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Low cloud physics are especially critical to climate and modern climate model projections. Explicit representation of low clouds in a global atmospheric model is achieved by pushing the computational limits of cloud superparameterization to resolve boundary-layer eddy scales relevant to marine stratocumulus (250m horizontal and 20 m vertical length scales). We call this extreme configuration “ultraparameterization”. Pilot test results from hindcast experiments will be shown, validated against top-of-atmosphere (TOA) radiation daily observations (CERES) and, against YOTC reanalyses during the VOCALS period. We demonstrate promising model behaviors and interesting sensitivities. The new computational challenges of running an ultraparameterized CAM at the GPU-accelerated petascale for explicit low cloud feedback science application will be discussed.