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Abstract
Air samples and updraft speeds were obtained simultaneously beneath the bases of convective clouds in northeastern Colorado and in South Dakota. Air samples were examined on the ground in a thermal diffusion nuclei chamber in the supersaturation range of 0.1–0.3% and the nuclei spectra determined. Using Twomey's technique, the maximum supersaturation was estimated, thereby estimating the number of cloud nuclei activated.
The updraft speeds were plotted against the concentration of activated cloud nuclei for varying storm intensities. The results indicate the dominate variable, in identifying storm intensity, is updraft strength, with 4–5 m sec−1 being the cutoff point between thunderstorm and hailstorms. The data gave no clear indication that cloud nuclei concentrations played a significant role in identifying storm intensity. Since N ∝ V 3k/(2k+4), where V is updraft strength, N number of activated nuclei and the values of k obtained from the analyzed air samples ranged from 0.4 to 2.9, it was determined that in the case of High Plains convection the resulting relation between activated nuclei and updraft strength is N ∝ V 0.25 to V 0.89.
Abstract
Air samples and updraft speeds were obtained simultaneously beneath the bases of convective clouds in northeastern Colorado and in South Dakota. Air samples were examined on the ground in a thermal diffusion nuclei chamber in the supersaturation range of 0.1–0.3% and the nuclei spectra determined. Using Twomey's technique, the maximum supersaturation was estimated, thereby estimating the number of cloud nuclei activated.
The updraft speeds were plotted against the concentration of activated cloud nuclei for varying storm intensities. The results indicate the dominate variable, in identifying storm intensity, is updraft strength, with 4–5 m sec−1 being the cutoff point between thunderstorm and hailstorms. The data gave no clear indication that cloud nuclei concentrations played a significant role in identifying storm intensity. Since N ∝ V 3k/(2k+4), where V is updraft strength, N number of activated nuclei and the values of k obtained from the analyzed air samples ranged from 0.4 to 2.9, it was determined that in the case of High Plains convection the resulting relation between activated nuclei and updraft strength is N ∝ V 0.25 to V 0.89.
Abstract
Based on airborne observations, estimates of air and moisture flux have been calculated for 18 hailstorms: eight in Colorado, one in Oklahoma, and nine in South Dakota. The average air flux for hailstorms in this study was 2.3 × 1011 gm sec−1, while the moisture flux averaged near 2.1 × 109 gm sec−1.
Precipitation efficiency for eight thunderstorms was found to be near 55%, though two severe hailstorms did exhibit slightly less efficiency.
Abstract
Based on airborne observations, estimates of air and moisture flux have been calculated for 18 hailstorms: eight in Colorado, one in Oklahoma, and nine in South Dakota. The average air flux for hailstorms in this study was 2.3 × 1011 gm sec−1, while the moisture flux averaged near 2.1 × 109 gm sec−1.
Precipitation efficiency for eight thunderstorms was found to be near 55%, though two severe hailstorms did exhibit slightly less efficiency.
Abstract
Abstract
Abstract
Haifall data collected from a fixed network in northeastern Colorado during three seasons (1960–62) included the estimated impact energy, duration of hailfall, most common stone size, maximum stone size, and number of stones per square inch. These basic data,X, along with the transformations, lnN, √X, 3√X, and 1/X were analyzed by computer methods to determine which parameters could be used in a statistical analysis of hail suppression experiment. The gamma distribution function was fitted to the hailfall data by the method of maximum likelihood. A chi-square goodness of fit test was applied to the data, and one transformation was tested using a sequential analysis technique.
All parameters except impact energy and number of hailstones per square inch were eliminated from the statistical analysis because of bias, non-homogeneity, or sparsity of samples. Transformations which produced the minimum mean coefficient of variation were logarithm of impact energy (InE) and square root of the number of stones per square inch (√N 1 − 6). It was determined that a target-control analysis was not feasible for the analysis of hail suppression experiment. A period of 3 to 5 years is believed necessary to detect changes of 10 to 25 per cent in the hail parameters. The gamma distribution function fitted only the √N 1 − 6 data. From the results it was concluded that a sequential analysis test alone could not adequately evaluate the effectiveness of a hall modification experiment.
