Backscatter and Extinction in Water Clouds
ARMY ELECTRONICS RESEARCH AND DEVELOPMENT COMMAND WSMR NM ATMOSPHERIC SCIENCES LAB
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Atmospheric clouds can adversely affect the operation of military electro-optical systems, particularly under slant path scenarios. The probing of clouds by the lidar technique, in which a short pulse of laser radiation scattered backwards by the cloud droplets is detected, is attractive for two reasons. First, it is a remote sensing technique, and measurements at ranges of several kilometers are possible. Second, it can give a two-dimensional picture of the cloud and trace its time-development. Unfortunately, in some cases meaningful information from a lidar backscatter signal is hard to obtain. If the backscatter signal could somehow be related to a more interesting quantity, for example the extinction coefficients, the value of the lidar measurement would increase substantially. In this report a linear relation between the volume extinction coefficient and backscatter coefficient of atmospheric cloud at visible and near-infrared wavelengths is derived. The relation is in good agreement within 50 percent with Mie calculations of extinction and backscatter coefficients based on 156 measurements of cloud droplet spectra in cumulus and stratus type clouds. The relation suggests that visible or near- infrared extinction coefficients in a cloud of unknown type could be inferred from lidar backscatter measurements alone, at least near the cloud boundary where the contribution of multiply scattered photons to the lidar return signal can be neglected. No similar size-distribution-independent relation between backscatter coefficient and liquid water content of cloud is found, suggesting that cloud liquid water content cannot be inferred solely from lidar backscatter measurements.
- Atmospheric Physics
- Lasers and Masers