The Infrared Sounder (IRS) is an infrared Fourier transform spectrometer that will be on board the Meteosat Third Generation series of the future European Organization for the Exploitation of ...Meteorological Satellites geostationary satellites and will have a unique four‐dimensional look at the atmosphere. After its planned launch in 2024, it will be able to measure the radiance emitted by the Earth at the top of the atmosphere using 1,960 channels in two spectral bands between 680–1,210 cm−1$$ {}^{-1} $$ (long‐wave infrared) and 1,600–2,250 cm−1$$ {}^{-1} $$ (mid‐wave infrared) at a spectral sampling of 0.6 cm−1$$ {}^{-1} $$. It will perform measurements over the full Earth disk with high spatial and temporal resolution of 4 km at nadir and 30 min over Europe. Thus, the huge amount of data from IRS will present challenges, particularly in data transmission, data storage, and assimilation into numerical weather prediction (NWP) models. To reduce the volume of data, various methods are available, including spatial sampling, principal component analysis, and channel selection. The latter technique will be discussed in this paper by proposing general channel selection to provide NWP models. The objective of this selection is to improve essential variables for NWP such as temperature, humidity, skin temperature, and ozone. This work has required the development of a large observation database and takes into account the main developments in assimilation techniques, including the use of full observation‐error covariance matrices or the assimilation of ozone in global models, for example. This study performs a specific analysis of the sensitivity of IRS observations and proposes a first general selection of 300 channels for NWP models. This selection allows us to reduce the analysis error in the troposphere by 48% in temperature, 65% in humidity, and 17% in ozone.
Location of the 300 selected Infrared Sounder (IRS) channels on a typical spectrum by main sensitivity.
In the framework of the EUMETSAT Polar System–Second Generation (EPS‐SG) preparation, a new generation of the Infrared Atmospheric Sounding Interferometer (IASI) instrument has been designed. The ...IASI‐New Generation (IASI‐NG) will measure radiances at a doubled spectral resolution compared to its predecessor and with a signal‐to‐noise ratio improved by a factor of 2. The large amount of data arising from IASI‐NG will present many challenges for data transmission, storage and assimilation. Moreover, the full set of measured radiances will not be exploitable in an operational numerical weather prediction (NWP) context. For these reasons, an appropriate IASI‐NG channel selection in needed, aiming to select the most informative channels for NWP models. Therefore, the standard iterative channel selection methodology, based on the optimal linear estimation theory and assuming spectrally correlated errors, has been applied to a set of simulated data of the IASI‐NG spectrum. The entire simulated IASI‐NG spectrum has been first investigated, while finally focusing the channel selection procedure on the most interesting wavelength ranges for the assimilation. Through this process, a total of 500 channels have been chosen to serve as a basis for future channel selections to be provided to NWP centres – 277 temperature, 23 surface‐sensitive and 200 water vapour channels. One‐dimensional variational (1D‐Var) assimilation experiments show that using this selected set of channels leads to a reduction of the standard deviation of the error in temperature (up to 30%) and water vapour (up to 50%) profiles with respect to the apriori information.
IASI‐NG will measure radiances at a doubled spectral resolution compared to IASI with a signal‐to‐noise ratio improved by a factor of 2. An appropriate channel selection is needed to select the most informative channels for NWP. The iterative channel selection methodology, based on Degrees of Freedom for Signal (DFS) and assuming spectrally correlated errors, has been applied. 500 channels have been chosen as a basis for future channel selections for NWP – 277 temperature, 23 surface‐sensitive and 200 water vapour channels.
The infrared sounder (IRS) instrument is an infrared Fourier‐transform spectrometer that will be on board the Meteosat Third Generation series of the future European Organization for the Exploitation ...of Meteorological Satellite's geostationary satellites. It will measure the radiance emitted by the Earth at the top of the atmosphere using 1,960 channels. The IRS will provide high spatial‐ and temporal‐frequency four‐dimensional information on atmospheric temperature and humidity, winds, clouds, and surfaces, as well as on the chemical composition of the atmosphere. The assimilation of these new observations represents a great challenge and opportunity for the improvement of numerical weather prediction (NWP) forecast skill, especially for mesoscale models such as the Applications de la Recherche à l'Opérationnel à Méso‐Echelle (AROME) at Météo‐France. The objectives of this study are to prepare for the assimilation of the IRS in this system and to evaluate its impact on the forecasts when added to the currently assimilated observations. By using an observing system simulation experiment constructed for a mesoscale NWP model. This observing system simulation experiment framework makes use of synthetic observations of both IRS and the currently assimilated observing systems in AROME, constructed from a known and realistic state of the atmosphere. The latter, called the “nature run”, is derived from a long and uninterrupted forecast of the mesoscale model. These observations were assimilated and evaluated using a 1 hr update cycle three‐dimensional variational data assimilation system over 2‐month periods, one in the summer and one in the winter. This study demonstrates the benefits that can be expected from the assimilation of IRS observations into the AROME NWP system. The assimilation of only 75 channels over oceans increases the total amount of observations used in the AROME three‐dimensional variational data assimilation system by about 50%. The IRS impact in terms of forecast scores was evaluated and compared for the summer and winter periods. The main findings are as follows: (a) over both periods the assimilation of these observations leads to statistically improved forecasts over the whole atmospheric column; (b) for the summer‐season experiment, the forecast ranges up to >$$ > $$48 hr are improved; (c) for the winter‐season experiment, the impact on the forecasts is globally positive but is smaller than the summer period and extends only to 24 hr. Based on these results, it is foreseen that the addition of future IRS observations in the AROME NWP systems will significantly improve mesoscale weather forecasts.
