High-resolution regional climate simulations over Iceland using Polar MM5

High-resolution regional climate simulations of Iceland for 1991-2000 have been performed using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (PSU-NCAR) Mesocale Model (MM5) modified for use in polar regions (Polar MM5) with three nested domains and short-duration integrations. The simulated results are compared with monthly mean surface observations from Iceland for 1991-2000 to demonstrate the high level of model performance; correlation coefficients exceed 0.9 for most variables considered. The simulation results are used to analyze the near-surface climate over Iceland. The simulated near-surface winds in winter are primarily katabatic. The land-sea-breeze circulation is clearly evident in summer. The land is colder than the ocean during winter. with a strong (weak) temperature gradient along the southern (northern) coast. This temperature pattern over the sloping terrain forces the katabatic wind. The diurnal cycle of near-surface air temperature is marked in summer over the land areas, which drives the land-sea breeze. The near-surface climate variations for extremes of the North Atlantic Oscillation (NAO) index during winter and summer result from the large-scale atmospheric advection conditions. The time-averaged mesoscale precipitation distribution over Iceland is reasonably well simulated by Polar MM5. Winter precipitation rates are double those during the summer, reflecting the much greater winter cyclonic activity. The simulated interannual precipitation variations during winter for 1991-2000 agree with those observed from snow accumulation measurements on the Vatnajokull ice cap. The winter precipitation decrease for 1991-2000 dominates the annual signal for all of Iceland except the northeastern and eastern parts where the precipitation increases. The large precipitation trends (decadal decrease of up to 50%) are caused by the eastward shift and weakening of the Icelandic low during the 1990s, as a result of changes in the NAO modulation of regional climate.