OBSERVATIONS OF THE SEA-BREEZE FRONT DURING CAPE .2. DUAL-DOPPLER ANDAIRCRAFT ANALYSIS

The three-dimensional kinematic structures of offshore and onshore flo w sea-breeze fronts observed during the CaPE experiment are shown usin g high resolution dual-Doppler and aircraft data. The fronts interact with horizontal convective rolls (HCRs) that develop within the convec tive boundary layer. Nearly perpendicular intersections between the HC Rs and sea-breeze front were observed during the offshore how case. Cl ose to the front, the HCR axes were tilted upward and lifted by the fr ontal updrafts. Consequently, a deeper updraft was created at the inte rsection points, providing additional impetus for cloud development. F urthermore, clouds forming at periodic intervals along the HCRs intens ified as they propagated over the front. During the onshore flow case, the HCR orientation was nearly parallel to the front. Extended sectio ns of the front ''merged'' with the HCRs. This process strengthened th e front and is explained as the merger of like-sign vortices associate d with both the front and HCRs. Clouds formed along the intensified po rtions of the front and at the locations of periodic enhancements on t he HCR, which were present prior to the merger. Documentation of two d istinct frontal boundaries is presented for the onshore flow case. The first is a kinematic sea-breeze front delineating the region of maxim um near-surface convergence between the sea-breeze air and the warmer, drier environmental air. The second is a thermodynamic sea-breeze fro nt, which delineates the location where the mean thermodynamic propert ies differ from the ambient air mass. It is generated by the interacti on of the HCRs with the sea breeze and extends a few kilometers ahead of the kinematic frontal position.The kinematic differences between th e two cases are quantitatively illustrated. The offshore flow case exh ibited stronger low-level convergence, larger vertical velocities, and larger radar reflectivity values. The source air for the clouds devel oping along the front originated from the ambient and moist sea-breeze air masses for the offshore and onshore flow cases, respectively.