Thursday, 13 February 2003
Estimating predictability and uncertainty in simulating the water cycle with a Regional Climate Model
The evaluation of the quality and usefulness of a climate modeling system is dependent upon an assessment of both the limited predictability of the climate system and the uncertainties stemming from model formulation. In this study a methodology is presented that is suited to assess the performance of a regional climate model (RCM), based on its ability to represent the natural interannual variability on monthly and seasonal time scales. The methodology involves carrying out extensive multi-year integrations, using different parameterizations of water cycle processes, and a shorter ensemble experiment whose members are initialized from different atmospheric and land surface initial states, all driven by observed (assimilated) lateral boundary conditions. The results of the ensemble experiments provide predictability bounds within which the modeling system, in its various forms, can be evaluated with confidence against observations. The model formulation experiments cover the time period of the ECMWF re-analysis project (ERA-15, 1979-1993) and provide a systematic evaluation of three versions of the CHRM RCM over Europe, analyzed against surface observations of precipitation, temperature and land surface variables. While the ensemble experiment demonstrates that the predictability of the climate system varies strongly between different seasons and regions, being weakest during the summer and over more continental regions, important sensitivities of the modeling system to parameterization choices are uncovered. In particular, mechanisms related to the representation of the water cycle (e.g. soil moisture charge/discharge and formation of cloud cover) are revealed, in which summer dry and hot conditions at the surface can co-exist with important net radiation errors, deriving from depressed solar radiation and a corresponding lack of evaporation. Memory effects on monthly and seasonal time scales, associated with soil moisture evolution, are also discussed.