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BUDGET OF ANGULAR MOMENTUM AND ENERGY IN TROPICAL CYCLONES

E. PalménAcademy of Finland

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Herbert RiehlUniversity of Chicago

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Abstract

The surface stress in tropical storms is computed as a function of the radius, from mean wind data in the troposphere. If only the symmetrical part of the circulation is considered, the stress ranges from 1 dyne per square centimeter in the outskirts to 20 dy/cm2 at a distance of 1 degree latitude from the center. Inclusion of the mean asymmetry of the wind field, resulting from superposition of steering current and vortex, yields values about 20 per cent higher.

The absolute angular momentum budget shows that most net inward momentum transport is carried by the symmetrical part of the circulation near the core, and that the asymmetrical part contributes up to 50 per cent at a distance of 6 deg lat from the center. Calculation of the angular momentum transport due to the earth's rotation in inflow and outflow layers shows that the vertical momentum transport is directed upward everywhere; outside the 2-deg radius, this transport is accomplished by eddies.

Generation and dissipation of kinetic energy are calculated. The generation depends on the vertical correlation between radial flow component and pressure gradient which, for production of kinetic energy, must be positive, i.e., the strongest inflow must occur at the strongest inward directed pressure gradient. This shows that kinetic energy production within the cyclone can take place only if the cyclone is of the warm core type.

Next, the importance of a local heat source at the ocean surface for production of the observed inward warming in the rain area is discussed, and a relation between the local heat source and the central pressure is given. A general energy balance shows that about 3 per cent of the latent heat released is converted to kinetic energy, and that a considerable export of potential plus internal energy takes place from tropical storms.

Abstract

The surface stress in tropical storms is computed as a function of the radius, from mean wind data in the troposphere. If only the symmetrical part of the circulation is considered, the stress ranges from 1 dyne per square centimeter in the outskirts to 20 dy/cm2 at a distance of 1 degree latitude from the center. Inclusion of the mean asymmetry of the wind field, resulting from superposition of steering current and vortex, yields values about 20 per cent higher.

The absolute angular momentum budget shows that most net inward momentum transport is carried by the symmetrical part of the circulation near the core, and that the asymmetrical part contributes up to 50 per cent at a distance of 6 deg lat from the center. Calculation of the angular momentum transport due to the earth's rotation in inflow and outflow layers shows that the vertical momentum transport is directed upward everywhere; outside the 2-deg radius, this transport is accomplished by eddies.

Generation and dissipation of kinetic energy are calculated. The generation depends on the vertical correlation between radial flow component and pressure gradient which, for production of kinetic energy, must be positive, i.e., the strongest inflow must occur at the strongest inward directed pressure gradient. This shows that kinetic energy production within the cyclone can take place only if the cyclone is of the warm core type.

Next, the importance of a local heat source at the ocean surface for production of the observed inward warming in the rain area is discussed, and a relation between the local heat source and the central pressure is given. A general energy balance shows that about 3 per cent of the latent heat released is converted to kinetic energy, and that a considerable export of potential plus internal energy takes place from tropical storms.

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