I. Vardavas

Shortwave Radiation Budget of the Southern Hemisphere Using ISCCP C2 and NCEP–NCAR Climatological Data

Abstract The seasonal shortwave radiation budget at the surface of the Southern Hemisphere was estimated, for 10° latitudinal zones, using a radiative transfer model with long-term mean monthly climatological data from the International Satellite Cloud Climatology Project (ISCCP; 1983–90) supplemented by data from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR). The computed radiation budget of the Southern Hemisphere is given at both top of the atmosphere (TOA) and at the surface and for each month. The model radiative fluxes at TOA were validated against Earth Radiation Budget Experiment (ERBE) scanner S4 satellite data (1984–89). Initially, large deviations were found for TOA fluxes in the south polar regions, where ISCCP cloud cover data are less certain. The cloud cover was thus modified in the polar regions using ground-based observations and better agreement was obtained. The surface radiative fluxes have been intercompared with other model res...

Aerosol shortwave direct radiative effect and forcing based on MODIS Level 2 data in the Eastern Mediterranean (Crete)

A spectral radiative transfer model was used to quantify the aerosol direct radiative effect and forcing over the island of Crete in the Eastern Mediterranean. Computations were performed for the 11-year period from 2000 to 2010. MODIS Level 2 data (daily, 10 km × 10 km spatial resolution) were used as input to the model. Output includes the radiative fluxes and the aerosol direct radiative ffect at the top of the atmosphere, within the atmosphere and at the surface. The corresponding forcing components were evaluated based on MODIS fine mode aerosol data. Results show a decreasing trend of the aerosol radiative effect. The analysis of the contribution of anthropogenic and natural aerosols shows major peaks of natural aerosol effects occurring mainly in spring and autumn, while a summer maximum is attributed to anthropogenic aerosol.

Longwave radiation budget of the southern hemisphere using ISCCP C2 climatological data

The longwave radiation budget at the top of the atmosphere (TOA) and at the surface of the Southern Hemisphere was estimated for 10° latitudinal zones, using a radiative transfer model and long-term mean monthly climatological data from the International Satellite Cloud Climatology Project (ISCCP) from 1983 to 1990. The model radiative fluxes at TOA were validated against Earth Radiation Budget Experiment (ERBE) scanner S4 satellite data (1984-1989) while at the surface they have been compared with other model results and available surface measurements. Large discrepancies were found between model and ERBE TOA fluxes over Antarctica that were resolved by increasing the winter polar cloudiness significantly. Generally, our model TOA fluxes agree within the standard deviation of the ERBE data. Important differences from previous works are also found in the polar regions, in terms of magnitudes, and seasonal/latitudinal trends in surface fluxes. The mean annual and hemispherical outgoing longwave radiation at TOA. upward longwave radiation, downwelling, and net upward longwave radiation at the surface, are computed to be 236, 387, 341, and 46 W m -2 , respectively.

The net radiation budget of the northern hemisphere

A radiative transfer model was used together with mean monthly long-term climatological data to infer the net radiation budget of the Northern Hemisphere for latitudinal zones of 10° width. Most of the data were taken from the International Satellite Cloud Climatology Project and supplemented by data from the collaboration between National Centers for Environmental Prediction and National Center for Atmospheric Research, for the vertical profiles of water vapor and temperature. Seasonal and latitudinal variations of net shortwave, longwave, and all-wave radiative fluxes and cloud radiative forcings were estimated at the top of the atmosphere (TOA) and at the Earths surface, and also radiative cooling/heating rates, together with their mean annual and mean hemispherical variations. The validation of the net TOA all-wave fluxes is based on a comparison with 5 years mean Earth Radiation Budget Experiment data for the shortwave and longwave fluxes in two of our previous studies. The net all-wave fluxes were found to be mainly determined by the net shortwave fluxes. Near radiative equilibrium was found on a mean annual basis for net all-wave radiation at TOA, while at the surface a gain of 99 W m -2 was found for the hemisphere. The estimated net all-wave forcings generally decrease from equator to pole, with local maxima in middle to high latitudes, and from summer to winter. The net shortwave cloud forcings were found to dominate the net longwave ones, by a factor of about 2. Mean annual hemispherical cloud forcings equal to 25 W m -2 and 13 W m -2 were computed at TOA and at the surface, respectively. A net all-wave atmospheric radiative cooling was computed, which decreases from equator to pole, with a local maximum in the subtropics, and from winter to summer, having a mean annual hemispherical value of 97 W m -2 . For all fluxes, forcings, and cooling/heating rates, detailed comparisons with the work of other authors were performed, and the main discrepancies are discussed.