The future infrared sounder (IRS) will provide high spatial‐ and temporal‐frequency four‐dimensional information on atmospheric temperature and humidity, winds, clouds, surfaces, and so on. In this study, an observing system simulation experiment was built in order to assess the potential benefit of assimilating IRS observations in addition to the data currently used in mesoscale numerical weather prediction model such as the Applications de la Recherche à l'Opérationnel à Méso‐Echelle (AROME) at Météo‐France. The use of IRS observations in the three‐dimensional variational assimilation AROME leads to a significant improvement in weather forecasts.
Nowadays, the assimilation of satellite observations, particularly radiances from infrared sounders, into numerical weather prediction (NWP) models plays a dominant role in improving weather ...forecasts. One of the keys to make optimal use of radiances is to simulate them with a radiative transfer model (RTM). At Météo‐France, the RTTOV RTM is used for NWP models. Currently, simulations are carried out taking into account single chemical profiles. However, neglecting the spatial and temporal variability of these gases can affect the accuracy of the simulations and thus the quality of the subsequent analyses and forecasts. To reduce the impact of this assumption on weather forecasts, we use a variational bias correction but it would be more appropriate to correct the bias directly at the source. Ozone is one of the atmospheric constituents with significant impacts on spectral radiances measured by hyperspectral infrared sounders. Thus, the objective of this paper is to replace the use of a single ozone profile with a realistic and variable ozone field in RTTOV for the simulation of infrared observation. The results show that the use of a variable ozone allows us to further reduce biases in simulation of ozone‐sensitive channels but also of channels sensitive mainly to other parameters such as CO2 for example. This has positive effects on the analyses and improves the fit of the short‐range forecasts (or analyses) to other observations such as radiosondes, microwave radiances, GNSS‐RO bending angles, etc. All of these positive impacts allow us to significantly improve weather forecasts.
Ozone is one of the atmospheric constituents with significant impacts on spectral radiances measured by hyperspectral infrared sounders. The objective of this paper is to replace the use of a single ozone profile with a realistic and variable ozone field in RTTOV for the simulation of infrared observation. The results show positive effects on analyses and allow us to better simulate other observations such as radiosondes or radiances, and to significantly improve weather forecasts.
During the first special observation period of the HyMeX program dedicated to heavy precipitation over the western Mediterranean, several Mesoscale Convective Systems (MCSs) formed over the sea and ...produced heavy precipitation over the coastal regions, as for example during IOP (Intensive Operation Period) 16a. On 26 October 2012, back‐building MCSs formed and renewed over the northwestern Mediterranean Sea while producing heavy rain over the French coastal urbanized regions. This paper analyses the storm evolution along with the ambient flow and the initiation and maintenance mechanisms of the offshore deep convection observed during IOP16a. The suites of water vapour lidars, wind profilers, radiosoundings and boundary‐layer drifting balloons over and along the coast of the northwestern Mediterranean offer a unique framework for validating the convective processes over the sea investigated using kilometre‐scale analyses and simulation.
The high‐resolution simulation shows clearly that the convective system is fed during its evolution over the sea by moist and conditionally unstable air carried by a southwesterly to southeasterly low‐level jet. The low‐level wind convergence in this southeasterly to southwesterly flow over the sea is the main triggering mechanism acting to continually initiate and maintain the renewal of convective cells contributing to the back‐building system. The convergence line appears when a secondary pressure low forms in the lee of the Iberian mountains. A sensitivity test turning off the evaporative cooling within the microphysical parametrization shows that the exact location of the main convergence area focusing the heaviest precipitation is determined by small‐scale feedback mechanisms of the convection to the environment.
This article focuses on the simulation and the assimilation of satellite infrared observations in convective‐scale numerical weather prediction (NWP) systems. A radiative transfer model that includes ...profiles for liquid‐water content, ice‐water content and cloud fraction was used to simulate cloud‐affected radiances as background equivalents. This approach avoids the use of cloud parameters (cloud‐top pressure and effective cloud fraction) deduced from a CO2 slicing algorithm and the modelling of clouds by single‐layer clouds. The advanced radiative transfer model was evaluated using infrared observations measured by the Infrared Atmospheric Sounding Interferometer (IASI). The observation‐screening procedure that was developed to improve the selection of usable cloudy scenes led to a good agreement between observations and background equivalents. For that purpose, a radiance analysis of collocated Advanced Very High Resolution Radiometer (AVHRR) pixels inside each IASI field of view was used. The goal of this preliminary work is to assess the feasibility of adding the cloud variables (liquid and ice‐water contents) to the state vector of the assimilation system. The approach is illustrated with one‐dimensional variational (1D‐Var) retrievals. The physical consistency of the 1D‐Var adjustments is verified with real observations. Then observing‐system simulation experiments (OSSE) are used to validate the 1D‐Var retrievals.
