Research Activities at Météo-France
By Philippe Nacass, Météo-France, Toulouse, France
Within Météo-France (the French Weather Service), the Centre National de Recherches Météorologiques (CNRM, or The National Center for Meteorological Research) is the department responsible for conducting most of the organization's meteorological research activities. The center is primarily oriented towards the needs of the public in the areas of meteorology, weather forecasts, and the physics and dynamics of the atmosphere. Its work also covers related fields, such as atmospheric chemistry (acid rain and ozone), surface oceanography, the physics and dynamics of snow cover (avalanches), surface hydrology (floods), and urban pollution.
The research aircraft ATR-42, a bi-turboprop operated as part of the SAFIRE project |
At CNRM, the development of new-generation atmospheric models is an ongoing effort. To carryout this mission, CNRM hosts approximately 225permanent staff (one-third being research scientists), and 45 students and visitors. At the national level, the research is conducted in close cooperation with many universities and atmospheric laboratories, such as the Centre National de Recherche Scientifique (CNRS, the National Center for Scientific Research). At the international level,CNRM collaborates with the European Center for Medium-Range Weather Forecasts (ECMWF) in the United Kingdom and the National Center for Atmospheric Research (NCAR) in the USA, among others. The CNRM also participates in international large-scale field experiments and multidisciplinary research programs, such as the International Geosphere-Biosphere Program and the World Climate Research Program.
Some of the data collection at Météo-France is done using atmospheric research ships and aircraft, which deploy a variety of sensors for state parameters,atmospheric chemistry, cloud physics, and remote sensing. Instruments used for these measurements have fundamental sources of uncertainty that can often be quantified. One source of error is distortion from the hull or fuselage of the ship or aircraft .Movement of air around these bodies can impact not only the flow speed and direction, but the concentrations of various constituents or particulate matter being measured. In fact, airflow distortion can generate errors that are larger than those inherent in the measurement sensor. To make the most accurate atmospheric measurements, these errors must be understood and minimized by a suitable selection of sampling location.
To better understand the influence of objects on measurement error, wind tunnel studies have traditionally been done, even though such tests are costly and time-consuming to perform. They are limited by the wind tunnel speed and the physical size of the model. Numerical modeling has also been employed to simulate the airflow disturbance over ships and around aircraft. From 1988 to 1994,potential flow codes were used to understand the flow characteristics in critical areas where sensors are placed. These codes provided reasonably accurate estimates of flow and particle behavior at locations outside the aircraft boundary layer.
In recent years, advances in CFD have permitted faster, more accurate representations of airflow around bodies. Airflow characteristics (speed and direction, for example) and particle trajectories in measurement regions can be predicted using CFD,allowing better placement of research instrumentation and measurements of greater accuracy. Since1995, Météo-France has used FLUENT for this purpose, tasking some individuals with full-time effort. CFD is currently being used to optimize measurement equipment and its positioning on a number of aircraft and ships.
The Aircraft Fleet at Météo -France
In cooperation with other French governmental organizations, Météo-France has operated several research aircraft over the years, such as a Piper-Aztec, a Merlin-IV, and a Fokker-27 [1]. For these aircraft, CFD has been used to correct in-flight measurements made by sensors and instruments designed and mounted on the fuselages prior to the introduction of CFD. In 2005, Météo-France, through SAFIRE [2], began to operate two new instrumented aircraft, an ATR-42 (a bi-turboprop) and a Falcon-20 (a bi-turbojet). For these aircraft, CFD was used to study the best position of the instruments, and to make sure that they, in turn,have no harmful influence on the aircraft for all possible flight attitudes.
The Design of New Airborne Instruments
The airflow disturbance in the volume of measurement for a manufactured sensor |
Météo-France has also used CFD to develop new aircraft instruments, including special sensors, and to optimize their shape. This effort has proved to be very important for the design of sensor inlets that measure airspeed or aerosol concentrations. For manufactured instruments, CFD is used to illustrate the airflow disturbance in the volume of measurement. The aerodynamic forces and moments on the outside body of these instruments are also calculated by FLUENT for certification by the French Aviation Administration.
Airflow Studies Around Aircraft
Pressure is used to compute the aerodynamic forces and moments on the pylons mounted under each wing of the ATR-42; the pylons are used to carry additional instruments |
CFD simulations for specific aircraft have proved to be valuable to the research community, both for optimal sensor placement and for interpretation of measured data. In most uncertainty analyses, it is assumed that the air reaching the sampling inlet or sensor is representative of the free stream atmospheric flow .However, the movement of air around an aircraft fuselage can impact not only the flow speed and direction, but the concentrations of various atmospheric constituents as well. The additional uncertainty caused by the fuselage is a function of the location and type of measurement being made. For studies of aerosol particles in clouds and in the atmosphere, for example, a wide range of particle sizes (0.001 to 1000µmin diameter) must be measured accurately. Their distribution at various locations around the aircraft varies significantly from free stream conditions, and CFD is useful for quantifying this discrepancy.
 
For studies of aerosol particles in clouds, the trajectories of particles ranging from 0.1µm (top) to 100µm (bottom) in diameter must be measured accurately |
Airflow Studies over Research Ships
Measurements made from ship borne instruments are biased due to the effect of the ship on the flow of air to the instruments as well as turbulence from the air/water interface. The presence of the ship causes the air flow to a particular instrument site to be either accelerated or decelerated, displaced vertically or, toad lesser degree, in the horizontal direction. Although recognized for some time,it is only recently that the problem has been addressed using 3D CFD models .These simulate the flow over particular ships, quantify the effects of flow distortion, and hence correct the ship-based measurements.
The presence of a research ship can distort the flow around it; contours of pressure coefficient are shown |
An illustration of how CFD has been used to correct ship-based measurements |
Since 1998, Météo-France has used FLUENT to model the flow around one of the research ships operated by a French Institute [3] for various relative wind directions and wind speeds. It has used the resulting estimates of error in the measured wind speeds to correct measurements of fluxes between the air and the sea. Comparison of the data recorded by the ship with the CFD results has suggested that the flow distortion on the measured wind speed is dependent on the incident wind speed. Pathline traces of the predicted flow field, beginning far upwind of the ship and passing through the instrument inlet, allows the vertical displacement of the flow reaching the site to be estimated. These and other CFD results have been useful to researchers who have obtained meteorological measurements from aircraft or ships in the past or to those making comparisons between ground, aircraft, ships, buoys and satellite data systems.
References:
- Fokker-27 was co-funded by Météo-France, the National Center for Scientific Research(CNRS), the French Space Center (CNES) and the National Geographical Institute (IGN).
- Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE,Facility for the French Aircraft Instrumented for Environmental Research), created in 2005 with Météo-France, CNRS and CNES.
- Institut Français de Recherche pour l’Exploitation de la Mer (IFREMER, French Research Institute for Exploitation of the Sea).
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