Since July 2018, ECMWF has been assimilating radiance data from the American satellite GOES‑16. In November 2016, the satellite was launched into a geostationary orbit whose position allows continuous coverage of the Americas, including the western Atlantic and eastern Pacific. The official GOES‑16 clear-sky radiance (CSR) product is not due to be available until May 2019, but close collaboration between ECMWF and the US National Oceanic and Atmospheric Administration (NOAA) has made it possible to start assimilating the data well before then.
The satellite is equipped with an instrument which provides images of the Earth for a number of spectral bands (channels) covering wavelengths from the visible to the thermal infrared. For data assimilation, we are primarily interested in the channels which cover the wavelength range 5.5–7.5 µm as they are sensitive to atmospheric water vapour. Compared to GOES‑13, its predecessor as the official ‘GOES‑EAST’ satellite, GOES‑16 has many additional capabilities thanks to the new ABI instrument (Advanced Baseline Imager). Notably, there is a significant improvement in spectral, spatial and temporal resolution, each of which has benefits for data assimilation.
As well as being sensitive to humidity, the water vapour channels also pick up strong signals from clouds, and any cloud-affected observations must be removed before the observations can be assimilated. For GOES‑16, this is performed by NOAA. The resulting CSRs are box‑averages of the clear-sky pixels.
Collaboration with NOAA
In January 2018, GOES-13 was retired and GOES-16 became the official GOES-EAST satellite. Since the official CSR product is not due to be available until May 2019, contact was made with NOAA to try to determine whether unofficial data could be made available to ECMWF before the 2018 hurricane season. NOAA/NESDIS/STAR were developing the CSR product, and ECMWF provided some assistance to ensure these data were in the correct format. Subsequently, the CSR data were made available to ECMWF and other centres via FTP, and the quality of the data was assessed and the analysis fed back to NOAA.
An example of the interaction between ECMWF and NOAA was related to horizontal stripes appearing in the images, which also affects the CSRs. Guidance from NOAA enabled us to implement a quality check whereby, if the standard deviation of the clear-sky pixels within an averaging box exceeds 4 K (in terms of brightness temperature), then the CSR is rejected from the assimilation to prevent degradation of the atmospheric analysis.
Several months after GOES‑13 was retired, GOES‑16 CSRs started to be assimilated operationally on 26 July 2018, following a period of passive monitoring during which bias correction coefficients were spun up and the quality was shown to be comparable to that of data from the Japanese satellite Himawari‑8. As well as CSRs, wind information from GOES‑16 is also assimilated directly. These winds are obtained by tracking cloud-features in consecutive visible and infrared images, hence this information is complementary to that provided by the CSR data. These retrievals are called atmospheric motion vectors (AMVs). In the period leading up to implementation, an assimilation experiment was performed to assess the impact on forecasts due to the assimilation of GOES-16 radiances. This demonstrated that the short-range forecast of humidity and wind was improved, as verified against independent satellite and in-situ observations.
GOES‑16 provides a full-disc scan every 15 minutes, unlike GOES-13, which produced one every 3 hours. Currently it is not possible to fully exploit the high temporal sampling in the ECMWF system, but we intend to investigate any benefits we may gain from it, particularly the potential improvements to the wind field which may be obtained by the 4D-Var system tracing water vapour features throughout the assimilation window. GOES-17, one of the three successors to GOES-16, has already been launched, and we plan to assess data from this satellite when it attains operational status, although some ongoing issues with the infrared channels may limit the volume of data we can use from this satellite.