Production and transport mechanisms of NO in the polar upper mesosphere and lower thermosphere 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 Experiment (SOFIE) instrument on-board 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 8 years of SOFIE observations, covering the period 2007–2015, and focus on the Southern Hemisphere (SH), rather than on dynamical variability in the Northern Hemisphere (NH) 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 in the thermosphere. Number densities are more similar during winter, though the altitude of peak NO density, which reaches between 102 and 106 km in WACCM and between 98 and 104 km in SOFIE, is most separated during winter. Using multiple linear regression (MLR) and superposed epoch analysis (SEA) methods, 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−1 in the 80–110 km region in both datasets. The downward-transported NO fluxes are, however, too low in WACCM, which is likely due to medium energy electrons (MEE) and D-region ion 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 Sassi et al, 2018;Siskind et al, 2015). In the Antarctic, Hendrickx et al (2018) 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.…”

response to reviewer 3

“…The standard TIME-GCM model uses a value for Reaction (R2) of 7 × 10 −13 cm 3 s −1 from Fell et al (1990). As discussed by Yonker (2013), Herron (1999) Figure 3 compares the diurnal NO variation between the driven and nudged models. It shows that both models have peak NO densities in the lower thermosphere of just over 10 8 cm −3 .…”

“…inferred a temperature dependence of this rate that appears to neglect the Fell et al(1990) reference. Instead, relying upon older studies,Herron (1999) recommends a room temperature value which is about double the room temperature measurement of Fell et al(1990). Thus in doubling the rate coefficient R2 we are essentially, as an academic exercise, using Herron's somewhat arbitrary room temperature value in lieu of Fell et al's (1990) measurement.…”

Mesospheric nitric oxide model from SCIAMACHY data

“…Chemistry-climate models struggle to simulate the NO amounts and distributions in the mesosphere and lower thermosphere (see, for example, Funke et al, 2017;Randall et al, 2015;Orsolini et al, 2017;Hendrickx et al, 2018). To remedy the situation, some models constrain the NO content at their top layer through observation-based parametrizations.…”

“…The dissociation energy of N 2 into ground state atoms N( 4 S) is about 9.8 eV (λ ≈ 127 nm) (Hendrie, 1954;Frost et al, 1956;Heays et al, 2017). This energy together with the excitation energy to N( 2 D) is denoted by hν in Reaction (R1) and can be provided by a number of sources, most notably by auroral or photoelectrons as well as by soft solar X-rays.…”