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Abstract
The ERS-1 microwave radiometer is dedicated to the correction of the wet tropospheric path delay of the altimeter signal. The accurate location of the microwave footprints is needed because (i) the derived correction must be obtained on the same air column as the altimeter, and (ii) the correction algorithm requires coincident microwave footprints. The authors benefited from the infrared radiometer on board the same platform to test a method for accurately determining the microwave antenna pointing. Preliminary results of this method are presented: although the microwave footprints are more than 20 km wide, their locations are obtained with an accuracy better than 3 km. These locations lie within the preflight tolerances. The microwave-based retrieval algorithms can therefore be used with confidence. This method could be applied to other platforms where both infrared-visible and microwave data are available.
Abstract
The ERS-1 microwave radiometer is dedicated to the correction of the wet tropospheric path delay of the altimeter signal. The accurate location of the microwave footprints is needed because (i) the derived correction must be obtained on the same air column as the altimeter, and (ii) the correction algorithm requires coincident microwave footprints. The authors benefited from the infrared radiometer on board the same platform to test a method for accurately determining the microwave antenna pointing. Preliminary results of this method are presented: although the microwave footprints are more than 20 km wide, their locations are obtained with an accuracy better than 3 km. These locations lie within the preflight tolerances. The microwave-based retrieval algorithms can therefore be used with confidence. This method could be applied to other platforms where both infrared-visible and microwave data are available.
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
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
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 lack of local humidity observations over a large portion of the globe hinders any improvement of humidity forecasting in meteorological models. However, satellite microwave radiometers routinely provide fields of precipitable water content over the means, with a horizontal resolution of a few tens of kilometers. The aim of this paper is therefore to establish the feasibility of the assimilation of microwave radiometer precipitable water content in a forecast model. This preliminary study presents the assimilation method, which is based on optimal interpolation, and results of analysis and forecast runs over 12 h for one case. The Special Sensor Microwave/Imager (SSM/I) radiometer (the scanning microwave radiometer aboard the U.S. Navy Defense Meteorological Satellite Program satellite) and the Prévision à Echéance Rapprochée, Intégrant des Données Observées et Télédetéctées (PERIDOT) model (operational limited-area model of the French Meteorological Office) were used to perform this test. We show that assimilation of SSM/I data has a significant impact on the forecast, since the resulting fields, especially the humidity field, are more coherent with the next SSM/I data and radiosonde observations. The SSM/I assimilation is performed first once only, then throughout a 24-h assimilation cycle. Also, the model sensitivity is established to the vertical error in humidity in the optimal interpolation.
Abstract
The lack of local humidity observations over a large portion of the globe hinders any improvement of humidity forecasting in meteorological models. However, satellite microwave radiometers routinely provide fields of precipitable water content over the means, with a horizontal resolution of a few tens of kilometers. The aim of this paper is therefore to establish the feasibility of the assimilation of microwave radiometer precipitable water content in a forecast model. This preliminary study presents the assimilation method, which is based on optimal interpolation, and results of analysis and forecast runs over 12 h for one case. The Special Sensor Microwave/Imager (SSM/I) radiometer (the scanning microwave radiometer aboard the U.S. Navy Defense Meteorological Satellite Program satellite) and the Prévision à Echéance Rapprochée, Intégrant des Données Observées et Télédetéctées (PERIDOT) model (operational limited-area model of the French Meteorological Office) were used to perform this test. We show that assimilation of SSM/I data has a significant impact on the forecast, since the resulting fields, especially the humidity field, are more coherent with the next SSM/I data and radiosonde observations. The SSM/I assimilation is performed first once only, then throughout a 24-h assimilation cycle. Also, the model sensitivity is established to the vertical error in humidity in the optimal interpolation.
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.
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.
Abstract
The Envisat microwave radiometer is designed to correct the satellite altimeter data for the excess path delay resulting from tropospheric humidity. Neural networks have been used to formulate the inversion algorithm to retrieve this quantity from the measured brightness temperatures. The learning database has been built with European Centre for Medium-Range Weather Forecasts (ECMWF) analyses and simulated brightness temperatures by a radiative transfer model. The in-flight calibration has been performed in a consistent way by adjusting measurements on simulated brightness temperatures. Finally, coincident radiosonde measurements are used to validate the Envisat wet-tropospheric correction, and this comparison shows the good performances of the method.
Abstract
The Envisat microwave radiometer is designed to correct the satellite altimeter data for the excess path delay resulting from tropospheric humidity. Neural networks have been used to formulate the inversion algorithm to retrieve this quantity from the measured brightness temperatures. The learning database has been built with European Centre for Medium-Range Weather Forecasts (ECMWF) analyses and simulated brightness temperatures by a radiative transfer model. The in-flight calibration has been performed in a consistent way by adjusting measurements on simulated brightness temperatures. Finally, coincident radiosonde measurements are used to validate the Envisat wet-tropospheric correction, and this comparison shows the good performances of the method.
Abstract
In this study, the relationship between trains of African easterly waves (AEWs) and downstream tropical cyclogenesis is studied. Based on 19 summer seasons (July–September from 1990 to 2008) of ERA-Interim reanalysis fields and brightness temperature from the Cloud User Archive, the signature of AEW troughs and embedded convection are tracked from the West African coast to the central Atlantic. The tracked systems are separated into four groups: (i) systems originating from the north zone of the midtropospheric African easterly jet (AEJ), (ii) those coming from the south part of AEJ, (iii) systems that are associated with a downstream trough located around 2000 km westward (termed DUO systems), and (iv) those that are not associated with such a close downstream trough (termed SOLO systems).
By monitoring the embedded 700-hPa-filtered relative vorticity and 850-hPa wind convergence anomaly associated with these families along their trajectories, it is shown that the DUO generally have stronger dynamical structure and statistically have a longer lifetime than the SOLO ones. It is suggested that the differences between them may be due to the presence of the previous intense downstream trough in DUO cases, enhancing the low-level convergence behind them. Moreover, a study of the relationship between system trajectories and tropical depressions occurring between the West African coast and 40°W showed that 90% of tropical depressions are identifiable from the West African coast in tracked systems, mostly in the DUO cases originating from the south zone of the AEJ.
Abstract
In this study, the relationship between trains of African easterly waves (AEWs) and downstream tropical cyclogenesis is studied. Based on 19 summer seasons (July–September from 1990 to 2008) of ERA-Interim reanalysis fields and brightness temperature from the Cloud User Archive, the signature of AEW troughs and embedded convection are tracked from the West African coast to the central Atlantic. The tracked systems are separated into four groups: (i) systems originating from the north zone of the midtropospheric African easterly jet (AEJ), (ii) those coming from the south part of AEJ, (iii) systems that are associated with a downstream trough located around 2000 km westward (termed DUO systems), and (iv) those that are not associated with such a close downstream trough (termed SOLO systems).
By monitoring the embedded 700-hPa-filtered relative vorticity and 850-hPa wind convergence anomaly associated with these families along their trajectories, it is shown that the DUO generally have stronger dynamical structure and statistically have a longer lifetime than the SOLO ones. It is suggested that the differences between them may be due to the presence of the previous intense downstream trough in DUO cases, enhancing the low-level convergence behind them. Moreover, a study of the relationship between system trajectories and tropical depressions occurring between the West African coast and 40°W showed that 90% of tropical depressions are identifiable from the West African coast in tracked systems, mostly in the DUO cases originating from the south zone of the AEJ.
Abstract
A large number of Atlantic tropical depressions are generated in the eastern basin in relation to the African easterly wave (AEW) and embedded mesoscale convective systems (MCSs) coming from the African continent. In this paper, the structures of strengthening and dissipating MCSs evolving near the West African coast are analyzed, including the role of the ocean surface conditions in their evolution.
Satellite infrared brightness temperature and meteorological radar data over seven summer seasons between 1993 and 2006 are used to subjectively select 20 cases of strengthening and dissipating MCSs in the vicinity of the Senegal coast. With these observed MCSs, a lagged composite analysis is then performed using Interim ECMWF Re-Analysis (ERA-Interim) and Climate Forecast System Reanalysis (CFSR).
It is shown that the strengthening MCS is generally preceded by prior passage of an AEW near the West African coast. This previous wave trough is associated with a convective cyclonic circulation in the low and middle troposphere, which enhances the southwesterly flow and then provides humidity to the strengthening MCS, located in the vicinity of the subsequent AEW trough. This is favored by the contraction of the wavelength associated with the two troughs. The sea surface contributes to the MCS enhancement through surface evaporation flux. But this contribution is found to be less important than advection of humidity from the previous wave trough. These conditions are almost not found in the dissipating MCS cases, which dissipate in a dry environment dominated by a subsident and anticyclonic circulation, with generally no interaction with a previous wave trough.
Abstract
A large number of Atlantic tropical depressions are generated in the eastern basin in relation to the African easterly wave (AEW) and embedded mesoscale convective systems (MCSs) coming from the African continent. In this paper, the structures of strengthening and dissipating MCSs evolving near the West African coast are analyzed, including the role of the ocean surface conditions in their evolution.
Satellite infrared brightness temperature and meteorological radar data over seven summer seasons between 1993 and 2006 are used to subjectively select 20 cases of strengthening and dissipating MCSs in the vicinity of the Senegal coast. With these observed MCSs, a lagged composite analysis is then performed using Interim ECMWF Re-Analysis (ERA-Interim) and Climate Forecast System Reanalysis (CFSR).
It is shown that the strengthening MCS is generally preceded by prior passage of an AEW near the West African coast. This previous wave trough is associated with a convective cyclonic circulation in the low and middle troposphere, which enhances the southwesterly flow and then provides humidity to the strengthening MCS, located in the vicinity of the subsequent AEW trough. This is favored by the contraction of the wavelength associated with the two troughs. The sea surface contributes to the MCS enhancement through surface evaporation flux. But this contribution is found to be less important than advection of humidity from the previous wave trough. These conditions are almost not found in the dissipating MCS cases, which dissipate in a dry environment dominated by a subsident and anticyclonic circulation, with generally no interaction with a previous wave trough.
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.
