A method has been devised to ensure stable, long-term calibration of Raman lidar measurements that are used to determine the altitude-dependent mixing ratio of water vapor in the upper troposphere and lower stratosphere. Because the lidar measurements yield a quantity proportional to the mixing ratio, rather than the mixing ratio itself, calibration is necessary to obtain the factor of proportionality. The present method involves the use of calibration data from two sources: (1) absolute calibration data from in situ radiosonde measurements made during occasional campaigns and (2) partial calibration data obtained by use, on a regular schedule, of a lamp that emits in a known spectrum determined in laboratory calibration measurements.
In this method, data from the first radiosonde campaign are used to calculate a campaign-averaged absolute lidar calibration factor (t1) and the corresponding campaign-averaged ratio (L1) between lamp irradiances at the water-vapor and nitrogen channel wavelengths. Depending on the scenario considered, this ratio can be assumed to be either constant over a long time (L = L1) or drifting slowly with time.
The absolutely calibrated water-vapor mixing ratio (q) obtained from the ith routine off-campaign lidar measurement run is given by
qi = Pi/ti = LPi/P′i
where Pi is water-vapor/nitrogen measurement signal ratio, ti is the unknown and unneeded overall efficiency ratio of the lidar receiver during the ith routine off-campaign measurement run, and P′i is the water-vapor/nitrogen signal ratio obtained during the lamp run associated with the ith routine off-campaign measurement run. If L is assumed constant, then the lidar calibration is routinely obtained without the need for new radiosonde data. In this case, one uses L = L1 = P1′/t1, where P1′ is the watervapor/ nitrogen signal ratio obtained during the lamp run associated with the first radiosonde campaign.
If L is assumed to drift slowly, then it is necessary to postpone calculation of qi until after a second radiosonde campaign. In this case, one obtains a new value, L2, from the second radiosonde campaign, and for the ith routine off-campaign measurement run, one uses an intermediate value of L obtained by simple linear time interpolation between L1 and L2.