Impact of Stochastic Physics and Model Resolution on the Simulation of Tropical Cyclones in Climate GCMs

Abstract

The role of model resolution in simulating geophysical vortices with the characteristics of realistic Tropical Cyclones (TCs) is well established. The push for increasing resolution continues, with General Circulation Models (GCMs) starting to use sub-10km grid spacing. In the same context it has been suggested that the use of Stochastic Physics (SP) may act as a surrogate for high resolution, providing some of the benefits at a fraction of the cost. Either technique can reduce model uncertainty, and enhance reliability, by providing a more dynamic environment for initial synoptic disturbances to be spawned and to grow into TCs. We present results from a systematic comparison of the role of model resolution and SP in the simulation of TCs, using EC-Earth simulations from project Climate-SPHINX, in large ensemble mode, spanning five different resolutions. All tropical cyclonic systems, including TCs, were tracked explicitly. As in previous studies, the number of simulated TCs increases with the use of higher resolution, but SP further enhances TC frequencies by ≈ 30%, in a strikingly similar way. The use of SP is beneficial for removing systematic climate biases, albeit not consistently so for interannual variability; conversely, the use of SP improves the simulation of the seasonal cycle of TC frequency. An investigation of the mechanisms behind this response indicates that SP generates both higher TC (and TC seed) genesis rates, and more suitable environmental conditions, enabling a more efficient transition of TC seeds into TCs. These results were confirmed by the use of equivalent simulations with the HadGEM3-GC31 GCM.

Impact of increased resolution on long-standing biases in HighResMIP-PRIMAVERA climate models

“…Increased atmospheric resolution, nonetheless, does improve the representation of weather or extremes, as found, for example, for tropical cyclones (Roberts M.J. et al, 2020a;Vidale et al, 2021;Zhang et al, 2021) and blocking frequency (Schiemann et al, 2020) in PRIMAVERA models as well as in numerical weather prediction systems (e.g., Lean et al, 2008).…”

Orographic resolution driving the improvements associated with horizontal resolution increase in the Northern Hemisphere winter mid-latitudes

“…the smallest scale reasonably represented by a numerical model, is estimated to be between three and five times the grid spacing (Klaver et al, 2020). Since increasing horizontal resolution implies smaller numerical truncation errors when solving the equations of motion, a finer grid positively affects the dynamics, thus leading to better resolved atmospheric eddies at a finer scale -as can be seen for tropical cyclones (Roberts et al, 2015;Vidale et al, 2021). However, while the dynamical aspects of a GCM certainly take advantage of a finer grid, numerical schemes and physical parameterizations might respond in a less coherent way to a resolution increase.…”

Reply on RC1

“…On this basis, the Horizon2020 PRIMAVERA project was conceived to "develop a new generation of advanced and well-evaluated high-resolution global climate models, capable of simulating and predicting regional climate with unprecedented fidelity, for the benefit of governments, business and society in general". Such new models have shown improvements in the representation of various aspects of weather and climate variability, including blocking frequency over the Pacific and Atlantic (Schiemann et al, 2020), the distribution of precipitation over Europe (Demory et al, 2020), tropical cyclones (Roberts M.J. et al, 2020a;Vidale et al, 2021;Zhang et al, 2021), air-sea interactions over the Gulf Stream (Bellucci et al, 2021), and Atlantic ocean heat transports (Roberts M.J. et al, 2020b). In this study, we provide a systematic assessment of the impact of ocean and atmospheric resolution on mean climate (Section 3), with a particular focus on the following long-standing biases: (i) the warm bias in the eastern tropical oceans, (ii) the double ITCZ, (iii) the warm Southern Ocean (SO), and (iv) the cold North Atlantic (Sections 4 and 5).…”