Stratopause Region Research Articles

Wavenumbers 3 and 4 Quasi 2‐day Wave Activities Observed by Multiple Meteor Radars in the Two Hemispheres During Austral Summer

AbstractCombining observations from three meteor radars in the Northern Hemisphere (NH) and a meteor radar in the Southern Hemisphere (SH) and reanalysis data, we present successive wavenumbers 3 and 4 (W3 and W4) quasi 2 day wave activities in austral summer. In the mesosphere and lower thermosphere, the wave exhibits two burst activities. The W3 dominates the first burst, while the W4 is the predominate mode in the second burst. The mode transition arises in the late January and is completed around the end of January. The W3 and W4 have the nearly same dominate period of about 46 hr and vertical wavelengths of 110 km in the SH and about 55–65 km in the NH, and the similar latitude and altitude distributions in the zonal and meridional winds and temperature, except stronger magnitude and farther extension in the NH for the W3. The analysis indicates that the unstable regions of the summer jets in the lower mesosphere and stratopause region have the boundaries which toward lower latitudes are largely consistent with the critical layers, and Eliassen‐Palm (EP) fluxes of the W3 and W4 increase rapidly near the unstable regions. This demonstrates that both the W3 and W4 originate from the jet instabilities. Nevertheless, the EP flux of the W3 shows a stronger transfer across the hemispheres than that of the W4. The W3 and W4 have the same period and vertical scale, which is different from previous reports.

The Relationship Between Upward Propagating Atmospheric Gravity Waves and Ionospheric Irregularities During Solar Minimum Periods

AbstractWe present a study on the relationship between upward propagating gravity waves and ionospheric irregularities. Gravity waves (GWs) are identified from Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite temperature measurements and nighttime ionospheric irregularities are observed from the three Swarm satellites. Using these two independent set of measurements, we investigate the relationship between gravity waves and ionospheric irregularities on both equinox and solstice seasons for the years 2018–2019. The results show, post‐sunset gravity wave characteristics are important in understanding the variability of global nighttime equatorial plasma density irregularities. We also find out that the pre‐midnight and post‐midnight characteristics of ionospheric irregularities show global westward shift in the occurrence of ionospheric irregularities. Upward propagating gravity waves also show three dominant peaks longitudinally with the strongest being between American and African longitudinal sectors. Strongest gravity wave amplitudes are associated with strongest plasma irregularities. GW suppression is observed around equatorial regions at stratopause region in our measurements which are commonly observed at polar and higher latitudinal regions in many studies. We also observe that, ionospheric irregularities are strongest when GWs are relatively less suppressed.

Two- and three-dimensional structures of the descent of mesospheric trace constituents after the 2013 sudden stratospheric warming elevated stratopause event

Abstract. We use the Specified Dynamics version of the Whole Atmosphere Community Climate Model Extended (SD-WACCMX) to model the descent of nitric oxide (NO) and other mesospheric tracers in the extended, elevated stratopause phase of the 2013 sudden stratospheric warming (SSW). The dynamics are specified with a high-altitude version of the Navy Global Environmental Model (NAVGEM-HA). Consistent with our earlier published results, we find that using a high-altitude meteorological analysis to nudge WACCMX allows for a realistic simulation of the descent of lower-thermospheric nitric oxide down to the lower mesosphere, near 60 km. This is important because these simulations only included auroral electrons and did not consider additional sources of NO from higher-energy particles that might directly produce ionization, and hence nitric oxide, below 80–85 km. This suggests that the so-called energetic particle precipitation indirect effect (EPP-IE) can be accurately simulated, at least in years of low geomagnetic activity, such as 2013, without the need for additional NO production, provided the meteorology is accurately constrained. Despite the general success of WACCMX in bringing upper-mesospheric NO down to 55–60 km, a detailed comparison of the WACCMX fields with the analyzed NAVGEM-HA H2O and satellite NO and H2O data from the Solar Occultation for Ice Experiment (SOFIE) and the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS) reveals significant differences in the latitudinal and longitudinal distributions at lower altitudes. This stems from the tendency for WACCMX descent to maximize at sub-polar latitudes, and while such sub-polar descent is seen in the NAVGEM-HA analysis, it is more transient than in the WACCMX simulation. These differences are linked to differences in the transformed Eulerian mean (TEM) circulation between NAVGEM-HA and WACCMX, most likely arising from differences in how gravity wave forcing is represented. To attempt to compensate for the differing distributions of model vs. observed NO and to enable us to quantify the total amount of upper-atmospheric NO delivered to the stratopause region, we use potential vorticity and equivalent latitude coordinates. Preliminary results suggest both model and observations are generally consistent with NO totals in the range of 0.1–0.25 gigamoles (GM).

