Influence of Horizontal Model Resolution on the Spatial Scale of Extreme Precipitation Events

Previous work has shown that strong ascending motion is a key driver of extreme precipitation events (EPEs). Thus, the horizontal spatial scales of this “extreme ascent” are likely important for determining the spatial scales of EPEs. Therefore, understanding how climate models capture horizontal scales of ascending anomalies is critical to understanding and assessing climate models’ simulations and projections of extreme precipitation. Analyzing daily output from 27 models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) and High-Resolution Model Intercomparison Project (HighResMIP), we show that horizontal model resolution is a key influence on the horizontal scales of extreme ascent. We compute the horizontal scale for a given EPE as the e-folding distance of the vertical velocity anomaly on the day of the EPE, which is scaled to produce an inverse wavenumber. We then composite these horizontal scales over all annual maximum EPEs between 1981 and 2000 for each model. We focused on the horizontal scale zonally averaged over the 40S-55S latitude band. Our analysis suggests that model horizontal resolution places an upper limit on the horizontal scale of extreme ascent. Models with around 150 longitude points (approximately 153 km resolution at 55S) have mean horizontal scales topping out at approximately 320 km, and this upper limit decreases to approximately 220 km for models with 500 longitude points (approximately 46 km resolution at 55S). Additional analysis shows that the horizontal scales for geopotential anomalies during EPEs have no clear resolution dependence. However, the horizontal scales of geopotential were generally larger (700-1100 km) than those for vertical velocity or precipitation anomalies, and more in line with theoretical expectations based on the Rossby radius of deformation. Additional insight is gained through analysis of grid-scale and convective precipitation. Altogether, these results suggest that the simulated large-scale dynamics associated with EPEs is realistic, but the models are convecting at the grid scale rather than sub-grid scale. This is unrealistic as convective precipitation is expected to contribute strongly to extreme precipitation events. Additional analysis ensured that our results were not just a product of grid box storms, and that they were not sensitive to internal variability or temporal resolution.