Extended Abstract (296K)4B.5
Evaluation and impact study of convective momentum parameterization using 3D cloud-resolving model and general circulation model
Xiaoqing Wu, NCAR, Boulder, CO; and M. W. Moncrieff, X. Z. Liang, and G. J. Zhang
Tropical cloud systems, through their release of latent heat and redistribution of heat, moisture and momentum, play an important role in the coupling of large-scale dynamic and thermodynamic processes. The effects of convection on the large-scale temperature and moisture fields are represented by convection schemes in all GCMs, but the effects of convection on the large-scale momentum field are usually neglected due to the complexity and inadequate understanding of the processes involved. Also, the development of convective momentum parameterization has been hindered by the lack of direct observation and accurate estimates of the "apparent momentum source" by cloud systems. Unlike heat and moisture, it is very difficult to get a reliable estimate of the apparent momentum source from observations. Cloud-resolving models (CRM) are a comprehensive alternative.
Firstly, our long-term three-dimensional (3D) cloud-resolving simulations of GATE cloud systems are used to evaluate two convective momentum parameterization schemes. Using the same large-scale conditions as the CRM, the Wu and Yanai scheme and the Zhang and Cho scheme broadly reproduce the CRM-generated apparent momentum source. In particular, the inclusion of cloud-scale pressure gradient has a large impact on the in-cloud momentum and the parameterized apparent momentum source, especially in the upper troposphere. The agreement between the CRM-produced and parameterized cloud mass flux contributes to this success.
Secondly, Wu and Yanai scheme is combined with the Arakawa-Schubert cumulus parameterization, and the Zhang and Cho scheme is added to the Zhang and McFarlane scheme in order to investigate the effects of convective dynamics and thermodynamics on the large-scale circulation of the atmosphere. Specifically, we included the Zhang and Cho convective momentum parameterization scheme in the NCAR Community Climate Model version 3 (CCM3) that uses the Zhang and McFarlane scheme. The 20-year simulation (1979 through 1998) shows strong impacts of convective momentum transport on the Inter-Tropical Convergence Zone (ITCZ). The global precipitation distribution is closer to the observed distribution than the standard CCM3 simulation. A detail analysis will be presented.
Session 4B, Convection III (Parallel with Sessions 4A, 4C, & 4D)
Tuesday, 30 April 2002, 8:30 AM-10:30 AM
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