Impacts of afternoon and evening sea-breeze fronts on local
by Arrillaga (University of Madrid, Spain), Vila-Guerau de Arellano (University of Wageningen, NL), Bosveld (KNMI), Klein Baltink (KNMI), Yague (University of Madrid, Spain) Sastre (University of Madrid, Spain) Roman-Cascon (University of Grenoble Alpes, France)
We investigated sharp disruptions of local turbulence and scalar transport due to the arrival of sea-breeze fronts (SBF). To this end, we employed a comprehensive 10-yr observational database from the Cabauw Experimental Site for Atmospheric Research (CESAR, The Netherlands). Sea-breeze (SB) days were selected using a ﬁve-
ﬁlter algorithm, that 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 (∼250 m h-1) and the sudden increase in average wind speed (> 2 m s-1). 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-1 in the observed surface-layer wind proﬁle compared to the proﬁle calculated using the Monin-Obukhov Similarity Theory (MOST). Our ﬁndings furthermore reveal the determinant role of the SB direction in the transport of water vapour, CO2 and 222 Rn. 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 222Rn evolution may also be inﬂuenced by other non-local processes such as the large-scale footprint from more remote sources.
Arrillaga, Vila-Guerau de Arellano, Bosveld, Klein Baltink, Yague, Sastre and Roman-Cascon, Impacts of afternoon and evening sea-breeze fronts on local