Abstract
Haifall data collected from a fixed network in northeastern Colorado during three seasons (1960–62) included the estimated impact energy, duration of hailfall, most common stone size, maximum stone size, and number of stones per square inch. These basic data,X, along with the transformations, lnN, √X, 3√X, and 1/X were analyzed by computer methods to determine which parameters could be used in a statistical analysis of hail suppression experiment. The gamma distribution function was fitted to the hailfall data by the method of maximum likelihood. A chi-square goodness of fit test was applied to the data, and one transformation was tested using a sequential analysis technique.
All parameters except impact energy and number of hailstones per square inch were eliminated from the statistical analysis because of bias, non-homogeneity, or sparsity of samples. Transformations which produced the minimum mean coefficient of variation were logarithm of impact energy (InE) and square root of the number of stones per square inch (√N 1 − 6). It was determined that a target-control analysis was not feasible for the analysis of hail suppression experiment. A period of 3 to 5 years is believed necessary to detect changes of 10 to 25 per cent in the hail parameters. The gamma distribution function fitted only the √N 1 − 6 data. From the results it was concluded that a sequential analysis test alone could not adequately evaluate the effectiveness of a hall modification experiment.
Abstract
In light of the significant icing hazard large drops pose to general aviation, two conditions have been previously associated with large-drop formation; these being a warm cloud-top temperature and a low droplet concentration. This paper identifies an additional condition associated with the development of large-drop regions. Wind shear is hypothesized as being a necessary but not sufficient condition for the formation of large drops. Wind shear at cloud top may cause turbulence, Kelvin-Helmholtz waves, and thus the inhomogeneous mixing leading to large drops.
This hypothesis was tested in 29 cases where the Wyoming King Air aircraft made a climb or descent through the top of stratiform clouds. The presence of a wind shear layer was defined by the magnitude of the wind shear and the value of the bulk Richardson number across the layer. In 23 of the 29 cases, wind shear was associated with large-drop regions. A χ2 statistical test was applied to the data. The null hypothesis, where wind shear and large drops were considered independent of each other, was rejected to a significance level of 0.01. From this it can be inferred that large drops and wind shear are related. The depth of the shear layer was usually small, less than 150 m. The validity of the condition of low droplet concentration is questioned since several cases of large drops were found in the presence of a high droplet concentration. These cases were marked by strong wind shear.
Abstract
In light of the significant icing hazard large drops pose to general aviation, two conditions have been previously associated with large-drop formation; these being a warm cloud-top temperature and a low droplet concentration. This paper identifies an additional condition associated with the development of large-drop regions. Wind shear is hypothesized as being a necessary but not sufficient condition for the formation of large drops. Wind shear at cloud top may cause turbulence, Kelvin-Helmholtz waves, and thus the inhomogeneous mixing leading to large drops.
This hypothesis was tested in 29 cases where the Wyoming King Air aircraft made a climb or descent through the top of stratiform clouds. The presence of a wind shear layer was defined by the magnitude of the wind shear and the value of the bulk Richardson number across the layer. In 23 of the 29 cases, wind shear was associated with large-drop regions. A χ2 statistical test was applied to the data. The null hypothesis, where wind shear and large drops were considered independent of each other, was rejected to a significance level of 0.01. From this it can be inferred that large drops and wind shear are related. The depth of the shear layer was usually small, less than 150 m. The validity of the condition of low droplet concentration is questioned since several cases of large drops were found in the presence of a high droplet concentration. These cases were marked by strong wind shear.
Abstract
A method for the analysis of single-Doppler radar data called band-velocity-processing (BVP) is presented. The BVP method was designed to exploit the two-dimensional nature of banded precipitation systems with crossband length scales of 5–50 km. The BVP method offers improved horizontal (crossband) resolution over existing volume scan methods (VAD/VVP), plus the ability to extract band-perpendicular vertical cross sections from a single volume scan. The BVP analysis yields a more complete kinematic structure than RHI analyses, providing estimates for both horizontal wind components, both horizontal divergence components, the shearing deformation, and the vertical shear vector. The bias and variance errors for the BVP method are shown to be acceptably small.
