Sensitivity of GCM simulations to land surface processes

A coupled NCEP GCM/SSiB (Simplified Simple Biosphere Model) model has been developed to investigate the interactions between land surface processes and climate. The NCEP global model is a forecasting model that is also used for climate studies. A version with spectral triangular 62 truncation (T62) is used for this study.

There are two soil layers in the original NCEP GCM, representing land surface processes. Using the original NCEP GCM and the NCEP GCM/SSiB, we have integrated the models for 4 months from May 1, 1987 to August 1, 1987, which is the boreal monsoon season. The most substantial differences between the NCEP GCM and the NCEP GCM/SSiB are the simulations in the African monsoon, the East Asian monsoon, and the Mexican monsoon. In the original NCEP GCM, the African monsoon is too week, especially in July. The monsoon rainfall does not fully develop until August. With the coupled NCEP GCM/SSIB, the African monsoon rainfall in July is closer to the observations. In East Asia, the monsoon onset in May is too strong in the original NCEP GCM and the rainfall area approaches farther to the north; whereas the NCEP GCM/SSiB correctly simulates the monsoon's onset. In these experiments, the soil moisture and albedo are similar in the two models.

Further experiments are also carried out to assess the impact of boundary layer schemes (first order closure and non-local vertical diffusion scheme) and initial soil moisture to climate simulation. The new soil moisture data is developed as a part of the Global Soil Wetness Project. The SSiB has been driven offline by observed and assimilated meteorological data to produce 1987 soil moisture climatology. The impacts of these two data sets have been tested to compare with the original model simulations.