1

Met Office, United Kingdom

AATSR Meteo product:global SST validation at the Met

Office

Lisa Horrocks

Jim Watts, Roger Saunders, Anne O’Carroll

Envisat Validation Workshop 9-13 December 2002

Envisat Validation Workshop / 2

Outline

Met Office contribution to AATSR validation Data chain and Meteo product availability Preliminary results:

– Meteo product SSTs

– Validation against buoys

– Validation against MOHSST

– Validation against HadISST

Conclusions, plans and recommendations

Envisat Validation Workshop / 3

Met Office commitment to AATSR validation

Validation of the AATSR Meteo product SST against in situ observations and analyses

– Gross checks on instrument performance in NRT

Validation activities:– comparisons against buoy SST (daily/weekly)

– comparisons against MOHSST (each month)

» 5° gridded in situ

– comparisons against HadISST (each month)

» 1° globally-complete monthly analysis

Web pages showing routine validation results

Envisat Validation Workshop / 4

AATSR monitoring website

Password required Updated daily at

0900 Monitoring plots

provided two days behind time

Buoy matchups updated weekly

Monthly summary plots

http://www.metoffice.com/research/nwp/satellite/infrared/aatsr/index.html

Envisat Validation Workshop / 5

Skin – bulk SST differences AATSR is sensitive to radiative skin temperature Validation data are measures of bulk SST (>1 m) Expect skin – bulk differences arising from:

– skin effect

– diurnal thermocline

Skin always cooler than “sub-skin” by >0.1 K– conduction through molecular surface layer

– at night, sub-skin = bulk

Sub-skin can get warmer than bulk by several K– strong insolation, low wind => thermal stratification

Envisat Validation Workshop / 6

groundstation

Level 1b processing

Level 2 processing:–Cloud detection–Spatial averaging–Retrieval of skin SST

Meteo product extraction

Second retrieval of skin SST

Skin effect model

Quality control

Diurnal thermocline model

Intercomparison of SSTs

ESA

Met Office

Data chain

BUFR encoding

FTP server

Buoy matchup

AATSR data averaging

Envisat Validation Workshop / 7

Near-real time data availability Routine service from 19 August 2002 ~8–10 orbits per day (Kiruna ftp server) ~4 blind orbits per day (until Svalbard scenario,

end-Nov) Some data gapsDates Days lost Reason for gap

25 August 1 no data available to ftp8-12 September 4 + ½ Envisat manoeuvre28-30 September ½ + 1 + ½ Kiruna hardware failure11-12 October ½ + 1 changes to BUFR tables22-24 October 3 technical problems at Kiruna1-3 November ¾ + 1 + ¾ technical problems at Kiruna8-9 November ½ + ¼ data supplied to ESRIN but not Kiruna9 November ¾ unknown18-20 November 2 + ¾ Envisat protection during Leonids

Require better communication of data supply changes or problems for operational service

Envisat Validation Workshop / 8

AATSR Meteo Product SST

Envisat Validation Workshop / 9

AATSR Meteo product coverageKiruna-only scenario: Atlantic “blind” at night

Envisat Validation Workshop / 10

Dual-view algorithm comparison

n = 38338

20 October 2002 (night)

dual-3 minus dual-2 SST Night time brightness

temperature data Retrieve SST using 3

channels and 2 channels Results show dual-2 ~0.2K

cooler than dual-3 Implications for day-night

biases in Meteo SST Similar discrepancy seen with

ATSR-2 “Correction” can be

determined

D3 – D2 / K

freq

uen

cy

Envisat Validation Workshop / 11

Validation against buoy SSTs

Quality control of buoy data– weekly buoy - NWP background test

– gross check of each reported SST against climatology

Matchup criteria:– Collocated to within 10 arc minutes

» (BUT caveat L2 AST lat/lon calculation error)

– Coincident within ± 3 hours

Weekly mean and SD of AATSR-buoy differences Monthly subset of matchup data uploaded to

NILU

Envisat Validation Workshop / 12

Meteo SST - buoy SST

coverage so far =1860

weekly coverage ~130

Means for period

19 Aug - 27 Nov:

all data = 0.03 K(s.d. 0.53 K)

night only = 0.02 K(s.d. 0.42 K)

day only = 0.03 K(s.d. 0.60 K)

Moored buoys

Drifting buoys

Envisat Validation Workshop / 13

Skin effect using buoy SSTs

19 Aug to 27 Nov 764 night time

matchups AATSR SST minus

buoy SSTvs wind speed

expect skin-buoy negative delta T

compare Fairall skin effect model

Dual-view 3-channel SST slightly too warm ?

Envisat Validation Workshop / 14

12 September “anomaly”

Daily mean Meteo – buoy SST = – 0.68 K (1.90) Anomaly traced to 2 out of (only) 10 matchups Located off coast of San Francisco

Buoy SST Meteo SST NWP SST

286.40 282.22286.77

287.20 282.96290.06

“Truth” confirmed by NWP background AATSR SST more than 4 K too cool Likelihood of undetected stratocumulus ?

Envisat Validation Workshop / 15

Validation against MOHSSTGridded in situ data at 5° resolution. October mean.

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AATSR bulk SST minus MOHSST / K

Envisat Validation Workshop / 16

Validation against HadISSTGlobally complete SST analysis at 1° resolution. October mean.

Envisat Validation Workshop / 17

Validation against HadISST

Skin - HadISST Bulk - HadISST

Sep (1-30) -0.07 (0.79) 0.08 (0.79)

Oct (1-31) -0.08 (0.77) 0.06 (0.77)

HadISST available 10 days after end of month– September and October completed

– global statistics (K):

Regional statistics also computed– not yet enough data to spot patterns

Envisat Validation Workshop / 18

Conclusions From preliminary results, the Meteo product

validates well against buoys and climate data: SSTs close to, or within, expectation

Interalgorithm differences ~0.2 K (as ATSR-2)– Dual-view 3-channel SSTs may be slightly warm

Evidence for undetected cloud in difficult areas Longer validation record required to detect

regional or seasonal trends Larger buoy matchup dataset required for

detailed investigation

Envisat Validation Workshop / 19

Future work and Recommendations

Plans for continued and expanded validation– continue routine monitoring and comparison in NRT

– AATSR/ATSR-2

– AATSR/microwave SSTs/AIRS

Recommend– more timely and more detailed communication from PDS

on data supply issues (changes or gaps)

– product format amendments

» BUFR version to include number of pixels (in Meteo)

– work to correct dual-view interalgorithm bias

– forum for discussion of ~1 year results