Comments on "Preliminary Tests of Multiscale Modeling with a Two-Dimensional Framework: Sensitivity to Coupling Methods"

Jung and Arakawa (2005, hereafter JA05) discuss an extension of the approach proposed by Grabowski and Smolarkiewicz (1999, hereafter GS99) and Grabowski (2001, hereafter G01) to represent smalland mesoscale processes in large-scale models of weather and climate. In this approach, a two-dimensional (2D; horizontal distance–height, x–z) small-scale model is applied in each column of the large-scale model in order to represent subgrid-scale processes, moist convection, and clouds in particular. GS99 named this approach the cloud-resolving convection parameterization and it has since been referred to as the superparameterization (SP). The two-dimensional small-scale models in G01’s approach apply periodic lateral boundary conditions. This implies that small-scale models from neighboring large-scale columns only communicate through the large-scale model and small-scale perturbations cannot coherently propagate across the large-scale model grid (see section 3 in G01). In JA05’s extension, the lateral boundary conditions in SP models are no longer periodic, enabling propagation of small-scale perturbations from one small-scale model to another (cf. Fig. 1 in JA05). JA05 present a simple 2D test of their extension and show that the modified approach does allow coherent propagation of smalland mesoscale features across the large-scale model grid. They also show that the modified approach is independent of a particular design of the coupled system (i.e., the number of large-scale model columns and the associated horizontal extent of the small-scale models) because similar results are obtained using either a small number of large-scale model columns (with large-extent subgrid-scale models) or a large number of large-scale model columns (each featuring subgrid-scale models with small horizontal extent). In contrast, JA05’s test of the original approach (GS99;G01) suggests that there is no coherent propagation of a cloud system across the large-scale model grid and convective organization changes when the design of the coupled system is modified. Although JA05’s criticism of the original SP proposal is valid in general, the particular test applied in JA05 is misleading as far as the validity of the original approach is concerned. The motivation for this note is to illustrate this key point. JA05’s test simulates the development and propagation of a 2D mesoscale convective system in the mean Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment (GATE) environment, defined by the initial sounding, the large-scale horizontal wind, and the large-scale advective tendencies of temperature and moisture. A part of JA05’s critique is that the formation and propagation of the mesoscale system in their test is poorly captured using G01’s approach, but is significantly improved in their extension. We repeated JA05’s test (albeit with some simplifications, see below) and obtained results that are different from those reported in JA05. In particular, the approach of GS99 and G01 performed significantly better than claimed in JA05. JA05 consider two different setups using the mean GATE forcing. One mimics maritime convection and the other one mimics convection over land (only results from the latter are shown in JA05). The difference * The National Center for Atmospheric Research is sponsored by the National Science Foundation.