CERES-GERB Joint Science Teams 2004 1 The Geostationary Earth Radiation Budget (GERB) experiment...

CERES-GERB Joint Science Teams 2004

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The Geostationary Earth Radiation Budget (GERB) experiment

GERB Status

Imperial College London

J E Harries (GERB PI)J E Russell (GERB Project Scientist)J Hanafin, (GERB Operations Scientist)


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The Geostationary Earth Radiation Budget (GERB) experiment

GERB Status

Imperial College London

Contents:1. Short review of the project2. Status3. Some early science


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1.Short review of the project

Why do we need ERB measurements?

• Understanding our climate and how it may change in the future is one of the scientific “Grand Challenges” of our time

• We make predictions about how climate might change in future using coupled, ocean-atmosphere General Circulation Models (GCMs).Ultimately the models must be tested and validated against observations of the real world.

• ERB variability is sensitive to climate processes on all timescales, especially those with cloud signature.

• Combination of CERES and GERB offers powerful new tool, building on past ERB measurements.


4Spatial Scale (km)

Tem

por

al

scal

e

2 20 200 2000 20000

15 min

1 hour

1 day

1 month

1 year

1 decade

DIURNAL CYCLE

Mesoscale convection complex

Synoptic processes

GERB TEMPORAL SAMPLING

Individual clouds

GE

RB

SP

AT

IAL

SA

MP

LIN

G

El Nino Southern Oscillation

Equatorial Wave disturbances (e.g. African Easterly waves)

Intraseasonal Oscillation

Hurricanes

SE

VIR

I S

PA

TIA

L S

AM

PLI

NG

SEVIRI TEMPORAL SAMPLING

North Atlantic Oscillation

Time and space scales of climate processes


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75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03

ERB (NIMBUS-6)

ERBE (ERBS) - Scanner

ERBE (NOAA-9) - Scanner

ERBE (NOAA-10) - Scanner

ScaRaB (METEOR)

CERES (TRMM)

ERB Instrument timeline

CERES: Clouds and the Earth’s Radiant Energy System

ScaRaB: Scanner for Radiation Budget

ERBE: Earth Radiation Budget Experiment

Red - Sun synchronous Green- Precessing Yellow - Geostationary

ERB (NIMBUS-7)

CERES (TERRA)

CERES (AQUA)

GERB


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•To make progress, accurate, inter-calibrated, better sampled observations are needed, to test improved models.

•MSG/SEVIRI/GERB is a European initiative to contribute to international effort.

•GERB developed by a European consortium of groups from UK, Belgium and Italy.

MSG/SEVIRI/GERB


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The GERB instrument


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The GERB instrument


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The GERB instrument


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256

dete

ctor

pix

els

262 steps

Satellite rotation period = 0.6 s

262 steps for full Earth disc = 157.2 s

Between each Earth scan, internal BB measurement taken for calibration

At correct viewing geometry, calibration monitor records scattered solar light as a relative measure over time

Average three scans in each channel to improve S/N

Total repeat time = 157.2*6 ~ 15 min.

2 channels: Total Total+quartz filter (SW)

GERB scanning


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GERB characteristicsWAVEBANDS Total: 0.32 mm - 100.0 mm

Shortwave, SW: 0.32 mm - 4.0 mm

Longwave, LW (by subtraction): 4.0 mm - 30.0 mm

RADIOMETRY SW LW

Absolute Accuracy: < 1.0 % < 1.0 %

Signal/Noise: 1250 400

Dynamic Range: 0-380 W m-2 sr-1 0-90 W m-2 sr-1

SPATIAL SAMPLING 44.6 39.3 km (NS EW) at nadir

TEMPORAL SAMPLING 15 minute SW and LW fluxes

CYCLE TIME Full Earth disc, both channels in 5 minutes

CO-REGISTRATION Spatial: 3 km wrt SEVIRI at satellite sub-point

Temporal: Within 15 min of SEVIRI at each pixel

INSTRUMENT MASS 25 kg

POWER 35 W

DIMENSIONS 476 mm 275 mm 345 mm


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SOLAR AND TERRESTRIAL RADIATION FIELDS

INSTRUMENT SIGNAL CREATION

GERB FILTERED RADIANCE AT SATELLITE

RECTIFICATION AND GEOLOCATION (GRIDDED FILTERED RADIANCES AT TOA)

GERB UNFILTERED RADIANCES/FLUXES

15 MINUTE INTEGRATION

GERB 15 MINUTE RADIANCES/FLUXES

The data processing chain

On board calibration

SEVIRI data

ON BOARD

RAL PROCESSING

RMIB PROCESSING


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2.Status

Overall Instrument performance:

• Excellent performance;

• Issues being worked on include:

Scanning;

Stray light;

Geolocation;

Validation and absolute accuracy;

Spectral response.


