High spatial and temporal variability of dry deposition in a coastal region

A real meteorological situation characterized by strong spatial and temporal variability of the meteorological fields in a coastal region of eastern Denmark is examined in view of the transport of a passive tracer and dry deposition. Model simulations using a full mesoscale NWP model (COAMPS TM ) at different horizontal resolutions are per- formed. A realistic simulation showed that the differences in the amount of dry deposited matter can reach one order of magnitude and larger, during the period of one afternoon, depending on the model horizontal resolution. In addition, the results of an idealized experiment with straight coastline indicate that the horizontal model resolution alone is responsible for most of the differences. This study confirms the importance of high spatial and temporal resolution modelling for environmental applications. Repeating occurrences of harmful algae blooms and their economic consequences have recently received scientific interest in the area of the Kattegat strait (1), Figure 1. The project named MEAD (Marine Effects of Atmospheric Deposition; see (2)) was organized to concentrate on this particular issue. In summer time, when the riverine input of the nutri- ents is small, the atmospheric deposition is expected to play an impor- tant role in supplying the nutrients, needed for growth of phytoplankton and, in extreme events, its blooms. The abundant sources of ammonia at Jutland (northern Denmark) combined with prevailing western winds pres- ent favourable conditions for nutrification of the Kattegat. In order to study such a particular issue one should first recognize the general coastal problem, which is high spatial and temporal variability of the atmosphere. Due to relatively fast rates of deposition of ammonia, large gradients in the local dry deposition in the vicinity of sources are known to occur