Observations of the Sea-Breeze Front during CaPE. Part II: Dual-Doppler and Aircraft Analysis

Nolan T. Atkins Department of Atmospheric Science, UCLA, Los Angeles, California

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Roger M. Wakimoto Department of Atmospheric Science, UCLA, Los Angeles, California

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Tammy M. Weckwerth Advanced Study Program, National Center for Atmospheric Research, Boulder, Colorado

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Abstract

The three-dimensional kinematic structures of offshore and onshore flow sea-breeze fronts observed during the CaPE experiment are shown using high resolution dual-Doppler and aircraft data. The fronts interact with horizontal convective rolls (HCRs) that develop within the convective boundary layer. Nearly perpendicular intersections between the HCRs and sea-breeze front were observed during the offshore flow case. Close to the front, the HCR axes were tilted upward and lifted by the frontal updrafts. Consequently, a deeper updraft was created at the intersection points, providing additional impetus for cloud development. Furthermore, clouds forming at periodic intervals along the NCRs intensified as they propagated over the front.

During the onshore flow case, the HCR orientation was nearly parallel to the front. Extended sections of the front “merged” with the HCRs. This process strengthened the front and is explained as the merger of like-sign vortices associated with both the front and HCRs. Clouds formed along the intensified portions of the front and at the locations of periodic enhancements on the HCR, which were present prior to the merger.

Documentation of two distinct frontal boundaries is presented for the onshore flow case. The first is a kinematic sea-breeze front delineating the region of maximum near-surface convergence between the sea-breeze air and the warmer, drier environmental air. The second is a thermodynamic sea-breeze front, which delineates the location where the mean thermodynamic properties differ from the ambient air mass. It is generated by the interaction of the HCRs with the sea breeze and extends a few kilometers ahead of the kinematic frontal position.

The kinematic differences between the two cases are quantitatively illustrated. The offshore flow case exhibited stronger low-level convergence, larger vertical velocities, and larger radar reflectivity values. The source air for the clouds developing along the front originated from the ambient and moist sea-breeze air masses for the offshore and onshore now cases, respectively.

Abstract

The three-dimensional kinematic structures of offshore and onshore flow sea-breeze fronts observed during the CaPE experiment are shown using high resolution dual-Doppler and aircraft data. The fronts interact with horizontal convective rolls (HCRs) that develop within the convective boundary layer. Nearly perpendicular intersections between the HCRs and sea-breeze front were observed during the offshore flow case. Close to the front, the HCR axes were tilted upward and lifted by the frontal updrafts. Consequently, a deeper updraft was created at the intersection points, providing additional impetus for cloud development. Furthermore, clouds forming at periodic intervals along the NCRs intensified as they propagated over the front.

During the onshore flow case, the HCR orientation was nearly parallel to the front. Extended sections of the front “merged” with the HCRs. This process strengthened the front and is explained as the merger of like-sign vortices associated with both the front and HCRs. Clouds formed along the intensified portions of the front and at the locations of periodic enhancements on the HCR, which were present prior to the merger.

Documentation of two distinct frontal boundaries is presented for the onshore flow case. The first is a kinematic sea-breeze front delineating the region of maximum near-surface convergence between the sea-breeze air and the warmer, drier environmental air. The second is a thermodynamic sea-breeze front, which delineates the location where the mean thermodynamic properties differ from the ambient air mass. It is generated by the interaction of the HCRs with the sea breeze and extends a few kilometers ahead of the kinematic frontal position.

The kinematic differences between the two cases are quantitatively illustrated. The offshore flow case exhibited stronger low-level convergence, larger vertical velocities, and larger radar reflectivity values. The source air for the clouds developing along the front originated from the ambient and moist sea-breeze air masses for the offshore and onshore now cases, respectively.

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