Patrick J. Espy

Stratospheric gravity wave characteristics and seasonal variations observed by lidar at the South Pole and Rothera, Antarctica

� 0.7 m s � 1 , and period of � 104 min. Approximately 44% of the observed waves show an upward phase progression while the rest display a downward phase progression in ground-based reference for both locations. Gravity wave potential energy density (GW-EP) at Rothera is � 4 times higher than the South Pole in winter but is comparable in summer. Clear seasonal variations of GW-EP are observed at Rothera with the winter average being 6 times larger than that of summer. The seasonal variations of GW-EP at the South Pole are significantly smaller than those at Rothera. The absence of seasonal variations in wave sources and critical level filtering at the South Pole is likely to be responsible for the nearly constant GW-EP. The minimum critical level filtering in winter at Rothera is likely to be a main cause for the winter enhanced GW-EP, as this would allow more orography-generated waves to reach the 30 to 45 km range. The stratospheric jet streams may also contribute to the winter enhancement at Rothera.

Mesospheric Na layer at extreme high latitudes in summer

Summertime observations of the mesospheric Na layer at high latitudes are reported from the 1993 Airborne Noctilucent Cloud (ANLC-93) campaign in the Canadian Arctic and at the Amundsen-Scott South Pole Station. Measurements at the South Pole reveal a layer that has a smaller column abundance and is significantly higher and thinner than at midlatitudes. Using a model that was essentially optimized to wintertime conditions at high northern latitudes, the South Pole layer can be modeled satisfactorily if the rate coefficient for the reaction between sodium bicarbonate and atomic hydrogen is set to k(NaHCO3 + H → Na + H2O + CO2) = 1.1 × 10−11 exp (−910/T) cm3 molecule−1 s−1. In particular, the model is able to reproduce the small scale height of about 2 km observed on the underside of the layer. It is then shown that this steep gradient in the atomic Na mixing ratio can be sustained against vertical eddy diffusion because of the sufficiently rapid chemical cycling between Na its major reservoir, NaHCO3. This justifies the assumption in the model that the vertical transport of Na species can be treated in terms of a single continuity equation describing total sodium. The observations from the campaigns in both hemispheres show that the Na abundance has a temperature dependence of about 2 × 108 cm−2 K−1 at temperatures below 170 K, in good accord with the model. About 40% of this dependence appears to be caused by the activation energies of the reactions that partition sodium between atomic Na and NaHCO3, and the remainder by the temperature dependence of the odd-oxygen/odd-hydrogen chemistry in the upper mesosphere.

Dynamical and chemical aspects of the mesospheric Na “wall” event on October 9,1993 during the Airborne Lidar and Observations of Hawaiian Airglow (ALOHA) campaign

On October 9, 1993, observations were made from the National Center for Atmospheric Research Electra aircraft during a flight from Maui, Hawaii, toward a low-pressure system NW of the island, a flight of 7 hours in total. The leading edge (wall) of a bright airglow layer was observed 900 km NW of Maui at 0815 UT, which was traveling at 75 m s−1 toward the SE, reaching Haleakala, Maui, about 3.25 hours later [see Swenson and Espy, 1995]. An intriguing feature associated with the event was the large increase in the mesospheric Na column density at the wall (∼180%). The enhancement was distributed over a broad region of altitude and was accompanied by significant perturbations in the Meinel (OH) and Na D line airglow emission intensities, as well as the temperature. This paper describes an investigation of the combined measurements from the aircraft and at Haleakala, including an analysis of the event using a gravity wave dynamic model. The modeled atmospheric variations associated with the leading edge of the “wall” wave are then applied to models of the neutral and ionic chemistry of sodium in order to establish whether the enhancement was caused by the release of atomic Na from a local reservoir species, as opposed to redistribution by horizontal convection. The most likely explanation for the Na release was the neutralization of Na+ ions in a sporadic E layer that was first transported downward by a large amplitude (≈10%) atmospheric gravity wave and then vertically mixed as the wave pushed the atmosphere into a super adiabatic state with associated convective instabilities and overturning.

Observations of gravity wave forcing of the mesopause region during the January 2013 major Sudden Stratospheric Warming

©2014. The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

Seasonal variations of the mesospheric Fe layer at Rothera, Antarctica (67.5°S, 68.0°W)

[1] Lidar observations of Fe densities between 75 and 105 km above Rothera, Antarctica, are used to characterize the seasonal variations of the mesospheric Fe layer near the Antarctic Circle and the differences are compared to the South Pole. The maximum Fe abundance occurs in late autumn (early May) at Rothera, rather than in midwinter. A secondary Fe enhancement occurs 6 months later in late spring (October–November) prior to the formation of polar mesospheric cloud (PMC) layers in summer. The midsummer Fe layer is 3 km lower at Rothera because Fe depletion by PMC layers near the mesopause is not as extensive or as complete as at the South Pole. These observations are modeled satisfactorily using a mesospheric one‐dimensional Fe chemistry model driven by a general circulation model and including a detailed micrometeoroid flux and ablation model. Our study shows that the autumnal maximum in the Fe abundance is caused primarily by the seasonal temperature maximum in the mesopause region, reinforced by the seasonal peak in the meteor input function (MIF). The Fe abundance at Rothera declines throughout the winter in response to the decrease in the MIF and the slowly falling temperatures. The modeled Fe injection rate is ∼5 times smaller while the eddy diffusivity values between 80 and 90 km are 4.1 times smaller than the corresponding values used in the South Pole model. This comparison demonstrates the sensitivity of the metal atom densities to the balance between injection by meteoric ablation and removal by downward transport.

