Abstract:
Atmospheric gravity waves are buoyancy waves that are excited by meteorological processes, such as deep convection, tropical cyclones, jet streams, and flow over topography. These waves become dynamically important, or even dominant, at the Earth's upper atmosphere and in its space environment. This is because of their global distribution, their exponential growth with altitudes as the air density decreases, and their contribution to the momentum and energy budgets. They are therefore thought to play a key role in connecting the terrestrial atmosphere with the space environment. However, it has been challenging for observations and numerical models alike to quantify their global effects because of the large scale range--from km to global scale--required to resolve them in a whole atmosphere context. In this talk, I will discuss our recent efforts to address this challenge by developing the high-resolution Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X). WACCM-X is one of the atmosphere components of the NCAR’s Community Earth System Model (CESM). It is a first-principle model that simulates processes from the Earth surface to the upper thermosphere, including radiative, physical, chemical, dynamical, and electrodynamical processes. With this newly developed capability, we can now assess the wave effects on the circulation, transport and composition of the whole atmosphere system. At the same time, the high-resolution simulations also reveal the importance of unresolved, smaller scale waves in the global momentum budget. We will explore innovative ways to account for such wave impacts.
Bio:
Dr. Hanli Liu is a senior scientist at the High Altitude Observatory, National Center for Atmospheric Research. He received a B.S. in Fluid Mechanics from the University of Science and Technology of China, and a Ph.D. in Atmospheric and Space Physics from the University of Michigan. He came to the Observatory in 1997 as a postdoctoral researcher, and joined the scientific staff in 1999. His research includes: theoretical, numerical, and interpretive studies of the dynamics, structure, and variability of the Earth's middle and upper atmosphere; coupling of different atmospheric regions on global and regional scales, including impacts of lower atmospheric forcing on space weather; atmospheric waves and geophysical turbulence. He is leading the thermosphere/ionosphere extension of the Whole Atmosphere Community Climate Model (WACCM-X).
Summary:
Focus: simulating the Earth’s atmosphere
WACCM-X: Whole Atmosphere Community Climate Model with Thermosphere/Ionosphere extension
Gravity waves, their propagation and effects
Atmosphere has many layers
Top layers: low earth satellites, drag from air
Ionosphere: blocks high energy solar/space particles
Middle/lower atmosphere transport processes transport gasses vertically and laterally
Challenges in modeling the atmosphere
Climate/global weather
Deep vertical domain: 0-700km, 10% of Earth radius,
Change in physical processes at different depths
1013 change in density from surface to edge
Surface: wind, heat, transport
Edge: radiation
Ion-neutral coupling
Short temporal/spatial sacles
Winds: 300m/s
Acoustic: 800m/s
NCAR Community Earth System Model (CESM)
Couples different domain-specific sub-models: Land, Ocean, Ice, Atmosphere
Atmosphere
CAM: lower atmosphere
WACCM: lower to mesosphere
WACCM-W: all layers, including space boundary
Components
Chemistry
Neutral physics
Iconosphere Physics
Variable resolution management
Current WACCM model incorporates many physical processes but has a number of key limitation
Motion of waves through the atmosphere
Carry momentum and energy fluxes
Gravity waves: caused by vertical movements in the atmosphere
Wind blowing up over mountain
Deep convective system pushing air up
20-2000km
Planetary waves: horizontal direction
E.g. Coriolis
~104km
Tides: motion due to day-night heating
~104km
Ocean tides: move atmosphere up and down
SIMA: System for Integrated Modeling of the Atmosphere
Models: WACCM-X, WACCM, CAM, MPAS, MUSICA
Currently can represent Local to Regional Scales
Not enough to represent gravity waves, which are global-scale
Doesn’t properly model: quasi-biennial oscillation (QBO), polar vortex
Need high-resolution whole atmosphere climate models
Sub-grid parameterizations effective to phenomena that depend on average effects (e.g. energy deposition)
Need a fine-grid model to capture phenomena where directionality matters
SIMA/WACCM-X development focused on high-resolution
Cube-sphere grid
25km, horizontal .1 scale vertical
200km / .25 for comparison
Fine-grained model captures fine-grained wave perturbations not visible from coarse model
Affects ionosphere and flow of plasmas in the upper atmosphere
Can model
Plasmasphere ducts
Thermosphere composition in southern vs northern hemisphere
Chemical composition in mesosphere/upper stratosphere polar regions
100km horizontal resolution: can resolve waves > 800km
~25km horizontal resolution: can resolve waves > 200km
Makes it possible to capture the mesa-alpha scale, but not yet the finer beta scale
Still many important waves at the unresolved finer scale
For climate models we need good sub-scale parameterizations for modeling gravity waves
Gravity wave impacts have a scale invariance, so we can extrapolate to finer scales
Enables a statistical approximation of the effect: ongoing work
Working on global models on convection-permitting scales
<5km / .05 scale global models
Will need ~1.25 ExaFlops, GPU enablement