P1.10
Open cell convection as seen by MODIS and synthetic aperture radar
Caren M. Fisher, Pennsylvania State University, University Park, PA; and G. S. Young, T. D. Sikora, and M. D. Stepp
The structure, behavior, and synoptic setting of open cell convection are analyzed using RADARSAT Synthetic Aperture Radar (SAR) wind speed imagery, Moderate Resolution Imaging Spectroradiometer (MODIS) multi-spectral imagery, meteorological analyses, and numerical weather prediction model forecasts. Of particular interest are those cases wherein the open cell convection induces sufficient perturbation in the surface wind field to be detectable in the SAR wind speed imagery. Many of these cases exhibit pronounced wind squalls and so can potentially impact maritime operations.
The study focuses on the Gulf of Alaska and adjacent regions of the Pacific Ocean and Bering Sea, a region with both a large number of open cell convection cases and ready access to SAR wind speed imagery. The analysis begins with a six year SAR-based climatology to determine the seasonal occurrence of open cell convection. This climatology reveals that open cell convection is predominantly a cool season phenomena in this region. This finding suggests that open cell convection may be linked to cold air outbreaks in some way, since this in the primary cool season synoptic phenomena of the Gulf of Alaska that is notably absent during the warm season.
Case studies of two dozen events across all seasons are employed to document the synoptic setting of the region's open cell convection events. Most occur during periods of cold advection, often in the region behind a cold front. This synoptic setting acts both to destabilize the boundary layer, fostering lower tropospheric convection, and to enhance lower tropospheric winds, which could be transported to the surface by convective downdrafts. Thus, the observed synoptic setting is in keeping with both the convective cloud structures observed in MODIS imagery and the surface wind squalls observed in the SAR wind speed imagery.
The case studies are also employed to demonstrate that squall-and-lull signatures observed within the SAR wind speed imagery are the footprints of open cell convection. In addition, the suite of data is exploited to explore how the structure and dynamics of open cell convection can lead surface squall formation. The observations suggest a two way coupling between the boundary layer convective structures and the surface wind pattern.
The individual squall-and-lull signatures have elliptical boundaries with an arc-shaped squall along the feature's downwind edge, and wind speed decreasing gradually across the feature, to its upwind edge. MODIS imagery reveals that the open cell convection Alaskan waters is typically composed of cumulus humulus and cumulus congestus clouds, with cloud top temperature measurements from MODIS yielding a considerable variation in cloud top height between the highest clouds (those on the leading or downwind edge of the cells) and the shallower clouds along the sides and rear of the cells. The MODIS imagery also reveals that the tallest clouds typically contain ice. This suggests that precipitation-driven downdrafts may cause the squall signatures seen within the SAR wind speed imagery on the leading (downwind) edge of the cells.
The elliptical gust-and-lull pattern observed in the SAR wind speed imagery tends to be aligned with the gust on the downwind or downshear edge of the cell. This is also where the tallest cumulus clouds are observed in the MODIS imagery. Thus the downwind and crosswind edges of an open convective cell are structurally reminiscent of the arc-shaped squall lines occasionally seen over tropical oceans. Indeed, these satellite observations of open cell convection suggest an interplay of moist convection, precipitation driven downdrafts, vertical transport momentum, and surface convergence, very similar to that previously documented for tropical squall lines in general. In situ observations will probably be required to ascertain the degree of dynamic similarity between these two phenomena.
Poster Session 1, Poster Session
Tuesday, 21 August 2007, 3:00 PM-5:00 PM, Hawthorne-Sellwood
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