Theoretical aspects of modeling backscattering by cirrus ice particles at millimeter wavelengths

Journal Article · Fri Dec 01 04:00:00 EST 1995 · Journal of the Atmospheric Sciences

DOI:https://doi.org/10.1175/1520-0469(1995)052<4367:TAOMBB>2.0.CO;2· OSTI ID:381779

Schneider, T L; Stephens, G L ^[1]

---

1. Colorado State Univ., Fort Collins, CO (United States)

This research attempts to understand how nonspherical ice particles backscatter electromagnetic radiation at millimetric wavelengths. The discrete dipole approximation (DDA) is employed to examine backscattering by single particles to (1) explore the limits of the Rayleigh approximation and (2) test the use and validity of spheroidal models to model semirealistic cirrus particles. It is shown that spheroids are reasonable models of cirrus ice particles at wavelengths of 3 and 8 mm. Furthermore, with careful consideration of optical size it is possible to exploit the Rayleigh approximation for spheroids under many circumstances. The sensitivity of backscattered radiation to variations in microphysical properties is examined, based on DDA calculations for ensembles of ice particles. The most important factor in the ice crystal size distribution is found to be the median diameter (D{sub m}) of the ice crystal volume distribution. In particular, for values of D{sub m} typical of cirrus, the contribution of crystals whose major dimension is D {<=} 100 {mu}m is masked by the signal of larger crystals. Simulations of ice water content-effective radar reflectivity factor relations (IWC-Z{sub i}) are also presented. Comparison with available empirical relations indicates a functional dependence of the IWC on D{sub m} (i.e., the relative number of large crystals) and also suggests upper and lower bounds on D{sub m}. It is demonstrated that the effective radar reflectivity cannot be used in an unambiguous way to determine the IWC. The difference between reflectivities at 3.16 and 8.66 mm are found to be insignificant. Implications for the remote sensing of ice clouds at millimeter wavelengths are discussed.

• OSTI ID: 381779
• Journal Information: Journal of the Atmospheric Sciences, Journal Name: Journal of the Atmospheric Sciences Journal Issue: 23 Vol. 52; ISSN 0022-4928; ISSN JAHSAK
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
BACKSCATTERING
CLOUDS
ICE
MICROWAVE RADIATION
PARTICULATES
REMOTE SENSING