LIDAR- Light Detection and LIDAR- Light Detection and RangingRanging

Lidar = “laser-radar”Lidar = “laser-radar” RADAR-wavelengths: mm, cmRADAR-wavelengths: mm, cm LIDAR-wavelengths: 250 nm-10 μmLIDAR-wavelengths: 250 nm-10 μm Principle: short energetic pulses of Principle: short energetic pulses of

electromagnetic radiation are emitted into the electromagnetic radiation are emitted into the atmosphere, hiting objects and scattered in all atmosphere, hiting objects and scattered in all directions. Parts of the radiation will be directions. Parts of the radiation will be backscattered to a receiver, which is usually backscattered to a receiver, which is usually close to the emitter.close to the emitter.

LIDAR continuedLIDAR continued

Since the radiation propagates at the speed of Since the radiation propagates at the speed of light, you can determine the distance to the light, you can determine the distance to the scattering object by measuring the time from scattering object by measuring the time from puls emission to signal reception.puls emission to signal reception.

LIDARs are well suited for observations of LIDARs are well suited for observations of fairly small objects, while RADARs cannot fairly small objects, while RADARs cannot observe objects smaller than raindrops. observe objects smaller than raindrops.

The LIDAR-pulses are scattered by molecules The LIDAR-pulses are scattered by molecules and aerosols which are always present in the and aerosols which are always present in the atmosphere. Consequently, the pulses are atmosphere. Consequently, the pulses are significantly reduced with height (this causes significantly reduced with height (this causes problem in fogy or cloudy conditions).problem in fogy or cloudy conditions).

The LIDAR equation 1The LIDAR equation 1

Backscattered flux from one single particle:Backscattered flux from one single particle:

ringbackscattefor ion Phasefunct)(

receiver object to scattering from distance

section cross scattering

particle) the(towardsflux incoming4

)()(

2

P

r

Fr

PFF

s

i

sis

The LIDAR equation 2The LIDAR equation 2

We can rewrite the equation in the following We can rewrite the equation in the following way:way:

section-crossr backscatte)(

radius with sphere a of surface4

4)(2

2

P

rr

rFF

s

si

The LIDAR equation 3The LIDAR equation 3

The incoming flux can be written in terms of The incoming flux can be written in terms of the transmitted power:the transmitted power:

rA

P

A

PF

t

t

t

ti

distanceat beamlaser theof areasection Cross

[W]power dtransmitte

The LIDAR equation 4The LIDAR equation 4

The received backscattered power can be The received backscattered power can be written in the following way: written in the following way:

power redbackscatte received

receiver theof area

4

)()(

0,

20,

r

r

t

rstrsr

P

A

rA

APPAFP

The LIDAR equation 5The LIDAR equation 5

We can define the mean scattering cross section:We can define the mean scattering cross section:

So the backscattered power for an average particle So the backscattered power for an average particle is given by is given by →→

particles ofdensity number

NNs

s

LIDAR equation 6LIDAR equation 6

The pulse length transmitted by the lidar The pulse length transmitted by the lidar system is system is ΔΔh. At any given time the receiver h. At any given time the receiver receives scattered energy from ½ pulse receives scattered energy from ½ pulse length. The total number of particles within length. The total number of particles within an effective scattering volume will therefore an effective scattering volume will therefore be: Nbe: N··AAtt··ΔΔh/2 h/2

t

srtor Ar

PAPP

2, 4

)(

Effective scattering volume:Effective scattering volume:

LIDARLIDAR equation 7equation 7

We can rewrite the equation again:We can rewrite the equation again:

But we must also take extinction into account:But we must also take extinction into account:

tcoefficien scattering volume24

)(20,

s

srtr r

hPAPP

r

err drrPP0

0, ')'(2exp

LIDAR equation 8LIDAR equation 8We end up with the final LIDAR equation:We end up with the final LIDAR equation:

AArr,P,Ptt and and ΔΔh are all well known parameters, but….h are all well known parameters, but….factor instrument

tcoefficien ringbackscatte volume

')'(2exp8

)(

02

C

drrr

hrCAPP

r

ert

r

2 unknown in the lidar-equation,2 unknown in the lidar-equation,ββee & & ββππ

ββee og og ββππ are depending on the consentration and are depending on the consentration and the optical properties of the scattering the optical properties of the scattering particles/molecules particles/molecules

For Rayleigh-scattering there is a simple For Rayleigh-scattering there is a simple relationship between the two parameters: relationship between the two parameters: ββππ//ββee=1.5=1.5

For Mie-scattering: The phasefunction is strongly For Mie-scattering: The phasefunction is strongly dependent on the sizeparameter dependent on the sizeparameter αα=2=2ππa/a/λλ. Strong . Strong forward scattering is characteristic.forward scattering is characteristic.

