Multi-scale modeling of the atmosphere
-Toward unification of the general circulation and cloud-resolving models

Prof Chien-Ming Wu (Dept. of Atmos. Sci., National Taiwan University) 

Abstract
========

As far as the representation of deep moist convection is concerned, 
only two kinds of model physics are used at present: highly 
parameterized as in the conventional general circulation models (GCMs) 
and explicitly simulated as in the cloud-resolving models (CRMs). 
Ideally, these two kinds of model physics should be unified so that a 
continuous transition of model physics from one kind to the other 
takes place as the resolution changes. In this talk, we will introduce 
the newly developed unified parameterization so that the physical 
parameterizations of a GCM converge to a global CRM as the horizontal 
resolution is refined. The key for this unified parameterization to 
succeed is to eliminate the assumption that convective clouds occupy 
only a small fraction of the grid size which is commonly used in the 
convectional parameterization of the GCM. 

As the horizontal resolution of the numerical models approaches 
cloud-resolving simulations, one of the difficult challenges is to 
handle the lower boundary as steep or as irregular as the surface 
topography. Most atmospheric models introduce the effect of surface 
topography through the use of a terrain-following vertical coordinate. 
While such a coordinate can formally bypass problems in implementing 
the lower boundary condition over the terrain, it is well known that 
the error due to the use of such coordinates is a prominent source of 
errors in calculating the horizontal pressure gradient over steep 
mountains. In the second part of the talk, we will introduce the block 
mountain approach for the numerical models that can handle complex 
topography without artificial smoothing. Since the block mountain is 
fixed at the coordinate surface, the model grid wonˇ¦t be distorted in 
dealing with steep slopes. Examples such as mountain waves and 
precipitation over complex terrain are presented to demonstrate this 
approach.