Retrieval of AOT in Arctic region using dual-view …earth.esa.int/workshops/atmos2009/participants/1305/pres...1. Introduction 3/21 The main purpose of the work is to develop, test

Retrieval of AOT in Arctic region Retrieval of AOT in Arctic region Retrieval of AOT in Arctic region using dualusing dualusing dual---view AATSR view AATSR view AATSR

observationsobservationsobservationsL. Istomina, W. von Hoyningen-Huene, A. Kokhanovsky,

J.P. Burrows

Institute of Environmental Physics, University of Bremen

2/2109.09.2009 Atmospheric Science Conference

Contents:

IntroductionInstrumentsDescription of the retrieval

Cloud screening over snowTheorethical basis of AOT retrieval over snow

Validation of the AOT retrieval algorithmApplications of the algorithmConclusions

1. Introduction

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The main purpose of the work is to develop, test and use the dual-view algorithm to retrieve AOT over Arctic using AATSR data. As a result we should create charts of AOT distribution over Arctic for some interesting events, such as aerosol pollution, which took place in some Arctic areas at the beginning of May, 2006.

Aerosol particles are known to cause significant effects on the radiative balance of the Earth, both directly, through scattering and absorption of short-wave and longwave radiation, and indirectly, by acting as condensation nuclei and therefore increasing the amount of droplets in a cloud, reducing their size and changing the lifetime of cloud.In polar regions, where the surface albedo can exceed 0.85 in areas covered by snow, aerosols may produce warming at the surface, if highly absorbing particles are suspended above these bright surfaces due to multiple reflection. Deposition of aerosols on the surface causes melting of snow and ice coverage. All these effects can have various consequences for the climate of the whole planet.

09.09.2009 Atmospheric Science Conference

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Monthly anomalies of the area of minimum extent of Arctic sea ice from 1978 to 2005. Thick line is 12-month running mean; thin straight line is least-squares linear fit. From Serreze and Francis, 2006, courtesy of National Snow and Ice Data Center, Boulder, CO.


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Area-weighted mean observed surface temperatures over the indicated latitude bands. The values are nine-year means relative to the 1880-1890 mean. From D. Shindell, G. Faluvegi, 2009


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Note the heavy aerosol loading over eastern Russia and Siberia that appears to intensify over the Arctic Ocean. It was caused by biomass burning of agricultural lands and boreal forest in Russia during May and transported across Scandinavia into the Arctic.

2006 Annual mean aerosol optical depth at 550 nm from MODIS


AATSR on Envisat(launched March in 2002)

AATSR swath 500 km

AATSR wavelengths 0.555, 0.659, 0.865, 1.6, 3.7, 10.85,12μm

AATSR spatial resolution 1km x 1km

2/3


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MPL acquires backscatter profiles on a 24-hour operation basis.Laser wavelength: 523.5 nm, maximum height: 60 km.Range resolution: 30 m, time resolution: 1 min.Pointing direction: vertical.Location: Ny Ålesund, N 78°55'44'', E 11°51'39''

AWI ground based AOT measurements. R. Treffeisen, M. StockLocation: Ny Ålesund


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3. Description of the AOT retrieval3a. Cloud screening over snow

Original spectrum from:ASTER spectral library, CalTech, 2008

The spectral shape of the pixel in VIS and NIR can be affected mainly by:- till 0.6μm – impurities of snow and AOT change,- 0.6 – 0.8μm – aerosols, 0.8 – 1.4μm – snow grain size variability, -1.4 – 1.8μm – snow grain size and possible AOT change.


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from MODIS UCSB Emissivity Library

In discussed channels snow and ocean are almost black, and emit as black bodies according to their T.

Emissivity (and BT) of snow varies depending on grain size and wavelength in less than 5% range.

Due to reflection contamination of 3.7μm channel, BT(3.7) of both cirrus and water clouds differ from BT(11,12) for more than 10%.


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The discussed cloud screening method was validated against MPL data for spring 2006, Spitsbergen (app. 100 scenes). In 95% of cases our method detects cloudy and clear sky correctly. The left 5% are thin cold clouds (as appears to be on MPL plot).Comparison to MODIS cloud mask also shows the reliability of presented cloud screenning method.

