assimilation of cross-track infrared sounder radiances at ...characteristics and spectral...
TRANSCRIPT
Baseline configuration The way forwardThe positive forecast impact from the baseline configuration does not reproduce in newer experiments based on a more up-to-date IFS version (Cy40r1) and using higher vertical resolution. Further optimization on the use of CrIS radiances is being carried out focussing on the following modifications:•Pre-selectionofthewarmestFOV•12channelsinthewatervapourabsorption
band added to the assimilation•Humidity-sensitivechannelsremovedfromthelong-waveCO2 absorption band
•Stringenttuningoftheclouddetectioninresponse to the low noise levels
•Observationerrorstandarddeviationrelaxedto0.5/0.3/1.2K
The following approaches will be considered when designing future experiments to improve the assimilation of CrIS radiances:•Accountforinter-channelerrorcorrelations
other than those introduced during the signal apodization
•Usecollocatedimagerinformationtoassisttheclouddetection:VIIRS-basedestimatesofcloud parameters are available for doing this
•Assimilatestratospheric-peakingchannelsover land
•StudytheimpactofCrISinadegradedassimilationsystem,wherenoAIRSradiancedata is available
Figure 7 Smoothed (line) and unsmoothed (dots) background departures on vertically-ranked channels in two single cases. Channels diagnosed cloud-contaminated in each case are shown in red.
Figure 8 Frequency distribution of background departure on a mid-tropospheric sounding channel before (black) and after (red) the cloud detection re-tuning.
Figure 10 Inter-channel observation error correlations as diagnosed using the Hollingsworth – Lönnberg method in a selection of 128 CrIS channels.
Figure 9 Pixel-number-specific background departure statistics after Warmest-FOV-based data selection: mean (black), standard deviation (blue) and count (green). Statistics are shown for channel 61.
Cloud detectionCloud-contaminated channels are rejected from the assimilation. Cloud contamination is diagnosed using a detection scheme of McNally & Watts (2003)anditisbasedonbrightnesstemperaturebackgrounddeparturesinasetofvertically-rankedchannelsinthe15 μmCO2 absorption band. Because of cloud contamination, typical count of usable data on a window channel
CrIS instrumentCrIS is a Michelson interferometer attachedtotheSuomi-NPPpolar-orbitingsatellite launched into an afternoon orbit inOctober2011.Theinfraredsounderprovides radiance information on 1,305 channels in the wavenumber range650–2550cm-1.Field-of-Viewdiameteris13.5kminnadirandglobalcoverageisproducedevery12hours.
ApproachWe aim at operational assimilation ofCross-trackInfraredSounder(CrIS)radiancesintheglobalNWPsystemof ECMWF. Experience gathered from earlier hyper-spectral radiances, includingthosefromtheAtmosphericInfraredSounder(AIRS)andInfraredAtmosphericSoundingInterferometer(IASI),isappliedwherepossible:theexperimental assimilation of new radiances is limited to cloud-free channels over sea and sea ice, and the emphasis is put on the efficient use of temperature sounding channels in the 15 μmCO2 absorption band.In contrast to the operational use ofIASIradiances,alargenumberofspectrally-adjacent CrIS channels are assimilated using a relatively aggressive specification for observation error standard deviation. This is hoped to compensate for the poorer spectral resolutionofCrISinstrument.Anon-diagonal observation error covariance matrix is applied to account for the effect of the signal apodization.
Assimilation of Cross-track Infrared Sounder radiances at ECMWFReima Eresmaa, Tony McNally and Niels BormannEuropean Centre for Medium-range Weather Forecasts, Reading, UK
The effect of spectral resolution
Used in experiments with the previously operational IFS version (Cy38r2) in resolution T511 / L91•Pre-selectthemiddleFOV(pixel5)fromeachField-of-Regard
•Assimilate122channelsfromthelong-waveCO2 absorption band
•Setobservationerrorstandarddeviationsto0.4/0.2/0.6K
•Assumesignalapodizationtobetheonlysourceofobservation error correlation
•Assimilatedataoverseaandseaiceonly•UseRTTOV-10forradiativetransfermodelling•Usevariationalbiascorrectionwithair-mass-and
scan-angle-dependent predictors•AssimilateoneFOVperthinningbox(1.25°by1.25°)
onlyA consistently positive medium-range forecast impact extending over the whole of troposphere is found in the extratropics, while a negative impact is seen in the tropical upper troposphere
ConclusionsAscomparedwithearlier hyper-spectral sounders, CrIS radiances are affected by a unique trade-off between noise characteristics and spectral resolution. Therefore, experience from the operational assimilation of AIRSandIASIradiancesisinsufficientto facilitate successful assimilation of CrIS radiances.We have put emphasis on strong exploitation of cloud-free sounding channels in the 15 μmCO2 absorption band. We demonstrate a positive medium-range forecast impact in the extratropics from assimilating CrIS radiances into the previously-operational IFS version at ECMWF. However,similarlevelofperformancehas not yet been produced in the currently-operational IFS version.Workisinprogresstofullyunderstand the nature and impact of CrIS radiance data in the ECMWF 4D-Varsystem.
isonly20%ofthatonahigh-peakingstratospheric channel.The low noise level of CrIS results in increased sensitivity to contamination by undetected clouds. Therefore, parameters of cloud detection need to be tuned more stringently for CrIS than for earlier hyper-spectral sounders. In an appropriately-tuned detection scheme cloud-free backgrounddeparturesconstituteanearlysymmetrical and Gaussian histogram.
