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Abstract
Methods of optimizing the Lovejoy and Austin technique to delineate areas of precipitation using visible and infrared satellite data are investigated. The technique involves training the satellite data by correlation with real-time radar data. The choice of statistical measures to define the precipitation/no-precipitation boundary between satellite classes is investigated. Subjective evaluation of the satellite-diagnosed precipitation fields indicates that minimizing the difference between the observed and diagnosed number of precipitation pixels produces the most realistic results. Maximization of some standard skill scores tends to overestimate the areas extent of the precipitation. Examples of the variability of the accuracy of the technique and the variation in optimum boundary or threshold are given. Cases illustrating the improvement produced by using different correlation tables for different synoptic systems are presented. Use of time-averaged correlation tables is investigated and found to be nearly as accurate as use of tables formed at one time, when evaluated within the training area. Fixed predefined tables were rather less accurate when evaluated within the training area, especially with respect to the diagnosed areal extent. A method is presented to combine the use of instantaneous and time-averaged correlation tables, together with a predefined table. Ideally, the method should incorporate the use of different correlation tables for different synoptic systems.
Abstract
Methods of optimizing the Lovejoy and Austin technique to delineate areas of precipitation using visible and infrared satellite data are investigated. The technique involves training the satellite data by correlation with real-time radar data. The choice of statistical measures to define the precipitation/no-precipitation boundary between satellite classes is investigated. Subjective evaluation of the satellite-diagnosed precipitation fields indicates that minimizing the difference between the observed and diagnosed number of precipitation pixels produces the most realistic results. Maximization of some standard skill scores tends to overestimate the areas extent of the precipitation. Examples of the variability of the accuracy of the technique and the variation in optimum boundary or threshold are given. Cases illustrating the improvement produced by using different correlation tables for different synoptic systems are presented. Use of time-averaged correlation tables is investigated and found to be nearly as accurate as use of tables formed at one time, when evaluated within the training area. Fixed predefined tables were rather less accurate when evaluated within the training area, especially with respect to the diagnosed areal extent. A method is presented to combine the use of instantaneous and time-averaged correlation tables, together with a predefined table. Ideally, the method should incorporate the use of different correlation tables for different synoptic systems.
Abstract
A severe thunderstorm which spawned at least four tornadoes, one of them anticyclonic, formed over central Iowa during the afternoon of 13 June 1976. This storm moved toward the east-northeast, approximately parallel to but slower than the mean tropospheric flow. The anticyclonic tornado (F3) and the most intense (F5) of the cyclonic tornadoes coexisted for 23 min and traveled on nearly parallel, cycloidal-like tracks, with the anticyclonic tornado 3–5 km southeast of the cyclonic. The major emphasis of this paper is on this pair of tornadoes and their relationship to the structure and evolution of the parent thunderstorm.
Radar recorded the development of a hook echo just prior to the genesis of the intense cyclonic tornado. A strengthening mesolow was centered somewhere south of this tornado soon after it formed. The mesolow is believed to have initiated a new updraft; the anticyclonic tornado formed in association with this updraft, south of the cyclonic tornado. It is hypothesized that the mesolow was responsible (through alteration of the storm-scale airflow) for the nearly simultaneous sharp right turns made by these tornadoes. Each of these tornadoes was observed to diminish in intensity soon after becoming associated with heavy rain.
It is argued that the parent thunderstom's distinctive airflow and thermodynamic structure at low levels provided a more favorable setting for the amplification of anticyclonic vorticity than is typical of most severe thunderstorms.
Abstract
A severe thunderstorm which spawned at least four tornadoes, one of them anticyclonic, formed over central Iowa during the afternoon of 13 June 1976. This storm moved toward the east-northeast, approximately parallel to but slower than the mean tropospheric flow. The anticyclonic tornado (F3) and the most intense (F5) of the cyclonic tornadoes coexisted for 23 min and traveled on nearly parallel, cycloidal-like tracks, with the anticyclonic tornado 3–5 km southeast of the cyclonic. The major emphasis of this paper is on this pair of tornadoes and their relationship to the structure and evolution of the parent thunderstorm.
