Shallow convection in orographic precipitation
An investigation into the dynamics of shallow orographic convection is performed through observations and numerical simulations using a cloud-resolving mesoscale model. Two-dimensional simulations reveal that embedded cellular convection in unstable orographic clouds is sensitive to several factors besides the moist stability, including the residence time of air parcels in the cloud, the cloud depth, and vertical shear. The presence of convective structures within an orographic cloud substantially enhances the intensity, efficiency, and accumulation of the simulated rainfall.Convective structures in operational radar images presented of three shallow convective precipitation events over the Oregon Coastal Range ranged from remarkably well-defined, organized, and stationary rainbands to disorganized and transient convective cells. Rainbands that developed in two of the cases appeared in similar locations, suggesting that they were fixed to small-scale topographic features. The difference between the more banded and cellular events appears to depend on the vertical shear and the susceptibility of the flow to convection upstream of the mountain. Quasi-idealized numerical simulations, which use a highly simplified topography yet still reproduce the convective formations in the three cases, suggest that the observed rainbands were governed by organized convective roll circulations that developed as a response to static instability in the orographic cap cloud.Additional simulations also suggest that orographic convection depends on both atmospheric factors and the mechanism by which convection is initiated. Thermally-initiated convection over a smooth mountain is disorganized and cellular except in flows with significant low-level vertical shear, in which there is weak static instability in the cloud and strong static stability in the unsaturated air surrounding it. Along with this shear-parallel mode, convective bands initiated by small-scale topography have a more dominant mode oriented parallel to the mean wind across the cloud layer. These comparatively robust bands appear to be triggered by lee waves generated by stable flow over small-scale topographic features just upstream of the orographic cloud, and considerably increase localized rainfall accumulations due to their stationarity. In more realistic flows containing both thermal and topographic perturbations, quasi-stationary bands only develop when both initiation mechanisms acting alone produce qualitatively similar banded features.
- Atmospheric sciences