SOMaR-4 Workshop Dependencies of the Doppler Velocity from X-Band Backscatter at Grazing Incidence 9-11th of May, 2017 / Lüneburg Michael Streßer Ruben Carrasco, Marius Cysewski, Jörg Seemann and Jochen Horstmann
Doppler Velocity Dependencies Outline 1.Part: The Past Radar Doppler Current Profiler Empirical Doppler Velocity Corrections 2.Part: The Future Wind drift Wave breaking Additional Doppler shift at extremely grazing incidence 2
Radar System and Doppler Processing X-band coherent-on-recieve 12 kw 8 foot antenna VV-polarization 7.5 m range resolution (50ns pulse duration) 0.8 azimuth resolution 1000 Hz / 2000 Hz PRF vv DD = cc ΔΦ f S 4 ππ ff RR f S : Pulse repetition frequency f R : Radar carrier frequency ΔΦ: Doppler phase shift 0.512 s ensemble averaging 0.97 0.64 CC ii = AA jj+1 iφ jj+1 AA jj iφ jj cccccccc = CC ii CC ii 3
Doppler Velocity Dependencies u Doppler = u wind + u cur + u orb + u break + u graz + u rem wind current wave orbital velocity wave breaking grazing angle remaining influences 4
Doppler Velocity Dependencies u Doppler = u wind + u cur + u orb + u break + u graz + u rem 5
The Radar-Doppler-Current-Profiler (RDCP) u Doppler = u wind + u cur + u orb + u break + u graz + u rem Ant.1 Ant.2 Low winds and sea sates Compensation of antenna movement Compensation of additional Doppler dependencies In-Situ empirical determination of the wind-spider in an homogenous area 6
The Radar-Doppler-Current-Profiler (RDCP) u Doppler = u wind + u cur + u orb + u break + u graz + u rem Antenna 12 Low winds and sea sates Compensation of antenna movement Compensation of additional Doppler dependencies 7
The Radar-Doppler-Current-Profiler (RDCP) u Doppler = u wind + u cur + u orb + u break + u graz + u rem Low winds and sea sates Compensation of antenna movement Compensation of additional Doppler dependencies 8
The Radar-Doppler-Current-Profiler (RDCP) u Doppler = u wind + u cur + u orb + u break + u graz + u rem Low winds and sea sates Compensation of antenna movement Compensation of additional Doppler dependencies 9
Long-term Measurements at FINO3 Acquisition plan per hour: polar images 10 min (260 revolutions) 330 10 min 30 3 min 75 3 min 270 3 min polar images 10 min (260 revolutions) 254.4 10 min 000.0 3 min 300.0 3 min 190.0 3 min 4 7 5 6 1 2 3 8 10
Empirical Model for Doppler Velocity Dependencies u Doppler (U 30,U x,u y,h s ) = u wind + u cur + u orb + u break + u graz + u rem Hs 2 m Hs 4 m 11
Empirical Doppler Velocity Corrections u Doppler (U 10,U x,u y,h s ) = u wind + u cur + u orb + u break + u graz + u rem 12
Empirical Doppler Velocity Corrections u Doppler (U 10,U x,u y,h s ) = u wind + u cur + u orb + u break + u graz + u rem 13
Empirical Doppler Velocity Corrections: Current Retrieval u Doppler (U 10,U x,u y,h s ) = u wind + u cur + u orb + u break + u graz + u rem Current component in radar view direction determined by ADCP at 8m depth cor rms bias 0.73 0.095 m/s -0.003 m/s vs. Doppler radar based current 14
Doppler Velocity Dependencies The Past Radar Doppler Current Profiler Empirical Doppler Velocity Corrections The Future Wind drift Wave breaking Additional Doppler shift at extremely grazing incidence
Wind Shear u Doppler = u wind + u cur + u orb + u break + u graz + u rem Generally accepted: Wind shear velocity is approx. 3-4 % of the wind speed (e.g. Wright and Keller 1971, Shemdin 1972) Wind stress depends on wave shape and local phase. Is there a need to consider a non-uniform wind drift? How can we parametrize this? 16
Wave Breaking u Doppler = u wind + u cur + u orb + u break + u graz + u rem Wave breaking detection The polarization ratio HH/VV is not always available. Alternative algorithms for a breaker detection? uu oooooo /cc pp is not explicitly determinable for irregular waves Spectral bandwidth at LGA 17
Additional Effects at Extremely Grazing Incidence u Doppler = u wind + u cur + u orb + u break + u graz + u rem Macro shadowing (resolved by the radar) Micro shadowing (smaller than the radar range resolution) Miret et al. 2014: the blow-up of the mean Doppler shift at grazing angle is associated to an electromagnetic sharp edge effect on the large surface crests rather than geometrical shadowing of the troughs. Is there an additional effect of sub-grid shadowing? 18
Discussion u Doppler = u wind + u cur + u orb + u break + u graz + u rem wind Wave coherent wind drift? current wave orbital velocity Detection methods? wave breaking grazing angle remaining Sub-grid influences shadowing? 19
References Wright, J. W., & Keller, W. C. (1971). Doppler spectra in microwave scattering from wind waves. Phys. Fl., 14(3), 466. https://doi.org/10.1063/1.1693458 Shemdin, O. H. O. H. (1972). Wind-Generated Current and Phase Speed of Wind Waves. Journal of Physical Oceanography. https://doi.org/10.1175/1520-0485(1972)002<0411:wgcaps>2.0.co;2 Miret, D., Soriano, G., Nouguier, F., Forget, P., Saillard, M., & Guerin, C. A. (2014). Sea surface microwave scattering at extreme grazing angle: Numerical investigation of the doppler shift. IEEE Transactions on Geoscience and Remote Sensing, 52(11), 7120 7129. https://doi.org/10.1109/tgrs.2014.2307893 20