Abstract
Abstract A detailed description is given of how the liquid water content (LWC) and the ice water content (IWC) can be determined accurately and absolutely from the measured water Raman spectra of clouds. All instrumental and spectroscopic parameters that affect the accuracy of the water-content measurement are discussed and quantified; specifically, these are the effective absolute differential Raman backscattering cross section of water vapor , and the molecular Raman backscattering efficiencies ηliq and ηice of liquid and frozen microparticles, respectively. The latter two are determined following rigorous theoretical approaches combined with Raman Lidar for Atmospheric Moisture Sensing (RAMSES) measurements. For ηice, this includes a new experimental method that assumes continuity of the number of water molecules across the vertical extent of the melting layer. Examples of water-content measurements are presented, including supercooled liquid-water clouds and melting layers. Error sources are discussed; one effect that stands out is interfering fluorescence by aerosols. Aerosol effects and calibration issues are the main reasons why spectral Raman measurements are required for quantitative measurements of LWC and IWC. The presented study lays the foundation for cloud microphysical investigations and for the evaluation of cloud models or the cloud data products of other instruments. As a first application, IWC retrieval methods are evaluated that are based on either lidar extinction or radar reflectivity measurements. While the lidar-based retrievals show unsatisfactory agreement with the RAMSES IWC measurements, the radar-based IWC retrieval which is used in the Cloudnet project performs reasonably well. On average, retrieved IWC agrees within 20% to 30% (dry bias) with measured IWC.
Highlights
Water content and phase of clouds are key to a better understanding of weather and climate processes (Baker 1997; Illingworth et al 2007), and so considerable research effort has been dedicated to measuring these quantities over the last decades, be it in situ or remotely
Because dsmac(p)/dV is known with high accuracy for both bulk liquid water and ice (Plakhotnik and Reichardt 2017), the key to absolute measurements of cloud water content with spectrometric Raman lidars is an accurate determination of the effective Raman backscattering cross section of water vapor, dsevfafp(p)=dV, and a quantitative understanding of relative Raman scattering hliq and hice
D28 peak sensitivity of about 87% is confirmed by all measurement series, and matches the experimental value determined previously with the operational Raman Lidar for Atmospheric Moisture Sensing (RAMSES) instrument in Lindenberg
Summary
Water content and phase of clouds are key to a better understanding of weather and climate processes (Baker 1997; Illingworth et al 2007), and so considerable research effort has been dedicated to measuring these quantities over the last decades, be it in situ or remotely. Because dsmac(p)/dV is known with high accuracy for both bulk liquid water and ice (Plakhotnik and Reichardt 2017), the key to absolute measurements of cloud water content with spectrometric Raman lidars is an accurate determination of the effective Raman backscattering cross section of water vapor, dsevfafp(p)=dV, and a quantitative understanding of relative Raman scattering hliq and hice. While the former is specific to each instrument, the latter is universal in nature and broadly applicable. The short illumination time and the transiently high count rates require low signal intensities, and have to be taken into account when the signals are corrected for nonlinearities
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