Direct Effect of Aerosols on Solar Radiation Over the Eastern Mediterranean Basin on a Daily 1° by 1° Resolution

The climatically sensitive Mediterranean basin, which is characterized by high solar radiation amounts and aerosol loadings, is the focus of this study that aims to determine the direct effect of aerosols on solar radiation (DRE) over the eastern basin. The objective is to reveal detailed patterns of aerosol DRE that are smoothed when obtained at coarse resolution. Therefore, computations of DRE are performed at a concurrent spatial and temporal resolution that has not been achieved to date. The DREs are computed for 1×1° latitude-longitude grids with the FORTH detailed spectral radiative transfer model (RTM) using daily input data for various atmospheric and surface parameters, such as clouds, water vapor, ozone and surface albedo, taken from the NASA-Langley Global Earth Observing System database. Key aerosol optical properties, namely aerosol optical depth, single scattering albedo and asymmetry parameter, necessary for the RTM runs, are taken from combined climatologies, like the Global Aerosol Data Set, and the satellite-derived datasets of Total Ozone Mapping Spectrometer and Advanced Very High resolution Radiometer that satisfy daily availability at the RTM required spectral and 1×1° resolution. The aerosol DREs are computed at the surface, the top-of-atmosphere and within the atmosphere, over the period 1985–1995.

Study of the Total and Ultraviolet Solar Radiation in the Alpine Region of Ioannina (Northwestern Greece)

The FORTH deterministic spectral radiative transfer model is used to estimate the incoming surface solar radiation (ISSR) at the alpine city of Ioannina in northwest Greece. The model is initialized with high spatial (10 km × 10 km and 5 km × 5 km) and temporal (daily) resolution satellite Level-2 Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) data for aerosol, cloud and other atmospheric and surface parameters. The daily total and ultraviolet (UV) solar radiation is computed with the model for the period 2008–2012. The model ISSR is successfully validated against corresponding ground-based measurements of total and UV radiation from a station located at Ioannina for the study period. The instantaneous total, UVA and UVB ISSR values are as high as 1070, 45 and 2.25 W/m2 in summer, exhibiting a significant day to day and seasonal variability, primarily driven by the top of atmosphere solar radiation and clouds. The primary role of clouds for ISSR, versus aerosols and ozone, is also verified by model sensitivity tests, which reveal however an increasingly stronger role of these two parameters for UV radiation.

Cloud Radiative Effects on Solar Radiation Over the Eastern Mediterranean Basin from 1984 to 2007

The cloud radiative effect (CRE) on solar shortwave (SW) radiation is investigated over the broader Mediterranean basin, for the 24-year period from 1984 to 2007, using a deterministic spectral radiation transfer model (RTM) and quality monthly cloud data taken from the ISCCP-D2 satellite database. The model simulates the interaction of solar radiation with all relevant physical parameters, namely ozone, carbon dioxide, methane, water vapour, aerosols and atmospheric molecules as well as surface reflection. The ISCCP cloud model input data include cloud cover and optical depths for low-, mid-, and high-level clouds, separately for ice and liquid water clouds. The model computes CREs at the top of atmosphere (CRETOA), within the atmosphere (CREatmab), and at the surface (effects on downwelling and absorbed solar radiation, CREsurf and CREsurfnet). The determined perturbation of regional SW radiation budget by clouds is important, undergoing significant intra-annual and inter-annual changes.

Day to Day Variability of Air Temperature over Greece for the Period 1957–2002

In the ongoing discussion of global warming and current climatic changes, an open issue is the increased variability of climatic parameters and the changing frequency and intensity of their extreme values. In this framework, the present work deals with the day-to-day variability of maximum (ΔTmax) and minimum (ΔTmin) air temperature and diurnal temperature range (Δ(DTR)) over the Greek area for the period 1957–2002. The data used are daily values from ten stations of the National Hellenic Meteorological Service (N.H.M.S.) which are uniformly distributed over the study area. The absolute values of the considered parameters, for each station, are analyzed in terms of intra-annual and inter-annual variation, whereas the existence of statistically significant trends on a seasonal and annual basis is also examined. The analysis reveals stations with common behaviour regarding temperature variability mainly determined by their geographical location and other geophysical characteristics. In many cases, especially for day-to-day variability of ΔTmin and Δ(DTR) there appear statistically significant trends, mainly increasing, either on a seasonal basis or annual.

Reply to comment by Kara and Barron on “Seasonal heat budgets of the Red and Black seas”

] We appreciate the chance to present our views on thecomment by Kara and Barron [2008] and contribute to thediscussion that began. The criticism raised by Kara andBarron [2008] focuses onthree issues: The first issue, whichalso draws most of their attention, is the representativenessof the coarse-resolution reanalysis used in our original paper[Matsoukas et al., 2007]. The second issue is our use ofmonthly temporal resolution, instead of, e.g., 6-hourlyproducts. Finally, the third issue deals with the calculationof the exchange coefficients in the parameterization of thelatent and sensible heat fluxes. In this Reply, we will try toaddress each issue separately, and then give our generalopinion on the Conclusions of Kara and Barron [2008].[