The new SMOS-IC L-band vegetation index (L-VOD):

Transcription

1 The new SMOS-IC L-band vegetation index (L-VOD): Overview and application to monitoring vegetation biomass at global scale J-P Wigneron, L. Fan, et al. With Bordeaux group: A. Al-Yaari, J. Swenson, F. Frappart, X. Li. CCI Biomass Workshop, Paris, Spt , 2018

2 VOD (definition) SMOS-IC L-VOD product (new, simplified, well-suited to applications) Applications of L-VOD to vegetation monitoring CCI Biomass Workshop, Paris, Spt , 2018

3 SMOS (Soil Moisture and Ocean Salinity), PI Y Kerr Spatial resolution: ~ 35-50km Revisit time: Max. 3 days Sensitivity ~ 2K over land Goal of accuracy in SM: ~ 0.04 m3/m3 Launch : Dec. 2009: ~ a 9-year data set Retrieval algorithm: using multiangular and dual polarization TB Soil moisture & vegetation opacity (VOD) based on the inversion of L-MEB, (L-band Microwave Emission of the Biosphere) Wigneron et al., (algorithm in ESA proposal, L-MEB, SMOS-IC product)

4 The Brightness temperature (TB) observations are sensitive to: soil moisture (SM), determines smooth soil reflectivity smooth biomass (VOD), determines vegetation extinction γ= exp( VOD/cos( )) temperature (TB = emissivity x temperature) canopy type (ω), soil roughness ( rough = C. smooth ), and texture PASSIVE RADAR TB* TB* S e S T S ATMOSPHERE v s / 2 Tb*(z=H) Vegetation Emission (1- )(1-ω) Extinction Extinction VEGETATION Tb*(z=0) VEGETATION Extinction Soil reflectivity S Soil emission SOIL Sol

5 L-MEB (L-band Microwave Emission of the Biosphere model) radiometer For vegetation, L-MEB is based on a zero order solution of radiative transfer equations ( - model): TB veg =(1-e -VOD/cos( ) )(1- )T veg (1+ soil e -VOD/cos( ) ) SKY ATM Theory - VOD = K E. H, accounts for extinction effects K Extinction = K Absorption + K Scattering K Absorption = K Emission - accounts for scattering effects (K Scattering / K Extinction ) H=Height of Crop VEG Experimental VOD (nadir) = b. VWC Jackson and Schmugge, 1991 with, SOIL VWC = vegetation water content (kg/m2) b~0.12 ( for crops)

6 L-MEB algorithm development /evaluation In situ: EMIRAD (TUD) at the INRA Avignon test site (soybean, corn) Smosrex, , Toulouse, with Lewis (CESBIO, CNRM, INRA, ONERA), soil, fallow Landes forest, (INRA), with EMIRAD-1 (TUD), coniferous forest Elbara, (ETH, U. of Bern), grass, deciduous forest Elbara -2, 2010 (ESA funded) at the Munich, VAS, Sodankyla sites, grass, mattoral, forest and airborne Carols, (Cnes, ESA), Smosmania (France) and Vas sites (Spain), EMIRAD, Landes forest LEWIS at Smosrex site MELBEX- EMIRAD INRA 01 - EMIRAD

7 Examples of results : SOYBEAN (INRA-1991) [Wigneron et al., RSE, ] Retrieved soil moisture Retrieved VOD, VWC and LAI = f(time) Retrieved VOD LAI VOD VOD VOD VWC R2=0.96 R2=0.88 time VWC LAI

8 Key features of SMOS to retrieve VOD: -multi-angular observations simultaneous retrievals of SM and VOD (Wigneron et al., 1995, 2000) -in SMOS-IC SM retrievals, no need of optical indices to estimate VOD (and vice versa) SMAP: In the SCA algorithm, VOD is estimated from NDVI AMSR-E: iterative approach based on only 2 observations (LPRM algorithm) -L-band (1.4GHz, ~ 30 cm): higher sensing capabilities through dense vegetation than C- (6 GHz, ~ 5 cm) and X- bands -passive observations are much less sensitive to structural effects of vegetation (row, vertical structure), soil (roughness, surface geometry), topography, etc. than radar observations the b and parameter are relatively constant for varying vegetation conditions = 0.07 for forests = 0.1 for other vegetation types (no need for parameter tuning)

