Updraft Dynamics within a Numerically Simulated Subtropical Rainband

Abstract

In this study a three-dimensional numerical cloud model is used to examine the early evolution of deep convective rainbands that occur in an environment of weak to moderate buoyancy and directionally varying lower-tropospheric vertical wind shear. A simulation based on a case observed on 8 June 1987 during the Taiwan Area Mesoscale Experiment produced a narrow bow-shaped rainband that comprised 1) short-lived updrafts 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 analysis of the dynamic portion of the perturbation pressure field are presented 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 noted to play an important role in the longevity and propagation of updrafts in midlatitude supercell storms. While this work was motivated by the desire to better understand mechanisms important to the. formation of heavy rainfall that occurs in association with prefrontal low-level jets over Taiwan, direct verification of the control simulation was hindered by the lack of available Doppler radar observations and difficulties in unambiguously determining initial conditions. Therefore, the 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 strongly dependent on the magnitude of the ambient vertical shear. At weaker vertical shears, low-level updrafts were generally weaker and not associated with strong vertical vorticity. Maximum rainwater mixing ratios were also significantly weaker for less ambient vertical shears despite the specification of identical initial profiles of temperature and moisture for all simulations. This suggests that the strong vertical shear associated with the low-level jet provides a mechanism for producing greater local rainfall rates by allowing enhanced forcing for low-level updrafts in the nearly saturated ambient environment.