TOMS Ozone Retrieval Sensitivity to

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3 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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TOMS Ozone Retrieval Sensitivity to

Assumption of Lambertian Cloud Surface

Part 1. Scattering Phase Function


Xiong Liu,
1

Mike Newchurch,
1,2

Robert Loughman
3
, and Pawan K. Bhartia
4



1. Department of Atmospheric Science, University of Alabama in Huntsville, Huntsville, Alabama, USA

2. National Center for Atmospheric Research, Boulder, Colorado, USA

3.
Cooperative Center for Atmospheric Science & Technology, University of Arizona, Tucson, AZ, USA

4. NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

Abstract
.

Cloud

surfaces

are

not

Lambertian
;

cloud

reflectivity

is

angularly

dependent

and

the

penetration

depth

is

greater

than

zero
.

We

study

the

effect

of

assuming

isotropic

scattering

in

the

TOMS

ozone

retrieval
.

The

non
-
isotropic

effect

results

from

the

difference

in

the

ozone

absorption

enhancement

above

clouds

due

to

rayleigh

scattering

and

multiple

cloud

reflection

between

the

simulated

scattering

clouds

and

assumed

Lambertian

clouds

in

the

ozone

retrieval
.

This

effect

varies

with

viewing

geometry,

cloud

optical

thickness,

cloud

type

(different

phase

functions),

cloud
-
top

height,

and

ozone

above

cloud
.

However,

f
or

most

conditions,

the

non
-
isotropic

effect

is

within

±
4

DU,

indicating

the

assumption

of

cloud

scattering

as

isotropic

is

fairly

good

for

clouds

with

optical

thickness



20
.

4

3

2

1

www.nsstc.uah.edu//atmchem

Figure

1
.

Possible

sources

of

ozone

retrieval

errors
.


Radiative Transfer Models



Treat

clouds

as

scattering

medium,

calculate

the

backscattered

radiance

at

the

top

of

the

atmosphere

using

Polarized

Plane
-
parallel

Gauss
-
Seidel

Radiative

Transfer

Code

(PPGSRAD)

at

N
7

TOMS

six

channels

(
312
,

317
,

331
,

339
,

360
,

and

380

nm)
.

Polarization

is

considered

for

clouds

with

optical

depth



150
.



Retrieve

ozone

using

TOMS

version
-
7

algorithm

(TOMSV
7
),

from

which

the

look
-
up

table

is

calculated

using

TOMRAD

code

at

10

pressure

levels

from

1
.
0

to

0
.
1

atm

to

reduce

radiation

interpolation

error
.



The

radiance

difference

between

TOMRAD

and

PPGSRAD

is

0
.
2
%
,

on

average,

for

clear

sky

conditions
.



Use

wavelength
-
weighted

ozone

absorption

coefficients,

rayleigh

scattering

coefficients

and

molecular

depolarization

factor

at

each

channel

(consistent

in

PPGSRAD

and

TOMRAD)
.



TOMS

standard

low
-
latitude

ozone

profile

and

temperature

profile

L
275

are

used

as

example

profiles
.




Optical

properties

of

water

clouds

are

computed

by

Bohren
-
Huffman’s

Mie

Code
.

Optical

properties

of

polycrystals

and

hexagon

column

crystals

are

computed

by

Ray

Tracing

Code
.


Methodology



Separate

the

effect

of

assuming

cloud

scattering

as

isotropic

on

TOMS

ozone

retrieval

from

the

effect

of

the

neglect

of

ozone

absorption

in

clouds
.

The

non
-
isotropic

effect

is

shown

in

this

poster
.



If

there

is

no

ozone

in

the

cloud

in

the

forward

calculation,

ozone

absorption

and

its

enhancement

do

not

occur
.

The

difference

between

the

retrieved

ozone

and

the

input

ozone

characterizes

the

non
-
isotropic

effect
.



We

study

how

the

non
-
isotropic

effect

varies

with

solar

zenith

angle

(SZA)

and

view

zenith

angle

(VZA)

(SZA



75

°
,

VZA




70

°
)

,

cloud

types

including

water

clouds

(WC),

hexagonal

column

ice

crystals

(HEX),

polycrystals

(POLY),

and

water

clouds

with

Henyey
-
Greenstein

phase

function

(WCHG)

,

optical

thickness

of

clouds,

cloud

location,

and

thickness
.



To

represent

those

tropical

high
-
reflecting

clouds,

a

typical

homogeneous

cloud

is

put

between

2

-

12

km

with

an

optical

thickness

of

40

(corresponding

to

cloud

reflectivity

of

~
80
%

for

water

clouds)
.

Motivation and Objectives



We

see

significant

total
-
ozone
-
column

excess

of

10
-
15

DU

over

tropical

high
-
altitude,

highly

reflecting

clouds

compared

to

clear

observations
.



After

accounting

for

errors

involving

incorrect

cloud

height,

tropospheric

ozone

climatology,

and

considering

potential

dynamical,

photochemical,

and

NIMBUS
-
7
(N
7
)/Earth

Probe

calibration

errors,

approximately

4
~
9

DU

excesses

over

cloudy

scenes

remain

unexplained
.



We

speculate

that

the

TOMS

algorithm

approximation

of

optically

thick

clouds

as

opaque

Lambertian

reflecting

surfaces

may

account

for

a

significant

portion

of

these

unexplained

excesses
.



Cloud

surfaces

are

not

Lambertian
;

the

reflection

of

clouds

is

angularly

dependent
.

Furthermore,

photons

penetrate

into

clouds

and

the

path

length

is

enhanced

due

to

in
-
cloud

multiple

scattering,

resulting

in

enhanced

ozone

absorption
.

