Space and time aliasing structure in monthly mean polar-orbiting satellite data

Monthly mean wind fields from the European Remote Sensing Satellite (ERS1) scatterometer are presented. A banded structure which resembles the satellite subtrack is clearly and consistently apparent in the isotachs as well as the u and v components of the rou- tinely produced fields. The structure also appears in the means of data from other polar-orbit- ing satellites and instruments. An experiment is designed to trace the cause of the banded structure. The European Centre for Medium-Range Weather Forecasts gridded surface wind analyses are used as a control set. These analyses are also sampled with the ERS1 temporal- spatial sampling pattern to form a simulated scatterometer wind set. Both sets are used to cre- ate monthly averages. The banded structures appear in the monthly mean simulated data but do not appear in the control set. It is concluded that the source of the banded structure lies in the spatial and temporal sampling of the polar-orbiting satellite which results in undersam- pling. The problem involves multiple timescales and space scales, oversampling and under- sampling in space, aliasing in the time and space domains, and preferentially sampled variability. It is shown that commonly used spatial smoothers (or filters), while producing visually pleasing results, also significantly bias the true mean. A three-dimensional spatial- temporal interpolator is designed and used to determine the mean field. It is found to produce satisfactory monthly means from both simulated and real ERS1 data. The implications to cli- mate studies involving polar-orbiting satellite data are discussed.

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