Spectral Nudging Impacts on Precipitation Downscaling in the Conformal Cubic Atmospheric Model, version CCAM-2504: Insights from Summer 2011

Abstract. This study evaluates the impacts of spectral nudging on rainfall when dynamically downscaling with the Conformal Cubic Atmospheric Model (CCAM). The study focuses on the extreme 2010 – 11 La Niña, in conjunction with the Madden – Julian Oscillation (MJO), across the CORDEX – Australasia domain at 12.5 km with CCAM nested in ERA-5 reanalysis. Sixteen simulations were performed, systematically varying nudging wavelength, vertical extent, frequency, and variable choice, and evaluated against GPM-IMERG precipitation and ERA5 reanalysis. Configurations at short nudging wavelengths (∼500 – 1500 km), with high-frequency updates (1 h), and including pressure, wind and temperature delivered the most robust performance. These setups reduced large-scale rainfall biases, improved spatial and temporal correlations, reproduced vertical structure and moisture convergence more realistically, and achieved the closest agreement with observed mean and extreme observed rainfall. In contrast, coarse-scale (3000 km), full-column constraints, or nudging limited to pressure or wind variables degraded performance, producing oversmoothed variability, misplaced convection, and unrealistic rainfall patterns. Overall, the results demonstrate that carefully tuned spectral nudging enhances the fidelity of both mean and extreme rainfall in CCAM, while preserving large-scale teleconnections associated with La Niña, MJO, and retaining mesoscale variability. This study strengthens confidence in CCAM downscaling for CORDEX – Australasia, with implications extending to other CORDEX domains and applications.