MODIS cloud mask for the 3rd of May 2006, 12:35


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3b. Theorethical basis of AOT retrieval over snow

00 0 0

0 0

( , ( )) ( , ( )) ( , , )( , , ) ( , , ) ( , , , ( ))

1 ( , , ) ( , )s s surf s

Aer s TOA s Ray s surfsurf s Hem

T M z T M z z zz z z z z z p z

z z zλ λ ρ λ

ρ λ ρ λ ρ λρ λ ρ λ

⋅ ⋅= − −

− ⋅

Kaufman et al., 1997; von Hoyningen-Huene et al., 2003:

0( , , , ( ))Ray s surfz z p zρ λ - the path reflectance for the Rayleigh scattering;( , ( ))T M zλ - total atmospheric transmission for the zenith angle z;

0( , )Hem zρ λ - hemispheric atmospheric reflectance;

0( , , )surf sz zρ λ - surface reflectance, in our case – the BRDF of snow;

2

2

( ) ( ) ( ) ( )( ) ( ) ( ) ( )

T O A

T O A

f f f fa tm sfc

n n n na tm sfc

TT

ρ λ ρ λ ρ λ λρ λ ρ λ ρ λ λ

− ⋅=

− ⋅

,,

, ,

( )( ) ( )( )( ) ( ) ( ) ( )

T O A

T O A

ff fnsfc sfc simT O A simn n f nsfc T O A sim sfc sim

ρ λρ λ ρ λρ λρ λ ρ λ ρ λ ρ λ

⎡ ⎤= ⋅ ⋅⎢ ⎥

⎢ ⎥⎣ ⎦

(1)

(2)


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Site Barrow (Alaska), 26.04.2003. AOT retrieved with dual-view algorithm and a-priori assumed snow BRDF model. Algorithm crashes on flat snow.

The same scene, AOT retrieved with the dual-view algorithm and corrected ratio of measured TOA reflectances instead of BRDF model.


The dual-view algorithm: schematic view

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Reading LUTs

Cloud screening TIRCloud?

Pixel loop

yesno

Cloud screening VIS & NIRCloud? yes

LUT interpolation for given geometryResult: Reflectance_vector[0;1.0]

Iterations loop to solve Eq. (2) Calculating transmittances, surface reflectances ratio,

including atmospheric correction.F(AOT)=0?

no

yesSolution saved

Nex

t pix

el

No solution

no


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3. Validation of the AOT retrieval algorithm

Comparison of AERONET AOT measurements to the dual-view AOT retrieval over snow for 4 AERONET stations. Only spring measurements were taken (as we only use “Arctic haze” aerosol type).


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Comparison of the discussed retrieval with BAER: snow covered Spitsbergen AOT retrieved with dual-view algorithm (AATSR), all other areas over ocean – with BAER (MERIS).

Global situation with the aerosol pollution transport over ocean for the 3rd of May, 2006, retrieved using MERIS data with BAER algorithm.


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4. Applications of the algorithm

The AOT retrieval over snow and open water has been performed for AATSR scene of Spitsbergen, during the Arctic smoke event on the 3rd of May 2006, 12:19.25, orbit number 2180. The red circles mark AOT over water, all the other values are AOT over snow. The difference between these two is about 0.1, which can be explained with the wrong accounting for ocean reflectance (assumed to be black) or presence of subpixel clouds or ice sheets.


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Ground-based AOT measurements, Ny Alesund

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

4.3.

2006

14.3

.200

624

.3.2

006

3.4.

2006

13.4

.200

623

.4.2

006

3.5.

2006

13.5

.200

623

.5.2

006

2.6.

2006

12.6

.200

622

.6.2

006

2.7.

2006

12.7

.200

622

.7.2

006

1.8.

2006

11.8

.200

621

.8.2

006

31.8

.200

610

.9.2

006

20.9

.200

630

.9.2

006

Date of measurement

AO

TTime sequence of AATSR scenes for Arctic smoke event (spring 2006) for Spitsbergen has been processed. The resulting AOT charts are shown on the left. One can notice the AOT increase during the beginning of May 2006, which is also present in ground-based data.

Ground based AOT measurements for Arctic smoke event 2006, Ny Ålesund station, R. Treffeisen, M. Stock


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AOT map for the Arctic region, 28.04.2006 and 3.05.2006


5. Conclusions

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A dual-view algorithm for AOT retrieval over snow (including automatic cloud screening) was established, validated and applied to the Arctic region for the haze event in spring 2006.The AOT retrieval was validated against AERONET data from 4 stations (Barrow, Resolute Bay, Hornsund, Longyearbyen). The correlation is rather good, but depends on the station location.Time series of AATSR data for Arctic smoke event at Spitsbergen were processed and the resulting AOTs were compared to ground-based AOT measurements at Ny Ålesund. The AOT peak at the beginning of May 2006 is visible in both AOT products.The presented AOT retrieval over snow was compared to BAER AOT retrieval over ocean (MERIS data) and shows quite good correspondence.


Developed cloud screening routine was validated against MPL data. The amount of correct cloudy/clear conditions detection is 95%.

AOT maps of different scale (up to the whole Arctic region) were retrieved for the Arctic smoke event in spring 2006. They can give qualitative impression on aerosol distribution in Arctic, with the polluted belt at 70° latitudes and not so polluted higher latitudes.

The presented cloud screening and dual-view AOT retrieval over snow could be the candidate for Sentinel 3 mission.


Documents

Retrieval of AOT in Arctic region using dual-view …earth.esa.int/workshops/atmos2009/participants/1305/pres...1. Introduction 3/21 The main purpose of the work is to develop, test