USE OF AIRS/AMSU DATA FOR WEATHER

AND CLIMATE RESEARCH

Joel Susskind

University of Maryland

May 12, 2005

USE OF AIRS/AMSU DATA FOR WEATHER AND CLIMATE RESEARCH

AIRS/AMSU/HSB launched on EOS Aqua May 5, 2002

AIRS is a multi-detector array grating spectrometer

2378 channels between 650 cm-1 and 2760 cm-1

Channel spacing (0.25 cm-1 - 1.1 cm-1)

Resolving power (0.5 cm-1 - 2.2 cm-1)

Footprint 13 km at nadir

3 x 3 array within AMSU A footprint - collocated with HSB

One sounding produced per AMSU A footprint

HSB failed on February 5, 2003

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OBJECTIVES OF AIRS/AMSU

Provide data to improve operational weather forecasting

Required global accuracy in up to 80% cloud cover:

1 K RMS error in 1 km layer mean tropospheric temperature

20% RMS error in tropospheric 1 km layer precipitable water

Provide long-term global coverage of surface and atmospheric parameters

Monitor climate variability and trends

Study processes affecting climate change

Extend 25 year TOVS Pathfinder data set

AIRS/AMSU PRODUCTS

Surface and Atmospheric Products - one set per FOR (45 km)

Sea/land skin temperature

Temperature profile T(p) to 1 mb

Water vapor profile q(p) to 100 mb

O3, CO, CH4 profiles

Cloud Cleared Radiances

Cloud Products - one set per FOV (13 km)Effective cloud fraction for up to two cloud layers

is geometric fractional cloud cover is cloud emissivity at 11 m

Cloud top pressure for up to two cloud layers

OLR

Computed using and retrieved parameters

Clear Sky OLRComputed using retrieved parameters with

Each product has a quality flag

Ts

Pc

, Pc ,

0

ˆ R i

M O N O C H R O M A T I C R A D I A T I V E T R A N S F E R E Q U A T I O N

C l e a r S k y

R B ( T s ) ( p s ) B T ( p )

d d np

d np H ( p s ) 1 R

( p s )

E m i t t e d b y s u r f a c e E m i t t e d b y a t m o s p h e r e R e f l e c t e d s u n l i g h t R e f l e c t e d t h e r m a l

( p ) ,

d d np

d e p e n d o n c o n s t i t u e n t p r o f i l e

p k ( p ) c ( p ) d p,0( p ) e

U n k n o w n s

s p e c t r a l s u r f a c e e m i s s i v i t y s p e c t r a l s u r f a c e b i - d i r e c t i o n a l r e f l e c t a n c e

T s s u r f a c e s k i n t e m p e r a t u r e T ( p ) t e m p e r a t u r e p r o f i l e q ( p ) O 3 ( p ) C O ( p ) C H 4 ( p ) C O 2 ( p )

P a r t i a l C l o u d C o v e r

R 1 jj

R , CLR j

j R , CLD , j j c l o u d t y p e s

MONOCHROMATIC WEIGHTING FUNCTIONS

W (p)

d d np

ddnp

dnp d 1 (ps )

If k(p), c(p) are constant and one gas is absorbing

v (p) e kcp

ddnp

kcp e kcp

x e x

Maximum value = .37 when x = 1, p 1

kc

If k increases with p, weighting function is narrower (line wing)If k decreases with p, weighting function is broader (line center)If k increases with T, and T increases with p, W(p) is narrowerIf c increases with p, W(p) is narrower – water vapor lines

R A D I A T I V E T R A N S F E R F O R C H A N N E L i

R i R f i ( ) d / f i ( ) d

if ( ) = s p e c t r a l r e s p o n s e f u n c t i o n

i = h a l f - w i d t h o f if ( ) I f

i i s n a r r o w ,

R i i B i ( T s ) i ( p s ) B i T ( p ) W i ( p ) d n p i H i i ( p s ) 1 i R i i ( p s )

w h e r e i ( p ) ( p ) f i ( ) d / f i ( ) d W i ( p ) W ( p ) f i ( ) d / f i ( ) d e t c .

