On resolution sensitivity in the Community Atmosphere Model

• Published in: Quarterly journal of the Royal Meteorological Society Vol. 146; no. 733; pp. 3789 - 3807
• Main Authors: Herrington, Adam R., Reed, Kevin A.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01-10-2020; Wiley Subscription Services, Inc; Royal Meteorological Society
• Subjects: Atmosphere; Atmospheric circulation; Atmospheric circulation models; Atmospheric models; Atmospheric stability; climate models; Cloud cover; Cloud formation; Clouds; Convection; Convective precipitation; Drying; Equations of motion; General circulation models; Meteorology & Atmospheric Sciences; physical parametrizations; Precipitation; Resolution; resolution sensitivity; Scaling; Stratiform clouds; Vertical velocities
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.3873

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.
Bar‐graph of the convective (solid) and stratiform (white) climatological precipitation rates in prior CAM/CCM convergence studies. Each window contains a single convergence experiment, with identical x‐axis; the approximate grid resolution. Colors indicate the model configuration; January ensemble (black), aqua‐planet configurations with SST profiles QOBS (blue) and CNTL (red) in Neale and Hoskins (2000) and annual means from an AMIP configuration (green; Gates et al., 1999). Studies included in this figure are Kiehl and Williamson (1991) (CCM1), Williamson et al. (1995) (CCM2), Williamson (2008) (CAM3), Rauscher et al. (2013); Zarzycki et al. (2014); Herrington and Reed (2017) (CAM4), Zarzycki et al. (2014) (CAM5) and this study (CAM6). Two additional studies are included that do no not use the NCAR models, Pope and Stratton (2002) (HadAM3) and Hagemann et al. (2006) (ECHAM5) in order to illustrate similarities in resolution sensitivity across different AGCMs. The asterisk (*) indicates that parameters are modified with resolution to reduce the dependence of cloud fraction and top‐ofatmosphere radiative balance on resolution.