Arctic Polar Vortex Research Articles

Observation of mesospheric air inside the arctic stratospheric polar vortex in early 2003

Abstract. During several balloon flights inside the Arctic polar vortex in early 2003, unusual trace gas distributions were observed, which indicate a strong influence of mesospheric air in the stratosphere. The tuneable diode laser (TDL) instrument SPIRALE (Spectroscopie Infra-Rouge par Absorption de Lasers Embarqués) measured unusually high CO values (up to 600 ppb) on 27 January at about 30 km altitude. The cryosampler BONBON sampled air masses with very high molecular Hydrogen, extremely low SF6 and enhanced CO values on 6 March at about 25 km altitude. Finally, the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) Fourier Transform Infra-Red (FTIR) spectrometer showed NOy values which are significantly higher than NOy* (the NOy derived from a correlation between N2O and NOy under undisturbed conditions), on 21 and 22 March in a layer centred at 22 km altitude. Thus, the mesospheric air seems to have been present in a layer descending from about 30 km in late January to 25 km altitude in early March and about 22 km altitude on 20 March. We present corroborating evidence from a model study using the KASIMA (KArlsruhe SImulation model of the Middle Atmosphere) model that also shows a layer of mesospheric air, which descended into the stratosphere in November and early December 2002, before the minor warming which occurred in late December 2002 lead to a descent of upper stratospheric air, cutting off a layer in which mesospheric air is present. This layer then descended inside the vortex over the course of the winter. The same feature is found in trajectory calculations, based on a large number of trajectories started in the vicinity of the observations on 6 March. Based on the difference between the mean age derived from SF6 (which has an irreversible mesospheric loss) and from CO2 (whose mesospheric loss is much smaller and reversible) we estimate that the fraction of mesospheric air in the layer observed on 6 March, must have been somewhere between 35% and 100%.

Evolution of stratospheric ozone during winter 2002/2003 as observed by a ground-based millimetre wave radiometer at Kiruna, Sweden

Abstract. We present ozone measurements from the millimetre wave radiometer installed at the Swedish Institute of Space Physics (Institutet för rymdfysik, IRF) in Kiruna (67.8° N, 20.4° E, 420 m asl). Nearly continuous operation in the winter of 2002/2003 allows us to give an overview of ozone evolution in the stratosphere between 15 and 55 km. In this study we present a detailed analysis of the Arctic winter 2002/2003. By means of a methodology using equivalent latitudes we investigate the meteorological processes in the stratosphere during the entire winter/spring period. During the course of the winter strong mixing into the vortex took place in the middle and upper stratosphere as a result of three minor and one major warming event, but no evidence was found for significant mixing in the lower stratosphere. Ozone depletion in the lower stratosphere during this winter was estimated by measurements on those days when Kiruna was well inside the Arctic polar vortex. The days were carefully chosen using a definition of the vortex edge based on equivalent latitudes. At the 475 K isentropic level a cumulative ozone loss of about 0.5 ppmv was found starting in January and lasting until mid-March. The early ozone loss is probably a result of the very cold temperatures in the lower stratosphere in December and the geographical extension of the vortex to lower latitudes where solar irradiation started photochemical ozone loss in the pre-processed air. In order to correct for dynamic effects of the ozone variation due to diabatic subsidence of air masses inside the vortex, we used N2O measurements from the Odin satellite for the same time period. The derived ozone loss in the lower stratosphere between mid-December and mid-March varies between 1.1±0.1 ppmv on the 150 ppbv N2O isopleth and 1.7±0.1 ppmv on the 50 ppbv N2O isopleth.

