Lamp mapping technique for independent determination of the water vapor mixing ratio calibration factor for a Raman lidar system

Abstract

We have investigated a technique that allows for the independent determination of the water vapor mixing ratio calibration factor for a Raman lidar system. This technique utilizes a procedure whereby a light source of known spectral characteristics is scanned across the aperture of the lidar system's telescope and the overall optical efficiency of the system is determined. Direct analysis of the temperature-dependent differential scattering cross sections for vibration and vibration-rotation transitions (convolved with narrowband filters) along with the measured efficiency of the system, leads to a theoretical determination of the water vapor mixing ratio calibration factor. A calibration factor was also obtained experimentally from lidar measurements and radiosonde data. A comparison of the theoretical and experimentally determined values agrees within 5%. We report on the sensitivity of the water vapor mixing ratio calibration factor to uncertainties in parameters that characterize the narrowband transmission filters, the temperature-dependent differential scattering cross section, and the variability of the system efficiency ratios as the lamp is scanned across the aperture of the telescope used in the Howard University Raman Lidar system.

A Raman lidar at La Reunion (20.8° S, 55.5° E) for monitoring water vapour and cirrus distributions in the subtropical upper troposphere: preliminary analyses and description of a future system

“…However, calibration issues are still pending and debated (Whiteman et al, 2011a;. Though an absolute calibration of the entire lidar system is theoretically possible (Vaughan et al, 1988;Sherlock et al, 1999b;Venable et al, 2011), the signal ratio is usually scaled to various external water vapour measurements (radiosonde, microwave radiometer, Global Positioning System (GPS),. .…”

Depolarization calibration and measurements using the CANDAC Rayleigh–Mie–Raman lidar at Eureka, Canada

“…An alternative is to use a lamp which scans or is projected over the whole entrance aperture at the first optic of the system. Notably, the lidar group at Howard University uses a mapping lamp applied to a water vapour lidar (Venable et al, 2011). CRL unfortunately does not have the capacity to install such a system at this time.…”

Study and mitigation of calibration factor instabilities in a water vapor Raman lidar

“…More recent empirical determination of water vapor Raman cross sections by Avila et al (2004) finally allowed achieving an absolute accuracy better than 10 %. The use of these improved cross sections in the independent calibration technique of Venable et al (2011) even resulted in agreement between the sensor-independent calibration and the traditional radiosonde-dependent technique of better than 5 %. The radiosonde-dependent technique is performed by calculating a constant, height-independent, normalization factor from the comparison of the lidar profile with a radiosounding (Ferrare et al, 1995;Leblanc et al, 2012).…”

“…Lately, Leblanc and McDermid (2008) developed a stability control procedure with calibrated spectral lamps, reaching a calibration factor standard deviation of 2 % over more than a year. Venable et al (2011) proposed an improved independent calibration technique which consists in scanning a known light source over the telescope aperture instead of a stationary lamp system. Advantages and drawbacks of these methods are further discussed in .…”