Microphysics of Clouds with the Relaxed Arakawa–Schubert Scheme (McRAS). Part II: Implementation and Performance in GEOS II GCM

Abstract

A prognostic cloud scheme named the Microphysics of Clouds with the Relaxed Arakawa?Schubert Scheme (McRAS) and the Simple Biosphere Model have been implemented in a version of the Goddard Earth Observing System (GEOS) II GCM at a 4° latitude ? 5° longitude ? 20 sigma-layer resolution. The McRAS GCM was integrated for 50 months. The integration was initialized with the European Centre for Medium-Range Weather Forecasts analysis of observations for 1 January 1987 and was forced with the observed sea surface temperatures and sea-ice distribution; on land, the permanent ice and vegetation properties (biomes and soils) were climatological, while the soil moisture and snow cover were prognostic. The simulation shows that the McRAS GCM yields realistic structures of in-cloud water and ice, and cloud-radiative forcing (CRF) even though the cloudiness has some discernible systematic errors. The simulated intertropical convergence zone (ITCZ) has a realistic time mean structure and seasonal cycle. The simulated CRF is sensitive to vertical distribution of cloud water, which can be affected hugely with the choice of minimum in-cloud water for the onset of autoconversion or critical cloud water amount that regulates the autoconversion itself. The generation of prognostic cloud water is accompanied by reduced global precipitation and interactive CRF. These feedbacks have a profound effect on the ITCZ. Even though somewhat weaker than observed, the McRAS GCM simulation produces robust 30?60-day oscillations in the 200-hPa velocity potential. Comparisons of CRFs and precipitation produced in a parallel simulation with the GEOS II GCM are included. Several seasonal simulations were performed with the McRAS?GEOS II GCM for the summer (June?July?August) and winter (December?January?February) periods to determine how the simulated clouds and CRFs would be affected by (i) advection of clouds, (ii) cloud-top entrainment instability, (iii) cloud water inhomogeneity correction, and (iv) cloud production and dissipation in different cloud processes. The results show that each of these processes contributes to the simulated cloud fraction and CRF. Because inclusion of these processes helps to improve the simulated CRF, it is inferred that they would be useful to include in other cloud microphysics schemes as well. Two ensembles of four summer (July?August?September) simulations, one each for 1987 and 1988, were produced with the earlier 17-layer GEOS I GCM with McRAS. The differences show that the model simulates realistic and statistically significant precipitation differences over India, Central America, and tropical Africa. These findings were also confirmed in the new 20-layer GEOS II GCM with McRAS in the 1987 minus 1988 differences.