J. P. Ortiz de Galisteo

Precipitable Water Vapor Characterization In The Gulf Of Cadiz Region (Southwestern Spain) Based On Sun Photometer, GPS And Radiosonde Data

[1]xa0Total precipitable water vapor (PWV) is characterized for the first time over southwestern Europe by means of ground-based measurements during the period 2001–2005. Existing data from three sites located in the Cadiz Gulf region, El Arenosillo, San Fernando, and Gibraltar, using three different techniques, Sun photometer (SP), GPS, and radiosondes, are used for the analysis. The 5 year data series gives a mean value of about 2 cm (SD = 0.7 cm) and a clear seasonal pattern. In the multiannual monthly means basis, the highest values are reached in August–September, with a mean value of 2.5–2.6 cm, whereas the lowest are obtained in January–February, with an average of 1.4–1.5 cm. The data in the three sites have been compared in order to assess regional variability. Differences could be due to real local variability but also could arise from the differences in the measurement techniques. From daily to monthly bases, water vapor behavior is similar in the three sites, with the largest differences ranging from 3% in summer to 14% in winter. Outstanding results from these analyses are the observed local minimum in July, occurring during the maximum of desert dust intrusions in the southern Iberian Peninsula, and the significant differences found between the El Arenosillo (SP) and San Fernando (GPS) measurements, related to the periodical replacement of the SP instrument at El Arenosillo. The observed differences highlight the importance of drift in each SP because of filter aging or other calibration problems. Finally, the Moderate Resolution Imaging Spectroradiometer (MODIS) near-infrared water vapor product has been compared to the data from the GPS station (San Fernando). MODIS retrieval slightly overestimates PWV in summer (5%–8%) and significantly underestimates in winter (−23%).

Comparison of total water vapor column from GOME-2 on MetOp-A against ground-based GPS measurements at the Iberian Peninsula

Water vapor column (WVC) obtained by GOME-2 instrument (GDP-4.6 version) onboard MetOp-A satellite platform is compared against reference WVC values derived from GPS (Global Positioning System) instruments from 2007 to 2012 at 21 places located at Iberian Peninsula. The accuracy and precision of GOME-2 to estimate the WVC is studied for different Iberian Peninsula zones using the mean (MBE) and the standard deviation (SD) of the GOME-2 and GPS differences. A direct comparison of all available data shows an overestimation of GOME-2 compared to GPS with a MBE of 0.7 mm (10%) and a precision quantified by a SD equals to 4.4mm (31%). South-Western zone presents the highest overestimation with a MBE of 1.9 mm (17%), while Continental zone shows the lowest SD absolute value (3.3mm) due mainly to the low WVC values reached at this zone. The influence of solar zenith angle (SZA), cloud fraction (CF), and the type of surface and its albedo on the differences between GOME-2 and GPS is analyzed in detail. MBE and SD increase when SZA increases, but MBE decreases (taking negative values) when CF increases and SD shows no significant dependence on CF. Under cloud-free conditions, the differences between WVC from GOME-2 and GPS are within the WVC error given by GOME-2. The changes of MBE and SD on Surface Albedo are not so evident, but MBE slightly decreases when the Surface Albedo increases. WVC from GOME-2 is, in general, more precise for land than for sea pixels.

The annual cycle of total precipitable water vapor derived from different remote sensing techniques: An application to several sites of the Iberian Peninsula

The annual cycle of precipitable water vapor is inferred from the MODIS thermal infrared (IR) and nearinfrared (NIR) satellite products under clear-sky conditions and for the period 2002-2008 at 18 sites of the Iberian Peninsula, with the aim to evaluate the capabilities of both algorithms. The paper presents these results in relation with ground observations using different techniques (GPS, sunphotometer, radiosounding), with a special emphasis on GPS. The differences in the monthly means mostly correspond to underestimations, ranging between a few percent to 40%, being generally larger in winter than in summer. For most sites, NIR performs better than IR, and more particularly during the winter. NIR usually presents overestimations in summer that can reach up to 30%. The time-coincident comparison shows that best MODIS/GPS agreement is found for the continental region (rms∼0.3) for both methods, whereas the largest biases and rms differences are found at the Mediterranean sites.

Columnar aerosol characterization over Scandinavia and Svalbard

An overview of sun photometer measurements of aerosol properties in Scandinavia and Svalbard was provided by Toledano et al. (2012) thanks to the collaborative effort of various research groups from different countries that maintain a number of observation sites in the European Arctic and sub-Arctic regions. The spatial coverage of this kind of data has remarkably improved in the last years, thanks, among other things, to projects carried out within the framework of the International Polar Year 2007-08. The data from a set of operational sun photometer sites belonging either to national or international measurement networks (AERONET, GAW-PFR) were evaluated. The direct sun observations provided spectral aerosol optical depth (AOD) and Angstrom exponent (AE), that are parameters with sufficient long-term records for a first characterization at all sites. At the AERONET sites, microphysical properties derived from inversion of sun-sky radiance data were also examined. AOD (500nm) ranged from 0.08 to 0.10 in...