and propagation of upd rafts in midlatitude supercell storms. While this work was motivated b y the desire to better understand mechanisms important to the formatio n of heavy rainfall that occurs in association with prefrontal low-lev el jets over Taiwan, direct verification of the control simulation was hindered by the lack of available Doppler radar observations and diff iculties in unambiguously determining initial conditions. Therefore, t he simulation results were viewed as idealized and interpreted within the context of a series of sensitivity experiments. These experiments revealed that updraft dynamics and convective organization were strong ly dependent on the magnitude of the ambient vertical shear. At weaker vertical shears, low-level updrafts were generally weaker and not ass ociated with strong vertical vorticity. Maximum rainwater mixing ratio s were also significantly weaker for lesser ambient vertical shears de spite the specification of identical initial profiles of temperature a nd moisture for all simulations. This suggests that the strong vertica l shear associated with the low-level jet provides a mechanism for pro ducing greater local rainfall rates by allowing enhanced forcing for l ow-level updrafts in the nearly saturated ambient environment.