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Abstract
Analyses of mean horizontal wind speed from four Doppler sodars separated by 15 to 38 km in the Gers region during the Mesogers 84 experiment gave information on the horizontal wind speed spectral behavior of the so-called mesoscale turbulence during 3 days of fair weather situations. The −5/3; spectral behavior is obtained. A study of the validity of the Taylor hypothesis at this scale shows that for the scales that are considered, this hypothesis is reasonable, but poses difficult questions about the transport speed, which seems to be close to the mean wind speed spatially averaged over the four sodars.
Vorticity spectra are found to support a vortical mode related to quasi-two-dimensional turbulence rather than due to purely internal inertia-gravity wave interactions.
Abstract
Analyses of mean horizontal wind speed from four Doppler sodars separated by 15 to 38 km in the Gers region during the Mesogers 84 experiment gave information on the horizontal wind speed spectral behavior of the so-called mesoscale turbulence during 3 days of fair weather situations. The −5/3; spectral behavior is obtained. A study of the validity of the Taylor hypothesis at this scale shows that for the scales that are considered, this hypothesis is reasonable, but poses difficult questions about the transport speed, which seems to be close to the mean wind speed spatially averaged over the four sodars.
Vorticity spectra are found to support a vortical mode related to quasi-two-dimensional turbulence rather than due to purely internal inertia-gravity wave interactions.
Abstract
In August 1979, the Dual Doppler Radar (Ronsard System) and the Doppler Sodar system of CNET were simultaneously used during an experiment on clear air convection. It provided the opportunity to develop a method for the spatial study of atmospheric structures in the PBL: the mean characteristics of the flow structures at scales larger than one kilometer (wind velocity and variance profiles, one-dimensional velocity spectra) are studied. Spatial properties of the atmosphere are then investigated, showing a heterogeneity of the horizontal wind field inside the observed area. Finally, a predominant alignment of the convective cells along the same direction is found in every case.
Abstract
In August 1979, the Dual Doppler Radar (Ronsard System) and the Doppler Sodar system of CNET were simultaneously used during an experiment on clear air convection. It provided the opportunity to develop a method for the spatial study of atmospheric structures in the PBL: the mean characteristics of the flow structures at scales larger than one kilometer (wind velocity and variance profiles, one-dimensional velocity spectra) are studied. Spatial properties of the atmosphere are then investigated, showing a heterogeneity of the horizontal wind field inside the observed area. Finally, a predominant alignment of the convective cells along the same direction is found in every case.
Abstract
This study concerns the evaluation of the boundary layer (BL) subgrid parameterization of a numerical weather forecast model on the synoptic scale. The method presented aims at separating the two possible origins of model defficiencies in representing a cloud-topped boundary layer: (i) large-scale data assimilation issues, and (ii) BL parameterization. The method combines two sources of data: model analyzed fields from the SEMAPHORE field experiment, and the Advanced Very High Resolution Radiometer (AVHRR) dataset from the NOAA-11 and -12 Satellite. The application focuses on an anticyclonic period during field experiment (10–17 November 1993), for which a special version of ARPEGE—The Météo-France Numerical Weather Prediction model—is used to analyze the data from the field experiment.
In the proposed method, the boundary layer is globally characterized by its height which is converted to an inversion layer temperature in the model. Low-level cloud-top temperatures of optically thick clouds are inferred from the satellite radiometers. The model and satellite-retrieved temperature fields are two independent fields that are used together to select regions for which spatial variations of boundary layer cloud-top temperature are approximately in phase. These regions are assumed to have correctly assimilated the large-scale data fields. It is found that they cover a significant part of the analyzed synoptic situations. In the selected regions, the two temperature fields can be examined to evaluate BL schemes and obtain insight on the physical processes responsible for cloudiness.
Abstract
This study concerns the evaluation of the boundary layer (BL) subgrid parameterization of a numerical weather forecast model on the synoptic scale. The method presented aims at separating the two possible origins of model defficiencies in representing a cloud-topped boundary layer: (i) large-scale data assimilation issues, and (ii) BL parameterization. The method combines two sources of data: model analyzed fields from the SEMAPHORE field experiment, and the Advanced Very High Resolution Radiometer (AVHRR) dataset from the NOAA-11 and -12 Satellite. The application focuses on an anticyclonic period during field experiment (10–17 November 1993), for which a special version of ARPEGE—The Météo-France Numerical Weather Prediction model—is used to analyze the data from the field experiment.
