Role of synoptic- and meso-scales on the evolution of the boundary-layer wind profile over a coastal region: the near-coast diurnal acceleration

The contributions of synoptic- and meso-scales to the boundary layer wind profile evolution in a coastal environment are examined. The analysis is based on observations of the wind profile within the first 200 m of the atmosphere continuously recorded during a 10 year period (2001–2010) at the 213-m meteorological tower at the Cabauw Experimental Site for Atmospheric Research (CESAR, The Netherlands). The analysis is supported by a numerical experiment based on the Weather Research and Forecasting (WRF) model performed at high horizontal resolution of 2 km and spanning the complete observational period (10 years). Results indicate that WRF is able to reproduce the inter-annual wind variability but with a tendency to be too geostrophic. At seasonal scales, we find a differentiated behavior between Winter and Summer seasons with the Spring and Autumn transition periods more similar to the Summer and Winter modes, respectively. 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 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 characterized by an oscillation of the near surface winds around the geostrophic direction with the maximum departure in the afternoon. Although also driven by thermal land-sea differences, this mesoscale component is not associated with the classical concept of a sea-breeze front. It originates from the thermal expansion of the boundary layer over land and primarily differs from the sea-breeze in its propagation speed resulting in a wind rotation far ahead of any coastal front. We refer to it as the near-coast diurnal acceleration (NCDA). The contribution of the NCDA depends on the specific orientation of the coast (NE-SW at CESAR). Our findings stress the importance of evaluating and understanding 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 and thus compensate for the scarceness of upper level wind observations.