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 intra-diurnal variability. The synoptic scale controls the shape of the near surface wind profile that is characterized by weaker and more a geostrophic 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 intra-diurnal 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.