The application of the BVP method to a narrow cold frontal rainband reveals several important kinematic features. The mesoscale structure of the low-level jet ahead of the narrow cold frontal rainband is examined. The band relative flow field exhibits a closed helical circulation cell at the front of the band.
Abstract
A method for the analysis of single-Doppler radar data called band-velocity-processing (BVP) is presented. The BVP method was designed to exploit the two-dimensional nature of banded precipitation systems with crossband length scales of 5–50 km. The BVP method offers improved horizontal (crossband) resolution over existing volume scan methods (VAD/VVP), plus the ability to extract band-perpendicular vertical cross sections from a single volume scan. The BVP analysis yields a more complete kinematic structure than RHI analyses, providing estimates for both horizontal wind components, both horizontal divergence components, the shearing deformation, and the vertical shear vector. The bias and variance errors for the BVP method are shown to be acceptably small.
The application of the BVP method to a narrow cold frontal rainband reveals several important kinematic features. The mesoscale structure of the low-level jet ahead of the narrow cold frontal rainband is examined. The band relative flow field exhibits a closed helical circulation cell at the front of the band.
Abstract
A remote-sensing technique called TRACIR (tracking air with circular-polarization radar) was developed recently for studying air-parcel trajectories in clouds. The technique uses a dual-circular-polarization radar to detect microwave chaff fibers that serve as tracers of the air motion. The radar is able to detect the chaff inside clouds and precipitation by measuring the circular-depolarization ratio, which is much higher for chaff than for hydrometeors. Chaff concentrations are also estimated by the technique, thus permitting turbulent diffusion in clouds to be examined. Demonstrations of TRACIR's capabilities are presented for three cases in which chaff was used to simulate the movement of cloud-seeding nuclei in clouds and precipitation. In two cases involving airborne chaff releases, the gradual drift and diffusion of chaff in a stratiform cloud are contrasted with its abrupt transport and dispersion in a convective cloud. In the third case study, the technique successfully detected a plume of chaff released from the ground in a snowstorm. In each case the radar data provided three-dimensional visualizations of the extent of the chaff region and maps of the chaff concentration with excellent spatial and temporal resolution.
Abstract
A remote-sensing technique called TRACIR (tracking air with circular-polarization radar) was developed recently for studying air-parcel trajectories in clouds. The technique uses a dual-circular-polarization radar to detect microwave chaff fibers that serve as tracers of the air motion. The radar is able to detect the chaff inside clouds and precipitation by measuring the circular-depolarization ratio, which is much higher for chaff than for hydrometeors. Chaff concentrations are also estimated by the technique, thus permitting turbulent diffusion in clouds to be examined. Demonstrations of TRACIR's capabilities are presented for three cases in which chaff was used to simulate the movement of cloud-seeding nuclei in clouds and precipitation. In two cases involving airborne chaff releases, the gradual drift and diffusion of chaff in a stratiform cloud are contrasted with its abrupt transport and dispersion in a convective cloud. In the third case study, the technique successfully detected a plume of chaff released from the ground in a snowstorm. In each case the radar data provided three-dimensional visualizations of the extent of the chaff region and maps of the chaff concentration with excellent spatial and temporal resolution.
Abstract
Thermodynamic and hydrometeor measurements from an aircraft flown through the melting layer of stratiform clouds over the California Valley are discussed and are compared with radar observations. An isothermal layer ∼200 m thick existed at 0°C, and radar bright bands up to 36 dB(Ze ) were measured. The largest concentrations of ice particles occurred near −5°C and snowflakes melted by ∼2°C. Aggregation, and possibly ice multiplication, contributed to the characteristics of the radar bright band.
Abstract
Thermodynamic and hydrometeor measurements from an aircraft flown through the melting layer of stratiform clouds over the California Valley are discussed and are compared with radar observations. An isothermal layer ∼200 m thick existed at 0°C, and radar bright bands up to 36 dB(Ze ) were measured. The largest concentrations of ice particles occurred near −5°C and snowflakes melted by ∼2°C. Aggregation, and possibly ice multiplication, contributed to the characteristics of the radar bright band.