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PSF scans

Measured PSF for Pixel 124


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PSF scans

1400 scanlines 1800 scanlines500 scanlines


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Mirror side offset correction

Observation of Earth limb with step size of 0.15’ (~1/28th NORMAL scan step)

’Optimum’ correction of ~2.7 pixels (0.4’)


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Scan mirror performance

•Basically excellent.

•MSG rotation period ~600ms.

•GERB de-spin mirror rotation period ~1200ms synchronised to MSG rotation by spacecraft Start of Line pulse (SOL) generated by MSG sun and earth sensors.

•De-spin mirror provides 40msec integration on a N-S earth ‘line’ (of 256 pixels) each rotation. Line position advanced by line (pixel) width (4.2 arcmin) each rotation.


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• Note: the line position precision of ~1/10th not yet achieved because of systematic diurnal errors in the SOL pulse - related to the in-orbit performance of the sun-, and particularly the earth- sensor. (Knowledge of the misalignments between the MSG and GERB frames should pin this down).

• Note: the SEVIRI nodding mirror causes variations in the MSG rotation rate at the ~0.01% level, which if not tracked would cause ~0.5 pixel line position errors. The mirror control system, however, tracks these changes. Any remaining errors are included in the 1/10th pixel line error above.


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Mirror performance 2003-2004 No of

events > 0.05V

No of events > 0.1V (~4 pixels)

No of events > 1V (~40 pixels)

2003 January 1 1 1 February 1 1 1 March - - - April 0 0 0 May 4 2 1 June 2 2 2 July 14 13 13 August 0 0 0 September - - - October - - - November 2 1 0 December 2 1 0 2004 January 4 4 4 February 2 0 0 ->17 March 5 5 1

Very good performance to date:•No of significant individual ‘sticky events’ v. small

•Worst month (July 2003) had 14 instances of mirror position error > 2 pixels, but 12 within one scan

•Other months have seen 1-4 files affected (i.e. < 0.025% of images collected)


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Geolocation Geolocation errors 24th May 2003


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Stray Light

Straylight in earth view


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Stray Light

Straylight visible as a stripe feature along image

MagnitudeIncreases rapidly at onset of feature, then asymptotes to ~100Wm-2 in TOTAL channel

Times affected:23:00 – 01:00 UTC

Dates affected:See graph


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Lunar scans

Fine scans both static and moon motion tracking have been made for calibration and science applications


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CERES inter-calibrationsCERES PAPS scanning pattern

Aqua pass at 12:39–12:47 on 06/21/2003; view from 2000km


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Reprocessed Average Detector Spectral Response

(mean shape over ASICs)

Mean spectral response (0.3-15 microns)

200

250

300

350

400

450

0 2 4 6 8 10 12 14 16

wavelength (microns)

res

po

ns

e


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(mean shape over ASICs)

Reprocessed measured average detector and witness sample response

250

270

290

310

330

350

370

390

410

430

450

0.000 10.000 20.000 30.000 40.000 50.000

w avelength (um )

No

rma

liz

ed

re

sp

on

se

`


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(mean shape over ASICs)Normal incidence reflectance varies < 1% at wavelengths shorter than 2.5m and by less than 10% at 10m.

0.2m - 0.7m in the visible, the reflectance of gold black < 2%.

Visible to ~3m the reflectance gradually decreases to a very low value < 0.3%”

Reflectance from 100 to 500m (40m thick samples of gold black) was 8 and 20% at 100m rising to 20 and 26% at 500m for 45 and 25 incidence respectively.


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Effect of reprocessed data on calibration

Ratio of Gain calculated from a SW source to that calculated from a LW source

Would be 1 for perfect knowledge of spectral response and instrument linearity

Green: old SR, Red: reprocessed SR


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3. Some early scienceDefinition of clear sky fluxes and cloud forcing studies (Jo Futyan)


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3. Some early science

Study of the radiative properties of aerosol using GERB and SEVIRI (Dr Helen Brindley)

Image courtesy NASA

GERB reflected shortwave fluxes MODIS visible image


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3. Some early science

Comparison of GERB and Met Office UM broadband fluxes: SINERGEE project (Prof Tony Slingo, Dr Richard Allen)


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4. SUMMARY• GERB is providing ERB from geostationary orbit with a 15

minute time resolution • Instrument commissioning tests indicate very stable instrument

gains and low measurement noise• After correction for a mirror side offset problem, instrument

pointing is accurate to within 0.1 of a pixel• MSG supplied geolocation information has some artefacts that

are affecting the data geolocation. These are expected to be solved in the next few months.

• Initial intercomparison between GERB and CERES data indicate the mean SW and LW radiances from the instrument agree to within the expected measurement accuracy. These data are now being analysed for any indication of scene dependent differences

• Data is expected to be released after validation later this year.


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CERES-GERB Joint Science Teams 2004 1 The Geostationary Earth Radiation Budget (GERB) experiment...

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Transcript of CERES-GERB Joint Science Teams 2004 1 The Geostationary Earth Radiation Budget (GERB) experiment...