Lidar measurements of water vapor concentrations in the troposphere

A multi-wavelength Raman lidar has been used to measure water vapor profiles in the troposphere under a wide range of geophysical conditions. The transportable LAMP lidar instrument has been used to make measurements at several locations, at the author's central Pennsylvania campus location, on shipboard between Arctic and Antarctic, and in a coastal environment at Point Mugu, CA. The Raman technique provides an accurate way to measure the concentration profile of water vapor by measuring the ratio of Raman vibrational (1st Stokes) signal of water vapor to that of nitrogen. The measurements have been made using the vibrational Raman backscatter intensity of the 660/607 ratio from 532 nm, the 407/387 ratio from 355 nm, and the 294/283 ratio from 266 nm. The Nd:YAG laser double (532 nm) and triple (355 nm) Raman backscatter signals have been found to be about equally useful in measuring the water vapor concentrations during night conditions. Having both sets of measurements allows additional examination of the results, particularly the small correction for aerosol differential extinction. The quadruple (266 nm) Raman scatter signals have been examined for daytime measurements where the troposphere is shielded from wavelengths below 300 nm (solar blind region) by the stratospheric ozone absorption. These measurements are complicated by the need to make corrections for the absorption of tropospheric ozone and other minor species. The Raman signals from molecular oxygen and nitrogen provide a suitable way to correct for the tropospheric species influences on the measurements. Measurements of the water vapor concentrations have been made over a wide range of atmospheric conditions and comparisons have been made with the current techniques used on meteorological balloons. From the studies which have been carried out, a convincing case can be made for the ability of lidar to accurately measure the water vapor concentrations from the surface to 8 km at night and from the surface to 2-3 km during daylight.<
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