9 -spatial calibration L-VOD / Biomass in 2011 Biomass (Baccini map ) - space for time substitution: L-VOD is used to monitor time changes in carbon stocks in Africa X-VOD Dynamics of carbon stocks in Africa over Biomass (Baccini map ) SMOS-IC L-VOD

10 F. Tian,, et al. "Coupling of ecosystem-scale plant water storage and leaf phenology observed by satellite", Nature EE Miombo Pre-rain Miombo forest C. Ryan, University of Edinburgh Time lag between L-VOD and LAI F. Tian, University of Copenhagen A high temporal decoupling between plant water storage and LAI in dry Tropical forests (especially in Miombo) L-VOD LAI Time variation in L-VOD and LAI (Miombo) F. Tian, University of Copenhagen

11 THANK YOU!

12 Retrieving of the # components of optical depth: [Saleh et al., RSE, 2006] retrieving VOD_GV (standing vegetation): for dry conditions : dry litter and no interception Green vegetation: gv =f (LAI) =f (VWC) ( gv =b VWC, b~0.1)

13 SMOS : le Système d Observation: visées multi-angulaires: un point au sol est vu sous différents angles de visée au fur et à mesure que le satellite avance: Champ de vue (FOV) Satellite Spacecraft velocity Earth Swath 1000 km m d N 30 Local incidence angle = 55 Nadir path

14 SMOS : un Système d Observation Multiangulaire 18 SMOS Brightness Temperature (L1C product), Browse product at 42.5

15 Le principe: Mesure de la réflectivité du sol = f(constante diélectrique ε) Humidité du sol SM (m3/m3) Texture structure du sol Cte diélectrique ε s_lisse Correction rugosité s Mesure radar ou passif Soil dielectric constant ε fort contraste sol sec (ε~5) et sol humide (ε~30) Soil dielectric constant = f(sm) à 1.4GHZ Ulaby et al. (1986) SM

16 Mesures dans les Micro ondes Le principe: mesure de la reflectivité du sol = f(constante dielectrique ε) Il existe un fort contraste entre la constante dielectrique d un sol sec (ε~5) et celle d un sol humide (ε~30) Mesures dites Actives (Radar) ou Passives actif reflectivité passif émissivité (= 1-Réflectivité) Mesures dans le domaine basse fréquence ~ 1.4 Ghz (bande L) et 10 Ghz (bande X): -faible sensibilité aux effets atmosphériques: (mesure tout temps & corrections précises) -faible atténuation du signal sol par la végétation Soil dielectric constant = f(sm) à 1.4GHZ Ulaby et al. (1986)

17 SMOS : Multi angular observation system multi-angular observations: -a given site on Earth is seen at different incidence angles as the satellite moves ahead. -larger angle ranges for sites close to the subsatellite track Field of view (FOV) Satellite Spacecraft velocity Earth Swath 1000 km m d N 30 Local incidence angle = 55 Nadir path

18 TB TB* TB* Гs (1-Гs) Ts atmosphere TB*(z=0 ) diffusion absorption ( ) diffusion absorption ( ) vegetation soil vegetation emission TB*(z=-d) Reflected (Гs) vegetation emission soil emission : (1-Гs) Ts

19 Vegetation attenuation increases as frequency increases Saturation of Biomass = f(vod) comes quicker at high frequencies X-band (~10 GHz, ~3 cm), AMSR-E C-band (~5 GHz, ~6 cm), AMSR-E, ASCAT ~100 t/ha L-band (1.4 GHz, ~30 cm), ESA/SMOS, NASA/SMAP, ~250 t/ha P-band (~0.4 GHz, ~75 cm), ESA/Biomass, ~ no saturation? Quicker saturation for active vs passive systems

20 (Most) Dense tropical forests in the Amazon basin (Guyana) = an extreme case Time variations in L- VOD, SM, EVI, rainfall -SM can be clearly related to rainfall Tian et al., 2017