Spectral calculations of Rayleigh‐scattering optical depth at Arctic and Antarctic sites using a two‐term algorithm

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AbstractA two‐term algorithm is defined to evaluate the Rayleigh‐scattering optical depth (ROD) in the Arctic and Antarctic atmospheres at ultraviolet, visible, and near‐infrared wavelengths. The first term accounts for the tropospheric contribution up to 8 km altitude and the second for the upper troposphere and low stratosphere (UTLS) region and the upper atmosphere up to 120 km altitude. In these calculations, the vertical profiles of pressure p(z) and temperature T(z) are used, as derived from the multiyear sets of radiosounding measurements performed from 2000 to 2006 at four Arctic stations (Cambridge Bay, Resolute Bay, Danmarkshavn, and Alert) and four Antarctic stations (Neumayer, McMurdo, Dome C, and South Pole). By analyzing the radiosonde data sets collected at the eight sites, the daily average values of tropospheric ROD are calculated and their seasonal variations versus the mean tropospheric temperature and surface level temperature are determined in order to evaluate the temperature correction functions for tropospheric ROD. Similarly, the vertical profiles of p(z) and T(z) defined in the stratosphere and upper atmosphere are examined in order to (i) calculate the “stratospheric” ROD contribution, (ii) analyze its annual temporal variations, (iii) calculate the average temperature in the UTLS region, and (iv) determine the most suitable temperature anomaly corrections at the eight chosen polar sites. The spectral evaluations of the tropospheric and “stratospheric” ROD terms are given at 21 selected wavelengths from 0.30 to 4.00 µm for each polar site. The present two‐term algorithm incorporates the tropospheric and “stratospheric” ROD terms, along with the correction factors derived for the actual pressure and temperature conditions, and can be used to calculate accurate overall ROD values at the given Arctic and Antarctic sites for the average annual thermal conditions of the atmosphere and the average seasonal variations characterizing the temperature conditions of the UTLS region. The ROD estimates obtained using the present method constitutes a nontrivial improvement over estimates made with the traditional method.