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

A prognostic cloud scheme named the Microphysics of Clouds with the Relaxed Arakawa-Schubert Scheme (McRAS) and the Simple Biosphere Model have been i mplemented in a version of the Goddard Earth Observing System (CEOS) II GCM at a 4 degrees latitude x 5 degrees longitude x 20 sigma-layer resolution. The McRAS GCM was integrated for 50 months. The integration was initialize d with the European Centre for Medium-Range Weather Forecasts analysis of o bservations for 1 January 1987 and was forced with the observed sea surface temperatures and sea-ice distribution; on land, the permanent ice and vege tation properties (biomes and soils) were climatological, while the soil mo isture and snow cover were prognostic. The simulation shows that the McRAS GCM yields realistic structures of in-cloud water and ice, and cloud-radiat ive forcing (CRF) even though the cloudiness has some discernible systemati c errors. The simulated intertropical convergence zone (ITCZ) has a realist ic time mean structure and seasonal cycle. The simulated CRF is sensitive t o vertical distribution of cloud water, which can be affected hugely with t he choice of minimum in-cloud water for the onset of autoconversion or crit ical cloud water amount that regulates the autoconversion itself The genera tion of prognostic cloud water is accompanied by reduced global precipitati on and interactive CRE These feedbacks have a profound effect on the ITCZ. Even though somewhat weaker than observed, the McRAS GCM simulation produce s robust 30-60-day oscillations in the 200-hPa velocity potential. Comparis ons of CRFs and precipitation produced in a parallel simulation with the GE OS 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) period s to determine how the simulated clouds and CRFs would be affected by (i) a dvection of clouds, (ii) cloud-top entrainment instability, (iii) cloud wat er inhomogeneity correction, and (iv) cloud production and dissipation in d ifferent cloud processes. The results show that each of these processes con tributes to the simulated cloud fraction and CRE Because inclusion of these processes helps to improve the simulated CRE it is inferred that they woul d be useful to include in other cloud microphysics schemes as wall. Two ensembles of four summer (July-August-September) simulations, one each for 1987 and 1888, were produced with the earlier 17-layer GEOS I GCM with McRAS. The differences show that the model simulates realistic and statisti cally significant precipitation differences over India, Central America, an d tropical Africa. These findings were also confirmed in the new 20-layer G EOS II GCM; with McRAS in the 1987 minus 1988 differences.