BRIDGE

This dataset contains MicroPulse Differential Absorption Lidar (MPD) data in NetCDF format which were collected from the MicroPulse Differential Absorption Lidar (MPD) for a subset of  the BRIDGE project in the summer of 2025. One MPD was deployed to Fukue, Japan for this field project. Data was collected over a period from May 13, 2025 - Sept 3, 2025.  The locations of the instruments are listed in the table below.  

Instrument description

The diode-laser-based (DLB) lidar architecture developed by NCAR in collaboration with Montana State University (MSU) uses continuous wave seed lasers that are amplified into pulses (5-10 µJ/pulse) at high repetition rates (8k Hz)^1,2. For high quality daytime operation, suppression of the solar background is achieved with a narrow receiver field of view (100 µrad) and extremely narrow-band (10-20 pm full width half max for water vapor and 40-60 pm for oxygen) optical filters. The transmitted laser beam is eye-safe and invisible (Class 1M) and the receiver uses single photon counting detectors.

The MPD retrievals rely on two different lidar techniques. Differential absorption lidar (DIAL) is used at 828 nm to measure molecular water vapor concentration and 770 nm to measure temperature of oxygen molecules (which are assumed to be representative of the temperature of all air molecules in the volume). In addition, the high spectral resolution lidar (HSRL) technique is employed at 770 nm to measure the aerosol backscatter ratio (the relative amount of molecular and aerosol scattering in a volume).

Water Vapor DIAL

The differential absorption lidar (DIAL) technique uses two separate laser wavelengths: an absorbing wavelength (online) and a non-absorbing wavelength (offline). The ratio of the range-resolved backscattered signals between the online and offline wavelengths is proportional to the amount of water vapor in the atmosphere. The technique requires knowledge of the absorption feature (obtained from molecular absorption database) and estimates of the atmospheric temperature and pressure (obtained from surface measurements and standard atmosphere models). The technique also requires the laser wavelength to be stable and confined to a narrow band or “single frequency”. For more information, see Spuler et al. (2021) and https://www.eol.ucar.edu/mpd.

Oxygen DIAL

Oxygen DIAL operates on the same principles as water vapor DIAL where two closely spaced laser wavelengths are transmitted with one tuned to an oxygen absorption line (online) and the other just off the line (offline). Where with water vapor DIAL, we use a known absorption feature to estimate the amount of water vapor, with oxygen DIAL we use a known amount of oxygen to measure the amount of (temperature dependent) oxygen absorption, and therefore the temperature. A key caveat to oxygen DIAL is that the amount of absorption observed is different on the return trip from molecular and aerosol scattered light. As a result, the integrated HSRL is needed to inform the oxygen DIAL retrieval. See Stillwell et al. (2020) and Hayman et al. (2024) for further information on this technique.

HSRL

HSRL is a technique for separating molecular and particulate scattered light in the lidar receiver. Return light is split into two different channels. One operates the same as a backscatter lidar, detecting all of the backscattered light (combined channel) while the other has a narrow band filter that blocks the spectrally narrow particulate (or aerosol) scattered light such that only molecular scattering is detected (thus called the molecular channel). With these two observations, we get a direct measurement of the relative amounts of molecular and particulate scattered light being collected by the instrument. See Hayman et al. (2017) and Stillewell et al. (2020) and Hayman et al. (2024) for more information. 

Multi-Pulse-Length Mode

The was the first deployment with multi-pulse-length operation, which further expands the MPD's observational coverage and capability. This method, uniquely enabled by the diode-laser architecture, transmits alternating long and short laser pulses on a shot-to-shot basis. The long pulses provide high signal-to-noise ratio and exceptional high-altitude performance, while the short pulses enable observations much closer to the surface and improve vertical resolution. A novel signal processing and denoising approach, which leverages a forward model, seamlessly merges the over-constrained data from all channels into a single, high-quality estimate of atmospheric state. 

As outlined above, there are eight observation channels associated with the MPD BRIDGE deployment.  In addition to the six wavelength/detector combinations, the water vapor channels each have a long pulse (1 μs) and short pulse (200 ns) channel associated with them.  The channels are as follows:

• WVOffline - 828 nm tuned off the water vapor absorption feature with 1 μs pulse length

• WVOfflineLow - 828 nm tuned off the water vapor absorption feature with 200 ns pulse length

• WVOnline - 828 nm tuned on the water vapor absorption feature with 1 μs pulse length

• WVOnlineLow - 828 nm tuned on the water vapor absorption feature with 200 ns pulse length

• O2OfflineComb - 770 nm tuned off the oxygen absorption feature with 1 μs pulse length, detected in the combined detector (combined channel in HSRL and offline in O2 DIAL)

• O2OfflineMol - 770 nm tuned off the oxygen absorption feature with 1 μs pulse length, detected in the molecular detector (molecular channel in HSRL)

• O2OnlineComb - 770 nm tuned onto the oxygen absorption feature with 1 μs pulse length, detected in the combined detector (online channel in O2 DIAL)

• O2OnlineMol - 770 nm tuned onto the oxygen absorption feature with 1 μs pulse length, detected in the molecular detector (online channel in O2 DIAL)

References

1. S. M. Spuler, M. Hayman, R. A. Stillwell, J. Carnes, T. Bernatsky, K. S. Repasky, “MicroPulse DIAL (MPD)–a diode-laser-based lidar architecture for quantitative atmospheric profiling,” Atmos. Meas. Techniques 14(6), 4593-4616 (2021).

2. Spuler et al., Field-deployable diode-laser-based differential absorption lidar (DIAL) for profiling water vapor, Atmos. Meas. Tech., 8, 1073-1087, 2015.

3. Dowell et al.,The High-Resolution Rapid Refresh (HRRR): An Hourly Updating Convection-Allowing Forecast Model. Part I: Motivation and System Description, Wea. Forecasting, 37, 1371–1395, doi:10.1175/WAF-D-21-0151.1, 2022.

4. Repasky, K. S., C. E. Bunn,  M. Hayman, R. A. Stillwell, and S. M. Spuler, 2019: Modeling the Performance of a Diode Laser-Based (DLB) Micro-Pulse Differential absorption Lidar (MPD) for Temperature Profiling in the Lower Troposphere. Opt. Express, 27(23), 33543-33563.  10.1364/OE.27.033543

5. M. Hayman, R. A. Stillwell, A. Karboski, W. J. Marais, and S. M. Spuler, “Global estimation of range resolved thermodynamic profiles from micropulse differential absorption lidar,” Opt. Express 32(8), 14442-14460 (2024).

6. M. Hayman, R. A. Stillwell, A. Karboski, S. M. Spuler, “Signal processing to denoise and retrieve water vapor from multi-pulse-length lidar data,”, Atmos. Meas. Techniques Discussion (2025). 10.5194/egusphere-2025-3523.