PROPERTIES OF CHANNELS

= effective average temperature within weighting function

At night radiance is weighted value

(brightness temperature) is weighted average between and

As decreases, decreases because see less of warm surface and more of . Also

decreases as peak of weighting function rises, but increases in stratosphere

increases brightness temperature, primarily for

Brightness temperatures can be higher than physical temperature

Ri i Bi (Ts ) i(ps ) Bi T(pi ) 1 i(ps ) iH 1 i Ri

T(pi ) Wi(p)

i(ps ) i Bi (Ts ) 1 i Ri

, 1 i(ps ) Bi T(pi )

i Ts T(pi )

i(ps ) i Ts T(pi )

H 2000 cm 1

T(pi )

A D V A N T A G E S O F H I G H S P E C T R A L R E S O L U T I O N

H i g h s p e c t r a l r e s o l u t i o n m e a n s a b s o r p t i o n f e a t u r e s d u e t o s i n g l e l i n e s c a n b e o b s e r v e d M a n y c h a n n e l s a r e o b s e r v e d A I R S h a s 2 3 7 8 c h a n n e l s w i t h / 1 2 0 0 A l l o w s f o r s e l e c t i v i t y o f c h a n n e l s t o b e u s e d B e s t c h a n n e l s a r e p r i m a r i l y s e n s i t i v e t o a b s o r p t i o n b y a s i n g l e s p e c i e s “ F i x e d ” g a s e s – C O 2 , N 2 O – f o r t e m p e r a t u r e s o u n d i n g H 2 O , O 3 , C H 4 , C O f o r c o n s t i t u e n t p r o f i l e s W i n d o w ( r e l a t i v e l y t r a n s p a r e n t ) c h a n n e l s f o r s u r f a c e p a r a m e t e r s B e s t c h a n n e l s a r e u s u a l l y i n l i n e w i n g s o r o n l i n e c e n t e r s

C h a n n e l s w i t h r e d u n d a n t i n f o r m a t i o n c a n b e u s e d t o g e t h e r t o r e d u c e n o i s e

IR AND MICROWAVE OBSERVATIONS ARE VERY COMPLEMENTARY

IR Strengths

• Best vertical resolution (accuracy) of T(p) in mid-lower troposphere• Water vapor profile information up to the tropopause• Best information about surface skin temperature• Trace gas profile information

IR Limitations

• Most channel observations are strongly affected by clouds

MW Strengths

• MW observations are not affected by most clouds• MW observations are critical in accounting for effects of clouds on IR observations

• Microwave soundings of T(p), q(p) can be produced in overcast conditions

OVERVIEW OF AIRS/AMSU RETRIEVAL METHODOLOGY

Physically based system

Independent of GCM except for surface pressure

Uses cloud cleared radiances to produce solution represents what AIRS would have seen in the absence of clouds

Basic steps

Microwave product parameters – solution agrees with AMSU A radiancesInitial cloud clearing using microwave product: producesAIRS regression guess parameters based on cloud cleared radiances Update cloud clearing using AIRS regression guess parameters: producesSequentially determine surface parameters, T(p), q(p), O3(p), CO(p), CH4(p), using

Apply quality control

Select retrieved state - coupled AIRS/AMSU or AMSU only retrieval parametersDetermine cloud parameters consistent with retrieved state and observed radiances Compute OLR, CLR sky OLR from all parameters via radiative transfer

ˆ R i0

ˆ R i

ˆ R i

ˆ R i0

ˆ R i

ˆ R i

OVERVIEW OF CLOUD CL EARING

Use radiances in 9 fields of view Rij channel i, FOV j

Allows up to 8 cloud formations R i = average radiance over 9 FOV’s

ˆ R i, CLR R i jj1

9 Ri,j R i R j j

j1

9 R i, j

If you have estimates of R i,CLR

j R N 1 R 1 R N 1 RCLR

RCLR,i R i,CLR R i R i,CLR computed from a surface and atmospheric state N = channel noise covariance - includes uncertainty in R i,CLR

R i,CLR should be an unbiased state – agree with AMSU radiances

QUALITY FLAGS

Order of increasing difficulty to pass

1) Stratospheric Temperature Test - temperature profile good above 200 mb

90% and cloud clearing passes minimal quality control

Use coupled AIRS/AMSU retrieval state if Stratospheric Temperature Test

is passed

2) Constituent profile test

Slightly more stringent cloud clearing quality control

3) Mid-tropospheric Temperature Test - T(p) good above 3 km

Tighter quality control on cloud clearing and T(p) convergence

flagged good

4) Lower Troposphere Temperature Test - T(p) good above surface

5) SST test - for non frozen ocean only

6) Tight SST test - for non frozen ocean only

ˆ R i

USE OF AIRS OBSERVATIONS FOR DATA ASSIMILATION

Could be (used by ECMWF, NCEP) or T(p),q(p) (used by Bob Atlas)

