Abstract

Daytime monitoring of water vapour is of peculiar interest because many significant meteorological phenomena occur more often during the day than at night [1]. Then, Raman lidar systems based either on KrF excimer (248 4 nm) or quadrupled Nd:YAG (266 nm) lasers have been developed. Mixing ratio of water vapour is determined by the ratio between Raman shifted H₂O and N₂ signals multiplied by the differential transmissivity q(z) between the two shifted wavelengths: (1) the differential transmissivity q(z) , mainly determined by ozone concentration, can be determined by a simultaneous acquisition of O₂ and N₂ signals (Raman DIAL method) [2]: where k, k′ are constant determined by calibration of the system and backscattering cross sections and γ by ozone absoprtion cross sections. When working with KrF laser, it must be considered that the absolute value of absorption cross section is nearly maximum for all the wavelengths involved, thus this method needs a very high S/N ratio and consequently it is utilisable only at the lower heights. When increasing the range of the measurement, uncertainty on m becomes dominated by the uncertainty on q(z). However, because of the high absorption, O₃ can be determined by the total extinction, with a systematic error depending mainly on aerosol extinction Thus the differential transmissivity can be obtained by this determination of ozone We will show, by a propagation error calculation, that under reasonable assumptions the systematic error on q is lower than the statistical error obtained when the Raman DIAL method is utilised, at least at the higher altitudes. This alternative determination of q(z) makes possible to extend the range of measurement, as it is shown in the figure where we show a result of a numerical simulation where a 2% water vapour volume mixing ratio is retrieved by the two methods. In the figure we present (a) the 1-σ variations of the DIAL retrieved water vapour mixing ratio(dotted line) and those retrieved by the extinction method (solid line); an aerosol extinction profile corresponding to an optical thickness of 0 2 on 1000 meters has been assumed for the simulations and this extinction has been underestimated of 50% in the retrieval by the extinction method.

© 2000 IEEE