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- Author or Editor: R. Bernard x
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Abstract
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Abstract
Simulated rains are produced by directing a jet of water toward the periphery of an automobile tire that rotates opposite to the flow of water. The air and mechanical turbulence created by the rotating tire surface break the jet up into a spray of water drops. Three rains having rainfall rates of 14, 48 and 2100 mm h−1 were investigated. The results show that these rains have 1) drop-size distributions that exhibit characteristics of those found in natural rains, and 2) drops that fall at approximately terminal velocity.
Abstract
Simulated rains are produced by directing a jet of water toward the periphery of an automobile tire that rotates opposite to the flow of water. The air and mechanical turbulence created by the rotating tire surface break the jet up into a spray of water drops. Three rains having rainfall rates of 14, 48 and 2100 mm h−1 were investigated. The results show that these rains have 1) drop-size distributions that exhibit characteristics of those found in natural rains, and 2) drops that fall at approximately terminal velocity.
Abstract
Nimbus-7 SMMR data of sea surface temperature, surface wind and precipitable water are compared to the ECMWF model daily analyzes for the first Special Observing Period of the FGGE Period (January-February 1979). The comparison of these fields shows some SMMR instrumental problems that were not resolved in the available geophysical data (SST and surface wind fields), but are still in discussion. Problems with analyzed fields are also revealed, particularly concerning the humidity field. The conclusion is that SMMR could not have been directly used for model assimilation, but there is a need for wind and humidity data assimilation. Scatterometers will be necessary for obtaining surface wind, but water vapor from microwave radiometers could help in testing the model hydrological cycle. The surface latent heat flux computed at the first guess of the ECMWF model is also compared to the flux obtained by applying Liu's method from Nimbus-7 SMMR data. Important quantitative differences between the two flux estimates are observed. Liu's method is tested by applying it to the model mean fields. The importance of the humidity field is shown by computing the latent heat flux using the model surface fields and the SMMR precipitable water field.
Abstract
Nimbus-7 SMMR data of sea surface temperature, surface wind and precipitable water are compared to the ECMWF model daily analyzes for the first Special Observing Period of the FGGE Period (January-February 1979). The comparison of these fields shows some SMMR instrumental problems that were not resolved in the available geophysical data (SST and surface wind fields), but are still in discussion. Problems with analyzed fields are also revealed, particularly concerning the humidity field. The conclusion is that SMMR could not have been directly used for model assimilation, but there is a need for wind and humidity data assimilation. Scatterometers will be necessary for obtaining surface wind, but water vapor from microwave radiometers could help in testing the model hydrological cycle. The surface latent heat flux computed at the first guess of the ECMWF model is also compared to the flux obtained by applying Liu's method from Nimbus-7 SMMR data. Important quantitative differences between the two flux estimates are observed. Liu's method is tested by applying it to the model mean fields. The importance of the humidity field is shown by computing the latent heat flux using the model surface fields and the SMMR precipitable water field.
Abstract
The possibility of using infrared surface temperatures from satellites (NOAA, GOES) for inferring daily evaporation and soil moisture distribution over large areas (102 to 105 km2) has been extensively studied during the past few years. The methods are based upon analysis of the surface energy budget, but treating surface transfers as over bare soils. In this context, we have developed a methodology using infrared surface data (from NOAA-7) as input data, in a one-dimensional boundary layer/vegetation/soil model, including a parameterization of transfers within the canopy, based on the formalism of Deardorff which allows the use of a small number of mesoscale surface vegetation parameters.
As shown from the model sensitivity tests, a single surface temperature measured near midday (provided by NOAA-7) is sufficient for obtaining the surface energy fluxes over dense vegetation and for deriving the only governing parameter that remains, the bulk canopy resistance to evaporation, a different concept from moisture availability used for bare soils. The objective of the model in predicting the area-averaged surface fluxes and canopy resistances over dense vegetation is analyzed in conjunction with experimental surface flux measurements for three cases with cloudless NOAA images over a flat monocultural region (the Beauce in France). In the absence of a current capability for routine daily soil moisture observation over an agricultural region, an area-averaged evaluation of the soil moisture can be derived from the canopy resistance obtained by this methodology, using an empirical expression relating this resistance to the root zone water content. Spatial gradient of water content between two areas of Beauce with different soil drainage properties is thus evaluated.
Abstract
The possibility of using infrared surface temperatures from satellites (NOAA, GOES) for inferring daily evaporation and soil moisture distribution over large areas (102 to 105 km2) has been extensively studied during the past few years. The methods are based upon analysis of the surface energy budget, but treating surface transfers as over bare soils. In this context, we have developed a methodology using infrared surface data (from NOAA-7) as input data, in a one-dimensional boundary layer/vegetation/soil model, including a parameterization of transfers within the canopy, based on the formalism of Deardorff which allows the use of a small number of mesoscale surface vegetation parameters.
