DUE GLOBVAPOUR Algorithm Theoretical Baseline Document L2 AATSR

Transcription

1 DU GLOBVAPOUR Algorthm Theoretcal Baselne Document L2 AATSR Issue 1 Revson 0 19 January 2012 Project nr: Project Coordnator: SRIN/AO/1-6090/09/I-OL Marc Schröder Deutscher Wetterdenst marc.schroeder@dwd.de

2 Issue: 1.0 Revson: 0 Page 2 Document Change Record Document, Verson Date Changes Orgnator DOC, draft 04 July 2011 Orgnal verson DOC Jan Complete rewrte to 1DVar, frst comments from DWD mplemented R. Preusker DOC Jan 2012 Mnor edtoral changes M. Schröder

3 Issue: 1.0 Revson: 0 Page 3 Table of Contents 1 Introducton Purpose Defntons, acronyms and abbrevatons Applcable Documents Reference Documents Structure of the document Algorthm overvew Algorthm descrpton Theoretcal descrpton Practcal applcaton Assumptons and lmtatons Conclusons... 7

4 Issue: 1.0 Revson: 0 Page 4 1 Introducton 1.1 Purpose Ths document provdes the Algorthm Theoretcal Baselne for the GlobVapour Level 2 AATSR product. 1.2 Defntons, acronyms and abbrevatons AATSR RTTOV GFS SST TCWV Advanced Along-Track Scannng Radometer Radatve Transfer for TIROS Operatonal Vertcal Sounder Global Forecast System Sea Surface Temperature Total Columnar Water Vapour 1.3 Applcable Documents [AD-1] [AD-2] DU GLOBVAPOUR Requrements Baselne Document (RBD), ssue 1, revson 0, dated 16 Aprl DU GLOBVAPOUR Techncal Specfcaton Document (TSD), ssue 1, revson 0, dated 16 Aprl [AD-3] DU GLOBVAPOUR Software Development Plan (SDP), ssue 1, revson 0, dated 16 Aprl [AD-4] DU GLOBVAPOUR Summary Report on xstng Algorthm Comparson and Valdaton Reports (SVR), ssue 1, revson 0, dated 29 July [AD-5] SRIN Statement of Work, OP-DUP-OPS-SW , ssue 1 rev. 1, [AD-6] DU GLOBVAPOUR Proposal, ssue 1 revson 3, dated 9 July Reference Documents [RD-1] [RD-2] Rodgers, C., 2000, Inverse Methods for Atmospherc Soundng: Theory and Practce World Scentfc, London Saha, Suranjana, and Coauthors, 2010: The NCP Clmate Forecast System Reanalyss.

5 Issue: 1.0 Revson: 0 Page 5 Bull. Amer. Meteor. Soc., 91, do: /2010 BAMS [RD-3] [RD-4] Mutlow, C.T., Zavody, A.M., Barton, I.J. and Llewellyn-Jones, D.T.; Sea-surface temperature-measurements by the Along Track Scannng Radometer on the RS-1 Satellte - early results. J. of Geophyscal Research-Oceans, 99, No.C11, , 1994 Saunders R.W., M. Matrcard and P. Brunel 1999: An Improved Fast Radatve Transfer Model for Assmlaton of Satellte Radance Observatons. Q.J.Royal Meteorol..Soc., 125, [RD-5] SA, The AATSR Product Handbook, Structure of the document Secton 2 gves an overvew of the AATSR nstrument and the retreval algorthm scheme developed wthn the GlobVapour project. The used methods, models and preparatory works are ntroduced. In secton 3 both the theoretcal bass of the algorthm as well as the practcal applcaton are detaled. Necessary assumptons and lmtatons are descrbed n secton 4. Secton 5 gves a concluson. 2 Algorthm overvew The algorthm for the retreval of TCWV from measurements of AATSR s based on the explotaton of the weak but artculated water vapour absorpton and emsson around 3.7 µm and 11 µm usng nverse modellng. The retreval s lmted to nght tme observatons above cloud-free ocean. The (Advanced) Along Track Scannng Radometer ((A)ATSR) are mult-channel magng radometer wth three bands n the thermal nfrared TIR (3.7 µm, 11 µm and 12 µm) and up to four other bands n the vsble and near nfrared spectral range. Ther concal scan prncple gves a dual-vew ( forward and Nadr ) of the arth's surface wth a 500 km wde swath. The spatal resoluton s approxmately 1 km for Nadr and 1.5 km for the forward vew. (A)ATSRs prncpal objectve were to provde the global Sea Surface Temperature (SST) wth the hghest level of accuracy and stablty. The ATSR nstruments fly on board the RS- satelltes, AATSR s mounted on NVISAT, all sun synchronous polar orbter. The retreval scheme for AATSR s usng these bands at forward and Nadr vew n a 1D varatonal approach. In practce: the nput s two tmes three bands plus two observaton geometres, the output are the best fttng surface temperature and the total column water vapor. To reduce ambgutes due to unknown surface emssvtes, the retreval s lmted to ocean scenes and to reduce ambgutes at 3.7 µm due to sun glnt the retreval s further lmted to the nght scenes. The hgh optcal thckness of clouds n the TIR allow only cloud free pxel to be used. The varatonal approach requres a forward radatve transfer model, for whch the necessary profles of temperature and humdty are taken from a 6 year GFS model clmatology [RD-2]. Thus no numercal weather model assmlaton s done and no other auxlary data s needed.