Common approximation for warm (liquid phase) Common approximation for warm (liquid phase) clouds: clouds: ββππ//ββee=0.63=0.63

ββee & & ββππ

The use of a backscattered LIDAR-signal to The use of a backscattered LIDAR-signal to calculate a vertical profile for the extinction calculate a vertical profile for the extinction koefficient is central within LIDAR theory.koefficient is central within LIDAR theory.

Vertical profiles of Vertical profiles of ββee You start by defining the variable S(r)You start by defining the variable S(r)

If there is an established relationship between If there is an established relationship between ββe e and and ββππ, , ββee(r) (r)

can be calculated from S(r). From can be calculated from S(r). From ββee,the size and number ,the size and number

concentration of the particles can be calculated.concentration of the particles can be calculated.

)(2

1)(

)(ln)( 2

rdr

d

dr

rdS

rPrrS

e

r

Differential Absorption Lidar Differential Absorption Lidar Technique (DIAL)Technique (DIAL)

Emission of laser pulses at two different Emission of laser pulses at two different wavelengths: One central in one of the wavelengths: One central in one of the absorption lines of the gas, the other absorption lines of the gas, the other corresponding to low absorption.corresponding to low absorption.

For clear sky the total extinction coefficient is For clear sky the total extinction coefficient is given by: given by:

kaaee )()( ,

gas for thet koefficien absorption

density gas

molec.air and aerosols todue extinction

a

,

k

ae

DIAL techniqueDIAL technique

The ALOMAR ozon lidar uses the DIAL The ALOMAR ozon lidar uses the DIAL techniquetechnique

DIAL technique cont.DIAL technique cont.

It is important to choose two wavelengths with It is important to choose two wavelengths with practically equal optical properties with respect to practically equal optical properties with respect to aerosols (and equal Rayleigh scattering).aerosols (and equal Rayleigh scattering).

By taking the log of the lidar equation for both By taking the log of the lidar equation for both wavelengths (wavelengths (λλ11 and and λλ22) and defining…) and defining…

drr

hrCA

P

rP r

ie

i

r

it

r )(28

)(ln

)(ln

02

DIAL technique continuedDIAL technique continued

For the right choice of For the right choice of λλ11 and and λλ22,scattering/ ,scattering/

backscattering will be practically equal for backscattering will be practically equal for the two wavelengths. We end up with:the two wavelengths. We end up with:

2

12,1

)(

)(ˆ

tr

tr

PrP

PrPP

r

a drkkrP0

212,1 )()()(2ˆln

DIAL technique cont.DIAL technique cont.

In this equation, In this equation, ρρaa(r) is the only unknown (r) is the only unknown

parameter → we can obtain a vertical parameter → we can obtain a vertical density profile.☺density profile.☺

With the newest laser technology, lasers can With the newest laser technology, lasers can be tuned exactly to the absorptionlines for be tuned exactly to the absorptionlines for different gases like Hdifferent gases like H22O, NOO, NO22,, SOSO22 and O and O33..

Studies of clouds and aerosols Studies of clouds and aerosols using depolarization-principlesusing depolarization-principles

The emitted power (PThe emitted power (Ptt) from the lidar can ) from the lidar can have either a vertical or horizontal have either a vertical or horizontal polarization.polarization.

The reciever can be constructed for The reciever can be constructed for reception of both polarization-components.reception of both polarization-components.

This will provide information about the This will provide information about the properties of the scattering particles – this properties of the scattering particles – this technique is called the technique is called the depolarization-depolarization-technique.technique.

Depolarization technique continuedDepolarization technique continued

Symmetric, spherical particles:Symmetric, spherical particles: Backscattered Backscattered radiation will keep its original polarization radiation will keep its original polarization

Non-spherical particles:Non-spherical particles: Backscattered radiation Backscattered radiation with opposite polarization compared to incident with opposite polarization compared to incident radiation is produced through multiple internal radiation is produced through multiple internal reflection.reflection.

Consider the case where the radiation incident on Consider the case where the radiation incident on a particle is vertically polarized (Pa particle is vertically polarized (Pverver). We can ). We can define the backscatterratio define the backscatterratio δδ=P=Phorhor/P/Pverver

By measuring By measuring δδ we can determine the phase of we can determine the phase of clouds. (Ice phase, clouds. (Ice phase, δδ=0.5-0.7; liquid phase, =0.5-0.7; liquid phase, δδ=0).=0).

