اثر گازهای جوّی سه‌اتمی ‌بر تراز تابشی در منطقه کویری مرکز ایران

The main driving force of the earth atmospheric system is solar radiation. Radiation flux determines the surface temperature and impacts life on the Earth through photosynthesis. A quantitative knowledge of the earth radiation field is important to evaluate the atmosphere–surface interactions and the global hydrological cycle. Various atmospheric radiative transfer models have been proposed in order to compute radiation levels accurately. The Santa Barbara DISORT Atmospheric Radiative Transfer model (SBDART) is one of the plane-parallel multiple scattering radiative models which is used in this study. SBDART includes all important effective processes on the ultraviolet, visible, and infrared wavelengths of radiation. The model was developed as a software tool to compute radiative transfer in clear and cloudy conditions within the Earth’s atmosphere and at the surface. This model incorporates the DISORT discrete ordinate method, low resolution atmospheric transmission models and Mie scattering results for light scattering by water droplets and ice crystals. In order to use the model in the Middle East with its dry climate, we evaluated the model for estimation of the net surface radiative flux in the central desert of Iran. The observation data included latent heat, ground heat and net radiative flux during August to September 2006 in the region having the latitude of 32°N and longitude of 54°E. The data wed were from the heights of 1.5 m and 3 m. The evaluation of the SBDART on the 22 clear days during that period shows good agreement between the simulation of diurnal cycle of the net radiation flux and the observations. Since the model considers the ground temperature as a constant value, it is not able to capture the discrepancy of longwave radiation flux due to the significant difference between maximum and minimum temperature in the desert area.
We have modified SBDART by implementing the diurnal cycle of ground temperature in the model to improve the simulation of the surface net radiation flux. The modified SBDART is used to study the effects of changes in water vapor, ozone and carbon dioxide on the radiative flux at the surface and in the atmosphere. Calculations are carried out for both upward and downward fluxes of shortwave, longwave and net radiation.
To solve the radiative transfer equations, the required parameters are the optical thickness and asymmetry factor due to gaseous absorption, and Rayleigh scattering. They depend on the atmospheric profiles; total amount and distribution of water vapour, ozone, carbon dioxide and other gases and type and concentration of aerosols.
In order to quantify the water vapour effects on shortwave and longwave radiation fluxes, we ran the model for the atmosphere with water vapour content of 1 g/cm2 and a dry atmosphere at 00:30 and 12:30 UTC. The differences between shortwave radiation fluxes in wet and dry atmospheres (for August 26th) vary from 5% at noon to 25% at sunrise and sunset. Moreover, longwave radiation flux in the wet atmosphere exceeds the flux in the dry atmosphere at all levels and reaches the maximum value of 67% near the ground level. A similar test was done to determine the effects of doubling the typical carbon dioxide value (360 ppm) on the radiation flux. The results show that downward radiation flux was raised by 2% and the upward flux decreased by 10% due to the increased CO2 level. Results also show that the net radiation flux in the atmosphere is greater for the double CO2 case, with maximum difference of 2% occurring at an altitude of 10km.
Depletion of the stratospheric ozone as well as its increase in the troposphere cause significant impacts on UV irradiance and tropospheric chemistry. In this study, we have also addressed the effects of changes of ozone on the radiation fluxes, by doubling and halving the typical total columnar ozone concentration of 296 DU. Results show that the increased ozone decreases the upward shortwave radiation flux by 2.2% and the downward flux by 1%. The reduction of the ozone columnar concentration decreases the net radiation flux in the atmosphere above an altitude of 5 km, with the maximum decrease of 1.7% occurring at an altitude of around 30 km.
In general, the results of this study show that the net surface radiative flux is most sensitive to variations in the value of atmospheric water vapor, carbon dioxide and ozone, respectively. Changes in the atmospheric water vapor highly impact the net surface radiative flux, while those of carbon dioxide and ozone lead to changes in the net atmospheric radiative flux, particularly between 10 km and 30 km above the surface.