Provide values for calibrating lidars using the integrated values from water clouds
Powerful lidars at short visible wavelengths can be calibrated by comparison of the profile of observed molecular backscatter with the modelled values derived from radiosonde or NWP model forecasts of temperature and pressure (to give molecule number density). Some lidars, especially ceilometers, may not be sensitive enough to detect the molecular backscatter or operate at longer wavelengths where the molecular signal strength is much reduced. In these cases it is possible to use the technique described in O'Connor et al. (2004).
The lidar extinction--to--backscatter ratio, S, derived from integrated backscatter for stratocumulus is, in the absence of drizzle, constrained to a theoretical value of 18.8 +/- 0.8 sr at a wavelength of 905 nm. The lidar can be calibrated by scaling the backscatter signal so that the observed lidar ratio matches the theoretical value when suitable conditions of stratocumulus are available. For a beam divergence of 1 to 1.5 mrad, multiple scattering introduces an uncertainty of about 10% into the calibration and for a narrow beam ground based lidar, with negligible multiple scattering, calibration may be possible to better than 5%.