Monday, 12 January 2009
Remote sensing of high latitude open cell convection
Hall 5 (Phoenix Convention Center)
This research examines open cell convection using two distinct but mutually supporting approaches. The first is a seven-year (1999-2006) climatology of the frequency of open cell convection and the thermodynamic and kinematic environment surrounding its development, based on synthetic aperture radar (SAR)-derived wind speed (SDWS) images and global reanalysis data. The second approach examines two-dozen case studies from 2006, which employ Moderate Resolution Imaging Spectroradiometer (MODIS) images, SDWS images, and mesoscale numerical prediction model output, to document the morphology of open cell convection. Both approaches are used to reexamine the tropical squall line – open cell convection similarity and corresponding conceptual model proposed in a motivating study. MODIS images showed that open cell convection clouds were typically cumulus congestus along the leading gust front arc, and shallower cumulus completing a ring. Individual open cells produced a squall-lull signature on SDWS images. (The squall is an area of stronger wind with a sharp gradient along its leading gust front.)
The climatology showed open cell convection was a cold season phenomenon, having occurred in environments with negative air-sea temperature difference, upward surface heat flux, lower tropospheric structure conducive to latent heat release, and moderate lower tropospheric vertical wind shear vector magnitude. Most case studies occurred during cold air advection southeast of a surface cyclone. MODIS images of the case studies showed that most cumulus congestus clouds were glaciated and most cumulus mediocris and humulus clouds were mixed phase. Lower troposphere static stability and vertical wind shear profiles from the case studies paralleled those of the climatology where comparisons could be made. The cases studies showed that the squall-lull orientation was parallel to the surface layer vertical wind shear vector and the cloud cell orientation was slightly to the right of the surface layer vertical wind shear vector. These findings were then used to revise the conceptual model referenced above.