The semiannual oscillation (SAO) in the tropical middle atmosphere and its gravity wave driving in reanalyses and satellite observations

Abstract. Gravity waves play a significant role in driving the semiannual oscillation (SAO) of the zonal wind in the tropics. However, detailed knowledge of this forcing is missing, and direct estimates from global observations of gravity waves are sparse. For the period 2002–2018, we investigate the SAO in four different reanalyses: ERA-Interim, JRA-55, ERA-5, and MERRA-2. Comparison with the SPARC zonal wind climatology and quasi-geostrophic winds derived from Microwave Limb Sounder (MLS) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite observations show that the reanalyses reproduce some basic features of the SAO. However, there are also large differences, depending on the model setup. Particularly, MERRA-2 seems to benefit from dedicated tuning of the gravity wave drag parameterization and assimilation of MLS observations. To study the interaction of gravity waves with the background wind, absolute values of gravity wave momentum fluxes and a proxy for absolute gravity wave drag derived from SABER satellite observations are compared with different wind data sets: the SPARC wind climatology; data sets combining ERA-Interim at low altitudes and MLS or SABER quasi-geostrophic winds at high altitudes; and data sets that combine ERA-Interim, SABER quasi-geostrophic winds, and direct wind observations by the TIMED Doppler Interferometer (TIDI). In the lower and middle mesosphere the SABER absolute gravity wave drag proxy correlates well with positive vertical gradients of the background wind, indicating that gravity waves contribute mainly to the driving of the SAO eastward wind phases and their downward propagation with time. At altitudes 75–85 km, the SABER absolute gravity wave drag proxy correlates better with absolute values of the background wind, suggesting a more direct forcing of the SAO winds by gravity wave amplitude saturation. Above about 80 km SABER gravity wave drag is mainly governed by tides rather than by the SAO. The reanalyses reproduce some basic features of the SAO gravity wave driving: all reanalyses show stronger gravity wave driving of the SAO eastward phase in the stratopause region. For the higher-top models ERA-5 and MERRA-2, this is also the case in the lower mesosphere. However, all reanalyses are limited by model-inherent damping in the upper model levels, leading to unrealistic features near the model top. Our analysis of the SABER and reanalysis gravity wave drag suggests that the magnitude of SAO gravity wave forcing is often too weak in the free-running general circulation models; therefore, a more realistic representation is needed.

Spatial variability in long-term temperature trends in the middle atmosphere from SABER/TIMED observations

Temperatures obtained from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on board the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) satellite during the period from 2003 to 2019 are analysed to obtain trends in the region from 50°S to 50°N using the method of Empirical Mode Decomposition (EMD). This method is used to remove long-term oscillations from the data to accurately estimate the linear trends. The novelty of the current study is in dividing the entire study region into sub-regions of 5° latitude and 10° longitude to investigate the associated longitudinal variability. The study shows that the global middle atmosphere, including the global mesopause and stratopause, is cooling as a whole. In the stratosphere and lower mesosphere the observed trends are −0.5 K/decade and in the upper mesosphere, the trends increase to −1.0 K/decade. Mesopause temperatures in Southern Hemisphere and stratopause temperatures in Northern Hemisphere are showing stronger cooling trends in comparison to the other hemisphere. Over tropical and subtropical regions, Southern Hemisphere mesopause is showing a stronger cooling trend over Indian Ocean and Asian region while the cooling trends are symmetric around the equator over Pacific and Atlantic Oceans. Two important results obtained in the present study are as follows: (1) A warming trend, reaching 2.5 K/decade, is observed around 77 km over the Equator and adjoining tropical latitudes, which is attributed to processes leading to the mesospheric semi-annual oscillation (MSAO). (2) Anomalous cooling is observed in the stratopause region over the Southern Indian Ocean (SIO) longitudes. Temperature trends are large and negative ranging from −1 to −2 K/decade over SIO while the trends over the Southern Pacific Ocean (SPO) and the Southern Atlantic Ocean (SAO) lying in the same latitudinal band are small and positive, reaching values of 1 K/decade. It is envisaged that the recent accelerated warming of the SIO is probably responsible for the cooling of the stratopause region above.

