Scatterometers provide high resolution all-weather wind vector fields over the sea. Wind speed and wind direction are provided with high quality under all weather conditions.
At present there are three types of scatterometers operational. The all-weather ESA ERS scatterometer observations have proven its significant impact for the forecasting of fast developing and severe weather. Moreover, scatterometer wind fields are provided on the ocean eddy scale and are important for driving the ocean’s motion. As a consequence, oceanographic applications have been initiated using winds from the SeaWinds scatterometer on the QuikScat satellite. Together, ASCAT, ERS-2 and SeaWinds provide good coverage over the oceans twice a day. They are used routinely in marine and weather forecasting. Below, the KNMI contribution to the EUMETSAT Satellite Application Facilities (SAFs), the ASCAT calibration, high spatial resolution wind products and their application are elaborated. In the coming years KNMI continues developing high resolution operational products for coastal areas.
Scatterometers uniquely define the mesoscale wind vector field at the sea surface by measuring the radar backscatter signal from wind-generated cm-sized, so-called, gravity-capillary sea waves. Because of the wavelength (5 cm), the signal of ASCAT is not affected by rain, and so it is an ‘all-weather’ system. The all-weather capability of the ASCAT scatterometer provides unique wind field products of the most intense and often cloud-covered wind phenomena, such as polar front disturbances and tropical cyclones (for example, see Figure 1). As such, it has been demonstrated that scatterometer winds are extremely useful in the prediction of extra-tropical cyclones and tropical cyclones1). Moreover, the high-resolution near-surface winds as provided by scatterometers are very relevant because these winds drive the ocean water circulation, which in turn plays a major role in the climate system and in marine life and its exploitation (e.g., fishery).
The methodology developed for the successful application and assimilation of ERS winds in Numerical Weather Prediction (NWP) (see Figure 2) has been generalised at KNMI to include the new scatterometer concepts, such as SeaWinds and ASCAT. In the coming years, further progress in high resolution processing, closer to the coast, and with improved geophysical interpretation will be elaborated.
Figure 5: Wind variance versus spectral wave number k for a climatological spectrum (k-2 ), for the KNMI SeaWinds 25-km wind product (SDP), and for collocated NWP 10-m winds (ECMWF) on a 10log-10log scale5). With respect to the SeaWinds curve, the NWP spectral curve drops faster in variance over the blue shaded spectral range of corresponding wavelengths from 1000 km to 100 km.
Scatterometer winds are used by all major NWP centres and by the oceanographic community. At KNMI efforts are currently ongoing to assimilate high resolution ASCAT scatterometer data in the NWP High-Resolution Limited Area Model (HiRLAM) and in the second half of 2007 operational implementation is planned. Emphasis is put on the assimilation of high-resolution products and on exploitation of winds on the short range, particularly for extreme weather events. Moreover, today these winds are being effectively used in near real-time on the KNMI meteorological work station by the shift forecasters. These services will particularly profit from the new ASCAT EUMETSAT Advanced Retransmission Service with a revolutionary timeliness of just 30 minutes after sensing.
With the launch of ASCAT the series of C-band scatterometers is continued with extended coverage of the ocean surface wind and ice. EUMETSAT started timely user services in collaboration with KNMI.