An Analysis of Precipitation Associated With the ITCZ in the CMIP5 AMIP and Historical-Coupled GCM Simulations: A Quantitative Assessment of Magnitude and Position

Abstract:

According to the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (AR5), the broad-scale features of precipitation as simulated by Phase 5 of the Coupled Model Intercomparison Project (CMIP5) are in modest agreement with observations, however, large systematic errors are found in the Tropics (IPCC AR5 Ch.9). This study provides a quantitative analysis of precipitation associated with the Intertropical Convergence Zone (ITCZ) over the equatorial northern Pacific. Results from 31 CMIP5 Atmospheric Model Intercomparison Project (AMIP) Global Circulation Model (GCM) runs and their historical ocean-coupled counterparts are compared with Global Precipitation Climatology Project (GPCP) and Tropical Rainfall Measuring Mission (TRMM) observations. Differences found between the GCMs and observations are quantitatively analyzed using two innovative feature-based approaches with the intent of separating these discrepancies in the precipitation fields into components of magnitude-based error due to parameterizations, and location-based error due to large scale dynamics. Results show widespread and varied results across all GCMs, with some models showing large under/over-simulation of precipitation magnitude while other models suffer from significant pole-ward or equator-ward position shift of the ITCZ. Linkage between these results with the models' deep/convective and shallow/stratiform parameterizations are discussed.