Physical processes occurring at spatial and temporal scales not resolved by Atmospheric General Circulation Models (AGCMs) are a major source of uncertainty (sensitive dependence) in explorations of natural climate variability, seasonal predictions, and climate change assessments, since the values of the corresponding AGCMs’ parameters are not uniquely defined.

The primary aim of the proposed research is to quantify the magnitude of this uncertainty for the seasonal precipitation and its variability in the present climate. The analysis will be conducted with the High Resolution Atmospheric Model (HIRAM) developed at the Geophysical Fluid Dynamics Laboratory (GFDL) for the entire globe at a 100 km horizontal resolution. This is remarkably high for this type of study.

We propose to perform ensembles of simulations by systematically perturbing certain parameters in HIRAM. These include the entrainment rate in convective clouds, the ice ‐ fall ‐ speed, the threshold for the occurrence of precipitation in both convective and stratiform clouds, and the critical droplet radius in stratiform clouds.

We will assess whether sensitive dependence and internal variability can be separated at the regional scale, and we will establish the functional form of the sensitivity to individual parameters.

The proposed research contains several innovative aspects: the focus on precipitation and its variability, the high horizontal resolution, and the outstanding quality of the atmospheric model used. This research will be of great value to the research community and provide an uncertainty estimate to be considered for risk assessment and water management worldwide