The Whole Atmosphere Community Climate Model-eXtended

The Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM-X) is a general circulation model and part of the atmosphere components of the NCAR Community Earth System Model (CESM). The chemistry module is interactive with dynamical transport and exothermic heating (Kinnison et al., 2007). Photochemistry associated with ion species (O+, NO+, O2+, N2+ , N+, and metastable O+ states) is part of the chemistry package. Details of an earlier version of WACCM-X can be found in Liu et al. (2010). A recent version of WACCM-X (v.2.0) includes a more self-consistent ionosphere module that calculates electron and ion temperatures, ionospheric electrodynamics of wind dynamo, and O+ transport in the ionospheric F region. WACCM-X neutral dynamics has also recently been used to drive the SAMI3 model, for ionosphere/plasmasphere studies. In MAGE, the high- and mid-latitude forcing of the WACCM-X, including ion convection pattern and auroral precipitation, which is currently specified by empirical models, will be dynamically evolved using the outputs from the REMIX module coupled with GAMERA, RCM, SAMI3 and IPWM components of MAGE.

Vertical wind perturbations in the lower thermosphere (February) as resolved by high-resolution WACCM (~25 km horizontal resolution). Gravity waves from lower atmosphere, including deep convective systems (including tropical cyclones) and jets are visible from the simulation. [Liu et al., 2014].

WACCM-X v 2.0 can be configured either for free running (FR) climate simulations (lower atmosphere unconstrained), or with the lower-middle atmosphere constrained by reanalysis data and data assimilation for weather simulations. The dynamical core of the model has also been recently improved to represent the species dependency of specific heats and mean atmosphere mass in the thermosphere. Validation of WACCM-X v 2.0 shows good agreement with thermospheric and ionospheric observations, including the climatology, short-term variability, and during solar flares and geomagnetically disturbed periods (Liu, Bardeen, et al., 2018; Liu, Liu, et al., 2018). The most recent model version also includes updated model physics package featuring enhanced physical, chemical and aerosol parameterizations (Gettelman et al., 2019). In addition, a new dynamical core is being developed based on the spectral element (SE) method (Lauritzen et al., 2018) for WACCM-X. The grid system used with this dynamical core is the cubed-sphere grid, which has the advantage of being globally quasi-uniform. This affords good scalability and is particularly important for high-resolution simulations. The potential and feasibility of high-resolution simulations using SE with global quasi-uniform resolution of ~25 km has been demonstrated in previous studies (Liu et al., 2014). It can resolve gravity waves associated with meteorological sources (e.g. orography, convection, and jets) down to meso-alpha range (~200km) and up into the lower thermosphere. The high-resolution capability of WACCM-X makes it possible to resolve mesoscale and large-scale waves that can propagate into the upper atmosphere and affect the thermosphere and ionosphere state of different temporal and spatial scales and its responses to geospace storms.

To better characterize the background atmosphere state and to enable research forecast, data assimilation capability has been developed by ingesting middle atmosphere temperature observations in WACCM-X. Data assimilation has been implemented in WACCM-X using the Data Assimilation Research Testbed (DART) ensemble Kalman filter (Pedatella et al., 2014; 2018). This provides the capability to constrain the model meteorology up to the lower thermosphere. Current capabilities of WACCM-X/DART include the assimilation of conventional meteorological observations in the troposphere and stratosphere, as well as satellite temperature observations from Aura MLS and TIMED/SABER between ~20-100 km. WACCM-X/DART has been demonstrated to capture the large scale variability in the mesosphere, including during sudden stratosphere warming and planetary wave events (Pedatella et al., 2014; Gu et al., 2017; Gan et al., 2018).


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