Role of synoptic- and meso-scales in the momentum budget at a coastal environment: multi-year observational and modeling study

The contributions of synoptic- and meso-scales to the local momentum budget in a coastal environment is examined. The analysis is based on observations of the wind profile within the first 200 m of the atmosphere continously recorded during a 10 year period (2001 - 2010) at a meteorological tower at the Cabauw Experimental Site for Atmospheric Research (CESAR, The Netherlands); and a numerical experiment based on the Weather Research and Forecastiong (WRF) model performed at high horizontal resolution of 2 km and spanning the complete observational period. In analyzing the whole period, results indicate that WRF is able to reproduce the inter-annual wind variability but with a tendency to be too geostrophic. We find a differentiated behavior between Winter and Summer seasons being the Spring and Autumn transition seasons. The winter momentum budget shows a weak intradiurnal variability. The synoptic scale controls the shape of the near surface wind profile that is characterized by a weaker and more ageostrophic winds near the surface than at higher altitudes within the planetary boundary layer (PBL) as a result of the frictional turning. In turn, during summer, mesoscale circulations associated with the differential heating of land and sea become important. As a result, the PBL winds show a stronger intradiurnal component that is responsible for an oscillation of the near surface winds around the geostrophic direction with the maximum departure in the afternoon. The contribution of
this mesoscale component depends on the specific orientation of the coast (NE-SW at CESAR). Our findings stress the importance to evaluate and understand the performance of mesoscale models with multi-year observational/simulated data sets in order to provide a statistically robust characterization of the limitations of surface layer and boundary-layer parameterizations.