Continuous photometric observations at ENEA base in Lampedusa to estimate precipitable water

Water vapour is a variable component of the atmosphere both in space and time. It is one of the most important components because of its effects in many fi elds: Meteorology, Climatology, Remote Sensing, Energy-Budget, Hydrology, etc. This work compares radiometric (sun photometer) readings, Global Positioning System (GPS) data and a meteorological model forecasted data. The aim is to understand if GPS measurements may help Numerical Weather Prediction (NWP) models. It is well known that GPS measurements are affected by the so-called tropospheric delay. Part of it, the so-called wet delay is related mainly to the amount of water vapour along the path of the GPS signal through the troposphere. Precise knowledge of the abundance of water vapour, in space and time, is important for NWP model because water vapour is the predecessor of precipitation. Despite the high variability of water vapour compared to other meteorological fi elds, like pressure and wind, water vapour observations are scarce, so that additional measurements of water vapour are expected to benefi t meteorology. A new sun photometer, which is part of the AERONET (AErosol and RObotic NETwork) program, has been installed at the ENEA (Ente per le Nuove tecnologie, l'Energia e l'Ambiente) base of Lampedusa Island. The sun photometer is quite close (less then 4 km) to an ASI (Agenzia Spaziale Italiana) GPS permanent receiver. A long record (summer period of the year 2000) of sun photometric measurements is available for the station at Lampedusa. We found that the GPS and sun photometric data are better correlated (std. dev. about 10 mm for the wet delay) than are the GPS measurements with the NWP model predictions. This is an indication that GPS delay data may contain information useful for weather prediction.