Impacts of Vegetation on the Intraseasonal Oscillation Simulated by the Community Atmosphere Model (CAM3)

The influences of vegetation on intraseasonal oscillation （ISO） were examined using the Community Atmosphere Model version 3 （CAM3）. Two 15-year numerical experiments were completed： the first was performed with a realistic vegetation distribution （VEG run）, and the second was identical to the VEG run except without land vegetation （NOVEG run）. Generally speak- ing, CAM3 was able to reproduce the spatial distribution of the ISO, but the ISO intensity in the simulation was much weaker than that observed in nature： the 1SO has a relatively much stronger signal. A comparison of the VEG run with the NOVEG run revealed that the presence of vegetation usually produces a weak ISO. The vegetation effects on ISO intensity were significant over West Africa and South Asia, especially in the summer half-year. Vegetation also plays an important role in modulating ISO propagation. The eastward propagation of the ISO in the VEG run was clearer than that in the NOVEG run over the West African and Maritime Continent regions. The northward propagation of the ISO in the VEG run was more consistent with observation than that in the NOVEG run.

The Dynamical and Climate Tests of an Atmospheric General Circulation Model Using the Second-Order Adams-Bashforth Method

The Asselin-Robert time filter used in the leapfrog scheme can degrade the accuracy of calculations.The second-order Adams-Bashforth method with the same accuracy as the leapfrog scheme is not subject to time splitting instability.A new semi-implicit atmospheric general circulation spectral model is developed on the basis of NCAR（National Center for Atmospheric Research）CAM3.0（Community Atmosphere Model 3.0）.In this new model,the second-order Adams-Bashforth method is used as an alternative to the leapfrog scheme,and a Crank-Nicholson scheme is incorporated for the treatment of fast gravity modes.In this paper,the new model is tested by the Held-Suarez test and an idealized baroclinic wave test.Results of the Held-Suarez test show that the second-order Adams-Bashforth model has similar climate states to those of many other global models and it converges with resolutions.Based on the idealized baroclinic wave test,the capability of different time differencing methods for keeping the initial steady-state are compared. This convinces a better ability of the second-order Adams-Bashforth method in maintaining the stability of the initial state.Furthermore,after the baroclinic wave is triggered through overlaying the steady-state initial conditions with the zonal perturbation,the second-order Adams-Bashforth method has an excellent property of convergence,and can represent the process of the baroclinic wave development much better than the original scheme in CAM3.0.A long-term integration of the new model during the period of 1980–1999 is also carried out and compared with that of CAM3.0.It is found that due to the reduction of simulation errors of prognostic variables,the second-order Adams-Bashforth method also has a better simulation ability for the diagnostic variables,such as precipitation.