The triggering of orographic rainbands by small-scale topography

AMS Citation:
Kirshbaum, D. J., G. H. Bryan, R. Rotunno, and D. R. Durran, 2007: The triggering of orographic rainbands by small-scale topography. Journal of the Atmospheric Sciences, 64, 1530-1549, doi:10.1175/JAS3924.1.
Resource Type:article
Title: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 wc and nearly zero buoyancy anomalies bc as air parcels undergo saturation. This relationship is quantified through a simple analytic condition involving wc, bc, and the static stability N 2m 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.
Peer Review:Refereed
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OpenSky citable URL: ark:/85065/d7js9qp3
Publisher's Version: 10.1175/JAS3924.1
  • Daniel Kirshbaum - NCAR/UCAR
  • George Bryan - NCAR/UCAR
  • Richard Rotunno - NCAR/UCAR
  • Dale Durran
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