Large nitric acid trihydrate particles and denitrification caused by mountain waves in the Arctic stratosphere

The contribution of stratospheric mountain waves to the formation of large nitric acid trihydrate (NAT) particles and subsequent denitrification of the Arctic polar vortex is calculated for the 1999/2000 winter using a three‐dimensional (3‐D) model. The model production mechanism involves the formation of NAT clouds with high particle number concentrations downwind of mountain wave ice clouds, as has been previously observed. These wave‐induced NAT clouds then serve as “mother clouds” for the release of low concentrations of sedimenting NAT particles, following the mechanism of Fueglistaler et al. (2002a). Our calculations show that wave‐induced NAT mother clouds can occupy up to 5–10% of the volume of air below the NAT temperature. NAT particles that have sedimented from the mother cloud bases can occupy in excess of 60% of the NAT supersaturated region. Integrated over the entire vortex at the end of the 1999/2000 simulation, it is estimated that denitrification due to this mechanism could potentially be responsible for as much as 80% of that observed. These results show that mountain waves may contribute to the occurrence of solid polar stratospheric clouds more than was previously thought and that they may play a significant role in denitrification.

Polar stratospheric cloud observations by MIPAS on ENVISAT: detection method, validation and analysis of the northern hemisphere winter 2002/2003

Abstract. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on ENVISAT has made extensive measurements of polar stratospheric clouds (PSCs) in the northern hemisphere winter 2002/2003. A PSC detection method based on a ratio of radiances (the cloud index) has been implemented for MIPAS and is validated in this study with respect to ground-based lidar and space borne occultation measurements. A very good correspondence in PSC sighting and cloud altitude between MIPAS detections and those of other instruments is found for cloud index values of less than four. Comparisons with data from the Stratospheric Aerosol and Gas Experiment (SAGE) III are used to further show that the sensitivity of the MIPAS detection method for this threshold value of cloud index is approximately equivalent to an extinction limit of 10-3km-1 at 1022nm, a wavelength used by solar occultation experiments. The MIPAS cloud index data are subsequently used to examine, for the first time with any technique, the evolution of PSCs throughout the Arctic polar vortex up to a latitude close to 90° north on a near-daily basis. We find that the winter of 2002/2003 is characterised by three phases of very different PSC activity. First, an unusual, extremely cold phase in the first three weeks of December resulted in high PSC occurrence rates. This was followed by a second phase of only moderate PSC activity from 5-13 January, separated from the first phase by a minor warming event. Finally there was a third phase from February to the end of March where only sporadic and mostly weak PSC events took place. The composition of PSCs during the winter period has also been examined, exploiting in particular an infra-red spectral signature which is probably characteristic of NAT. The MIPAS observations show the presence of these particles on a number of occasions in December but very rarely in January. The PSC type differentiation from MIPAS indicates that future comparisons of PSC observations with microphysical and denitrification models might be revealing about aspects of solid particle existence and location.

Climatological features of stratospheric streamers in the FUB-CMAM with increased horizontal resolution

Abstract. The purpose of this study is to investigate horizontal transport processes in the winter stratosphere using data with a resolution relevant for chemistry and climate modeling. For this reason the Freie Universität Berlin Climate Middle Atmosphere Model (FUB-CMAM) with its model top at 83 km altitude, increased horizontal resolution T42 and the semi-Lagrangian transport scheme for advecting passive tracers is used. A new approach of this paper is the classification of specific transport phenomena within the stratosphere into tropical-subtropical streamers (e.g. Offermann et al., 1999) and polar vortex extrusions hereafter called polar vortex streamers. To investigate the role played by these large-scale structures on the inter-annual and seasonal variability of transport processes in northern mid-latitudes, the global occurrence of such streamers was calculated based on a 10-year model climatology, concentrating on the existence of the Arctic polar vortex. For the identification and counting of streamers, the new method of zonal anomaly was chosen. The analysis of the months October-May yielded a maximum occurrence of tropical-subtropical streamers during Arctic winter and spring in the middle and upper stratosphere. Synoptic maps revealed highest intensities in the subtropics over East Asia with a secondary maximum over the Atlantic in the northern hemisphere. Furthermore, tropical-subtropical streamers exhibited a higher occurrence than polar vortex streamers, indicating that the subtropical barrier is more permeable than the polar vortex barrier (edge) in the model, which is in good correspondence with observations (e.g. Plumb, 2002; Neu et al., 2003). Interesting for the total ozone decrease in mid-latitudes is the consideration of the lower stratosphere for tropical-subtropical streamers and the stratosphere above ~20 km altitude for polar vortex streamers, where strongest ozone depletion is observed at polar latitudes (WMO, 2003). In the lower stratosphere the FUB-CMAM simulated a climatological maximum of 10% occurrence of tropical-subtropical streamers over East-Asia/West Pacific and the Atlantic during early- and mid-winter. The results of this paper demonstrate that stratospheric streamers e.g. large-scale, tongue-like structures transporting tropical-subtropical and polar vortex air masses into mid-latitudes occur frequently during Arctic winter. They can therefore play a significant role on the strength and variability of the observed total ozone decrease at mid-latitudes and should not be neglected in future climate change studies.

