Research Topics
Image Waves 01

Waves at the beach - a common known result of air-sea-interaction. -image: HZG-

Momentum transfer at the air-sea interface

Near surface processes in the ocean must be well understood to better comprehend and estimate the momentum exchange between atmosphere and ocean.
In this respect, the role of surface waves, their influence on the near surface airflow dynamics as well as wave current interaction are of importance. However, our physical understanding in particular, with respect to their coupling and at higher wind speeds remains incomplete.
To better quantify and understand their interactions, several measurement techniques for high-resolution remote sensing of near-surface wind, current, and wave fields were developed. The methods are based on radars, lasers, and video cameras, covering scales between millimeters and hundreds of kilometers:

Accurate measurements of wind fields at high wind speeds are important for estimating momentum transfer under high-energy conditions when observations are sparse. A large spatial coverage (>100 km) with a high resolution (up to 1 km) of the sea surface wind fields can only be obtained by spaceborne synthetic aperture radars (SAR).
These measurements, however, were limited to moderate winds and resulted in huge errors under extreme conditions such as tropical cyclones.
To overcome these limitations, new algorithms were developed, tested, and validated. They were applied to spaceborne SARs operating at C-band with either co-polarization or cross-polarization.
Comparison with QuikSCAT winds and Stepped Frequency Microwave Radiometer (SFMR) during reconnaissance flights showed that SAR cross-polarization data are, with a root mean square error of 3.8 ms-1, significantly better suited for SAR wind retrieval, especially at wind speeds above 20 ms-1. This result led to the inclusion of cross polarization in future satellite scatterometer missions.

See also the site of our Radar Hydrography group

Horstmann, J., Falchetti, S., Wackerman, C., Maresca, S.,Caruso, M.J.,Graber, H.C.: Tropical Cyclone Winds Retrieved From C-Band Cross-Polarized Synthetic Aperture Radar, IEEE Transactions on Geoscience and Remote Sensing, 53, no. 5, c2887-2898, 2015a, doi:10.1109/TGRS.2014.2366433

Horstmann, J., Nieto Borge, J.C., Seemann, J., Carrasco, R., Lund, B., Wind, Wave and Current retrieval utilizing X-Band Marine Radars, Chapter 16 in Coastal Ocean Observing Systems, 281-304, 2015b, doi:10.1016/B978-0-12-802022-7.00016-X

van Zadelhoff, G.-J., Stoffelen, A., Vachon, P. W., Wolfe, J., Horstmann, J., and Belmonte Rivas, M.: Retrieving hurricane wind speeds using cross-polarization C-band measurements, Atmos. Meas. Tech., 7, 437-449, 2014, doi:10.5194/amt-7-437-2014

Image Marine Radar

Marine Radar onboard HZG's research vessel Ludwig Prandtl -image: HZG-

In order to estimate the spatial variability of surface wave fields, to observe wave-current interaction, or wave energy dissipation, it is necessary to observe waves with sufficient spatial and temporal resolution over larger areas.
For this purpose, radar remote sensing techniques such as the coherent-on-receive marine radar with a range of up to 3 km were developed by this division Operational Systems. Utilizing the radar Doppler speed measurements, a robust method for retrieving significant wave heights was developed and validated. For the first time, it is now possible to retrieve significant wave heights from marine radars without any calibration and with a high accuracy of 0.21 m.
HZG’s coherent-on-receive marine radar system is one of the most reliable systems available worldwide, and have therefore been used by several research partners in numerous campaigns in Germany, France, Norway, Taiwan, and the USA. They have been proven to measure surface wind, waves, currents, and bathymetry from moving vessels, fixed platforms, and coastal stations. The HZG marine radar systems can now also be used
(i) as a Surface Feature Monitoring System (SuFMoS) for real-time observations of fronts, internal waves, wind gusts and sea ice,
(ii) for retrieving bathymetry in shallow water,
(iii) for retrieving surface winds and gusts, and
(iv) current measurements.
Ongoing developments at HZG are focused on the retrieval of individual waves.

See also the site of our Radar Hydrography group

Carrasco, R., Streßer, M., and Horstmann, J.: A simple method for retrieving significant wave height from Dopplerized X-band radar, Ocean Sci., 13, 95-103, 2017a, doi:10.5194/os-13-95-2017

Carrasco, R., Horstmann, J., Seemann, J.,Significant Wave Height Measured by Coherent X-Band Radar, IEEE Transactions on Geoscience and Remote Sensing, 55, no. 9, 5355-5365, 2017b, doi:10.1109/TGRS.2017.2706067

Horstmann, J., Nieto Borge, J.C., Seemann, J., Carrasco, R., Lund, B., Wind, Wave and Current retrieval utilizing X-Band Marine Radars, Chapter 16 in Coastal Ocean Observing Systems, 281-304, 2015b, doi:10.1016/B978-0-12-802022-7.00016-X

Huang, W.,Carrasco, R., Shen, C., Gill, E.W., Horstmann, J., Surface Current Measurements Using X-Band Marine Radar With Vertical Polarization, IEEE Transactions on Geoscience and Remote Sensing, 54, no. 5, 2988-2997, 2016. doi:10.1109/TGRS.2015.2509781

Lund, B., Haus, B.K., Horstmann, J., Graber, H.C., Carrasco, R., e, Near-Surface Current Mapping by Shipboard Marine X-nd Radar: A Validation, Journal of Atmospheric and Oceanic Technology, submitted 2017.

