John Smith Defense
Exploration of a whole atmosphere lidar concept for whole atmosphere science: Advances in resonance Doppler lidar technologies
Candidate: John A. Smith
Adviser: Prof. Xinzhao Chu
Although resonance Doppler lidars have made substantial contributions to our understanding of the mesosphere and lower thermosphere (MLT), today's whole atmosphere models demand a more complete picture which will require mobile instruments with superior resolution and coverage (0-200+ km). Higher resolution is needed to capture smaller scale impacts on transport of energy and momentum in the MLT and greater coverage to understand the ways in which disturbances originating in the lower atmospheres can govern both large and small scale dynamics in the MLT and higher into the thermosphere. A whole atmosphere lidar is needed. Efforts to extend the coverage of wind and temperature in the lower atmosphere with resonance lidars have so far been of limited scientific use since a large gap in coverage still exists where scattering by the Cabannes-Mie/Rayleigh process is too weak and unbound meteoric metals do not extend low enough. Many direct detection techniques suffer from measurement contamination from aerosols also and a measurement sensitivity which can vary depending on the temperature and pressure of the scattering volume, making measurements unreliable. We have a two-pronged approach to address the need for a whole atmosphere instrument in the community.
First, by improving receiver efficiencies of resonance lidars we can reduce photon noise and resolve smaller scale features at higher temporal resolutions. A critical gap in temperatures derived by the Rayleigh lidar technique can be filled by increasing the available signal between where the Rayleigh signal diminishes and where the scattering from resonant species such as Fe picks up in the MLT region. Higher signal levels will also lower the detection threshold for tenuous layers of metal in the thermosphere and enable studies of thermospheric Fe and Na up to 200 km with far better detail. We show how optical design methods and certain alignment considerations were used to improve the sensitivity of existing resonance-fluorescence lidars by several factors. Cases of the implementation of this technique at Boulder, Colorado and Cerro Pachón, Chile show that our procedures in the design and alignment of resonance receivers has improved signal levels by as much as a factor of five - sufficient to directly resolve seasonal vertical eddy flux in the MLT region for the first time and detect extremely tenuous layers of Na in the thermosphere above Cerro Pachón which were previously unstudied.
Second, a novel receiver design concept using a field-widened Mach-Zehnder interferometer (MZI) is investigated which will enable a resonance lidar to conduct wind measurement below the MLT region. The MZI has emerged as a promising direct detection wind technique since it avoids the problem of contamination from aerosols and has been shown to have the best theoretical performance among molecular direct detection techniques. When used in the receiver of an Fe-Doppler lidar system, we show that a particular ratio formulated from the signals in each channel and frequency can be made nearly as sensitive to a unit Doppler shift as the most sensitive implementations of the MZI, yet is also insensitive to aerosol backscatter ratio and temperature and pressure of the scatter volume. Summation of the two channels results in a signal which is independent of the intermediate MZI and can therefore be used harmoniously with the existing resonance Doppler 3-frequency technique, thereby enabling routine coverage of wind from the near surface to more than 115 km with a single lidar. The implications for such measurements are an improved understanding of, for example, gravity wave spectra propagation, Brewer-Dobson circulation and the 'cold pole' problem and the tracing of inertia gravity waves to their source regions.