Use of Altimeter and SAR Wave Data at the Met Office and Recent Comparison with Wave Model and Buoys
Martin Holt(1) and Jian-Guo Li(1)
Marine observations are relatively scarce in comparison with atmospheric ones due to limitation of observation tools and hindrance of the deep water. For centuries marine scientists have relied on sporadic cruise measurements and scattered buoys. The advent of remote sensing by aircrafts and satellites in late of the 20th century has greatly enlarged our ocean surface data set. UK Met Office has been involved closely with other international scientific organizations in the use of satellite wave observations. Altimeter data assimilation in ocean and wave models started in the 1980s with data from the SEASAT and Geosat satellites. Operational altimeter data assimilation began with the ERS-1 satellite launched in 1991. The most recent application of satellite data in the Met Office uses data from the ERS-2 and ENVISAT satellites. Radar altimeter wave data have enriched our knowledge of the global ocean waves and helped in improvement of our ocean wave models. On the other hand, ocean wave models have also contributed to the calibration and validation of satellite observations.
The operational wave spectral model suite run in the Met Office provides analyses and forecasts of sea state on grid spacing of approximately 60 km for the global model and 12 km for the nested regional models. As standard the models operate with a spectral resolution of 13 frequency and 16 directional bins, which represents waves with a range of periods between 3 and 25 seconds. The wave model is forced by hourly winds at 10m above mean sea-level generated in the Met Office Unified Model, which include observations from satellite, ship and data buoy networks in their 4-DVar assimilation schemes. The general performance of this wave model is comparable to other operational wave models, such as the 3rd generation WAM model.
The modelled two-dimensional wave energy spectra have been compared with measured ocean wave energy spectra from 11 moored buoys and the Advanced Synthetic Aperture Radar (ASAR) aboard the Envisat satellite. Comparisons are also made with the ocean wave height observed by the radar altimeter (RA2) onboard the same satellite Envisat as the ASAR instrument over the two years 2004 and 2005. Measured ocean wave energy spectra from two moored buoys (ID 51028 near Christmas Island at 0.02° S 153.87° W and ID 42001 in the Gulf of Mexico at 25.86° N 89.67° W) are selected for illustration here. Altimeter and ASAR data close to these two buoys are also collected for buoy-model-satellite inter-comparisons. Apart from comparison of the total significant wave height (SWH), we also compare the SWH in 4 frequency sub-ranges. This spectral breakdown of ocean wave energy sheds some light on the spectral characteristics of the wave model and the observations.
Results indicate that the wave model SWHs are generally in agreement with all the 3 different observations (buoy, altimeter and ASAR). Model sub-range SWHs are more close to the buoy ones than to the ASAR ones, which have larger variations than the modelled ones in both ends of the wave spectra. Some improvement of the ASAR data since late 2004 is also showed.