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- Author or Editor: Toby N. Carlson x
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Abstract
An elevated mixed layer is a principal component of the conceptual model recently proposed by Carlson and others to explain the evolution of a severe storm environment over the Southern Plains of the United States during springtime. Elevated mixed layers are most likely to be found downwind of strongly heated arid land areas (often plateaus), which favor the growth of a deep mixing layer with high potential temperature. The lower and lateral boundaries of elevated mixed layers are distinctly defined respectively by a statically stable layer, referred to as a lid inversion, and a midlevel front. These boundaries mark the zonesof transition between the airstream defining the elevated mixed layer and other airstreams of differing geographical origin and, consequently, thermodynamic properties.
In this study, the Sawyer-Eliassen secondary circulation equation is used to diagnose the transverse ageostrophic circulations that are associated with the dynamical forcing implied by the above conceptual model of the elevated mixed layer structure. The diagnoses are based upon subjective analyses of the elevated mixed layer identified in the SESAME IV dataset at 2100 GMT 9 May 1979 and upon analytically specified patterns reproducing many of the main features of the subjective analyses. The outcome of the diagnoses indicates that a combination of confluence and anticyclonic shear forces a thermally direct circulation centered in the midlevel front and an indirect cell centered in the upper region of the elevated mixed layer,which results in a zone of rising motion between the cells at the edge of the elevated mixed layer. Additional tests, which compare the ageostrophic circulations derived from analytically specified fields in which the elevated mixed layer structure is defined in detail and in which it is highly smoothed, indicate that the above circulation pattern is enhanced by increased baroclinicity in the midlevel frontal zone and the diminished static stability in the elevated mixed layer.
Abstract
An elevated mixed layer is a principal component of the conceptual model recently proposed by Carlson and others to explain the evolution of a severe storm environment over the Southern Plains of the United States during springtime. Elevated mixed layers are most likely to be found downwind of strongly heated arid land areas (often plateaus), which favor the growth of a deep mixing layer with high potential temperature. The lower and lateral boundaries of elevated mixed layers are distinctly defined respectively by a statically stable layer, referred to as a lid inversion, and a midlevel front. These boundaries mark the zonesof transition between the airstream defining the elevated mixed layer and other airstreams of differing geographical origin and, consequently, thermodynamic properties.
In this study, the Sawyer-Eliassen secondary circulation equation is used to diagnose the transverse ageostrophic circulations that are associated with the dynamical forcing implied by the above conceptual model of the elevated mixed layer structure. The diagnoses are based upon subjective analyses of the elevated mixed layer identified in the SESAME IV dataset at 2100 GMT 9 May 1979 and upon analytically specified patterns reproducing many of the main features of the subjective analyses. The outcome of the diagnoses indicates that a combination of confluence and anticyclonic shear forces a thermally direct circulation centered in the midlevel front and an indirect cell centered in the upper region of the elevated mixed layer,which results in a zone of rising motion between the cells at the edge of the elevated mixed layer. Additional tests, which compare the ageostrophic circulations derived from analytically specified fields in which the elevated mixed layer structure is defined in detail and in which it is highly smoothed, indicate that the above circulation pattern is enhanced by increased baroclinicity in the midlevel frontal zone and the diminished static stability in the elevated mixed layer.
Abstract
Vertical motions have been computed for a 6-day period during which an upper tropospheric cold Low moved through the eastern Caribbean, and a kinetic energy budget for the region has been constructed. During the first 3 days, the kinetic energy inside the volume increased. The computations indicate that the increase was caused by lateral advection of kinetic energy into the volume plus a small internal conversion of potential to kinetic energy. The kinetic energy decreased during the last 3 days, as the circulation became indirect. Visual agreement between the vertical motions and the observed weather was good.
Abstract
Vertical motions have been computed for a 6-day period during which an upper tropospheric cold Low moved through the eastern Caribbean, and a kinetic energy budget for the region has been constructed. During the first 3 days, the kinetic energy inside the volume increased. The computations indicate that the increase was caused by lateral advection of kinetic energy into the volume plus a small internal conversion of potential to kinetic energy. The kinetic energy decreased during the last 3 days, as the circulation became indirect. Visual agreement between the vertical motions and the observed weather was good.
Abstract
A method for objective analysis of aircraft observations in tropical cyclones has been developed. Quasi-horizontal fields of motion, temperatures, mixing ratios, and D-values are analyzed using a modified version of the method of successive corrections. The weighting functions are specified so that the high degree of circular symmetry characteristic of tropical cyclones is incorporated in the analyses. The analyses are performed on a 25 by 25 Cartesian grid of 5 n mi spacing which is centered on the storm. A special feature is the analysis of vertical motions as determined from aircraft flight characteristics. Three Atlantic storms are analyzed in detail: Hurricanes Inez (1966), Debbie (1969), and Ginger (1971). The analyses show the significant larger-scale features and major asymmetries of these storms. Both Inez and Debbie, which were well organized hurricanes, display characteristic vertical motion patterns in which a ring of strong ascent is found immediately surrounding the eye, with marked descent just outside of the annulus of strong ascent. Maximum ascent and descent rates were each indicated to be a few meters per second in these storms. Ginger was a marginal hurricane with poorly organized eye structure and relatively weak and disorganized patterns of vertical motion.
Abstract
A method for objective analysis of aircraft observations in tropical cyclones has been developed. Quasi-horizontal fields of motion, temperatures, mixing ratios, and D-values are analyzed using a modified version of the method of successive corrections. The weighting functions are specified so that the high degree of circular symmetry characteristic of tropical cyclones is incorporated in the analyses. The analyses are performed on a 25 by 25 Cartesian grid of 5 n mi spacing which is centered on the storm. A special feature is the analysis of vertical motions as determined from aircraft flight characteristics. Three Atlantic storms are analyzed in detail: Hurricanes Inez (1966), Debbie (1969), and Ginger (1971). The analyses show the significant larger-scale features and major asymmetries of these storms. Both Inez and Debbie, which were well organized hurricanes, display characteristic vertical motion patterns in which a ring of strong ascent is found immediately surrounding the eye, with marked descent just outside of the annulus of strong ascent. Maximum ascent and descent rates were each indicated to be a few meters per second in these storms. Ginger was a marginal hurricane with poorly organized eye structure and relatively weak and disorganized patterns of vertical motion.
Abstract
Rapid soil-surface drying, which is called “decoupling,” accompanied by an increase in near-surface air temperature and sensible heat flux, is typically confined to the top 1–2 cm of the soil, while the deeper layers remain relatively moist. Because decoupling depends also on a precise knowledge of fractional vegetation cover, soil properties, and soil water content, an accurate knowledge of these parameters is essential for making good predictions of temperature and humidity. Accordingly, some simulations centered on the Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed Southern Great Plains site in Kansas and Oklahoma using a high-resolution substrate layer (Simulator for Hydrology and Energy Exchange at the Land Surface), the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, and derived and default values for soil water content and fractional vegetation cover are presented. In so doing, the following points are made: 1) decoupling occurs only within certain threshold ranges of soil water content that are closely related to the soil type and 2) a knowledge of fractional vegetation cover derived from concurrent observations is necessary for capturing the spatial variation in rapid soil drying in forecast models.
Abstract
Rapid soil-surface drying, which is called “decoupling,” accompanied by an increase in near-surface air temperature and sensible heat flux, is typically confined to the top 1–2 cm of the soil, while the deeper layers remain relatively moist. Because decoupling depends also on a precise knowledge of fractional vegetation cover, soil properties, and soil water content, an accurate knowledge of these parameters is essential for making good predictions of temperature and humidity. Accordingly, some simulations centered on the Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed Southern Great Plains site in Kansas and Oklahoma using a high-resolution substrate layer (Simulator for Hydrology and Energy Exchange at the Land Surface), the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model, and derived and default values for soil water content and fractional vegetation cover are presented. In so doing, the following points are made: 1) decoupling occurs only within certain threshold ranges of soil water content that are closely related to the soil type and 2) a knowledge of fractional vegetation cover derived from concurrent observations is necessary for capturing the spatial variation in rapid soil drying in forecast models.
Abstract
Numerical models of the atmosphere and aerosols are used to investigate mobilization and transport of Saharan dust over West Africa and the tropical Atlantic Ocean for 23–28 August 1974. We have found that mobilization during this period was related to the passage of a shallow easterly wave and was not initiated by dry convective mixing of a midlevel easterly jet, as has been previously suggested, since high static stability beneath the midlevel easterly jet inhibited significant boundary layer development and transport of momentum in the jet down to the surface. Instead, mobilization was done by dry convective mixing of low-level jets associated with the easterly wave. Another easterly wave present in the domain during the period did not contribute significantly to dust mobilization while over Africa yet became a strong tropical storm over the Atlantic Ocean in early September. The periodicity of the outbreak was reinforced by scavenging of dust by precipitation associated with the easterly waves.
The model simulations show that the aerosol at any one point can be a complicated mixture of particles lifted at different times and different places. Bimodal size distributions developed when dust was mobilized within a dust plume that was generated on a previous day. An elevated layer of dust developed over the ocean as the northeast trade winds advected clean air underneath the dust-laden air as it moved westward. The size and spatial distributions of aerosol in the marine layer depended upon the undercutting process, the amount of background mineral aerosol present, and transport across the marine layer inversion by sedimentation and turbulent mixing.
Abstract
Numerical models of the atmosphere and aerosols are used to investigate mobilization and transport of Saharan dust over West Africa and the tropical Atlantic Ocean for 23–28 August 1974. We have found that mobilization during this period was related to the passage of a shallow easterly wave and was not initiated by dry convective mixing of a midlevel easterly jet, as has been previously suggested, since high static stability beneath the midlevel easterly jet inhibited significant boundary layer development and transport of momentum in the jet down to the surface. Instead, mobilization was done by dry convective mixing of low-level jets associated with the easterly wave. Another easterly wave present in the domain during the period did not contribute significantly to dust mobilization while over Africa yet became a strong tropical storm over the Atlantic Ocean in early September. The periodicity of the outbreak was reinforced by scavenging of dust by precipitation associated with the easterly waves.
The model simulations show that the aerosol at any one point can be a complicated mixture of particles lifted at different times and different places. Bimodal size distributions developed when dust was mobilized within a dust plume that was generated on a previous day. An elevated layer of dust developed over the ocean as the northeast trade winds advected clean air underneath the dust-laden air as it moved westward. The size and spatial distributions of aerosol in the marine layer depended upon the undercutting process, the amount of background mineral aerosol present, and transport across the marine layer inversion by sedimentation and turbulent mixing.
Abstract
This study examines the influence of differences in ground moisture over the southern Great Plairs and the Mexican plateau on the formation and evolution of the dryline, the elevated mixed layer, and the local planetary boundary layer. These features are examined in a series of numerical experiments in which dry and wet surface conditions over the southern plains and Mexico are simulated by the model
Results of the numerical simulations show that the dry soil conditions of northern Mexico are critical to the formation of the lid, and the variable soil conditions of the southern Great Plains are important for the processes of differential surface heating and generation of low-level instability through strong surface evaporation. The processes interact dynamically to alter the prestorm conditions and subsequent convective patterns observed over Texas and Oklahoma in the SESAME IV case.
Abstract
This study examines the influence of differences in ground moisture over the southern Great Plairs and the Mexican plateau on the formation and evolution of the dryline, the elevated mixed layer, and the local planetary boundary layer. These features are examined in a series of numerical experiments in which dry and wet surface conditions over the southern plains and Mexico are simulated by the model
Results of the numerical simulations show that the dry soil conditions of northern Mexico are critical to the formation of the lid, and the variable soil conditions of the southern Great Plains are important for the processes of differential surface heating and generation of low-level instability through strong surface evaporation. The processes interact dynamically to alter the prestorm conditions and subsequent convective patterns observed over Texas and Oklahoma in the SESAME IV case.
Abstract
A method for inferring the distribution of surface heat and evaporative fluxes and the ground moisture availability and thermal inertia (ground conductive capacity) is used to analyze two urbanized areas, Los Angeles and St. Louis. The technique employs infrared satellite temperature measurements in conjunction with a one-dimensional boundary-layer model. Results show that there is a marked reduction of evaporation and moisture availability and a corresponding elevation of sensible heat flux over urbanized areas and over cropped areas with low vegetative cover. Conversely, low heat flux and high evaporation characterize vegetated and, especially, forested areas. Warm urban centers appear directly related to a reduction in vegetation, which normally allows for a greater fraction of available radiant energy to be converted into latent heat flux. The distribution of thermal inertia was surprisingly ill-defined and its variation between urban and rural areas was quite small. Thus, the increased heat storage within the urban fabric, which has been proposed as the underlying cause of the nocturnal heat island, may be caused mainly by enhanced daytime surface heating which occurs because of surface dryness, rather than by large spatial variations in the conductivity of the surface.
Abstract
A method for inferring the distribution of surface heat and evaporative fluxes and the ground moisture availability and thermal inertia (ground conductive capacity) is used to analyze two urbanized areas, Los Angeles and St. Louis. The technique employs infrared satellite temperature measurements in conjunction with a one-dimensional boundary-layer model. Results show that there is a marked reduction of evaporation and moisture availability and a corresponding elevation of sensible heat flux over urbanized areas and over cropped areas with low vegetative cover. Conversely, low heat flux and high evaporation characterize vegetated and, especially, forested areas. Warm urban centers appear directly related to a reduction in vegetation, which normally allows for a greater fraction of available radiant energy to be converted into latent heat flux. The distribution of thermal inertia was surprisingly ill-defined and its variation between urban and rural areas was quite small. Thus, the increased heat storage within the urban fabric, which has been proposed as the underlying cause of the nocturnal heat island, may be caused mainly by enhanced daytime surface heating which occurs because of surface dryness, rather than by large spatial variations in the conductivity of the surface.
The purpose of this study is to demonstrate the feasibility of determining the soil-water content fields required as initial conditions for land surface components within atmospheric prediction models. This is done using a model of the hydrologic balance and conventional meteorological observations, land cover, and soils information.
A discussion is presented of the subgrid-scale effects, the integration time, and the choice of vegetation type on the soil-water content patterns. Finally, comparisons are made between two The Pennsylvania State University/National Center for Atmospheric Research mesoscale model simulations, one using climatological fields and the other one using the soil-moisture fields produced by this new method.
The purpose of this study is to demonstrate the feasibility of determining the soil-water content fields required as initial conditions for land surface components within atmospheric prediction models. This is done using a model of the hydrologic balance and conventional meteorological observations, land cover, and soils information.
A discussion is presented of the subgrid-scale effects, the integration time, and the choice of vegetation type on the soil-water content patterns. Finally, comparisons are made between two The Pennsylvania State University/National Center for Atmospheric Research mesoscale model simulations, one using climatological fields and the other one using the soil-moisture fields produced by this new method.
Abstract
There is great interest in wind-borne mineral dust because of the role that dust plays in climate by modulating solar radiation and cloud properties. Today, much research focuses on North Africa because it is Earth’s largest and most persistently active dust source. Moreover, this region is expected to be greatly impacted by climate change, which would affect dust emission rates. Interest in dust was stimulated over 50 years ago when it was discovered that African dust was frequently transported across the Atlantic in great quantities. Here we report on the initial discovery of African dust in the Caribbean Basin. We show that there were three independent “first” discoveries of African dust in the 1950s through the 1960s. In each case, the discoverers were not seeking dust but, rather, they had other research objectives. The meteorological context of African dust transport was first elucidated in 1969 with the characterization of the Saharan air layer (SAL) and its role in effecting the efficient transport of African dust over great distances to the Western Hemisphere. The link between dust transport and African climate was established in the 1970s and 1980s when dust transport to the Caribbean increased greatly following the onset of severe drought in the Sahel. Here we chronicle these events and show how they contributed to our current state of knowledge.
Abstract
There is great interest in wind-borne mineral dust because of the role that dust plays in climate by modulating solar radiation and cloud properties. Today, much research focuses on North Africa because it is Earth’s largest and most persistently active dust source. Moreover, this region is expected to be greatly impacted by climate change, which would affect dust emission rates. Interest in dust was stimulated over 50 years ago when it was discovered that African dust was frequently transported across the Atlantic in great quantities. Here we report on the initial discovery of African dust in the Caribbean Basin. We show that there were three independent “first” discoveries of African dust in the 1950s through the 1960s. In each case, the discoverers were not seeking dust but, rather, they had other research objectives. The meteorological context of African dust transport was first elucidated in 1969 with the characterization of the Saharan air layer (SAL) and its role in effecting the efficient transport of African dust over great distances to the Western Hemisphere. The link between dust transport and African climate was established in the 1970s and 1980s when dust transport to the Caribbean increased greatly following the onset of severe drought in the Sahel. Here we chronicle these events and show how they contributed to our current state of knowledge.
Abstract
Simsphere, a soil–vegetation–atmosphere–transfer (SVAT) model developed at The Pennsylvania State University, can be downloaded from the web for use by students and researchers. In existence for several decades, Simsphere has figured in both the classroom and in research at several universities. As such, Simsphere has been supported by a knowledgeable group of academic users and has been applied in a variety of applications, such as in remote sensing of surface soil water content and in the assessment of water and ozone stresses on plants. This paper describes the model and how it can be downloaded and run.
Abstract
Simsphere, a soil–vegetation–atmosphere–transfer (SVAT) model developed at The Pennsylvania State University, can be downloaded from the web for use by students and researchers. In existence for several decades, Simsphere has figured in both the classroom and in research at several universities. As such, Simsphere has been supported by a knowledgeable group of academic users and has been applied in a variety of applications, such as in remote sensing of surface soil water content and in the assessment of water and ozone stresses on plants. This paper describes the model and how it can be downloaded and run.
