A spectral transform dynamical core with an 85 spectral truncation resolution
(T85) within the Community Atmosphere Model (CAM), version 4, is evaluated within
the recently released Community Earth System Model, version 1.0 (CESM) global
climate model. The spectral dynamical core option provides a well-known base
within the climate model community from which to assess climate behavior and
statistics, and its relative computational efficiency for smaller computing
platforms allows it to be extended to perform climate length simulations using
high-resolution configurations in the near term.
To establish the characteristics of the CAM4 T85, an ensemble of simulations
covering the present day observational period using forced sea surface
temperatures and prescribed sea-ice extent are evaluated. Overall, the
T85 ensemble attributes and biases are similar to a companion ensemble of
simulations using the one degree finite volume (FV1) dynamical core, relative
to observed and model derived datasets. Notable improvements with T85 compared
to FV1 include the
representation of wintertime Arctic sea level pressure and summer precipitation
over the Western Indian subcontinent. The mean and spatial patterns of the land
surface temperature trends over the AMIP period are generally well simulated with
the T85 ensemble relative to observations, however the model is not able to
capture the extent nor magnitude of changes in temperature extremes over the
boreal summer, where the changes are most dramatic. Biases in the wintertime
Arctic surface temperature and annual mean surface stress fields persist with
T85 as with the CAM3 version of T85.
