Temperature variability over the Po Valley, Italy, according to radiosounding data

Temperature variations registered above the southeast part of the Po Valley, Italy have been examined by applying the principal component analysis of radiosoung profiles recorded during the period from 1987 to 2010. Two data sets, considered to describe intra- and inter-annual oscillations, respectively were extracted from the measurements data and the results show that both types of fluctuations can be projected onto four empirical orthogonal functions (EOF), interpreted as vertical distributions of oscillation amplitudes and four uncorrelated time series that represent the evolution of corresponding EOFs. It was found that intra-annual oscillations composed of periods between 30 and 120 days, together with inter-annual variations of 1- to 7-year period contribute to the highest extent (about 70 percents) the temperature oscillations up to 20 km, changing in both cases the phase in the tropopausal region. The other three EOFs indicate prevailing weight of the oscillations in the upper troposphere-low stratosphere region and are characterized by longer periods in both types of fluctuations. The intra-annual variations can be accounted for an interaction between Madden-Julian and Arctic oscillations, while the spectral features of inter-annual fluctuations could be associated with those of Quasi Biennial, El Nino and North Atlantic global oscillations

The two-layered radiative transfer model for snow reflectance and its application to remote sensing of the Antarctic snow surface from space

The two-LAyered snow Radiative Transfer (LART) model has been proposed for snow remote sensing applications. It is based on analytical approximations of the radiative transfer theory. The geometrical optics approximation has been used to derive the local snow optical parameters, such as the probability of photon absorption by ice grains and the average cosine of single light scattering in a given direction in a snowpack. The application of the model to the selected area in Antarctica has shown that the technique is capable of retrieving the snow grain size both in the upper and lower snow layers, with grains larger in the lower snow layer as one might expect due to the metamorphism processes. Such a conclusion is confirmed by ground measurements of the vertical snow grain size variability in Antarctica

A Refined Calibration Procedure of Two-Channel Sun Photometers to Measure Atmospheric Precipitable Water at Various Antarctic Sites

Abstract Two-channel sun photometers can be easily employed at Antarctic sites, where harsh environmental conditions prevail, to carry out measurements of precipitable water W. In the very dry air conditions observed in the Antarctic atmosphere, water vapor does not produce strong absorption features along the sun path. Therefore, these instruments need to be calibrated using analytical forms different from the square root regime, which can be determined by simulating the output voltages measured at Antarctic sites, for the spectral near-IR curves of extraterrestrial solar irradiance, instrumental responsivity parameters, and atmospheric transmittance, relative to various measurement periods. For this purpose, average models of the Antarctic atmosphere from the ground level up to the 30-km altitude were considered for different solar zenith angles and relative humidity conditions. The ratios between the output voltages simulated in the band and window channels were plotted as a function of total water vapor content Cw, for each site and each period, to define the best-fit calibration curves, which were subsequently normalized to the field measurements to take into account the aging effects on the filter transmission characteristics. Each of the five calibration curves was found to present a slope coefficient decreasing gradually with Cw from values higher than 0.8 to about 0.6. Using these curves, measurements of W were obtained, which differ appreciably at both sea level and high-altitude sites from those given by the square root calibration curves, avoiding large overestimation errors of 10%–40% at the high-altitude sites and underestimation errors of 5%–15% at the sea level site

Ground-Based Water Vapor Retrieval in Antarctica: An Assessment

The atmospheric water vapor is an important indicator of the Earth’s climate state and evolution. We therefore aimed at calculating the content and long-term variation of the precipitable water vapor at five coastal Antarctic stations, i.e., Casey, Davis, Mawson, McMurdo, and Mario Zucchelli. To do that, we processed the 12-year time series of GPS and radiosounding (RS) observations acquired at those stations, with the purpose of ensuring the utmost accuracy of the results adopting homogeneous, consistent, and up-to-date processing strategies for both data sets. Using the two fully independent techniques, rather consistent contents and seasonal variations of precipitable water were detected, mainly ranging from 1 (Austral winter) to 10 mm (Austral summer). At each site, correlation coefficients varying from 0.86 to 0.91 were found between the GPS and RS time series, with mean discrepancies ≤0.75 mm. There is no clear indication regarding the possible dry or wet biases of one technique with respect to the other, with only a notable GPS wet bias identified at Mawson and a dry bias at Casey that, nevertheless, correspond to an average difference of < 1 mm on the two series; the biases at the other sites are much smaller. Although extremely small, i.e., ranging from−0.03 to 0.04 mm/year, the linear trends of the series are not always consistent in sign. In accordance with the major climate models, the RS linear trends are mostly positive, whereas depending on the site, GPS exhibits a (very small) decrease or increase in water vapor