Principles of depolarizationPrinciples of depolarization

Mekanical chopperMekanical chopper

A mekanical chopper is a disk rotating at high A mekanical chopper is a disk rotating at high frequency.frequency.

It is used for blocking intense backscattered It is used for blocking intense backscattered radiation from the lower levels of the atmosphere. radiation from the lower levels of the atmosphere. This radiation would otherwise “overload” the This radiation would otherwise “overload” the receiver system.receiver system.

The received signal is not reliable within the time The received signal is not reliable within the time interval where the chopper starts to open until it is interval where the chopper starts to open until it is completely open. This timeinterval should completely open. This timeinterval should therefore be as short as possible .therefore be as short as possible .

BACKGROUNDBACKGROUND

I reality there will always be a background I reality there will always be a background signal and noise in addition to the signal signal and noise in addition to the signal described by the lidar-equation.described by the lidar-equation.

NNcountcount= N(r) + N = N(r) + N backgr. backgr. (N = counted photones)(N = counted photones)

The background consists of scattered sunlight, The background consists of scattered sunlight, moonlight and starlight etc.moonlight and starlight etc.

We can reduce the background by reducing We can reduce the background by reducing the LIDAR “field of view (FOV)” since Nthe LIDAR “field of view (FOV)” since N

backgr backgr is is proportional to the FOV. But, the FOV must proportional to the FOV. But, the FOV must always be larger than the divergence of the always be larger than the divergence of the emitted beam.emitted beam.

BACKGROUND continuedBACKGROUND continued

The background can also be reduced by The background can also be reduced by reducing the receiver bandwidth.reducing the receiver bandwidth.

This can be achieved by using filters.This can be achieved by using filters. However, the receiver bandwidth cannot be However, the receiver bandwidth cannot be

more narrow than the laser bandwidth.more narrow than the laser bandwidth.

END OF LIDAR LECTUREEND OF LIDAR LECTURE

PSC – Polar Stratospheric PSC – Polar Stratospheric CloudsClouds

Also known as mother-of-pearl cloudsAlso known as mother-of-pearl clouds Developes when stratospheric cooling reaches a Developes when stratospheric cooling reaches a

critical level (193 K, -80critical level (193 K, -80ooC)C) For these temperature water will condense, despite For these temperature water will condense, despite

the very low water vapour concentration in the the very low water vapour concentration in the stratosphere (~ 1 ppmv)stratosphere (~ 1 ppmv)

This occurs over arctic and antarctic areas in winterThis occurs over arctic and antarctic areas in winterPSCs can be divided into two types :PSCs can be divided into two types :

- Type 1:- Type 1: Condensed water and nitric acid, sometimes Condensed water and nitric acid, sometimes combined with sulfuric acidcombined with sulfuric acid

- Type 2:- Type 2: Consist of practically pure ice crystals (H Consist of practically pure ice crystals (H22O)O)

Beautiful but dangerous….Beautiful but dangerous….

PSCs and ozone depletionPSCs and ozone depletion

Chemical reactions take place on the Chemical reactions take place on the surface of the cloud droplets in the PSCs:surface of the cloud droplets in the PSCs:

Then, when sunlight returns in spring Then, when sunlight returns in spring ☼☼

322 HNOClHClClONO

23

2

OClOClO

ClClradiationuvCl

OZONE

DEPLETION!!!

Polar VortexPolar Vortex

Large-scale cyclonic circulation in the UTLS Large-scale cyclonic circulation in the UTLS (“Upper Troposphere, Lower Stratosphere”) in (“Upper Troposphere, Lower Stratosphere”) in polar regions.polar regions.

The air is isolated from stratospheric air from The air is isolated from stratospheric air from lower latitudes for long periods each winter. lower latitudes for long periods each winter. This produces cold and inert air in the center of This produces cold and inert air in the center of the cyclones.the cyclones.

The isolation is due to strong zonal winds The isolation is due to strong zonal winds circulating around the poles and preventing in- circulating around the poles and preventing in- and outflow of airand outflow of air

Cont. Polar VortexCont. Polar Vortex

The isolation is more efficient over antarctic The isolation is more efficient over antarctic than over arctic areas, probably because the than over arctic areas, probably because the topography around the north pole tends to topography around the north pole tends to break up the circulation pattern.break up the circulation pattern.

Polar vortex right now:Polar vortex right now: A potential vorticity > 70A potential vorticity > 70·10·10-6 -6 KmKm22/kgs is /kgs is

characteristic within the vortex:characteristic within the vortex:

Polar vortex right now:Polar vortex right now:

A temperature lower than 193 K is A temperature lower than 193 K is characteristic within the vortex:characteristic within the vortex: