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- Author or Editor: Simon Chang x
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Abstract
An efficient, multilayer model for predicting the diurnal variations in the thermal and momentum fields in the planetary boundary layer (PBL) is proposed for incorporating into mesoscale or large-scale dynamical models. The ground temperature is given by a soil slab heated (or cooled) by net radiation and sensible heat from the atmospheric surface layer and a ground thermal reservoir. The surface heat flux can be generated by two mechanisms: 1) the convective mixing depending on the temperature difference between the ground and the screen level and 2) the mechanical mixing depending on the wind stress. Following Blackadar (1976), a prediction equation is employed for the screen-level temperature. In the PBL, the heat and momentum exchanges are computed by a Richardson number adjustment scheme. Heat and momentum exchanges occur mainly due to thermal instability under convectively unstable conditions and due to shear instability under convectively stable conditions. A case study shows good agreement between model results and observation. Additional experiments are performed to test the scheme under calm and stronger wind situations. Since no explicit diffusion coefficient is needed in the adjustment scheme, the model time step is not restricted by computational stability requirements of the diffusion term. This PBL parameterization scheme is therefore very appealing for use in numerical models that use large time steps yet have good vertical resolutions in the PBL.
Abstract
An efficient, multilayer model for predicting the diurnal variations in the thermal and momentum fields in the planetary boundary layer (PBL) is proposed for incorporating into mesoscale or large-scale dynamical models. The ground temperature is given by a soil slab heated (or cooled) by net radiation and sensible heat from the atmospheric surface layer and a ground thermal reservoir. The surface heat flux can be generated by two mechanisms: 1) the convective mixing depending on the temperature difference between the ground and the screen level and 2) the mechanical mixing depending on the wind stress. Following Blackadar (1976), a prediction equation is employed for the screen-level temperature. In the PBL, the heat and momentum exchanges are computed by a Richardson number adjustment scheme. Heat and momentum exchanges occur mainly due to thermal instability under convectively unstable conditions and due to shear instability under convectively stable conditions. A case study shows good agreement between model results and observation. Additional experiments are performed to test the scheme under calm and stronger wind situations. Since no explicit diffusion coefficient is needed in the adjustment scheme, the model time step is not restricted by computational stability requirements of the diffusion term. This PBL parameterization scheme is therefore very appealing for use in numerical models that use large time steps yet have good vertical resolutions in the PBL.
Abstract
In this paper a simple model of the planetary boundary layer (PBL) is proposed. The surface layer is modeled according to established similarity theory. Above the surface layer a prognostic equation for the mixing length is introduced. The time-dependent mixing length is a function of the PBL characteristics, including the height of the capping inversion, the local friction velocity and the surface heat flux. In a preliminary experiment, the behavior of the PBL is compared with observations from the Great Plains Experiment.
Abstract
In this paper a simple model of the planetary boundary layer (PBL) is proposed. The surface layer is modeled according to established similarity theory. Above the surface layer a prognostic equation for the mixing length is introduced. The time-dependent mixing length is a function of the PBL characteristics, including the height of the capping inversion, the local friction velocity and the surface heat flux. In a preliminary experiment, the behavior of the PBL is compared with observations from the Great Plains Experiment.
Abstract
The Nimbus-7 Total Ozone Mapping spectrometer (TOMS) was used to map the distribution of total Ozone in and around western Atlantic tropical cyclones from 1979 to 1982. It was found that the TOMS-observed total Ozone distribution within the subtropics during the tropical cyclone seasonal correlated well with the tropopause topoghraphy, similar to earlier middle-latitudinal observations. This relationship made it possible to use TOMS to monitor the propagation of upper-tropospheric subtropical transient waves and the mutual adjustment between the tropical cyclone and the upper-tropospheric waves during their interaction. These total ozone patterns reflected the three-dimensional upper-tropospheric transport processes that were conducive for storm intensification and weakening. It was also found from satellite observations and numerical model simulations that modification of the environmental distribution of total ozone by the tropical cyclones was primarily caused by the secondary circulation associated with the tropical cyclone's outflow jet and the intrusion of stratospheric air in the eyes of tropical cyclones.