Gravity wave mixing effects on the OH*-layer

Based on an advanced numerical model for excited hydroxyl (OH*) we simulate the effects of gravity waves (GWs) on the OH*-layer in the upper mesosphere. The OH* model takes into account (1) production by the reaction of atomic hydrogen (H) with ozone (O3), (2) deactivation by atomic oxygen (O), molecular oxygen (O2), and molecular nitrogen (N2), (3) spontaneous emission, and (4) loss due to chemical reaction with O. This OH* model is part of a chemistry-transport model (CTM) which is driven by the high-resolution dynamics from the KMCM (Kühlungsborn Mechanistic general Circulation Model) which simulates mid-frequency GWs and their effects on the mean flow in the MLT explicitly. We find that the maximum number density and the height of the OH*-layer peak are strongly determined by the distribution of atomic oxygen and by the temperature. As a results, there are two ways how GWs influence the OH*-layer: (1) through the instantaneous modulation by O and T on short time scales (a few hours), and (2) through vertical mixing of O (days to weeks). The instantaneous variations of the OH*-layer peak altitude due to GWs amount to 5–10 km. Such variations would introduce significant biases in the GW parameters derived from airglow when assuming a constant pressure level of the emission height. Performing a sensitivity experiment we find that on average, the vertical mixing by GWs moves the OH*-layer down by ~2 to 7 km and increases its number density by more than 50%. This effect is strongest at middle and high latitudes during winter where secondary GWs generated in the stratopause region account for large GW amplitudes.

Validation of water vapor measured by SABER on the TIMED satellite

The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) measured water vapor (H2O) had errors of unknown origin that were recently identified and corrected. The cause of the errors was determined to be unaccounted for spectral out-of-band (OOB) radiance in the H2O channel centered at 6.8 μm arising from ozone emission in the 9.6 μm band. The corrected SABER radiance profiles have been used to produce a long-term H2O data base labeled as version 2.07 (v2.07). Water vapor volume mixing ratio (VMR) vertical profiles are available in the SABER data archive covering the stratosphere and mesosphere extending from near the tropopause at ~100 hPa (~16 km) to the mesopause region at ~ 0.006 hPa (~83 km), and over the time period from 25 January 2002 to the present day. The random error of the v2.07 product is smaller than 4% at 60 km and below, while above this altitude it rapidly increases to 30% (at 80 km), mainly due to low signal-to-noise. The estimated systematic error of SABER v2.07 H2O is about 10–20%. Coincidence analysis between SABER v2.07, MLS v4.2, ACE v3.5-3.6, MIPAS ESA reprocessed v6, and SOFIE v1.3 shows overall excellent agreement in the mean profile with the mean difference being within ±10% in most cases. In the stratopause region SABER H2O tends to be biased high relative to each of the other data sets used for comparisons especially in the SH polar winter where the mean difference reaches 20% or greater. In polar summer above 80 km, SABER H2O is biased low by ~20% compared to the other measurements. SABER H2O therefore reflects the polar winter and spring descent very well but in the summer PMC region the enhancement is weaker than expected. SABER H2O long-term series in the latitude range 50°S-50°N shows close agreement with MLS on a series of pressure levels throughout the stratosphere and mesosphere on inter-annual to decadal time scales. On these time scales also, throughout the years 2002–2005, SABER and MIPAS long-term time series agree well in the equatorial region which serves as an unprecedented validation for this time period. SABER H2O also captures the “tape recorder” phenomenon in the tropical tropopause region very well.

