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- Author or Editor: C. W. Newton x
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Abstract
A detailed analysis is presented to show the atmospheric structure during the earlier formative stages of a deep, upper trough. Over the western United States, a well-marked west-east frontal layer extends through the whole troposphere; in upper levels, this disappears within a short distance eastward. Individual changes in vorticity, vertical shear, horizontal temperature gradient, and vertical stability are discussed for air particles streaming eastward, out of the frontal layer. Factors in individual frontolysis vary in relative importance at different levels. In particular, variations of the vertical and horizontal components of vorticity and potential temperature gradient are chiefly effected, in the middle troposphere, by horizontal gradients of vertical motion and vertical gradients of the non-geostrophic wind component normal to the frontal layer. Near the maximum-wind level and near the earth's surface, divergence processes and deformation fields in the horizontal flow are of predominant importance. A general scheme is suggested for the vertical and transverse motions during frontolysis (or frontogenesis), in which the varying processes leading to changes in wind and temperature fields at different levels are interrelated in a three-dimensionally consistent manner.
Abstract
A detailed analysis is presented to show the atmospheric structure during the earlier formative stages of a deep, upper trough. Over the western United States, a well-marked west-east frontal layer extends through the whole troposphere; in upper levels, this disappears within a short distance eastward. Individual changes in vorticity, vertical shear, horizontal temperature gradient, and vertical stability are discussed for air particles streaming eastward, out of the frontal layer. Factors in individual frontolysis vary in relative importance at different levels. In particular, variations of the vertical and horizontal components of vorticity and potential temperature gradient are chiefly effected, in the middle troposphere, by horizontal gradients of vertical motion and vertical gradients of the non-geostrophic wind component normal to the frontal layer. Near the maximum-wind level and near the earth's surface, divergence processes and deformation fields in the horizontal flow are of predominant importance. A general scheme is suggested for the vertical and transverse motions during frontolysis (or frontogenesis), in which the varying processes leading to changes in wind and temperature fields at different levels are interrelated in a three-dimensionally consistent manner.
Abstract
Based on recently published mean maps, the torques upon the atmosphere owing to pressure differences between the west and east faces of mountain ranges are computed for the whole earth, for four midseason months. Considering the year as a whole, mountain torques are eastward in high and low latitudes, and westward in middle latitudes. Individual mountain complexes have mixed effects; the greatest drain of momentum is by the relatively low mountains of eastern Siberia while the average effect of the Himalayas is small. The momentum drain by the Andes is greatest in summer, while in winter their influence is overcome by the eastward torque of Africa.
Seasonal variations are greatest near latitudes 15°, where in the winter of each hemisphere there is an eastward pressure torque that reverses to westward in summer. Relative to frictional torques, mountain torques are greatest in middle northern latitudes in April and July, when they approach the same magnitude.
Abstract
Based on recently published mean maps, the torques upon the atmosphere owing to pressure differences between the west and east faces of mountain ranges are computed for the whole earth, for four midseason months. Considering the year as a whole, mountain torques are eastward in high and low latitudes, and westward in middle latitudes. Individual mountain complexes have mixed effects; the greatest drain of momentum is by the relatively low mountains of eastern Siberia while the average effect of the Himalayas is small. The momentum drain by the Andes is greatest in summer, while in winter their influence is overcome by the eastward torque of Africa.
Seasonal variations are greatest near latitudes 15°, where in the winter of each hemisphere there is an eastward pressure torque that reverses to westward in summer. Relative to frictional torques, mountain torques are greatest in middle northern latitudes in April and July, when they approach the same magnitude.
Abstract
The life cycle of a destructive cyclone over the United States during 16 to 21 November 1948 is investigated. The vorticity advection at 300 millibars, as an indicator of divergence in the upper troposphere, is shown to be closely related to the organization, maintenance and final collapse of the surface cyclone.
The early period of most rapid cyclogenesis is investigated with reference to orographic factors. It is found that the cyclogenesis took place when the region of strongest upward motion at 500 mb, directly beneath the upper-tropospheric jet stream, became superimposed over the band of maximum descending motion at the earth's surface, giving maximum vertical stretching in the lower troposphere.
Computations of the contributions of horizontal convergence, and of lateral differences of friction, to circulation changes indicate that, during the period of most rapid development, the latter was of about equal importance with the former in increasing the circulation. The mechanism of circulation production, by friction, and orographic influences on the path taken by the cyclone, are discussed qualitatively.
Abstract
The life cycle of a destructive cyclone over the United States during 16 to 21 November 1948 is investigated. The vorticity advection at 300 millibars, as an indicator of divergence in the upper troposphere, is shown to be closely related to the organization, maintenance and final collapse of the surface cyclone.
The early period of most rapid cyclogenesis is investigated with reference to orographic factors. It is found that the cyclogenesis took place when the region of strongest upward motion at 500 mb, directly beneath the upper-tropospheric jet stream, became superimposed over the band of maximum descending motion at the earth's surface, giving maximum vertical stretching in the lower troposphere.
Computations of the contributions of horizontal convergence, and of lateral differences of friction, to circulation changes indicate that, during the period of most rapid development, the latter was of about equal importance with the former in increasing the circulation. The mechanism of circulation production, by friction, and orographic influences on the path taken by the cyclone, are discussed qualitatively.
Abstract
Using upper-air soundings taken by the Thunderstorm Project and surface and serological data available through teletype distribution, a detailed three-dimensional analysis of a prefrontal squall line is presented and certain new observational features of squall-line structure are shown.
It is shown that squall-line thunderstorms appear in some cases to form first over the cold-front surface and subsequently move into the warm sector. Serial ascents taken in such a case show that there is a distinct cold front at the forward edge of the thunderstorm area, which coincides with the squall line observed at the ground. It is suggested that the squall-line activity can be accounted for partly as a result of this front, and partly by the continuous generation of new thunderstorms as a result of convergence-divergence patterns produced by the vertical transfer of horizontal momentum in pre-existent thunderstorms. This is augmented by solenoidal circulations due to unbalance between the “thermal wind” and the actual vertical wind shear when the latter is diminished by vertical mixing of horizontal momentum. An essential source of energy for maintaining squall-line activity appears to be the kinetic energy of air brought down from higher levels.
Abstract
Using upper-air soundings taken by the Thunderstorm Project and surface and serological data available through teletype distribution, a detailed three-dimensional analysis of a prefrontal squall line is presented and certain new observational features of squall-line structure are shown.
It is shown that squall-line thunderstorms appear in some cases to form first over the cold-front surface and subsequently move into the warm sector. Serial ascents taken in such a case show that there is a distinct cold front at the forward edge of the thunderstorm area, which coincides with the squall line observed at the ground. It is suggested that the squall-line activity can be accounted for partly as a result of this front, and partly by the continuous generation of new thunderstorms as a result of convergence-divergence patterns produced by the vertical transfer of horizontal momentum in pre-existent thunderstorms. This is augmented by solenoidal circulations due to unbalance between the “thermal wind” and the actual vertical wind shear when the latter is diminished by vertical mixing of horizontal momentum. An essential source of energy for maintaining squall-line activity appears to be the kinetic energy of air brought down from higher levels.
Instantaneous height tendencies computed from different analyses of the same 500-mb chart illustrate that discrepancies in analysis or observations can have a serious effect on the accuracy of numerical forecasts. The present status of the Northern-Hemisphere aerological network is reviewed, stressing the inhomogeneity of the radiosonde network, the need for observations in fringe areas surrounding the present dense networks, and the desirability of more radio-wind stations. The suggestion is made that machine prognoses incorporating variations of fictitious observations could be used to evaluate the improvements needed and the desirability of setting up new aerological stations in particular locations.
Instantaneous height tendencies computed from different analyses of the same 500-mb chart illustrate that discrepancies in analysis or observations can have a serious effect on the accuracy of numerical forecasts. The present status of the Northern-Hemisphere aerological network is reviewed, stressing the inhomogeneity of the radiosonde network, the need for observations in fringe areas surrounding the present dense networks, and the desirability of more radio-wind stations. The suggestion is made that machine prognoses incorporating variations of fictitious observations could be used to evaluate the improvements needed and the desirability of setting up new aerological stations in particular locations.
Abstract
A mean cross section is constructed from a number of individually analyzed daily cross sections, using the frontal layer as a reference for the coordinates. This preserves the thermal gradients observed in the individual cases, and minimizes the effect of observational errors. Computed wind velocity, wind shear, and vorticity patterns are discussed, particularly with regard to the strong anticyclonic shear and low absolute vorticity found to the south of the jet stream near the tropopause level.
Abstract
A mean cross section is constructed from a number of individually analyzed daily cross sections, using the frontal layer as a reference for the coordinates. This preserves the thermal gradients observed in the individual cases, and minimizes the effect of observational errors. Computed wind velocity, wind shear, and vorticity patterns are discussed, particularly with regard to the strong anticyclonic shear and low absolute vorticity found to the south of the jet stream near the tropopause level.
Abstract
An attempt is made to study, by means of a detailed synoptic analysis, the three-dimensional air-motion in a typical outbreak of a polar air-mass. The mean vertical velocities at different levels over the entire cold-air region south of latitude 45°N are computed by use of the principle of continuity of mass. The maximum mean subsidence is found to be nearly 2 km per day at about the 600-mb level. In addition, the distribution of vertical velocity in different parts of the cold tongue is computed by means of trajectories on isentropic surfaces, and some examples of characteristic three-dimensional air trajectories are given. The results show that a typical outbreak of cold air can be regarded as an effective cell for meridional exchange of mass, heat and angular momentum.