In the proposed method, the boundary layer is globally characterized by its height which is converted to an inversion layer temperature in the model. Low-level cloud-top temperatures of optically thick clouds are inferred from the satellite radiometers. The model and satellite-retrieved temperature fields are two independent fields that are used together to select regions for which spatial variations of boundary layer cloud-top temperature are approximately in phase. These regions are assumed to have correctly assimilated the large-scale data fields. It is found that they cover a significant part of the analyzed synoptic situations. In the selected regions, the two temperature fields can be examined to evaluate BL schemes and obtain insight on the physical processes responsible for cloudiness.
Abstract
Doppler sodar information on the wind field in relation to frontal slope observed on reflectivity facsimile records was considered during Mesogers 84 experiment. The data have been analyzed to examine and quantify the correlation between vertical (updraft/downdraft) velocities and the frictional velocity in the surface layer. Based on theoretical considerations, the frontal slope modifications due to divergence of frontal friction have been quantified, and the associated drag coefficients are tentatively established. It has been observed that the downdraft velocities tend to increase with the horizontal wind speed in the surface layer. Frontal slope is directly proportional to the magnitude of momentum transfer (in the direction of front propagation) caused by friction velocity in the surface layer. Estimated values of drag are realistic.
Abstract
Doppler sodar information on the wind field in relation to frontal slope observed on reflectivity facsimile records was considered during Mesogers 84 experiment. The data have been analyzed to examine and quantify the correlation between vertical (updraft/downdraft) velocities and the frictional velocity in the surface layer. Based on theoretical considerations, the frontal slope modifications due to divergence of frontal friction have been quantified, and the associated drag coefficients are tentatively established. It has been observed that the downdraft velocities tend to increase with the horizontal wind speed in the surface layer. Frontal slope is directly proportional to the magnitude of momentum transfer (in the direction of front propagation) caused by friction velocity in the surface layer. Estimated values of drag are realistic.
Abstract
Satellite platforms NOAA-11 and -12 Advanced Very High Resolution Radiometer (AVHRR) data are used during the daytime to study large sheets of stratocumulus over the North Atlantic Ocean. The application concerns an anticyclonic period of the Structure des Echanges Mer–Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherché Expérimentale (SEMAPHORE) campaign (10–17 November 1993). In the region of interest, the satellite images are recorded under large solar zenith angles. Extending the SEMAPHORE area, a region of about 3000 × 3000 km2 is studied to characterize the atmospheric boundary layer. A statistical cloud classification method is applied to discriminate for low-level and optically thick clouds. For AVHRR pixels covered with thick clouds, brightness temperatures are used to evaluate the boundary layer cloud-top temperature (CTT). The objective is to obtain accurate CTT maps for evaluation of a global model. In this application, the full-resolution fields are reduced to match model grid size. An estimate of overall temperature uncertainty associated with each grid point is also derived, which incorporates subgrid variability of the fields and quality of the temperature retrieval. Results are compared with the SEMAPHORE campaign measurements. A comparison with “DX” products obtained with the same dataset, but at lower resolution, is also presented. The authors claim that such instantaneous CTT maps could be as intensively used as classical SST maps, and both could be efficiently complemented with gridpoint error-bar maps. They may be used for multiple applications: (i) to provide a means to improve numerical weather prediction and climatological reanalyses, (ii) to represent a boundary layer global characterization to analyze the synoptic situation of field experiments, and (iii) to allow validation and to test development of large-scale and mesoscale models.