Observation and Attribution of Temperature Trends Near the Stratopause From HALOE.

This study considers time series of temperature versus pressure, T(p), from the Halogen Occultation Experiment (HALOE) across the stratopause region, where the effects of radiative forcings from the greenhouse gases (CO2 and H2O) and from ozone are most pronounced. Trend analyses of HALOE T(p) values for 1993-2005 are for six levels from 2.0 to 0.3 hPa with a vertical resolution of about 4 km and for eight latitude zones from 65°S to 65°N. The analyses account for the forcing effects from the 11-yr solar cycle. HALOE trends at 2.0 hPa are of the order of -1.0 K/decade across the tropics and subtropics but then become smaller (-0.5 K/decade) at the middle latitudes. Near-global T(p) trends are of order -0.5 K/decade but have a minimum of -0.2 K/decade at 1.0 hPa; they are clearly negative in the southern but slightly positive in the northern hemisphere. The combined radiative forcings from CO2, H2O, and ozone vary between -0.4 and -0.7 K/decade for 1993-2005 and are hemispherically symmetric. The HALOE temperature trend and total radiative cooling profiles differ from those reported from observations and calculations for 1980-2000, mainly because the ozone trends changed from clearly negative in the 1980s through mid-1990s to slightly positive during the time of HALOE. Trends at low latitudes for the tracer, methane (CH4), increase from 2% to 4%/decade from 50 to 10 hPa and then to ~6%/decade by 5 hPa. Analyses of time series of CH4 across the stratopause reveal subseasonal scale variability within the northern hemisphere that reduces the significance of the T(p) trends.

Rayleigh Lidar observed atmospheric temperature characteristics over a western Indian location: intercomparison with satellite observations and models

General characteristics of sub-tropical middle atmospheric temperature structure over a high altitude station, Mt. Abu (24.5°N, 72.7°E, altitude ~1670 m, above mean sea level (amsl)) are presented using about 150 nights observational datasets of Rayleigh Lidar. The monthly mean temperature contour plot shows two distinct maxima in the stratopause region (~45–55 km), occurring during February-March and September-October, a seasonal dependence similar to that reported for mid- and high-latitudes respectively. Semi-Annual Oscillation (SAO) are stronger at an altitude ~60 km in the mesospheric temperature in comparison to stratospheric region. A comparison with the satellite (Halogen Occultation Experiment, (HALOE)) data shows qualitative agreement, but quantitatively a significant difference is found between the observation and satellite. The derived temperatures from Lidar observations are warmer ~2–3 K in the stratospheric region and ~5–10 K in the mesospheric region than temperatures observed from the satellite. A comparison with the models, COSPAR International Reference Atmosphere (CIRA)-86 and Mass Spectrometer Incoherent Scatter Extended (MSISE)-90, showed differences of ~3 K in the stratosphere and ~5–10 K in the mesosphere, with deviations somewhat larger for CIRA-86. In most of the months and in all altitude regions model temperatures were lower than the Lidar observed temperature except in the altitude range of 40–50 km. MSISE-90 Model temperature overestimates as compared to Lidar temperature during December-February in the altitude region of 50–60 km. In the altitude region of 55–70 km both models deviate significantly, with differences exceeding 10–12 K, particularly during equinoctial periods. An average heating rate of ~2.5 K/month during equinoxes and cooling rate of ~4 K/month during November-December are found in altitude region of 50–70 km, relatively less heating and cooling rates are found in the altitude range of 30–50 km. The stratospheric temperature derived from the Lidar and columnar ozone observed by the Total Ozone Mapping Spectrometer (TOMS) over Mt. Abu shows good correlation (r 2 = 0.61) and indicates the association of ozone with the temperature.

