Abstract. The micropulse differential absorption lidar (MPD) was developed at Montana State University (MSU) and the National Center for Atmospheric Research (NCAR) to perform range-resolved water vapor (WV) measurements using low-power lasers and photon-counting detectors. The MPD has proven to produce accurate WV measurements up to 6 km altitude. However, the MPD's ability to produce accurate higher-altitude WV measurements is impeded by the current standard differential absorption lidar (DIAL) retrieval methods. These methods are built upon a fundamental methodology that algebraically solves for the WV using the MPD forward models and noisy observations, which exacerbates any random noise in the lidar observations. The work in this paper introduces the adapted Poisson total variation (PTV) specifically for the MPD instrument. PTV was originally developed for a ground-based high spectral resolution lidar, and this paper reports on the adaptations that were required in order to apply PTV on MPD WV observations. The adapted PTV method, coined PTV-MPD, extends the maximum altitude of the MPD from 6 to 8 km and substantially increases the accuracy of the WV retrievals starting above 2 km. PTV-MPD achieves the improvement by simultaneously denoising the MPD noisy observations and inferring the WV by separating the random noise from the non-random WV. An analysis with 130 radiosonde (RS) comparisons shows that the relative root-mean-square difference (RRMSE) of WV measurements between RS and PTV-MPD exceeds 100 % between 6 and 8 km, whereas the RRMSE between RS and the standard method exceeds 100 % near 3 km. In addition, we show that by employing PTV-MPD, the MPD is able to extend its useful range of WV estimates beyond that of the ARM Southern Great Plains Raman lidar (RRMSE exceeding 100 % between 3 and 4 km); the Raman lidar has a power-aperture product 500 times greater than that of the MPD.
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Demonstration of a combined differential absorption and high spectral resolution lidar for profiling atmospheric temperature
This work presents the first demonstration of atmospheric temperature measurement using the differential absorption lidar (DIAL) technique. While DIAL is routinely used to measure atmospheric gases such as ozone and water vapor, almost no success has been found in using DIAL to measure atmospheric temperature. Attempts to measure temperature using a well-mixed gas like oxygen (
- NSF-PAR ID:
- 10128444
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Optics Express
- Volume:
- 28
- Issue:
- 1
- ISSN:
- 1094-4087; OPEXFF
- Page Range / eLocation ID:
- Article No. 71
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract A recent laboratory study suggests that water vapor displays structured absorption features over the 290–350 nm region, with maximum and minimum cross‐sections of 8.4 × 10−25and 1.4 × 10−25 cm2/molecule at room temperature (Pei et al. 2019,
https://doi.org/10.1029/2019jd030724 ; Du et al., 2013,https://doi.org/10.1002/grl.50935 ). To observe water vapor absorption features in the ultraviolet (UV) region in the atmosphere, a United States Department of Agriculture reference spectroradiometer was upgraded with a new fore‐optical module, enabling it to measure direct solar beam and sky radiance at given azimuth and elevation angles. This double Czerny‐Turner spectroradiometer enables wavelength scanning from 290 to 410 nm, with a nominal bandwidth of 0.1 nm. It can operate with a step‐size of 0.0005 nm and a full width at half maximum of 0.1 nm. It has an out‐of‐band rejection ratio of approximately 10−10. This high resolution spectroradiometer can be used as a reference instrument for UV radiation measurements and for monitoring atmospheric gases such as O3, SO2, and NO2. A series of field observations were made using this spectroradiometer in the University at Albany campus. A residual analysis method is developed to analyze absorption by atmospheric components and to retrieve atmospheric optical depth. The residual optical depth was calculated by subtracting the optical depths of Rayleigh scattering, aerosol extinction, and absorption of typical atmospheric gases such as O3, SO2, and NO2from the retrieved total optical depth. Multiple case studies show that residual optical depth from the observed UV spectra is sensitive to the atmospheric water vapor amount. The greater the water vapor path, the larger the magnitude of residual optical depth. The ozone amount was inferred from the residual analysis; it is comparable to the satellite measurements. For example, in a case with water vapor path of 13 mm on October 24, 2019, the inferred ozone amount from residual analysis is 2.7% lower than retrievals from the Ozone Monitoring Instrument‐Total Ozone Mapping Spectrometer. -
