The Megha-Tropiques satellite was launched in 2011 with a microwave sounder called SAPHIR onboard. This instrument probes the atmosphere with six channels around the 183.31 GHz water vapour absorption band. Its observations are sensitive to water vapour as well as to hydrometeors. This instrument was proven to be useful for data assimilation by different numerical weather prediction centres, in particular for clear-sky assimilation. At Météo-France, SAPHIR observations have been routinely assimilated in clear sky since 2015 in the ARPEGE global model. The present article introduces a framework to complement this clear-sky assimilation route by a new cloudy and rainy assimilation route for satellite microwave brightness temperatures. This framework is based on several steps including a Bayesian inversion of the SAPHIR brightness temperatures into relative humidity retrievals, which are then assimilated within the ARPEGE global model. This study presents the methodology of assimilation, including the development of two error models, one for the Bayesian inversion, and one for the observation errors of relative humidity retrievals within the ARPEGE 4D-Var data assimilation system. The forecast scores obtained with this methodology over a three-month period indicate a positive impact of SAPHIR cloudy and rainy observations within the ARPEGE system, in particular on tropical temperature and wind forecasts for which the improvements range from 0.5 to 1.7% on standard deviations with respect to the ECMWF analysis and up to a +60 h lead time.
This study focuses on the ability of ERA-Interim to represent wind variability in the middle atmosphere. The originality of the proposed approach is that wind measurements are deduced from the trajectories of zero-pressure balloons that can reach high-stratospheric altitudes. These balloons are mainly used to carry large scientific payloads. The trajectories of balloons launched above Esrange, Sweden, and Teresina, Brazil, from 2000 to 2011 were used to deduce zonal and meridional wind components (by considering the balloon as a perfect tracer at high altitude). Collected data cover several dynamical conditions associated with the winter and summer polar seasons and west and east phases of the quasi-biennial oscillation at the equator. Systematic comparisons between measurements and ERA-Interim data were performed for the two horizontal wind components, as well as wind speed and wind direction in the [100, 2]-hPa pressure range to deduce biases between the model and balloon measurements as a function of altitude. Results show that whatever the location and the geophysical conditions considered, biases between ERA-Interim and balloon wind measurements increase as a function of altitude. The standard deviation of the model–observation wind differences can attain more than 5 m s−1 at high altitude (pressure P < 20 hPa). A systematic ERA-Interim underestimation of the wind speed is observed and large biases are highlighted, especially for equatorial flights.
Observing the Earth with a microwave radiometer on board a geostationary satellite has generated interest for several decades. Such a mission would add a high observation rate in the microwave spectrum, offered by a geostationary orbit, to the sounding capabilities of the current observing system. The instrumental concept under study considers a microwave radiometer with six channels with different observation errors within the 183.31 GHz water vapour absorption band. Observing System Simulation Experiments (OSSEs) are conducted to examine if these very frequent microwave observations would be beneficial to mesoscale numerical weather prediction (NWP) and complement the current or soon‐available satellite observations. The OSSE framework is built up on (i) simulated observations from a known ‘truth’ which is a long and uninterrupted forecast from the Météo‐France ARPEGE global model, and (ii) the Météo‐France AROME mesoscale model in which the simulated observations are assimilated using a 1 h update cycle 3D‐Var data assimilation system. Benefits that may be expected from such a microwave sounder mission are evaluated in the context of a dense observing system including observations from the future hyper‐spectral InfraRed Sounder on board Meteosat Third Generation. In particular, impacts of microwave observations with observation errors ranging from 1.25 to 5 K are studied. One of the main findings of this study is that fine‐scale NWP systems do not only need observations that are frequent in space and time, but that these observations must be accurate as well.
International audienceThe feasibility study of the HALESIS (High-Altitude Luminous Events Studied by Infrared Spectro-imagery) project is presented. The purpose of this experiment is to measure the atmospheric perturbation in the minutes following the occurrence of transient luminous events (TLEs) from a stratospheric balloon in the altitude range of 20–40 km. The instrumentation will include a spectro-imager embedded in a pointing gondola. Infrared signatures of a single blue jet were simulated under the assumption of local thermodynamic equilibrium (LTE), and were then compared with a panel of commercially available instrument specifications
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