Validation of Land Surface Temperatures derived from AATSR data at the Valencia Test Site

Transcription

1 Validation of Land Surface Temperatures derived from AATSR data at the Valencia Test Site César Coll, Vicente Caselles, Enric Valor, Raquel Niclòs, Juan M. Sánchez and Joan M. Galve Thermal Remote Sensing Unit, Department of Thermodynamics, Faculty of Physics, University of Valencia SPAIN Page 1

2 Objective To collect a database of ground measurements of land surface temperature (LST) concurrent to AATSR ( ) To validate LST derived from AATSR split-window algorithms ( ) Plan of presentation Test site Experimental data Validation of AATSR split-window algorithms Conclusions Page 2

3 The Valencia test site THERMAL REMOTE SENSING GROUP AATSR LST product validation site. ASTER and MODIS cal/val site. Flat, large and homogeneous area of rice fields (full cover in July and August) AATSR Brightness temperature, 11 µm nadir Valencia Mediterranean Sea Test site N = 39 pixels T max = ºC T min = ºC T aver = ºC σ = 0.20 ºC Test site AATSR RGB (11/07/03) Brightness temperature (ºC) 39º N 0º Page 3

4 Terra/ASTER (03/08/04) RGB (15 m) Brightness temperature (ºC) ch µm (90 m) number of pixels N = 4400 pixels T max = ºC T min = ºC T aver = ºC σ= 0.45 ºC temperature (ºC) Page 4

5 July 20, 2002 August 12, 2004 Field campaigns in July and August : 17 ground LST/AATSR concurrent measurements 2005: underway Page 5

6 Ground measurements of LST Concurrent to cloud-free, morning AATSR overpasses (~10:30 UTC) Thermal infrared radiometers: 2 CIMEL CE 312 radiometers (4 bands). Accuracy: ±0.1 ºC - ±0.2 ºC. 1 Everest 112.2L IR thermometer (8-13 µm). Accuracy: ±0.5 ±0.7 ºC. 1 AGA 80 IR thermometer (8-13 µm). Accuracy: ±0.7 ±0.9 ºC. Calibration and inter-comparison of radiometers in the field. Emissivity correction of radiometric temperatures (emissivity and downwelling sky radiance measurements) relative response band 4 band 1 CE 312 band 3 band wavelength (µm) Page 6

7 Methodology Temperatures measured along transects in the 1 km 2 square. Test site 1 ( ): Test site 2 (2004): 0º17 50 W, 39º14 27 N 0º17 43 W, 39º15 01 N Average temperatures for each transect/radiometer (±1.5 minutes around satellite overpass). Standard deviation of temperatures, σ(var): natural variability. Estimation of error budget: σ(t) = [σ(var) 2 + σ(cal) 2 + σ(em) 2 ] 1/2 calibration error emissivity correction error Average temperatures for all transects (Everest and AGA discarded when σ(t)>1.0 ºC or T-T CE >1.0 ºC) Temperature (ºC) CE Everest AGA 25 10:25 10:30 10:35 10:40 10:45 10:50 Time (UTC) Page 7

8 Ground LST database (± uncertainties) coincident with AATSR overpasses. Year Date (day/month) Overpass LST ± σ(t) (ºC) time (UTC) CE1 CE2 Everest AGA Average 10/07 10: ± ± ±0.6 13/07 10: ± ± ± ±0.9 29/07 10: ± ± ± ±0.7 08/08 10: ± ± ±0.7 14/08 10: ± ± ±0.5 08/07 10: ± ± ± ± ±0.7 11/07 10: ± ± ± ±0.7 14/07 10: ± ± ± ±0.6 24/07 10: ± ± ± ±0.6 30/07 10: ± ± ± ±0.6 12/08 10: ± ± ± ±0.6 28/06 10: ± ± ±0.6 08/07 10: ± ± ±0.6 14/07 10: ± ± ± ±0.7 27/07 10: ± ± ±0.4 30/07 10: ± ± ±0.4 12/08 10: ± ± ± ±0.6 T CE1 T CE2 0.6 ºC T 1.0 ºC LST: ºC σ(t): ±0.4 ±0.9 ºC Page 8