Driving of the SAO by gravity waves as observed from satellite

Abstract. It is known that atmospheric dynamics in the tropical stratosphere have an influence on higher altitudes and latitudes as well as on surface weather and climate. In the tropics, the dynamics are governed by an interplay of the quasi-biennial oscillation (QBO) and semiannual oscillation (SAO) of the zonal wind. The QBO is dominant in the lower and middle stratosphere, and the SAO in the upper stratosphere/lower mesosphere. For both QBO and SAO the driving by atmospheric waves plays an important role. In particular, the role of gravity waves is still not well understood. In our study we use observations of the High Resolution Dynamics Limb Sounder (HIRDLS) satellite instrument to derive gravity wave momentum fluxes and gravity wave drag in order to investigate the interaction of gravity waves with the SAO. These observations are compared with the ERA-Interim reanalysis. Usually, QBO westward winds are much stronger than QBO eastward winds. Therefore, mainly gravity waves with westward-directed phase speeds are filtered out through critical-level filtering already below the stratopause region. Accordingly, HIRDLS observations show that gravity waves contribute to the SAO momentum budget mainly during eastward wind shear, and not much during westward wind shear. These findings confirm theoretical expectations and are qualitatively in good agreement with ERA-Interim and other modeling studies. In ERA-Interim most of the westward SAO driving is due to planetary waves, likely of extratropical origin. Still, we find in both observations and ERA-Interim that sometimes westward-propagating gravity waves may contribute to the westward driving of the SAO. Four characteristic cases of atmospheric background conditions are identified. The forcings of the SAO in these cases are discussed in detail, supported by gravity wave spectra observed by HIRDLS. In particular, we find that the gravity wave forcing of the SAO cannot be explained by critical-level filtering alone; gravity wave saturation without critical levels being reached is also important.

Tidal signatures in temperatures derived from daylight lidar soundings above Kühlungsborn (54°N, 12°E)

We have developed a new Rayleigh–Mie–Raman (RMR) lidar at the mid-latitude station in Kühlungsborn (54°N, 12°E) for analyzing geophysical phenomena at day and night, e.g., temperature tides and Noctilucent Clouds. For this study we have used about 3100h of data since April 2011 with additional data from summer 2010. The RMR lidar was in operation day and night in addition to the existing daylight-capable potassium resonance lidar. We show for the first time an overview of the altitude structure and seasonal variation of temperature tides, observed with lidars between 40 and 100km altitude at a mid-latitude site. There is a gap around 80km altitude due to a decreasing signal-to-noise ratio during the day. We derive mean tidal amplitudes and phases with 24-, 12-, and 8-h period. In most of the months, the diurnal component dominates the other tidal components with mean amplitudes of 1–2K in the stratopause region (45–55km altitude), where it is up to three times higher than semidiurnal and terdiurnal tidal amplitudes. The diurnal tide is damped at ∼60km altitude. In the mid-mesosphere (65–70km) diurnal, semidiurnal, and terdiurnal tidal components have comparable mean amplitudes of about 1–1.5K, except around the equinoxes. Around the mesopause the diurnal tide dominates again, with mean amplitudes of about 4K, but with a large variability. The seasonal variation shows a conspicuous structure below ∼65km altitude with tidal amplitudes small in summer and large around the equinoxes. This structure vanishes above ∼65km. There, the amplitude increases in summer. The measured tidal amplitudes and phases are compared with the MERRA (Modern Era Retrospective analysis for Research and Applications) reanalysis data. Repeated soundings in subsequent years allow to examine the year-to-year variation. The data from March in both 2012 and 2013 show a prominent diurnal tide at around 45km altitude with amplitudes about three times larger than in the other months. The short-term variability can be examined from continuous lidar operations during clear-sky periods. In a case study we show a large variability of the tidal amplitudes, especially the 8-h variation. This can only be examined due to a good temporal coverage of the lidar data.

