Improving Ocean Surface Temperature for NWP using All-Sky Microwave Imager Observations
| Title | Improving Ocean Surface Temperature for NWP using All-Sky Microwave Imager Observations
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Report
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| Date Published |
08/2024
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| Series/Collection |
EUMETSAT/ECMWF Fellowship Programme Research Report
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| Document Number |
RR64
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| Author | |
| Abstract | In this report, we demonstrate the utility of information from low frequency Microwave (MW) channels for improving ocean surface temperature in an Numerical Weather Prediction (NWP) system. This is not only important for current instruments, but will also enhance the exploitation of upcoming sensors such as the Copernicus Imaging Microwave Radiometer (CIMR) from the European Space Agency (ESA) and the Microwave Imager / Ice Cloud Imager (MWI/ICI) from the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT). Knowledge of the ocean surface Skin Temperature (SKT) is vital to the accurate use of satellite radiances in weather forecasting and it helps improve the quality of forecasts for the ocean and atmosphere. A primary source of the SKT information in this context is externally generated retrievals from satellite radiances. As we move to a full earth system approach, the aim is to retrieve SKT and other surface information as an integrated part of the use of satellite radiances within the NWP system. In the European Centre for Medium Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS), the SKT used for assimilating satellite radiances is currently derived using an input Sea Surface Temperature (SST) from a combination of external sources. A cool skin and warm layer parametrization is then applied to transform this into a SKT valid at the time of the observation. However, the use of external SST products brings issues such as a complex and non-optimal use of the satellite radiances, and latency of up to 69 hours between the validity time of the SST observations and the time of its use in weather forecasting. This further motivates the direct retrieval of SKT within weather forecasting. In the current work, a sink variable approach has been activated to allow the ocean SKT to update as part of the 4D variational assimilation (4D-Var). This method has been implemented for two MW imagers assimilated under all-sky conditions: Advanced Microwave Scanning Radiometer 2 (AMSR2) and the Global Precipitation Mission (GPM) Microwave Imager (GMI) which have been chosen as they include 6.9v/h and/or 10.65v/h GHz channels. These channels have much stronger sensitivity to SKT than the higher frequency MW channels currently being assimilated, and much greater coverage than IR channels, which can be strongly affected by cloud. The 6.9v/h and 10.65v/h GHz channels are being used for the first time over open ocean and the work is intended for operational use in cycle 50r1 of the ECMWF-IFS. A series of new Quality Control (QC) methods have been implemented to allow the use of the 6.9v/hand 10.65v/h GHz channels: frequency dependent coastline screening, sun-glint screening and reflected Radio Frequency Interference (RFI) detection and screening from geostationary satellite sources. The inclusion of these channels has an overall neutral impact on the atmospheric forecast scores and associated departure statistics. However, there are slight improvements in the upper tropospheric humidity seen in both the forecast scores and independent observations (including IR) likely due to the improved representation of heavy rainfall. The increments in SKT generated by the new approach indicate errors in the existing SKT, which is used as the background. These increments illustrate known deficiencies in the input data sources, for example, a large-scale cooling of Tropical Instability Waves (TIWs) in the Eastern Tropical Pacific (ETP) confirms a warm bias in this area in the existing SKT. The increments are generally larger and more wide spread during the day-time indicating a potential error either in the input SST used to grow the diurnal cycle, the SKT parametrization or both. The improved handling of the surface contribution to the microwave radiances also helps the atmosphere, for example some improvements in the vector winds, although overall the impact is small. The next step in this work is to include the SKT increments into the coupled atmosphere-ocean system to improve the representation of the ocean surface in the ocean model and hence improve ocean and 2 EUMETSAT/ECMWF Fellowship Programme Research Report RR64 Improving Ocean Surface Temperature for NWP using All-Sky Microwave Imager Observations atmosphere forecasts directly through the new SKT information. This approach also shares the SKT information derived from the low frequency MW across the data assimilation system so that it can improve the use of all observations with sensitivity to the surface including both MW and IR (e.g. AMSU-A, IASI, etc.). The use of the coupled system for MW SKT retrievals is a pathfinder for a fully integrated processing of SKT information from all satellite radiances from both current and future sensors. |
| URL | https://www.ecmwf.int/en/elibrary/81580-improving-ocean-surface-temperature-nwp-using-all-sky-microwave-imager |
| DOI | 10.21957/c16be07b23 |
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