Assimilation of surface sensitive infrared channels over land at Environment Canada

L. Garand, S.K. Dutta, and S. HeillietteDAOS Meeting Montreal, QcAugust 15-16, 2014

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Outline

• Context & motivation

• Approach

• Assimilation results

________________________

• Also: Update on PCW mission

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Context & motivation

Env. Can. moving to ensemble-variational system with:

•Flow-dependent background errors including surface skin temperature correlations with other variables

•Analysis grid at 50 km, increments interpolated to model 25 km grid (15 km in 2015)

•~140 AIRS and IASI channels assimilated: many sensitive to low level T, q, and Ts. RTM is RTTOV-10.

Favorable context to attempt assimilating surface-sensitive IR channels over land and sea-ice vs earlier work with GOES (Garand et al. JAM, 2004) with analysis grid at 150 km and no hyperspectral IR.

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Ensemble spread of Ts (Feb-Mar 2011)

00 UTC 06 UTC

12 UTC 18 UTC

Maximum ~15h localIn SH (summer)

Maximum at nightTibetan area (winter)

Ts background error over land in 4Dvar isConstant: 3 K.

In EnVar, B is 0.5(BENKF+ BNMC)

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Ensemble T(~964 hpa) spread (Jan-Feb 2011)

00 UTC 06 UTC

12 UTC 18 UTC

Only minor variations withlocal time

Values < 0.5 K in SHseem underestimated

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Key factors to consider

• Reliable cloud mask • Spectral emissivity definition (CERES, U-Wisconsin)• Highly variable topography • Radiance bias correction• Background and observation error definition

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Approach guided by prudence

Assimilate under these restrictive conditions over land and sea ice:

•Estimate of cloud fraction < 0.01•High surface emissivity (> 0.97)•Relatively flat terrain (local height STD < 100 m)•Diff between background Ts and retrieval based on inverting RTE limited to 4K

Radiance bias correction approach:•For channels flagged as being surface sensitive, use only ocean data to update bias coefficients

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Assigned observation error to radiances

15.0m 4.13m 15.3m 4.50m

Assigned = f std(O-P) Desroziers = <(O-P)(O-A)>

f typically in range 1.6-2.0

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Limitation linked to topography

Criterion used: local STD of topography < 50 m (on 3X3 ~50 km areas)

RED: accepted, white std > 100 m, blue 100>std>50 m

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Limitation linked to surface emissivity

Surface emissivityAIRS ch 787

Emissivity based onCERES land types(~15 km) + weighted averagefor water/ice/snowfraction.

Accept only emissivity > 0.97 ; Bare soil, open shrub regions excluded.

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Examples of emissivity from CERES

10.7 micron 3.9 micron

water: 0.990 water: 0.977

A large portion of land masses have surface emissivity > 0.97

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First attempt: negative impact in region 60-90 N/S

00 hr 72hr

Possible cause: cloud contamination ?

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Second attempt: added constraints

• Cut latitudes > 60 deg

• Local gradient of topography < 50 m (was 100 m) Strong impact on (O-B) stats for surface channels

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T std error diff. (CNTL-EXP) NH-Extratropics 6 Feb to 31March 2011

vs ERA Interim vs own analysis

Consistent positive impact vs ERA Interim and own

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T std diff (CNTL-EXP) vs ERA-Interim

Tropics SH Extratropics

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Zonal T STD difference (CNTL-EXP)vs ERA-Interim

72-h 120-h

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Time series of T std diff at 850 hPa

NH extratropics TropicsCNTLEXP

EXP superior to CNTL 54-66 % of cases at days 3-5

(b)(a)

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850 hPa TT anomaly cor.

NH extratropics TropicsCNTLEXP

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120-h N-Extr vs ERA-Interim

CNTLEXP

U HR

GZ T

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Validation vs raobs 120-hFeb-Mar 2011 (59 days)

CNTLEXP

SH-extratropics NH-extratropics

U V U V

GZ T GZ T

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Validation vs raobs 120-hFEB-Mar 2011 (59 days)

North America EuropeCNTLEXP

U V U V

GZ T GZ T

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Added yield: about 15%(for surface sensitive channels)

Number of radiances assimilated for surface channel AIRS 787CNTL: ~1400/6h EXP: ~1600/6h

CNTLEXP

Region: world, EXP excludes surface-sensitive channels at latitudes > 60Radiance thinning is at 150 km

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Std/bias of (O-P) and (O-A), AIRS 787CNTL EXP

O-P

O-A

No major impact on bias. Strong assimilation over land,with std (O-P) of ~1.7 K, std (O-A) of 0.40 K (not shown)

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Conclusion

• Very encouraging results, especially in NH where most of new data are assimilated

Next steps:

• Relax limitations on emissivity, use MODIS-derived atlas• Run a summer cycle• Seek operational implementation

Longer term• Evaluate problems specific to high latitudes

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Update on Polar Communications and Weather (PCW) mission

A possible 2-sat HEO system

Goal:Fill communication and meteorologicalobservation gaps in the Arctic

Most likely configuration:2 satellites in highly elliptical orbits

Status:Seeking approval from Governmentin early 2015. Could startoperating in 2021.

Meteorological instrument:Similar to ABI or FCI

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Weather:High-Level Requirements

Capabilities

• ‘GEO-like’ continuous imaging of Arctic circumpolar region

• ‘GEO-like’ spatial (1-3 km) and temporal (15 min) resolution

• ‘Next-generation’ meteorological imager (ABI, FCI, 16+ channels)

• Near-real time processing to L1c for delivery to Environment Canada

• Compatibility with GEO imagers as part of WMO Global

Observing System.

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Orbit Comparison(2-sat Constellation)

Molniya (12-h)Apogee: 39,800 km

Tundra (24-h)Apogee: 48,300 km

TAP (16-h)Apogee: 43,500 km

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Science Studies: Uniqueness of HEO System vs LEO

23 LEO satellites needed to get 15 minImagery at 60 N, versus 2 HEO

Trichchenko and Garand, Can. J. Remote Sensing, 2012

Also, capability to get image triplets for AMV vastly superior to LEO

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Science Studies:Impact on NWP

OSSE showing significant impact of PCW AMVs filling gap in polar areas

Garand et al., JAMC, 2013

Positive impact (blue) at 120-hIn both polar regions from 4 satellites

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Example of PCW Geometry Matching[16-hr TAP Orbit] for intercalibration

VIIRS/SNPP

GEO

Trishchenko and Garand, GSICS Newsletter, 2012