9 AATSR brightness temperatures T 11 and T 12, nadir (L1b data, 3 3 pixels average and σ) Year Date (day/month) Viewing angle (º) T 11 (ºC) σ(t 11 ) (ºC) T 12 (ºC) σ(t 12 ) (ºC) 10/ / / / / / / / / / / / / / / / / σ: ºC (except for 08/07/04) σ: ºC (except for 08/07/04) Page 9

10 Validation of AATSR split-window algorithms AATSR LST operational product - Prata (2000) (ATBD) LST = a f,i,pw + b f,i (T 11 -T 12 ) n + (b f,i + c f,i )T 12 Coefficients depend on: a f,i,pw = 0.4[sec(θ)-1]pw + f a v,i + (1-f) a s,i b f,i = f b v,i + (1-f) b s,i c f,i = f c v,i + (1-f) c s,i n = cos(θ/5) i : land cover type. 13 classes or biomes (i=1 13). f : fractional vegetation cover (0 f 1). For each class, separate coefficients for the vegetated (subscript v) and for the bare surface (subscript s). pw : precipitable water (in cm). RAL processor f, i : global classification and fractional vegetation cover maps (0.5º 0.5º). pw: climatology Coefficients from Prata (2002) Page 10

11 LST validation RAL processor i = 6 (broadleaf trees with groundcover) f = pw = cm (July) f = pw = cm (August) With i = 6 and f =1 (full cover) Average Diff. = 1.3 ºC Stand. Dev. = 0.7 ºC Range of Diff. = [ 2.4 ; 0.4] ºC Year Date (dd/mm) Ground LST (ºC) AATSR LST (ºC) Ground AATSR LST (ºC) 10/ / / / / / / / / / / / / / / / / Average difference (ºC) -3.5 Standard deviation (ºC) 0.9 Range of differences (ºC): [ 4.8 ; 1.6] Page 11

12 AATSR LST algorithm optimised for the test site i = 8 (broadleaf shrubs with groundcover) f = 1 (full cover) T 11, T 12 : L1b data LST = 0.4[sec(θ)-1]pw (T 11 -T 12 ) n T 12 (Eq. 1) (pw=2.5 cm) Quadratic, emissivity-dependent algorithm (Coll and Caselles, 1997) LST = T 11 + a 0 + a 1 (T 11 -T 12 ) + a 2 (T 11 -T 12 ) 2 + α(1-ε) β ε ε=(ε 11 +ε 12 )/2; ε=ε 11 -ε 12 Coefficients calculated for AATSR and taking ε=0.985; ε=0 for the test site: LST = T (T 11 -T 12 ) (T 11 -T 12 ) 2 (Eq. 2) Page 12

13 LST validation Year Ground AATSR Date Ground AATSR LST (ºC) LST (ºC) (dd/mm) LST (ºC) Eq. (1) Eq. (2) Eq. (1) Eq. (2) 10/ / / / / / / / / / / / / / / / / Average difference (ºC) Standard deviation (ºC) Range of differences (ºC): [ 1.0 ; 1.5] [ 1.1 ; 1.6] Page 13

14 LST validation LST (ºC) ground RAL Eq. (1) Eq. (2) LST (ºC) LST (ºC) /07/02 18/07/02 02/08/02 17/08/ /07/03 18/07/03 2/08/03 17/08/03 Date (dd/mm/yy) 25 22/06/04 07/07/04 22/07/04 06/08/04 21/08/04 Page 14

15 Conclusions Database of ground LSTs for the validation of AATSR derived LSTs in the Valencia test site. Field campaigns in the summers of (17 matchups) and 2005 (underway). Thermal homogeneity of the site analysed with satellite brightness temperatures at different scales (AATSR 1 km: σ < 0.2 ºC; ASTER 90 m: σ < 0.5 ºC). LST validation: The RAL processor overestimates the ground LSTs by 3.5 ºC. Optimised algorithms yield differences in the range between 1.0 and 1.5 ºC. High sensitivity of split-window LSTs to land cover (i) and vegetation fraction (f). The 0.5º 0.5º resolution of land cover/vegetation fraction classification used by the operational AATSR algorithm is too coarse in order to account for the heterogeneity of land surfaces. 1 km resolution is recommended for i and f. Page 15