Radar recorded the development of a hook echo just prior to the genesis of the intense cyclonic tornado. A strengthening mesolow was centered somewhere south of this tornado soon after it formed. The mesolow is believed to have initiated a new updraft; the anticyclonic tornado formed in association with this updraft, south of the cyclonic tornado. It is hypothesized that the mesolow was responsible (through alteration of the storm-scale airflow) for the nearly simultaneous sharp right turns made by these tornadoes. Each of these tornadoes was observed to diminish in intensity soon after becoming associated with heavy rain.
It is argued that the parent thunderstom's distinctive airflow and thermodynamic structure at low levels provided a more favorable setting for the amplification of anticyclonic vorticity than is typical of most severe thunderstorms.
Abstract
A numerical method of solving the equation of radiative transfer for a plane parallel, horizontally homogeneous medium is presented. The method is applicable for problems with nonconservative scattering as well as for conservative scattering problems. Comparison of results for the reflected and transmitted radiation from this method with existing solutions for conservative Rayleigh scattering shows that, for optical depths up to 1-0, the present scheme is accurate to within ±0.007 unit total intensity and ±1.0 per cent polarization for an incident flux of π units per unit normal area. Results are presented for the reflected and transmitted intensity and per cent polarization for optical depths 2.0 and 4.0, for a particular problem of conservative Rayleigh scattering.
Abstract
A numerical method of solving the equation of radiative transfer for a plane parallel, horizontally homogeneous medium is presented. The method is applicable for problems with nonconservative scattering as well as for conservative scattering problems. Comparison of results for the reflected and transmitted radiation from this method with existing solutions for conservative Rayleigh scattering shows that, for optical depths up to 1-0, the present scheme is accurate to within ±0.007 unit total intensity and ±1.0 per cent polarization for an incident flux of π units per unit normal area. Results are presented for the reflected and transmitted intensity and per cent polarization for optical depths 2.0 and 4.0, for a particular problem of conservative Rayleigh scattering.
Abstract
In this paper, calculations are presented of the change in reflected flux by the earth-atmosphere system in response to increases in the atmospheric aerosol loading for a range of complex indices of refraction, solar elevation angle and ground albedo. Results show that, for small values of ground albedo, the reflected solar flux may either increase or decrease with increasing aerosol loadings, depending upon the complex part of the index of refraction of the aerosols. For high ground albedos (A > 0.4), an increase in aerosol levels always results in a decrease of reflected flux (i.e., a warming of the earth-atmosphere system).
The first part of the paper concerns itself with the computational techniques employed in this study. The method employs a numerical solution to the equation of radiation transfer and is essentially a modification of an older technique used by the authors. The modifications are detailed, and comparisons with the older technique are presented.
Abstract
In this paper, calculations are presented of the change in reflected flux by the earth-atmosphere system in response to increases in the atmospheric aerosol loading for a range of complex indices of refraction, solar elevation angle and ground albedo. Results show that, for small values of ground albedo, the reflected solar flux may either increase or decrease with increasing aerosol loadings, depending upon the complex part of the index of refraction of the aerosols. For high ground albedos (A > 0.4), an increase in aerosol levels always results in a decrease of reflected flux (i.e., a warming of the earth-atmosphere system).
The first part of the paper concerns itself with the computational techniques employed in this study. The method employs a numerical solution to the equation of radiation transfer and is essentially a modification of an older technique used by the authors. The modifications are detailed, and comparisons with the older technique are presented.
Abstract
A three-dimensional hot-film probe, a Vector Vane, and an Aerovane were used to measure the mean wind speed and turbulence structure in the atmospheric surface layer at a location on the south shore of Long Island. A comparison was recently made of the characteristics of the three instruments to determine their capabilities in measuring the various meteorological parameters of interest. Results from the comparison indicated that the mean wind speed measured by the three instruments was the same.
