Envisat RA-2 IGDR product anomaly
04 February 2010
ECMWF atmospheric model pressure grids are operationally used to derive dry troposphere and inverse barometer corrections for the ENVISAT RA-2 height measurements.
Following the upgrade of the ECMWF model to a higher spatial resolution done on 26 January 2010 for the Envisat OFL products, anomalies have been identified on the IGDR products.
The largest impact is seen in the inverse barometer correction, which has static variations of up to 2 cm amplitude at a wavelength of ~ 50 km. This effect is most noticeable in regions within 200 km of the coast, but is not negligible outside those regions. The anomalous short-wavelength variations can be detected comparing IB values before and after the model evolution. The dry tropospheric correction is affected as well (~4 times smaller amplitude), but not the wet tropospheric correction.
The IGDR products have been affected by the ECMWF model resolution change from cycle 86 (pass 404).
The solution is to retain all the model's spectral information when interpolating from the high-resolution to low-resolution grids, which is an option when creating the operational grids. This change was implemented on 02-Feb-2010, and the first FGDR and IGDR products that have reduced pressure-induced errors are from cycle 86 (pass 613).
A smaller issue remains that the interpolation of the high-resolution ECMWF surface pressure grids to a lower resolution does not allow us to fully remove spectral leakage of land features onto the ocean. The remaining errors have a smaller amplitude (~20% of the short-wavelength features seen in the first figure),is also static in time, and almost exclusively affects regions within about 100 km of the coast. Work is ongoing to solve the remaining issue as well, and users will be kept updated on the progress.
The affected IGDR products will be reprocessed. A short note will be issued when those are available. All issues related to the ECMWF model resolution change will be corrected prior to OFL Level 2 GDR production for this time period.