Accuracy of T(p),q(p) degrades slowly with increasing cloud fraction

There is a trade-off between accuracy and spatial coverage

ECMWF, NCEP uses radiances “unaffected by clouds”

Passes internal threshold tests

They should try assimilating clear column radiances “unaffected by clouds”

Bob Atlas assimilated T(p) for all levels flagged as good

Treats AIRS T(p) as radiosonde reports

R̂i

R̂i

AIRS EXPERIMENTS WITH FVSSI

Global data assimilation system used:fvSSI: fvGCM - Resolution: 1x1.25 SSI (NCEP) analysis-T62

Period of assimilation: 1 January - 31 January, 2003

Experiments:

Control: All Conventional Data + ATOVS Radiance (NOAA-14, 15, 16) + CTW + SSM/I TPW+ SSM/I Wind Speed + QuikScat + SBUV Ozone

Control + AIRS Retrieved Temperature Profiles (Global, passing level quality control)

Forecasts:25 forecasts run every day beginning on January, 6 2003

AIRS LEVEL 3 PRODUCTS

Different geophysical parameters are gridded according to different tests

Stratospheric Temperature Test

T(p) 200 mb and above

Constituent Profile Test

q(p), O3(p), CO(p) at all p

Mid-Tropospheric Temperature Test

T(p) beneath 200 mb, MSU2R/MSU4, land (including ice and coasts) surface skin

temperature (and emissivity)

Sea Surface Temperature Test

Non-frozen ocean surface skin temperature (and emissivity)

Cloud parameters, OLR and clear sky OLR use all AIRS cases

Interannual Differences of T(P)

AIRS AIRS-ECMWF AIRS-TOVS Jan 2004-2003 Jan 2004-2003 Jan 2004-2003 mean STD mean STD correlation mean STD correlation

1000 mb -0.05 1.44 0.14 0.90 0.82 -0.02 1.24 0.45

850 mb -0.09 1.69 0.04 0.71 0.93 -0.04 1.28 0.69

700 mb -0.28 1.54 -0.05 0.45 0.97 -0.05 1.06 0.77

600 mb -0.15 1.55 0.07 0.42 0.98

500 mb -0.36 1.54 -0.05 0.39 0.97 -0.19 1.02 0.75

400 mb -0.45 1.50 -0.15 0.39 0.95 0.00 0.98 0.73

300 mb -0.10 1.33 0.03 0.46 0.94 0.02 0.85 0.83

200 mb -0.06 1.99 -0.13 0.53 0.99 -0.60 0.98 0.92

150 mb 0.23 2.07 0.06 0.53 0.99

100 mb -0.10 2.57 0.16 0.84 0.99 0.04 1.02 0.97

70 mb -1.01 2.35 -0.21 0.81 0.99 -0.37 1.09 0.96

50 mb -0.53 2.37 0.04 1.03 0.99 -0.29 1.12 0.95

30 mb 0.10 3.05 0.09 0.91 0.99 0.54 1.39 0.90

10 mb -0.06 2.74 0.02 0.72 0.99 0.15 1.67 0.81

1 mb -1.47 4.13 0.26 1.76 0.99

MSU2R -0.19 1.31 -0.10 0.72 0.81 -.03 0.81 0.74

MSU4 -0.42 2.00 -0.12 0.36 0.99 -.04 0.66 0.97

AIRS AIRS-Spencer-Christy AIRS-TOVS Interannual Difference of MSU2R/MSU4

DATA AVAILABILITY

Results shown are based on the AIRS Version 4.0 algorithm

We (SRT) currently have results for January 2003 and January, August, September 2004

Goddard DAAC began analyzing AIRS/AMSU data near real time April 1, 2004

DAAC is also processing backwards from March 31, 2004 at 5 days per day

Level 1B (radiances), Level 2 (spot by spot retrievals), and Level 3 (gridded) data is available

Level 3 is 1°x 1° daily, 8 day mean, and monthly mean

Ascending (1:30 pm local time) and descending (1:30 am local time) data are separate

To order data to go

http://daac.gsfc.nasa.gov/data/datapool/AIRS/index.html

Use collection 003 (Version 4.0)

Earlier DAAC products (collection 002) used AIRS Version 3.0

Do not use earlier results