Highlights of Coastal Waves 1996

Abstract

Some of the highlights of an experiment designed to study coastal atmospheric phenomena along the California coast (Coastal Waves 1996 experiment) are described. This study was designed to address several problems, including the cross-shore variability and turbulent structure of the marine boundary layer, the influence of the coast on the development of the marine layer and clouds, the ageostrophy of the flow, the dynamics of trapped events, the parameterization of surface fluxes, and the supercriticality of the marine layer.Based in Monterey, California, the National Center for Atmospheric Research (NCAR) C-130 Hercules and the University of North Carolina Piper Seneca obtained a comprehensive set of measurements on the structure of the marine layer. The study focused on the effects of prominent topographic features on the wind. Downstream of capes and points, narrow bands of high winds are frequently encountered. The NCAR-designed Scanning Aerosol Backscatter Lidar (SABL) provided a unique opportunity to connect changes in the depth of the boundary layer with specific features in the dynamics of the flow field.An integral part of the experiment was the use of numerical models as forecast and diagnostic tools. The Naval Research Laboratory's Coupled Ocean Atmosphere Model System (COAMPS) provided high-resolution forecasts of the wind field in the vicinity of capes and points, which aided the deployment of the aircraft. Subsequently, this model and the MIUU (University of Uppsala) numerical model were used to support the analysis of the field data.These are some of the most comprehensive measurements of the topographically forced marine layer that have been collected. SABL proved to be an exceptionally useful tool to resolve the small-scale structure of the boundary layer and, combined with in situ turbulence measurements, provides new insight into the structure of the marine atmosphere. Measurements were made sufficiently far offshore to distinguish between the coastal and open ocean effects. COAMPS proved to be an excellent forecast tool and both it and the MIUU model are integral parts of the ongoing analysis. The results highlight the large spatial variability that occurs directly in response to topographic effects. Routine measurements are insufficient to resolve this variability. Numerical weather prediction model boundary conditions cannot properly define the forecast system and often underestimate the wind speed and surface wave conditions in the nearshore region.

The Turbulence Structure of the Stable Atmospheric Boundary Layer Around a Coastal Headland: Aircraft Observations and Modelling Results

“…The close correlation between the spatial distribution of the surface stress in the region of the expansion fan and that of the coldest sea-surface temperatures has been noted in a number of papers (Rogers et al, 1998;Dorman et al, 1999;Tjernström and Grisogono, 2000;Brooks, 2001) -cold upwelling is observed along the whole West Coast, driven by Ekman pumping, but is enhanced in the lee of a headland by the strong horizontally divergent flow in the expansion fan. The colder SST results in increased stability within the lower BL but appears to have little impact on the dynamics of the flow Tjernström, 1999).…”

“…The NCAR C-130 Hercules has an extensive suite of instrumentation for the measurement of meteorological, turbulent, radiative, and microphysical variables; details can be found in Rogers et al (1998). All data are sampled at 25 Hz; turbulence quantities are calculated using this high-rate data, while mean quantities are derived from one second averages.…”

“…The parameterizations of surface fluxes are uncertain, particularly over the oceans where results derived from measurements over land remain largely unverified; as a consequence, numerical models tend to perform poorly for stable conditions (Nappo and Bach, 1997). Several recent studies have reported significant discrepancies between bulk estimates and directly measured surface fluxes under stable conditions (Rogers et al, 1998;Edson et al, 2000;Oost et al, 2000).…”

“…Eleven research flights were made by the National Centre for Atmospheric Research C-130 Hercules during June and early July 1996. An overview is given by Rogers et al (1998) and results have been presented in an extensive series of publications (Burk et al, 1999;Tjernström, 1999, Ström, 1999Dorman et al, 1999;Dorman and Winant, 2000;Tjernström and Grisogono, 2000;Edwards et al, 2001;Haack et al, 2001;Brooks, 2001;Söderberg and Tjernström, 2001). To date the analysis of aircraft data from CW96 and associated modelling studies have focused on the mesoscale dynamics and variability.…”

Global distribution and seasonal variability of coastal low-level jets derived from ERA-Interim reanalysis

“…Rogers et al, 1998;Rahn and Garreaud, 2010) as well as modelling and theoretical studies (e.g. Burk and Thompson, 1996;Mun˜oz and Garreaud, 2005).…”