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Abstract
Abstract. Coastal outflow describes the horizontal advection of pollutants from the continental boundary layer (BL) across a coastline. The outflow can ventilate polluted continental BLs and thus regulate air quality in highly populated coastal regions. This paper investigates the factors controlling coastal outflow and quantifies their importance as a ventilation mechanism. Tracers in the Met Office Unified Model (MetUM) are used to examine the magnitude and variability of coastal outflow over the eastern United States during summer 2004. Over the 4 week period examined, ventilation of tracer from the continental BL via coastal outflow occurs with the same magnitude as vertical ventilation via convection and advection. The relative importance of tracer decay rate, cross-coastal advection rate, and a parameter based on the relative continental and marine BL heights on coastal outflow is assessed by reducing the problem to a time-dependent box model. The ratio of the advection rate and decay rate is a dimensionless parameter which determines whether tracers are long-lived or short-lived. Long- and short-lived tracers exhibit different behaviours with respect to coastal outflow. Short-lived tracers exhibit large diurnal variability in coastal outflow but long-lived tracers do not. For short-lived tracers, increasing the advection rate increases the diurnally averaged magnitude of coastal outflow, but this has the opposite effect for very long-lived tracers. By using the box-model solutions to interpret the MetUM simulations, a land width is determined which represents the distance inland over which emissions contribute significantly to coastal outflow. A land width of between 100 and 400 km is found to be representative for a tracer with a lifetime of 24 h.
Highlights
Coastal outflow is a potentially important mechanism for the ventilation of continental boundary layers (BLs) and regulation of air quality in coastal regions
The tracer mass crossing the coast into the coastal outflow layer and marine BL (MBL) depends on the ratio of the height of the residual layer to the MBL height, γ = Hmax/HMBL, typically greater than 1
We introduce the concept of a representative land width as the width of the coastal strip with emissions that best explains the observed variability of coastal outflow using the box model
Summary
Coastal outflow is a potentially important mechanism for the ventilation of continental boundary layers (BLs) and regulation of air quality in coastal regions. A month-long mesoscale model simulation of summer 2004 over the eastern side of North America is used to investigate the relative importance of coastal outflow and vertical ventilation for pollutant levels within the continental BL. This suggests that while the export associated with localised convection or coastal outflow events may be small, over the whole summer period it plays an important role in the ventilation of pollutants from the BL They used the MOZART chemical transport model driven by National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analyses at 1.9 × 1.9◦ resolution, which is too coarse to capture mesoscale flows such as sea breeze circulations. Since both the synoptic and diurnal variability are represented in the simulation, it is reasonable to suppose that the variability in tracer transport can be simulated realistically
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