Sun photometric measurements of atmospheric water vapor column abundance in the 940-nm band

Sun photometers operating in the strong water vapor absorption 940-nm combination-vibrational band have been used to determine water column abundance in the atmosphere when the path to the Sun is clear of clouds. We,describe a procedure to perform a rapid determination of the water column abundance, using sun photometers with an accuracy that is easily comparable to that of the radiosondes. The effect of parameters, such as filter bandwidth, atmospheric lapse rate, and the water vapor amount, on the accuracy of the retrieved abundance is examined. It is seen that a narrow filter band of approximately 10-nm bandwidth, positioned at the peak of absorption, is quite insensitive to the type of atmosphere present during calibration or measurement with less than 1% variability under extreme atmospheric conditions. A comparison of the retrieved water column abundance using sun photometers with contemporaneously measured values using radiosondes and microwave radiometers shows that the latest version of the radiative transfer algorithm used in this procedure, MODTRAN-3, gives far superior results in comparison with earlier versions because of the use of improved absorption coefficients in the 940-nm bands. Results from a network of sun photometers spread throughout the globe will be discussed.