In the framework of the Hydrological cycle in the Mediterranean Experiment (HyMeX) project, measurements from more than 1000 ground‐based Global Positioning System (GPS) receivers located in the ...northwestern Mediterranean area are reprocessed using a single piece of GPS software for the period from 1 September 2012 to 31 March 2013. A special screening procedure is developed for the removal of outliers in the GPS Zenith Total Delay (ZTD) data. ZTD data are converted to integrated water vapour (IWV) using surface pressure information from an AROME‐WMED operational analysis. The reprocessed ZTD and IWV data are used to assess the accuracy of the near‐real time E‐GVAP ZTD data assimilated in operational numerical weather prediction systems and to validate the IWV data from the AROME‐WMED operational analysis and AROME‐WMED reanalysis 1, and from radiosonde observations. The mean differences between E‐GVAP and reprocessed ZTD data are not negligible and lie in the range from −3 to +3 mm. The standard deviations of differences are between 4 and 8 mm. The comparisons of IWV from AROME‐WMED analyses and the reprocessed GPS data show high quality of the analyses where operational GPS data are assimilated and lower quality where no GPS data are assimilated. Small but significant biases are found in the radiosonde data during daytime (−0.5 to +1.4 kg m−2), but their origin is not determined so far. Thanks to the high spatial density of the reprocessed GPS stations, both the large‐scale and small‐scale variations in IWV can be documented. The case of HyMeX Intensive Observing Period 8 is presented as an example of a heavy precipitation event. This work suggests that improved quality of the humidity fields can be expected of the future AROME‐WMED reanalysis 2 as a result of the assimilation of the reprocessed GPS data.
During Intensive Observation Period 13 (15 to 16 October 2012) of the first Special Observing Period of the Hydrological cycle in the Mediterranean Experiment (HyMeX), Southern Italy (SI) was ...affected by two consecutive heavy precipitation events (HPEs). Both HPEs were associated with multi‐cell V‐shaped retrograde regeneration mesoscale convective systems (MCSs). The life cycle of two MCSs in connection with their dynamic and thermodynamic environments were analysed using a combination of ground‐based, airborne and spaceborne observations and numerical simulations. Rain gauges revealed that heavy precipitation occurred in two phases: the first one from 1300 to 1700 UTC (35 mm h−1) was caused by a V‐shaped system initiating over the Tyrrhenian Sea in the early morning of 15 October. Convection was triggered by the low‐level convergence between the southwesterlies ahead of an upper‐level trough positioned over southeastern France and very moist southerlies from the Strait of Sicily. The convection was favoured by high convective available potential energy (1500 J kg−1) resulting from warm and moist conditions at low levels associated with high sea surface temperatures in the Strait of Sicily. In addition, humidity at mid‐level was enriched by the presence of an elevated moisture plume from tropical Africa, favouring the efficiency of the convection to produce more precipitation. The second phase of heavy precipitation (2300 UTC on 15 October to 0200 UTC on 16 October, 34 mm h−1) was caused by a MCS initiating over Algeria around 1300 UTC, which subsequently travelled over the Strait of Sicily toward Sicily and SI. Convection was maintained by the combination of large low‐level moisture contents and a marked convergence ahead of the cold front. Unlike other MCSs forming in the same region earlier on that day, this huge V‐shaped system did affect SI because the strong upper‐level flow progressively backed from southwesterly to south southwesterly.
The Infrared Atmospheric Sounding Interferometer (IASI) provides 8461 channels in the infrared spectrum. In current numerical weather prediction (NWP) models, it is not feasible to assimilate all ...channels and it is known that the information content between adjacent channels is redundant. This issue has been addressed in NWP centres by employing a channel selection strategy. The goal of this article is to add new channels to the existing IASI operational channel selection, aimed at improving the data assimilation in cloudy conditions and the simultaneous retrieval of cloud microphysical variables, specifically liquid and ice water contents. Cloudy profiles from the French convective‐scale model Applications of Research to Operations at MEsoscale (AROME) are used in the study to focus on the retrieval of cloud variables over the Mediterranean region. Three channel selection methodologies were evaluated in this study: a statistical approach based on the degrees of freedom of the signal (DFS), a physical method based on the channel spectral sensitivity to the cloud variables and a random selection. To validate the new selections, an idealized framework is used with observing system simulation experiments (OSSE) in the context of one‐dimensional variational retrievals. The current operational IASI selection has already been shown to provide good retrievals of cloud variables. However, all the different channel selections improve the results with small differences in the 1D‐Var retrievals. Based on the physical and DFS methods, the final sets of 134 channels sensitive to cloud variables are proposed for future investigation in operational implementation. Additional tests on temperature and water‐vapour retrieval results, air‐mass dependence and cloud microphysical parametrization have also been conducted.
PDF
