High-Resolution Modeling of Atmospheric Dispersion and Turbulence Transport in the Coastal Marine Boundary Layer

Summary Due to the highly variable conditions in the marine boundary layer close to a coastline, accounting for the atmospheric deposition of nitrous compounds to the coastal ocean requires high resolution modeling of both the dispersion processes in the atmosphere and the deposition. For the deposition, no global theory exist for deposition in the case of heterogeneous meteorological conditions, although a simplified theory was suggested by Geernaert (2000, personal communication). The aim of this project is partly to study the dispersion of atmospheric gases in highly complex conditions, through high-resolution modeling, and to device a parameterization of the atmospheric deposition for use in coarser scale models. In this paper, we report on the initial attempts in these fields. High-resolution modeling of the dispersion of a passive tracer for the Kattegat using the US Navy Coupled Ocean/Atmosphere Mesoscale Prediction System - (COAMPS) model (Hodur 1997) illustrates the complexity of the flow in the strait between Sweden and Denmark. For summer conditions, the low-level flow in the strait favors a northerly or southerly flow even for background flow across the strait - a channeling effect due to the often stably stratified boundary layer over the water. Dispersion of a tracer that was continuously released at the surface over northern Jutland illustrates the fate of pollutants for a single day. The plume meanders strongly but remains unmixed and reaches the Swedish coast at different locations for different times of the day. Highly simplified simulations with a very high-resolution model were utilized to study the variability in the turbulent fluxes close to the coast. Near-shore variations in surface momentum flux of the order of 50% compared to that far off shore were found. It was also found that the distance off-shore at which the surface fluxes approached the free undisturbed conditions are of the order of ~100 km, thus the Kattegat is not wide enough to ever be treated as an open ocean – coastal effects will influence deposition all over this water body. Aim of the research This study is part of the MEAD project. The over-all aim of this project is to model the conditions that are favorable for harmful algae blooms in the Kattegat sea between the west coast of Sweden and Denmark. The aims of the current project are: 1. Study dry deposition in horizontally inhomogeneous conditions, using field experiment data and high-resolution meteorological models.