V. L. Harvey

Stratospheric effects of energetic particle precipitation in 2003-2004

Upper stratospheric enhancements in NOx (NO and NO2) were observed at high northern latitudes from March through at least July of 2004. Multi-satellite data analysis is used to examine the temporal evolution of the enhancements, to place them in historical context, and to investigate their origin. The enhancements were a factor of 4 higher than nominal at some locations, and are unprecedented in the northern hemisphere since at least 1985. They were accompanied by reductions in O-3 of more than 60% in some cases. The analysis suggests that energetic particle precipitation led to substantial NOx production in the upper atmosphere beginning with the remarkable solar storms in late October 2003 and possibly persisting through January. Downward transport of the excess NOx, facilitated by unique meteorological conditions in 2004 that led to an unusually strong upper stratospheric vortex from late January through March, caused the enhancements.

Intra‐seasonal variability of polar mesospheric clouds due to inter‐hemispheric coupling

] Polar mesospheric cloud (PMC) observations haverevealed that interannual variability near the polar summermesopause can be forced by planetary wave activity in thewinter stratosphere. We use data from the Aeronomy of Icein the Mesosphere (AIM) satellite to investigate couplingbetween the Arctic winter stratosphere and PMC variabilityin the Antarctic summer of 2007–2008. We find a highcorrelation between zonal mean PMC frequency and Arcticwinter zonal mean winds from the Goddard EarthObserving System, as well as Microwave Limb Sounderzonal mean temperatures. The time lag between changes inthe winter stratosphere and the connected response in PMCsvaries from 2 to 8 days. We suggest that the differences inlag times are related to the evolution of cloud altitudesthroughout the season. The results here are the first to showevidence for intra-seasonal PMC variability forced by inter-hemispheric coupling.

Simulation of energetic particle precipitation effects during the 2003–2004 Arctic winter

Energetic particle precipitation (EPP) during the 2003–2004 Arctic winter led to the production and subsequent transport of reactive odd nitrogen (NOx = NO + NO2) from the mesosphere and lower thermosphere (MLT) into the stratosphere. This caused NOx enhancements in the polar upper stratosphere in April 2004 that were unprecedented in the satellite record. Simulations of the 2003–2004 Arctic winter with the Whole Atmosphere Community Climate Model using Specified Dynamics (SD-WACCM) are compared to satellite measurements to assess our understanding of the observed NOx enhancements. The comparisons show that SD-WACCM clearly displays the descent of NOx produced by EPP but underestimates the enhancements by at least a factor of four. Comparisons with NO measurements in January and February indicate that SD-WACCM most likely underestimates EPP-induced NO production locally in the mesosphere because it does not include precipitation of high energy electrons. Comparisons with temperature measurements suggest that SD-WACCM does not properly simulate recovery from a sudden stratospheric warming in early January, resulting in insufficient transport from the MLT into the stratosphere. Both of these factors probably contribute to the inability of SD-WACCM to simulate the stratospheric NOx enhancements, although their relative importance is unclear. The work highlights the importance of considering the full spectrum of precipitating electrons in order to fully understand the impact of EPP on the atmosphere. It also suggests a need for high-quality meteorological data and measurements of NOx throughout the polar winter MLT.

A multi tracer analysis of thermosphere to stratosphere descent triggered by the 2013 Stratospheric Sudden Warming

Arctic winter observations in 2013 by the Solar Occultation for Ice Experiment (SOFIE) show significant transport from the lower-thermosphere to the stratosphere of air enriched in nitric oxide, but depleted in water and methane. The transport is triggered by the Stratospheric Sudden Warming (SSW) on 11 January and is continuously tracked for over 3 months. Ultimately, evidence for lower thermospheric air is seen at 40 km in mid-April. Area integrated nitric oxide (NO) fluxes are compared with previous events in 2004, 2006, and 2009, to show that this event is the second largest in the past 10 years. The SOFIE data are combined with a meteorological analysis to infer descent rates from 40 to 90 km. The descent profile initially peaks near 75 km, shifting downward by approximately 5 km per 10 days. Our work demonstrates the utility of SOFIE tracer measurements in diagnosing vertical transport from the stratosphere to the edge of space.