Ultraviolet radiation levels over Bulgarian high mountains

The UV-index (UVI) is a measure of the erythemally effective solar radiation reaching the Earth surface and it was introduced to alert people about the need of Sun protection. The present study applies a model that estimates the UVI over the high Bulgarian mountains for clear sky conditions considering the Total Ozone Content (TOC), which was taken from satellite measurements. The results show that during the periods from May to August at altitudes above 2 000 m a.s.l. very high UVI's (greater than 8) were observed for more than 18 days per month. The UVI values were very high practically for every day of July at altitudes higher than 1 500 m. Extremely high UVI result from episodes with TOC lower than 290 DU during June and July at the highest mountain parts with elevations greater than 2 500 m. High radiation risks were observed during April, especially when the preceding polar vortex was strong and the mountains were snow covered

Analysis of multi-year near-surface ozone observations at the WMO/GAW "Concordia" station (75°06′S, 123°20′E, 3280 m a.s.l. – Antarctica)

Abstract This work focuses on the near-surface O3 variability over the eastern Antarctic Plateau. In particular, eight years (2006–2013) of continuous observations at the WMO/GAW contributing station "Concordia" (Dome C–DMC: 75°06′S, 123°20′E, 3280 m) are presented, in the framework of the Italian Antarctic Research Programme (PNRA). First, the characterization of seasonal and diurnal O3 variability at DMC is provided. Then, for the period of highest data coverage (2008–2013), we investigated the role of specific atmospheric processes in affecting near-surface summer O3 variability, when O3 enhancement events (OEEs) are systematically observed at DMC (average monthly frequency peaking up to 60% in December). As deduced by a statistical selection methodology, these OEEs are affected by a significant interannual variability, both in their average O3 values and in their frequency. To explain part of this variability, we analyzed OEEs as a function of specific atmospheric variables and processes: (i) total column of O3 (TCO) and UV-A irradiance, (ii) long-range transport of air masses over the Antarctic Plateau (by Lagrangian back-trajectory analysis – LAGRANTO), (iii) occurrence of "deep" stratospheric intrusion events (by using the Lagrangian tool STLEFLUX). The overall near-surface O3 variability at DMC is controlled by a day-to-day pattern, which strongly points towards a dominating influence of processes occurring at "synoptic" scales rather than "local" processes. Even if previous studies suggested an inverse relationship between OEEs and TCO, we found a slight tendency for the annual frequency of OEEs to be higher when TCO values are higher over DMC. The annual occurrence of OEEs at DMC seems related to the total time spent by air masses over the Antarctic plateau before their arrival to DMC, suggesting the accumulation of photochemically-produced O3 during the transport, rather than a more efficient local production. Moreover, the identification of recent (i.e., 4-day old) stratospheric intrusion events by STEFLUX suggested only a minor influence (up to 3% of the period, in November) of "deep" events on the variability of near-surface summer O3 at DMC

First Retrievals of Surface and Atmospheric Properties Using EnMAP Measurements over Antarctica

The paper presents the first retrievals of clean snow properties using spaceborne hyperspectral observations via the Environmental Mapping and Analysis Program (EnMAP). The location close to the Concordia station at the Dome C Plateau (Antarctica) was selected. At this location, the atmospheric effects (except molecular light scattering and absorption) are weak, and the simplified atmospheric correction scheme could be applied. The ice grain size, snow specific surface area, and snow spectral and broadband albedos were retrieved using single-view EnMAP measurements. In addition, we propose a technique to retrieve trace gas concentrations (e.g., water vapor and ozone) from EnMAP observations over the snow surfaces. A close correspondence of satellite and ground-measured parameters was found