evaluate the temperature sensitivity of differential absorption lidar (DIAL) H20 measurements. This paper estimates the temperature sensitivity of H20 lines in the ...
Temperature sensitivity of differential absorption lidar measurements of water vapor in the 720-nm region Edward V. Browell, Syed Ismail, and Benoist E. Grossmann
Recently measured properties of water vapor (H2 0) absorption lines have been used in calculations to evaluate the temperature sensitivity of differential absorption lidar (DIAL) H2 0 measurements. This paper estimates the temperature sensitivity of H2 0 lines in the 717-733-nm region for both H2 0 mixing ratio and number density measurements, and discusses the influence of the H2 0 line ground state energies E", the H2 0 absorption linewidths, the linewidth temperature dependence parameter, and the atmospheric temperature and pressure variations with altitude and location on the temperature sensitivity calculations. Line parameters and temperature sensitivity calculations for sixty-seven H2 0 lines in the 720-nm band are given which can be directly used in field experiments. Water vapor lines with E" values in the 100-300-cm- range were1 found to be optimum for DIAL measurements of H2 0 number densities, while E" values in the 250-500-cm- range were found to be optimum for H2 0 mixing ratio measurements.
1. Introduction
Accurate measurements of atmospheric water vapor (H 2 0) are needed to aid in the understanding of many earth system processes including the hydrologic cycle, the radiation budget, global circulation and dynamics, and various meteorological processes. Differential absorption lidar (DIAL) is an active remote sensing technique that has been used to measure H 2 0 profiles from ground-based and airborne platforms1 -5 in the atmosphere. The DIAL technique can provide long range measurements of H 2 0 profiles, and it has the potential for making H 2 0 measurements from space (see Ref. 6 and references therein). In the DIAL measurement of H 2 0, it is necessary to choose H 2 0 absorption lines that are insensitive to variations in the atmospheric temperature in the measurement region. This is done to prevent the uncertainty in the knowledge of the local temperature from producing a significant error in the DIAL measurement. Since atmospheric temperature is a function of altitude, geographic location, and season, it is necessary to select the most temperature insensitive H 2 0 lines for a specific mea-
When this work was done Benoist Grossmann was with Old Dominion University Research Foundation, Norfolk, Virginia 23508; he is now with Thomson CSF, Activites des Technologies Emergentes, B.P. 55, F-78283 Guyancourt CEDEX, France. The other authors are with NASA Langley Research Center, Atmospheric Sciences Division, Hampton, Virginia 23665-5225. Received 16 February 1990.
surement region. Earlier temperature sensitivity analyses 3 7 8 were applied to some specific DIAL measurements and were based on a H 2 0 line temperature dependence parameter which is more directly applicable to pure rotational transitions. In this paper a full discussion and evaluation of the temperature sensitivity of H 2 0 lines for DIAL measurements of H 2 0 number densities and mixing ratios are presented. In this analysis it has been assumed that the off-line cross section aoff is negligible, and therefore, only the on-line contributes to the temperature sensitivity of the DIAL measurement. This assumption is valid for the 720-nm band of H 2 0 where regions of negligible o.ff (5 km), and E" values in the 350-550-cm- 1 range are more suitable for measurements in the boundary layer. The temperature neutral points and the temperature insensitive regions with
