Production and transport mechanisms of NO in observations and models

Abstract

Abstract. A reservoir of Nitric Oxide (NO) in the lower thermosphere efficiently cools the atmosphere after periods of enhanced geomagnetic activity. Transport from this reservoir to the stratosphere within the winter polar vortex allows NO to deplete ozone levels and thereby affect the middle atmospheric heat budget. As more climate models resolve the mesosphere and lower thermosphere (MLT) region, the need for an improved representation of NO related processes increases. This work presents a detailed comparison of NO in the Antarctic MLT region between observations made by the Solar Occultation for Ice 5 Experiment (SOFIE) instrument onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite and simulations performed by the Whole Atmosphere Community Climate Model with Specified Dynamics (SD-WACCM). We investigate 7 years of SOFIE observations and focus on the Southern hemisphere, rather than on dynamical variability in the Northern hemisphere or a specific geomagnetic perturbed event. The morphology of the simulated NO is in agreement with observations though the long term mean is too high and the short term variability is too low. Number densities are more similar during winter, though 10 the altitude of peak densities, which reaches between 102 -106 km in WACCM and between 98-104 km in SOFIE, is most separated during winter. Using multiple linear regressions and superposed epoch analyses we investigate how well the NO production and transport are represented in the model. The impact of geomagnetic activity is shown to drive NO variations in the lower thermosphere similarly across both datasets. The dynamical transport from the lower thermosphere into the mesosphere during polar winter is found to agree very well, with a descent rate of about 2.2 km/day in the 80 -110 km region in 15 both datasets. The downward transported NO fluxes are however too low in WACCM, which is likely due to medium energy electrons and D-region chemistry that are not represented in the model.

Evaluation of the Mesospheric Polar Vortices in WACCM

“…Recent studies have shown that either constraining the model up to 90 km or assimilating mesospheric data improves the representation of the EPP IE (Pedatella, Liu, et al, ; Sassi et al, ; Siskind et al, ). In the Antarctic, Hendrickx et al () show EPP‐NO x underestimates despite MLT descent rates in WACCM that agree with observations. This result bolsters the argument that missing NO production is responsible for the model NO underestimates, although they also note that the model could have too much NO destruction or too strong horizontal diffusion.…”

Observations and Modeling of Increased Nitric Oxide in the Antarctic Polar Middle Atmosphere Associated With Geomagnetic Storm‐Driven Energetic Electron Precipitation

“…Hendrickx et al () compared seven years (2007–2015) of SOFIE NO observations in the SH with simulations performed by the standard version of WACCM incorporating SD. SD‐WACCM includes a Kp‐driven auroral electron parameterization but lacks the specific D ‐region ion‐neutral chemistry and MEE precipitation that are in WACCM‐SIC.…”

“…However, above the mesopause MEE precipitation has a negligible effect, and similar to our findings using WACCM‐SIC, their modeled NO concentrations in the lower thermosphere are almost a factor 2 higher than observations. Hendrickx et al () suggest that the differences between their observations and model simulations could be attributed, at least in part, to uncertainties and challenges in determining reaction rates and branching ratios in several NO reactions involving excited state (N( 2 D)) and ground state (N( 4 S)) nitrogen. In the standard parameterization in WACCM (Jackman et al, ; Marsh et al, ) an ion pair produces 1.25 N atoms with branching ratios of 0.55 N( 4 S) and 0.70 N( 2 D).…”

“…In the standard parameterization in WACCM (Jackman et al, ; Marsh et al, ) an ion pair produces 1.25 N atoms with branching ratios of 0.55 N( 4 S) and 0.70 N( 2 D). However, as pointed out by Hendrickx et al (), other recommended values of the N( 2 D) branching ratio range from 0.50 to 0.60 (Solomon et al, ; Yonker, ). Our simulations use a different parameterization from standard WACCM, with the detailed and verified mechanisms implemented in WACCM‐SIC (Kovács et al, ) based on comprehensive analysis and revision of the D ‐region chemistry in the SIC model using updated rate coefficients (Pavlov, ; Verronen et al, ).…”

“…Setting the Prandtl number below the recommended value of 2 increased mesospheric NO in our WACCM-SIC simulations, producing better agreement between the modeled and observed data for both winters and also increased thermospheric NO abundance. Hendrickx et al (2018) compared seven years (2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) of SOFIE NO observations in the SH with simulations performed by the standard version of WACCM incorporating SD. SD-WACCM includes a Kp-driven auroral electron parameterization but lacks the specific D-region ion-neutral chemistry and MEE precipitation that are in WACCM-SIC.…”