Solar forcing of the polar atmosphere

AbstractWe present highly resolved, annually dated, calibrated proxies for atmospheric circulation from several Antarctic ice cores (ITASE (International Trans-Antarctic Scientific Expedition), Siple Dome, Law Dome) that reveal decadal-scale associations with a South Pole ice-core 10Be proxy for solar variability over the last 600 years and annual-scale associations with solar variability since AD 1720. We show that increased (decreased) solar irradiance is associated with increased (decreased) zonal wind strength near the edge of the Antarctic polar vortex. The association is particularly strong in the Indian and Pacific Oceans and as such may contribute to understanding climate forcing that controls drought in Australia and other Southern Hemisphere climate events. We also include evidence suggestive of solar forcing of atmospheric circulation near the edge of the Arctic polar vortex based on ice-core records from Mount Logan, Yukon Territory, Canada, and both central and south Greenland as enticement for future investigations. Our identification of solar forcing of the polar atmosphere and its impact on lower latitudes offers a mechanism for better understanding modern climate variability and potentially the initiation of abrupt climate-change events that operate on decadal and faster scales.

On the signal of the 11-year sunspot cycle in the stratosphere over the Antarctic and its modulation by the Quasi-Biennial Oscillation (QBO)

In earlier studies it was shown that during northern winters as well as northern summers the QBO must be introduced to identify a solar signal. A clear solar signal emerges only when the data are stratified according to the different phases of the QBO. This signal is often of opposite sign in the respective phases of the QBO. During the easterly phase of the QBO the height differences between solar maxima and minima imply an intensification of the polar vortex (positive Annular Mode) throughout the year during solar maxima: During the northern winter half-year positive height differences (anomalies) are observed from about 90 ◦ St o 60 ◦ N, with negative anomalies further north; and during the southern winter half-year positive anomalies exist from 90 ◦ Nt o 45 ◦ S, with negative anomalies further south. The structure of the anomalies is very symmetric with a change between hemispheres in May/June and October/November. The consistently positive height and temperature anomalies during solar maxima/easterly phase over the tropics and subtropics imply anomalous downwelling, i.e. a weakening of the Brewer-Dobson circulation throughout the year – during solar maxima. The antarctic polar vortex and the connected Final Warmings are influenced by the 11-year solar cycle in much the same way as the arctic polar vortex. During solar maxima/westerly phase the midwinter warmings over the Arctic and the Final Warmings over the Antarctic lead to the negative mode of the Annular Mode (weaker polar vortex). They influence the whole stratosphere with downwelling (adiabatic warming) over the polar regions and connected upwelling (adiabatic cooling) outside the respective polar regions, reaching far into the other (summer) hemisphere. This leads to an intensification of the Brewer-Dobson circulation – during solar maxima. Zusammenfassung

Ozone perturbations in the Arctic summer lower stratosphere as a reflection of NOX chemistry and planetary scale wave activity

Ozone concentration perturbations in the high‐latitude lower stratosphere in the Northern Hemisphere were observed by Improved Limb Atmospheric Spectrometer (ILAS) after the polar vortex breakdown at the beginning of May 1997 and until the end of June of that same year. Simulations and a passive tracer experiment using the Center for Climate System Research/National Institute for Environmental Studies (CCSR/NIES) nudging chemical transport model (CTM) show that the low‐ozone perturbations observed in May were caused by the Arctic polar vortex debris, while those after the end of May resulted from a dynamical elongation due to zonal wave number 2 planetary waves of the low‐ozone region in the summer polar stratosphere, which had been developed by the catalytic ozone destruction cycle of NOX. These low‐O3 air masses of different origin were advected or elongated from the polar region to the ILAS measurement points. An episodic event of a dynamical O3 perturbation in June 1997 on a chemically induced meridional O3 gradient is described. These results show that a timing of the polar vortex breakdown and activity of planetary waves after the breakdown may affect the O3 background gradient in the summer lower stratosphere at middle and high latitudes.