Støle-Hentschel, S., Seemann, J., Nieto Borge, J.C., Trulsen, K., Analyzing Coherent Spatio-Temporal Radar Measurements: Transfer Function and Sea Surface Reconstruction, J. Geophys. Res., submitted 2017

Vicen-Bueno, R., Horstmann, J., Terril, E., de Paolo, T., Dannenberg, J., Real-Time Ocean Wind Vector Retrieval from Marine Radar Image Sequences Acquired at Grazing Angle, J. Atmos. Oceanic Technol., 30, 127–139, 2013, doi:10.1175/JTECH-D-12-00027.1

Shen, C, Huang, W., Gill, E.W., Carrasco, R., Horstmann, J., An Algorithm for Surface Current Retrieval from X-band Marine Radar Images, Remote Sens., 7, 7753-7767, 2015 doi:10.3390/rs70607753


Scheme of the PIV / LIV experiment-image:HZG-

To investigate the role of the wind stress on surface waves (momentum flux) over the ocean, a technique was developed to measure the detailed centimeter-scale airflow in the vicinity of the ocean surface.
A combined particle image velocimetry (PIV) and laser-induced fluorescence (LIF) technique was developed at the University of Delaware and used in the laboratory taking into consideration seventeen different wind wave conditions.
Dimensional airflow velocity fields were obtained as low as 100 µm above the air-water interface. The data were further analyzed at HZG showing that the mean velocity profile follows the law of the wall when the wind stress is too weak to generate surface waves. With waves present, turbulent structures are directly observed in the airflow, whereby low-horizontal-velocity air is ejected away from the surface and high-velocity fluid is swept downward. Airflow separation is observed above young wind waves, and the resulting spanwise vorticity layers detached from the surface produce intense wave-coherent turbulence.
On average, the airflow over young waves is sheltered downwind of wave crests. The aforementioned PIV technique has been adjusted at HZG to make first measurements in the open ocean, which for the first time, were performed aboard RV Flip in October 2017.

See also the site of our Small-scale Physics and Turbulence group

Buckley, M.P. and F. Veron, Structure of the Airflow above Surface Waves. J. Phys. Oceanogr., 46, 1377–1397, 2016, doi:10.1175/JPO-D-15-0135.1

Buckley, M.P. & Veron, Airflow Measurements at a Wavy Air-Water Interface using PIV and LIF , F. Exp Fluids, 58, 161, 2017, doi:10.1007/s00348-017-2439-2

Image Surface Current Field in the River Elbe

Surface current field in the Elbe River at Lauenburg retrieved with a quadcopter drone. -image:HZG-

Small-scale physical processes in the ocean are often characterized by sharp fronts that have strong horizontal shear.
In order to sufficiently resolve these processes in space (a couple of meters) and time (minutes), the techniques developed for marine radar image sequences were adjusted to the needs of video sequences of water surface waves. Video sequences were acquired in the range of visible light with a small, nadir-looking video camera attached to an off-the-shelf quadcopter drone.
The actively controlled gimbal stabilized the video camera. The video data are corrected for lens distortion and are geocoded to a rectilinear coordinate system at water level. The resulting video data allow for measuring wave direction, wave length, and phase velocity.
These properties facilitate estimates of surface current vectors resulting from the difference of the observed phase velocity and linear dispersion relation of surface gravity waves.

See also the site of our Radar Hydrography group

Streßer, M., Carrasco, R., Horstmann, J., Video-Based Estimation of Surface Currents Using a Low-Cost Quadcopter, IEEE Geoscience and Remote Sensing Letters, 14, no. 99, 1-5, 2017, doi:10.1109/LGRS.2017.2749120

image wind_wave_web_fig new interpretation air-sea-interaction

Plot showing the growth rate for the wind-wave instability for different wavenumbers (inverse wavelength). The dashed lines are a set of new approximations that quickly converge to the exact solution (solid line). -image:HZG-

A key component of air-sea momentum transfer is in the growth of surface gravity waves from the airflow above. A new theoretical advance in understanding this process has been developed and a new physical interpretation of the wind-wave interaction instability is described.

This interpretation breaks the instability down into its simplest form, which consists of a two-way interaction between the surface wave and the airflow critical layer. It also links the physics of the wind-wave instability to those of stratified shear layers, such as the well-known Kelvin-Helmholtz instability.
This theoretical basis is also used for constructing an efficient solution method for computations of the most unstable modes likely to arise in different wind profiles.

See also the site of our Small-scale Physics and Turbulence group

Carpenter, J.R., A. Guha & E. Heifetz (2017): A physical interpretation of the wind-wave instability as interacting waves. Journal of Physical Oceanography, 47, 1441-1455, doi:10.1175/JPO-D-16-0206.1