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Abstract
An attempt is made to explain the mechanism of a hook-echo formation on the southwestern edge of an eastward-moving cumulonimbus cell. The conditions necessary for originating a hook-echo circulation through this proposed mechanism are: significant absolute vorticity within the subcloud layer, intense updraft to bring the low-level moist air into the cloud, and a vertical wind shear which steers the cumulonimbus cell with a velocity which is considerably different from that of the low-level winds. The Magnus effect involving the steering current and the spiraling up-draft is considered to be the force which directs the hook-echo circulation at low levels toward the southwestern edge of the major thunderstorm cell. A kinematic diagram with the absolute tangential speed and the radius of the cyclone as coordinates is also presented, as well as some speculation on the conservation of absolute circulation and absolute vorticity.
Abstract
An attempt is made to explain the mechanism of a hook-echo formation on the southwestern edge of an eastward-moving cumulonimbus cell. The conditions necessary for originating a hook-echo circulation through this proposed mechanism are: significant absolute vorticity within the subcloud layer, intense updraft to bring the low-level moist air into the cloud, and a vertical wind shear which steers the cumulonimbus cell with a velocity which is considerably different from that of the low-level winds. The Magnus effect involving the steering current and the spiraling up-draft is considered to be the force which directs the hook-echo circulation at low levels toward the southwestern edge of the major thunderstorm cell. A kinematic diagram with the absolute tangential speed and the radius of the cyclone as coordinates is also presented, as well as some speculation on the conservation of absolute circulation and absolute vorticity.
Abstract
Doppler winds measured by an instrumented aircraft are of great value in determining the wind field accompanying large-scale atmospheric disturbances. When they are utilized in interpreting and computing the wind fields of so-called mesoscale disturbances with their horizontal dimensions of a few to a few hundred miles, slight errors in the vector quantities forming the navigation triangle result in fictitious winds which differ considerably from the real winds. In the first part of this paper, the wind velocity errors due to the backscattering water particles illuminated by Doppler beams, designated as wet, beams, are discussed. The influence of wet beams upon Doppler winds was calculated theoretically under various conditions to allow an estimate of maximum wind velocity error. Following the solution of wet-beam cases, theoretical consideration was given to the fluctuation of the measured winds caused by the constant errors in the true air speed, the aircraft heading, the Doppler ground speed, and the Doppler drift angle. For the purpose of investigating whether this type of error occurs or not, test flights were made over Florida and Oklahoma along a number of loops with varying diameters. Results of the evaluation revealed that the error in aircraft heading is of least importance and that the other errors can be determined and corrected with a high degree of accuracy provided only a few specific loops are flown during each mission. It has become feasible to calculate both divergence and vorticity associated with mesoscale disturbances from the calibrated Doppler winds measured along well-designed flight tracks. It is expected that the basic research presented in this paper will stimulate the use of Doppler wind systems in the determination of the detailed structure of winds accompanying mesoscale meteorological systems.
Abstract
Doppler winds measured by an instrumented aircraft are of great value in determining the wind field accompanying large-scale atmospheric disturbances. When they are utilized in interpreting and computing the wind fields of so-called mesoscale disturbances with their horizontal dimensions of a few to a few hundred miles, slight errors in the vector quantities forming the navigation triangle result in fictitious winds which differ considerably from the real winds. In the first part of this paper, the wind velocity errors due to the backscattering water particles illuminated by Doppler beams, designated as wet, beams, are discussed. The influence of wet beams upon Doppler winds was calculated theoretically under various conditions to allow an estimate of maximum wind velocity error. Following the solution of wet-beam cases, theoretical consideration was given to the fluctuation of the measured winds caused by the constant errors in the true air speed, the aircraft heading, the Doppler ground speed, and the Doppler drift angle. For the purpose of investigating whether this type of error occurs or not, test flights were made over Florida and Oklahoma along a number of loops with varying diameters. Results of the evaluation revealed that the error in aircraft heading is of least importance and that the other errors can be determined and corrected with a high degree of accuracy provided only a few specific loops are flown during each mission. It has become feasible to calculate both divergence and vorticity associated with mesoscale disturbances from the calibrated Doppler winds measured along well-designed flight tracks. It is expected that the basic research presented in this paper will stimulate the use of Doppler wind systems in the determination of the detailed structure of winds accompanying mesoscale meteorological systems.
