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Atmospheric temperature and humidity profiles are important for weather prediction, but climate change has increased the interest in upper-air observations asking for very high-quality reference measurements. This paper discusses an experimental approach to determine the radiation-induced error on radiosonde air temperature measurements. On the one hand, solar shortwave and thermal longwave radiation profiles were accurately measured during radiosonde ascents from the surface to 35-km altitude. On the other hand, air temperature was measured with several thermocouples on the same flight, simultaneously under sun-shaded and unshaded conditions. The radiation experiments reveal that thermal radiation errors on the very thin thermocouple of the Meteolabor SRS-C34 radiosonde are similar during night- and daytime. They produce a radiative cooling in the lower troposphere and the upper stratosphere, but a radiative heating in the upper troposphere and lower stratosphere. Air temperature experiments with several thermocouples, however, show that solar radiation produces a radiative heating of about +0.2°C near the surface, which linearly increases to about +1°C at 32 km (~10 hPa). The new solar radiation error profile was then applied to SRS-C34 measurements made during the Eighth WMO Intercomparison of High Quality Radiosonde Systems, held in Yangjiang, China, in July 2010. The effects of thermal and solar radiation errors are finally shown in contrast to the 10 other internationally used radiosonde systems, which were flown during this international campaign.

The detection of low-level clouds from ground-based infrared (IR) radiometry is usually based on the IR brightness temperature (IRBT) contrast between the warm clouds and the cold clear-sky background. This method works as long as the brightness temperature contrast subsists. It is not the case for cirrus clouds, which are usually optically thin and exhibit a low brightness temperature, comparable to that of clear sky in similar atmospheric conditions. In this paper, we propose a detection scheme to discriminate between clear sky and cirrus sky based on the fluctuations of the IRBT rather than on the absolute IRBT values. For this, we use the detrended fluctuation analysis (DFA) method on IRBT data when no liquid clouds were present. We compute the exponent α for nine IRBT time series covering clear- and cirrus-sky situations. We find that, for the fluctuation range of 1-5 min, α ≃ 0.1 for clear sky, α ≃ 0.5 for stratiform cirrus layers, and α ≃ 1 for broken cirrus layers. We suggest a threshold value of α ≃ 0.25 for the discrimination between clear sky and cirrus sky. We also examine the relationship between DFA and the classical spectral analysis and find that spectral analysis could be an alternative to the DFA method in detecting the presence of cirrus clouds.

An uncooled commercial infrared camera is used to retrieve horizontal winds at cloud-base height. The camera is equipped with a microbolometer array of 320 times 240 pixels covering a field of view of 32deg times 24deg. It operates in the atmospheric window from 7.5 to 14 mum . In this wavelength range, the camera has day and night measurement capabilities, independent of the illumination conditions. Optically thick clouds appear as blackbody radiators and offer a large brightness temperature contrast with the much colder clear-sky background. We develop a method to follow the infrared structures in the clouds as they pass through the field of view of the camera. Taking a picture every 6 s, the angular velocity of the cloud base is calculated as the ratio of the angular displacement of the cloud structures between two pictures over the elapsed time. To transform the angular velocity into a metric velocity, we use the temperature profile retrieved by a microwave radiometer called All-Sky MUlti-WAvelength Radiometer. The case study of September 10, 2007 is shown. The retrieved wind is of 18.0 plusmn 2.4 m/s at an altitude of 4164 plusmn 546 m above the instrument, blowing from northwest. These results agree well with radiosondings launched in the vicinity of the experiment site.

The lightning performance of 63 kV and 90 kV shielded and unshielded power transmission lines, protected by surge arresters are considered. A specialized software package is used for the computation of the line's fast front transients. A Monte Carlo statistical method, together with a three-dimensional electrogeometric model is used to determine the lightning stroke locations. Different arrester installation configurations are analyzed and compared. The energy duties of the transmission line surge arresters are computed statistically.