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 5km 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 5km-resolution model to be used alongside other ensemble forecasts to allow researchers and forecasters to further assess these simulations.

Kilometer-scale simulations of trade-wind cumulus capture processes of mesoscale organization

“…The strong sensitivity of cloud fraction and organization to resolution highlights the need for scale-aware parametrization at these scales. Including a reduced and simplified parametrization of shallow convection can improve the representation of convection in the gray zone (Tomassini et al, 2022). However, given the models reproduce the variations in mesoscale organization and associated processes, they are a useful tool for understanding processes driving mesoscale organization and interactions with larger scales.…”

Kilometer‐Scale Simulations of Trade‐Wind Cumulus Capture Processes of Mesoscale Organization

“…The strong sensitivity of cloud fraction and organization to resolution highlights the need for scale‐aware parametrization at these scales. Including a reduced and simplified parametrization of shallow convection can improve the representation of convection in the gray zone (Tomassini et al., 2022). However, given the models reproduce the variations in mesoscale organization and associated processes, they are a useful tool for understanding processes driving mesoscale organization and interactions with larger scales.…”

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.…”