Understanding the details of the descent of mesospheric air following the 2013 elevated stratopause event

Abstract:

We use the Specified Dynamics version of the Whole Atmosphere Community Climate Model Extended (SD-WACCMX) to model the descent of nitric oxide (NO) and other mesospheric tracers in the extended, elevated stratopause phase of the 2013 Sudden Stratospheric Warming (SSW). The dynamics are specified with a high altitude version of the Navy Global Environmental model (NAVGEM-HA). Consistent with earlier published results, we find that using a high altitude meteorological analysis to nudge WACCMX allows for a realistic simulation of the descent of lower thermospheric nitric oxide down to the lower mesosphere, near 60 km. This is important because these simulations only included auroral electron precipitation, and did not consider additional sources of NO from medium energy electron precipitation. This suggests that the so-called energetic particle precipitation indirect effect (EPP-IE) can be accurately simulated, at least in years of low geomagnetic activity such as 2013, without the need for additional NO production, provided the meteorology is accurately constrained.

Despite the general success in WACCMX in simulating mesospheric NOx, a detailed comparison of the WACCMX fields with the analyzed NAVGEM H2O and satellite NO data from AIM and ACE reveals significant differences in the latitudinal and longitudinal distributions in the 45-55 km region. This stems from the tendency for WACCMX descent to maximize at sub polar latitudes, while the satellite data more often show maximum descent in the core of the vortex. Our previous work linked these differences to differences in the Transformed Eulerian Mean (TEM) circulation between NAVGEM-HA and WACCMX. Here, we use potential vorticity and equivalent latitude analysis to compensate for the different distributions of model vs observed NOx and to enable us to constrain the budgets of MLT NOx delivered to the stratosphere. Preliminary results suggest that both model and observations are generally consistent with MLT NOx totals of 0.2 - 0.4 gigamoles (GM). While only a small fraction of the overall total stratospheric NOx, it would be more significant in the polar region. Significant uncertainties are associated with these estimates which will be discussed.

This work was sponsored by the NASA Aeronomy of Ice in the Mesosphere (AIM) program