Precipitable water in the Sahel measured using sun photometry

Abstract

A sunphotometer network in Mali and Niger was established in 1984 to measure the temporal and spatial variability of aerosols and water vapor. A new technique developed by Reagan was applied to the master calibration instrument based on a Mauna Loa Langley calibration data set. Measurements obtained with the calibrated instrument for 14 days in July 1987 at Goddard Space Flight Center (Greenbelt, MD), retrieved the vertical column precipitable water ( P W) to within an average of 0.28 cm of the radiosonde measured values under a 2–5 cm P W range and a large range of aerosol optical thicknesses. The calibration was transferred to the sunphotometers in the Sahel via inter-calibration. Precipitable water retrieved for the Sahelian sites show close agreement with the sparse radiosonde data available and is consistent with published data. The spatial variability of the precipitable water was found to be significantly lower than the aerosol optical depth spatial variability, and there is no correlation between the magnitude of precipitable water and aerosol optical depth. The network shows a dry season monthly average P W value of 1–2 cm and a wet season range of 3.5–4.5 cm.