Ers-2 Altimetry in Operational Noaa Forecast Models

NOAA's Laboratory for Satellite Altimetry has been producing real-time altimetry products from ERS-1 and ERS-2 since 1995. Significant enhancements are made to ESA's fast-delivery data sets, most notably the addition of orbit information provided by the Delft University of Technology. In late 1998, several enhancements were made to the NOAA/Delft altimetry products: ESA's fast-delivery data now include measured wet troposphere corrections from the on-board radiometer; the Delft DGM-E04 gravity model (specifically tuned for ERS) is used in the orbit determination system; and an iterative feedback scheme is now performed to refine the real-time orbits by incorporating altimetric heights and crossovers. Within NOAA, several collaborative efforts are utilizing real-time ERS-2 data in their operational forecast models. At the largest scales, the Climate Modeling Branch at the National Centers for Environmental Prediction (NCEP) is assimilating TOPEX/Poseidon and ERS-2 altimetry into a coupled model for El Niño / Southern Oscillation prediction. Timeliness is not as critical as absolute accuracy in this modeling effort. Thus, we are combining the ERS-2 data with the more accurate TOPEX/Poseidon altimetry to further reduce orbit and environmental correction errors before assimilation in the ENSO model. At finer resolution, the NCEP Ocean Modeling Branch is assimilating TOPEX/Poseidon and ERS-2 altimetry into the Coastal Ocean Forecast System, which produces nowcast and 24-hour forecast fields for the Gulf Stream/western North Atlantic region. Evaluations carried out during a recent demonstration project established that addition of sea surface height anomalies derived from TOPEX/Poseidon added significant detail to the real-time products: sea surface heights, 1-meter currents, and subsurface temperatures. Parallel forecast experiments are now in progress to determine the impact of the addition of ERS-2 altimetry. The increased spatial resolution provided by ERS-2 is expected to further improve the model's skill at determining mesoscale features. Recently, a unique use of altimetry data was begun with NOAA's National Hurricane Center. Products containing surface height anomalies, upper ocean heat content, and upper layer thickness will be available operationally to the forecasters this hurricane season for the Western Atlantic Ocean. The upper ocean heat content and upper layer thickness are inferred from sea surface heights, which augment sea surface temperature and in-situ subsurface measurements. Again, the increased spatial resolution provided by ERS-2's sampling will greatly benefit the determination of these oceanic parameters, thereby improving the prediction of tropical cyclone intensity. REAL-TIME ALTIMETRY FROM ERS-2 One of the unique aspects of the European Space Agency's (ESA) Earth Remote Sensing satellite (ERS) missions is the rapid delivery of its data products to end users. For "low bit-rate" data sets, which include measurements from the onboard altimeter and radiometer systems, the latency between down-link telemetry and reception by the user is no greater than six hours, Fig. 1. The primary receiving stations at Kiruna, Sweden and Maspalomas, Spain typically show latencies of 2-3 hours, while the remote stations at Gatineau and Prince Albert, Canada exhibit somewhat longer delays. The data acquired by the Prince Albert station became a part of the fast-delivery data stream in July, 1999, after a leased line was installed between the two Canadian ground stations. When fast-delivery altimeter data arrives at NOAA's Laboratory for Satellite Altimetry (LSA) it does not contain an accurate orbit, but rather only crude satellite state vectors. Through a long-standing and highly successful collaboration with the Delft University of Technology's Institute for Earth-Oriented Space Research (DEOS) we obtain precise orbit information for the ERS satellites on a daily basis. The Delft/NOAA system has been continually improved, with orbit errors now estimated at 7-10 cm radially, as described further below. Once the altimetric range is combined with the satellite's orbital elevation it is possible to generate sea surface height measurements. However, atmospheric path delay corrections must also be obtained. The operational weather forecasting system at NOAA's National Centers for Environmental Prediction (NCEP, formerly NMC) provides estimates of sea level pressure and total precipitable water, which can be converted to wet and dry troposphere corrections. These fields are routinely available within nine hours past the end of a day, Fig. 1. Once the satellite orbits and atmospheric correction fields have been received, the final NOAA Real-time Geophysical Data Record (RGDR) is generated within an hour, so that the net latency for creation of the daily files is ten hours past midnight of the previous day.