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ERA*: An eddy-scale ocean forcing product

AuthorsPortabella, Marcos ; Trindade, Ana ; Stoffelen, Ad; Lin, Wenming
Issue Date24-Apr-2017
PublisherEuropean Geosciences Union
CitationGeophysical Research Abstracts 19: EGU2017-17971 (2017)
AbstractHigh resolution satellite derived sea surface wind data, such as those from scatterometers, are increasinglyrequired for operational monitoring and forecasting of the ocean. We present the development of ERA*, whichkeeps the time and space coverage of atmospheric model fields, but adds the accurately observed local mean andvariability of wind scatterometers, to make these datasets suitable for, among others, high-resolution ocean modelforcing.Recent attempts of combining scatterometer data and numerical weather prediction (NWP) outputs, i.e.blended ocean forcing products, allows for an increased temporal resolution (e.g., daily) but generally onlyresolves NWP spatial scales of∼200 km. Therefore, information on the wind-current interaction, the diurnal windcycle and the wind variability in moist convection areas is lost in such products. Moreover, known systematicNWP model (parameterization) errors are in fact propagated at times and locations where no scatterometer windsare available. The alternative, direct forcing from NWP results in even more extensive physical drawbacks. Wepropose to maintain the increased temporal coverage in a gridded wind and stress product (ERA*), but alsoto maintain the most beneficial physical qualities of the scatterometer winds, i.e. 25-km spatial resolution,wind-current interaction, variability due to moist convection, etc., and, at the same time correct the large-scaleNWP parameterization and dynamical errors. Additionally, we correct these winds for the effects of atmosphericstability and mass density, using stress equivalent 10 m winds, U10S.In fact, collocations of scatterometer and global NWP winds show physical differences, where the localmean and variability of such differences are rather constant in time and thus could be added to the ERA-interimtime record in order to better represent physical interaction processes and avoid NWP model errors. Correction ofboth wind vector biases and wind vector variability is expected to affect ocean forcing. Moreover, the collocationprocess provides the NWP winds sampled like a scatterometer and, therefore, provides information on thescatterometer wind sampling error.Prior to merging different scatterometer data sources, a comprehensive characterization of the scatterometercorrections is required. Through a Monte Carlo simulation, we provide an assessment of the corrections andsampling errors (which depend on scatterometer sampling) for the tandem scatterometer data set ASCAT-A/B.The local NWP biases are reduced at the cost of a somewhat increased variance, and the total error mitigation isconstrained to regions covered by the scatterometer at least 3 times over 5 days.The new ERA* gridded ocean forcing product is validated against continuous 10-min moored buoy winddatasets and the Ku-band QuikSCAT scatterometer 25 km product. Globally, there is a 9% reduction of the vectorroot-mean-square error in ERA* w.r.t to ERA interim. Despite the limited sampling in the tropics, ERA* showsareas of increased wind variability, likely associated to moist convection. Furthermore, there is a remarkableimprovement in the ERA* zonal wind component quality in the tropics.In coastal areas, ERA* still shows slightly larger biases than ERA interim. This is most likely due to boththe presence of increased wind variability and an inadequate scatterometer gridding. Future work will focus on theimprovement of ERA* in coastal areas
DescriptionEuropean Geosciences Union General Assembly 2017, 23-28 April 2017, Vienna, Austria.-- 1 page
Publisher version (URL)https://meetingorganizer.copernicus.org/EGU2017/posters/24345
Identifiersissn: 1607-7962
Appears in Collections:(ICM) Artículos
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