Improving the QBO Forcing by Resolved Waves with Vertical Grid
Refinement in E3SMv2
Abstract
The quasi-biennial oscillation (QBO) is the strongest mode of
variability in the tropical stratosphere and plays an important role in
stratospheric dynamics and chemistry. The QBO is notably deficient in
many climate models, including the Energy Exascale Earth System Model
(E3SM) developed by the US Department of Energy. In this work, we
utilize vertical grid refinement to reduce the lower stratospheric grid
spacing from roughly 1 km to 500 m to facilitate more realistic synoptic
wave activity in lower stratosphere in E3SM version 2. The refinement
results in a simulated QBO with a reasonable amplitude and
easterly-westerly transition, but still has a longer period than
observed, slower easterly downward propagation speed, and shallower
vertical depth. Similar refinement in the multi-scale modeling framework
(MMF) configuration of E3SM yields similar improvements. By analyzing
the forcing contributions from different wave types, we find that most
of the QBO forcing still comes from parameterized gravity wave drag from
convection. The improved QBO forcing contributions from resolved waves,
especially equatorial Kelvin waves and resolved small scale gravity
waves can be attributed to the grid refinement.