In this work a threshold technique for cloud detection and classification is applied to 9 years NOAA-AVHRR
imagery in order to obtain a cloud climatology of the Canary Islands region (Northeast Atlantic Ocean). Once
the clouds are classified, a retrieval method is used to estimate cloud macro- and micro-physical parameters, such
as, effective particle size, optical thickness and top temperature. This retrieval method is based on the inversion
of the simulated radiances obtained by a numerical radiative transfer model, libRadtran, using artificial neural
networks (ANNs). The ANNs, whose architecture was based on Multilayer Perceptron model, were trained with
simulated theoretical radiances using backpropagation with momentum method, and their architectures were
optimized through genetic algorithms. The global procedure was performed for both day and night overpasses
and, from a set of more than 9000 images, maps of relative frequency were calculated. These results were
compared with ISCCP data for the 21-year period 1984-2004. The relationships between the retrieved cloud
properties and some climate and atmospheric variables were also considered.
Spectral measurements of direct solar ultraviolet irradiance are very important for many applications in the field of atmospheric sciences. Despite its usefulness, few UV monitoring sites include such measurements in their regular observational programs. Standardization of measurement methodologies and calibration techniques is required in order to reach the quality standard of global irradiance measurements. This study presents preliminary results from an intercomparison campaign of seven UV spectroradiometers of different types that took place at the high altitude site of Izana (28.3°N, 16.5°W, 2367 m above sea level), in Tenerife, Canary Islands in June 2005. The campaign is focused primarily on spectral measurements of direct solar irradiance. Among the objectives is to improve the quality of direct solar irradiance spectral measurements, through instrumental modifications and standardization of calibration techniques, as well as to assess the significance of the differences in the field of view of the spectroradiometers with respect to aerosols and to solar zenith angle. Under the low aerosol conditions prevailing during this campaign, we aimed to establish the differences among the various instruments under "ideal" conditions. Moreover, continuous measurements under stable total ozone and aerosol optical depth will be used to determine the extraterrestrial solar flux, through the application of the Langley extrapolation method. A first comparison of sky radiance measurements of the zenith light and of various directions on the sky show effects of sensitivity to polarization of one type of instruments and the variability of the provisional radiance calibration of 4 instruments.
We present measurements of the vertical structure of the aerosol extinction coefficient in the lower troposphere, up to five kilometers. Lidar profiles were collected at Armilla (680 m asl) and Pitres (1252 m asl) during the VELETA-2002 campaign, organized to analyze the effect of altitude and aerosols on ground-level UV spectral irradiance. Single-wavelength lidar signals are inverted to derive vertically resolved aerosol extinction coefficient and integrated to provide aerosol optical depth (AOD) at 532 nm. These results are compared with measurements of the aerosol optical depth at the same wavelength provided by Licor LI-1800 spectroradiometers located at several altitudes. Lidar traces show that most of the aerosol loading is present in the first 2.5 km layer before a high-dust Saharan air mass overflew the site. On the 17th of July evening, an elevated aerosol layer was detected between 2.5 and 3.5 km and during the following three days the aerosol vertical profile of the lower atmosphere showed Sahara dust layers, producing relatively high values for the optical depth.
In this work we study the complete evolution of an episode of Saharan dust invasion over Canary Island. To describe adequately this phenomena two magnitudes are calculated from NOAA-AVHRR satellite data: aerosols optical depth (AOD) in the channel 1 and the ratio channel 1-channel 2 (R12) which gives us rough information about the aerosols size distribution. The aerosols optical depth, calculated to 500 nm with a ground based sunphotometer, situated above the inversion layer is used too.