reporter:葉宗穎 YEH-ZONG-ZING Supervisor:鄭志文教授 Prof. Zhi-Wen Zheng Preliminary validation and error analysis on surface currents provided by CODAR system surrounding Taiwan 台灣周遭岸基雷達資料之初步驗證與誤差來源分析 reporter:葉宗穎 YEH-ZONG-ZING Supervisor:鄭志文教授 Prof. Zhi-Wen Zheng

Outline Background Main Goal How to do Final Goal Data process procedure Result Future work Conclusion Reference

CODAR is abbreviation of Background CODAR is abbreviation of => Coastal Ocean Dynamics Application Radar

Background What is CODAR? The frequency of radiowave ranges from 3MHZ to 30MHZ, and corresponding wavelength range from 100 meter to 10 meter. Vertical polarization polarity high frequency radiowave can propagate along ocean surface to distant place, and isn't influenced by mist. Therefore, high frequency radar can be used in any kind of weather.

Background Photos:

Background Principle of the operation of CODAR: =>The measurement of radar is using the time difference of electromagnetic waves launching to receiving to get the distance between radar and wave equals to (speed of light multiplied by time difference back and forth) divided by two.

Radar wave departures from emissive source after getting in touch with ocean wave, it will scatter toward counter direction. If ocean wavelength is half radar wavelength, at the moment, the scattering at counter direction will cause constructive superposition and be a strong echo signal, this is called Bragg's scattering.

The backward scattering signal of high frequency will cause Doppler Shift with emissive radar wave (like the following picture), including two obvious first order energy peaks and the other secondary peaks. Through two first order peaks corresponding Doppler Shift , we can speculate the velocity at radial direction.

The advantage of using CODAR data (why do we use CODAR) Background The advantage of using CODAR data (why do we use CODAR) =>Constructing CODAR around Taiwan, it is more convenient and efficient to get and deal with the information about ocean for us.

Background (CODAR surrounding Taiwan) Taiwan Ocean Research Institution (TORI) is responsible for the construction and preservation of CODAR. They also offers information of surface currents around Taiwan, in particular, the Kuroshio current east of Taiwan.

Background (CODAR surrounding Taiwan) However, so far, most of the attention was paid for the construction of CODAR system surrounding Taiwan, the analysis and validation about CODAR that had not been developed completely.

MAIN Goal To give a more comprehensive understanding of the quality of CODAR data surrounding Taiwan through systematic validation and error analysis.

How to do Because in-situ measurement are limited and sparse ,to give a comprehensive comparison, in this study, we use different data source to attempt to validate CODAR data in different view. => Maximum Cross Correlation (MCC) (retrieved currents based on satellite based data)

★Maximum Cross Correlation (MCC): Patterns are identified using cross correlations depicted in the first and second template boxes Position of the template in the first image is fixed, correlations are computed as the template in the second image is moved to different positions within the dashed line The template position with the greatest cross correlation indicates the most likely displacement of the feature

FINAL Goal Using the surface current velocity measured by MCC to compare with the velocity measured by CODAR, confirm whether CODAR around Taiwan works properly or not. Let CODAR be more common, and apply in more aspects, like ecology environment, disaster helping, etc.

Data process procedures First, using the information measured by CODAR at the east of Taiwan through MATLAB to draw distribution and direction about current around Taiwan. Second, download the satellite data from Ocean Color Browse based on sea surface temperature (SST) at east of Taiwan. Then, use seadas check and correct data. Last, using matlab to write MCC script. And then, compare with each other. By the way, we have better chose the data at the same or the latest time. It can improve the accuracy.

Results (MCC currents) SST image1 and SST image2 => 2013/6/27/17:55------------2013/6/28/04:50

Data processed through MCC

Results (MCC currents) SST image1 and SST image2 => 2013/7/10/5:15--------------2013/7/10/17:25

Data processed through MCC

Results (MCC currents) SST image1 and SST image2 => 2013/9/6/18:00-----------------2013/9/7/4:55

Data processed through MCC

Results (MCC currents) SST image1 and SST image2 => 2013/9/7/4:55-----------------2013/9/7/17:05

Data processed through MCC

MCC V.S CODAR 2013/6/27/17:552013/6/28/04:50 2013/6/27/23:00

MCC V.S CODAR 2013/7/10/5:152013/7/10/17:25 2013/7/10/11:00

MCC V.S CODAR 2013/9/6/18:002013/9/7/4:55 2013/9/6/11:00

MCC V.S CODAR 2013/9/7/4:552013/9/7/17:05 2013/9/7/10:00

Conclusion Because of insufficient time, I only validate and analysis roughly. Thus, when you guys see my figure of result, you will think why these figure are so different. One of reasons is the difference of data collecting periods, CODAR will measure once per hour. In contrast, MCC will measure once almost per 12 hours. (Why MCC will be measured for a long time, because of the repeat cycle of satellite.) During this process, we find CODAR system is expansibility in the future. Relative to satellite data, it won’t be influenced by clouds, so CODAR can provide continued and systematic observation.

Future work Develop some methods to decrease inaccuracies. Whether there are other methods that make data be more accuracy, less error.

Reference Ocean Surface Currents Mapped by Radar----D. E. Barrick, M. W. Evans, B. L. Weber Extracting Multiyear Surface Currents from Sequential Thermal Imagery Using the Maximum Cross-Correlation Technique-------MELISSA M. BOWEN WILLIAM J. EMERY JOHN L. WILKIN PAUL C. TILDESLEY AND IAN J. BARTON REBECCA KNEWTSON HF RADIO MEASUREMENTS OF OCEAN CURRENTS -----ROBERT H. STEWART and JOSEPH W. JOY Mapping mesoscale currents by optimal interpolation of satellite radiometer and altimeter data------John L. Wilkin
Melissa M. Bowen
William J. Emery Mapping Mesoscale Surface Currents by Optimal Interpolation of Satellite Radiometer and Altimeter Data------John Wilkin (1), Melissa Bowen (2), William Emery (2) Ocean Color Browse : http://oceancolor.gsfc.nasa.gov/cgi/browse.pl?per=DAY&day=15979&sub=level3&prm=CHL&set=10&ndx=0&mon=15949&sen=am&rad=0&frc=0&rgn=Taiwan&cn=26.17&cs=21.83&cw=119.43&ce=123.41&dnm=D