Winter Lower Stratosphere Research Articles

Dependence of normal modes of the barotropic vortex equation on the mean flow structure and numerical simulation parameters

We present the results of numerical simulations of normal modes of the mean flow due to the superposition of cyclonic and anticyclonic vortices at high latitudes. Such a flow structure is often observed in the upper troposphere — the lower stratosphere in winter. Our aim is to identify normal modes in the oscillation spectrum that resemble torsional oscillations. We solve the problem numerically, using a barotropic quasi-geostrophic model. Additionally, we estimate the dependence of the normal modes on experimental parameters (the number of spherical harmonics in the stream function field expansion, the parameterization of viscosity and hyperviscosity). The simulation results show that flow instability almost always increases with increasing amplitude of the anticyclonic vortex to varying degrees at different viscosities and different numbers of harmonics in the field expansion. The spatial structure of the most unstable normal modes changes most chaotically when the experiment parameters and the mean flow change. This significantly complicates the interpretation of real oscillations in terms of normal modes, including the interpretation of torsional oscillations. Axisymmetric normal modes are often present in the spectrum, but they do not have all the properties of torsional oscillations and do not dominate the spectrum.

Зависимость нормальных мод баротропного уравнения вихря от структуры среднего течения и параметров численного моделирования

We present the results of numerical simulations of normal modes of the mean flow due to the superposition of cyclonic and anticyclonic vortices at high latitudes. Such a flow structure is often observed in the upper troposphere — the lower stratosphere in winter. Our aim is to identify normal modes in the oscillation spectrum that resemble torsional oscillations. We solve the problem numerically, using a barotropic quasi-geostrophic model. Additionally, we estimate the dependence of the normal modes on experimental parameters (the number of spherical harmonics in the stream function field expansion, the parameterization of viscosity and hyperviscosity). The simulation results show that flow instability almost always increases with increasing amplitude of the anticyclonic vortex to varying degrees at different viscosities and different numbers of harmonics in the field expansion. The spatial structure of the most unstable normal modes changes most chaotically when the experiment parameters and the mean flow change. This significantly complicates the interpretation of real oscillations in terms of normal modes, including the interpretation of torsional oscillations. Axisymmetric normal modes are often present in the spectrum, but they do not have all the properties of torsional oscillations and do not dominate the spectrum.

ГЕОДЕЗІЯ, КАРТОГРАФІЯ І АЕРОФОТОЗНІМАННЯ

The aim of this work is to evaluate the accuracy of determining the wet component of zenith tropospheric delay (ZTD) from GNSS-measurements and the accuracy of determining the hydrostatic component according to the Saastamoinen model in comparison with the radio sounding data as well. Zenith tropospheric delay is determined mainly by two methods - traditional, using radio sounding or using atmospheric models, such as the Saastamoinen model, and the method of GNSS measurements. Determination of the hydrostatic component of the zenith tropospheric delay was performed by radio sounding data obtained at the aerological station Praha-Libus in 2011-2013 and in 2018. Data were processed for the middle decades of January and July of each year at 0h o’clock of the Universal Time. The wet component was calculated from GNSS observations. By a significant number of radio soundings at the Praha-Libus aerological station, hydrostatic and wet components of zenith tropospheric delay (ZTD) and the same number of ZTD values derived for the corresponding time intervals from GNSS measurements at the GOPE reference station were determined. The values of the wet component of ZTD were determined and compared with the corresponding data obtained from radio soundings. We found that the error of the hydrostatic component in winter does not exceed 10 mm in absolute value, and in summer it is approximately 1.5 times smaller. This is due to differences in the stratification of the troposphere and lower stratosphere in winter and summer. As for the wet component of ZTD, its errors do not exceed: in winter 15 mm, in summer – 35 mm. The resulting differences in summer have a negative sign, indicating a systematic shift, and in winter – both negative and positive. Today, there are many studies aimed at improving the accuracy of determining zenith tropospheric delay by both Ukrainian and foreign authors, but the problem of the accuracy of the hydrostatic component remains open. The study provides recommendations for further research to improve the accuracy of zenith tropospheric delay.

