Assessing ocean-model sensitivity to wind forcing uncertainties

International audienceIn this paper, we assess the short-term forecast error of a mesoscale primitive-equation open-ocean model, induced by uncertainties in wind forcing. Statistics calculated from an ensemble of ocean states show that temperature forecast error is strongest at the top of the ensemble-mean thermocline, as a consequence of vertical displacement of the mixed-layer base around its ensemble mean. Horizontal pattern of the temperature error in the mixed-layer is mainly explained by horizontal advection and surface heat flux fluctuations. These two mechanisms and entrainment through the mixed-layer bottom are presented as the three processes responsible for thermal forecast error growth in the modeled upper ocean

Development and Qualification of Instrumented Unmanned Planes for Turbulence Observations in the Atmospheric Surface Layer

The development of new observation systems like drones, present an opportunity to measure differently the turbulence in the atmospheric boundary layer. One of the main advantage of the unmanned plane lies in its capacity to fly at very low heights which is not possible with piloted airplanes, and thus to in situ investigate the turbulence in a way complementary to instrumented towers/masts. In the recent years, we have developed in Toulouse (France) two platforms of different size. The first one, called OVLI-TA, is a small unmanned aerial system (UAS) (3kg, payload included). It is instrumented with a 5-hole probe on the nose of the airplane, a Pitot probe, a fast inertial measurement unit (IMU), a GPS receiver, as well as temperature and moisture sensors in specific housings. After wind tunnel calibrations, the drone’s flight tests were conducted in Lannemezan (France), where there is an equipped 60m tower, which constitutes a reference to our measurements. The drone then participated to the international project DACCIWA (Dynamics-Aerosol-Chemistry-Clouds Interactions In West Africa), in Benin. Moreover, another project is carried out about the instrumentation of a so-called “Boreal” drone, which weights 25 kg and can embark 5 kg of sensors and IMU with data fusion. The scientific payload relates to atmospheric turbulence, GNSS reflectometry and gravimetry. In addition, this UAS has a long endurance (up to 10 h) and is more robust to fly in turbulent conditions. We will present the instrumental packages of the two UASs, the results of qualification flights as well as the first scientific results obtained in the DACCIWA campaign. We will also give some examples of envisaged deployment and observation strategy in future campaigns

Isopycnal Empirical orthogonal Functions (EOFs) in the North and Tropical Atlantic and their use in estimation problems.

International audienceWe investigated the vertical variability of the thermal/dynamical structure of the North and tropical Atlantic ocean from a set of historical hydrographic data [Reynaud et al., 1998]. The analysis was performed in 10° geographic bins in terms of vertical empirical orthogonal functions (EOFs) in isopycnal coordinates as first proposed by Gavart and De Mey [1997] (hereinafter referred to as GDM97). In the isopycnal coordinate system the state vector is made up of isopycnal displacements (η) and the compensated potential temperature along isopycnals (θ). GDM97 showed that in the Azores-Madeira region the isopycnal EOFs consistently help to better separate physical processes in the water column and to better project the dynamic height anomaly downward in comparison with the classical EOF analysis in depth coordinates. In the present study, we extended their work geographically and the connection with state estimation and data assimilation was more formalized. From our results, smooth spatial variations of isopycnal EOFs were found in the North Atlantic and confirmed the large-scale relevance of isopycnal EOFs in the ocean. North of 20°N the dominant isopycnal EOF was associated with a quasihomogeneous vertical displacement of the isopycnal surfaces (η ≃ 1) with a quasiconservation of compensated potential temperature on those surfaces (θ ≃ 0). That mode was strongly modified by thermocline-intensified baroclinic effects along the path of the Gulf Stream up to 40°W. Some physical interpretations of the vertical EOFs were also suggested in several areas as far as the isopycnal formulation was concerned (e.g., some thermal fronts, the Mediterranean outflow, water lenses, or meddies). This work confirmed the validity of the ideas of GDM97 over the entire North Atlantic ocean. The potential use of the isopycnal EOFs in state estimation problems was discussed. They are possible candidates to reduce the order of state estimation problems provided that they have good observability properties. These observability properties were studied in the case of satellite altimetry; that is, we examined how the dominant isopycnal EOFs were connected to sea-level anomalies. In most bins it was found that the order of an estimation problem could be truncated to one (primarily north of 20°N) or two modes. The dominant EOF was found to account for most of the surface dynamic height variability with a rather normally distributed error of only a few centimeters. Dynamically more robust results were also presented, performing a univariate isopycnal analysis in η, leaving coherent thermal effects out. As a result, the dominant character of mode 1 was further enhanced everywhere. These univariate isopycnal EOFs can be used for purely dynamical state estimation. More generally, we believe that our results can be used to set up reduced-order data assimilation schemes in the North Atlantic or in other regions with good historical data

