introduction_lidar

L.1 Introduction to Aerosol Lidar: Lidar principle, lidar equation, scattering of radiation in the atmosphere

This lecture is about remote measurements of aerosol particles
? Strictly speaking remote measurements is not the same as remote sensing → Remote measurements: active radiation source → Remote sensing: passive radiation source ? Aerosol particles or simply particles: → Solid and/or liquid components of the atmosphere → In contrast: Aerosols are the mixture of aerosol particles and the gaseous components of the atmosphere

LITERATURE
E. D. Hinkley Laser Monitoring of the Atmosphere Springer Verlag, Berlin, 1976 R. M. Measures Laser Remote Sensing, Fundamentals and Applications Krieger Publishing Company, Malabar, Florida, 1992 V. A. Kovalev and W. E. Eichinger Elastic Lidar. Theory, Practice, and Analysis Methods Wiley VCH, Weinheim, 2004 C. Weitkamp LIDAR --- Range-resolved Remote Sensing of the Atmosphere Springer Verlag, Heidelberg, 2005

The challenge Global warming (IPCC 2001)
Forecasts of global warming for the 21st century Each bar is
Forecasts: climate models using different expected social & industrial development scenarios one model forecast (2100), different emission scenarios
different emission scenarios (SRES)
Dispersion between different models (bars) shows model uncertainties

The challenge Cloud Radiative Forcing (ΔCRF)
Change in TOA fluxes in 10 models due to clouds for CO2 doubling
3
Change in cloud radiative forcing at TOA (W m-2)
2
SW LW NET
1
? more aerosol and low cloud cool the climate by reflecting more sunlight to space ? more high clouds warm the climate by reducing the IR loss to space
0
-1
-2
Le Treut & McAvaney (2000) – after IPCC TAR
-3
1
2
3
4
5
6
7
8
9
10
Models
dispersion of the predictions D similar to IPCC external factors

The challenge AMIP present climatology
Atmospheric Models Intercomparison Project – 14 climate Models
Vertically integrated cloud water (liquid and solid phase) AMIP2 JJA zonal seasonal mean
Present Climate
0.25
0.20
0.15
0.10
But all models are tuned to give the same mean TOA radiative flux
Kg m-2
0.05
90N
80
60
40
20
0
-20
-40
-60
-80 90S
Latitude
Cloud parameterisation deficient D can’t model clouds consistently in the present climate

The challenge Required observations
Clouds
? Geometry (top, base, multiple layers, fractional cover/overlap) ? Vertical profiles of ice/liquid water content and ice particle size ? Super-cooled cloud layers ? Small scale (1km) fluctuations in cloud properties. ? Light precipitation ? Vertical motions
Aerosol
? Height and optical depth of aerosol layers, aerosol size and type
Radiation
? Short-wave (SW) and long-wave (LW) radiances at TOA ? Spectrally resolved top of the atmosphere LW radiances ? Water vapour and temperature profiles

The challenge Radiative forcing (IPCC 2001)
Importance, uncertainties and understanding of EXTERNAL FACTORS forcing climate change
Global mean radiative forcing of the climate system for the year 2000, relative to 1750
3
Radiative forcing (W m-2)
Warming
2
Halocarbons N2O CH4 CO2 Tropospheric ozone
Aerosols Black carbon from fossil fuel burning
1
Mineral dust
Aviation-induced Contrails Cirrus
Solar
0
Cooling
Stratospheric ozone -1
Organic carbon Biomass Sulphate from burning fossil fuel burning
Aerosol indirect effect
Land-use (albedo) only
-2
High Medium Medium Low Very Low Very Low Very Very Low Low Very Low Very Low Very Very Low Low
Level of Scientific Understanding
Large uncertainties for aerosol effects – especially indirect effect whereby aerosols change cloud properties

Why Vertically Resolved Observations?
? Different Transport Processes in the Boundary Layer and the Free Troposphere ? Different Lifetime and Transport Range of the Particles Boundary Layer : 2-4 Days ~ 2000 km Free Troposphere: at least 3 weeks ~10000 km
Podgorny and Ramanathan, JGR, 106, 24097-24105, 2001
clear sky
aerosols underneath clouds
aerosols above clouds

Importance of particles in the (free) troposphere
●
Inluence on radiative transport (Quijano et al., JGR 2000): ? “vertical position” of particle layers
CLEAR DUST
DUST CLEAR DUST
DUST CLEAR

There are few Investigations (Particularly Over Long Periods of Time) on Particle Properties in the Free Troposphere
.
A. Stohl: Intercontinental Transport of Air Pollution, Springer-Verlag, 2004
→ lack of vertical resolution of passive remote sensing methods ? satellite, sun photometer
→ In-situ measurements with aircraft only during field campaigns ? low statistical significance → aerosol measurements with Multiwavelength Raman Lidar

Lidar …
? ? ? ? The ?basic“ principle System setups Lidar equation Solution for the so-called elastic backscatter lidar: Klett algorithm ? Solution for the so-called Raman lidar: Raman-lidar technique

Transmission loss → extinction coefficient: scattering and absorption of radiation
Scattering volume: Particles and molecules
Radiation pulse (wavelength λ) from lidar
L A S E R
Reflection → backscatter coefficient

Return signal
L A S E R
Analysis of Signals Intensity of backscattered radiation (light) as a function of - time - height - wavelength
Receiver Telescope

LIght Detection And Ranging LI A R D
TIME
Pulse Laser Receiver
Some Numbers (Speed of light: 3 x 108 m/s) Target at distance z1 = 1 km → return signal at t2 = 6.6 x 10-6 s (6.6 μs) Target at distance z1 = 12 km → return signal at t2 = 80 x 10-6 s (80 μs)

LIght D Detection A And R Ranging LI
HEIGHT

Lidar Equation
Backscatter Coefficient (m-1sr-1)
Extinction Coefficient (m-1)
Nmol : molecule conc. [1/m3] Npar : particle conc. [1/m3] σ: scatt. cross-section [m2] λo : emitted [m] λ: received [m] λo ≠ λ

The Basics of a Lidar:
laser, telescope and a bit more optics, detector
laser semi-transparent mirror light-beam to radar target
photo detector
monitor
light-beam from radar target
amplifier of electrical signal
photo detector
filter
mirror telescope

POLLY System at IfT, Leipzig (Germany)

POLLY System at IfT, Leipzig (Germany)
rain sensor c ove r on the roof status: open
air-conditioning
power su pply and heat ex cha nge r E of laser
H
bread board with optical setup
I G H T ≈ 170 cm
sla ve computer for housekeeping
main co mputer with DAQ and main program
Weight: 500 kg
Width ≈ 160 cm
h t d a e cm r B 80 ≈