On the Transformation of a Tropical Easterly Wave into a Tropical Depression: A Simple Numerical Study

Yoshio Kurihara Geophysical Fluid Dynamics Laboratory/N0AA, Princeton University, Princeton, NJ 08542

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Mitsuhiro Kawase Geophysical fluid Dynamics Program, Princeton University, Princeton, NJ 08542

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Abstract

Through the time integration of a simple numerical model, the transfomation of a tropical easterly wave into a tropical depression was investigated. The initial condition selected for the model is a slowly decaying adiabatic linear normal mode resembling an easterly wave. It was found that the addition of a CISK (conditional instability of the second kind) type heating effect to the model results in the growth of the wave. Addition of the nonlinearity effect alone, which includes the nonlinear zonal advection and vertical stretching of relative vorticity, has little impact on the wave evolution. However, if the above nonlinearity effect is combined with the heating effect, it can cause the contraction of the disturbance and enhance the development The initial free wave undergoes significant structural changes during its transformation into a developing system.

Experiments were also performed for three different waves under various basic flow conditions. The obtained results agree with a conclusion of the three-dimensional simulation experiments by Tuleya and Kurihara; the coupling between the upper-level wind and the propagating disturbance at low levels may be an important mechanism in the formation of a tropical depression in the trough region of an easterly wave.

Abstract

Through the time integration of a simple numerical model, the transfomation of a tropical easterly wave into a tropical depression was investigated. The initial condition selected for the model is a slowly decaying adiabatic linear normal mode resembling an easterly wave. It was found that the addition of a CISK (conditional instability of the second kind) type heating effect to the model results in the growth of the wave. Addition of the nonlinearity effect alone, which includes the nonlinear zonal advection and vertical stretching of relative vorticity, has little impact on the wave evolution. However, if the above nonlinearity effect is combined with the heating effect, it can cause the contraction of the disturbance and enhance the development The initial free wave undergoes significant structural changes during its transformation into a developing system.

Experiments were also performed for three different waves under various basic flow conditions. The obtained results agree with a conclusion of the three-dimensional simulation experiments by Tuleya and Kurihara; the coupling between the upper-level wind and the propagating disturbance at low levels may be an important mechanism in the formation of a tropical depression in the trough region of an easterly wave.

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