The estimated spectral densities of the Vector Vane were approximately equal to those of the hot-film probe to a cyclic frequency of 1 Hz. The standard deviations of the velocity fluctuations were equal.
Comparison of the longitudinal velocity fluctuations measured by the Aerovane and Vector Vane were not significantly different to a frequency of 0.3 Hz. The Aerovane underestimated the lateral velocity fluctuations.
Abstract
A three-dimensional hot-film probe, a Vector Vane, and an Aerovane were used to measure the mean wind speed and turbulence structure in the atmospheric surface layer at a location on the south shore of Long Island. A comparison was recently made of the characteristics of the three instruments to determine their capabilities in measuring the various meteorological parameters of interest. Results from the comparison indicated that the mean wind speed measured by the three instruments was the same.
The estimated spectral densities of the Vector Vane were approximately equal to those of the hot-film probe to a cyclic frequency of 1 Hz. The standard deviations of the velocity fluctuations were equal.
Comparison of the longitudinal velocity fluctuations measured by the Aerovane and Vector Vane were not significantly different to a frequency of 0.3 Hz. The Aerovane underestimated the lateral velocity fluctuations.
Abstract
The relationship between precipitation and infrared and visible satellite data is investigated in the vicinity of the United Kingdom. The investigation uses histograms of Meteosat data, built up from many half-hourly fields, which represent the frequency distribution of the pixels as a function of temperature and brightness. Separate histograms are produced for pixels classified as “precipitation” and “no precipitation” by coincident radar observations. The study is conducted separately for four distinct synoptic types: cold fronts, warm fronts, cold-air convection, and mesoscale convective systems (MCSs). A method is presented that uses this information to delineate areas of precipitation.
It is found that the use of combined infrared and visible satellite data yields better results than using infrared alone for all four synoptic types and is better than visible date alone for the majority. Use of visible data alone is better than using infrared data by itself, except for warm-front cases. The results indicate that the ability of the satellite data to delineate precipitation decreases in the following order of synoptic regime: cold frontal, MCS, warm frontal. The most difficult regime to delineate is cold-air convection.
Abstract
The relationship between precipitation and infrared and visible satellite data is investigated in the vicinity of the United Kingdom. The investigation uses histograms of Meteosat data, built up from many half-hourly fields, which represent the frequency distribution of the pixels as a function of temperature and brightness. Separate histograms are produced for pixels classified as “precipitation” and “no precipitation” by coincident radar observations. The study is conducted separately for four distinct synoptic types: cold fronts, warm fronts, cold-air convection, and mesoscale convective systems (MCSs). A method is presented that uses this information to delineate areas of precipitation.
It is found that the use of combined infrared and visible satellite data yields better results than using infrared alone for all four synoptic types and is better than visible date alone for the majority. Use of visible data alone is better than using infrared data by itself, except for warm-front cases. The results indicate that the ability of the satellite data to delineate precipitation decreases in the following order of synoptic regime: cold frontal, MCS, warm frontal. The most difficult regime to delineate is cold-air convection.
Abstract
The effect of surface characteristics on the daytime change in the potential for development of deep convection resulting from surface flux of heat and moisture is evaluated by conceptual, scaling, and numerical modeling approaches. It is shown that deep convection depends significantly on the Bowen ratio; for smaller Bowen ratio, the thermodynamic potential for deep convection increases. The elevation and the intensity of the capping stable layer have an opposing impact on deep convection: increasing moisture accumulation through evapotranspiration was supportive but was counteracted by the enhancement of dry entrainment. Based on an approximate treatment of the effect of cloudiness on solar irradiance, it was found that development of fair weather cumulus has a secondary effect on deep convection potential. Observational and operational aspects of the influence of surface conditions on evapotranspiration and development of deep convection are presented.
