Environmental Conditions Controlling the Morphology of Shallow Orographic Convection

Observations have shown that shallow orographic convection can organize into either scattered cells or quasi-stationary bands. The latter morphology, which has only been documented in a few studies, tends to enhance flash-flooding risk by focusing rainfall over narrow mountain watersheds. These bands owe their organization to updrafts associated with standing lee waves formed by flow over small-scale topographic features. While previous studies have explained the mechanisms by which such bands form, they have not determined the environmental conditions that distinguish bands from cells. This deficiency limits the ability of local forecasters to warn their communities of potentially hazardous meteorological events. To address this knowledge gap, this study performs a five-year radar-based climatology of cool-season convection over the Oregon Coastal Range. All convection events are categorized as cellular or banded, and banded events are found to occur far less frequently than cellular events. A composite upstream sounding is created for each convection morphology. To assess whether differences in the two soundings explain the corresponding differences in cloud morphology, these soundings are used to initialize high-resolution quasi-idealized simulations of moist flows over the Coastal Range. When run over an unheated surface, both the cellular and banded soundings give rise to stationary cloud bands, suggesting that the soundings alone do not dictate the cloud morphology. A more important factor, suggested by buoy observations, is the upstream ocean—air temperature contrast: cellular events tend to have warmer ocean SSTs than surface air temperatures, while banded events have the opposite. Additional numerical experiments reveal that positive ocean—air temperature differences give rise to stronger turbulence in the impinging boundary layer, which readily seeds cellular moist convection initiation over the windward slope. The large amplitudes of these cells interfere with the standing lee wave circulations nominally responsible for band formation, leading to a more cellular pattern. In contrast, when the ocean is cooler than the air, the boundary-layer turbulence is much weaker and does not disturb the lee wave convection initiation, allowing convective bands to develop and persist. Altogether, the convection morphology is dictated by whether moist convection initiates upstream of the nominal band initiation point (i.e. the location where bands would form in the absence of upstream turbulence). If so, the cells suppress the ability of small-scale lee waves to organize the convection into bands. If not, bands tend to prevail.