The Impact of Surface Hydrology on the Simulation of Tropical Intraseasonal Oscillation in NCAR (CCM2) Atmospheric GCM.

Abstract

The impact of surface hydrological processes in the simulation of tropical climate by the Community Climate Model Version 2 (CCM2) has been studied. Two ten year climate simulations have been analysed to study the effect of surface hydrology feedback. In one of the simulations the surface moisture was determined interactively (VAR_HYD), while in the other simulation the surface moisture was fixed to annual climatological values (FIX_HYD). The simulated values of soil moisture in the VAR_HYD simulation are higher than the annual climatological values, both during northern summer and winter seasons. The impact of surface hydrology on precipitation was larger during northern summer than during the northern winter season. The impact of surface hydrology specification was found to be not entirely local in the model. It appears to have a remote impact on the other parts of the tropics. The precipitation increased over the Indian region, and off-equatorial West Pacific in the interactive hydrology simulation (vis-a-vis FIX_HYD) and reduced over the equatorial West Pacific regions. The simulation of precipitation also improves over the East Pacific region with the incorporation of interactive hydrology. The differences in precipitation between the VAR_HYD and the FIX_HYD simulations are associated with differences in vertical moist static stability and 500 hPa vertical velocity. From harmonic analysis of OLR it was found that the power and frequency in the intraseasonal scales is simulated more realistically with the incorporation of interactive surface hydrology. The interactive hydrology was found to be necessary for the simulation of meridional propagations of convective zones over the Indian region (on intraseasonal time-scales). The meridional migration is not observed in the simulation with fixed surface hydrology, and is on account of non-monotonic variation of vertically integrated moist static energy. The specification of continental surface hydrological processes did not however have a significant impact on the simulation of the equatorially trapped waves, such as the Madden-Julian Oscillation (MJO), though the propagations are discontinuous in the FIX_HYD simulation.