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 nearsurface 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 Vatnajökull 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.
Possible dynamical influences on the diurnal behavior of ozone are investigated. A time dependent one‐dimensional photochemical model is developed for this purpose. All model calculations are made at 70°N during summer, because of available data on turbulence and zonal winds at Poker Flat, Alaska, and ozone observations from the Solar Mesosphere Explorer (SME) satellite. The model includes vertical diffusion induced by breaking gravity waves, as parameterized by Lindzen (1981). We show that the vertical diffusion can vary as much as 1 order of magnitude within a day as a result of large changes in the zonal wind induced by atmospheric thermal tides. It is found that by introducing a dissipation time scale for turbulence produced by breaking gravity waves, the agreement with Poker Flat echo data is improved. Comparisons of results from photochemical model calculations where the vertical diffusion is a function of height only with those in which Kzz is changing in time show large differences in the diurnal behavior of ozone between 70 and 90 km. By including the dynamical effect, much better agreement with the SME data is obtained. The results are, however, sensitive to the background zonally averaged wind. The influence of including time‐varying Kzz on the OH densities is also large, especially between 80 and 90 km. This suggests that dynamical effects are important in determining the diurnal behavior of the airglow emission from the Meinel bands.
Daily solar irradiance measurements in the spectral interval 120‐305 nm have been made since 6 October 1981 with an instrument on the Solar Mesosphere Explorer. The instrument operates with a spectral resolution of about 0.75 nm. Analysis of the observed data for the period 6 December 1981 to 3 June 1983 (20 solar rotations) shows that during this period there was an apparent decrease in irradiance at all wavelengths observed (−19.7% ± 9.7% at Ly‐α) but the decrease was not significantly different from zero at wavelengths longer than 210 nm. The cross correlations between daily values of the solar irradiance and 10.7 cm flux varied from 0.7 (Ly‐α) to 0.5 (210‐215 nm) and ∼0 (290‐295nm). Calculations of the % range (i.e., highest to lowest value) of the irradiance within each solar rotation showed that for Ly‐α the range varied between 6% and 30% over the 20 solar rotations studied. At longer wavelengths the % range was smaller—about 7% at 180 nm and about 2% beyond 240 nm. The percent range values indicate representative variations useful as input data for model calculations of stratosphere/mesosphere responses to short period solar variability.
Total ozone measurements using a Dobson spectrophotometer have been performed on a regular basis at Reykjavík (64°08′N, 21°54′W), Iceland, since 1957. The data set for the entire period of observations has been critically examined. Due to problems related to the calibration of the instrument the data record of ozone observations is divided into two periods in the following analysis (1957–1977 and 1977–1990). A statistical model was developed to fit the data and estimate long‐term changes in total ozone. The model includes seasonal variations, solar cycle influences, quasi‐biennial oscillation (QBO) effects, and linear trends. Some variants of the model are applied to investigate to what extent the estimated trends depend on the form of the model. Trend analysis of the revised data reveals a statistically significant linear decrease of 0.11 ± 0.07% per year in the annual total ozone amount during the earlier period and 0.30 ± 0.11% during the latter. The annual total ozone decline since 1977 is caused by a 0.47 ± 0.14% decrease per year during the summer with no significant change during the winter or fall. On an annual basis, ozone varies by 3.5 ± 0.8% over a solar cycle and by 2.1 ± 0.6% over a QBO for the whole observation period. The effect of the 11‐year solar cycle is particularly strong in the data during the early months of the year and in the westerly phase of the QBO. The data also suggest a strong response of total ozone to major solar proton events. Comparisons of the total ozone data made at Reykjavík during the last 13 years with total ozone as measured by the total ozone mapping spectrometer (TOMS) on the Nimbus 7 satellite show an excellent agreement for solar zenith angles less than 80° at times of satellite overpass. Comparisons with results of the zonally averaged TOMS data at this latitude suggest a significant longitudinal dependence of the linear trends in stratospheric ozone. This is in accordance with recent analysis of the satellite data.
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