Dependence of equatorial precipitation activity
on the vertical profile of radiative cooling
in an aqua-planet experiment

An aqua-planet experiment with a simplified GCM (general circulation model) is performed to examine the effect of vertical profile of radiative cooling on the appearance of equatorial precipitation structures in GCMs. Results are demonstrated by animation of time series and composite analysis of circulation structure referring to precipitation events.

One of the possible causes for the difference of the appearance of precipitation patterns in GCMs is the difference of vertical profile of condensation heating. Numaguti and Hayashi (1991) argued that, by performing an aqua-planet experiment, the eastward moving grid-scale precipitation structures are maintained by the wave-CISK (conditional instability of the second kind) dynamics. In order for the wave-CISK dynamics to operate, the vertical profile of condensation heating has a great influence. If the eastward moving grid-scale precipitation structures in a GCM are indeed maintained by the wave-CISK dynamics, they will become more evident when the vertical profile of condensation heating is more consistent with the wave-CISK dynamics. With this expectation in mind, we perform a series of experiments in which the profile of radiative cooling is changed through tuning the absorption coefficients of longwave radiation scheme, expecting that the vertical profile of condensation heating changes accordingly.

When the Kuo scheme is employed as the cumulus parameterization, an expected change of the behavior of the precipitation feature is obtained. Eastward motion of grid-scale precipitation structures becomes more evident, when the radiative cooling is tuned to have a peak in the upper troposphere. A pronounced westward phase tilt, which is consistent with the prediction of the wave-CISK theory, is observed. On the other hand, westward motion becomes more evident, when the radiative cooling has a peak in the lower troposphere. No phase tilt is observed, suggesting a structure consistent with the CIFK (conditional instability of the first kind) theory. When the moist convective adjustment (MCA) scheme is employed as the cumulus parameterization, the vertical profile of condensation heating does not show the expected change through the change of the absorption coefficients of longwave radiation, and the behavior of the equatorial grid-scale precipitation structure does not show any prominent difference.

Main text:

1. Introduction
2. Model and Setup
3. Results with Kuo's scheme
   1. Temporal and zonal mean structure
   2. Longitudinal and temporal variation of rainfall along the equator and its spectrum
   3. Time development of precipitation and circulation in the equatorial region
   4. Composite structure of disturbance associated with grid-scale precipitation
4. Results with adjustment scheme
   1. Temporal and zonal mean structure
   2. Longitudinal and temporal variation of rainfall along the equator and its spectrum
   3. Time development of precipitation and circulation in the equatorial region
   4. Composite structure of disturbance associated with grid-scale precipitation
5. Summary and Discussions

Acknowledgement, Appemdix, Corresponding address:

• Acknowledgements
• References
• Corresponding address

Appendix:

1. Brief review of wave-CISK
   1. Equatorial wave-CISK
   2. Mechanism of emergence of propagating growing mode
   3. The effect of asymmetric heating at upward and downward motion
2. Methods of analysis
   1. Composite analysis
   2. Space-time spectral analysis
3. Comparison with Numaguti and Hayashi (1991)