Estimation of radial sea currents in the northern Adriatic Sea close to the Venice Lagoon inlets #ID2141

Transcription

1 Estimation of radial sea currents in the northern Adriatic Sea close to the Venice Lagoon inlets #ID2141 ASI CSK Workshop Stefano Zecchetto ISAC-CNR, Uos Padova Gianfranco Fornaro IREA-CNR, Napoli 1

2 Estimation of radial sea currents in the northern Adriatic Sea close to the Venice Lagoon inlets #ID2141 Summary 1. Project description 2. Tools 3. Cases selection and geophysical characterization 4. Image processing 5. Results 6. Critical points and future activities 2

3 Project description This project is aimed to assess if and with what accuracy the radial sea surface currents can be evaluated using the CosmoSkymed SAR images. The radial sea surface current will be computed from the map of the Doppler shift obtained from level 0 Stripmap Himage Cosmo-SkyMED images, and compared with the radial sea surface current obtained from a 3-D oceanographic model (SHYFEM) The test area includes part of the northern Adriatic Sea and of the Venice Lagoon (45 10' 45 40' N, 12 00' 13 00' E); the main focus is on the three lagoon inlets because here the sea currents reach the highest values of the Mediterranean Sea. 3

4 Project description Motivations This area has been chosen because the currents at the Venice lagoon inlets reach the maximum velocities of the Mediterranean currents (up to 1.5 m/s) for the availability of in-situ data set (current, tide, wind) for the availability of hydrodynamic model routinely running 4

5 Project description Institutes involved Istituto Scienze dell'atmosfera e del Clima (ISAC), CNR, Uos. Padova Istituto di Scienze Marine (ISMAR), CNR, Uos. Venezia Istituto per il Rilevamento Elettromagnetico dell'ambiente (IREA), CNR, Uos. Napoli Istituzione Centro Previsioni e Segnalazioni Maree (ICPSM), Comune di Venezia, Venezia 5

6 Project Tools Shallow water HYdrodynamic Finite Element Model 3-D version (SHYFEM, 6

7 Project Tools Shallow water HYdrodynamic Finite Element Model 3-D version (SHYFEM) 7

8 Project Tools: Cosmo-SkyMed Stripmap Himage images Area imaged ~50 km x 50 km Grid size: 2.7 m in azimuth, 2.3 m in range Polarization: HH (archived images) Compromise between the area size and the spatial resolution 8

9 Two examples of selected images 1 April GMT Ws=8 m/s Wdir=50 Current: outflow 27 July GMT Ws=6.6 m/s Wdir=118 Current: outflow 9

10 Geophysical characterization: sea current field 1 April GMT Ws=8 m/s Wdir=50 Current: outflow 27 July GMT Ws=6.6 m/s Wdir=118 Current: outflow 10

11 Processing of the selected images Development of the algorithm for DC estimation Analysis of the images characteristics Calibration of DC variations 11

12 Development of the algorithm for DC estimation DC estimation carried out locally after azimuth focusing to mitigate the problems associated with the range migration Raw data Range Compression Azimuth Focusing Madsen DC Estimation 1 f DC = arg [ R ( 1 ) ] 2πT R(n) azimuth self-correlation, T=1/PRF -540 Hz -440 Hz DC estimation on block of 512x512 pixels (~1.4 km x 1.2 km). Frequency Resolution of each block: 6 Hz (~ 10 cm/s)

13 Spurious DC variations: test on a internal area The analysis of the azimuth spectrum has shown variations of the DC with the range. Such variations are not annotated in the data.

14 Calibration of DC variations Calibrated DC f DC ( x,r ) =f DC ( x,r ) a br cr A 1 N a = ai Parameters are N i =1 estimated over 1 N b = bi several (say N) range N i =1 lines and averaged 1 N c = ci N i =1 2

15 Calibration of DC variations Before DC calibration SCS-B Provided by ASI After DC calibration E' stata inoltrata ad ASI la richiesta di accedere a stime di variazione del Centroide Doppler che, oltre al contributo di tipo geometrico, portino in conto anche di effetti di distorsione elettronica del fascio, ovvero di accedere a dati e misure d interesse per la valutazione della distorsione elettronica del fascio.

16 Range starting window sampling On several images it has been experienced the presence of a power decrease at far range, probably due to a non optimal use of the range starting sampling window associated with the range first time. Antenna pattern in range extracted from the image of 25 Jan Jan Uncalibrated SSC slant range geometry

17 Radial sea current maps From SHYFEM From CSK 1 April GMT Ws=8 m/s Wdir=50 Current: outflow 17

18 Radial sea current maps From CSK From SHYFEM 1 April GMT Ws=8 m/s Wdir=50 Current: outflow 18

19 Radial sea current maps From SHYFEM From CSK 27 July GMT Ws=6.6 m/s Wdir=118 Current: outflow 19

20 Critical points the results obtained in this project do not correspond to the extensive work carried out. The main reason is the the presence in the CSK derived Doppler frequency maps of a spurious DC component caused by the non orthogonality between the lookup direction and the satellite speed. This component often hinders the geophysical signal present in the image. access to the estimates of the DC variability due, besides the geometric contribution, to the electronic distortion of the radar beam, is essential spatial and frequency resolution of the CSK Doppler frequency map. We have used block of size 512x512 samples thus leading to a resolution of about 1.4 km in azimuth (approximately SN) and 1.2 km in ground range (approximately WE). This resolution may be not sufficient to resolve features of similar spatial scale, especially at the lagoon inlets where the surface current is higher. Furthermore, the frequency resolution estimated of 6 Hz, leads to a sea surface current resolution of about 10 cm s -1, which appears for the images analysed too coarse to resolve the features of the radial currents, which have been found to have a range of the order of 20 cm s-1 20

21 Possible future activity Once solved the problems of the DC variability due to the geometric contribution and to the electronic distortion of the radar beam Select areas of stronger sea surface currents (outside the Mediterranean Sea) Select CSK Stripmap Himage images at VV polarization 21