As shown from the model sensitivity tests, a single surface temperature measured near midday (provided by NOAA-7) is sufficient for obtaining the surface energy fluxes over dense vegetation and for deriving the only governing parameter that remains, the bulk canopy resistance to evaporation, a different concept from moisture availability used for bare soils. The objective of the model in predicting the area-averaged surface fluxes and canopy resistances over dense vegetation is analyzed in conjunction with experimental surface flux measurements for three cases with cloudless NOAA images over a flat monocultural region (the Beauce in France). In the absence of a current capability for routine daily soil moisture observation over an agricultural region, an area-averaged evaluation of the soil moisture can be derived from the canopy resistance obtained by this methodology, using an empirical expression relating this resistance to the root zone water content. Spatial gradient of water content between two areas of Beauce with different soil drainage properties is thus evaluated.
Abstract
A method is proposed for testing microwave measurements from spaceborne sensors by computing collocated simulated brightness temperatures from the ECMWF numerical weather meteorological model using an atmospheric radiative transfer model and the Stogryn model for the surface emissivity. In this paper this method is tested on Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave/Imager (SSM/I) data. The comparison with observed brightness temperature underlines a rather good agreement for SSM/I but a significant discrepancy for SMMR in the vertical polarization, confirming that the SMMR was biased (up to 10 K). Correcting functions are proposed for SMMR 18- and 21-GHz channels, which are validated by comparing Wilheit and Chang's water vapor algorithm results and radiosonde measurements. Small biases in the SSM/I 19- and 22-GHz brightness temperatures comparison can be similarly eliminated. This calibration is validated in the same way by applying water vapor and surface wind retrieval algorithms and comparing the results with corresponding local measurements. The SSM/I algorithms were developed using the same radiative transfer and surface emissivity model, applied to a dataset based on five months of ECMWF analyses. This study establishes the validity of the model, and the reliability of ECMWF analyses, both for validating microwave brightness temperatures and for developing retrieval algorithms in view of future sensors.
Abstract
A method is proposed for testing microwave measurements from spaceborne sensors by computing collocated simulated brightness temperatures from the ECMWF numerical weather meteorological model using an atmospheric radiative transfer model and the Stogryn model for the surface emissivity. In this paper this method is tested on Scanning Multichannel Microwave Radiometer (SMMR) and Special Sensor Microwave/Imager (SSM/I) data. The comparison with observed brightness temperature underlines a rather good agreement for SSM/I but a significant discrepancy for SMMR in the vertical polarization, confirming that the SMMR was biased (up to 10 K). Correcting functions are proposed for SMMR 18- and 21-GHz channels, which are validated by comparing Wilheit and Chang's water vapor algorithm results and radiosonde measurements. Small biases in the SSM/I 19- and 22-GHz brightness temperatures comparison can be similarly eliminated. This calibration is validated in the same way by applying water vapor and surface wind retrieval algorithms and comparing the results with corresponding local measurements. The SSM/I algorithms were developed using the same radiative transfer and surface emissivity model, applied to a dataset based on five months of ECMWF analyses. This study establishes the validity of the model, and the reliability of ECMWF analyses, both for validating microwave brightness temperatures and for developing retrieval algorithms in view of future sensors.
Abstract
The ozone evolution in the lower stratosphere of the Southern Hemisphere during the period 5–10 August 1994 is analyzed. The analysis focuses on the ozone “collar” (the band of maximum values in ozone mixing ratio around the Antarctic ozone “hole” at these altitudes) and the development of “collar filaments.” Ozone mixing ratios provided by the Microwave Limb Sounder (MLS) on board the Upper Atmosphere Research Satellite and by an ER-2 aircraft participating in the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft campaign are compared with values at corresponding locations in high-resolution isentropic maps obtained by using the numerical scheme of “contour advection with surgery” (CAS).
The CAS reconstructed ozone maps provide a view of the way in which air masses are exported from the outskirts of the collar to form the “tongues” of higher mixing ratios observed at lower latitudes on MLS synoptic maps. There is an overall consistency between the datasets insofar as the collar location is concerned. This location seems to be primarily defined by the local properties of the flow. Nevertheless the CAS reconstructed collar tends to become weaker than that depicted by MLS data. By means of radiative calculation estimates, it is argued that diabatic descent may be responsible for maintaining the ozone concentration approximately constant in the collar while filaments isentropically disperse collarlike mixing ratios from this region toward lower latitudes.