Abstract
Satellite platforms NOAA-11 and -12 Advanced Very High Resolution Radiometer (AVHRR) data are used during the daytime to study large sheets of stratocumulus over the North Atlantic Ocean. The application concerns an anticyclonic period of the Structure des Echanges Mer–Atmosphère, Propriétés des Hétérogénéités Océaniques: Recherché Expérimentale (SEMAPHORE) campaign (10–17 November 1993). In the region of interest, the satellite images are recorded under large solar zenith angles. Extending the SEMAPHORE area, a region of about 3000 × 3000 km2 is studied to characterize the atmospheric boundary layer. A statistical cloud classification method is applied to discriminate for low-level and optically thick clouds. For AVHRR pixels covered with thick clouds, brightness temperatures are used to evaluate the boundary layer cloud-top temperature (CTT). The objective is to obtain accurate CTT maps for evaluation of a global model. In this application, the full-resolution fields are reduced to match model grid size. An estimate of overall temperature uncertainty associated with each grid point is also derived, which incorporates subgrid variability of the fields and quality of the temperature retrieval. Results are compared with the SEMAPHORE campaign measurements. A comparison with “DX” products obtained with the same dataset, but at lower resolution, is also presented. The authors claim that such instantaneous CTT maps could be as intensively used as classical SST maps, and both could be efficiently complemented with gridpoint error-bar maps. They may be used for multiple applications: (i) to provide a means to improve numerical weather prediction and climatological reanalyses, (ii) to represent a boundary layer global characterization to analyze the synoptic situation of field experiments, and (iii) to allow validation and to test development of large-scale and mesoscale models.
Abstract
The vertical velocity variance during convective activity was measured with an acoustic Doppler sounder. Here we show that this technique provides measurement of the heat flux profile in the well-mixed surface layer and gives a reference height h′ related to the inversion height ZI during the mornings.
The paper also shows that the structure function of temperature fluctuations
Abstract
The vertical velocity variance during convective activity was measured with an acoustic Doppler sounder. Here we show that this technique provides measurement of the heat flux profile in the well-mixed surface layer and gives a reference height h′ related to the inversion height ZI during the mornings.
The paper also shows that the structure function of temperature fluctuations
Abstract
The present paper describes a new type of floating platform that was specifically designed for estimating air–sea fluxes, investigating turbulence characteristics in the atmospheric surface boundary layer, and studying wind–wave interactions. With its design, it can be deployed in the open ocean or in shallow-water areas. The system is designed to be used from a research vessel. It can operate for ~10 h as a drifting wave rider and 3 h under power. Turbulence and meteorological instrument packages are placed at a low altitude (1–1.5 m). It was deployed for validation purposes during the Front de Marée, Variabilité (FROMVAR), 2011 experiment off the west coast of Brittany, France. Wind friction velocity and surface turbulent buoyancy flux were estimated using eddy covariance, spectral, bulk, and profile methods. The comparisons of the four methods show a reasonable agreement except for the spectral buoyancy flux. This suggests that the platform design is correct. Also, the wind measured at a fixed height above the sea shows spectral coherence with wave heights, such that wind and swell are in phase, with the largest wind values on top of swell crests. This result in qualitative agreement with current model predictions supports the capability of the Ocean Coupled to Atmosphere, Research at the Interface with a Novel Autonomous platform (OCARINA) to investigate wind–swell interactions.
Abstract
The present paper describes a new type of floating platform that was specifically designed for estimating air–sea fluxes, investigating turbulence characteristics in the atmospheric surface boundary layer, and studying wind–wave interactions. With its design, it can be deployed in the open ocean or in shallow-water areas. The system is designed to be used from a research vessel. It can operate for ~10 h as a drifting wave rider and 3 h under power. Turbulence and meteorological instrument packages are placed at a low altitude (1–1.5 m). It was deployed for validation purposes during the Front de Marée, Variabilité (FROMVAR), 2011 experiment off the west coast of Brittany, France. Wind friction velocity and surface turbulent buoyancy flux were estimated using eddy covariance, spectral, bulk, and profile methods. The comparisons of the four methods show a reasonable agreement except for the spectral buoyancy flux. This suggests that the platform design is correct. Also, the wind measured at a fixed height above the sea shows spectral coherence with wave heights, such that wind and swell are in phase, with the largest wind values on top of swell crests. This result in qualitative agreement with current model predictions supports the capability of the Ocean Coupled to Atmosphere, Research at the Interface with a Novel Autonomous platform (OCARINA) to investigate wind–swell interactions.
Abstract
After a brief description of humidity measurement and a short presentation of methods of microwave refractometry for evaporation flux, a new X-band refractometer system is presented. Based on a new design and a new material for the microwave cavity, it does not need calibration for refractive index variations because of its reduced thermal time constant.
