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Abstract
If distributions of precipitation in the atmosphere could be understood in terms of their associated three-dimensional winds, then radar, radiosonde, and surface observations might be utilized in new ways as sources of information about the winds. In this paper, continuity equations for precipitation content of horizontally uniform updrafts, or to the cores of simple cells such as represented by showers formed in the absence of vertical wind shear. A parabolic profile of updrafts is assumed. When the precipitation profile is steady-state and the fall speed of precipitation is constant, the amount of precipitation per unit volume of air increases rapidly downward in mid-atmosphere. Near the surface, changes in the vertical are very small. Maxima of precipitation content occur aloft before steady conditions obtain throughout the updraft layer; in the steady case, maxima aloft may occur when the terminal fall speed of precipitation increases only slightly faster than the updrafts and with little increase of fall speed during their growth, as is often the case with snow. An explanation is offered for the difference between the vertical distributions of precipitation associated with widespread systems and with showers, and means for practical utilization of the results are suggested.
Abstract
If distributions of precipitation in the atmosphere could be understood in terms of their associated three-dimensional winds, then radar, radiosonde, and surface observations might be utilized in new ways as sources of information about the winds. In this paper, continuity equations for precipitation content of horizontally uniform updrafts, or to the cores of simple cells such as represented by showers formed in the absence of vertical wind shear. A parabolic profile of updrafts is assumed. When the precipitation profile is steady-state and the fall speed of precipitation is constant, the amount of precipitation per unit volume of air increases rapidly downward in mid-atmosphere. Near the surface, changes in the vertical are very small. Maxima of precipitation content occur aloft before steady conditions obtain throughout the updraft layer; in the steady case, maxima aloft may occur when the terminal fall speed of precipitation increases only slightly faster than the updrafts and with little increase of fall speed during their growth, as is often the case with snow. An explanation is offered for the difference between the vertical distributions of precipitation associated with widespread systems and with showers, and means for practical utilization of the results are suggested.
Abstract
The eye region of Hurricane Edna (1954) is studied with the principal aid of radar and dropsonde data. Vertical sections show that over the eye there was a thick layer derived from the wall cloud which bounded the eye on the northeast. Precipitation fell from this upper layer into drier air beneath. A reasonable mechanism is thereby suggested by which large moisture values can become associated with air in the eye without producing the wet bulb potential temperatures or high winds characteristic of the rain-filled masses outside the eye.
Radar data giving the height of the “bright band” or melting level show that the warm core structure of Edna was most pronounced within the radius if maximum surface winds. The result is qualitatively confirmed by soundings and by comparison of surface winds and the speeds of radar weather elements in various portions of the storm. The radar photographs also show that heavy precipitation near the eye of Edna was bounded sharply in the western semicircle along an east-west line through the center of the storm. This boundary must be associated with a rather large change of vertical air speeds and therefore has special dynamic significance.
Abstract
The eye region of Hurricane Edna (1954) is studied with the principal aid of radar and dropsonde data. Vertical sections show that over the eye there was a thick layer derived from the wall cloud which bounded the eye on the northeast. Precipitation fell from this upper layer into drier air beneath. A reasonable mechanism is thereby suggested by which large moisture values can become associated with air in the eye without producing the wet bulb potential temperatures or high winds characteristic of the rain-filled masses outside the eye.
Radar data giving the height of the “bright band” or melting level show that the warm core structure of Edna was most pronounced within the radius if maximum surface winds. The result is qualitatively confirmed by soundings and by comparison of surface winds and the speeds of radar weather elements in various portions of the storm. The radar photographs also show that heavy precipitation near the eye of Edna was bounded sharply in the western semicircle along an east-west line through the center of the storm. This boundary must be associated with a rather large change of vertical air speeds and therefore has special dynamic significance.
Abstract
In order to extend our understanding of the relationships between the atmospheric distributions of water substance and the wind, a two-dimensional continuity equation for water substance is studied in terms of model wind fields. Water-budget parameters are defined, and digital methods are used to develop time-dependent solutions of the continuity equation with the assumptions that the fall speed of condensate with respect to air is uniform, that condensation in rising saturated air and evaporation in subsaturated air occur instantaneously, and that the assumed wind field does not vary with time. It is shown that, for given distributions of the wind and condensation rate, the shape of the developing water distribution is determined only by the ratio of the updrafts to the fall speed of condensate.
The distributions of water substance derived from the continuity equations show features analogous to radar observations of water substance in the real atmosphere. For example, when the falling speed of condensate is larger than the maximum speed of updrafts, relatively small amounts of condensed water exist aloft, and vertical profiles of condensed water content correspond to those observed by radar in widespread precipitation. When the maximum updrafts are larger, condensate occurs aloft in large amounts: in these cases, the vertical protiles display upper-level maxima similar to radar observations of thunderstorms and early stages of shower development.
When fall speeds are large, nearly all the water condensed is also precipitated. However, the amount of precipitation decreases to about 35 per cent of the condensed water when fall speeds are comparatively quite small; in these cases, condensate aloft is so distributed that much of it evaporates in the downdraft part of the assumed wind field.
The methods used in this study can readily be adapted to study a variety of wind fields and to incorporate less restrictive postulates regarding the physical processes of precipitation.
Abstract
In order to extend our understanding of the relationships between the atmospheric distributions of water substance and the wind, a two-dimensional continuity equation for water substance is studied in terms of model wind fields. Water-budget parameters are defined, and digital methods are used to develop time-dependent solutions of the continuity equation with the assumptions that the fall speed of condensate with respect to air is uniform, that condensation in rising saturated air and evaporation in subsaturated air occur instantaneously, and that the assumed wind field does not vary with time. It is shown that, for given distributions of the wind and condensation rate, the shape of the developing water distribution is determined only by the ratio of the updrafts to the fall speed of condensate.
The distributions of water substance derived from the continuity equations show features analogous to radar observations of water substance in the real atmosphere. For example, when the falling speed of condensate is larger than the maximum speed of updrafts, relatively small amounts of condensed water exist aloft, and vertical profiles of condensed water content correspond to those observed by radar in widespread precipitation. When the maximum updrafts are larger, condensate occurs aloft in large amounts: in these cases, the vertical protiles display upper-level maxima similar to radar observations of thunderstorms and early stages of shower development.
When fall speeds are large, nearly all the water condensed is also precipitated. However, the amount of precipitation decreases to about 35 per cent of the condensed water when fall speeds are comparatively quite small; in these cases, condensate aloft is so distributed that much of it evaporates in the downdraft part of the assumed wind field.
The methods used in this study can readily be adapted to study a variety of wind fields and to incorporate less restrictive postulates regarding the physical processes of precipitation.