Traditionally, quantitative lidar techniques like differential absorption lidar (DIAL) and high-spectral-resolution lidar (HSRL) utilize high-power-aperture product designs, which partially compensates for the need to take discrete deriva-tives of noisy data in post-processing (for number density for DIAL and extinction for HSRL) and provides for high-performance measurements, i.e., higher resolu-tion, accuracy, or precision. Conversely, low-power-aperture product lidar designs are easier to make eye-safe, reliable, and cost-effective, which are important attributes for network development and field deployment. The atmospheric science community has expressed the need for high-quality, quantitative, robust, network deployable, and cost-effective sensors for a variety of applications such as improved numerical weather forecasting – in essence requiring the best of both worlds without the accompanying drawbacks. In response to this need, the National Center for Atmospheric Research and Montana State University have been developing the MicroPulse DIAL (MPD) architecture for thermodynamic profiling in the lower atmosphere. The MPD architecture takes advantage of the benefits of low-power, low-cost laser diodes, and fiber optics to achieve quantitative profiling leverag-ing narrowband filtering and efficient elastic scattering. A field-deployable MPD instrument capable of humidity, quantitative aerosol, and temperature profiling has recently been developed. This presentation will describe the current status of this thermodynamic profiler and the initial results from a recent field deployment. Emphasis will be given to the analysis of the temperature data including compar-isons to co-located radiosondes to describe current performance.more » « less
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We demonstrate thermodynamic profile estimation with data obtained using the MicroPulse DIAL such that the retrieval is entirely self contained. The only external input is surface meteorological variables obtained from a weather station installed on the instrument. The estimator provides products of temperature, absolute humidity and backscatter ratio such that cross dependencies between the lidar data products and raw observations are accounted for and the final products are self consistent. The method described here is applied to a combined oxygen DIAL, potassium HSRL, water vapor DIAL system operating at two pairs of wavelengths (nominally centered at 770 and 828 nm). We perform regularized maximum likelihood estimation through the Poisson Total Variation technique to suppress noise and improve the range of the observations. A comparison to 119 radiosondes indicates that this new processing method produces improved temperature retrievals, reducing total errors to less than 2 K below 3 km altitude and extending the maximum altitude of temperature retrievals to 5 km with less than 3 K error. The results of this work definitively demonstrates the potential for measuring temperature through the oxygen DIAL technique and furthermore that this can be accomplished with low-power semiconductor-based lidar sensors.
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Characterization of the thermal gradients within supersonic and hypersonic flows is essential for understanding transition, turbulence, and aerodynamic heating. Developments in novel, impactful non-intrusive techniques are key for enabling flow characterizations of sufficient detail that provide experimental validation datasets for computational simulations. In this work, Resonantly Ionized Photoemission Thermometry (RIPT) signals are directly imaged using an ICCD camera to realize the techniques 1D measurement capability for the first time. The direct imaging scheme presented for oxygen-based RIPT (O2RIPT) uses the previously established calibration data to direct excite various resonant rotational peaks within the S-branch of the
C 3Π, (v = 2) ←X 3Σ(v ′ = 0) absorption band of O2. The efficient ionization of O2liberates electrons that induce electron avalanche ionization of local N2molecules generating N2+, which primarily deexcites via photoemissions of the first negative band of. When sufficient lasing energy is used, the ionization region and subsequent photoemission signal is achieved along a 1D line thus, if directly imaged can allow for gas temperature assignments along said line; demonstrated here of up to five centimeters in length. The temperature gradients present within the ensuing shock train of a supersonic under expanded free jet serves as a basis of characterization for this new RIPT imaging scheme. The O2RIPT results are extensively compared and validated against well-known and established techniques (i.e., CARS and CFD). The direct imaging capability fully realizes the technique’s fundamental potential and is expected to be the standard of implementation going forward. The direct imaging capability can play instrumental roles in future scientific studies that rely upon acute characterization of thermal gradients within a medium that cannot be easily resolved by a point. Furthermore, the removal of the spectrometer greatly reduces the cost, complexity, and optical alignment associated with prior RIPT measurements.
