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

Abstract

[1] A large set of radiosoundings was analyzed to study the dependence of Rayleighscattering 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 CO 2 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 mm 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 p a and air temperature T a were defined. The dependence of ROD on the daily ground-level values of air pressure p o and air temperature T o measured at each site can be accounted for by using an algorithm in which the pressure dependence is given with good approximation by ratio (p o /p a ), and the temperature linear dependence is expressed by the difference (T a À T o ) multiplied by a spectral slope coefficient k(l) 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.

Seasonal Variations of the Relative Optical Air Mass Function for Background Aerosol and Thin Cirrus Clouds at Arctic and Antarctic Sites

“…The rather low values of e(z) correspondingly found usually in both polar atmospheres for cloudless conditions exert a weak influence on n(z). Conversely, appreciable variations in the monthly mean temperature profiles are observed during the local summer months at all the Arctic and Antarctic sites, in agreement with the results by Tomasi et al [24]. The monthly mean vertical profiles of T(z) obtained from the radiosounding datasets at the eight polar sites are presented in Figure 1, showing that T(z) gradually decreases with altitude until reaching a minimum at the tropopause level found between 8 and 10 km.…”

“…As shown by Tomasi et al [24], the monthly mean vertical profiles of pressure p(z) regularly decrease in an exponential fashion as a function of altitude over the whole range, while relative humidity decreases in general with height until reaching values of a few percent at the tropopause level and low stratosphere altitudes [25]. The rather low values of e(z) correspondingly found usually in both polar atmospheres for cloudless conditions exert a weak influence on n(z).…”

“…The radiosounding datasets were based on measurements made at Ny-Ålesund (~79°N) during June-July of 2000-2003 [24], and at Mario Zucchelli (~75°S) during the austral summer season from 1987 to 1998 [25]. Above 30 km altitude, the profiles were completed with (i) monthly mean vertical profiles of the three thermodynamic parameters obtained by Tomasi et al [31] from multi-year Michaelson Interferometer for Passive Atmospheric Sounding (MIPAS)-Environmental Satellite (ENVISAT) limb-scanning measurements over the 33-60 km altitude range at 80°N in July and at 75°S in January, and (ii) the vertical profiles p(z) and T(z) defined in the AFGL Subarctic Summer model over the 70-120 km altitude range [27].…”

“…A set of appropriate values of m was calculated by Tomasi and Petkov [22] (hereinafter referred to as TP2014) at Arctic and Antarctic sites over the 0° ≤ θ ≤ 87° range. The dependence of air refractive index on the vertical profiles of pressure p(z), temperature T(z) and water vapor partial pressure e(z) in the polar atmospheres was taken into account by using the algorithm of Tomasi et al [23] applied to the average vertical profiles of p(z), T(z) and e(z) derived from the radiosounding data collected at Ny-Ålesund (~79°N) in Spitsbergen (Svalbard) during June and July in the years from 2000-2003 [24], and from those recorded at Mario Zucchelli (~75°S) during the austral summer season from 1987 to 1998 [25]. Table 1.…”

Analysis of a 4 year radiosonde data set at Dome C for characterizing temperature and moisture conditions of the Antarctic atmosphere

“…This was done by multiplying the daytime RH data by the SH dry bias correction factor F SH calculated by adjusting the C08 midlatitude algorithm to the Antarctic atmosphere conditions of Dome C. The original algorithm of C08 yields the difference F SH − 1 as a function of solar zenith angle SZA over the range 54° ≤ SZA ≤ 86°, in the following general form: where coefficient α = 0.067 was assumed to represent the effect of solar heating on the RS80 Humicap sensor, and the effective optical depth τ eff of the atmosphere was kept equal to 0.2 in the midlatitude atmosphere, over the entire short‐wave spectrum. To adapt to the Antarctic atmosphere conditions, a realistic value of τ eff = 0.1 was assumed in place of 0.2, since it is given by the sum of (1) the Dome C yearly average value of Rayleigh‐scattering optical depth equal to 0.091 at 0.50 μ m wavelength [ Tomasi et al , 2010] (in place of the sea‐level value of 0.143 relative to the midlatitude standard atmosphere [ Tomasi et al , 2005]) and (2) the summer average aerosol optical depth at Dome C equal to 0.03 at visible wavelengths [ Tomasi et al , 2007]. The A‐Humicap algorithm defined in for τ eff = 0.1 provides a value of the percentage SH dry bias equal to 7.5% RH for SZA = 62°.…”