The Triggering of Orographic Rainbands by Small-Scale Topography

Abstract

The triggering of convective orographic rainbands by small-scale topographic features is investigated through observations of a banded precipitation event over the Oregon Coastal Range and simulations using a cloud-resolving numerical model. A quasi-idealized simulation of the observed event reproduces the bands in the radar observations, indicating the model's ability to capture the physics of the band-formation process. Additional idealized simulations reinforce that the bands are triggered by lee waves past small-scale topographic obstacles just upstream of the nominal leading edge of the orographic cloud. Whether a topographic obstacle in this region is able to trigger a strong rainband depends on the phase of its lee wave at cloud entry. Convective growth only occurs downstream of obstacles that give rise to lee-wave-induced displacements that create positive vertical velocity anomalies w c and nearly zero buoyancy anomalies b c as air parcels undergo saturation. This relationship is quantified through a simple analytic condition involving w c , b c , and the static stability N 2 m of the cloud mass. Once convection is triggered, horizontal buoyancy gradients in the cross-flow direction generate circulations that align the bands parallel to the flow direction.

Multifractal properties of embedded convective structures in orographic precipitation: toward subgrid-scale predictability

“…In this inviscid formulation, the linear model predicts maximum growth for infinitesimally small horizontal wavelengths. Admitting that smallscale disturbances are ubiquitous in the real atmosphere, this does not agree with the observed finite spacing between bands observed in nature, e.g., in western Kyushu in Japan (Yoshizaki et al, 2000), in the Cévennes region in southern France (Miniscloux et al, 2001;Cosma et al, 2002) and the Oregon Coastal Range (Kirshbaum and Durran, 2005b;Kirshbaum et al, 2007b). Kirshbaum et al (2007a) developed a…”

Numerical Study of Horizontal Shear Instability Waves along Narrow Cold Frontal Rainbands

“…The U5-S2 and U10-S2 cases, wherein the frontal updrafts are tilted strongly upshear, exhibit horizontally contiguous MAULs above the cold air. In such a situation, the strong local vertical shear above the cold air can align the convective updrafts in the MAUL in the shear direction, as discussed by Bryan et al (2007). In contrast, MAULs are identified only in narrow regions immediately ahead of the cold air in U15-S2 and U20-S2, wherein the frontal updrafts are more upright.…”

“…In contrast, MAULs are identified only in narrow regions immediately ahead of the cold air in U15-S2 and U20-S2, wherein the frontal updrafts are more upright. Convective-scale AFVs can hardly develop in such a situation because the residence time of air parcels within the MAUL becomes too short to allow significant convections to develop (e.g., Fuhrer and Schär 2005;Kirshbaum et al 2007). Though not shown, U5-S1 also exhibits a MAUL above the cold air.…”

Finescale Orographic Precipitation Variability and Gap-Filling Radar Potential in Little Cottonwood Canyon, Utah

“…This was also observed during IMPROVE-2 when postfrontal convective cells strengthened and upstream precipitation became more persistent during flow over the Cascade Mountains . Although not occurring exclusively in postfrontal or cold-sector environments, a variety of banded, cellular, and mesoscale precipitation features are generated or modified during unstable or potentially unstable flow interaction with complex terrain (e.g., Maddox et al 1978;Caraceña et al 1979;Parsons and Hobbs 1983;Sénési et al 1996;Kirshbaum and Durran 2005;Kirshbaum et al 2007;Godart et al 2011).…”