NUMERICAL-SIMULATION OF THE EFFECTS OF MESOSCALE CONVERGENCE ON CONVECTIVE RAIN SHOWERS

A nonhydrostatic axisymmetric cloud model is used to quantify the effe cts of persistent mesoscale convergence on cumulus development and con vective rainfall. The model was initialized by environmental condition s adopted from sounding and Doppler radar velocity data sampled on 19 August 1992 in central Alberta. The sounding showed a moist warm air m ass with a moderate amount of convective available potential energy an d the wind field had boundary layer convergence but almost no vertical shear in the lowest 5 km, The simulated rainfall intensity and accumu lation compared well with radar observations. The dependence of the co nvective rainfall on the characteristics of the convergence zone is in vestigated by intercomparing model simulations with different converge nce magnitudes, convergence depths, and convergence profiles. Increasi ng the magnitude or the depth of convergence causes stronger convectio n and more precipitation. Rainfall increases monotonically (but nonstr ictly linearly) with the convergence magnitude. Doubling the convergen ce magnitude from 1 x 10(-4) to 2 x 10(-4) s(-1) increases the rainfal l by a factor of 2.6, while rainfall increases by only 2.3 times when the convergence is doubled from 1.25 x 10(-4) to 2.5 x 10(-4) s(-1). T he nonlinear effects become even more apparent when changing the depth of convergent layers. Even when keeping the vertical mass Aux constan t, the depth of the convergence affects greatly the timing and amount of the surface rainfall, This is related to the fact that humidity ten ds to decrease with height and therefore the upward moisture flux is w eakest for the deepest convergence layer for a fixed upward momentum A ux. The model suggests that rainfall is mostly controlled by the amoun t of vapor converging into the column below cloud base.