Impact of Stratospheric Warmings on Ozone Depletion and PSC Generation at Mid Latitudes
The polar stratosphere in the winters 2007/2008 and 2008/2009 was greatly disturbed by several stratospheric sudden warmings (SSW). The major stratospheric warming of February 2008 had a deep impact on the whole middle atmosphere: The warming was accompanied by updrafts and cooling of the upper troposphere, lower stratosphere and mesosphere. The strong cooling led to polar stratospheric clouds over England. These results demonstrate the vertical coupling of the whole atmosphere by a SSW, even at mid latitudes. During the same period our microwave radiometers in Bern (Switzerland) measured a more than 30% depletion in volume mixing ratio (VMR) of stratoshperic ozone and water vapor VMR enhancements of 20%. This ozone depletion is explained by the temperature increase of 50 K and more in the upper stratosphere and by transport of cold, ozone-poor air from the polar vortex, which was shifted to Switzerland in the lower stratosphere. The large temperature increase affects the catalytic NOx cycle, which more efficiently destructs ozone at higher temperatures. The increase of water vapor was a consequence of different transport processes depending on altitude. Air in the lower stratosphere was of polar origin whilst higher up air was transported to Switzerland from subtropical regions. At the end of January 2009 another SSW occurred and resulted in a splitting of the polar vortex. The troposphere was directly affected as the two vortices entrained heavy snow falls over Oklahoma and Kentucky as well as over France and England. Our radiometers again observed a significant decrease in ozone as well as changes in water vapor in the stratosphere. Hence SSW's are very suitable to demonstrate the coupling of lower and upper atmosphere. Latest results will be presented at the assembly.
Coulpling of the Themosphere/Ionosphere with the Lower Atmosphere through atmospheric waves
Observations suggest that short-term variability in the thermosphere and ionosphere can be driven by meteorological perturbations such as planetary waves and gravity waves. The mechanism of this driving, however, is not yet well understood. This talk will review some of our recent efforts in this aspect. In particular, TIME-GCM simulations have shown that the total electron contents (TEC) and peak F2 layer electron density can vary rather significantly in response to transient planetary waves and secondary generation of gravity waves in the thermosphere. We will present a detailed analysis of the key dynamical and electrodynamical processes that produce the thermospheric and ionospheric variability.
Baroclinic Conditions of the Arctic Wintertime Middle Atmosphere and Anomalous Temperature Excursions
The arctic stratosphere and mesosphere are dynamically altered throughout the winter months as wave activity and its interaction with the mean flow is a regular occurrence. An extreme interaction leads to polar vortex breakdown and a complete alteration in temperature from the lower stratosphere through the mesosphere. However, there are more regular occurrences where the dynamical interactions can alter just the upper stratosphere and mesosphere without modification to the lower stratosphere. We investigate these occurrences and their associated baroclinic conditions that can result in dramatic (50K) changes in upper stratosphere and mesosphere temperatures. Due to the flexible lower boundary, strong temperature gradients, and low air density, the circulation development of the arctic middle atmosphere is less well known than that below 30 km. Under certain conditions the wintertime polar vortex may become distorted and weakened, producing baroclinic conditions near the stratopause and into the lower mesosphere. Ageostrophic vertical circulations are enhanced during these times resulting in regions of adiabatic heating and cooling that further strenghten the baroclinic state . Events have been identified over the years where the stratopause warms by up to 50 K (and the mesosphere cooled by a similar magnitude) over nominal conditions. These heating/cooling events cannot be caused by advection and are based on the associated changing dynamical conditions. We propose that strong vertical circulation generates the required adiabatic heating and cooling. We have identified a number of these events over the past ten years in UKMO fields of the upper stratosphere and in middle atmosphere temperature measurements provided by the SABER instrument on the TIMED satellite. In order to determine the vertical circulations induced, we apply a technique used in tropospheric meteorology that requires only temperature and geopotential height fields known as the Q-vector analysis. The Q-vector allows us to qualitatively determine regions of ascending and descending air. The Q-vector analysis is well- suited to this region of the atmosphere as there are relatively few observational variables available. The UKMO assimilated data set provides the necessary fields up to approximately 2 hPa. Measurements from SABER/TIMED satellite instruments provide temperature measurements well into the mesosphere. Using these data sets, we identify these middle atmospheric baroclinic events and using Q-vector analysis show the vertical air motion induced.
Winter-time vertical winds in the upper mesosphere/lower thermosphere in the High Arctic
Vertical winds in the upper mesosphere/lower thermosphere are estimated from Doppler shifts of green-line airglow O (1S) emission using a ground based Michelson Interferometer stationed at Resolute Bay (74° N, 95° W). Measured vertical winds are compared to simultaneously measured horizontal winds and synoptic geomagnetic indices in an effort to better understand the vertical motions associated with competing high latitude phenomena; namely the downward motion caused by the polar vortex circulation and the upward motion associated with geomagnetic activity. Specifically, we present vertical winds associated with both geomagnetic quiet-time and active-time periods and discuss the dynamics associated with both situations.
Sources for Traveling Ionospheric Disturbances
A number of different sources have been suggested for the gravity waves that cause TIDs:- auroral electrojets, jet streams, and mesospheric weather systems. In this presentation we will examine whether any of these sources fits the known properties of the TIDs. It turns out that the best fit seems to be with mesospheric weather systems, but there still appear to be a number of minor inconsistencies between the source properties and the observations.
Weather Effects on the D-region Electron Density
Studies of D-region ionization are complicated by the low electron densities and the altitude range involved. The D-region bottom-side densities are less than 100 cm-3 and the D-region altitudes are inaccessible to most in-situ measurements. Available methods, such as sounding rockets and incoherent scatter radar, can provide detailed profiles for specific times and locations, but mesoscale characterization of D-region weather effects is difficult to obtain. Specifically the horizontal structuring of these densities and to which drivers they are most sensitive is unclear. The response of the D-region to solar inputs, background radiation sources, and wind transport from high latitudes needs to be better understood to improve both our understanding and modeling efforts. The Agile beacon monitor network measures signal strength from radio beacons from three important frequency ranges. The measurements in three frequency ranges, VLF (3-30kHz), LF (30-300 kHz), and HF (0.3-30 MHz), cooperatively help define the D region more precisely. The daytime D-region is perhaps best known for absorption of frequencies below 30 MHz. Measurements of radio signal absorption are useful in describing the D-region response to solar flares and the winter absorption anomaly. Description of the D- region bottom-side and nighttime D-region density requires a different methodology. VLF and LF propagation analysis is sensitive to densities in the 0.1 to 10 cm-3 range. Networks of receivers over these frequency ranges provide an approach for observing the horizontal spatial distribution of the lower D-region density. The D-region electron densities may be inferred by interpreting signal levels at VLF, LF, and HF using D-region models and propagation analysis. This paper describes how the model electron density profiles are modified to include weather effects. Variations are observed in day and night data even during the quietest solar conditions; some variations are consistent with atmospheric gravity wave and planetary wave scales.