Inter-hemispheric coupling during northern polar summer periods of 2002–2010 using TIMED/SABER measurements

It has been found that for more than one polar summer season between 2002 and 2010, the northern polar mesospheric region near and above about 80km was warmer than normal. The strongest warming effect of this type was observed to occur during northern summer 2002. Observational and theoretical studies imply that these “anomalies” were preceded by unusual dynamical processes in the southern hemisphere. We have analyzed temperature distributions measured by the SABER limb scanning infrared radiometer aboard the NASA TIMED satellite between 2002 and 2010 at altitudes from 15 to 110km and for latitudes between 83°S and 83°N. We describe the approach to trace the spatial extent of inter-hemispheric temperature correlations demonstrating the global features that were unique for the “anomalous” northern polar summers. From our analysis of SABER data from 2002 to 2010, the anomalous heating for the northern mesopause region during northern summer was accompanied by stratospheric heating in the equatorial region. In the winter hemisphere it is accompanied by heating in the lower stratosphere and mesopause region, and cooling in the stratopause region. Also, all the elements of the temperature anomaly structure appear to develop and fade away nearly simultaneously, thereby suggesting either a global influence or a short lagging period (less than 7 days).

Ozone distribution in the middle latitude mesosphere as derived from microwave measurements at Lindau (51.66°N, 10.13°E)

[1] Mesospheric ozone measurements between 50 and 80 km altitude were carried out in middle latitude at Lindau (51.66°N, 10.13°E), Germany, by means of the microwave technique between 1993 and 2006, with an interruption from 1997 to 1998. We utilize data obtained between 1998 and 2006. The measurements show a tendency of typical ozone features that occurs in each year. The most marked patterns are a late summer maximum of ozone in the middle and lower mesosphere which is shifted into autumn and winter in the stratopause region, and a distinct nighttime maximum around 72 km during the winter season, whereas no annual maximum occurs there during daytime. A clear annual asymmetry of the nighttime ozone distribution exists in this domain, marked by a decline of the mean ozone values in January/February and an increase to a subsidiary annual maximum a few kilometers higher in March/April. This asymmetry at the height of the well-known middle mesospheric maximum of ozone (MMM, often called the tertiary ozone maximum) results from the asymmetric occurrence rate of sudden stratospheric warmings (SSWs) occurring more frequently after winter solstice than before. Additionally, the asymmetric annual variation of water vapor with lowest values just around spring equinox influences the annual variation of ozone. A strong influence on the nighttime ozone concentration is the zonal wind. The night-to-day ratios (NDRs) in the middle to upper mesosphere display a distinct winter anomaly marked by values more than twice as high as in summer. The NDR is modulated by pronounced oscillations with a planetary time scale. The maximum effect occurs at 65–70 km, clearly below the height of the MMM.

Ozone trends in the mid-latitude stratopause region based on microwave measurements at Lindau (51.66° N, 10.13° E), the ozone reference model, and model calculations

We compared 8 years of ozone measurements taken at Lindau (51.66° N, 10.13° E) at altitudes between 40 and 60 km using the microwave technique with the CIRA ozone reference model that was established 20 years ago ( Keating et al., 1990). We observed a remarkable decrease in ozone density in the stratopause region (i.e., an altitude of 50 km), but the decrease in ozone density in the middle mesosphere (i.e., up to 60 km in altitude) is slight. Likewise, we observed only a moderate decrease in the atmospheric region below the stratopause. Other studies have found the strongest ozone decrease at 40 km and a more moderate decrease at 50 km, which is somewhat in contradiction to our results. This decrease in ozone density also strongly depends on the season. Similar results showed model calculations using the GCM COMMA-IAP when considering the increase in methane. In the lower mesosphere/stratopause region, the strongest impact on the concentration of odd oxygen (i.e., O 3 and O) was observed due to a catalytic cycle that destroys odd oxygen, including atomic oxygen and hydrogen radicals. The hydrogen radicals mainly result from an increase in water vapor with the growing anthropogenic release of methane. The finding suggesting that the stratopause region is apparently attacked more strongly by the water vapor increase has been interpreted in terms of the action of this catalytic cycle, which is most effective near the stratopause and amplified by a positive feedback between the ozone column density and the ozone dissociation rate, thereby chemically influencing the ozone density. However, the rising carbon dioxide concentration cools the middle atmosphere, thereby damping the ozone decline by hydrogen radicals.

