Characterizing polar atmospheres and their effect on Rayleigh‐scattering optical depth

Abstract

A large set of radiosoundings was analyzed to study the dependence of Rayleigh‐scattering optical depth (ROD) on pressure and temperature features of the polar clear‐sky atmosphere. This set consists of 1320 radiosoundings, launched throughout the year at six Arctic sites, and of 940 radiosoundings launched at five Antarctic sites. The vertical profiles of pressure, temperature, and relative humidity given by these radiosoundings were corrected for lag errors and dry biases and then completed up to 120 km altitude, using COSPAR International Reference Atmosphere (CIRA) monthly vertical profiles of pressure and temperature, water vapor mixing ratio data derived from various satellite observations, and standard CO2 vertical profile relative to 2007, when the ground‐level concentration was 380 ppmv. Calculations of the volume Rayleigh‐scattering coefficient were made for all the radiosoundings to study its seasonal variations with height due to pressure and temperature changes occurring in the troposphere and stratosphere. It was found that the surface‐level pressure and temperature conditions can account to a large extent for these seasonal effects. Therefore, average spectral evaluations of ROD were made at 88 selected wavelengths from 0.20 to 4.0 μm for all the radiosoundings, subdivided into eight latitude/altitude site classes, representing 70°N, 75°N, 80°N, 70°S, 75°S, 80°S, and 90°S latitudes, for which the average ground values of air pressure pa and air temperature Ta were defined. The dependence of ROD on the daily ground‐level values of air pressure po and air temperature To measured at each site can be accounted for by using an algorithm in which the pressure dependence is given with good approximation by ratio (po/pa), and the temperature linear dependence is expressed by the difference (Ta − To) multiplied by a spectral slope coefficient k(λ) which varies by site classification. This algorithm was estimated to provide values of ROD with accuracy within ±0.5% at the six sea‐level Arctic sites, ±0.5% at the three Antarctic coastal sites, and ±0.7% at the two Antarctic Plateau sites (Dome C and South Pole). When used to analyze the Sun photometer measurements, the present evaluations of ROD are estimated to provide aerosol optical depth values at visible wavelengths with relative errors of 1%–2% at the Arctic sites, 1%–4% at the coastal Antarctic sites, and 3%–13% at the Antarctic Plateau sites, for background aerosol extinction conditions.