Numerical Simulation of the Interaction between the Sea-Breeze Front and Horizontal Convective Rolls. Part II: Alongshore Ambient Flow

Robert G. Fovell Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California

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Peter S. Dailey Litton-TASC, Inc., Reading, Massachusetts

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Abstract

A three-dimensional, high-resolution model is employed to examine the interaction between the sea-breeze front (SBF) and horizontal convective rolls (HCRs) aligned parallel to the front. This study extends the perpendicular case that was the focus of Part I. In this situation, the SBF systematically encounters roll downdrafts and updrafts as it propagates inland.

The sea-breeze circulation is found to significantly influence HCR strength and development. In turn, the rolls are found to dramatically modulate the overall convective activity, alternately suppressing and enhancing SBF-associated convection. Suppression occurs as the SBF merges with a roll downdraft. This is in part due to the downdraft's introduction of dry air into the mixed layer that becomes part of the SBF cloud's inflow.

Following suppression, the SBF accelerates as convective heating above the frontal head diminishes. This leads to reinvigorated convection above the front prior to its contact with the next roll updraft, which itself sports a strong, deep cloud of its own by this time. This brings about two strong updrafts obscured by a single, merged cloud shield. During this time, a strong yet brief midlevel downdraft occurs in between the two updrafts; forcing mechanisms for this feature are discussed. The SBF propagation speed also declines significantly during this period; the near-surface portion of the front actually becoming retrograde for a period of a few minutes. Two other, less dramatic roll encounters are also examined.

Corresponding author address: Prof. Robert G. Fovell, Department of Atmospheric Sciences, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095-1565. Email: rfovell@ucla.edu

Abstract

A three-dimensional, high-resolution model is employed to examine the interaction between the sea-breeze front (SBF) and horizontal convective rolls (HCRs) aligned parallel to the front. This study extends the perpendicular case that was the focus of Part I. In this situation, the SBF systematically encounters roll downdrafts and updrafts as it propagates inland.

The sea-breeze circulation is found to significantly influence HCR strength and development. In turn, the rolls are found to dramatically modulate the overall convective activity, alternately suppressing and enhancing SBF-associated convection. Suppression occurs as the SBF merges with a roll downdraft. This is in part due to the downdraft's introduction of dry air into the mixed layer that becomes part of the SBF cloud's inflow.

Following suppression, the SBF accelerates as convective heating above the frontal head diminishes. This leads to reinvigorated convection above the front prior to its contact with the next roll updraft, which itself sports a strong, deep cloud of its own by this time. This brings about two strong updrafts obscured by a single, merged cloud shield. During this time, a strong yet brief midlevel downdraft occurs in between the two updrafts; forcing mechanisms for this feature are discussed. The SBF propagation speed also declines significantly during this period; the near-surface portion of the front actually becoming retrograde for a period of a few minutes. Two other, less dramatic roll encounters are also examined.

Corresponding author address: Prof. Robert G. Fovell, Department of Atmospheric Sciences, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095-1565. Email: rfovell@ucla.edu

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