HR: 10:35h
AN: A32A-02 INVITED     [Abstracts]
TI: Downscaling With a Variable Resolution GCM
AU: * McGregor, J L
EM: John.McGregor@csiro.au
AF: CSIRO Atmospheric Research, PB1, Aspendale, Vic 3195 Australia
AB: It has become rather popular in the last decade to use limited-area models as regional climate models (RCMs) to downscale from GCM simulations. This technique has proved rather successful for many locations. However, the technique can have difficulties for situations in which there is not an adequate progression of synoptic situations through the domain, such as may occur in the tropics A recent development is the application of variable-resolution GCMs as a downscaling technique. An obvious advantage is the avoidance of lateral boundary reflections, which can produce spurious vertical velocities and associated spurious rainfall near the boundaries of a limited-area model. Another advantage is that variable-resolution GCMs can avoid lateral boundary discrepancies which may arise if the GCM and RCM have dissimilar temperature or moisture biases. The model developed at CSIRO for these studies is the Conformal-Cubic Atmospheric Model (C-CAM), which obtains its variable resolution by means of the Schmidt transformation. For downscaling purposes, C-CAM has largely superseded the CSIRO limited-area model, DARLAM. The talk will commence with a description of the main features of the numerics and parameterizations of C-CAM. A large number of downscaling simulations have been performed with C-CAM, for a range of resolutions. For modest grid stretching, the model can be run in stand-alone mode. However, for strongly stretched applications it is desirable to include some form of nudging, at least in the coarsest regions, so as to ensure a reasonable climatology in the coarse region. In C-CAM, this nudging is provided just by the winds, either from a previous C-CAM simulation, from an atmosphere-ocean GCM, or from reanalyses, depending on the type of experiment being carried out. The wind nudging can either be "far-field" (only over the coarsest region) or "global" (at all grid points, but applied more weakly). The latter option is somewhat akin to the spectral nudging used by some of the limited-area modeling community to ensure their simulations produce a sequence of synoptic situations resembling the large-scale GCM or reanalysis. Both methods have been used at CSIRO for various applications. To provide an insight into the issues, a set of "big-brother" simulations has been performed, where a high-resolution global simulation is compared with a number of downscaled versions of itself. The remainder of the presentation will include examples from several C-CAM simulations. a) Climate change simulations for 1960-2100 with 60 km resolution over Australia, using SSTs and far-field forcing from the CSIRO Mk3 GCM for the A2 scenario. b) 40-year simulations over Australia, with both types of wind-nudging from NCEP reanalyses. As might be expected, the interannual variability is captured better by the simulation with "global" forcing. c) Several multi-decadal climate simulations over Tasmania at 14 km resolution. d) Simulations over Asia at 60 km resolution. e) Simulations of the transport of trace gases.
DE: 3337 Numerical modeling and data assimilation
DE: 1610 Atmosphere (0315, 0325)
DE: 1620 Climate dynamics (3309)
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting