A compressed super-parameterization: test of NAM-SCA under single-column GCM configurations

Abstract. The NAM-SCA (nonhydrostatic anelastic model with segmentally-constant approximation) is a type of cloud-resolving model (CRM) with a highly-inhomogeneous distribution of finite volumes of varying sizes in a horizontal direction under a two-dimensional geometrical configuration. These finite-volume positions and sizes are dynamically adapted to achieve best performance with a limited number of finite volumes. The concept can be compared with "compression" techniques used for digital images. The physics are also limited only to a minimum microphysics.

The present paper reports on a successful implementation of NAM-SCA into single column versions of two global atmospheric models as a compressed super-parameterization. ECHAM¹ and ACCESS² are chosen as the host models. A stand-alone single-column model (SCM) is also developed, in which the other physics are prescribed by observations. A simple radiation scheme is added as required.

Overall, it is found that, under this configuration, the lowest-resolution (Δ x = 16 km) NAM-SCA with the smallest domain size (L = 32 km) often works the best in terms of the errors for the precipitation rate, apparent heat source and moisture sink. Neither increase of resolution nor domain size leads to better performance in these respects until both cross the thresholds (Δ x ≤ 1km and L ≥ 256 km). These results suggest that even a simple parameterization (with a small degree of freedom) can perform in a reliable way as long as it is constructed in a physically consistent manner.

On the other hand, the prediction errors tend to be smaller for higher resolutions and larger domains. This tendency is clearer for the GATE³ case than the TWP-ICE⁴case.



¹ECMWF (European Centre for Medium Range Weather Forecasts) Hamburg version Atmospheric Model.
²Australian Community Climate and Earth-System Simulator.
³The Global Atmospheric Research Program’s (GARP) Atlantic Tropical Experiment.
⁴Tropical Warm Pool-International Cloud Experiment.