Origin of the 2016 QBO Disruption and Its Relationship to Extreme El Niño Events

AbstractThe descent of the westerly phase of the quasi‐biennial oscillation (QBO) in equatorial stratospheric zonal wind was interrupted by the development of easterlies near 40 hPa (~23 km altitude) in early 2016. We use tropical meteorological analyses of wind and temperature to describe in detail the special circumstances by which equatorward‐propagating planetary waves produced this unprecedented disruption in the QBO. Our findings show that the subtropical easterly jet in the winter lower stratosphere during the 2015–2016 winter was anomalously weak owing to (1) the timing of the QBO relative to the annual cycle and (2) an extreme El Niño event. The weak jet allowed an unusually large flux of westward momentum to propagate from the extratropical Northern Hemisphere to the equator near the 40 hPa level. Consequently, the QBO westerlies at that level experienced sustained easterly acceleration from extratropical wave breaking, leading to the observed wind reversal.

Observations of filamentary structures near the vortex edge in the Arctic winter lower stratosphere

Abstract. The CRISTA-NF (Cryogenic Infrared Spectrometers and Telescope for the Atmosphere – New Frontiers) instrument is an airborne infrared limb sounder operated aboard the Russian research aircraft M55-Geophysica. The instrument successfully participated in a large Arctic aircraft campaign within the RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interactions) project in Kiruna (Sweden) from January to March 2010. This paper concentrates on the measurements taken during one flight of the campaign, which took place on 2 March in the vicinity of the polar vortex. We present two-dimensional cross-sections of derived volume mixing ratios for the trace gases CFC-11, O3, and ClONO2 with an unprecedented vertical resolution of about 500 to 600 m for a large part of the observed altitude range (≈ 6–19 km) and a dense horizontal sampling along flight direction of ≈ 15 km. The trace gas distributions show several structures, for example a part of the polar vortex and a vortex filament, which can be identified by means of O3–CFC-11 tracer–tracer correlations. The observations made during this flight are interpreted using the chemistry and transport model CLaMS (Chemical Lagrangian Model of the Stratosphere). Comparisons of the observations with the model results are used to assess the performance of the model with respect to advection, mixing, and the chemistry in the polar vortex. These comparisons confirm the capability of CLaMS to reproduce even very small-scale structures in the atmosphere, which partly have a vertical extent of only 1 km. Based on the good agreement between simulation and observation, we use artificial (passive) tracers, which represent different air mass origins (e.g. vortex, tropics), to further analyse the CRISTA-NF observations in terms of the composition of air mass origins. These passive tracers clearly illustrate the observation of filamentary structures that include tropical air masses. A characteristic of the Arctic winter 2009/10 was a sudden stratospheric warming in December that led to a split of the polar vortex. The vortex re-established at the end of December. Our passive tracer simulations suggest that large parts of the re-established vortex consisted to about 45% of high- and mid-latitude air.

Interannual variability of ozone in the winter lower stratosphere and the relationship to lamina and irreversible transport

We use the high‐resolution dynamic limb sounder (HIRDLS) high‐vertical resolution ozone profiles in the northern hemisphere lower stratosphere to examine the meridional transport out of the tropics. We focus on February 2005–2007 when there are differences in the dynamical background in the lower stratosphere due to the states of the quasi‐biennial oscillation and polar vortex. HIRDLS data reveal a large number of low ozone laminae that have the characteristics of tropical air at midlatitudes. More laminae are observed in February in 2006 than in 2005 or 2007. Because laminae can form, move out of the tropics, and return to the tropics without mixing into the midlatitude ozone field, the number of laminae is not directly related to the net transport. We use equivalent latitude coordinates to discriminate between reversible and irreversible laminar transport. The equivalent latitude analysis shows greater irreversible transport between the tropics and lower midlatitudes in both 2005 and 2007 compared to 2006 despite the higher number of laminae observed in 2006. Our conclusion that there was more irreversible transport of tropical air into the lower midlatitudes in 2005 and 2007 is supported by equivalent length analysis of mixing using microwave limb sounder N2O measurements. This study shows that reversibility must be considered in order to infer the importance of lamination to net transport.

