Comparison of Envisat ASAR Ocean Wave Spectra with Buoys and Altimeter Observations via a Wave Model
Jian-Guo Li(1) and Martin Holt(1)
UK Met Office,
Exeter EX1 3PB,
The Advanced Synthetic Aperture Radar (ASAR) on board the European Space Agency Envisat satellite is one of a few instruments which can measure the directional characteristics of the ocean wave field on a global scale and provide an unprecedented resource for ocean wave models. Assessment of the ASAR data quality is, however, difficult due to lack of independent observations of similar temporal and spatial scales. The ocean wave heights measured by the radar altimeter instrument (RA2) onboard the same satellite as the ASAR can only be used for partial validation of the ASAR data as they do not have any spectral and directional information, not to mention that the ASAR spectra are displaced from the RA2 measurements due to the slant beam of the ASAR instrument. Moored ocean data buoys are another independent observation and some of them offer wave spectral and directional information though their spatial coverage is very limited. Direct comparison of ASAR data with buoy observations is not practical as few ASAR spectra fell on the exact position of any buoy, prohibiting any meaningful statistical application. An indirect comparison of these three independent observations is made possible by an intermediate wave model, which provides coherent links to fill the time and space gaps among these 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 the Envisat ASAR and selected buoys, respectively. Comparisons are also made with the ocean wave height observed by the Envisat RA2 altimeter over the two years 2004 and 2005. 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 these ocean wave data. 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.