Atmospheric temperature measurement using a pure rotational Raman lidar: comment

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  • Atmospheric temperature measurement using a pure rotational Raman lidar: comment John A. Cooney

    Drexel University, Department of Physics & Atmospheric Science, Philadelphia, Pennsylvania 19104. Received 4 November 1983. 0003-6935/84/050653-02$02.00/0. 1984 Optical Society of America The authors of this paper1 are to be congratulated. The

    success of the effort along with the punctilious attention to detail shown therein has greatly enhanced the development of methods for acquisition of accurate atmospheric temperature profiles.

    The purpose of this Letter is to comment on the severity of the elastic cross talk entering the Raman channels and to suggest why such cross talk is a matter of somewhat less concern than the authors suppose.

    As noted in Ref. 1 the presence of an enhanced aerosol overburden increases both the elastic backscatter and transmission losses. Due to the finite rejection of radiation at the elastic wavelength (Rayleigh and Mie backscatter) in the Raman channels some of the so-called online radiation penetrates and becomes part of the signal acquired in the Raman channel. The greater the aerosol overburden the greater the penetration into the Raman channel and so the greater the amount of unwanted signal which can act to reduce the accuracy of the measurement.

    It is noted in Ref. 1 that because of the ratio of the Raman to Rayleigh cross section along with nominal overburdens (10.0-km visibility), and because of the need to keep the errors on the temperature measurement below the 1.0% level, transmission of the elastic line in the Raman channel must be of the order of 10~6. For the small wavelength intervals which separate the pertinent Raman lines from the elastic line rejection by factors of 106 are quite difficult to achieve by ordinary narrowband interference filters. When using a ruby laser (6943 ) as the existing source useful Raman lines suggest that the Raman channels be set in the vicinity of 6910 and 5990 . Interference filters were used in a series of experiments2 which led to temperature uncertainties of 1.0 K and an absolute temperature difference of mean value integrated all along the profile to 2.3 km of 2.5 K between lidar and radiosonde methods of profile acquisition. Here, however, the filter rejection was only ~105.

    The seeming inconsistency of the 1% error requirement of 106 rejection, on the one hand, and the

  • position that the only source of deviation from an errorless Raman signal is due to this elastic intrusion. Let the channel I signal be I1 = A1 + 1, where A1 is the errorless measurement of the Raman signal in Raman channel I, and 1 is the elastic intrusion. Similarly for channel II, let I2 = A2 + z The error 1, is assumed to be