Implementation and Exploration of Parametrizations of Large-Scale
Dynamics in NCAR's Single Column Atmosphere Model SCAM6
Abstract
A single column model with parameterized large-scale dynamics is used to
better understand the response of steady-state tropical precipitation to
relative sea surface temperature under various representations of
radiation, convection, and circulation. The large-scale dynamics are
parametrized via the weak temperature gradient (WTG), damped gravity
wave (DGW), and spectral weak temperature gradient (Spectral WTG) method
in NCAR’s Single Column Atmosphere Model (SCAM6). Radiative cooling is
either specified or interactive, and the convective parameterization is
run using two different values of a parameter that controls the degree
of convective inhibition. Results are interpreted in the context of the
Global Atmospheric System Studies (GASS) Intercomparison (Daleu et al.
2016). Using the settings given in Daleu et al. (2016), SCAM6 under the
WTG and DGW methods produces erratic results, suggestive of numerical
instability. However, when key parameters are changed to weaken the
strength with which the circulation acts to eliminate tropospheric
temperature variations, SCAM6 performs comparably to single column
models in the GASS Intercomparison. The Spectral WTG method is less
sensitive to changes in convection and radiation than are the other two
methods, performing at least qualitatively similarly across all
configurations considered. Under all three methods, circulation
strength, represented in 1D by grid-scale vertical velocity, is
decreased when barriers to convection are reduced. This effect is most
extreme under specified radiative cooling, and is shown to come from
increased static stability in the column’s reference
radiative-convective equilibrium profile. This argument can be extended
to interactive radiation cases as well, though perhaps less
conclusively.