COLA Report 19

Wave-CISK, The Evaporation-Wind Feedback, and the Intraseasonal Oscillation: Phase Propagation and Scale Selection

Ben P. Kirtman and Anandu D. Vernekar

August 1995


Abstract

The scale selection and phase propagation characteristics of the wave-CISK and the E-W feedback instability mechanisms are examined in a simple five layer model with and without positive only heating. With wave-like internal heating, the model gives classical wave-CISK vertical mode coalescence where shallow heating yields the slowest propagation speeds and the smallest horizontal scales are the most unstable. When the E-W feedback is introduced into the model with linear heating, the phase propagation and the scale selection characteristics change. The E-W feedback modes have larger phase speeds and, in the case of shallow heating, the most unstable mode propagates as much as 15 m s-1 faster than the corresponding wave-CISK mode. The growth rate of the wave-CISK modes increases linearly with increasing zonal wavenumber, whereas the growth rate of the E-W feedback modes increases with increasing zonal wavenumber but at a significantly slower rate. In addition, the growth rate of the E-W feedback modes is bounded from above as the spatial scale becomes small; in contrast, the growth rate of the wave-CISK modes is unbounded. A number of numerical simulations with positive definite heating confirm the phase propagation characteristics of the model with linear heating. The scale selection characteristics, however, of the model with positive definite heating are distinct from the model with linear heating. It is shown that, while the a single precipitation/circulation cell wave-CISK mode is the most unstable configuration, any number of precipitation/circulation cells can equilibrate. The E-W feedback modes, on the other hand, always select a zonal wavenumber dominated spatial structure with a single precipitation/circulation cell regardless of initial condition.


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last update: 31 August 1995
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