Abstract
The effect of surface characteristics on the daytime change in the potential for development of deep convection resulting from surface flux of heat and moisture is evaluated by conceptual, scaling, and numerical modeling approaches. It is shown that deep convection depends significantly on the Bowen ratio; for smaller Bowen ratio, the thermodynamic potential for deep convection increases. The elevation and the intensity of the capping stable layer have an opposing impact on deep convection: increasing moisture accumulation through evapotranspiration was supportive but was counteracted by the enhancement of dry entrainment. Based on an approximate treatment of the effect of cloudiness on solar irradiance, it was found that development of fair weather cumulus has a secondary effect on deep convection potential. Observational and operational aspects of the influence of surface conditions on evapotranspiration and development of deep convection are presented.
Abstract
Recent studies of particulate and gaseous materials in the atmosphere have raised important questions about diffusion at distances of 10–100 km. A photometric densitometer, initially developed for a quantitative study of oil-fog concentrations at ground level, has been adapted for use in an aircraft. Real-time measurements of ground-level and airborne particle concentrations are presented to distances of 120 km, and the implications of these data in terms of large-scale dispersion are discussed.
Abstract
Recent studies of particulate and gaseous materials in the atmosphere have raised important questions about diffusion at distances of 10–100 km. A photometric densitometer, initially developed for a quantitative study of oil-fog concentrations at ground level, has been adapted for use in an aircraft. Real-time measurements of ground-level and airborne particle concentrations are presented to distances of 120 km, and the implications of these data in terms of large-scale dispersion are discussed.
Abstract
Theoretical computations of the intensity and polarization of diffusively transmitted sunlight are presented for two wavelengths, λ = 4290 Å and λ = 5000 Å. The computations are for atmospheres containing various distributions of aerosols, as well as normal molecular constituents, and allow for all significant orders of scattering. The theoretical computations are compared with observations, and it is shown that inclusion of aerosols in the theoretical models results in considerably better agreement between observation and theory than can be achieved by assuming a pure molecular atmosphere for the theoretical computations.
Abstract
Theoretical computations of the intensity and polarization of diffusively transmitted sunlight are presented for two wavelengths, λ = 4290 Å and λ = 5000 Å. The computations are for atmospheres containing various distributions of aerosols, as well as normal molecular constituents, and allow for all significant orders of scattering. The theoretical computations are compared with observations, and it is shown that inclusion of aerosols in the theoretical models results in considerably better agreement between observation and theory than can be achieved by assuming a pure molecular atmosphere for the theoretical computations.
Abstract
It can be shown, theoretically, that the polarization properties of laser light scattered by a volume of air containing aerosols include considerable information as to the size distribution of the aerosols. A theoretical inversion model, utilizing the above information, is developed, which uses the Stokes parameters of the angularly scattered laser light as input data. These input data are generated theoretically from assumed size distribution functions of the aerosols. Both “perfect” measurements and measurements into which random errors are introduced are employed. These data are then used in the inversion model to generate predicted size distribution functions. Numerical experiments are performed with 0, 1 and 2% random error in the observations, in order to determine what accuracy is required in the lidar measurements. Comparisons between the actual and predicted functions are then made in order to assess the accuracy of the model.
Abstract
It can be shown, theoretically, that the polarization properties of laser light scattered by a volume of air containing aerosols include considerable information as to the size distribution of the aerosols. A theoretical inversion model, utilizing the above information, is developed, which uses the Stokes parameters of the angularly scattered laser light as input data. These input data are generated theoretically from assumed size distribution functions of the aerosols. Both “perfect” measurements and measurements into which random errors are introduced are employed. These data are then used in the inversion model to generate predicted size distribution functions. Numerical experiments are performed with 0, 1 and 2% random error in the observations, in order to determine what accuracy is required in the lidar measurements. Comparisons between the actual and predicted functions are then made in order to assess the accuracy of the model.