Wave forcing in the stratosphere under doubled‐CO2 conditions in a 100‐year coupled chemistry‐climate model study

The impact of doubling atmospheric CO2 on the resolved‐wave forcing of the stratospheric flow, and thus on the Brewer‐Dobson circulation (BDC), is investigated with 100‐year timeslice simulations using a chemistry‐climate model, the IGCM‐FASTOC, coupled to a mixed‐layer slab ocean. The Arctic lower stratosphere in winter warms by up to 4 K, with associated weakening of the polar vortex and enhancement of the BDC. This change is related to a significant increase in the wave forcing near the vortex core starting in January, followed by an increased wave forcing at the lower edge of the polar vortex in February. Maximum wave forcing is found both to occur earlier in the winter and to be distributed over a longer period of time in the 2 × CO2 climate. The sensitivity to surface conditions is studied by repeating the CO2‐doubling experiments with prescribed interannually varying and fixed annual cycle monthly mean surface temperatures. In the absence of interannual variability, the BDC response is strongly attenuated. With interannual variability, the monthly mean prescribed surface temperatures lead to a similar dynamical response in the stratosphere as found with the interactive surface but with reduced magnitude.

Intensive radiosonde observations of gravity waves in the lower atmosphere over Yichang (111°18´ E, 30°42´ N), China

Abstract. The characteristics of dynamical and thermal structures and inertial gravity waves (GWs) in the troposphere and lower stratosphere (TLS) over Yichang (111°18´ E, 30°42´ N) were statistically studied by using the data from intensive radiosonde observations in August 2006 (summer month) and January 2007 (winter month) on an eight-times-daily basis. The background atmosphere structures observed in different months exhibit evident seasonal differences, and the zonal wind in winter has a prominent tropospheric jet with a maximum wind speed of about 60 ms−1 occurring at the height of 11.5 km. The statistical results of the inertial GWs in our two-month observations are generally consistent with previous observations in the mid-latitudes. In the summer month, the mean intrinsic frequency and vertical wavelength of the inertial GWs in the troposphere are still larger than those in the lower stratosphere with the absence of intensive tropospheric jets, suggesting that the Doppler shifting due to the tropospheric jets cannot completely account for the differences between the GWs in the troposphere and lower stratosphere. Compared with the observations in the summer month, some interesting seasonal characteristics of the GWs are revealed by the observations in the winter month: 1) more and stronger tropospheric GWs are observed in the winter month; 2) less and weaker GWs are observed in the lower stratosphere in winter; 3) the ratio of the mean GW kinetic energy density to potential energy density is smaller than 1 in winter, which contrasts to that in summer. Most of the seasonal differences can be explained by the intensive tropospheric jets in winter. In both the summer and winter months, the fitted spectral slopes of the vertical wave number spectra for GWs are generally smaller than the canonical spectral slope of −3. Correlation analyses suggest that the tropospheric jet induced wind shear is the dominant source for GWs in both the troposphere and lower stratosphere. Moreover, the tropospheric (lower stratospheric) GWs are found to be modulated by the quasi-7-day (10-day) PW, and the impacts of the diurnal tide on the GWs are relatively weak.

Solar occultation satellite data and derived meteorological products: Sampling issues and comparisons with Aura Microwave Limb Sounder

Derived Meteorological Products (DMPs, including potential temperature, potential vorticity (PV), equivalent latitude (EqL), horizontal winds and tropopause locations) from several meteorological analyses have been produced for the locations and times of measurements taken by several solar occultation instruments and the Aura Microwave Limb Sounder (MLS). MLS and solar occultation data are analyzed using DMPs to illustrate sampling issues that may affect interpretation and comparison of data sets with diverse sampling patterns and to provide guidance regarding the kinds of studies that benefit most from analyzing satellite data in relation to meteorological conditions using the DMPs. Using EqL or PV as a vortex‐centered coordinate does not alleviate all sampling problems, including those in studies using “vortex averages” of solar occultation data and in analyses of localized features (such as polar stratospheric clouds) and other fields that do not correlate well with PV. Using DMPs to view measurements with respect to their air mass characteristics is particularly valuable in studies of transport of long‐lived trace gases, polar processing in the winter lower stratosphere, and distributions and transport of O3 and other trace gases from the upper troposphere through the lower stratosphere. The comparisons shown here demonstrate good agreement between MLS and solar occultation data for O3, N2O, H2O, HNO3, and HCl; small biases are attributable to sampling effects or are consistent with detailed validation results presented elsewhere in this special section. The DMPs are valuable for many scientific studies and to facilitate validation of noncoincident measurements.

