Development and evaluation of a new merged sea surface height product from multi-satellite altimeters

Abstract

Satellite altimetry data has been a backbone of global oceanography research since the early 1990s. Because a satellite altimeter measures sea surface height (SSH) at the nadir point, it produces a one-dimensional profile of SSH. Along the track, an altimeter provides SSH sampling at an interval of about 7 km. However, the distance between adjacent tracks is larger than 100 km, and satellites have repeat times of 10–30 d. These characteristics limit the usefulness of altimetry data from a single satellite for scientific analysis of the space-time structure of ocean circulation. During the past two decades, there have usually been three to four altimeters in orbit at any one time. Mapping along-track measurements from multiple altimeters onto a uniform grid has been a major research theme. A popular merged data product, known as AVISO, has been developed and extensively applied to a wide variety of ocean research. One of key properties of a merged data product is its effective resolution. The effective resolution represents the minimum spatial size of an eddy or other mesoscale circulation that the data product can resolve. The AVISO data product has an effective resolution of 300 km in equatorial regions to 100 km in high latitudes. Nowadays, there are more than five satellite altimeters in orbit, including the Chinese HY-2A that was launched in 2011, and thus the density of the tracks has been much improved. However, the effective resolution of the AVISO data product has not increased accordingly. Effective resolution of mapped data is closely related to the background error correlation coefficient scale in the mapping scheme. The effective resolution is no more than twice the correlation coefficient scale even when the amount of satellite altimeter is more than 5. As the background fields contains the correct large scale information, the meso-scale or small-scale information in the background error will play a dominant role and the background error correlation coefficient scale becomes even smaller. Therefore, The option of background determines the effective resolution of mapped data. The above theory is the basement of our new proposed mapping scheme, which effectively improved the effective resolution. Another key technique is how to deal with the observation errors: to reduce time-smoothing by introducing evolution errors in observation errors and effectively mapping multi-mission satellite altimeters from different times. In this study, a new mapping system has been developed, aiming to maximize the effective resolution of the mapped data product. The formulation is based on a variational method and optimal estimation theory. The system has been applied to the South China Sea region. The product uses a regular grid with spacing of 0.08°, in comparison to 0.25° for AVISO. The product merged data from five satellites, Sentinel-3A, Jason-3, Cryosat-2, Saral/AltiKa and HY-2A. Systematic evaluations show that this new product has an effective resolution of 125 km, whereas the AVISO data product an effective resolution of 240 km for the same region. The accuracy is also significantly improved. The new proposed mapping scheme is important in the application of satellite altimeters on the study of meso-scale ocean eddies. In addition, the option of background fields and the processing method of observation error are strictly general and can be used in other schemes of high-resolution data mapping.