6 Issue: 1.0 Revson: 0 Page 6 In a frst step, the AATSR L1B data s screened for clouds and land surfaces, usng the standard flags delvered wth the L1B fle. ([RD-5]) 3 Algorthm descrpton 3.1 Theoretcal descrpton Measurements around 3.7 µm, 11 µm and 12 µm provde a hgh senstvty to the surface temperature and addtonally a margnal senstvty to the atmospherc water vapour. If the spectral emssvty s known n the respectve channels and f the vertcal profle of water vapour s prmarly determned by the surface temperature, the total column water vapour can be roughly estmated n a jont retreval together wth the surface temperature. For nght tme ocean pxel, these requrements are fulflled. The retreval s based on a smplfed varatonal approach, that mnmzes a weghted dfference between a measurement Y and a modelled measurement F(X): J 1 2 T 1 ( X ) = ( Y F( X )) S ( Y F( X )) (1) where the state vector X, that mnmzes the cost functon J(X), s the most probable estmate (f least squares a supposed to be the maxmum lkelhood estmator). Here the state vector s composed of the surface temperature and the TCWV. The measurement vector Y s composed of the 2x3 top of atmosphere brghtness temperatures from AATSR. S s the error covarance matrx S = S + K S K (2) M T B whch s composed of the measurement error co-varance S M and model parameter error co-varance S B. K B s the Jacoban of the forward model F wth respect to ts parametersatons B. Ths approach s not usng any weather or clmate model outputs, to be clmatologcally ndependent and to be not ncestuous. On the other hand the neglgence of a pror nformaton, wll cause a mss of a best estmate n a Bayesan sense. Nevertheless, the uncertanty of the retreved state vector S R s estmated usng: B B S 1 T 1 = K S K (3) R K s the Jacoban of the forward model wth respect to the state varables. The mnmsaton of equaton (1) s done by an teratve gradent descent: X S 1 T 1 ( K S K ) T 1 ( K S ( Y F( X ) ) + 1 = X + S ) = (4)

7 Issue: 1.0 Revson: 0 Page 7 untl ether the resdual ( F X ) ) Y s smaller than 0.1 K or the number of teratons s larger than ( 5 (these numbers can be subject of changes n future). In the later case, the algorthm s regarded as non-convergng. The dagonal elements of the last S are the uncertantes of TCWV and SST. Currently the forward operator s a Look Up Table (LUT) flled wth radatve transfer calculatons from RTTOV [RD-4] for a fxed set of temperature and humdty profles (see next paragraph). The top of atmosphere brghtness temperatures are calculated by means of a 5-dmensonal lnear nterpolaton wth respect to the LUT dmensons: (Vewng angle, TCWV, SST, CO2, T-RH profle). Sx representatve temperature and humdty profles as well as ther varance have been extracted from 5 years ( ) of global cloud free ocean GFS datasets [RD-2]. The varance has been used to estmated forward model uncertanty. The sea surface temperature s used as the determnng quantty. Ths approach s very smple but applcable, snce most water vapour s n the lowest part of the atmosphere and n partcular above the ocean the amount of humdty s manly determned by the sea surface and ar temperature due the Clausus-Clapeyron relaton. 3.2 Practcal applcaton The processor s wrtten n IDL wth nlne C code. It needs AATSR L1B data as nput and produces NetCDF output fles, contanng the SST and TCWV as well as the correspondng uncertantes. No further ancllary data s needed. 4 Assumptons and lmtatons The retreval algorthm s applcable over ocean only durng nght tme for cloud free pxel. There are two man sources of uncertanty: 1. The unknown profle of humdty and temperature, but ths uncertanty s pxelwse quantfed. 2. Undetected thn clouds, whch lead to dry based TCWV. A later verfcaton of the retreved SST could help to flter out these cases 5 Conclusons The GlobVapour AATSR L2 s based on the explotaton of the relatvely weak absorpton and emsson of water vapor n the thermal nfrared, as measured by AATSR bands 3.7 µm, 11 µm and 12 µm forward and Nadr. The output s used for the generaton of grdded L3 products, namely daly nghtme compostes and monthly means. The algorthm s totally complementary to MRIS (daytme/land vs. nghttme/ocean) and partly supplemental to SSM/I due to ts much hgher spatal resoluton.