Precipitable water vapor characterization in the Gulf of Cadiz region (southwestern Spain) based on Sun photometer, GPS, and radiosonde data

Total 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%).