14-16 April 2003International Limb Workshop, Bremen On two potential sources of systematic error in...

14-16 April 2003 International Limb Workshop, Bremen EnvironmentCanada

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On two potential sources of systematic error in retrievals using limb-scattered sunlight

Chris A. McLinden (1) and Christopher E. Sioris (2)

(1) Meteorological Service of Canada, Toronto, Ontario, Canada ([email protected])

(2) Atomic and Molecular Physics Division, Harvard-Smithsonian Center for Astrophysics, Cambridge,

Massachusetts, USA


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Outline

1. Allowing for a varying atmosphere along the line-of-sight (LOS) when performing retrievals

2. Systematic errors in plane-parallel models, including so-called “pseudo-spherical” limb scattering models.


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Varying atmosphere along the LOS (1)

Perform multiple plane-parallel calculations, each one for a particular atmosphere & SZA along the LOS

From these, calculate LOS sources function – integrate to obtain limb radiance

Using vector RT model of McLinden et al.

(Can. J. Phys., 80, 375-383, 2002)


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- in practice, slices narrower near tangent point

OS or SCIAN 1

2


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Want to estimate magnitude of errors when atmosphere is not homogeneous along LOS, but assumed to be in retrieval (‘2D’ forward, ‘1D’ inversion)

If species diurnal, can a ‘2D’ retrieval improve result for scans near terminator and SZA variations along LOS ?


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5 6 7 8 9 2 3 4 5 6 7 8 91.0 10.0R adiance (uW /nm /cm 2/sr)

10.0

15.0

20.0

25.0

30.0

Tan

gent

Alti

tude

(km

)

0.0E+0 2.0E+12 4.0E+12 6.0E+12O zone Num ber Density (/cm 3)

N o H ole

H ole on N ear S ide

H ole on Far S ide

65S , O ctSZA=80dAZ=90

5 6 7 8 9 2 3 4 5 6 7 8 91.0 10.0Radiance (uW /nm /cm 2/sr)

10.0

15.0

20.0

25.0

30.0

Tan

gent

Alti

tude

(km

)

65S , O ctSZA=80dAZ=90cirrus o.d.=0.1 at 13km

N o C irrus

C irrus a t TP only

C irrus in a ll A tm s


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0.0E+0 2.0E+12 4.0E+12 6.0E+12O zone Num ber Density (/cm 3)

10.0

20.0

30.0

40.0

Alti

tude

(km

)

N o H ole

H ole on N ear S ide

H ole on Far S ide

65S, O ctSZA=80dAZ=90

- Flittner algorithm used-‘2D’ forward calculation, ‘1D’ inversion


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0

5

10

15

20

25

30

35

40

45

0.0E+00 5.0E+07 1.0E+08 1.5E+08 2.0E+08 2.5E+08 3.0E+08 3.5E+08 4.0E+08

NO2 number density (molec/cm3)

Alti

tude

(km

)

perturbed standard


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0

5

10

15

20

25

30

35

40

45

-4% -2% 0% 2% 4% 6% 8% 10%

Overestimation

Alti

tude

(km

)

-Sioris NO2 inversion algorithm used-Uses an iterative onion peel technique

SZA=27º, dAZ=92º;Aug., 25ºN, albedo=0


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0

5

10

15

20

25

30

35

40

45

-80% -70% -60% -50% -40% -30% -20% -10% 0% 10%

Overestimation

Alti

tude

(km

)

linear nearest neighbour



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1012141618202224262830323436384042

-100% -80% -60% -40% -20% 0% 20% 40% 60% 80% 100%

Overestimation

Alti

tude

(km

) f ar 14

tan pt 14

far 13

tan pt 13



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0.0E+0 4.0E+8 8.0E+8 1.2E+9 1.6E+9 2.0E+9N um ber D ensity (/cm 3)

15.0

20.0

25.0

30.0

35.0

40.0

Alti

tude

(km

)

H A LO E

O SIR IS 1D

O SIR IS 2D

Comparison of ‘1D’ and‘2D’ OSIRIS inversionswith HALOE coincidence

LOS scaling factors fromphotochemical box model

4 June 2002 lat=4ºN, lon=55ºW


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Systematic Errors in “Pseudo-Spherical” Limb Radiances (1)

- Comparison of several “pseudo-spherical” (PS) radiative transfer model calculations with 3-D, spherical Monte-Carlo (MC) model calculations show systematic errors

- These errors increase with (a) tangent height and (b) surface albedo


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SZA=39º; albedo=0.95


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-PS calculations based on plane-parallel source function & integrated through spherical-shell atmosphere

-These errors are driven by an overestimation of the surface reflected component – either the surface proper or the effective Rayleigh “surface”


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-root of problem is how much of the surface a point in the atmosphere “sees”:

-PP atmosphere: point sees 2 [str]

-spherical atmosphere: point sees <2 [str]; this decreases with altitude


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THE

E

zR

R1sin*

Point under consideration

Direct

Surface reflected *


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- try simple, near-transparent atmosphere correction factor using ratio of PS to spherical solid angles

Where is cos(SZA) and is surface albedo0

0

0

21

*)cos1(21


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-2 0 2 4 6 8 1010

20

30

40

50

60T

ang

ent

He

ight

(km

)

Percent Difference

=0.0=0.5=1.0

800 nm

SZA=39º


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-Trace gas retrievals should be minimally impacted since there will be cancellation/removal of this effect- e.g., the Flittner radiance-ratio method there will be a large degree of cancellation due to form of retrieval vector; - in DOAS it will not cancel but it would be included or removed in closure polynomial or high pass filter


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-Aerosol retrievals may suffer systematic errors if the retrieval involves a model-measurement matching of radiances

-If aerosol signal small, the PP errors can lead to amplified aerosol errors; unclear how “optimal”-type solutions would handle this


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-If aerosol signal is: S = I(measure) – I(model w/o aerosol)

-But P-P aerosol signal is:

SPP = I(measure) – I(model w/o aerosol) / c

Where c=

0

0

21

*)cos1(21


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Systematic Errors in “Pseudo-Spherical” Limb Radiances


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400 500 600 700 80020

25

30

35

40

45

50

Wavelength (nm)

Tan

gent

Hei

ght

(km

)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5


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400 500 600 700 80020

25

30

35

40

45

50Error in Aerosol "Signal": SZA=60o, albedo=0.3

Wavelength (nm)

Ta

nge

nt H

eig

ht (

km)

-0.5

-0.45

-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0


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Conclusions & Future Work

-systematic PS errors may be important in OS aerosol retrievals + other RT applications including photochemistry (& other planetary atmospheres)

-attempt to obtain “universal” correction valid at all wavelengths, tangent heights, albedo


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0

0

21

*)cos1(21

c

)()(38

31412

/)( 01 EEe

14-16 April 2003International Limb Workshop, Bremen On two potential sources of systematic error in...

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