Remko Scharroo

One- and Two-Dimensional Wind Speed Models for Ka-Band Altimetry

abstractSARAL—the Satellite with ARgos and ALtiKa—is the first satellite radar altimetry mission to fly a Ka-band instrument (AltiKa). Ocean backscatter measurements in the Ka band suffer larger signal attenuation due to water vapor and atmospheric liquid water than those from Ku-band altimeters. An attenuation algorithm is provided, based on radar propagation theory, which is a function of atmospheric pressure, temperature, water vapor, and liquid water content. Because of the nature of the air–sea interactions between wind and surface gravity waves, the shorter wavelength Ka-band backscatter exhibits a different relationship with wind speed than at Ku band, particularly at moderate to high wind speeds. This paper presents a new one-dimensional wind speed model, as a function of backscatter only, and a two-dimensional model, as a function of backscatter and significant wave height, tuned to AltiKa’s backscatter measurements. The performance of these new Ka-band altimeter wind speed models is assessed thr...

Cyclone Xaver seen by geodetic observations

Cyclone Xaver pounded the North Sea on 5-6 December 2013 and reached its maximum in the German Bight the second day. Combined geodetic measurements from the SARAL/AltiKa satellite and from a local geodetic network detect cross-shelf and alongshore variations and loading vertical deformation. The cross-shelf root mean square differences between observations and predictions are 30 cm for surge height, 2 m for significant wave height and 4 m/s for wind speed, with significant biases. The different wind forcing mainly causes the predicted heights differences. The smallest standard deviation difference between observed and predicted vertical displacements is from HyFlux2 forced by DWD wind, with a 52% reduction in the standard deviation by subtracting prediction from observations. The GPS network monitors the anticlockwise surge path with maximum land subsidence of 3-5 cm. The tide gauge network monitors both the anticlockwise path of the external wave and the surge associated with strong north-westerly winds.

Ensuring that the Sentinel-3A altimeter provides climate-quality data

Sentinel-3A, launched in February 2016, is part of ESAs long-term commitment to climate monitoring from space. Its suite of instruments for measuring surface topography includes a Microwave Radiometer (MWR) and SRAL, the first delay-Doppler instrument to provide global coverage. SRAL promises fine spatial resolution and reduced noise levels that should together lead to improved performance over all Earth surfaces. The Sentinel-3 Mission Performance Centre (S3MPC) has been developing the methodology to evaluate the accuracy of retrievals, monitor any changes and develop solutions to known problems. The S3MPC monitors internal temperatures, path delays and the shape of the generated pulses to assess the instruments health. The MWR records over known reference surfaces are compared with those from other spaceborne instruments. Over the ocean the SRALs return pulses are analysed to give range to the sea surface, wave height and signal strength (which can be interpreted as wind speed). The metocean data are regularly contrasted with records from in situ measurements and the output from meteorological models, which rapidly highlights the effects of any changes in processing. Range information is used to give surface elevation, which is assessed in three ways. First, flights over a dedicated radar transponder provide an estimate of path delay to within ~10 mm (r.m.s.). Second, measurements are compared to GPS-levelled surfaces near Corsica and over Lake Issyk-kul. Third, there are consistency checks between ascending and descending passes and with other missions. Further waveform analysis techniques are being developed to improve the retrieval of information over sea-ice, land-ice and inland waters.