Abstract
An attempt is made to study, by means of a detailed synoptic analysis, the three-dimensional air-motion in a typical outbreak of a polar air-mass. The mean vertical velocities at different levels over the entire cold-air region south of latitude 45°N are computed by use of the principle of continuity of mass. The maximum mean subsidence is found to be nearly 2 km per day at about the 600-mb level. In addition, the distribution of vertical velocity in different parts of the cold tongue is computed by means of trajectories on isentropic surfaces, and some examples of characteristic three-dimensional air trajectories are given. The results show that a typical outbreak of cold air can be regarded as an effective cell for meridional exchange of mass, heat and angular momentum.
Abstract
In typical squall-line situations wherein the wind veers strongly with height, individual convective storms move as much as 60 deg right or 30 deg left of the direction of the mean wind in the cloud layer. It is shown that, on the average, the radar echoes having largest diameters move farthest to right of the wind.
This behavior is consistent with physical considerations and with supply-and-demand requirements of the storm water budget. For a given rainfall intensity, the amount of water precipitated by a storm is proportional to its area or to the diameter squared. The amount of water vapor intercepted is proportional directly to the diameter, and to the velocity of the storm relative to the winds of the lower-tropospheric moist layer. A large storm must intercept more vapor in proportion to its diameter than a small one, requiring a larger migration velocity relative to the moist layer. This requirement is satisfied if (wind veering with height) large storms move toward the right of the mean wind.
Based on these considerations, a simple expression is derived for the direction of storm motion as related to storm diameter. This describes the mean behavior fairly well, but there is considerable residual scatter. With this taken into account, an expression is given for the probability of storm passage over a given point as related to the initial storm location and size.
Some characteristic patterns of development are illustrated. New convective elements tend to form on or amalgamate with the right-hand side or end of an existing storm cluster or squall line, somewhat on the up-wind side relative to the mean wind. This pattern of generation contributes to the movement of large storm clusters strongly toward the right of the winds, and also to make large storms move consistently more slowly than small ones.
Abstract
In typical squall-line situations wherein the wind veers strongly with height, individual convective storms move as much as 60 deg right or 30 deg left of the direction of the mean wind in the cloud layer. It is shown that, on the average, the radar echoes having largest diameters move farthest to right of the wind.
This behavior is consistent with physical considerations and with supply-and-demand requirements of the storm water budget. For a given rainfall intensity, the amount of water precipitated by a storm is proportional to its area or to the diameter squared. The amount of water vapor intercepted is proportional directly to the diameter, and to the velocity of the storm relative to the winds of the lower-tropospheric moist layer. A large storm must intercept more vapor in proportion to its diameter than a small one, requiring a larger migration velocity relative to the moist layer. This requirement is satisfied if (wind veering with height) large storms move toward the right of the mean wind.
Based on these considerations, a simple expression is derived for the direction of storm motion as related to storm diameter. This describes the mean behavior fairly well, but there is considerable residual scatter. With this taken into account, an expression is given for the probability of storm passage over a given point as related to the initial storm location and size.
Some characteristic patterns of development are illustrated. New convective elements tend to form on or amalgamate with the right-hand side or end of an existing storm cluster or squall line, somewhat on the up-wind side relative to the mean wind. This pattern of generation contributes to the movement of large storm clusters strongly toward the right of the winds, and also to make large storms move consistently more slowly than small ones.
By means of hourly rainfall data from the Hydroclimatic Network, the motions of large rainstorms, of the kind associated with squall lines, are examined in relation to the winds aloft. Very little correlation is found between the speed of movement of the rainstorms and the wind speed at any level, although the fastest moving storms were associated with strong winds aloft. Significant correlation is found between direction of motion of rainstorms, and wind direction at 700 mb or higher levels. On the average, the rainstorms move with an appreciable component toward right of the wind direction. The difference between these results, and those from other studies based on small precipitation areas, is ascribed to propagation. The mechanism involved is discussed briefly.
By means of hourly rainfall data from the Hydroclimatic Network, the motions of large rainstorms, of the kind associated with squall lines, are examined in relation to the winds aloft. Very little correlation is found between the speed of movement of the rainstorms and the wind speed at any level, although the fastest moving storms were associated with strong winds aloft. Significant correlation is found between direction of motion of rainstorms, and wind direction at 700 mb or higher levels. On the average, the rainstorms move with an appreciable component toward right of the wind direction. The difference between these results, and those from other studies based on small precipitation areas, is ascribed to propagation. The mechanism involved is discussed briefly.