The Influence of Sea Surface Temperatures on the Northern Winter Stratosphere: Ensemble Simulations with the MAECHAM5 Model

Abstract The role of interannual variations in sea surface temperatures (SSTs) on the Northern Hemisphere winter polar stratospheric circulation is addressed by means of an ensemble of nine simulations performed with the middle atmosphere configuration of the ECHAM5 model forced with observed SSTs during the 20-yr period from 1980 to 1999. Results are compared to the 40-yr ECMWF Re-Analysis (ERA-40). Three aspects have been considered: the influence of the interannual SST variations on the climatological mean state, the response to El Niño–Southern Oscillation (ENSO) events, and the influence on systematic temperature changes. The strongest influence of SST variations has been found for the warm ENSO events considered. Namely, it has been found that the large-scale pattern associated with the extratropical tropospheric response to the ENSO phenomenon during northern winter enhances the forcing and the vertical propagation into the stratosphere of the quasi-stationary planetary waves emerging from the troposphere. This enhanced planetary wave disturbance thereafter results in a polar warming of a few degrees in the lower stratosphere in late winter and early spring. Consequently, the polar vortex is weakened, and the warm ENSO influence clearly emerges also in the zonal-mean flow. In contrast, the cold ENSO events considered do not appear to have an influence distinguishable from that of internal variability. It is also not straightforward to deduce the influence of the SSTs on the climatological mean state from the simulations performed, because the simulated internal variability of the stratosphere is large, a realistic feature. Moreover, the results of the ensemble of simulations provide weak to negligible evidence for the possibility that SST variations during the two decades considered are substantially contributing to changes in the polar temperature in the winter lower stratosphere.

On the formation of HNO3 in the Antarctic mid to upper stratosphere in winter

We address the previously unresolved puzzle of nitric acid formation in the polar winter mid to upper stratosphere, first indicated by Limb Infrared Monitor of the Stratosphere observations in the Arctic winter of 1978–1979. Several theoretical studies over the past 2 decades have tried to reproduce these observations with varying success. More recently, the onset, altitude range, and duration of the formation process have been clarified by the Cryogenic Limb Array Etalon Spectrometer onboard UARS and by a series of ground‐based observations taken at the South Pole during the Antarctic winters of 1993, 1995, and 1999. Using the Stony Brook‐SSt. Petersburg two‐dimensional photochemical model, we have reexplored HNO3 formation via both the ion cluster chemistry and heterogeneous chemistry on sulfate aerosols considered by earlier investigators. By including what we believe to be a realistic flux of NOy from the mesosphere, we find that the model can generate observed mixing ratios through a combination of ion‐clusterenhanced chemistry in the upper to mid stratosphere, augmented by heterogeneous chemistry on sulfate aerosol below ∼40 km. Results are presented which clarify the relative role of various processes and assumed NOy fluxes. These also point up the need to incorporate more accurate downward NOy fluxes in models being used to simulate the polar stratosphere. Finally, we emphasize the need to consider the influence of repartitioning of NOy or NOx into HNO3 before observed variations in amounts of the former reaching the mid to lower stratosphere in winter and early spring can properly be used as tracers to reflect variations in thermospheric‐mesospheric NOy production or transport.

