On resolution sensitivity in the Community Atmosphere Model

Abstract

Advances in high-performance computing make it possible to run atmospheric general circulation models (AGCMs) over an increasingly wider range of grid resolutions, using either globally uniform or variable-resolution grids. In principle, this is an exciting opportunity to resolve atmospheric process and scales in a global model and in unprecedented detail, but in practice this grid flexibility is incompatible with the non-or weakly converging solutions with increasing horizontal resolution that have long characterized AGCMs. In the the Community Atmosphere Model (CAM), there are robust sensitivities to horizontal resolution that have persisted since the model was first introduced over thirty years ago; the atmosphere progressively dries and becomes less cloudy with resolution, and parametrized deep convective precipitation decreases at the expense of stratiform precipitation. This study documents a convergence experiment using CAM, version 6, and argues that a unifying cause, the sensitivity of resolved dynamical modes to native grid resolution, feeds back into other model components and explains these robust sensitivities to resolution. The increasing magnitudes of resolved vertical velocities with resolution are shown to fit an analytic scaling derived for the equations of motion at hydrostatic scales. This trend in vertical velocities results in an increase in resolved upward moisture fluxes at cloud base, balanced by an increase in stratiform precipitation rates with resolution. Compensating, greater magnitude subsiding motion with resolution has previously been shown to dry out the atmosphere and reduce cloud cover. Here, it is shown that both the increase in condensational heating from stratiform cloud formation and greater subsidence drying contribute to an increase in atmospheric stability with resolution, reducing the activity of parametrized convection. The impact of changing the vertical velocity field with native grid resolution cannot be ignored in any effort to recover convergent solutions in AGCMs, and, in particular, the development of scale-aware physical parametrizations.

Projecting climate change in South America using variable‐resolution Community Earth System Model: An application to Chile

“…These differences are undoubtedly influenced by the lack of long term, spatially complete in situ networks throughout the South American Andes Poveda et al (2020), but may also be due to long-standing model biases. This includes: drizzle effects from sub-grid-scale parameterizations (e.g., too-often saturated model columns from the cumulus parameterization [Chen and Dai, 2018]), an long-standing erroneous double intertropical convergence zone (Wehner et al, 2014), and nonconvergence of mean and extreme precipitation with resolution refinement (Wehner et al, 2014;Herrington and Reed, 2017;Chen and Dai, 2019;Herrington and Reed, 2020). That said, there are multiple lines of evidence showing that more refined model resolutions, particularly over mountainous and glaciated regions, enhance model veracity in CESM (Wehner et al, 2014;Rhoades et al, 2016Rhoades et al, , 2018van Kampenhout et al, 2019).…”

“…While telescoping ESMs such as VR‐Andes add value to modelling studies and provide improvements over RCMs, they are not without problems. These problems arise due to a lack of development of scale‐aware physics parameterizations (e.g., convection, entrainment, turbulence, and cloud macrophysics) and the use of the hydrostatic approximation in the dynamical core at sub‐10 km resolutions (Arakawa and Jung, 2011; Herrington and Reed, 2017, 2020).…”

Sowing Storms: How Model Timestep Can Control Tropical Cyclone Frequency in a GCM

“…A final wrinkle is defining the role of convective parameterization itself in high-resolution simulations. Rauscher et al (2016) and Herrington and Reed (2020) both quantify how vertical velocities become more intense as grid spacing decreases. In practice, this more intense resolved scale motion explains why large-scale precipitation increases as resolution increases -effectively, the resolved scales are augmenting convective parameterizations in convective regimes.…”

“…χ m becomes slightly larger as timestep is decreased (from dt 1800 to dt 450 ), representing a small drying of the middle troposphere. Links between resolved-scale precipitation and subsidence (and associated drying) as a function of model resolution are discussed in Herrington and Reed (2020) and are likely relevant here. Average relative vorticity also decreases slightly as timestep is decreased, indicating a tendency toward more anticyclonic motion in dt 450 .…”

Evaluation of a Quasi-steady state approximation of the cloud Droplet Growth Equation (QDGE) scheme for aerosol activation in global models using multiple aircraft data over both continental and marine environments

“…Ghan et al (2011)). The ongoing increase in computing power (Herrington and Reed, 2020) reduces the need to apply rigorous approximations of physicochemical processes in climate models. In the following, we will introduce a Quasi-steady state approximation of the cloud Droplet Growth Equation (QDGE) that provides an efficient alternative to parameterizations of activated in climate models.…”