First measurements of ClOOCl in the stratosphere: The coupling of ClOOCl and ClO in the Arctic polar vortex

The first measurements of ClOOCl in the stratosphere have been acquired from a NASA ER‐2 aircraft, deployed from Kiruna, Sweden (68°N, 21°E), during the joint SOLVE/THESEO‐2000 mission of the winter of 1999/2000. ClOOCl is detected by thermal dissociation into two ClO fragments that are measured by the well‐known technique of chemical conversion, vacuum ultraviolet resonance fluorescence. Ambient ClO is detected simultaneously. Observations of the ratio [ClOOCl]/[ClO]2 (estimated uncertainty of ±25%, 1 σ) are used with a time‐dependent photochemical model, to test the model representation of the ratios of kinetic parameters J/kProd and kLoss/kProd for day and nighttime observations, respectively. Here, kProd and kLoss are the rate constants for ClOOCl production and loss, respectively, and J is the photolysis rate of ClOOCl. The observations are in good agreement with J based upon the 2002 JPL recommended cross sections for ClOOCl [Sander et al., 2003], if the true value of kProd is given by either the 2000 JPL recommendation [Sander et al., 2000] or the work of Trolier et al. [1990]. The larger values of kProd given by Bloss et al. [2001] and the 2002 JPL recommendation are consistent with the observations only if J is increased by a significant amount. This is accomplished if J is calculated with the larger ClOOCl cross sections measured by Burkholder et al. [1990]. The J values for ClOOCl based on the Huder and DeMore [1995] cross sections are too small, by factors of ∼1.6 to 2.5 for all values of kProd, based on the observations. Nighttime results suggest that, for 190 < T < 200 K, the values for KEq (the equilibrium constant, equal to the ratio of kProd/kLoss) of Cox and Hayman [1988] and Avallone and Toohey [2001] are in best agreement with the observations.

Measurements of large stratospheric particles in the Arctic polar vortex

During the SAGE III Ozone Loss and Validation Experiment/Third European Stratospheric Experiment on Ozone 2000 mission, the Multiangle Aerosol Spectrometer Probe (MASP) was flown aboard the ER‐2 aircraft sampling the Arctic polar vortex. On several of the flights the ER‐2 flew directly through polar stratospheric clouds (PSCs), and on six of these flights the MASP sampled large particles (2–22 μm diameter). MASP large particle observations are correlated in time with the large particle signal from the NOAA NOy measurements. This is direct evidence supporting the inference of particle size made by the NOy instrument. On 20 January 2000, the day of the most intense PSC encounter, the shapes of the large particle size distributions above 9 μm observed by the MASP and simulated based on the NOy instruments are in reasonable agreement. The MASP observations show a dramatic increase in large particle volume as the temperature approaches the nitric acid trihydrate equilibrium temperature. Our study suggests that large particles containing nitric acid are present in the Arctic polar stratosphere and may lead to denitrification.

