D5 Advanced Active Remote Sensing

Topic

Advanced active remote sensing encompasses lidar systems and cloud radar—two families of instruments that complement conventional weather radars where the latter fall short: they possess greater sensitivity to small particles, such as aerosols and cloud droplets, and achieve much finer vertical resolution. Backscatter lidar is the most basic instrument in advanced active remote sensing; it emits laser light pulses at two distinct wavelengths—one near the visible green spectrum and the other in the near-infrared—and measures the amount of light returned by the aerosols and clouds it traverses. From this returned light, backscatter lidar derives vertical profiles of the atmosphere's backscatter and extinction coefficients, as well as the precise altitude of clouds and the boundary layer, all with a vertical resolution of ten to thirty meters.

Doppler lidar builds upon backscatter lidar but incorporates the same principle used by Doppler radar: it measures the frequency shift experienced by light backscattered by air molecules and tropospheric particles. From this Doppler frequency shift, it derives vertical wind speed with a precision of one to five-tenths of a meter per second, within seconds and reaching ranges of five to fifteen kilometers.

When lidar and cloud radar fly together on the same satellite, they complement each other even further; a pioneering satellite mission combined both instruments for the first time, and its more recent successor refined this combination, enabling the simultaneous characterization of aerosols, thin clouds, and precipitation—features a single instrument could not distinguish individually. The cloud radar flown on these missions operates at a much higher frequency than conventional weather radar—close to 94 gigahertz—making it sensitive to cloud droplets and ice crystals that conventional radar fails to detect. The same backscatter lidar principle—simplified and made more affordable—gave rise to ceilometers: low-cost lidars deployed in dense networks (with nearly a thousand covering Europe). Without requiring the sophistication of research-grade lidar, they measure the height of the boundary layer base and cloud bases at each station in the network.

All these instruments, from research-grade backscatter lidar to the simplest ceilometer, rely on the same fundamental relationship known as the lidar equation: the power returned to the instrument depends on how much the atmosphere backscatters the incoming light and how much it attenuates that light along the round-trip path. Consequently, the higher the backscatter coefficient of an atmospheric layer, the more power that layer returns; conversely, the higher its extinction coefficient, the less power remains in the signal to penetrate further into deeper layers.

The expected outcome of advanced active remote sensing is a set of vertical aerosol and cloud profiles with temporal and spatial resolution far superior to that of any conventional weather radar. These systems can "see" precisely where conventional weather radar is blind—detecting thin clouds that fail to saturate the radar signal or are too tenuous to produce sufficient microwave backscatter. Thus, advanced active remote sensing does not replace conventional weather radar but rather completes the picture that the latter leaves unfinished.

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