Confronting the Convective Gray Zone in the Global Configuration of the Met Office Unified Model

Abstract

In atmospheric models with kilometer‐scale grids the resolution approaches the scale of convection. As a consequence the most energetic eddies in the atmosphere are partially resolved and partially unresolved. The modeling challenge to represent convection partially explicitly and partially as a subgrid process is called the convective gray zone problem. The gray zone issue has previously been discussed in the context of regional models, but the evolution in regional models is constrained by the lateral boundary conditions. Here we explore the convective gray zone starting from a defined global configuration of the Met Office Unified Model using initialized forecasts and comparing different model formulations to observations. The focus is on convection and turbulence, but some aspects of the model dynamics are also considered. The global model is run at nominal 5 km resolution and thus contributions from both resolved and subgrid turbulent and convective fluxes are non‐negligible. The main conclusion is that in the present assessment, the configurations which include scale‐aware turbulence and a carefully reduced and simplified mass‐flux convection scheme outperform both the configuration with fully parameterized convection as well as a configuration in which the subgrid convection parameterization is switched off completely. The results are more conclusive with regard to convective organization and tropical variability than extratropical predictability. The present study thus endorses the strategy to further develop scale‐aware physics schemes and to pursue an operational implementation of the global 5 km‐resolution model to be used alongside other ensemble forecasts to allow researchers and forecasters to further assess these simulations.

Impact of Domain Size on Tropical Precipitation Within Explicit Convection Simulations

“…Persistent model biases, such as overly intense precipitation within the ITCZ, are seen in both simulations. Improvements to underpinning physical parameterization schemes therefore remain a priority to enable the benefits of convection‐permitting global models (Tomassini et al., 2023), and will accelerate progress toward their practical operational implementation.…”

The Impact of the Explicit Representation of Convection on the Climate of a Tidally Locked Planet in Global Stretched-mesh Simulations

“…All four experiments have 63 levels with a model top at about 41 km. This corresponds to the same vertical resolution below 41 km as that used by the Met Office's UM in global climate simulations (Walters et al 2019;Tomassini et al 2023). While the vertical resolution may impact the degree of convective aggregation (Jenney et al 2023), the simulations for tidally locked terrestrial planets have not been sensitive to it (Wei et al 2020;Yang et al 2023).…”

Modulation of Tropical Convection‐Circulation Interaction by Aerosol Indirect Effects in Convective Self‐Aggregation Simulations of a Gray Zone Global Model

“…As the length scales of these processes depend on the prevailing meteorological situation and are all intertwined, deciding the adequate model formulation when the model grid length approaches the scale of turbulence and convection is ambiguous (i.e., the "gray zone" problem; Arakawa et al, 2011;Honnert et al, 2020;Tomassini, 2018). Tomassini et al (2023) argued that the terms "convection-permitting" or "turbulence-permitting" are adequate to designate models operating in the gray zone of convection and turbulence in contrast to the terms "convection-resolving" and "turbulence-resolving" because they reflect the fact that convection and turbulence are partly resolved and partly unresolved in these regimes. In the CWBGFS, turbulence is essentially unresolved and has to be parameterized, while the convective motions with coherent structures are partially simulated and partially parameterized through the URAS.…”

“…Tomassini et al. (2023) argued that the terms “convection‐permitting” or “turbulence‐permitting” are adequate to designate models operating in the gray zone of convection and turbulence in contrast to the terms “convection‐resolving” and “turbulence‐resolving” because they reflect the fact that convection and turbulence are partly resolved and partly unresolved in these regimes. In the CWBGFS, turbulence is essentially unresolved and has to be parameterized, while the convective motions with coherent structures are partially simulated and partially parameterized through the URAS.…”