QBO modulation of the semiannual oscillation in MAECHAM5 and HAMMONIA

The quasi‐biennial oscillation (QBO) signature in the equatorial upper stratosphere and mesosphere is analyzed from MAECHAM5 and HAMMONIA general circulation models. Our results show that this region is significantly influenced by the stratospheric QBO. In the upper stratosphere the QBO modulates the altitude of maximum descent of the stratospheric semiannual oscillation (SSAO) westerly phases. Our results also suggest that the QBO modulates the altitude of maximum descent and also the strength of the SSAO easterly phase. We explore the role of large‐scale and small‐scale waves and also momentum advection in the forcing of the QBO signature in the SSAO domain. The results show how the vertical propagation of the QBO signature to the middle and upper mesosphere depends on the vertical phase structure of the SAO and consequently on the seasonal cycle. During the solstices when MSAO westerlies prevail in the middle and upper mesosphere no QBO signature can be detected above the stratopause region. However, during the equinoxes, when MSAO easterlies dominate in the middle and upper mesosphere, the QBO signature extends throughout the mesosphere and low thermosphere. The QBO directly modulates MSAO easterlies by modifying the altitude at which they are generated in the upper mesosphere. A QBO signature is also detected on the MSAO westerly phase occurring in the mesopause region during the equinoxes.

Possible ozone influence on the quasi-biennial oscillation in the equatorial stratosphere

Crossspectral analysis between the time series of zonal wind velocity in the equatorial stratosphere and the flux of solar UV radiation revealed high coherence between the variations of these two parameters on a quasibiennial scale (24-30 months) in the atmo� spheric layer 500-1 hPa (~6-48.5 km). A lag of the quasibiennial variations in wind velocity behind the quasibiennial variations in the solar radiation flux, the maximum (~ 24 months) of which is located at a level of 100 hPa (16.5 km), decreases with height, and in the stratopause region (~1 hPa), the wind velocity variations are approximately in phase with the varia� tions in the solar radiation flux. This points at the pos� sibility of the influence of quasibiennial variations in the shortwave solar radiation on the quasibiennial oscillation of the zonal wind in the neighborhood of the equatorial stratopause. It has been shown that the meridional gradient and the curvature degree of the meridional profile of ozone concentration in this layer undergo quasibiennial variations such that could be a cause of synchronization between the quasibiennial variations in the wind velocity and thermal variations according to the solar wind equation. 1. The quasibiennial oscillation (QBO) manifests itself most clearly in the change of the direction of the

Solar cycle signal in a general circulation and chemistry model with internally generated quasi‐biennial oscillation

Simulations with the HAMMONIA general circulation and chemistry model are analyzed to improve the understanding of the atmospheric response to solar cycle variations and the role of the quasi‐biennial oscillation of equatorial winds (QBO) for this response. The focus is on the Northern Hemisphere winter stratosphere. Owing to the internally produced QBO, albeit with a too short period of 24 months, the model is particularly suited for such an exercise. The simulation setup with only solar and QBO forcing allows an unambiguous attribution of the simulated signals. Two separate simulations have been performed for perpetual solar maximum and minimum conditions. The simulations confirm the plausibility of dynamical mechanisms, suggested earlier, that propagate the solar signal from the stratopause region downward to the troposphere. One feature involved in this propagation is a response maximum of temperature and ozone in the lower equatorial stratosphere. In our model, this maximum appears as a pure solar signal independent of the QBO and of other forcings. As observed, the simulated response of the stratospheric polar vortex to solar cycle forcing depends on the QBO phase. However, in the model this is statistically significant only in late winter. The simulation for early and mid winter suffers probably from a too strong internal variability of the polar vortex in early winter.

