Nicole Braun

Sea-Surface Current Features Observed by Doppler Radar

A coherent X-band radar was operated from the shore to measure velocities at the sea surface. The test site at a tidal inlet (Lister Tief) in the German Bight lay protected from swell and long wind waves. A vertical-vertical antenna was used to obtain the backscattered radar signal caused by Bragg scattering. To cover a sector of the circle, the antenna was rotated in 1Dstrok steps. At each step, the data acquisition was performed to obtain a time series of the backscattered radar signal. The maximum range is 1905 m with a range resolution of 7.5 m and an azimuthal resolution of 3.5diam. Maps of the averaged backscattered radar power and radial Doppler velocities are obtained. The radar backscatter maps show the modulation of the radar cross section due to changes in the surface roughness. The radar Doppler velocity maps show the modulations of the surface scatterer speed. The maps show many features, for example, the impact of subsurface sand waves, eddies, current shears, slicks, ship wakes, atmospheric effects, and small-scale current changes behind sheet pile walls. As the radar backscatter is caused by short wind waves, the wind influence on the Doppler signal was calculated for a first attempt to obtain true current motions. The results were compared with in situ current measurements. By subsequently measuring the surfaces current from two radar positions and performing the wind correction, the current vector fields are assessed.

The effect of artificial rain on wave spectra and multi-polarisation X band radar backscatter

We have carried out wind-wave tank measurements using wave-height and wave-slope gauges and a coherent 9.8 GHz (X band) scatterometer, when the water surface was agitated by heavy rain (160 mm h- 1 to 300 mm h -1 ) and by wind (2 ms- 1 to 12 ms -1 ). The upwind-looking scatterometer was operating at co- (VV- and HH-) and cross- (HV-) polarisation at a steep incidence angle of 28°. In the presence of rain, the power spectral density of the wind-wave spectra is enhanced at frequencies above about 5 Hz and it is reduced at lower frequencies. This is the net effect of surface roughness production by the rain-induced splash products and of wave damping by the rain-induced turbulence. We measured isotropic (rain-dominated) wave spectra at low wind speeds and anisotropic (wind-dominated) wave spectra at high wind speeds, with a transition wind speed that increases with rain rate. The radar backscattering at co-polarisation at low wind speeds is mainly caused by rain-induced ring waves, whereas at cross-polarisation, at all wind speeds, other rain-induced splash products, like crowns, stalks, and cavities, are the dominant scatterers. We have found a rain-induced increase of the radar backscatter at co-polarisation at wind speeds of up to 9 ms- 1 and at cross-polarisation at all wind speeds. At cross-polarisation the radar backscatter slightly depends on rain rate. Using our results an analysis of spaceborne synthetic aperture radar (SAR) images of tropical rain cells was performed.

Radar backscattering measurements of artificial rain impinging on a water surface at different wind speeds

At the wind-wave tank of the University of Hamburg the authors carried out laboratory measurements with artificial rain and wind. The radar backscatter from the rain- and wind-roughened water surface was measured with a coherent 9.8 GHz (X-band) scatterometer at different polarizations (VV, HH, and HV) and incidence angles (28/spl deg/, 40/spl deg/, and 55/spl deg/). At VV-polarization the radar Doppler spectra show two Doppler-shifted maxima at all incidence angles. At HH-polarization a third maximum is visible at zero Doppler shift. With increasing incidence angle the two Doppler-shifted maxima are weakened, whereas the third (non Doppler-shifted) maximum increases. The authors explain this as being an effect of the different backscatter mechanisms caused by the different splash products, like Bragg scattering at ring waves and scattering at the static splash products like stalks. They measured that the relative radar cross sections at HV-polarization increase with increasing high rain rate (50 mm/h and 100 mm/h), which they attribute to enhanced edge and multiple scattering.

Multi‐frequency scatterometer measurements on water surfaces agitated by artificial and natural rain

Images of rain events over the ocean acquired by a multi‐frequency/multi‐polarization Synthetic Aperture Radar (SAR) show different radar contrasts at different frequencies and polarizations. In order to better understand these effects, field and laboratory experiments were performed at different rain rates and wind speeds with scatterometers working at different radar frequencies, polarizations, and incidence angles. Our results show that the dominant scattering mechanism on a rain‐roughened water surface, observed at VV polarization, at all incidence angles is Bragg scattering from ring waves. At HH polarization the radar backscatter is caused by both ring waves and non‐propagating splash products, with the dominating effect depending on incidence angle. The reduction and enhancement of the surface roughness by ring waves and sub‐surface phenomena, respectively, result in a transition wavenumber between reduction of the radar backscattering and its enhancement of about 100 rad m−1. We assume that this transition wavenumber depends on the drop‐size distribution of the rain. Taking into consideration the different dependencies of the radar backscatter at different frequencies and polarizations on rain rate, we suggest a method to estimate rain rates by calculating the ratio of the radar cross‐sections at L band, VV polarization and at C band, HV polarization. Provided an availability of SAR data at the respective frequency–polarization combinations, this method allows for investigating the nature of small‐scale (convective) rain events over the ocean.

