¶ Doppler Lidar (DL)
¶ Background
The Doppler Lidar (DL) is an active remote sensing instrument that provides range- and time-resolved measurements of radial velocity and attenuated backscatter. The principle of operation is similar to radar in that pulses of energy are transmitted into the atmosphere; the energy scattered back to the transceiver is collected and measured as a time-resolved signal. From the time delay between each outgoing transmitted pulse and the backscattered signal, the distance to the scatterer is inferred.
¶ Additional Information
The DL operates in the near-IR (1.5 microns) and is sensitive to backscatter from micron-sized aerosols. Aerosols are ubiquitous in the low troposphere and are ideal tracers of atmospheric winds. Thus, in contrast to radar, the DL is capable of measuring wind velocities under clear-sky conditions with very good precision (typically ~10 cm/sec). The DL also has full upper-hemispheric scanning capability, enabling the three-dimensional mapping of turbulent flows in the atmospheric boundary layer.
* For more information see Doppler Lidar
* For more information on RADAR theories and scan strategies, see Radar General Information.
¶ Expected Operations
¶ Metrics
The 3 main variables being monitored are
- Attenuated Backscatter - Energy returned to the instrument by scattering off of aerosols and particles in the atmosphere
- Intensity - Signar to Noise Ratio +1.
- Radial Velocity - The velocity towards or away the instrument along a radial
The variables being monitored for both the FPT and PPI modes are the same.
Fixed Point - Vertically Pointing
Planned Position Indicator - PPI
¶ Plots
¶ FPT Plots
Doppler Velocity
A vertical profile of doppler velocity towards (negative) and away (positive) from the lidar.
Backscatter
A vertical profile plot of the power returned from the particles in the atmosphere.
Intensity
A vertical profile plot of the signal-to-noise ratio + 1.
¶ Boundary Layer Plots
All the above plots are also plotted out from 0-3 km to get a better look at the boundary layer.
¶ Calibration Plots
The instrument needs to be very precise with the azimuth direction to correctly infer horizontal wind speeds. To ensure the system is correctly calibrated and aligned, two calibration scans occur each day.
| Calibration Scan Type | ARM Datastream | Reason |
| Point and Stare | dlcal1 | Calibration for Backscatter Values |
| Point Target Scan | dlcal2 | Calibration for Azimuthal Alignment |
Point and Stare
The instrument will point and stare at a single azimuth and elevation. This will look for changes in the backscatter as a function of time and can find issues with the basic lidar operation.
Scan Point Target
The instrument will perform a sector scan over a small azimuth angle range containing a known target. The target's location is well known and can be used to ensure the instruments azimuth angle is correctly set to true North. The location of the target should not fluctuate much from scan-to-scan, but may vary due to local atmospheric conditions. A problem will need to be systematic and occur over a few scans before we know it is a true issue.
Plots of the radial velocity, backscatter, and intensity from these scans are included as individual plots.
The azimuth, distance and intensity of the point target are tracked over a week to try and help indicate if the instruments azimuth is off. The points should be consistent and within the range indicated on the plots.
¶ PPI Plots
The other mode of operation is a Planned Position Indicator (PPI) where the instrument scans at a constant elevation over a full 360 degree scan. This produces a cone. The initially measured wind speeds are along the radial and contain a component of both the vertical and horizontal wind speeds. By measuring the radial wind speed at a known fixed elevation angle, the horizontal wind speeds can be derived for each full 360 degree scan. This process to derived horizontal wind speed from a cone of radial wind speeds is known as a Velocity Azimuth Display (VAD).
DL Derived Winds
Currently, the azimuthal resolution was greatly reduced so that there are 8 azimuth bins. Due to this decrease, the winds derived from the DL are not as good as they used to be and do not compare as well with the Sonde data. This plot is very useful if the data is of a higher resolution.
Using the Planned Position Indicator (PPI) scan horizontal wind speed and direction can be derived at each level. The wind speed and direction can be compared with a co-located Radiosonde to check for consistency. There are a number of assumptions inferred when creating a wind field by using a VAD analysis.
- Measurements around the PPI cone are sampling the same continuous layer
- Vertical wind speed and direction measured in the FPT mode are used in the process of converting radial winds into horizontal winds
- Wind speed and direction are continuous and consistent for each vertical layer
For more information on the calculations, please see http://www.knmi.nl/~beekhuis/rad_doppler.html.
PPI Videos
Movies of the PPI data (velocity, backscatter, and intensity) are included in the PPI plots. These are very hard to interpret at the low resolution. It is important to view these plots to make sure there is nothing extremely wrong, but the main indicator of the health and status of this instrument will be deduced from the vertical point data (FPT).
¶ Comparison Plots
¶ Profile Comparison Plot
This plot shows the attenuated backscatter plotted up relative to other profiling instruments onsite, including KAZR, MPL, VCEIL, RL, and more! The image below shows the DL on 5th plot with the cloud base height from the ceilometer overplotted in white. The stronger areas of backscatter indicate cloud base. These areas line up fairly well with the ceilometer overlay.
¶ Site-Specific Information
The Doppler Lidar is located at SGP C1.
¶ Known Issues
¶ Known Behaviors
List of known issues that may not need to be mentioned in DQAs:
Decrease in PPI Resolution
Around August 2012, the mentor decreased the resolution of lidar down to about 8 beams @ 60 degree elevation. This is repeated once every 15 minutes and each beam has a 2 second average stare.
¶ Past Problems
Past problems that do need to be mentioned in DQAs are below:
MAO DL
Data has historically gone missing days at a time for unknown reasons. The current theory (per DQPR 4228) is that the data logging software seems to freezes up leading to zero signal in the background data. The lack of signal causes NANs to appear in the backscatter and SNR fields. A complete restart of the lidar and onboard computer seems to fix the problem. While the instances of missing data are still somewhat rare, it is best to submit a DQPR when data are unavailable for longer than a few days-worth of time. For more information, please see DQPRs 3997, 4228, and DQRs D140702.5 and D140905.8.