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G. Caniaux, L. Prieur, H. Giordani, F. Hernandez, and L. Eymard

Abstract

A hydrographic survey was performed in January–February 1997 to document the winter circulation of the North Atlantic Current system in the Newfoundland Basin, as part of the Fronts and Atlantic Storm Tracks Ex-periment (FASTEX). Eighty-seven conductivity–temperature–depth (CTD) stations were occupied along a four-section trapezoid, which spanned the “Northwest Corner” and the branching of the North Atlantic Current along 35°W. Realistic sea surface temperature analyses were produced every 15 days, using all available data collected in this area during the two months of the FASTEX experiment. These maps were combined with sea level anomaly fields from the TOPEX/Poseidon and ERS-2 satellites at the same time intervals to analyze the features of the main currents in the area and their evolution. These combined analyses, providing a coherent overview of the fronts and jets identified along the ship track, and the CTD stations are further used to estimate their transports. The general pattern is a 15 Sv (Sv ≡ 106 m3 s−1) transport by the North Atlantic Current at 47°N, 43°W, the existence of a recirculating gyre inside the Northwest Corner, and a complex branching of the circulation associated with significant surface fronts. The recirculating gyre forms a closed circulation, in which a very deep warm eddy, 100 km wide, was sampled at the end of February: its mixed layer was 800 m deep and its transport was 27 Sv. Along 35°W, three fronts were identified between 45° and 52°N: the Northern Subarctic Front, the Southern Subarctic Front, and the Mid-Atlantic Front, whose origins are precisely located. The currents associated with these fronts transport 26 Sv toward the east before crossing the Mid-Atlantic Ridge and supplying the eastern part of the North Atlantic basin. An important transport (14 Sv) was calculated near 46°N, 37°W, which mostly fed the current associated with the Mid-Atlantic Front.

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J-F. Mahfouf, A. O. Manzi, J. Noilhan, H. Giordani, and M. DéQué

Abstract

This paper describes recent developments in climate modeling at Météo-France related to land surface processes. The implementation of a simple land surface parameterization, Interactions between Soil Biosphere Atmosphere (ISBA), has gained from previous validations and calibrations at local scale against field datasets and from aggregation procedures devised to define effective land surface properties. Specific improvements for climate purposes are introduced: spatial variability of convective rainfall in canopy drainage estimation and subsurface gravitational percolation. The methodology used to derive climatological maps of land surface parameters at the grid-scale resolution of the model from existing database for soil and vegetation types at global scale is described. A 3-yr integration for the present day climate with a T42L30 version of the climate model has been performed. Results obtained compare favorably with available observed climatologies related to the various components of the continental surface energy and water budgets. Differences are due mostly to a poor simulation of the precipitation field. However, some differences suggest specific improvements in the surface scheme concerning representation of the bare soil albedo, the surface runoff, and the soil moisture initialization. As a first step prior to tropical deforestation experiments presented in Part II, regional analyses over the Amazon forest indicate that the modeled evaporation and net radiation are in good agreement with data collected during the Amazon Region Micrometeorological Experiment campaign.

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A. Weill, L. Eymard, G. Caniaux, D. Hauser, S. Planton, H. Dupuis, A. Brut, C. Guerin, P. Nacass, A. Butet, S. Cloché, R. Pedreros, P. Durand, D. Bourras, H. Giordani, G. Lachaud, and G. Bouhours

Abstract

An accurate determination of turbulent exchanges between the ocean and the atmosphere is a prerequisite to identify and assess the mechanisms of interaction that control part of the variability in the two media over a wide range of spatial and temporal scales. An extended dataset for estimating air–sea fluxes (representing nearly 5700 h of turbulence measurements) has been collected since 1992 during six dedicated experiments performed in the Atlantic Ocean and the Mediterranean Sea. This paper presents the methodology used through the successive experiments to progress in this field. The major developments concern (i) flux instrumentation, with the deployment of a microwave refractometer to get the latent heat flux in most meteorological conditions; (ii) the analysis of airflow distortion effects around the ship structure and sensors through both computational fluid dynamics and physical simulations in a water tank, then the derivation of correction for these effects; (iii) the application of both inertial dissipation and eddy-correlation methods from the various experiments, allowing the authors to assess and discuss flux-determination methods on ships, and particularly bulk parameterization; (iv) the validation and analysis of mesoscale surface flux fields from models and satellites by using ship data, showing some deficiencies in operational model fields from ECMWF, the need of high-quality fluxes to interpret ocean–atmosphere exchanges, and the potential advantage of satellite retrieval methods. Further analysis of these datasets is being performed in a unique database (the ALBATROS project, open to the international scientific community). It will include refinement of airflow distortion correction and reprocessing of earlier datasets, the investigation of fluxes under specific conditions (low wind), and the effect of sea state among others. It will also contribute to further validation and improvements of satellite retrievals in various climatic/meteorological conditions.

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