The impact of energetic electron precipitation on mesospheric hydroxyl during a year of solar minimum

©2016. The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

The climatology of zonal wave numbers 1 and 2 planetary wave structure in the MLT using a chain of Northern Hemisphere SuperDARN radars

The zonal wave components 1 and 2 were extracted from the meridional wind along the latitude band of 51–66°N for the years 2000–2008 using eight Super Dual Auroral Radar Network (SuperDARN) radars spanning longitudes from 25°E to 150°W. Each extracted zonal component represents the superposition of all temporal periods with that zonal structure and indicates the total planetary wave energy available with that wave number. The Hovmoller diagrams show stationary as well as eastward and westward traveling planetary waves propagating in the background wind. The method used to detect the zonal planetary wave components in the SuperDARN data are detailed and validated using UK Meteorological Office data, which allows the evolution of S1 and S2 planetary wave energy between the stratosphere and mesosphere to be assessed. The climatology of zonal wave number 1 and 2 planetary wave activity in the mesosphere-lower thermosphere (MLT) is presented and compared to the activity in the stratosphere. The MLT climatology of the mean wind anomalies shows stronger planetary wave activity during winter and weaker activity during summer with enhancement around midsummer and autumn equinox. The climatology of the mean wind displays similar amplitudes apart from very strong S1 planetary wave amplitudes during summer. In addition planetary wave activity during winters with major and minor stratospheric warming events are examined and contrasted.

Toward an Improved Representation of Middle Atmospheric Dynamics Thanks to the ARISE Project

This paper reviews recent progress toward understanding the dynamics of the middle atmosphere in the framework of the Atmospheric Dynamics Research InfraStructure in Europe (ARISE) initiative. The middle atmosphere, integrating the stratosphere and mesosphere, is a crucial region which influences tropospheric weather and climate. Enhancing the understanding of middle atmosphere dynamics requires improved measurement of the propagation and breaking of planetary and gravity waves originating in the lowest levels of the atmosphere. Inter-comparison studies have shown large discrepancies between observations and models, especially during unresolved disturbances such as sudden stratospheric warmings for which model accuracy is poorer due to a lack of observational constraints. Correctly predicting the variability of the middle atmosphere can lead to improvements in tropospheric weather forecasts on timescales of weeks to season. The ARISE project integrates different station networks providing observations from ground to the lower thermosphere, including the infrasound system developed for the Comprehensive Nuclear-Test-Ban Treaty verification, the Lidar Network for the Detection of Atmospheric Composition Change, complementary meteor radars, wind radiometers, ionospheric sounders and satellites. This paper presents several examples which show how multi-instrument observations can provide a better description of the vertical dynamics structure of the middle atmosphere, especially during large disturbances such as gravity waves activity and stratospheric warming events. The paper then demonstrates the interest of ARISE data in data assimilation for weather forecasting and re-analyzes the determination of dynamics evolution with climate change and the monitoring of atmospheric extreme events which have an atmospheric signature, such as thunderstorms or volcanic eruptions.

Modelling the descent of nitric oxide during the elevated stratopause event of January 2013

Using simulations with a whole-atmosphere chemistry-climate model nudged by meteorological analyses, global satellite observations of nitrogen oxide (NO) and water vapour by the Sub-Millimetre Radiometer instrument (SMR), of temperature by the Microwave Limb Sounder (MLS), as well as local radar observations, this study examines the recent major stratospheric sudden warming accompanied by an elevated stratopause event (ESE) that occurred in January 2013. We examine dynamical processes during the ESE, including the role of planetary wave, gravity wave and tidal forcing on the initiation of the descent in the mesosphere-lower thermosphere (MLT) and its continuation throughout the mesosphere and stratosphere, as well as the impact of model eddy diffusion. We analyse the transport of NO and find the model underestimates the large descent of NO compared to SMR observations. We demonstrate that the discrepancy arises abruptly in the MLT region at a time when the resolved wave forcing and the planetary wave activity increase, just before the elevated stratopause reforms. The discrepancy persists despite doubling the model eddy diffusion. While the simulations reproduce an enhancement of the semi-diurnal tide following the onset of the 2013 SSW, corroborating new meteor radar observations at high northern latitudes over Trondheim (63.4°N), the modelled tidal contribution to the forcing of the mean meridional circulation and to the descent is a small portion of the resolved wave forcing, and lags it by about ten days.

Observational evidence for temporary planetary wave forcing of the MLT during fall equinox

We present direct observations of zonal wave numbers 1 and 2 planetary wave activity in the mesopause region derived from a longitudinal chain of high-latitude Northern Hemisphere (51–66°N) Super Dual Auroral Radar Network radars. Over a 9 year period (2000–2008), the planetary wave activity observed shows a consistent increase around the fall equinox. This is shown to be coincident with a minimum in the magnitude of the stratospheric winds and consequently a minimum in the stratospheric gravity wave filtering and the subsequent momentum deposition in the mesopause region. Despite this, the observed meridional winds are shown to be perturbed poleward and mesopause temperatures rise temporarily, suggesting that westward momentum deposition from planetary waves temporarily becomes the dominant forcing on the mesopause region each fall equinox.