Spatial and temporal distribution of long-term short-wave surface radiation over Greece

摘要

The short-wave (SW) surface radiation budget (SRB, downward and absorbed fluxes) over Greece and surrounding areas has been computed for the first time, using a physical deterministic radiative-transfer model. The model computations were performed ona monthly-mean basis and at 2.5° x 2.5° latitude-longitude resolution over the 17-year period 1984-2000. Higher spatial resolution of 1° x 1° was achieved using the new NASA-Langley dataset but only for the shorter period 1985-1995. In the first case(2.5° x 2.5° resolution), the model input data were taken from global datasets, such as the International Satellite Cloud Climatology Project (ISCCP), the TIROS Operational Vertical Sounder (TOVS) or the Global Aerosol Dataset (GADS), as well as fromthe National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) and European Centre for Medium Range Weather Forecasts (ECMWF) Global Re-analysis Projects. In the second case (1° x 1° resolution), the model inputdata were taken from the NASA-Langley dataset supplemented by others (e.g., GADS). It is found that the annual mean values of the downward SW radiation at the surface (DSR) increase from about 150 W m~(-2) in the north of Greece to 210 W m~(-2) in the south. (Seasonal means are 50 and 100 W m~(-2), respectively, in the winter and 270 and 330 W m~(-2) in summer). The 1° x 1° DSR fluxes reveal significant geographical features, indicating an important longitudinal variation, with smaller values along the central mountain axis and in the northern part of the country. The annual mean DSR flux, averaged over the study area, is 181.9 ± 14.4 W m~(-2), calculated using the ISCCP-D2 (1984-2000) data or 188.8 ± 14.2 W m~(-2) using the NASA-Langley (1985-1995) data. The corresponding averaged mean surface absorbed fluxes are 164.7 ± 13.1 W m~(-2) and 170.7 ± 13.2 W m~(-2). Computed time series of annual mean solar radiation arriving at the surface, and a linear fit applied to them, show an increase with time which may possibly be related to climatic change. Time series of annual amplitude of DSR (maximum minus minimum monthly values) indicate a year-to-year variation of DSR of about 245 W m~(-2), whereas a linear fit shows an increase with time but no systematic increase in summer maxima or decrease in winter minima. The model-computed DSR fluxes are in good agreement with surface measurements, made at eight stations of the Hellenic National Meteorological Service and four other high-standard stations run by Greek research organizations, with correlation coefficients in the range 0.91-0.99 and standard deviations smaller than 20 W m~(-2). The ratio of direct to diffuse DSR (an indicator of clear skies) peaks during the summer months at a value of 1.5 inThessaloniki (northern Greece), at about three in Athens and nine in Crete.