Dependence of the upward terrestrial radiance within the (3.5–4.0) μm spectral range on thermodynamic and composition parameters of the atmosphere

Calculations of the upward infrared radiance reaching outer space within the (3.5–4.0) μm wavelength range were carried out to define the dependence features of the signals measured by radiometers onboard satellites on the temperature, moisture and composition parameters of the atmosphere. In particular, simulations of upwelling radiance were performed for application to the measurements taken by the AVHRR radiometers mounted aboard the NOAA-7 to NOAA-16 satellites and the SEVIRI instrument mounted aboard the Second Generation Meteosat (MSG-1) satellite launched in 2002. The calculations were made using a modified version of computer code LOWTRAN 7 for a large set of atmospheric vertical profiles of temperature and humidity parameters, each one represented with a set of 324 isothermal layers from sea-level to 100 km height and derived from one of 23 atmospheric models relative to different latitudes and seasons. For all these atmospheric configurations, we determined the temperature deficit ΔT, as given b ythe difference between the surface temperature and apparent emission temperature of the surface, the latter quantity being obtained in terms of black-body emission theory from the satellite measurement of upward radiance. Parameter ΔT was found to depend mainly on the total atmospheric content of water vapour and the shape of the vertical profile of temperature within the ground layer: it was found to vary considerably passing from cases of marked thermal inversions to cases of adiabatic or superadiabatic temperature gradients. Considering sets of atmospheric models where precipitable water was assumed to remain constant, ΔT was found to decrease appreciably as the temperature gradient increases from negative values (in the presence of thermal inversions) to positive ones (for adiabatic and superadiabatic lapse rates at the ground), presenting negative slopes that become gradually more marked as the ground layer depth increases. Considering sets of atmospheric models where the moisture parameters were assumed to vary widely, ΔT was found to change linearly as a function of precipitable water, with slope coefficients varying slowly from positive to negative values, as the temperature gradient increases from negative to positive values. As a consequence of these dependence features, the ratio between ΔT and precipitable water was found to increase as a function of surface temperature, following patterns closely best-fitted by second-order polynomial curves. Dependence features of ΔT on the mean atmospheric concentrations of methane and nitrous oxide were also determined. Similarly, ΔT was found to vary linearly as a function of aerosol optical thickness in the visible for polydispersions of maritime, rural, urban and tropospheric aerosols, presenting the most marked slope in the case of maritime aerosols. An overall procedure is proposed for calculating ΔT, taking into account i) the latitudinal and seasonal conditions of the atmosphere, ii) estimates of ground-level temperature and precipitable water, as derived from satellite and/or ground-based measurements of meteorological parameters, iii) aerosol optical thickness at visible wavelengths, and iv) CH4 and N2O atmospheric concentrations