In response to a community-identified need for ground-based thermodynamic (TD) profiling of the troposphere, we present the further development and validation of a differential absorption LiDAR (DIAL) technique to retrieve temperature. This paper showcases the accuracy of temperature retrievals using a perturbative technique, combining a DIAL measurement of a temperature-sensitive oxygen (O2) absorption profile with a high spectral resolution LiDAR measurement of the backscatter ratio profile near 770 nm. This study introduces three key advancements. First, the spectroscopic model used to represent the absorption of light by O2 is enhanced via a more complete physical representation, improving measurement accuracy. Second, the error estimation and masking are developed using the bootstrapping technique. Third, we present a comparison of temperature profiles from our laboratory-based instrument with collocated radiosondes, evaluating the accuracy of our updated measurements. It is essential to clarify that the instrument described in this paper does not operate as a stand-alone TD profiler, as it is not capable of measuring water vapor (WV). Instead, we focus on demonstrating the perturbative retrieval technique with temperature profiles inferred using ancillary radiosonde WV profiles. Results from a full TD profiling instrument will be presented in a future publication. The laboratory-based LiDAR instrument was operated over a 6-month period between April 21, 2022, and September 22, 2022. During this time, we launched 40 radiosondes, providing reference data to validate the accuracy of the DIAL-based temperature profiles. The results indicate that DIAL-based temperature retrievals are within ±2.5°C between 0.4 and 3 km (3.5 km) during daytime (nighttime) operation, using a 300-m range resolution and a 60-min time resolution.
KEYWORDS: LIDAR, Aerosols, Backscatter, Analog electronics, Signal to noise ratio, Linear filtering, Sensors, Motion estimation, Digital filtering, Receivers
The performance of two direct-detection atmospheric lidar systems with very different methods of generating and detecting laser radiation is compared as the result of a field experiment held in March 2015, in Chico, California. During the noncontinuous, 11-day test period, in which the systems operated side by side, the micropulse lidar was operated at its maximum pulse repetition frequency (15 kHz) and integrated elastic backscatter over the interpulse period of the analog direct-detection lidar (0.1 s). Operation at the high pulse repetition frequency resulted in second-trip echoes that contaminated portions of the data. The performance of the micropulse lidar varied with background brightness—as expected with a photon-counting receiver—yet showed equal or larger backscatter intensity signal-to-noise ratio throughout the experiment. Examples of wind fields and time series of wind vectors from both systems during the Chico experiment are presented. In addition, scans over the ocean that were collected by the micropulse lidar during a subsequent deployment on the northern California coast are presented. We conclude by reviewing the advantages and disadvantages of each system and make some suggestions to improve the design and performance of future systems.
Although operating at the same near-infrared 1.5- m wavelength, the Raman-shifted Eye-safe Aerosol Lidar (REAL) and the Scanning Aerosol Micro-Pulse Lidar-Eye-safe (SAMPLE) are very different in how they generate and detect laser radiation. We present results from an experiment where the REAL and the SAMPLE were operated side-by-side in Chico, California, in March of 2015. During the non-continuous, eleven day test period, the SAMPLE instrument was operated at maximum pulse repetition frequency (15 kHz) and integrated over the interpulse period of the REAL (0.1 s). Operation at the high pulse repetition frequency resulted in second trip echoes which contaminated portions of the data. The performance of the SAMPLE instrument varied with background brightness--as expected with a photon counting receiver|--yet showed equal or larger backscatter intensity signal to noise ratio throughout the intercomparison experiment. We show that a modest low-pass filter or smooth applied to the REAL raw waveforms (that have 5x higher range resolution) results in significant increases in raw signal-to-noise ratio and image signal-to-noise ratio--a measure of coherent aerosol feature content in the images resulting from the scans. Examples of wind fields and time series of wind estimates from both systems are presented. We conclude by reviewing the advantages and disadvantages of each system and sketch a plan for future research and development activities to optimize the design of future systems.
Improvements to the original NCAR/NSF Raman-shifted Eye-safe Aerosol Lidar (REAL) made between 2008 and 2013 are described. They are aimed mainly at optimizing and stabilizing the performance of the system for long-term, unattended, network-controlled, remote monitoring of the horizontal vector wind field and boundary layer height, and observing atmospheric boundary layer phenomena such as fine-scale waves and density current fronts. In addition, we have improved the polarization purity of the transmitted laser radiation and studied in the laboratory the effect of the beam-steering unit mirrors on the transmitted polarization as part of a longer-term effort to make absolute polarization measurements of aerosols and clouds.
A multi-dimensional scanning lidar has been developed for tracking and monitoring aerosol plumes in urban
settings. The reliability of the system has been demonstrated and plans for additional units are in place to
create a unique scanning lidar network. The paper discusses the current capabilities of the instrument and
research underway to extract more information, such as quantitative aerosol properties, from the network.
The ability to detect relative changes in backscatter polarization from a scanning high-pulse-energy lidar system at 1.54-µm wavelength is demonstrated. The new capability was tested during the dissemination of various biological aerosol simulants and other particulate emissions at the U.S. Army's Dugway Proving Ground. Results demonstrate that the lidar is sensitive to different types of aerosols, and departures from the atmospheric background depolarization ratio are consistent with the limited amount of information available on the degree of particle sphericity. We conclude that the polarization-sensitive coatings of the beam-steering unit mirrors are presently the largest source of error and that this error is minimized when scanning with a near-zero elevation angle. This is an encouraging result for aerosol source surveillance applications, where the depolarization information may be useful in determining the aerosol generation mechanism or provide an additional scalar variable for use in delineating the plume from the background.
An InGaAsSb/AlGaAsSb phototransistor has been validated for lidar atmospheric remote sensing. The validation was performed using the Raman-shifted eye-safe aerosol lidar (REAL) at the National Center for Atmospheric Research. Although the device is optimized for detection around the 2-µm wavelength, the validation was performed at 1.543 µm, where mature commercial detectors are available. Simultaneous measurement of the atmospheric backscatter signals using the custom-built phototransistor and commercial InGaAs avalanche photodiode indicated good agreement between both devices. The validation included detecting 11-km-range hard targets, 5-km atmospheric structure consisting of cirrus clouds, and a near-field boundary layer. Far-field low intensity and spatially narrow atmospheric features were also detectable with the new phototransistor. Preliminary results related to systematic effects are discussed in the first attempt of incorporating a phototransistor in a lidar system.
Effective monitoring of the atmosphere for potentially hazardous aerosol plumes in urban areas requires a lidar that produces high signal-to-noise backscatter returns, fine spatial resolution, rapid updates, eye-safety at all ranges, and long-range operation. A scanning elastic backscatter lidar with high pulse energy that meets these requirements was recently developed at NCAR. The latest upgrades to the lidar system include the use of a new Raman cell for wavelength conversion and a two-channel receiver for backscatter depolarization ratio measurements. Highlights from recent field tests of the system are presented and plans to improve the prototype, as well as construct an unattended and continuously operating version, are discussed.
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