The retrieval of cirrus cloud properties from aircraft multi‐spectral reflectance measurements during EUCREX'93

Abstract

We present details of a scheme for retrieving cirrus cloud optical thickness and effective particle size from nadir‐viewing reflectance measurements. Two near‐infrared wavelengths are used, one with negligible ice absorption (1.04 μm) and one with significant ice absorption (1.55 μm). Four ice crystal shapes are used in the scheme: ice spheres, hexagonal columns, hexagonal plates and randomized polycrystals. We highlight the sensitivity of the retrievals to the shape of the phase function, which follows from the particular ice crystal shape assumed. Five aircraft‐based retrievals are presented, all cases occurring during the 1993 intensive field campaign of the European Cloud Radiation Experiment (EUCREX'93). The retrieved effective sizes are compared with the in situ measurements taken from the same cases. In these comparisons, account has to be taken of the fact that (a) in situ replicator data indicate that the presence of small (<100 μm) ice crystals can have a significant effect on the effective crystal size, and (b) the measured crystal sizes consistently show a systematic decrease with height in all cases, the effective sizes at cloud base being typically two or three times larger than at cloud top. Taking these considerations into account, we conclude that the assumption of polycrystals gives the most consistent agreement with the in situ measurements.

The Characteristics of Ice Cloud Properties Derived from CloudSat and CALIPSO Measurements

“…Lidar multiple scattering is estimated by the fast multiple-scattering model of Hogan (2006). In the absence of lidar observations, the area-size relation taken from Francis et al (1998) is used to derive the extinction coefficient (Delanoë and Hogan 2010), and IWCs are then derived as a function of the radar reflectivity factor Z e and temperature. For example, when only lidar data are available, IWCs can still be obtained by introducing a mass-size relationship of Brown and Francis (1995).…”

Comparison of Airborne In Situ, Airborne Radar–Lidar, and Spaceborne Radar–Lidar Retrievals of Polar Ice Cloud Properties Sampled during the POLARCAT Campaign

“…On the other hand, when the radar is not sensitive enough to detect the cloud, the retrieved extinction is converted to IWC using lookup tables. The corresponding area-size relationship is taken from Francis et al (1998), who used the same aircraft dataset as Brown and Francis (1995). The retrieval technique uses the optimal estimation framework and the Gauss-Newton method (Rodgers 2000) to minimize iteratively the difference between the forward-modeled observations and real observations.…”

Influence of ice crystal shape on retrieval of cirrus optical thickness and effective radius: A case study

“…This owes to the fact that the single‐scattering properties of nonspherical ice crystals may differ substantially from those of surface, cross‐sectional or volume equivalent spheres [ Kinne and Liou , 1989; Takano and Liou , 1989; Macke , 1993]. Contrarily, in global climate models cirrus are mostly represented by a fixed ice particle size and shape [ Francis et al , 1998; Garrett et al , 2003]. However, as observed in several field experiments, a variety of ice crystal sizes and shapes exist [ Garrett et al , 2003; Lawson et al , 2006; Schmitt and Heymsfield , 2007].…”