Abstract

The backscatter and extinction of laboratory generated cloud and drizzle sized drops have been measured at CO₂ laser wavelengths (predominately at λ = 10.591 μm). Measurements of volume backscatter coefficient σ_b and volume extinction coefficient σ_e for laboratory cloud of predominantly <20-μm radius droplets are dependent on the form of the size distribution in agreement with numerical prediction. For drops of ⪞20 μm at λ = 10.591 μm the relation between σ_e and σ_b has the appealingly simple size distribution independent form of σ_e/σ_b = 8π/G_∞,

is the asymptotic value of the backscatter gain, where n and k are the real and imaginary indices of refraction. The linear relation is in good agreement with extinction and backscatter measurements made on laboratory generated drizzle sized drops (r > 20 μm). This suggests that the extinction coefficient at CO₂ laser wavelengths could be inferred from lidar backscatter return signals without requiring knowledge of the size distribution for drizzle and spherical precipitation sized water drops.

© 1986 Optical Society of America

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