In

addition,

clouds

might

be

ice

(not

water)

clouds
.



Possible

sources

of

ozone

retrieval

errors

are

illustrated

in

Figure

1
.

We

use

radiative

transfer

codes

to

address

the

effects

of

these

aspects

on

TOMS

ozone

retrieval

for

thick

clouds
.


Effect

of

Assuming

isotropic

cloud

scattering



Figure

2

(left)

shows

the

retrieved

total

ozone

as

a

function

of

viewing

geometry

for

a

water

cloud

between

2
-
12

km

with

optical

depth

(OD)

of

40
.

The

non
-
isotropic

effect

varies

with

viewing

geometry,

but

is

±
4
.
5

DU

for

the

L
275

DU

ozone

profile

illustrated

here
.




The

non
-
isotropic

effect

decreases

with

increasing

cloud

OD

as

shown

in

Figure

2

and

Table

1
.

When

OD



30
,

the

patterns

of

the

non
-
isotropic

effect

vs
.

viewing

geometry

are

very

similar,

but

differs

significantly

when

OD



20

(
Figure

2
)
.

When

OD

increases,

clouds

act

more

like

Lambertian

surfaces
;

therefore

the

non
-
isotropic

decreases
.

In

addition,

at

smaller

cloud

OD,

the

partial

cloud

model

is

used

in

the

ozone

retrieval,

introducing

additional

ozone

retrieval

errors
.




The

non
-
isotropic

effect

shows

different

patterns

vs
.

viewing

geometry

for

WC

(
Figure

2

left),

HEX

(
Figure

3

left),

POLY

(
Figure

3

right),

and

WCHG
.

For

POLY,

the

total

ozone

is

±
3
.
5

DU

of

the

original

275

DU
.

For

HEX,

the

total

ozone

is

within

±
4
.
5

DU

of

275

DU

except

at

a

few

angles

where

the

the

error

could

be

-
7
.
5

or

+
10
.
9

DU
.

For

WCHG,

the

total

ozone

is

similar

to

WC

except

at

anti
-
solar

side

and

SZA



70
,

where

the

error

ranges

from

-
10

to

-
7

DU

(
Table

1
)
.



The

non
-
isotropic

effect

varies

with

the

ozone

profiles
.

The

more

ozone

above

cloud,

the

larger

the

non
-
isotropic

effect

(
Table

1
)
.




The

non
-
isotropic

effect

varies

a

great

deal

with

cloud
-
top

height
.

The

three

clouds

(
2
-
12

km,

6
-
12

km,

10
-
12

km)

show

almost

the

same

pattern,

but

the

three

clouds

(
2
-
4

km,

2
-
8

km,

2
-
12

km)

shows

very

different

patterns

of

non
-
isotropic

effect

(
Figure

4
)
.

Figure

2
.

Non
-
isotropic

effect

for

water

clouds

at

OD=
40

(left)

and

OD=
10

(right)
.

*

indicates

the

solar

zenith

angle
.

Figure

3
.

Non
-
isotropic

effect

for

ice

clouds
.

Hexagon

column

crystals

(left)
.

Polycrystals

(right)
.

Non
-
isotropic

effect



The

non
-
isotropic

effect

is

mainly

due

to

the

difference

in

the

ozone

absorption

enhancement

resulting

from

Rayleigh

scattering

and

cloud

reflection

between

simulated

scattering

clouds

in

PPGSRAD

and

assumed

Lambertian

clouds

in

TOMRAD
.

That

is

why

it

varies

with

cloud
-
top

height,

cloud

optical

thickness,

ozone

profiles,

and

phase

function
.



The

non
-
isotropic

effect

is

within

±
4

DU

for

most

cloudy

conditions

(
Table

1
)

with

OD



20
,

approximately

within

the

accuracy

of

TOMS

ozone

retrieval,

indicating

assuming

cloud

scattering

as

isotropic

is

fairly

good
.

Figure

4
.

Non
-
isotropic

effect

for

water

clouds

between

2

-

4

km

(left)

and

2

-

8

km

(right)
.

Table

1
.

The

range

of

non
-
isotropic

effect,

the

average

and

standard

deviation

over

viewing

geometry

(SZA
:

0
°
,

15
°
,

30
°
,

45
°
,

60
°
,

70
°
,

and

75
°
;

VZA

from

0
°

to

70
°

every

5
°
;

AZA

from

0
°

to

180
°

every

30
°
)

for

different

conditions
.

Summary and Conclusions




Motivated

by

the

desire

to

explain

the

excess

observed

ozone

over

cloudy

areas

from

our

previous

studies,

we

use

radiative

transfer

models

to

study

the

ozone

retrieval

errors

due

to

the

treatment

of

optically

thick

clouds

as

opaque

Lambertian

reflecting

surfaces
.



We

separate

ozone

retrieval

errors

due

to

the

assumption

of

isotropic

cloud

scattering

from

those

errors

due

to

the

neglect

of

enhanced

ozone

absorption

in

clouds
.




The

non
-
isotropic

effect

results

from

the

difference

in

the

ozone

absorption

enhancement

above

cloud

due

to

the

Rayleigh

scattering

and

multiple

cloud

reflection

between

the

simulated

scattered

clouds

and

Lambertian

clouds
.

The

non
-
isotropic

effect

varies

with

viewing

geometry,

cloud

optical

thickness,

different

types

of

clouds

(different

phase

function),

cloud
-
top

height,

and

ozone

above

cloud
.

However,

f
or

most

conditions,

the

non
-
isotropic

effect

is

within

±
4

DU,

indicating

the

assumption

of

isotropic

cloud

scattering

is

fairly

good

for

clouds

with

optical

thickness



20
.