Ten-year global distribution of downwelling longwave radiation

Abstract

Abstract. Downwelling longwave fluxes, DLFs, have been derived for each month over a ten year period (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993), on a global scale with a spatial resolution of 2.5×2.5 degrees and a monthly temporal resolution. The fluxes were computed using a deterministic model for atmospheric radiation transfer, along with satellite and reanalysis data for the key atmospheric input parameters, i.e. cloud properties, and specific humidity and temperature profiles. The cloud climatologies were taken from the latest released and improved International Satellite Climatology Project D2 series. Specific humidity and temperature vertical profiles were taken from three different reanalysis datasets; NCEP/NCAR, GEOS, and ECMWF (acronyms explained in main text). DLFs were computed for each reanalysis dataset, with differences reaching values as high as 30 Wm −2 in specific regions, particularly over high altitude areas and deserts. However, globally, the agreement is good, with the rms of the difference between the DLFs derived from the different reanalysis datasets ranging from 5 to 7 Wm −2 . The results are presented as geographical distributions and as time series of hemispheric and global averages. The DLF time series based on the different reanalysis datasets show similar seasonal and inter-annual variations, and similar anomalies related to the 86/87 El Niño and 89/90 La Niña events. The global ten-year average of the DLF was found to be between 342.2 Wm −2 and 344.3 Wm −2 , depending on the dataset. We also conducted a detailed sensitivity analysis of the calculated DLFs to the key input data. Plots are given that can be used to obtain a quick assessment of the sensitivity of the DLF to each of the three key climatic quantities, for specific climatic conditions corresponding to different regions of the globe. Our model downwelling fluxes are validated against available data from ground-based stations distributed over the globe, as given by the Baseline Surface Radiation Network.Correspondence to: I. Vardavas (vardavas@iesl.forth.gr) There is a negative bias of the model fluxes when compared against BSRN fluxes, ranging from −7 to −9 Wm −2 , mostly caused by low cloud amount differences between the station and satellite measurements, particularly in cold climates. Finally, we compare our model results with those of other deterministic models and general circulation models.

Variability in clear‐sky longwave radiative cooling of the atmosphere

“…Soden [2000] found an interannual variability range in tropical mean surface longwave flux greater than 10 W m À2 which was ascribed to boundary layer cloud. However, changes in downwelling longwave flux at the surface estimated by Pavlakis et al [2004] for all-sky conditions do not show a larger variability than those inferred from the present study for clear skies and at present agreement between satellite estimates of changes in cloudiness has not been established [Wielicki et al, 2002;Trenberth, 2002;Norris, 2005;Wylie et al, 2005]. Since changes in cloudiness are inextricably linked to precipitation it will be important to assess the clear and cloudy sky contribution to atmospheric radiative cooling in the future.…”

Annual cycle of aerosol direct radiative effect over southeast Italy and sensitivity studies

“…In summary, Figure 16 reveals that simulated and experimental net flux values (grey and black solid line, respectively) are in good accordance, especially during spring and summer when Lecce appropriate ground temperatures are used. Recent sensitivity studies by Pavlakis et al [2004], on downwelling longwave fluxes at the surface, have also revealed that their model was able to reproduce the observed downwelling longwave flux at a specific station very well, by using local meteorological data. In particular, they have found that the downwelling longwave flux can increase in some regions by up to 11 W/m 2 for a 2 K temperature, in accordance with the results of this paper.…”

Global atmospheric downward longwave radiation at the surface from ground‐based observations, satellite retrievals, and reanalyses

“…However, satellites have difficulty in providing temperature and humidity profiles of the lower atmosphere with the accuracy required for estimation of L d [Ellingson, 1995;Wang and Liang, 2009c]. Therefore, satellite cloud products and temperature and humidity estimates from atmospheric analysis or reanalysis systems have been combined to calculate L d [Gupta et al, 1999;Gupta et al, 2010;Nussbaumer and Pinker, 2012;Pavlakis et al, 2004;Schmetz et al, 1986;Zhang et al, 2004]. Some satellite algorithms have empirically related satellite-derived total water vapor amount to L d [Naud et al, 2012;Zhou et al, 2007].…”