McNally & Watts(2003):Aclouddetectionalgorithm for high-spectral-resolution infrared sounders. Q. J. R. Meteorol. Soc., 129,3411—3423.
200hPa
500hPa
850hPa
Northernextratropics
Z T W R Z T W R Z T W R
Tropics Southernextratropics
Jacobian (K/K)
Pres
sure
(hPa
)
667.50
680.00
677.50
736.25740.00
667.50
677.50
680.00
736.25740.00
10–1
100
101
102
103
10–1
100
101
102
1030.20.100.20.10 650 675 700 725 750 775
Wavenumber (1/cm)
220
240
260
280
220
240
260
280
Brig
htne
ss T
empe
ratu
re (K
)
650 675 700 725 750 775Wavenumber [1/cm]
650 700 750 800 850 900Wavenumber (1/cm)
230
250
270
290
Brig
htne
ss te
mpe
ratu
re (K
) 0.6 K
0.2 K
0.4 K
Figure 4 Long-wave CrIS spectrum highlighting assimilated channels. Blue bars indicate the assumed observation error standard deviations.
-1.0 -0.5 +0.5 +1.0Normalized OmB sdev (%)
0
200
400
600
800
1000
Pres
sure
(hPa
)
TEMP QTEMP T
AIREP T
-1 -0.5 +0.5 +1Normalized OmB sdev (%)
6
8
10
12
14
18
20
22
ATM
S ch
anne
l num
ber
5
7
9
11
13
5
4
3
AM
SU-A
/ M
HS
chan
nel n
umbe
rMetop-A
AMSU-A
NPP ATMS
NPP ATMS
Metop-A
MHS
Figure 5 Normalized global background departure standard deviations for some conventional (left) and space-borne microwave radiance (right) observations in a 90-day assimilation experiment. Negative values imply improved background fit when CrIS radiances are assimilated.
Figure 6 Medium-range forecast impact from adding CrIS radiances in the ECMWF 4D-Var system. Z, T, W, and R refer to geopotential, temperature, vector wind, and relative humidity, respectively. Green (red) indicates positive (negative) impact. Dot size indicates the magnitude of the impact.
AAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAA
AA
AAAAAAA A
AAA
AA
AAAA
AAAAAAAAAAAAAA
A
AAAAA
AA A
A
A
A
A
A
A
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
IIIIII III I I I
IIIIIII
IIIIIIIII
II
I
I
I
II
CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC
CCCCCCCCCCCCCCCCCCCCCCC C CC C C
CC
CC
C
C
CC
CC
C C
800 1000 1200 1400 1600Wavenumber (1/cm)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Om
B St
. Dev
. (K)
AIRSAIASIICrISC
Figure 1 Global background departure standard deviation on selected AIRS (blue), IASI (red) and CrIS (black) channels.
Figure 2 Temperature Jacobians on selected IASI (left) and CrIS (right) channels in a tropical reference atmosphere.
Figure 3 Excerpts of IASI (left) and CrIS (right) long-wave spectra.
1007550250 125Channel rank index
-3
-2
-1
0
1
Dep
artu
re (K
)
1007550250 125Channel rank index
-3
-2
-1
0
1
Dep
artu
re (K
)
-0.6 -0.4 -0.2 0 0.2 0.4 0.6FG Departure (K)
1
10
100
1000
Coun
t
BeforeAfterGaussian
−0.9
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Channel number
Chan
nel n
umbe
r
1 18 23 28 37 42 47 52 57 62 67 72 77 82 87 94 101
113
119
124
139
173
210
457
900
1025
1182328374247525762677277828794
101113119124139173210457900
1025
650
660.
6266
3.75
666.
8867
2.5
675.
6267
8.75
681.
8868
568
8.12
691.
2569
4.38
697.
570
0.62
703.
7570
8.12
712.
572
072
3.75
726.
8873
6.25
757.
578
0.62
935
1442
.515
98.7
5
1 2 3 4 5 6 7 8 9Pixel number
5
10
15
20
25
Coun
t (th
ousa
nds)
-0.05
0
0.05
0.1
0.15
Om
B M
ean
or S
t. D
ev. (
K)
In comparison with earlier hyper-spectral infrared sounders, lower instrument noise levels of CrIS compensate for the effect of the relatively poor spectral resolution. While past experience fromtheassimilationofAIRSandIASIradiances provides a natural starting point for the use of CrIS radiances, modifications are required to properly account for the different trade-off between noise and spectral purity.We aim at maximizing the meteorological information extracted
from apodized radiances by using almost all channels in the most critical spectral range at 670 –730 cm-1.Usingalargenumberof spectrally-adjacent channels means that observation error correlation introduced by the signal apodizationhastobetakenintoaccount in the assimilation.Furthermore, the low noise levels are reflected in an aggressive specification of observation error standard deviations during the assimilation.