CloudSat to demonstrate the importance of considering the sub-grid variability of cloud and 34 precipitation when using the COSP to evaluate GCM simulations. We carry out two sensitivity 35 tests: SPCAM5 COSP and SPCAM5-Homogeneous COSP. In the SPCAM5 COSP run, the sub-36 grid cloud and precipitation properties from the embedded cloud resolving model (CRM) of 37 SPCAM5 are used to drive the COSP simulation, while in the SPCAM5-Homogeneous COSP 38 run only grid mean cloud and precipitation properties (i.e., no sub-grid variations) are given to 39 the COSP. We find that the warm rain signatures in the SPCAM5 COSP run agree with the 40 MODIS and CloudSat observations quite well. In contrast, the SPCAM5-Homogeneous COSP 41 run which ignores the sub-grid cloud variations, substantially overestimates the radar reflectivity 42 and probability of precipitation compared to the satellite observations, as well as the results from 43 the SPCAM5 COSP run. The significant differences between the two COSP runs demonstrate 44 that it is important to take into account the sub-grid variations of cloud and precipitation when 45 using COSP to evaluate the GCM to avoid confusing and misleading results. 46 47 48 Geosci. Model Dev. Discuss., https://doi