Mesure des flux d’aérosols et d’énergie au-dessus des océans par un drone à grand rayon d’action : le projet MIRIAD

Invited article in La Météorologie: The National Centre for Meteorological Research at Météo-France is leading a new project to study the atmosphere at the interface of Earth’s surface using instrumented drones. The project is titled MIRIAD (Système de Mesures scIentifiques de flux de suRface en mIlieu mAritime embarqué sur Drone). It is conducted in partnership with the Laboratoire d’Aérologie of the Midi-Pyrénées Observatory and AJS, Inc. (a private company based in the Toulouse region and a member of the Aerospace Valley, specialized in the development and construction ofmid-size, long-range UAVs). The project is funded by the Occitanie region and the European Union

Un drone au-dessus des océans : le projet Miriad

Le Centre National de Recherches Météorologiques (CNRM) de Météo-France est porteur d'un projet dédié à l'étude de la couche limite marine de l'atmosphère à l'aide de drones instrumentés : MIRIAD (Système de Mesures scIentifiques de flux de suRface en mIlieu mAritime embarqué sur Drone).International audienc

BOREAL-A Fixed-Wing Unmanned Aerial System for the Measurement of Wind and Turbulence in the Atmospheric Boundary Layer

International audienceAn instrumentation package for wind and turbulence observations in the atmospheric boundary layer on an unmanned aerial vehicle (UAV) called BOREAL has been developed. BOREAL is a fixed-wing UAV built by BOREAL company, which weighs up to 25 kg (5 kg of payload) and has a wingspan of 4.2 m. With a light payload and optimal weather conditions, it has a flight endurance of 9 h. The instrumental payload was designed in order to measure every parameter required for the computation of the three wind components, at a rate of 100 s-1, which is fast enough to capture turbulence fluctuations: a GPS-inertial measurement unit (IMU) platform measures the three components of the groundspeed a well as the attitude angles; the airplane nose has been replaced by a five-hole probe in order to measure the angles of attack and sideslip, according to the so-called radome technique. This probe was calibrated using computational fluid dynamics (CFD) simulations and wind tunnel tests. The remaining instruments are a Pitot tube for static and dynamic pressure measurement and temperature/humidity sensors in dedicated housings. The optimal airspeed at which the vibrations are significantly reduced to an acceptable level was defined from qualification flights. With appropriate flight patterns, the reliability of the mean wind estimates, through self-consistency and comparison with observations performed at 60 m on an instrumented tower could be assessed. Promising first observations of turbulence up to frequencies around 10 Hz and corresponding to a spatial resolution to the order of 3 m are hereby presented

Implementation of a multivariate data assimilation scheme for isopycnic coordinate ocean models: Application to a 1993-96 hindcast of the North Atlantic Ocean circulation

A 1993–1996 hindcast of the North Atlantic circulation is presented and analyzed. Multivariate satellite observations (sea surface height and sea surface temperature) are assimilated into an isopycnic coordinate ocean model (Miami Isopycnic Coordinate Ocean Model [MICOM]) using a reduced order Kalman filter (the SEEK filter). The objective is to validate the assimilation system in hindcast mode before running a near real‐time exercise as part of the European DIADEM project. The analysis scheme is upgraded to make the statistical parameterization as realistic as possible, and this is shown to be particularly important as soon as a multivariate data set is used. Second, the paper describes the modifications to the SEEK filter that are necessary to meet the requirements of a layered model and to benefit from the advantages of the isopycnic coordinate. In particular, the need for an initialization procedure is explained, and this dynamical adjustment is shown to be more natural and robust in the context of the isopycnic coordinate. The hindcast experiment is validated by comparisons with observations, by studying innovation statistics, and by comparing the results with independent observations (expendable bathythermograph [XBT] profiles)