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King Air airplane in background with King Air Research Aircraft: Atmospheric Science

Wyoming Cloud Lidar

 

WYOMING CLOUD LIDAR


The University of Wyoming Cloud Lidar installed aboard the UWKA and NSF/NCAR C-130

 

 

 

 

 

 

 

 

 

 

 

 

Wyoming Cloud Lidar Overview

The Wyoming Cloud Lidar (WCL) is an airborne observational system for the study of cloud structure and composition. It is installed principally on the University of Wyoming King Air (UWKA), but can also be flown on the NSF/NCAR C-130 research aircraft. Operating at 355 nm (WCL-I) and 351 nm (WCL-II) wavelength, the lidar provides high spatial resolution cloud base measurements, as well as cloud and aerosol extinction coefficient and depolarization profiles. Coupled with the in situ observations of hydrometeors and air motions from the same aircraft these data yield unique information for analysis of cloud and precipitation processes.

The WCL may be made available to users of the UWKA or NSF/NCAR C-130 through the NSF allocation process or by special requests. Click here for an overview of the request process.

The lidars were developed by Remote Sensing Group in the University of Wyoming's Department of Atmospheric Science.

Major funding for the acquisition, development and research use of the WCL has been derived from NSF, ONR, NASA and UW.

History

The zenith-pointing WCL-I was developed in 2007 and has been successfully deployed and tested on the UWKA during the Wyoming Airborne Integrated Cloud Observations Experiment (WAICO) in 2008 and 2009, and on the NSF/NCAR C-130 during the Ice Clouds Experiment-Layer Clouds (ICE-L) in 2007. Since then, it has been regularly deployed in in many projects aboard both aircraft.

The nadir-pointing WCL-II was developed in 2008 and  successfully tested during  the WAICO09 experiments.

In 2011, the WCL-II was upgraded by adding two new receiving channels which can enlarge the dynamic range of the receiving signals.

Citation

University of Wyoming - Flight Center, 2007: The University of Wyoming Cloud Lidar (WCL). University of Wyoming, College of Engineering, Department of Atmospheric Science, doi:10.15786/M25W9D.
Format Citation (ReFindit). Download metadata: XML JSON

For further information concerning the WCL, please contact Dr. Min Deng


WCL Key Features and Hardware

An Ultra Pulsed Nd:YAG Laser from the Big Sky Laser Technologies Inc. providing a 20 Hz 16 mJ output at 355 nm is used for the WCL-I. Operating at 355 nm not only makes it easy to achieve eye-safe operation, it also provides a stronger molecular backscattering signal than a lidar operating at 532 or 1064 nm with the same laser energy. This is important for calibrating backscattering coefficients. The laser beam is expanded 5 times to a diameter of 15 mm before emittance into the atmosphere, making the system eye-safe beyond a distance of ~65 m. To improve lidar linear depolarization measurements, a 1/2 λ wave plate is placed after the beam expander and coupled with a cubic polarization beam splitter in the receiver path.

The receiver in the WCL-I is based on a 75mm refractive lens with a 12.5mm collimated beam that enters into the cubic polarization beam splitter. The field of view is controlled by a pinhole located at the focal plane of the receiving lens. The PMT packages include narrow band filters (0.3 nm), a focus lens, and a compact PMT. To provide the ruggedness and stability needed for the WCL to operate in a turbulent environment, the receiver is designed to share the same optical bench with the transmitter. The PMT's gain can easily be adjusted with bias control voltage. Signals from the PMTs are sent to the LICEL data acquisition system. The data system has a combined A/D and photon counting capability. To provide high-resolution spatial measurements, only strong signals digitized by A/D at 40 MHz are saved at single shot or averaging of number of shots. Thus, the WCL can provide measurements at ~4.5 m horizontal and 3.75 m vertical resolution from the UWKA, for an average cruise speed of ~90 m/s.

Technical Specifications

Table of technical specifications for the WCL-I and II
Platform
  • University of Wyoming King Air
  • NSF/NCAR C-130
Zenith Lidar (WCL-I) Nadir Lidar (WCL-II)
Transmit Laser Wavelength 355 nm 351 nm
Pulse Energy 16 mj 200 μj
Pulse Length 8 ns 30 ns
Pulse Repetition Frequency (PRF) 20 Hz 1 KHz
Laser Beam Divergence 1 mrad 0.3 mrad
Polarization (pol) Radiated Linear Linear
Receiver Diameter 75 mm  108 mm
Receiver Field of View 2 mrad 1 mrad
Receiving Channels 2 4
Polarization Received H&V H&V
Detector PMT PMT
Range Resolution 3.75 m and up 1.5 m and up
Temporal Resolution 0.05 s and up 0.01 s and up
Data Acquisition System LICEL GAGE

WCL Data Processing and Data Format

Real-Time DSP

The WCL is software-controlled, with all output recorded in digital form by the Data Acquisition System (DAQ). The WCL DAQ performs the following operations:

Processing

  • Pulse averaging
  • Calibrate received signal strength in mV
  • Data stored on a hard disk in custom WCL format

Outputs

  • Backscatter Data Profiles (Range corrected)
  • Maximum number of recorded profiles per second (pps) depends on the laser pulse repeat rate(WCL-I: 20 Hz, WCL-II: 1K Hz)

Post Flight Processing

After a flight the raw data are archived in the native WCL format.
Post flight processing is available for:

  • Overlap factor correction
  • Producing backscattering signal strength images; images are available on the WCL project web page
  • Visualization of backscattering signal strength, uncalibrated depolarization ratio

Post Experiment Processing

  • Calculate WCL products (Backscattering signal strength, Depolarization ratio)
  • Archive WCL products in NetCDF format
  • The final data product is usually ready for further analysis and distribution no later than 2-3 months after an experiment.

Processed Data Format

  • WCL NetCDF (WCL user notes)

Data Availability

  • Data from past experiments are available to researchers upon request.

Measurement Limitations

The area around the flight level where the laser beam enters into the field of view (FOV) of the receiving telescope progressively is the "overlap zone", which is about 100 m in length, depending on the transmitted laser beam divergence, the FOV of the receiving telescope and the angle between the axes of the laser beam and telescope. The processed data are already corrected for this by using calculated overlap factors. However the correction may not be completely accurate for all of the profiles due to the drift in the laser beam's direction.

Also, the near-range signals can saturate when the aircraft is flying through dense clouds. In this situation, the clouds cannot be fully penetrated by the lidar, and the signals can be dramatically attenuated with increasing distance.

WCL Display Interface

The WCL data acquisition system uses GUIs for lidar control and data display, which connect to the data system server via a LAN.

Example of WCL real-time display interface

Example of WCL real-time display interface

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