Abstract
The ozone evolution in the lower stratosphere of the Southern Hemisphere during the period 5–10 August 1994 is analyzed. The analysis focuses on the ozone “collar” (the band of maximum values in ozone mixing ratio around the Antarctic ozone “hole” at these altitudes) and the development of “collar filaments.” Ozone mixing ratios provided by the Microwave Limb Sounder (MLS) on board the Upper Atmosphere Research Satellite and by an ER-2 aircraft participating in the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft campaign are compared with values at corresponding locations in high-resolution isentropic maps obtained by using the numerical scheme of “contour advection with surgery” (CAS).
The CAS reconstructed ozone maps provide a view of the way in which air masses are exported from the outskirts of the collar to form the “tongues” of higher mixing ratios observed at lower latitudes on MLS synoptic maps. There is an overall consistency between the datasets insofar as the collar location is concerned. This location seems to be primarily defined by the local properties of the flow. Nevertheless the CAS reconstructed collar tends to become weaker than that depicted by MLS data. By means of radiative calculation estimates, it is argued that diabatic descent may be responsible for maintaining the ozone concentration approximately constant in the collar while filaments isentropically disperse collarlike mixing ratios from this region toward lower latitudes.
Abstract
Recent analyses of global climate models suggest that uncertainty in the coupling between midlatitude clouds and the atmospheric circulation contributes to uncertainty in climate sensitivity. However, the reasons behind model differences in the cloud–circulation coupling have remained unclear. Here, we use a global climate model in an idealized aquaplanet setup to show that the Southern Hemisphere climatological circulation, which in many models is biased equatorward, contributes to the model differences in the cloud–circulation coupling. For the same poleward shift of the Hadley cell (HC) edge, models with narrower climatological HCs exhibit stronger midlatitude cloud-induced shortwave warming than models with wider climatological HCs. This cloud-induced radiative warming results predominantly from a subsidence warming that decreases cloud fraction and is stronger for narrower HCs because of a larger meridional gradient in the vertical velocity. A comparison of our aquaplanet results with comprehensive climate models suggests that about half of the model uncertainty in the midlatitude cloud–circulation coupling stems from this impact of the circulation on the large-scale temperature structure of the atmosphere, and thus could be removed by improving the climatological circulation in models. This illustrates how understanding of large-scale dynamics can help reduce uncertainty in clouds and their response to climate change.
Abstract
Recent analyses of global climate models suggest that uncertainty in the coupling between midlatitude clouds and the atmospheric circulation contributes to uncertainty in climate sensitivity. However, the reasons behind model differences in the cloud–circulation coupling have remained unclear. Here, we use a global climate model in an idealized aquaplanet setup to show that the Southern Hemisphere climatological circulation, which in many models is biased equatorward, contributes to the model differences in the cloud–circulation coupling. For the same poleward shift of the Hadley cell (HC) edge, models with narrower climatological HCs exhibit stronger midlatitude cloud-induced shortwave warming than models with wider climatological HCs. This cloud-induced radiative warming results predominantly from a subsidence warming that decreases cloud fraction and is stronger for narrower HCs because of a larger meridional gradient in the vertical velocity. A comparison of our aquaplanet results with comprehensive climate models suggests that about half of the model uncertainty in the midlatitude cloud–circulation coupling stems from this impact of the circulation on the large-scale temperature structure of the atmosphere, and thus could be removed by improving the climatological circulation in models. This illustrates how understanding of large-scale dynamics can help reduce uncertainty in clouds and their response to climate change.
Abstract
This paper compares surface sensible heat flux and soil moisture values derived by inverting two boundary layers models with a surface/vegetation formulation, using surface temperature measurements made from NOAA-7 satellite (the AVHRR) with measured values for a wheat-growing area of the Beauce in France. The vegetation parameterization enables the models to reproduce the dramatic increase in surface sensible heat flux and decrease in soil moisture which occurred over a 5-day period during the field experiment. A bare soil model proved incapable of capturing the increase of the sensible heat flux during the 5-day period even though it yielded similar values of root-zone moisture.
The vegetation model responds sensitively to small changes in canopy temperature by producing large changes in surface sensible heat flux due to the parameterization of the foliage resistance and the fact that the foliage is considered a layer of zero thermal inertia. Both the vegetation and bare soil models showed a continuous moisture decrease to values near or below the wilting point in the upper part of the root zone.
The sensitivity of the results to errors in the initial sounding values or measured surface temperature were tested by varying the initial sounding temperature, dewpoint and windspeed, and the measured surface temperature by amounts corresponding to typical measurement error. Accordingly, we found that an unlucky combination of such errors can totally mask even large variations in surface heat flux from day to day, such as was measured during the field experiment. The vegetation component, therefore, is apparently more sensitive to error than the bare soil model.