The impact of spatial averaging on calculated polar ozone loss: 1. Model experiments

We have used an off‐line chemical transport model (CTM) to investigate the effect of model resolution on the calculated ozone destruction in the Arctic lower stratosphere in winter and spring. The CTM was forced using the meteorology of the 1994–1995 winter for horizontal resolutions from 5.6° × 5.6° to 1.4° × 1.4°. Using both a full chemistry scheme and a simple chemistry scheme based only on the ClO‐ClO catalytic ozone destruction cycle, we find very little sensitivity of the polar ozone loss to model resolution. This lack of sensitivity is due to the effectively complete activation of chlorine at all model resolutions during this cold winter, which removes any potential effect of spatial averaging on the ClO distribution. Our model results are in strong contradiction with the recent calculations of Edouard et al [1996]. The differences may be due to differences in tracer diffusion between the studies. Comparison of the filamentary structures produced in the ClOx fields from the simple and full chemistry schemes show that chemistry plays an important role in determining the lifetime of these features in the model. Narrow filaments, which are no longer apparent in the full chemistry integrations, persist for much longer in the simple chemistry case at the same resolution.

Filamentation and layering of an idealized tracer by observed winds in the lower stratosphere

The isentropic transport of a passive tracer on synoptic time scales in the winter lower stratosphere is modelled with the use of a high‐resolution transport model, which is forced by winds derived from global meteorological analyses. The study has focused on a meteorological situation which occured in late January 1992. Repeated poleward intrusions of mid‐latitude air are shown to lead to the filamentation of a tracer distribution, which was initially compact and located inside the polar vortex. The effect of wind resolution on the filamentation process is examined. By performing isentropic advection on many closely spaced independent levels, the vertical structure of these filaments can be studied.

Denitrification mechanism of the polar winter stratosphere by major volcanic eruptions

The uptake of HNO3 by volcanic sulfuric acid droplets is investigated using a one dimensional sulfate aerosol model and recent laboratory data. HNO3 uptake by volcanic sulfate particles coupled with the sedimentation of the particles may lead to irreversible transfer of HNO3 from higher to lower altitudes in the polar stratosphere during winter. This denitrification mechanism is found to be strongly dependent on the sulfate aerosol loading and temperature. Model calculations suggest that anomalously large volcanic eruptions have caused a severe denitrification of the Arctic winter lower stratosphere. There is conflicting evidence for this mechanism in ice core nitrate records.

Antarctic slope winds and the polar stratospheric vortex

A wave mean-flow interaction model is used to investigate the coupling of the Antarctic slope winds and the polar vortex in the lower stratosphere in winter. The model is restricted to the Antarctic domain but synoptic waves of fixed frequency and zonal wavenumber can be prescribed at the northern boundary. They represent southward moving midlatitude perturbations. Cooling at the slope of the Antarctic continent induces southeasterly flow near the surface which acts as a source of westerly angular momentum to the atmosphere. The momentum so generated must be transported out of the Antarctic domain by these synoptic waves. It is found that the polar vortex is intense in the troposphere and lower stratosphere, and that the slope winds are weak when these waves are ineffective in transporting angular momentum. If, on the other hand, the momentum transport is sufficiently strong, the polar vortex is relatively weak and the slope winds are vigorous. Such a coupling of vortex and slope winds has been observed for the troposphere [Yasunari and Kodama (1992) J. geophys. Res. (in press)] and our results suggest that it extends into the lower stratosphere.

Analysis of long‐period waves using the mesosphere‐stratosphere‐troposphere radar at Poker Flat, Alaska

Using the mesosphere‐stratosphere‐troposphere radar at Poker Flat, Alaska, the long‐period waves (greater than 1.5 but less than 36 days) in the troposphere/lower stratosphere and mesosphere were analyzed for the first 340 days of 1984. A 16‐day wave was significant the whole year in all regions resolved by the radar and had maxima in the winter lower stratosphere, consistent with the (1,3) Rossby normal mode. Contrary to the theory of the (1,3) normal mode, the observed 16‐day wave had a maximum in the summer mesosphere. Two possible explanations are given: (1) The 16‐day wave was generated in the winter hemisphere, then propagated vertically and toward the summer pole following the westerly mean winds and (2) gravity waves from the summer troposphere modulated by the 16‐day tropospheric wave propagated vertically into the mesosphere where the resulting momentum deposition varied in a 16‐day cycle. The period of the 16‐day wave varied from 12 to 19 days in the summer mesosphere. The quasi 2‐day period in the high summer mesosphere migrated from about 51 to 47 hours over midsummer to midautumn.

