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ECE 583Lectures 22

LIDARDifferent types of LIDAR

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•DIAL

•Raman LIDAR

•Wind LIDAR

•Spaceborne LIDAR

• HSRL LIDAR

Types of LIDAR

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DIAL

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N – Gaseous Species Densityk – Gaseous Species Absorption Cross Section per Unit Density

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AEROSOLES

OZONE

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DIAL Lidar Limitations:

•DIAL requires high power, complex and expensive lasers with reliability limitation to generate and control transmitted energy precisely on spectral absorption lines.

•Like all lidar DIAL does not operate through dense clouds and DIAL retrievals are problematic even for thin clouds.

•DIAL involve a differential signal analysis where the answer is a small different between two large quantities. Thus DIAL requires a much larger signal-to-noise signal and lidar system than aerosol lidar.

•DIAL to date requires large expensive, non-eyesafe Lidar systems that need the support of large engineering teams.

•Space borne DIAL is turning out to be a “Big Lie” of science similar to controlled nuclear fusion. Ground and airborne system rely on long pulse averaging. Space platform move at ~7000 m/s compared to ~200 m/s for aircraft. Currently both NASA and ESA have no plans for space DIAL.

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•DIAL

•Raman LIDAR

•Wind LIDAR

•Spaceborne LIDAR

• HSRL LIDAR

Types of LIDAR

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Limitation: Raman scattering cross sections are a thousand to ten thousand times smaller than Rayleigh scattering cross sections.

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The Raman Lidar (RL) is an active, ground-based laser remote sensing instrument that measures vertical profiles of water-vapor mixing ratio and several cloud- and aerosol-related quantities. Lidar (light detection and ranging) is the optical analog of radar, using pulses of laser radiation to probe the atmosphere. This system is fully computer automated, and will run unattended for many days following a brief (~5-minute) startup period. The self-contained system (requiring only external electrical power) is housed in a climate-controlled 8'x8'x20' standard shipping container.Output Dataraw backscattered profiles at 355, 387, and 408 nm

DOE ARM Raman Nitrogen and Water Vapor Lidar

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Raman Lidar Profiling of Water Vapor and Aerosolsover the ARM SGP Site

Richard Ferrare, David Turner, Lorraine Heilman, Wayne Feltz,Randy Peppler, Tim Tooman, Rangasayi Halthore

http://yard.arm.gov/~turner/doe_aerosols.html

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• Fixed site• Vertical only• Nearly Continuous Operation• Nd:YAG (355 nm) (day/night)

– 12 W• 61 cm telescope• Wavelengths

– Rayleigh/Aerosol (355 nm)– Depolarization (355 nm)– Raman water vapor (408 nm)– Raman nitrogen (387 nm)

• 39 meter range resolution• low, high sensitivity channels

Additional information: http://www.arm.gov/docs/instruments/static/rl.html

Southern Great Plains (SGP) CART Raman Lidar

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• Between April 1998 through present, system up ~50% of time• Prior to Feb 1999, electrical power interruptions caused most downtime

Raman Lidar System Status

Sep Nov Jan Mar May Jul Sep Nov Jan Mar May July Sep Nov0

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Laser Repairs

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Replacement of Laser Rodand Flashlamps; installation of UPS

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Raman Lidar Water Vapor Measurements

• Mixing Ratio

• Relative Humidity

– computed from lidar mixing ratio and AERI+Model temperatures

• Precipitable Water– computed from integral of lidar mixing ratio

=⎛⎝⎜

⎞⎠⎟

=⎛⎝⎜

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massof watervapormassof dryair

RamanH O signalRaman N signal

differentialtransmission

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=⎛⎝⎜

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mixing ratiosaturation mixing ratio

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• Scattering Ratio

• Backscatter Coefficient– computed from scattering ratio and molecular backscatter

coefficient from AERI+Model density• Extinction Coefficient

– computed from derivative of the Raman N2 signal • Extinction/Backscatter Ratio [ωoPa(180o)/4π]-1

• Optical Thickness– computed from integral of extinction coefficient

Raman Lidar Aerosol Measurements

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Rayleigh Mie signalRaman N signal

differentialtransmission

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CART Raman Lidar/AERI+Model Clear-Sky ProductPeriod: April 1998-present (~50% coverage in time)Location:DOE ARM SGP CF site (Lamont , Oklahoma) (36o 37 ' N, 97o 30 ' W)Data: available via ftp from ARM Experiment Center (http://www.arm.gov)Color images at http://yard.arm.gov/~turner/raman_lidar_quicklooks.htmlMore information at http://yard.arm.gov/~turner/doe_aerosols.htmlMeasurement Altitude

RangeVerticalResolution

NominalTemporalResolution

Error Precision Detection Limit

AerosolBackscattering(355 nm)

0.060-8 km 78 m 10 min 5-10% 2% 0.0002-0.0004 km-sr-1

Aerosol Extinction(355 nm)

0.1-8 km 150-500 m 10 min 5-10% 5% 0.02-0.03 km-1

Aerosol OpticalThickness (355 nm)

- - 10 min 5% or0.03

5% 0.03

Water Vapor MixingRatio

0.060-8 km(night)0.060-4 km(day)

78 m 2-10 min 5% 2% 0.002 g/kg

Relative Humidity 0.060-8 km(night)0.060-4 km(day)

78 m 2-10 min 5% 5% 1%

Precipitable WaterVapor

- - 10 min 5% 5% 2 mm

Linear Depolarization 1-14 km 39 m 1-10 min 10% 2%Temperature(AERI+Model)

0-3 km(AERI)3-15(Model)

100 m- 1 km

8 min 1 K 1 K

Cloud Base Height 0.060-14 km 78 m 1-10 min 78 m 39 m 0.060 km

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Examples of “Quick-look Images”

http://dev.ec.arm.gov/~turner/raman_lidar_quicklooks.html

Water Vapor Mixing Ratio

Relative Humidity

Aerosol Backscatter

Aerosol Extinction

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Examples of “Quick-look Images”

http://dev.ec.arm.gov/~turner/raman_lidar_quicklooks.html

Linear Depolarization

Cloud Mask

Temperature

Potential Temperature

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Examples of “Quick-look” Diagnostics

http://dev.ec.arm.gov/~turner/raman_lidar_quicklooks.html

Raman Lidar

Radiosonde

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Examples of “Quick-look” Diagnostics

http://dev.ec.arm.gov/~turner/raman_lidar_quicklooks.html

Precipitable Water Vapor

Water Vapor Mixing Ratio (60 m)

Aerosol Optical Thickness

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Mean Aerosol Extinction Profiles

• Seasonal variability in aerosol optical thickness• Vertical distributions vary with aerosol optical thickness and season

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Mean Water Vapor Profiles

• Seasonal variability in precipitable water vapor• Smaller variations in vertical distribution

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•DIAL

•Raman LIDAR

•Wind LIDAR

•Spaceborne LIDAR

•HSRL LIDAR

Types of LIDAR