Microphysics of Clouds with the Relaxed Arakawa-Schubert Scheme (McRAS). Part I: Design and evaluation with GATE Phase III data

A prognostic cloud scheme named McRAS (Microphysics of Clouds with Relaxed Arakawa-Schubert Scheme) has been designed and developed with the aim of im proving moist processes, microphysics of clouds, and cloud-radiation intera ctions in GCMs. McRAS distinguishes three types of clouds: convective, stra tiform, and boundary layer. The convective clouds transform and merge into stratiform clouds on an hourly timescale, while the boundary layer clouds m erge into the stratiform clouds instantly. The cloud condensate converts in to precipitation following the autoconversion equations of Sundqvist that c ontain a parametric adaptation for the Berperon-Findeisen process of ice cr ystal growth and collection of cloud condensate by precipitation. All cloud s convect, advect, as well as diffuse both horizontally and vertically with a fully interactive cloud microphysics throughout the life cycle of the cl oud, while the optical properties of clouds are derived from the statistica l distribution of hydrometeors and idealized cloud geometry. An evaluation of McRAS in a single-column model (SCM) with the Global Atmos pheric Research Program Atlantic Tropical Experiment (GATE) Phase III data has shown that, together with the rest of the model physics, McRAS can simu late the observed temperature, humidity, and precipitation without discerni ble systematic errors. The time history and time mean in-cloud water and ic e distribution, fractional cloudiness, cloud optical thickness, origin of p recipitation in the convective anvils and towers, and the convective updraf t and downdraft velocities and mass fluxes all simulate a realistic behavio r. Some of these diagnostics are not verifiable with data on hand. These SC M sensitivity tests show that (i) without clouds the simulated GATE-SCM atm osphere is cooler than observed; (ii) the model's convective scheme, RAS, i s an important subparameterization of McRAS; and (iii) advection of cloud w ater substance is helpful in simulating better cloud distribution and cloud -radiation interaction. An evaluation of the performance of McRAS in the Go ddard Earth Observing System II GCM is given in a companion paper (Part II) .