Mesospheric Nitric Oxide Transport in WACCM

Energetic particle precipitation (EPP) causes ionization of the main constituents of the Earth's atmosphere which leads to the production of nitric oxide (NO) throughout the polar mesosphere and lower thermosphere (MLT). Due to the long lifetime of NO during winter, it can also be transported deeper into the atmosphere by the mesospheric residual circulation (the indirect EEP effect). This study investigates the mesospheric indirect NO response to EEP using Whole Atmosphere Community Climate Model (WACCM) version 6. In comparison to observations from the instrument Solar Occultation For Ice Experiment (SOFIE) on the AIM (Aeronomy of Ice in the Mesosphere) satellite, a wintertime underestimation is found in the modeled mesospheric NO amount. WACCM's temperature profile is found to be vertically shifted compared to observations by SOFIE and by The Sounding of the Atmosphere using Broadband Emission Radiometry instrument on the Thermosphere Ionosphere Mesosphere Energetics Dynamics satellite (SABER). The discrepancies in NO are therefore attributed to the model's ability to simulate the dynamics responsible for the indirect EEP effect. The drivers of this transport are investigated by sensitivity runs of WACCM's gravity wave forcing. Changing the amplitude of the non-orographic gravity waves and the Prandtl number improves the modeled vertical distribution of NO and temperature in the MLT region. Plain Language Summary Nitric oxide (NO) is produced by energetic particle precipitation in the polar regions of the mesosphere and lower thermosphere, and its distribution rapidly increases with altitude. During winter the lifetime of NO is long enough for it to be transported by the downward atmospheric circulation in the mesosphere far from where it was produced. This makes the level of NO in the mesosphere dependent on both the local production and on transport processes. When comparing simulated NO in the model Whole Atmosphere Community Climate Model (WACCM) to observations by the satellite instrument Solar Occultation For Ice Experiment, too little NO is found in the mesosphere even when the mesospheric production is accounted for in the model. This may be related to model errors in the transport from the thermosphere. Comparing model temperatures to those observed by two different satellite instruments confirms that there are some dynamic deficiencies. The mesospheric circulation is driven by breaking gravity waves, so by changing where and how the wave energy and momentum are deposited in WACCM, the modeled circulation can be altered to improve the vertical NO distribution and the temperature profile in the polar mesosphere and lower thermosphere. The best correspondence between modeled and observed nitric oxide and temperature is found when the amplitude of the gravity waves is reduced or when the eddy diffusion is increased.