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- Author or Editor: Herbert Riehl x
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Abstract
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Abstract
It was a main objective of the Venezuela experiments in 1969 and 1972 to examine the links between low and high atmosphere furnished by mesoscale rain areas and embedded undilute towers and, as far as possible, to attain a quantitative description of the role of cumulonimbi in tropical weather systems. A mesoscale model could be defined with precipitation yield of 2.5 cm in 2 h over an area of 2000 km2.
The ratio of cumulonimbus precipitation to the condensate in undilute updrafts is 0.5 compared to zero in the trades and 1.0 in hurricane cores. Heat is eroded from cumulonimbus towers to the environment in the upper troposphere. Freezing processes, previously not considered, contribute substantially to heating the upper troposphere. These two mechanisms compensate for about half the radiation cooling in the tropical belt. The other half must be derived from sinking motion with downward mass transport attaining one-quarter of the ascent in the towers.
The synoptic-scale rain areas and their mesosystems are subject to external forcing in the Caribbean area, mainly extratropical troughs intruding into the tropics from the north and large cloud lines progapating northward from southern South America. The inflow into the mesosystems usually is at their forward edge in low levels. But in the middle troposphere the air overtakes the radar echoes, suggesting a mechanism for limiting their life.
The thermal structure of rain areas is stable, confirming that the mesosystem rain areas cold-cored with respect to their environment at least to the middle troposphere. Only the concentration of ascent in undilute towers permits the tropical rain system to do swork maintaining the general circulation.
Abstract
It was a main objective of the Venezuela experiments in 1969 and 1972 to examine the links between low and high atmosphere furnished by mesoscale rain areas and embedded undilute towers and, as far as possible, to attain a quantitative description of the role of cumulonimbi in tropical weather systems. A mesoscale model could be defined with precipitation yield of 2.5 cm in 2 h over an area of 2000 km2.
The ratio of cumulonimbus precipitation to the condensate in undilute updrafts is 0.5 compared to zero in the trades and 1.0 in hurricane cores. Heat is eroded from cumulonimbus towers to the environment in the upper troposphere. Freezing processes, previously not considered, contribute substantially to heating the upper troposphere. These two mechanisms compensate for about half the radiation cooling in the tropical belt. The other half must be derived from sinking motion with downward mass transport attaining one-quarter of the ascent in the towers.
The synoptic-scale rain areas and their mesosystems are subject to external forcing in the Caribbean area, mainly extratropical troughs intruding into the tropics from the north and large cloud lines progapating northward from southern South America. The inflow into the mesosystems usually is at their forward edge in low levels. But in the middle troposphere the air overtakes the radar echoes, suggesting a mechanism for limiting their life.
The thermal structure of rain areas is stable, confirming that the mesosystem rain areas cold-cored with respect to their environment at least to the middle troposphere. Only the concentration of ascent in undilute towers permits the tropical rain system to do swork maintaining the general circulation.
Abstract
The question, Why are there discrete synoptic weather systems in the tropics without a basic current that can become baroclinically unstable, is considered. Release of kinetic energy through vertical circulations and maintenance of the surface flow against friction by mean values indicate that existence of a simple general circulation cell should be possible. But, in the descending branch the requisite poleward temperature gradient cannot be maintained with radiation cooling. An additional mechanism for heat removal must be sought.
Extratropical troughs extending deep into the tropics may provide such a mechanism. At first their role in setting off tropical rain areas is investigated dynamically. It is found that a large imbalance of forces develops in the northerly current on the western side of the troughs which causes the equator-bound air to turn sharply clockwise. Absolute vorticity approaches zero and intense high-tropospheric divergence is initiated acting as a siphon on the lower layers.
Then the troughs are investigated for their capability of exchanging heat, mass and angular momentum between the tropics and higher latitudes during summer. Two or three discrete troughs around the hemisphere suffice to accomplish these functions mainly through the presence of cyclonic flow trajectories west and anticyclonic trajectories east of the trough axis. Thus, the entire exchange across the latitude circles can be seen as a unified process.
Abstract
The question, Why are there discrete synoptic weather systems in the tropics without a basic current that can become baroclinically unstable, is considered. Release of kinetic energy through vertical circulations and maintenance of the surface flow against friction by mean values indicate that existence of a simple general circulation cell should be possible. But, in the descending branch the requisite poleward temperature gradient cannot be maintained with radiation cooling. An additional mechanism for heat removal must be sought.
Extratropical troughs extending deep into the tropics may provide such a mechanism. At first their role in setting off tropical rain areas is investigated dynamically. It is found that a large imbalance of forces develops in the northerly current on the western side of the troughs which causes the equator-bound air to turn sharply clockwise. Absolute vorticity approaches zero and intense high-tropospheric divergence is initiated acting as a siphon on the lower layers.
Then the troughs are investigated for their capability of exchanging heat, mass and angular momentum between the tropics and higher latitudes during summer. Two or three discrete troughs around the hemisphere suffice to accomplish these functions mainly through the presence of cyclonic flow trajectories west and anticyclonic trajectories east of the trough axis. Thus, the entire exchange across the latitude circles can be seen as a unified process.
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Abstract
Several simple integrations are performed to determine to what extent a steady, symmetrical hurricane model can be used to approximate observed storm structure. A two-layer model with inflow and outflow is considered. In the outflow, the absolute angular momentum about the vertical axis at the hurricane center is conserved. In the inflow, conservation of potential vorticity is assumed. For specified outer boundary conditions this assumption determines the distribution of momentum transport from air to ocean and there-with the radial profile of the tangential wind component. The local heat source at the ocean surface is assumed to supply the energy for the generation of hurricane winds. Given this heat source and the vertical wind shear between inflow and outflow layers from the dynamic model, a relation must exist between heat source and momentum sink at the air-water interface, if a particular wind field is to exist in steady state. This relation is computed.
Various empirical tests are performed to assess the degree of reality of the model. Observations from hurricanes between 1945 and 1958 are used. Most observational material is taken from research missions conducted by aircraft of the National Hurricane Research Project. The crude model gives a rather good approximation to several hurricanes, especially the more intense ones. But it cannot explain an measurements taken by NHRP; of course, steady state did not exist in all situations investigated by aircraft.
It may be concluded that computations with steady, symmetrical models are relevant at least to partial understanding of hurricanes.
Abstract
Several simple integrations are performed to determine to what extent a steady, symmetrical hurricane model can be used to approximate observed storm structure. A two-layer model with inflow and outflow is considered. In the outflow, the absolute angular momentum about the vertical axis at the hurricane center is conserved. In the inflow, conservation of potential vorticity is assumed. For specified outer boundary conditions this assumption determines the distribution of momentum transport from air to ocean and there-with the radial profile of the tangential wind component. The local heat source at the ocean surface is assumed to supply the energy for the generation of hurricane winds. Given this heat source and the vertical wind shear between inflow and outflow layers from the dynamic model, a relation must exist between heat source and momentum sink at the air-water interface, if a particular wind field is to exist in steady state. This relation is computed.
Various empirical tests are performed to assess the degree of reality of the model. Observations from hurricanes between 1945 and 1958 are used. Most observational material is taken from research missions conducted by aircraft of the National Hurricane Research Project. The crude model gives a rather good approximation to several hurricanes, especially the more intense ones. But it cannot explain an measurements taken by NHRP; of course, steady state did not exist in all situations investigated by aircraft.
It may be concluded that computations with steady, symmetrical models are relevant at least to partial understanding of hurricanes.
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