Abstract
The Nimbus-7 Total Ozone Mapping spectrometer (TOMS) was used to map the distribution of total Ozone in and around western Atlantic tropical cyclones from 1979 to 1982. It was found that the TOMS-observed total Ozone distribution within the subtropics during the tropical cyclone seasonal correlated well with the tropopause topoghraphy, similar to earlier middle-latitudinal observations. This relationship made it possible to use TOMS to monitor the propagation of upper-tropospheric subtropical transient waves and the mutual adjustment between the tropical cyclone and the upper-tropospheric waves during their interaction. These total ozone patterns reflected the three-dimensional upper-tropospheric transport processes that were conducive for storm intensification and weakening. It was also found from satellite observations and numerical model simulations that modification of the environmental distribution of total ozone by the tropical cyclones was primarily caused by the secondary circulation associated with the tropical cyclone's outflow jet and the intrusion of stratospheric air in the eyes of tropical cyclones.
Abstract
Special Sensor Microwave/Imager (SSM/I) observations were used to examine the spatial and temporal changes of the precipitation characteristics of tropical cyclones. SSM/I observations were also combined with the results of a tropical cyclone numerical model to examine the role of inner-core diabatic heating in subsequent intensity changes of tropical cyclones. Included in the SSM/I observations were rainfall characteristics of 18 named western North Atlantic tropical cyclones between 1987 and 1989. The SSM/I rain-rate algorithm that employed the 85-GHz channel provided an analysis of the rain-rate distribution in greater detail. However, the SSM/I algorithm underestimated the rain rates when compared to in situ techniques but appeared to be comparable to the rain rates obtained from other satellite-borne passive microwave radiometers.
The analysis of SSM/I observations found that more intense systems had higher rain rates, more latent heat release, and a greater contribution from heavier rain to the total tropical cyclone rainfall. In addition, regions with the heaviest rain rates were found near the center of the most intense tropical cyclones. Observational analysis from SSM/I also revealed that the greatest rain rates in the inner-core regions were found in the right half of fast-moving tropical cyclones, while the heaviest rain rates in slow-moving tropical cyclones were found in the forward half. The combination of SSM/I observations and an interpretation of numerical model simulations revealed that the correlation between changes in the inner core diabatic beating and the subsequent intensity become greater as the tropical cyclones became more intense.
Abstract
Special Sensor Microwave/Imager (SSM/I) observations were used to examine the spatial and temporal changes of the precipitation characteristics of tropical cyclones. SSM/I observations were also combined with the results of a tropical cyclone numerical model to examine the role of inner-core diabatic heating in subsequent intensity changes of tropical cyclones. Included in the SSM/I observations were rainfall characteristics of 18 named western North Atlantic tropical cyclones between 1987 and 1989. The SSM/I rain-rate algorithm that employed the 85-GHz channel provided an analysis of the rain-rate distribution in greater detail. However, the SSM/I algorithm underestimated the rain rates when compared to in situ techniques but appeared to be comparable to the rain rates obtained from other satellite-borne passive microwave radiometers.
The analysis of SSM/I observations found that more intense systems had higher rain rates, more latent heat release, and a greater contribution from heavier rain to the total tropical cyclone rainfall. In addition, regions with the heaviest rain rates were found near the center of the most intense tropical cyclones. Observational analysis from SSM/I also revealed that the greatest rain rates in the inner-core regions were found in the right half of fast-moving tropical cyclones, while the heaviest rain rates in slow-moving tropical cyclones were found in the forward half. The combination of SSM/I observations and an interpretation of numerical model simulations revealed that the correlation between changes in the inner core diabatic beating and the subsequent intensity become greater as the tropical cyclones became more intense.
