Impacts of afternoon and evening sea-breeze fronts on local turbulence, and on CO2 and radon-222 transport

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We investigated sharp disruptions of local turbulence and scalar transport due to the arrival of sea-breeze fronts (SBFs). To this end, we employed a comprehensive 10-year observational database from the Cabauw Experimental Site for Atmospheric Research (CESAR, the Netherlands). Sea-breeze (SB) days were selected using a five-filter algorithm, which accounts for large-scale conditions and a clear mesoscale-frontal signal associated with the land-sea contrast. Among those days (102 in all, 8.3%), based on the value of the sensible-heat flux at the onset of SB, we identified three atmospheric boundary-layer (ABL) regimes: convective, transition and stable. In the convective regime, the thermally driven convective boundary layer is only slightly altered by a small enhancement of the shear when the SBF arrives. Regarding the transition regime, we found that the ABL afternoon transition is accelerated. This was quantified by estimating the contributions of shear and buoyancy to the turbulent kinetic energy. Other relevant disruptions are the sharp reduction in ABL depth (similar to 250 m/hr) and the sudden increase in average wind speed (> 2 m/s). In the stable regime, the arrival of the SB leads to disturbances in the wind profile at the surface layer. We observed a deviation of more than 1 m/s in the observed surface-layer wind profile compared with the profile calculated using Monin-Obukhov Similarity Theory (MOST). Our findings furthermore reveal the determinant role of the SB direction in the transport of water vapour, CO2 and Rn-222. The return of continental air masses driven by the SB circulation generates sharp CO2 increases (up to 14 ppm in half an hour) in a few SB events. We suggest that the variability in Rn-222 evolution may also be influenced by other non-local processes such as the large-scale footprint from more remote sources.
©2018 Wiley This project was funded by the Spanish government project CGL2015-65627-C3-3-R (MINECO/FEDER). Jon A. Arrillaga is supported by the Predoctoral Training Program for Non-Doctorate Researchers of the Department of Education of the Basque government (PRE_2016_2_0160, MOD = B). The first author developed part of the research during a visit to Wageningen University, supported by a EGONLABUR mobility grant from the Basque government (EP_2016_1_0048). We thank the Royal Netherlands Meteorological Institute (KNMI) for the meteorological data from Cabauw. We are also grateful to the Energy Research Centre of the Netherlands (ECN) for the CO2 and 222Rn data and we also thank OSI-SAF for the satellite full-resolution Metop data provided.
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