Generation of Internal-and External-Mode Motions from Internal Heating: Effects of Vertical Shear and Damping

H. Lim Department of Meteorology, Naval Postgraduate School, Monterey. CA 93943

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C-P. Chang Department of Meteorology, Naval Postgraduate School, Monterey. CA 93943

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Abstract

Tropical heating due to latent heat release has been proposed as a source that may influence midlatitude weather through teleconnection patterns. However, such heating is usually-internal (with the maximum in the midtroposphere) and, by itself, inefficient in exciting external, barotropic-type responses, which are necessary for the teleconnection mechanism. To study this problem, a simple two-level model is used to investigate the effect of vertical shear, differential damping and the planetary boundary layer on the characteristics of the atmospheric response to internal heating.

All three effects are found to enable a transfer of energy from the internal-mode motions, which are directly forced by the internal heating, to external-mode motions. To generate external-mode divergent motions, it is necessary to have a planetary boundary layer or other equivalent effects that force vertical motion at the bottom of the atmosphere. The efficiency of generation increases with the horizontal scale. On the other hand, vertical shear is normally the main effect for generating external-mode rotational motions, and the efficiency of generation decreases with the horizontal scale. In addition, this efficiency depends strongly on the relative vertical shear (vertical shear divided by vertical-mean wind). When the relative vertical shear is greater than unity, the external mode finally attains an amplitude larger than that of the internal mode.

The solution of an initial-value problem shows that in tropical regions, the process of energy transfer from internal to external-mode motions takes about two weeks to complete. This rather slow rate of energy transfer to external-mode motions implies that in a given vertical wind shear, the responses to a transient heating lasting for only a few days will have stronger internal mode (more baroclinic) structure than those to a steady-state heating.

Abstract

Tropical heating due to latent heat release has been proposed as a source that may influence midlatitude weather through teleconnection patterns. However, such heating is usually-internal (with the maximum in the midtroposphere) and, by itself, inefficient in exciting external, barotropic-type responses, which are necessary for the teleconnection mechanism. To study this problem, a simple two-level model is used to investigate the effect of vertical shear, differential damping and the planetary boundary layer on the characteristics of the atmospheric response to internal heating.

All three effects are found to enable a transfer of energy from the internal-mode motions, which are directly forced by the internal heating, to external-mode motions. To generate external-mode divergent motions, it is necessary to have a planetary boundary layer or other equivalent effects that force vertical motion at the bottom of the atmosphere. The efficiency of generation increases with the horizontal scale. On the other hand, vertical shear is normally the main effect for generating external-mode rotational motions, and the efficiency of generation decreases with the horizontal scale. In addition, this efficiency depends strongly on the relative vertical shear (vertical shear divided by vertical-mean wind). When the relative vertical shear is greater than unity, the external mode finally attains an amplitude larger than that of the internal mode.

The solution of an initial-value problem shows that in tropical regions, the process of energy transfer from internal to external-mode motions takes about two weeks to complete. This rather slow rate of energy transfer to external-mode motions implies that in a given vertical wind shear, the responses to a transient heating lasting for only a few days will have stronger internal mode (more baroclinic) structure than those to a steady-state heating.

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