Pattern formation of convective activity over the aqua-planet with globally uniform sea surface temperature (SST)

Abstract

Pattern formation of convective activities over the Earth has been investigated by using numerical models. Several numerical experiments have been conducted by using 12-level T21 and T42 global spectral models at the University of Tokyo. In order to focus on a distribution of convective activity over the tropics, the Earth was assumed to be entirely covered by an ocean with a globally uniform sea surface temperature (“Jovian-type” Aqua-planet experiment). A simple radiational process was assumed, where horizontally uniform cooling was assumed to compensate for convective heating and maintain the globally averaged temperature. Time integration was conducted from an initial condition where temperature and moisture fields are assumed to be uniform. As moisture fields are assumed to be relatively dry, the model atmosphere tends to be moistened during time integration. The following interesting results have been obtained: 1. When the evaporation rate is large and the moisture supply from the ocean is intense, convective activities tend to be distributed all over the Earth. 2. Then, convective activities tended to be suppressed on the equator. 3. As the evaporation rate decreased, convective activities in the extra-tropics tended to be suppressed and a band structure with respect to the equator (double ITCZ structure) becomes noticeable in the T42-run. On the contrary, a single ITCZ structure is simulated in the T21-run. This is because disturbances off the equator cannot be resolved in the T21-run. 4. With respect to the propagation of convective activity, westward propagation is noted during the period of large evaporation and eastward propagation is noted after the evaporation decreases below a certain level. 5. Two time-scales were found in this experiment. One scale is about 50 days, where the zonally averaged fields are adjusted. This is due to the establishment of a symmetric circulation. The other scale is about 200 days, where organization of convective activities is realized. This is related to a decrease of evaporation and precipitation. Finally, additional numerical experiments by using another parametarization scheme for convection and radiational processes have been conducted and it is concluded that the results obtained in this study are not particularly sensitive to these two schemes.