Cloud and Aerosol Spectroscopy with Raman Lidar

Abstract

spectrometer for height-resolved measurements of the Raman backscatter-coefficient spectrum of water in its gaseous and condensed phases is presented. The spectrometer is fiber coupled to the far-range receiver of the Raman Lidar for Atmospheric Moisture Sensing (RAMSES) of the German Meteorological Service and consists of a Czerny?Turner spectrograph (500-mm focal length) and a 32-channel single-photon-counting detection system based on a multianode photomultiplier. During a typical measurement (transmitter wavelength of 355 nm), the spectrum between 385 and 410 nm is recorded with a spectral resolution of 0.79 nm; the vertical resolution is 15 m and the height range is 15 km. The techniques outlined are those that are applied to calibrate the spectrum measurement and to monitor fluorescence by atmospheric aerosols that have the potential to interfere with the water observation. For the first time, Raman spectra of liquid-water, mixed-phase, and cirrus clouds are reported, and their temperature dependence is investigated by means of band decomposition. The spectrum-integrated condensed-water Raman backscatter coefficient strongly depends on cloud particle volume, but it is not tightly correlated with the cloud optical properties (particle extinction and backscatter coefficient), which implies that retrieval of cloud water content from optical proxies is likely impossible. Aerosol measurements are also discussed. Depending on type, aerosols may show no backscattering in the spectrometer range at all, or a featureless spectrum that stems quite likely from fluorescence. Finally, the example of a cloud forming in an aerosol layer demonstrates that the new instrument not only opens up new perspectives in cloud research but also contributes to studies of cloud?aerosol interaction.