EVALUATING CLIMATE MODEL SIMULATIONS OF PRECIPITATION - METHODS, PROBLEMS AND PERFORMANCE

Climate system modelling has been used extensively to investigate the role of human activities in causing global change. Model evaluation as sesses the ability of the models used to simulate current climate. Thi s article reviews the methodology of model evaluation with examples fr om recent studies involving precipitation. This crucial element of cli mate is difficult to model since the majority of precipitation occurs at scales less than that of the gridboxes of the highest resolution mo dels. Detailed and reliable evaluation requires investigation of inter annual variability as well as of climatological means on a variety of spatial scales. This sort of detailed analysis requires time-series of observed global precipitation at monthly time-steps or less. No singl e currently available global dataset of precipitation fulfils all the requirements for model evaluation, making the comparison of modelled g lobal precipitation fields with 'reality' difficult. A number of recen t precipitation evaluation projects are reviewed and a hierarchy of ev aluation methods is provided based on spatial and temporal scale and w hether or not tests for statistical significance are applied. Most stu dies to date have not tested for statistical significance, although wh en models improve with higher resolution and better physical parameter izations, statistical significance testing of differences will become increasingly more essential. The problems of evaluating modelled preci pitation are being tackled by international projects such as the Globa l Precipitation Climatology Project, the WetNet Precipitation Intercom parison Projects and the Atmospheric Model Intercomparison Project. Th e results of evaluation studies to date emphasize that model simulatio ns of future changes to the magnitude, timing and spatial pattern of g lobal precipitation be viewed as scenarios and not as predictions.