Water vapor measurements at ALOMAR over a solar cycle compared with model calculations by LIMA

Microwave water vapor measurements between 40 and 80 km altitude over a solar cycle (1996–2006) were carried out in high latitudes at Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) (69.29°N, 16.03°E), Norway. Some smaller gaps and three interruptions of monitoring in the winters 1996/1997 and 2005/2006 and from spring 2001 to spring 2002 occurred during this period. The observations show a distinct year‐to‐year variability not directly related to solar Lyman‐α radiation. In winter the water vapor mixing ratios in the upper domain were anticorrelated to the solar activity, whereas in summer, minima occurred in the years after the solar maximum in 2000/2001. In winter, sudden stratospheric warmings (SSWs) modulated the water vapor mixing ratios. Within the stratopause region a middle atmospheric water vapor maximum was observed, which results from the methane oxidation and is a regular feature there. The altitude of the maximum increased by approximately 5 km as summer approached. The largest mixing ratios were monitored in autumn. During the summer season a secondary water vapor maximum also occurred above 65 km most pronounced in late summer. The solar Lyman‐α radiation impacts the water vapor mixing ratio particularly in winter above 65 km. In summer the correlation is positive below 70 km. The correlation is also positive in the lower mesosphere/stratopause region in winter due to the action of sudden stratospheric warmings, which occur more frequently under the condition of high solar activity and the enhancing the humidity. A strong day‐to‐day variability connected with planetary wave activity was found throughout the entire year. Model calculations by means of Leibniz‐Institute Middle Atmosphere model (LIMA) reflect the essential patterns of the water vapor variation, but the results also show differences from the observations, indicating that exchange processes between the troposphere and stratosphere not modeled by LIMA could have influenced the long‐term variability. We show results of measurements, compare these with calculations, and discuss the chemical and dynamical backgrounds of the variation of water vapor in the middle atmosphere.

Overview and characterization of retrievals of temperature, pressure, and atmospheric constituents from the High Resolution Dynamics Limb Sounder (HIRDLS) measurements

The retrieval algorithm for the High Resolution Dynamics Limb Sounder (HIRDLS) instrument onboard NASA's Earth Observing System (EOS) Aura satellite is presented. The algorithm is based on optimal estimation theory, using a modified Levenberg‐Marquardt approach for the iterative solution. Overview of the retrieval scheme, convergence criteria, and the forward models is given. Treatments of clouds and aerosols as well as line‐of‐sight gradients in temperature are described. The retrievals are characterized by high vertical resolution of 1 km and negligible a priori contribution for all products in regions of high signal‐to‐noise ratio (SNR) (most of the retrieval ranges). It is shown that these characteristics hold for all latitudes along a HIRDLS orbit. The weighting functions are narrow and show good sensitivity to temperature or gas perturbations in regions of high SNR. The retrieval error predicted by the algorithm consists of radiometric noise, pointing jitter error, smoothing error, and forward model error. For temperature, these components are shown for a midlatitude profile as well as for a full orbit. The predicted temperature error varies from 0.5 K to 0.8 K from the upper troposphere to the stratopause region, consistent with the empirical estimates given by Gille et al. (2008). For O3 and HNO3, the predicted errors and their useful pressure ranges are, respectively, 10–5% from 50 to 1 hPa and 5–10% from 100 to 10 hPa. These results are based on version V004 of the retrieved data, released in August 2008 to the Goddard Earth Sciences Data and Information Services Center (http://daac.gsfc.nasa.gov).

Upper stratospheric ozone decrease events due to a positive feedback between ozone and the ozone dissociation rate

Abstract. Ozone measurements taken with a ground based microwave instrument at Lindau (51.66° N, 10.13° E) over some years showed strong ozone decrease events within the stratopause region, particularly during the winter half-year. These events are characterized by a marked drop of the ozone mixing ratio from two to three ppmv to less than half a ppmv in extreme cases. Simultaneous water vapor measurements at the same place, also carried out by a microwave instrument, showed a strong increase of its mixing ratio and the temperature was also enhanced during these episodes. The theoretical analysis brought evidence that these events result from a positive feedback in the complex radiatively-chemical system between the ozone column density and the ozone dissociation rate.