Abstract
Over maritime areas, meteorological satellite data generally have been used only in the study of synoptic scale systems. However, TIROS 7 pictures of summertime stratus over the northern Pacific, when rectified and combined with mesoscale radiation analyses, give valuable clues regarding the mesoscale dynamics of the lower atmosphere. Radiation data distinguish stratus from middle level cloud layers. Stratus bands, averaging 15 km. wide and oriented 16° to the left of the geostrophic wind, are thought to be related to horizontal roll vortices. Reflectance cross sections cross these bands indicate sharp variations in the thickness of the stratus.
Precisely gridded photographs show the relation of marked clear areas in the stratus to the topography of the Aleutian Islands. Only when execeding a critical size do isolated mountainous islands produce clear wakes. One island appears to be shedding a possible vortex street. A derived model for the flow of the stable air over long mountain ridges in the Alentians includes such phenomena as upstream blocking, windward slope bolster eddies, and extreme turbulent mixing on the Ice slopes causing clear areas to extend over 100 km. domnstream. Radiosonde data indicate that the mixed layer downstream is approximately the depth of the obstructing ridge.
Abstract
Over maritime areas, meteorological satellite data generally have been used only in the study of synoptic scale systems. However, TIROS 7 pictures of summertime stratus over the northern Pacific, when rectified and combined with mesoscale radiation analyses, give valuable clues regarding the mesoscale dynamics of the lower atmosphere. Radiation data distinguish stratus from middle level cloud layers. Stratus bands, averaging 15 km. wide and oriented 16° to the left of the geostrophic wind, are thought to be related to horizontal roll vortices. Reflectance cross sections cross these bands indicate sharp variations in the thickness of the stratus.
Precisely gridded photographs show the relation of marked clear areas in the stratus to the topography of the Aleutian Islands. Only when execeding a critical size do isolated mountainous islands produce clear wakes. One island appears to be shedding a possible vortex street. A derived model for the flow of the stable air over long mountain ridges in the Alentians includes such phenomena as upstream blocking, windward slope bolster eddies, and extreme turbulent mixing on the Ice slopes causing clear areas to extend over 100 km. domnstream. Radiosonde data indicate that the mixed layer downstream is approximately the depth of the obstructing ridge.
Abstract
An extensive aerial survey was made over a large portion of the area affected by the outbreak of tornadoes on Palm Sunday on Apr. 11, 1965. The destruction from the tornadoes extended over parts of six Midwestern States. Aerial and ground damage surveys were combined with eyewitness reports to determine the exact location and time of each tornado occurrence and its path. Radar pictures of the squall line clouds were used to verify the direction and speed of the tornado-producing clouds. Almost simultaneously with the first tornado touchdown in eastern Iowa, TIROS IX took pictures of the Midwest United States that showed a large tongue of cloud-free dry air behind the cold front. The vertical structure of the cold dome is discussed in connection with its role in the development of the tornadoes.
Two predictive parameters, namely, the best lifted index (BLI) and material differential advection (MDA) were developed and evaluated with data gathered on this outbreak of tornadoes.
The wind speed of a tornado in relation to its parent tornado cyclone is discussed in terms of an anemometer trace showing a peak gust speed of 151 mi hr–1. An indirect wind-speed estimate was also attempted by examining characteristic cycloidal marks left on the fields along the tornado paths. The ground speeds computed ranged from 166 to 180 mi hr–1 for one tornado.
Abstract
An extensive aerial survey was made over a large portion of the area affected by the outbreak of tornadoes on Palm Sunday on Apr. 11, 1965. The destruction from the tornadoes extended over parts of six Midwestern States. Aerial and ground damage surveys were combined with eyewitness reports to determine the exact location and time of each tornado occurrence and its path. Radar pictures of the squall line clouds were used to verify the direction and speed of the tornado-producing clouds. Almost simultaneously with the first tornado touchdown in eastern Iowa, TIROS IX took pictures of the Midwest United States that showed a large tongue of cloud-free dry air behind the cold front. The vertical structure of the cold dome is discussed in connection with its role in the development of the tornadoes.
Two predictive parameters, namely, the best lifted index (BLI) and material differential advection (MDA) were developed and evaluated with data gathered on this outbreak of tornadoes.
The wind speed of a tornado in relation to its parent tornado cyclone is discussed in terms of an anemometer trace showing a peak gust speed of 151 mi hr–1. An indirect wind-speed estimate was also attempted by examining characteristic cycloidal marks left on the fields along the tornado paths. The ground speeds computed ranged from 166 to 180 mi hr–1 for one tornado.