Abstract
This paper compares surface sensible heat flux and soil moisture values derived by inverting two boundary layers models with a surface/vegetation formulation, using surface temperature measurements made from NOAA-7 satellite (the AVHRR) with measured values for a wheat-growing area of the Beauce in France. The vegetation parameterization enables the models to reproduce the dramatic increase in surface sensible heat flux and decrease in soil moisture which occurred over a 5-day period during the field experiment. A bare soil model proved incapable of capturing the increase of the sensible heat flux during the 5-day period even though it yielded similar values of root-zone moisture.
The vegetation model responds sensitively to small changes in canopy temperature by producing large changes in surface sensible heat flux due to the parameterization of the foliage resistance and the fact that the foliage is considered a layer of zero thermal inertia. Both the vegetation and bare soil models showed a continuous moisture decrease to values near or below the wilting point in the upper part of the root zone.
The sensitivity of the results to errors in the initial sounding values or measured surface temperature were tested by varying the initial sounding temperature, dewpoint and windspeed, and the measured surface temperature by amounts corresponding to typical measurement error. Accordingly, we found that an unlucky combination of such errors can totally mask even large variations in surface heat flux from day to day, such as was measured during the field experiment. The vegetation component, therefore, is apparently more sensitive to error than the bare soil model.
Abstract
The Moderate Resolution Imaging Spectroradiometer (MODIS) white-sky surface albedos are compared with similar products generated on the basis of the Multiangle Imaging SpectroRadiometer (MISR) surface bidirectional reflectance factor (BRF) model parameters available for the year 2005. The analysis is achieved using global-scale statistics to characterize the broad patterns of these two independent albedo datasets. The results obtained in M. Taberner et al. have shown that robust statistics can be established and that both datasets are highly correlated. As a result, the slight but consistent biases and trends identified in this paper, derived from statistics obtained on a global basis, should be considered sufficiently reliable to merit further investigation. The present paper reports on the zonal- and seasonal-mean differences retrieved from the analysis of the MODIS and MISR surface albedo broadband products. The MISR − MODIS differences exhibit a systematic positive bias or offset in the range of 0.01–0.03 depending on the spectral domain of interest. Results obtained in the visible domain exhibit a well-marked and very consistent meridional trend featuring a “smile effect” such that the MISR − MODIS differences reach maxima at the highest latitudes in both hemispheres. The analysis of seasonal variations observed in MISR and MODIS albedo products reveals that, in the visible domain, the MODIS albedos generate weaker seasonal changes than MISR and that the differences increase poleward from the equatorial regions. A detailed investigation of MODIS and MISR aerosol optical depth retrievals suggests that this large-scale meridional trend is probably not caused by differences in the aerosol load estimated by each instrument. The scale and regularity of the meridional trend suggests that this may be due to the particular sampling regime of each instrument in the viewing azimuthal planes and/or approximations in the atmospheric correction processes. If this is the case, then either MODIS is underestimating, or MISR overestimating, the surface anisotropy or both.
Abstract
The Moderate Resolution Imaging Spectroradiometer (MODIS) white-sky surface albedos are compared with similar products generated on the basis of the Multiangle Imaging SpectroRadiometer (MISR) surface bidirectional reflectance factor (BRF) model parameters available for the year 2005. The analysis is achieved using global-scale statistics to characterize the broad patterns of these two independent albedo datasets. The results obtained in M. Taberner et al. have shown that robust statistics can be established and that both datasets are highly correlated. As a result, the slight but consistent biases and trends identified in this paper, derived from statistics obtained on a global basis, should be considered sufficiently reliable to merit further investigation. The present paper reports on the zonal- and seasonal-mean differences retrieved from the analysis of the MODIS and MISR surface albedo broadband products. The MISR − MODIS differences exhibit a systematic positive bias or offset in the range of 0.01–0.03 depending on the spectral domain of interest. Results obtained in the visible domain exhibit a well-marked and very consistent meridional trend featuring a “smile effect” such that the MISR − MODIS differences reach maxima at the highest latitudes in both hemispheres. The analysis of seasonal variations observed in MISR and MODIS albedo products reveals that, in the visible domain, the MODIS albedos generate weaker seasonal changes than MISR and that the differences increase poleward from the equatorial regions. A detailed investigation of MODIS and MISR aerosol optical depth retrievals suggests that this large-scale meridional trend is probably not caused by differences in the aerosol load estimated by each instrument. The scale and regularity of the meridional trend suggests that this may be due to the particular sampling regime of each instrument in the viewing azimuthal planes and/or approximations in the atmospheric correction processes. If this is the case, then either MODIS is underestimating, or MISR overestimating, the surface anisotropy or both.
