Effects of resolution and physics on precipitation in the NCAR Community Climate Model

Simulations of precipitation at different horizontal resolutions from the National Center for Atmospheric Research Community Climate Model (NCAR CCM2) version 2, are compared to four observed global climatologies. Simulations were available at the following resolutions: R15, T21, T31, T42, T63, T106, and T170 and range from 2 years in length (for the highest resolution T170) to 10 years in length. These runs all had prescribed, climatological sea surface temperatures and soil moisture which varied by month. An additional simulation of CCM2 at T42 coupled to a soil-vegetation atmospheric transfer (SVAT) package was also used. We also show the climatology of the latest version of the NCAR CCM (CCM3) and compare this to the T42 simulations of CCM2 as well as to the observations. The main results include the following: (1) The models fall within the uncertainty of the observations. (2) Little systematic difference is seen in the simulation of precipitation as one goes from T42 to T170 resolution. (3) The SVAT interactive land surface package simulation does as well as a specified surface hydrology. (4) The simulation by CCM3 shows a marked improvement over all CCM2 simulations at the global scale and particularly within the tropics.