Abstract
AbstractUsing large‐eddy simulations (LES) systematically has the potential to inform parameterizations of subgrid‐scale processes in general circulation models (GCMs), such as turbulence, convection, and clouds. Here we show how LES can be run to simulate grid columns of GCMs to generate LES across a cross section of dynamical regimes. The LES setup approximately replicates the thermodynamic and water budgets in GCM grid columns. Resolved horizontal and vertical transports of heat and water and large‐scale pressure gradients from the GCM are prescribed as forcing in the LES. The LES are forced with prescribed surface temperatures, but atmospheric temperature and moisture are free to adjust, reducing the imprinting of GCM fields on the LES. In both the GCM and LES, radiative transfer is treated in a unified but idealized manner (semigray atmosphere without water vapor feedback or cloud radiative effects). We show that the LES in this setup reaches statistically steady states without nudging to thermodynamic GCM profiles. The steady states provide training data for developing GCM parameterizations. The same LES setup also provides a good basis for studying the cloud response to global warming.
Highlights
General circulation model (GCM) predictions of the equilibrium climate sensitivity—the equilibrium global mean surface temperature change after doubling of CO2 concentrations—range between 2 and 5 K across climate models (Knutti et al, 2017)
The large-eddy simulations (LES) are forced with prescribed surface temperatures, but atmospheric temperature and moisture are free to adjust, reducing the imprinting of GCM fields on the LES
It is tempting to allow the surface temperatures in LES to vary by coupling the simulations with a slab ocean; we found that doing so leads to significant drift of the LES away from the GCM state in deep convection regions, presumably because of inadequacies of the simple turbulence and convection parameterizations employed in the GCM
Summary
General circulation model (GCM) predictions of the equilibrium climate sensitivity—the equilibrium global mean surface temperature change after doubling of CO2 concentrations—range between 2 and 5 K across climate models (Knutti et al, 2017). The large-scale forcing approach has been used in recent years to study boundary layer dynamics and cloud feedbacks in selected locations and over long times (e.g., Dal Gesso & Neggers, 2018; Schalkwijk et al, 2015; Van Laar et al, 2019). Another approach to force LES with GCMs is one-way nesting (e.g., Dipankar et al, 2015; Heinze et al, 2017), where states at lateral boundaries of the LES domain are relaxed toward those in a host model.
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