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

Monthly mean wind fields from the European Remote Sensing Satellite (ERS-1) scatterometer are calculated and 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 wind with a typical variation (amplitude) of 4 ms^-1. An experiment is designed to trace the cause of the banded structure; the European Centre for Medium Range Weather Forecasting (ECMWF) surface wind analyses are used to form a control (synoptic) set and a simulated scatterometer wind set sampled with the ERS-1 temporal/spatial sampling pattern. Both sets are used to create monthly averages. The banded structures appear in the simulated scatterometer set but do not appear in the control set. It is concluded that the source of the banded structure is in the spatial and temporal sampling of the polar-orbiting satellite. The nature of the problem and some means to control it are discussed. The problem is rather complicated and involves multiple time and space scales, over and under sampling in space, aliasing of time to space domain, and intraseasonal variability which is preferentially sampled. Therefore, common means of controlling the undesirable effects of aliasing address only part of the problem. While some of these control methods produce visually pleasing results, they may bias the true mean.