The Action of Rain Impinging on a Water Surface

Investigations on Radar Backscattering and Wave Damping

To describe the earth’s climate system, the ocean atmosphere interaction has to be known. One interaction is given by rain events where fluxes of heat and water as well as gas and aerosol transfer occur. Thus rain rates have to be measured over the oceans and the interaction of rain with a water surface agitated by wind has to be understood. By using the Wind-Wave-Tank of the University of Hamburg, radar backscatter measurements have been performed. The results of these measurements help to interpret Synthetic Aperture Radar (SAR) images of the ocean surface. With the help of SAR sensors, rain can be detected over the ocean and through the development f future sensors, rain rates can be obtained. The counteracting effect of enhancement and reduction of surface roughness were investigated by measuring the wave amplitude and wave slope inside the rain-area to compare this effect with the radar backscatter intensity on SAR images, which varies strongly inside convective rain cells. The intensity and lifetime of the rain induced subsurface turbulence was measured using video cameras and an Acoustic Doppler Velocimeter. While the radar backscatter and subsurface turbulence was investigated intensively in the last years, it would be a challenging task to use the wind and rain setup of the Wind-Wave-Tank to investigate the combined influence of rain and wind on the air sea gas-transfer.

References:

Braun, N., M. Gade, and P.A. Lange, 2002: The effect of artificial rain on wave spectra and multi-polarisation X band radar backscatter, Int. J. Remote Sens., 23, 4305-4322.

Impinging Rain Drops

The University's wind-wave tank is equipped with a rain generator that is capable of producing strong rain of up to 300 mm/h. Laboratory measurements have been carried out to study the wave field under the simultaneous action of wind and rain.

This image was taken when the (reference) wind speed was 5 m/s (wind blowing from the right), and when the rain rate was 300 mm/h (with rain drops of 2.9 mm in diameter).

Sequence of images of a single rain drop impinging into the still water surface. After the drop impact, a crown evolves (left image; the cavity below the water surface cannot be seen). After the crown has collapsed, a stalk and a secondary drop (middle image) are generated. Finally, the radially spreading ring waves (right image; the vertical line is caused by the next falling drop) are the only products of the drop impact that remain on the water surface.

Measurements of the Surface Roughness

Inside the area agitated by rain, a resistance type wire gauge and laser slope gauge were mounted at 14.5 m fetch. The water surface elevation and slope were measured respectively. The figure on the left shows the spectral power densities of the wave amplitudes. Where no wind is acting, the drop impact causes maxima at about 4 Hz and higher harmonics, panel a), rain rate: 160 mm/h. At a wind speed of 4 m/s the maximum amplitude of the wind wave spectra (solid line) is reduced if rain is agitating the water surface (dashed line). This reduction will be smaller at higher wind speeds, see panel c) at 8 m/s and panel d) at 12 m/s. The enhancement of the surface roughness at higher frequencies can be observed at all measured wind speeds. The transition frequency between reduction and enhancement of the sea surface roughness is about 5 Hz (measured from these spectra of encounter). These measurements were performed using only one drop size . Future investigations should use a mixture of raindrop sizes to simulate a natural drop-size distribution.

Measurements of the Radar Backscatter

Using the X band scatterometer which is mounted at the roof of the wind wave tank, time-series of the relative radar backscatter were measured. The scatterometer allows for changing the incidence angle and the polarizations of the transmitted and received radar waves. From the time series, the radar Doppler-spectra were calculated and analyzed. By interpreting the radar Doppler spectra, different splash features were found to cause different maxima at different radar polarizations and incidence angles. One interesting result obtained is the rain rate dependence of the radar backscatter at cross polarization. This can be used in further SAR missions to estimate rain rates over the oceans.

Investigations of the Subsurface Turbulence

Sequence of images of a single dyed rain drop impinging into the still water surface. Apart from the splash products at the water surface (as described above), sub-surface turbulence is generated by the impinging rain drop. Note the ring vortex in the last image, that is generated by the drop and that propagates downward.

To explain the wave damping mechanism of rain, the subsurface turbulence was investigated by using an Acoustic Doppler Velocimeter (ADV, see small image). The velocimeter was mounted inside the area agitated by rain. Using different wind velocities and sensor heights, profiles of the turbulent velocities were obtained. An example is shown on the right for a rain rate of 40 mm/h and for a wind speed of 4 m/s. The green circles show that the rain-induced turbulence decreases with depth and is small compared to the orbital velocities of wind waves (blue stars). In both cases (upper panel: turbulence component in along wind direction, lower panel: turbulence component in vertical direction) the orbital motion of the waves is reduced by wind (red crosses). In the along wind direction at a water depth below 7 cm, the velocity fluctuation due to wave motion is enhanced by the action of rain. This results from a downward mixing of velocity fluctuation by the rain induced turbulence. By abruptly stopping the rain, the lifetime of rain induced turbulence was measured to be about 60 sec. An interesting task for future investigation would be to measure the average velocity profile below the water surface in along wind direction and compare these results with X band measurements of the surface drift in order to gain more insight into the rain-induced surface drift enhancement.