Image sequence analysis of water surface waves in a hydraulic wind wave tank

The spatio-temporal distribution and evolution of wave fields is measured by the acquisition of optical video image sequences. An experiment at the wind wave flume of the University of Hamburg was performed to measure small-scale processes at the water surface. From the image sequences, image spectra are calculated by a 3D Fast Fourier Transformation. This spectral description is limited to stationary and homogeneous wave fields and therefore other techniques which are designed for local scales are applied.

Sea-surface current measurements with an X band radar

A measurement system is under development, which uses a coherent X band radar operating from land based stations. By measuring the radar backscattering over several tidal cycles, tidal current patterns are observed in a test area which is 500 X 700 m in size. The radar is operated from two stations to obtain radial Doppler-velocity components which are corrected for Bragg-wave velocities and wind-drift and combined to current vectors. While the operation of radar from space and current gauges from ships is elaborate, the herein presented measuring system offers a low cost facility with high mobility. It has the potential for current monitoring as well as for case studies of changes in current dynamics which have an impact on erosion and sand transport, coastline changes and safety of navigation.

Multi-frequency/multi-polarization measurements of radar backscatter under different rain and wind conditions

The authors have performed radar backscattering measurements with a scatterometer in a wind-wave tank under different wind and rain conditions. The aim of these experiments was to gain further insight into those mechanisms which are responsible for the combined rain- and wind-induced modifications of the water-surface roughness and, thus, of the radar backscatter. The results from their measurements imply that the ratios of the co-polarization and cross-polarization backscatter may help to estimate rain rates, and therefore enable the authors to interpret radar images of rain cells more quantitatively.

Laboratory measurements of artificial rain impinging on a water surface

In the wind-wave tank of the University of Hamburg laboratory measurements have been carried out in the presence of artificial rain and wind. By using a resistance type wire gauge the surface wave elevation, and by using a laser slope gauge the surface slope was measured. The radar backscattering from the water surface was measured with a coherent 9.8 GHz (X band) scatterometer. First results of this study show that the wave spectra are affected differently by the rain in different spectral domains. The wave spectra are enhanced in the frequency range of about 5 Hz to 100 Hz, while they are reduced at frequencies smaller than about 5 Hz. By comparing wave slope spectra measured in downwind and crosswind direction we found that the rain affects the orientation of the wave spectra: the transition region from an isotropic to an anisotropic wave spectrum increases with wind speed. By operating the scatterometer at different polarizations (VV, HH, and HV) we measured a strong increase of the relative backscattered radar power (/spl sigma//sub rel/) in the presence of rain at wind speeds below approximately 8 m/s at all polarizations. At wind speeds above 8 m/s, only /spl sigma/(rel) at cross-polarization increases in the presence of rain. The strongest increase, however, was measured at cross-polarization, which is in accordance with observations which we obtained from the analysis of multifrequency-SAR data.

Laboratory measurements of artificial rain impinging on a wind-roughened water surface

Laboratory measurements in the wind-wave tank of the University of Hamburg with artificial rain have been carried out by using a coherent X-band scatterometer, a resistance-type wire gauge and a two-dimensional laser slope gauge. The authors used raindrops with a diameter of 2.9 mm, rain rates between 160 mm/h and 300 mm/h, and wind speeds between 2 m/s and 12 m/s. They show that, by comparing the along-wind and the crosswind slope spectra, a wind dependent orientation of the entire wave field (consisting of wind- and rain-generated waves) can be found. From the X-band Doppler spectra measured at low wind speeds they infer that the backscattered radar power (BRP) at co-polarization is caused by the rain-generated ring waves, whereas at cross-polarization it is associated with the cavities and stalks generated by the impinging rain drops. Moreover, they measured a strong increase of the relative BRP, /spl sigma//sub rel/, at co-polarizations (VV, HH) in the presence of rain at wind speeds below approximately 8 m/s. However, at cross-polarization (HV) /spl sigma//sub rel/ was enhanced more strongly in the presence of rain than at co-polarization for all wind speeds (i.e. up to 12 m/s) and rain rates.

Optical image sequence analysis and interpretation of stationary and instationary wave fields

The spatio-temporal distribution and evolution of wave fields is measured by the acquisition of optical video image sequences. As one possible application of the optical image sequence acquisition system an experiment at the wind wave flume of the University of Hamburg has been performed to measure small scale processes at the water surface. From the image sequences image spectra are calculated by a three dimensional fast Fourier transformation (3D FFT). The spectral description by the 3D FFT is limited to stationary wave fields and therefore spectrograms of the wave field are presented which allow to describe instationary/inhomogeneous processes.