Momentum and Mass Transport by Convective Bands: Comparisons of Highly Idealized Dynamical Models to Observations

Abstract

The effects of quasi-two-dimensional convective bands on the environmental flow are investigated by comparing the observed mass and momentum fluxes and horizontal pressure changes to those predicted by the Moncrieff archetypal model (M92). The model idealizes the organized convection as two-dimensional and steady state, with three flow branches?a front-to-rear jump updraft, a front overturning updraft, and a rear overturning current, which can be an updraft or a downdraft. Flow through the branches satisfies mass continuity and Bernoulli's equation. The vertical divergence of line-normal momentum flux averaged over the volume is constrained to be zero. Coriolis and buoyancy effects are neglected. The model predicts the vertical mass flux, the vertical divergence of the vertical flux of line-normal momentum, and the pressure change across the line (independent of height). A simple equation for the vertical transport of line-parallel momentum follows from the model assumptions. Case studies show a systematic linkage of fluxes and structure and a relationship of some of these changes to differences in the environmental sounding. The M92 successfully replicates the general shapes of the vertical mass flux and line-normal momentum flux profiles, and to some degree how they change with environment. The M92 correctly predicts both the magnitude and shape of the curves in cases occurring in near-neutral environments (low buoyancy or high shear) and with system width-to-depth ratios close to the dynamically based value of 4:1. The model is less successful for systems in more unstable environments or those with large horizontal extent, probably due to the neglect of the generation of horizontal momentum by the buoyancy distribution. The observed sign of the average pressure changes across the line is consistent with that predicted by the model in the upper half of the system, where some case studies suggest that buoyancy effects should be minimized. Letting the model (4:1 aspect ratio) represent the dynamically active part of a mesoscale system, the rearward advective broadening of the inert anvil region is simply related to the (rearward) outflow speed of the jump updraft, U1. Since U1 increases as tropospheric shear decreases, the model correctly associates broad mesoscale systems with small tropospheric shear. Success in predicting the vertical flux of line-parallel momentum was fair.