Level 2 Processing of HUT-2D Data: Preliminary Results

Transcription

1 Level Processing of HU-D Data: Preliminary Results SMOS-BEC Level eam. SMOS Barcelona Expert Centre Pg. Marítim de la Barceloneta 37-49, Barcelona SPAIN URL: SMOS cal/val Workshop - March Lisbon (Portugal)

2 Outline - he scenario - Brightness emperature analysis - Retrieval Configuration - Results Nominal case Other forward models - Future Work SMOS cal/val Workshop - March Lisbon (Portugal) M. alone / 5

3 he Campaign wo series of flights over the Gulf of Finland August, 13 SD1 August, 15 SD SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 3 / 5

4 he Scenario SSS front from 0 to 4 psu SWH almost constant at 0. m SS front from 19 to 3 C QuikSCA wind Speed 3.73 m/s SD m/s SD SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 4 / 5

5 Brightness emperatures Analysis Cross-rack mean value and std. of meas - mod SD1 SD ascending flights descending flights SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 5 / 5

6 Brightness emperatures Analysis Snapshot-dependent mean value and std. of meas - mod SD1 SD ascending flights descending flights SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 6 / 10 5

7 Pre-Processing o improve the quality of the measurements have been considered: - 0-m Square cells. - all the flights. - º, 4º, 8º angular bins. SD1 SD SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 7 / 5

8 he Campaign he drawback is: errors up to 5 K errors up to 1 K SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 8 / 5

9 he Processing For each bin-width, SSS has been retrieved using cost functions χ = 1 N obs N obs n= 1 F meas n σ F F n model n + ( SS SS ) ( U U ) σ SS aux + 10 σ 10aux U 10, (1) χ = N obs n= 1 F meas n σ F F n model n + ( SS SS ) ( U U ) σ SS aux + 10 σ 10aux U 10, () and three different brightness temperatures Case A Case B Case C No correction applied Constant bias corrected External Calibration applied σss = 0.5 C σws = m/s σswh = m SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 9 / 5

10 he Processing Nominal case: - Klein and Swift model for dielectric constant - Hollinger measurements linear fit for wind effect - Linear Approximation for the atmospheric contribution H V dn up = = surf _ H surf _ V e e.1 = cos( θ ) = τ cos τ cos h cos ( θ ) τ cos ( ) cos ( ) ( θ θ τ θ ) ( ) τ cos ( ) cos ( ) ( θ θ τ θ ) ( ) + + dn 0.03 h dn 1 em 1 em V H e e + + up up + + sky sky e e ( 1 em ) ( 1 em ) V H e e τ cos ( θ ) τ cos ( θ ) τ = sky = 3.7K ( h h emp ) cos ( θ ) surf - Uniform and specularly reflected galactic noise SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 10 / 5

11 he External B Calibration Using the auxiliary data as input to compute a brightness temperature image. he mean of the difference between the measured temperature and the modeled one is considered bias: B = meas B mod ( SSS SS, U, θ ) ( SSS, SS, U,θ ) orig, orig 10orig B aux aux 10aux he new brightness temperature is given by the subtraction between the measured brightness temperature and the bias: corrected B = meas B B SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 11 / 5

12 Selected Results Bias correction effect X/Y -5-5 SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 1 / 5

13 Selected Results Bias correction effect I SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 13 / 5

14 Selected Results Binning effect X/Y -5-5 SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 14 / 5

15 Selected Results Binning effect I SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 15 / 5

16 Nominal configuration - Conclusions Even after averaging measurements, large biases have been detected in the measurements. Retrieve without correcting for this bias resulted in very poor results, the positive offset of the brightness temperature translates in a saturation of the retrieved salinity toward its minimum (0 psu). Results improve when the constant bias provided at level 1 is subtracted to the measurements; the best retrieval is performed when the external brightness temperature calibration is applied. he binning also largely affects the retrieval performance. Increasing the bin width results in an underestimation of the V-pol and overestimation of the H-pol, producing a positive bias in the retrieved SSS when using X/Y. Using I, the error with respect to the modeled I introduced by the bin width is very low and the improvement of the radiometric sensitivity permit better results SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 16 / 5

17 Other forward models Bh Bv = = θ U 55 θ 0. 1 U Bh Bv = = θ 14 θ 51 SWH SWH Hollinger WISE SWH Bh = θ U Bh = θ U θ SWH Bv = θ U Bv =.1 1 θ U θ SWH WISE WS WISE WS + SWH SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 17 / 5

18 Other forward models WISE WS X/Y -5-5 SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 18 / 5

19 Other forward models WISE WS I SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 19 / 5

20 Other forward models WISE SWH X/Y (r) (o) -5-5 SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 0 / 5

21 Other forward models WISE SWH I SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 1 / 5

22 Other forward models WISE WS+SWH X/Y -5-5 SMOS cal/val Workshop - March Lisbon (Portugal) M. alone / 5

23 Other forward models WISE WS+SWH I (r) (o) SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 3 / 5

24 Other forward models - Conclusions WISE WS model results are in agreement with the nominal model. SWH model produces a positive offset in the retrieved SSS with respect to the WS ones. Anyway a gradient between the first and the second part of the plot can be observed, even if shifted up. WS + SWH model give results which are comparable with the WS models ones. It must be considered that SWH is calculated as 4σ(HS), which is valid only for fully developped sea. SWH models are empiricals, coming from fitting data of a very different environment SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 4 / 5

25 Future work More averaging of Brightness temperatures Salinities SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 5 / 5

26 External Salinity Calibration Once the Sea Surface Salinity map is retrieved, to correct for the errors introduced by the forward models inaccuracies ( ε r, sea state dependence ), a Calibration Factor is calculated as in rain radar [*] : CF = SSS inst SSS retr he CF has been calculated taking into account only the points observed more than 40 times (eliminating the noisiest points). he final retrieved SSS is thus given by: SSS = CF corr SSS retr [*]Seo, D.J. and Breidenbach, J.P., Real-time correction of spatially non-uniform bias in radar rainfall data using rain gauge measurements, Journal of Hydrometeorology, vol. 3, pp , 00 SMOS cal/val Workshop - March Lisbon (Portugal) M. alone 6 / 5

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