The new device has been combined with a sonic anemometer and traditional mean meteorological measurements on a 12-m shipborne mast. It has been found to be very efficient for obtaining humidity fluctuations and fluxes in the CATCH 97 (Couplage avec l’ATmosphère en Conditions Hivernales) and FETCH 98 (Flux, Etat de la mer et Télédétection en condition de fetCH variable) experiments under various wind and stability conditions. The inertial subrange is of very high quality. To first order, the evaporation flux and refractive index flux are very similar. In extreme meteorological conditions, such as those encountered during CATCH, the sensible heat flux contribution must be determined independently.
A great advantage of the system is that the contamination by salt, as is typical for other devices at sea, has been found to be negligible for the conditions encountered.
Abstract
After a brief description of humidity measurement and a short presentation of methods of microwave refractometry for evaporation flux, a new X-band refractometer system is presented. Based on a new design and a new material for the microwave cavity, it does not need calibration for refractive index variations because of its reduced thermal time constant.
The new device has been combined with a sonic anemometer and traditional mean meteorological measurements on a 12-m shipborne mast. It has been found to be very efficient for obtaining humidity fluctuations and fluxes in the CATCH 97 (Couplage avec l’ATmosphère en Conditions Hivernales) and FETCH 98 (Flux, Etat de la mer et Télédétection en condition de fetCH variable) experiments under various wind and stability conditions. The inertial subrange is of very high quality. To first order, the evaporation flux and refractive index flux are very similar. In extreme meteorological conditions, such as those encountered during CATCH, the sensible heat flux contribution must be determined independently.
A great advantage of the system is that the contamination by salt, as is typical for other devices at sea, has been found to be negligible for the conditions encountered.
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.
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.
The Vasco-Cirene program explores how strong air-sea interactions promoted by the shallow thermocline and high sea surface temperature in the Seychelles-Chagos thermocline ridge results in marked variability at synoptic, intraseasonal, and interannual time scales. The Cirene oceanographic cruise collected oceanic, atmospheric, and air-sea flux observations in this region in January–February 2007. The contemporaneous Vasco field experiment complemented these measurements with balloon deployments from the Seychelles. Cirene also contributed to the development of the Indian Ocean observing system via deployment of a mooring and 12 Argo profilers.
Unusual conditions prevailed in the Indian Ocean during January and February 2007, following the Indian Ocean dipole climate anomaly of late 2006. Cirene measurements show that the Seychelles-Chagos thermocline ridge had higher-than-usual heat content with subsurface anomalies up to 7°C. The ocean surface was warmer and fresher than average, and unusual eastward currents prevailed down to 800 m. These anomalous conditions had a major impact on tuna fishing in early 2007. Our dataset also sampled the genesis and maturation of Tropical Cyclone Dora, including high surface temperatures and a strong diurnal cycle before the cyclone, followed by a 1.5°C cooling over 10 days. Balloonborne instruments sampled the surface and boundary layer dynamics of Dora. We observed small-scale structures like dry-air layers in the atmosphere and diurnal warm layers in the near-surface ocean. The Cirene data will quantify the impact of these finescale features on the upper-ocean heat budget and atmospheric deep convection.
The Vasco-Cirene program explores how strong air-sea interactions promoted by the shallow thermocline and high sea surface temperature in the Seychelles-Chagos thermocline ridge results in marked variability at synoptic, intraseasonal, and interannual time scales. The Cirene oceanographic cruise collected oceanic, atmospheric, and air-sea flux observations in this region in January–February 2007. The contemporaneous Vasco field experiment complemented these measurements with balloon deployments from the Seychelles. Cirene also contributed to the development of the Indian Ocean observing system via deployment of a mooring and 12 Argo profilers.
Unusual conditions prevailed in the Indian Ocean during January and February 2007, following the Indian Ocean dipole climate anomaly of late 2006. Cirene measurements show that the Seychelles-Chagos thermocline ridge had higher-than-usual heat content with subsurface anomalies up to 7°C. The ocean surface was warmer and fresher than average, and unusual eastward currents prevailed down to 800 m. These anomalous conditions had a major impact on tuna fishing in early 2007. Our dataset also sampled the genesis and maturation of Tropical Cyclone Dora, including high surface temperatures and a strong diurnal cycle before the cyclone, followed by a 1.5°C cooling over 10 days. Balloonborne instruments sampled the surface and boundary layer dynamics of Dora. We observed small-scale structures like dry-air layers in the atmosphere and diurnal warm layers in the near-surface ocean. The Cirene data will quantify the impact of these finescale features on the upper-ocean heat budget and atmospheric deep convection.
