Practical Considerations for the Computations of Radar Altimeter Path Delay
ECMWF, UNITED KINGDOM
Estimation of the path delay of the altimeter radar signal due to the existence of the atmosphere is very important to compensate for it in the sea surface height measurements by radar altimeters. Such correction can be divided into two components: dry (DTC) and wet (WTC). The only way to estimate the DTC is by using atmospheric fields produced from numerical weather prediction (NWP) models. The computations of the DTC from the model fields are straightforward and do not involve a large number of fields. Such computations are almost non-sensitive to the various possible choices.
On the other hand, the WTC can be estimated using microwave radiometers that are typically installed together with the radar altimeter (MWR) on the same platform (e.g. Envisat and Jason satellites). Another way to estimate the WTC is by using atmospheric fields produced NWP models. This approach is usually favoured due to its availability at all times. Some low-cost satellite missions lack the MWR facility (e.g. Cryosat-2) and, therefore, rely on model estimations. However, the WTC computations require the integration across the atmospheric column and, therefore, require model fields at various levels of the atmosphere. With continuous increase in the number of the model levels (for example, in 2013 the number of levels of ECMWF model is increasing from 91 to 137 levels), the amount of data transfer required for the WTC estimation becomes extremely large especially after the expected higher horizontal resolution in a couple of years. The use of what is called the standard "pressure-level" (PL) fields, which are only around 25 levels, seems to be an attractive and "maintenance-free" solution. However, this comes with a price tag as far as the accuracy is concerned. The results from these two options and a proposal for a third approach will be summarised with some recommendations.