SIMULATED CLIMATOLOGY OF A GENERAL CIRCULATION MODEL WITH A HYDROLOGIC CYCLE

Abstract

A numerical experiment with a general circulation model with a simple hydrologic cycle is performed. The basic framework of this model is identical with that adopted for the previous study [35] except for the incorporation of a simplified hydrologic cycle which consists of the advection of water vapor by large-scale motion, evaporation from the surface, precipitation and an artificial adjustment to simulate the process of moist convection. This adjustment is performed only when the relative humidity reaches 100 percent and the lapse rate exceeds the moist adiabatic lapse rate. The radiative flux is computed for the climatological distribution of water vapor instead of using the distribution calculated by the prognostic equation of water vapor. A completely wet surface without any heat capacity is chosen as the lower boundary. The initial conditions consist of a completely dry and isothermal atmosphere. A state of quasi-equilibrium is obtained as a result of the time integration of 187 days. A preliminary analysis of the result is performed for the 40-day period from 148th day to 187th day. According to this analysis, the hemispheric mean of the rate of precipitation is about 1.06 m./yr. which is close to the estimate of the annual mean rainfall obtained by Budyko [5]. In the Tropics rainfall exceeds evaporation and in the subtropics the latter exceeds the former in qualitative agreement with observation. The difference between them, however, is too exaggerated, and an extremely large export of water vapor from the dry subtropics into the wet, Tropics by the meridional circulation takes place. In the troposphere, relative humidity increases with decreasing altitude. In the stratosphere it is very low except at the tropical tropopause, and the mixing ratio of water vapor is extremely small in qualitative agreement with observation. Although water vapor is transported from the troposphere into the stratosphere, it is then transported toward low latitudes and condenses at the tropical tropopause where the temperature is very low and the relative humidity is high. Based upon a harmonic analysis of the flow field and the surface pressure field, it, is concluded that the effect of condensation tends to increase the wave number of the tropospheric flow and surface pressure field. Also, the incorporation of the moist process in the model seems to increase the intensity of meridional circulation in the Tropics. As a result of this increase, the transport of momentum and heat by the meridional circulation in the Tropics is much larger than that obtained from the previous study. In middle latitudes, the poleward transport of total energy in the moist-model atmosphere is less than that in the dry-model atmosphere because of the effect of the poleward transport of Intent energy or the heat of condensation. The latitudinal distributions of radiative fluxes at the top of the atmosphere and at the earth's surface coincide very well with those obtained by London [17] for the actual atmosphere. Bowen's ratio increases with increasing latitude and its magnitude coincides reasonably well with that obtained by Budyko [5] or Jacobs [11] for the ocean surface.