Sensitivity of the Rainfall Simulation in NCAR-CAM3.0 to Various Model Components

The dependence of rainfall simulation to various model components has been investigated, in NCAR-CAM3.0. The investigation was carried out with two model configurations, viz. aqua-planet and actual planet modes.
It was noticed that, dynamical core has impact on the simulation of, spatial distribution and temporal variation of rainfall. One of the reasons behind this impact was the time step size (temporal resolution), which is controlled by the numerical stability of the dynamical core, and varies from one core to another. The impact was observed in the first time step of model integration. This can be traced to threshold conditions imposed on the cloud base mass flux in the parameterization of deep convection.
Investigation of the sensitivity of rainfall simulation to moist physics revealed interesting features. Simulation was found to be sensitive to, convective relaxation time scale, CAPE threshold, evaporation efficiency of the precipitating rainfall and deep convective precipitation production efficiency. It was observed that, increasing of the convective relaxation time scale affects the partitioning of the total rainfall between its different components, i.e. deep convective precipitation, shallow convective precipitation and large scale rainfall This in turn leads to change in the vertical thermodynamical structure of the atmosphere. It is observed that, a higher value of convective relaxation time scale improves the simulation of hierarchial structure in the ISV, MJO, Kelvin waves and seasonal migration of ITCZ over Indian region.