Molecular-Backscatter Lidar Profiling of the Volume-Scattering Coefficient in Cirrus

Abstract

Backscatter and polarization lidars have already been used extensively to investigate ice clouds (see chapters 2 and 10). A severe limitation is that trustworthy values of the volume-scattering coefficient, one of the most important parameters in the description of the impact of cirrus on climate, cannot be derived from data taken with these lidars. Even the retrieved cirrus backscatter-coefficient profile is often questionable. A discussion of achievements and limitations of the lidar method can be found in the literature (e.g., Fernald et al. 1972; Klett 1981; Fernald 1984; Klett 1985; Sasano et al. 1985; Bissonnette 1986; Ansmann et al. 1992b; Kovalev 1995). The procedure, with all its subsequent modifications and improvements, suffers from the fact that two physical quantities, the particle backscatter coefficient and the particle extinction coefficient, must be determined from only one lidar signal. The uncertainties in the estimated optical parameters are especially large in cirrus, in which the relationship between particle extinction and backscattering can vary strongly in space and time. The situation improved significantly when the first molecular (Raman)-backscatter lidar experiments demonstrated that accurate extinction profiling throughout the entire troposphere is possible (Ansmann et al. 1990, 1992b). After the Pinatubo eruption, it was shown that even at stratospheric heights profiles of the volume-scattering coefficient can easily be obtained with a Raman lidar (Ansmann et al. 1991, 1993a, 1997; Ferrare et al. 1992; Gross et al. 1995; Donavan und Carswell 1997). Two types of molecular-backscatter lidars for extinction measurements are available. The Raman lidar measures lidar return signals elastically backscattered by air molecules and particles and inelastically (Raman) backscattered by nitrogen and/or oxygen molecules (Cooney et al. 1969; Melfi 1972; Ansmann et al. 1992a; Whiteman et al. 1992; Reichardt et al. 1996). Interference-filter polychromators and double-grating monochromators (Arshinov et al. 1983; Wandinger et al. 1998) are used to separate the aerosol signal from the vibrational-rotational or pure rotational Raman signals, to reduce the sky background radiation, and, for the Raman channels, to block the strong elastic-backscatter radiation at the laser wavelength. The suppression has to be better than 10-8. The second type of a molecular-backscatter lidar is the High Spectral Resolution Lidar (HSRL).