The Impact Of The Lower Troposphere, The Atmospheric Aerosols On The Radiation Intensity And Color Of The Sky Of Twilight

The radiance and color of the twilight sky were calculated by use of the pseudo-spherical model of DISORT (Discrete-Ordinate-method) radiative transfer software package-UVSPEC. The absorption of H2O, CO2, O3 and multiple-scattering were taken into account in the radiative transfer model. The computation employed the U.S. Standard Atmosphere 1976 for the wavelength range from 300nm to 850nm. DISORT selected 16 streams. Meteoric dust extinction was selected in 30~ 100km, background stratospheric extinction was selected in 10~30km, tropospheric extinction provided by UVSPEC was selected in 2~10km. However, the aerosol extinction in 0~2km was related to surface visibility. Radiances of four Aerosol Optical Depths (AOD) of 550nm were calculated respectively. The variations of intensity and color index (7C) during twilight at different solar zenith angles (SZA) and view elevation angles with AODs in lower atmosphere were investigated. The results showed that the red /blue color index during twilight changed evidently with AODs in lower atmosphere. When AOD was small, there was a maximum of the radiance at 450nm in the high view elevation angle. So the sky color at the zenith is blue. However, the maximum shifted to red wavelength (about 700nm) with the fall of view elevation angle. When the AOD was rather big, the radiances decreased evidently and the variations of radiance with the wavelength were not obvious. So the sky color is dull. There was a monotone relation between the color index and the AOD when the view elevation angles range from 5 degree to 15 degree and the AOD was less than 1.5. Thus, in principle, the proposed color index would be used to measure and retrieve the AOD in lower atmosphere.