Backscatter and extinction measurements in cloud and drizzle at CO_2 laser wavelengths

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 σb and volume extinction coefficient σe 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 σe and σb has the appealingly simple size distribution independent form of σe/σb = 8π/G∞, G∞=(n-1)2+k2(n+1)2+k2is 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.