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- Author or Editor: A. Weill x
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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
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
