Variability in the summertime coastal marine atmospheric boundary‐layer off California, USA

Abstract

AbstractAn analysis of boundary‐layer structure and surface‐layer turbulence from measurements off the California coast is presented from data collected by research aircraft during two field experiments: the Coastal Waves 1996 (CW96) and the Monterey Area Ship Track (MAST) experiments. CW96 covers the near‐coast region, in particular in the vicinity of major headlands, whereas MAST extends offshore.Along the US west coast, coastal modification of the along‐coast flow occurs on two main horizontal scales. Firstly, a large‐scale variability is due to the interplay between the shallow near‐coast marine atmospheric boundary‐layer (MABL) and the coastal terrain, typically higher than the MABL depth. The MABL depth decreases smoothly towards the coast while the wind speed increases to a coastal jet in response to the sloping MABL inversion. Secondly, the flow is affected by supercritical flow dynamics. As the wind speed increases and the MABL depth decreases towards the coast, the MABL flow becomes supercritical in a shallow‐water sense. As supercritical shallow‐water flow interacts with major headlands, expansion fans form, affecting both the wind speed and the MABL depth.The combination of CW96 and MAST data reveals significant differences between the flow along the coast and that far offshore. MABL winds are stronger near the coast whereas aloft the winds are weaker than offshore. The near‐coast MABL is also better mixed. Turbulence increases towards the coast while the sensible‐heat flux decreases and often changes sign, leading to stable stratification near the coast. A length‐scale determining the across‐coast influence of the expansion fans is defined from simple inviscid and irrotational shallow‐water theory. Data from four days of the CW96 experiment show that this simple theory describes the low‐level wind speed adequately. Surface‐layer turbulence also scales with this simple length‐scale, but the results are complicated by upwelling of cold water, giving rise to strong internal boundary‐layers. From terms in the turbulent‐energy budgets it appears that dissipation of turbulence exceeds local production. Copyright © 2004 Royal Meteorological Society