Characterizing Convection Schemes Using Their Responses to Imposed
Tendency Perturbations
Abstract
Convection is usually parameterized in global climate models, and there
are often large discrepancies between results obtained with different
convection schemes. Conventional methods of comparing convection schemes
using observational cases or directly in 3D models do not always clearly
identify parameterization strengths and weaknesses. In this paper we
evaluate the response of parameterizations to various perturbations
rather than their behavior under particular strong forcing. We use the
linear response function method proposed by Kuang (2010) to compare
twelve physical packages in five atmospheric models using single-column
model (SCM) simulations under idealized radiative-convective equilibrium
conditions. The models are forced with anomalous temperature and
moisture tendencies. The temperature and moisture departures from
equilibrium are compared with published results from a cloud-resolving
model (CRM). Results show that the procedure is capable of isolating the
behavior of a convection scheme from other physics schemes. We identify
areas of agreement but also substantial differences between convection
schemes, some of which can be related to scheme design. Some aspects of
the model linear responses are related to their RCE profiles (the
relative humidity profile in particular), while others constitute
independent diagnostics. All the SCMs show irregularities or
discontinuities in behavior that are likely related to switches or
thresholds built into the convection schemes, and which do not appear in
the CRM. Our results highlight potential flaws in convection schemes and
suggest possible new directions to explore for parameterization
evaluation.