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Observations of the Sea-Breeze Front during CaPE. Part I: Single-Doppler, Satellite, and Cloud Photogrammetry Analysis

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  • 1 Department of Atmospheric Science, University of California, Los Angeles, Los Angeles, California
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Abstract

An analysis of two sea-breeze events on 6 August (an onshore flow event) and 12 August (an offshore flow event) 1991 is presented using single-Doppler observations, satellite images, and cloud pictures collected during the Convection and Precipitation/Electrification (CaPE) Experiment. Documentation of the alongfrontal variability at the leading edge of the sea-breeze circulation is presented for the first time. The horizontal structure of the front was strongly modulated by the near-perpendicular intersections of horizontal convective rolls developing in the ambient air out ahead of the sea breeze on 12 August. These intersection points also appeared to be preferred locations for cloud development along the front. Horizontal convective rolls were also documented on 6 August; however, their orientation was nearly parallel to the sea-breeze front. As a result, extended sections of these rolls appeared to have merged with the front as it propagated inland rather than having distinct intersection points. First cloud development along the front occurred at periodic locations and only along the sections where the rolls and front had merged. Further elucidation of the frontal characteristics under synoptic flow that is onshore versus offshore is discussed. The absence of a sharp frontal discontinuity and nonuniform propagation speed for the 6 August case were noted. There were substantial differences in the character of the radar-detected fine line on these two days. When compared with the 6 August line, the 12 August case was wider, easily identifiable, and characterized by higher reflectivities. Indeed, there were times on 6 August when identifying the location of the sea-breeze front based on radar reflectivity in the clear air was difficult.

Abstract

An analysis of two sea-breeze events on 6 August (an onshore flow event) and 12 August (an offshore flow event) 1991 is presented using single-Doppler observations, satellite images, and cloud pictures collected during the Convection and Precipitation/Electrification (CaPE) Experiment. Documentation of the alongfrontal variability at the leading edge of the sea-breeze circulation is presented for the first time. The horizontal structure of the front was strongly modulated by the near-perpendicular intersections of horizontal convective rolls developing in the ambient air out ahead of the sea breeze on 12 August. These intersection points also appeared to be preferred locations for cloud development along the front. Horizontal convective rolls were also documented on 6 August; however, their orientation was nearly parallel to the sea-breeze front. As a result, extended sections of these rolls appeared to have merged with the front as it propagated inland rather than having distinct intersection points. First cloud development along the front occurred at periodic locations and only along the sections where the rolls and front had merged. Further elucidation of the frontal characteristics under synoptic flow that is onshore versus offshore is discussed. The absence of a sharp frontal discontinuity and nonuniform propagation speed for the 6 August case were noted. There were substantial differences in the character of the radar-detected fine line on these two days. When compared with the 6 August line, the 12 August case was wider, easily identifiable, and characterized by higher reflectivities. Indeed, there were times on 6 August when identifying the location of the sea-breeze front based on radar reflectivity in the clear air was difficult.

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