DCMIP2016: a review of non-hydrostatic dynamical core design and intercomparison of participating models

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126

152

It is the purpose of this paper to provide a comprehensive documentation of the new NCAR (National Center for Atmospheric Research) version of the spectral element (SE) dynamical core as part of the Community Earth System Model (CESM2.0) release. This version differs from previous releases of the SE dynamical core in several ways. Most notably the hybrid sigma vertical coordinate is based on dry air mass, the condensates are dynamically active in the thermodynamic and momentum equations (also referred to as condensate loading), and the continuous equations of motion conserve a more comprehensive total energy that includes condensates. Not related to the vertical coordinate change, the hyperviscosity operators and the vertical remapping algorithms have been modified. The code base has been significantly reduced, sped up, and cleaned up as part of integrating SE as a dynamical core in the CAM (Community Atmosphere Model) repository rather than importing the SE dynamical core from High‐Order Methods Modeling environment as an external code.

Section: Resultsmentioning

confidence: 99%

NCAR Release of CAM‐SE in CESM2.0: A Reformulation of the Spectral Element Dynamical Core in Dry‐Mass Vertical Coordinates With Comprehensive Treatment of Condensates and Energy

Lauritzen

Nair

Herrington

et al. 2018

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126

152

“…Idealized models like aquaplanets represent a vital component of a hierarchy of model complexity aimed at improving our understanding of comprehensive GCMs and the physical underpinnings of the climate system [ Held , ; National Research Council , ; Stevens and Bony , ; Medeiros et al ., ]. Aquaplanet model intercomparison projects (MIP) focusing on atmospheric GCMs (AMIP) [ Gates , ] and model dynamical cores (DCMIP) [ Ullrich et al ., ] have been extremely effective at isolating the root causes of model differences. Likewise, intermediate‐complexity “aquaplanets” that introduce seasonality and idealized land masses to examine their impacts on the seasonal motion of tropical rain bands (“Tropical Rain belts with an Annual cycle and Continent ‐ MIP”, or TRACMIP) [ Voigt et al ., ] have identified key physical processes that control global hydroclimate.…”

Section: Introductionmentioning

confidence: 99%

Sensitivities of the hydrologic cycle to model physics, grid resolution, and ocean type in the aquaplanet Community Atmosphere Model

Benedict

Medeiros

Clement

et al. 2017

Precipitation distributions and extremes play a fundamental role in shaping Earth's climate and yet are poorly represented in many global climate models. Here, a suite of idealized Community Atmosphere Model (CAM) aquaplanet simulations is examined to assess the aquaplanet's ability to reproduce hydroclimate statistics of real‐Earth configurations and to investigate sensitivities of precipitation distributions and extremes to model physics, horizontal grid resolution, and ocean type. Little difference in precipitation statistics is found between aquaplanets using time‐constant sea‐surface temperatures and those implementing a slab ocean model with a 50 m mixed‐layer depth. In contrast, CAM version 5.3 (CAM5.3) produces more time mean, zonally averaged precipitation than CAM version 4 (CAM4), while CAM4 generates significantly larger precipitation variance and frequencies of extremely intense precipitation events. The largest model configuration‐based precipitation sensitivities relate to choice of horizontal grid resolution in the selected range 1–2°. Refining grid resolution has significant physics‐dependent effects on tropical precipitation: for CAM4, time mean zonal mean precipitation increases along the Equator and the intertropical convergence zone (ITCZ) narrows, while for CAM5.3 precipitation decreases along the Equator and the twin branches of the ITCZ shift poleward. Increased grid resolution also reduces light precipitation frequencies and enhances extreme precipitation for both CAM4 and CAM5.3 resulting in better alignment with observational estimates. A discussion of the potential implications these hydrologic cycle sensitivities have on the interpretation of precipitation statistics in future climate projections is also presented.

“…We assume

z

is a monotone function of

s

, so the pseudodensity

\partial π false/ \partial s

can be defined in terms of the hydrostatic pressure

π

and density:

\frac{\partial π}{\partial s} \frac{\partial s}{\partial z} = \frac{\partial π}{\partial z} = - ρ g

with

ρ = - \frac{\partial π}{\partial s}/ \frac{\partial ϕ}{\partial s} .

Vertical integrals of mass and other density weighted quantities are transformed to terrain following coordinates via

\int_{z_{surf}}^{z_{top}} ρ X d z = \frac{1}{g} \int_{s_{top}}^{s_{surf}} \frac{\partial π}{\partial s} X d s .

We denote the three‐dimensional physical coordinate‐independent differential operators in the usual notation,

\nabla \cdot boldv, \nabla p, \nabla \times boldv

and use

\nabla_{s} p, \nabla_{s} \times w \hat{k}, \nabla_{s} \cdot u, \nabla_{s} \times u

to denote the two‐dimensional operators on

s

surfaces (Ullrich et al, ).…”

Section: Continuum Equations In Terrain Following Coordinatesmentioning

confidence: 99%

An Energy Consistent Discretization of the Nonhydrostatic Equations in Primitive Variables

Taylor

Guba

Steyer

et al. 2020

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We derive a formulation of the nonhydrostatic equations in spherical geometry with a Lorenz staggered vertical discretization. The combination conserves a discrete energy in exact time integration when coupled with a mimetic horizontal discretization. The formulation is a version of Dubos and Tort (2014, https://doi.org/10.1175/MWR-D-14-00069.1) rewritten in terms of primitive variables. It is valid for terrain following mass or height coordinates and for both Eulerian or vertically Lagrangian discretizations. The discretization relies on an extension to Simmons and Burridge (1981, https://doi.org/10.1175/1520-0493(1981)109%3C0758:AEAAMC%3E2.0.CO;2) vertical differencing, which we show obeys a discrete derivative product rule. This product rule allows us to simplify the treatment of the vertical transport terms. Energy conservation is obtained via a term‐by‐term balance in the kinetic, internal, and potential energy budgets, ensuring an energy‐consistent discretization up to time truncation error with no spurious sources of energy. We demonstrate convergence with respect to time truncation error in a spectral element code with a horizontal explicit vertically implicit implicit‐explicit time stepping algorithm.