Abstract
The mutual adjustment between upper-tropospheric troughs and the structure of western Atlantic Tropical Cyclones Florence (1988) and Irene (1981) are analyzed using satellite and in situ data. Satellite-observed tracers (e.g., cirrus clouds, water vapor, and ozone) art used to monitor the circulation within the tropical cyclones' environment. The tropical cyclones' convection is inferred from satellite flown passive microwave and infrared sensors. In addition, numerical model simulations are used to analyze and interpret these satellite observations. The study suggests that the initiation and maintenance of intense convective outbreaks in these tropical cyclones during their mature stage are related to the channeling and strengthening of their outflow by upper-tropospheric troughs. The convection can be enhanced in response to the outflow jet-induced import of eddy relative angular momentum and ascending motion associated with the thermally direct circulation. The channeling and strengthening of the outflow occurs when the upper-tropospheric troughs are located in a favorable position relative to the tropical cyclones. Both Florence and Irene intensify after the onset of these intense convective episodes. Satellite observations also suggest that the cessation in the convection of the two tropical cyclones occurs when the upper-tropospheric troughs move near or over the tropical cyclones, resulting in the weakening of their outflow and the entrainment of dry upper-tropospheric air into their inner core.
Abstract
The mutual adjustment between upper-tropospheric troughs and the structure of western Atlantic Tropical Cyclones Florence (1988) and Irene (1981) are analyzed using satellite and in situ data. Satellite-observed tracers (e.g., cirrus clouds, water vapor, and ozone) art used to monitor the circulation within the tropical cyclones' environment. The tropical cyclones' convection is inferred from satellite flown passive microwave and infrared sensors. In addition, numerical model simulations are used to analyze and interpret these satellite observations. The study suggests that the initiation and maintenance of intense convective outbreaks in these tropical cyclones during their mature stage are related to the channeling and strengthening of their outflow by upper-tropospheric troughs. The convection can be enhanced in response to the outflow jet-induced import of eddy relative angular momentum and ascending motion associated with the thermally direct circulation. The channeling and strengthening of the outflow occurs when the upper-tropospheric troughs are located in a favorable position relative to the tropical cyclones. Both Florence and Irene intensify after the onset of these intense convective episodes. Satellite observations also suggest that the cessation in the convection of the two tropical cyclones occurs when the upper-tropospheric troughs move near or over the tropical cyclones, resulting in the weakening of their outflow and the entrainment of dry upper-tropospheric air into their inner core.
Abstract
Data from the Special Sensor Microwave/Imager (SSM/I) on board a Defense Meteorological Satellite Program satellite are used to study the precipitation patterns and wind fields associated with Hurricane Florence (1988). SSM/I estimates indicate that the intensification of Florence was coincident with the increase in total latent beat release. Additionally, an increase in the concentration and areal coverage of heavier rain rates near the center is observed. SSM/I marine surface winds of Florence are examined and compared to in situ data, and to an enhanced objective isotach analysis over the Gulf of Mexico. Results indicate that the SSM/I winds are weaker than those depicted in the enhanced objective analysis and slightly stronger than in situ observations. Finally, center positions of Florence are estimated using the 85-GHz brightness temperature imagery. Much improved estimates are achieved using this imagery compared to using GOES infrared imagery. These results concur with previous studies that applications of SSM/I data could be valuable in augmenting current methods of tropical cyclone analysis.
Abstract
Data from the Special Sensor Microwave/Imager (SSM/I) on board a Defense Meteorological Satellite Program satellite are used to study the precipitation patterns and wind fields associated with Hurricane Florence (1988). SSM/I estimates indicate that the intensification of Florence was coincident with the increase in total latent beat release. Additionally, an increase in the concentration and areal coverage of heavier rain rates near the center is observed. SSM/I marine surface winds of Florence are examined and compared to in situ data, and to an enhanced objective isotach analysis over the Gulf of Mexico. Results indicate that the SSM/I winds are weaker than those depicted in the enhanced objective analysis and slightly stronger than in situ observations. Finally, center positions of Florence are estimated using the 85-GHz brightness temperature imagery. Much improved estimates are achieved using this imagery compared to using GOES infrared imagery. These results concur with previous studies that applications of SSM/I data could be valuable in augmenting current methods of tropical cyclone analysis.