Is a high‐altitude meteorological analysis necessary to simulate thermosphere‐stratosphere coupling?

We compare simulations of mesospheric tracer descent in the winter and spring of 2009 with two versions of the Whole Atmosphere Community Climate Model (WACCM), both with specified dynamics. One is constrained with data from the Modern-Era Retrospective Analysis for Research and Applications which extends from 0 to 50 km; the other uses the Navy Operational Global Atmospheric Prediction System-Advanced Level Physics High Altitude (NOGAPS-ALPHA) which extends up to 92 km. By comparison with Solar Occultation for Ice Experiment data we show that constraining WACCM to NOGAPS-ALPHA yields a dramatic improvement in the simulated descent of enhanced nitric oxide (NO) and very low methane (CH4). We suggest that constraining to NOGAPS-ALPHA compensates for an underestimate of nonorographic gravity wave drag in WACCM. Other possibilities, such as missing energetic particle precipitation or underestimated eddy diffusion, are less likely for the Arctic winter and spring of 2009.

Simulated solar cycle effects on the middle atmosphere: WACCM3 Versus WACCM4

The Whole Atmosphere Community Climate Model version 4 (WACCM4) is used to quantify solar cycle impacts, including both irradiance and particle precipitation, on the middle atmosphere. Results are compared to previous work using WACCM version 3 (WACCM3) to estimate the sensitivity of simulated solar cycle effects to model modifications. The residual circulation in WACCM4 is stronger than in WACCM3, leading to larger solar cycle effects from energetic particle precipitation; this impacts polar stratospheric odd nitrogen and ozone, as well as polar mesospheric temperatures. The cold pole problem, which is present in both versions, is exacerbated in WACCM4, leading to more ozone loss in the Antarctic stratosphere. Relative to WACCM3, a westerly shift in the WACCM4 zonal winds in the tropical stratosphere and mesosphere, and a strengthening and poleward shift of the Antarctic polar night jet, are attributed to inclusion of the QBO and changes in the gravity wave parameterization in WACCM4. Solar cycle effects in WACCM3 and WACCM4 are qualitatively similar. However, the EPP-induced increase from solar minimum to solar maximum in polar stratospheric NOy is about twice as large in WACCM4 as in WACCM3; correspondingly, maximum increases in polar O3 loss from solar min to solar max are more than twice as large in WACCM4. This does not cause large differences in the WACCM3 versus WACCM4 solar cycle responses in temperature and wind. Overall, these results provide a framework for future studies using WACCM to analyze the impacts of the solar cycle on the middle atmosphere.

Chemical definition of the mesospheric polar vortex

We present a simple chemical definition to demark the edge of the mesospheric polar vortices. Because this vortex definition does not rely on the wind field, it is useful in the mesosphere where wind observations are sparse and reanalysis winds are unreliable. The chemical definition is also insensitive to double jets that complicate vortex identification in the mesosphere. The algorithm is based on horizontal gradients of carbon monoxide (CO) and mirrors the widely used vortex edge definition in the stratosphere based on potential vorticity (PV) gradients. Here the approach is used to identify the Arctic vortex in the mesosphere during a 10 year (2004–2014) record of Microwave Limb Sounder data. Vortex size and shape comparisons are made where the CO and PV methods overlap in the upper stratosphere. A case study is presented during the NH 2008–2009 winter that demonstrates the fidelity of the CO gradient method on individual days and emphasizes the impact of double jets on methods to identify the polar vortex. We recommend transitioning from a PV or stream function-based vortex definition in the stratosphere to using a CO gradient definition above 0.1 hPa (~60 km). The CO gradient method identifies a coherent region of high CO at 80 km that is confined to mid-to-high latitudes 99.8% of the time during Arctic winter. Taking advantage of the CO gradient method to identify the polar vortex adds ~20 km of reliable vortex information (from 60 to 80 km) in a region of the atmosphere where reanalyses are most suspect.

