Theoretical and numerical investigations of the intensity of the lidar signal specular reflected from a set of oriented ice plates

Abstract

The algebraic formula of the backscattering coefficient has been derived within the scope of the physical optics method. The formula relates the lidar return anomalous high amplitude of observed experimentally with the principal parameters of a polydisperse crystalline medium. It is shown that at the lidar small-angular scanning relative to the normal to the ice-plate preferred orientation, the average size of the plate can be defined and the flutter size can be evaluated from the behavior of the lidar return variation.

Doppler lidar measurements of oriented planar ice crystals falling from supercooled and glaciated layer clouds

“…From space, Hu (2007) analysed returns from the CALIPSO lidar, which initially pointed 0.3 • from the nadir, and estimated that half of all optically thick ice cloud was affected by specular reflection, with important repercussions for retrievals of cloud optical properties. The statistics from these studies are summarized in Table I. A number of theoretical calculations have been performed to estimate the specular backscatter from a population of oriented crystals (Platt, 1978;Popov and Shefer, 1994;Iwasaki and Okamoto, 2001); however knowledge of the amount of faceted surface on the crystal and the distribution of crystal orientations is required, and these quantities are not well constrained at present. As an example, to produce a backscatter of 10 −4 m −1 sr −1 only requires a concentration of ∼ 1 m −3 ice discs 500 µm in diameter if they are all perfectly aligned.…”

“…Similarly, Hogan and Illingworth (2003) used integrated backscatter measurements to identify regions of strong specular reflection, and found it to occur exclusively between −10 and −20 • C for 20 days of lidar data. Thomas et al (1990) A number of theoretical calculations have been performed to estimate the specular backscatter from a population of oriented crystals (Platt 1978, Popov and Shefer 1994, Iwasaki and Okamoto 2001; however knowledge did not occur at other temperatures Hogan and Illingworth (2003) 20 days continuous data Observed 18% of time in cloud layers at 4 km, not observed at cirrus altitudes Noel and Sassen (2005) 6 cirrus cases Observed 22% of time for Hu (2007) 1 month of CALIPSO data Half of all optically thick ice cloud profiles affected Chepfer et al (1999) 3 months POLDER data 37-50% of all ice-only cloud show specular glint Noel and Chepfer et al (2004) POLDER, 31 ice cloud cases 80% of ice-only cloud showed specular glint of the amount of faceted surface on the crystal, and distribution of crystal orientations is required, and these quantities are not well constrained at present. As an example, to produce a backscatter of 10 −4 m −1 sr −1 only requires a concentration of ∼ 1m −3 ice discs 500µm in diameter if they are all perfectly aligned.…”