1987–1989 total ozone and ozone sounding observations in Northern Scandinavia and Antarctica, and the climatology of the lower stratosphere during 1965–1988 in Northern Finland

The total ozone observations of Tromsö (Northern Norway), Sodankylä (Northern Finland) and Murmansk (Northwestern Soviet Union) for 1987–1989 have been studied. Comparisons of the total ozone with stratospheric temperatures observed at Sodankylä have been made. These values have also been compared with the long-term mean total ozone at Tromsö and the long-term means of stratospheric temperatures at Sodankylä. No severe ozone depletions were observed. The exceptionally high total ozone values at these stations in February 1989 were connected to abnormally high stratospheric temperatures. The comparison of total ozone observed at roughly the same southern latitudes revealed great differences in the springtime. The 1989 ozone sounding observations of Sodankylä, Bear Island and Ny Ålesund (Spitzbergen) did not reveal any indications of pronounced ozone depletion. A comparative study of ozone, temperature and relative humidity indicated that the springtime variability of ozone in the lower stratosphere was clearly connected to meteorological variability. The lower tropospheric ozone had two distinct maxima, one in spring with large-scale photochemical causes and the other in summer connected with the emissions of hydrocarbons and oxides of nitrogen in Europe. Temperature observations made at Sodankylä over 24 yr revealed the existence of a potential for polar stratospheric cloud formation in the lower stratosphere in winter and early spring. A trend analysis of 50 hPa temperature revealed a negative trend of −0.16 K/yr in January and a positive trend of 0.15 K/yr in April; the annually-averaged trend was only −0.02 K/yr for this 24-yr period. When the January–February mean temperatures are separated according to the phase of the QBO in the tropical stratosphere, correlations between temperatures and sunspot numbers are found.

In situ measurements of midlatitude ClO in winter

In situ measurements of ClO in the winter lower stratosphere are presented for six flights of the NASA ER‐2 aircraft from 38°N to 61°N. Enhanced abundances, increasing in severity with date, were observed below 20 km, where HCl and ClONO2 dominate the inorganic chlorine budget. The greatest mixing ratios, over 150 pptv, were encountered on February 20 and 21, 1989, as the vortex experienced a major warming. Although the timing of these ClO enhancements and the evidence that vortex air can reach midlatitudes suggest that heterogeneous conversion of chlorine compounds within the vortex influences chemistry at midlatitudes, the enhancements observed early in the winter could have been caused by unknown chemistry occurring outside the vortex. In either case, photochemical loss of ozone due to catalytic reactions involving ClO at these mixing ratios may be responsible in part for the ozone decreases observed at high latitudes in the northern hemisphere.

On the relationship between the thermal structure of the stratosphere and the seasonal distribution of ozone

An attempt is made to put the phenomenon of Antarctic ozone hole in a global perspective by pointing out that there is a much larger and more persistent "hole" over the tropics. The thermal structure of the Antarctic lower stratosphere in winter is more similar to the tropical lower stratosphere than the Arctic lower stratosphere in winter. The dynamical transport that is responsible for creating the two column ozone minima has similar relationship to the thermal structure in the lower stratosphere in the two regions.

The Influence of Wave Damping on the Winter Lower Stratosphere

Abstract The mean circulation of the winter stratosphere depends strongly on the interaction between the planetary waves and the zonally averaged flow. The theory of wave-mean flow interactions is used to determine the acceleration of the mean zonal flow by linear stationary waves as a function of the wave's amplitude, vertical structure and damping rate. The theory can be used to determine the expected response to changes in wave damping processes and mean drag. Sensitivity experiments in which the above terms are modified in a version of the NCAR Community Climate Model are used to show that the model responds as expected from the theory. Different forms of wave damping are shown to have similar effects on the mean flow as long as their time scales are comparable. Wave damping significantly in excess of the internally determined relative damping of the model is required in order to produce a reasonable simulation of the lower winter stratosphere. In the context of this study it cannot be determined whet...