Effects of the September 2005 Solar Flares and Solar Proton Events on the Middle Atmosphere in WACCM

This work investigates middle atmosphere effects of the September 2005 solar flares and solar proton events (SPEs). X-17 and X-6.2 flares occurred on 7 and 9 September, respectively, while two moderate SPEs occurred on 10 and 15 September. Flare ionization and dissociation were calculated in the Whole Atmosphere Community Climate Model (WACCM) using the Flare Irradiance Spectral Model. Proton measurements from the Geostationary Operational Environmental Satellite system were used to compute solar proton ionization. SPEs are shown to have a larger impact than solar flares on the polar stratosphere and mesosphere; however, flares have a larger influence on the sunlit and equatorial lower thermosphere. The two flares differed significantly with respect to photon spectrum. The larger, X-17 flare was stronger during the impulsive phase, while the X-6.2 flare was stronger during the gradual phase. This resulted in the X-17 flare causing more initial ionization but for a shorter duration. The simulated flare impacts also differed because specific wavelengths of the flares influenced the atmosphere above the model top. Model-measurement comparisons show that WACCM captures the overall timing and spatial distribution of the observed electron enhancements, indicating a reasonable simulation of flare and SPE-induced ionization. Both the SPEs and flares caused odd nitrogen increases in the mesosphere. Odd hydrogen produced in the lower mesosphere by the SPEs led to short-lived ozone decreases of nearly 100%. The flares caused small temperature increases in the lower thermosphere but had no effect on the stratosphere. Plain Language Summary This is the first study to use a global climate model to investigate the effects of solar flares on the Earth’s atmosphere from the surface up to about 140 km. The model simulations confirm that a flare’s energy spectrum, which differs from flare to flare, determines how the atmosphere is affected. For the flares investigated in this work, which occurred in September of 2005, the simulations show that the solar photons ionized the atmosphere for several hours, causing large increases in electron density. The ionization led to small increases in temperature and also initiated a cascade of chemical reactions that resulted in the production of reactive nitrogen oxides in the mesosphere and thermosphere. There were no significant effects of the flares on the stratosphere. This work is important because it adds to our growing understanding of how impulsive solar events such as solar flares affect the Earth’s atmosphere and provides a foundation for understanding their cumulative impacts on the atmosphere and possibly climate.

Modeling and mechanisms of polar winter upper stratosphere/lower mesosphere disturbances in WACCM

This work focuses on the characteristics and mechanisms of upper stratosphere/lower mesosphere (USLM) disturbances in the polar winter as simulated by a free-running 40 year run of the Whole Atmosphere Community Climate Model (WACCM) version 4. USLM disturbances have been shown to precede the development of every major sudden stratospheric warming (SSW), thus potentially increasing the predictability of SSWs. WACCM4 is shown to internally and spontaneously generate polar USLM disturbances that are consistent with an established USLM climatology based on the UK Meteorological Office stratospheric assimilation. Arctic USLM disturbances in WACCM4 occur on average 2.65 times per winter season; are most frequently generated in the months of December, January, and February; and are preferentially located in the longitude band between 30°E and 120°E, poleward of 40° latitude, all in good agreement with observations. Analysis of composite USLM events corroborates the underlying mechanism responsible for their formation as planetary wave breaking between ~45 km and 65 km that decelerates the mean flow, induces vertical air motion, and causes regions of adiabatic heating in a limited longitude band. These conditions are suitable for the growth of a baroclinic instability at the stratopause level. Using the Trenberth localized three-dimensional Eliassen-Palm flux along with the Charney-Stern criteria, a WACCM4 case study of an independent USLM event implicates baroclinic instability as a critical process in the development of the USLM thermal structure.