Estimation of Arctic polar vortex ozone loss during the winter of 1999–2000 using vortex‐averaged airborne differential absorption lidar ozone measurements referenced to N2O isopleths

The NASA Langley UV differential absorption lidar (DIAL) system flew on the NASA DC‐8 aircraft during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment/Third European Stratospheric Experiment on Ozone 2000 (SOLVE/THESEO 2000) mission from 30 November 1999 to 15 March 2000. The UV DIAL system measured ozone (O3) profiles at altitudes from about 1 km above the aircraft up to about 26 km with a vertical resolution of 750 m and a horizontal resolution of 70 km below 19 km and 140 km above 19 km. In comparison with electrochemical concentration cell ozonesonde profiles, the UV DIAL O3 measurements agreed to within 5% up to 20 km and 10% from 20 to 25 km. Ozone loss during the season was determined using the UV DIAL O3 data along with air mass subsidence determined using N2O as a conservative tracer at five levels from 50 to 250 ppbv [Greenblatt et al., 2002]. O3 mixing ratios were determined inside the polar vortex, away from the collar region along these five levels during the mission. The maximum O3 loss determined from 30 November to 12 March was 1.55 ± 0.3 ppmv at the 440–450 K potential temperature (Θ) level, while the loss there between 20 January and 15 March was 1.3 ± 0.3 ppmv. These results are comparable to many of the other reported losses for these periods, but lower than several. Some of the determinations of higher losses used a different method to determine descent during the season. These results indicate that a series of vertical profiles of O3 that sample much of the vortex during the winter, along with determinations of the descent of air masses inside the vortex, can give a reasonable estimate of the O3 changes during the season.

N2O and NOy observations in the 1999/2000 Arctic polar vortex: Implications for transport processes in a CTM

We compare the evolution of simulated NOy and N2O with observations made during the 1999/2000 NASA Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment/Third European Stratospheric Experiment on Ozone‐2000 to understand the different effects of transport on these two species and evaluate model transport processes in the Arctic lower stratosphere during mid to late winter. The simulations were made with the Goddard Space Flight Center chemistry and transport model, driven by meteorological analyses from the Goddard Data Assimilation Office. In January, model NOy and N2O distributions agree well with observations, provided a substantial loss of NOy due to polar stratospheric cloud sedimentation is included in the model simulation. In late winter, model NOy is ∼2–3 times larger than observed, despite the continued reasonable agreement of N2O. Anomalously large vertical transport of NOy due to overly vigorous descent in the CTM is primarily responsible for the large late winter NOy mixing ratios. However, the rapid descent is not apparent in the evolution of modeled N2O and presumably other similar species due to compensating horizontal transport of N2O across the vortex edge, which results in the reasonable agreement between modeled and measured N2O at ER‐2 altitudes throughout the winter. The need to properly represent the seasonal evolution of NOy when studying lower stratospheric polar wintertime photochemistry places strong constraints on the meteorological data used to drive global chemistry and transport models.

Weak impact of mixing on chlorine deactivation during SOLVE/THESEO 2000: Lagrangian modeling (CLaMS) versus ER‐2 in situ observations

During the second and third segment of the SOLVE campaign in February and March 2000, the Arctic polar vortex began to be disturbed by planetary waves and upper stratospheric warming events. The perturbations of the vortex were associated with transport of air from low and middle latitudes into the polar region. Filaments with a lifetime exceeding two weeks were generated in regions of strong baroclinicity and peeled off the vortex edge.The Chemical Lagrangian Model of the Stratosphere (CLaMS) is used for the interpretation of filamentary structures in chemical tracer fields measured on board the ER‐2 during the March flights across the edge of the polar vortex. Both the mixing and the impact of mixing on the chemistry are considered. The isentropic version of CLaMS is initialized on 10 February at four isentropic levels: θ = 400, 425, 450, and 475 K. A comparison of the measured CH4/Halon‐1211 correlation curves and time series with corresponding CLaMS results obtained for spatial resolution of about 45 km indicates weak mixing between vortex and midlatitude air without pronounced anomalous mixing events. Thus the Arctic vortex in the altitude range 400–475 K was well isolated during the considered period without significant mass exchange across the vortex edge. The mixing intensity in CLaMS is controlled by the finite time Lyapunov exponent λ measuring the deformation rate of the horizontal wind and switching on mixing in the flow regions where λ exceeds a critical value λc. The CLaMS simulations suggest a temporally and spatially inhomogeneous mixing in the lower stratosphere with a lateral (across the wind) effective diffusion coefficient of the order 103 m2 s−1. The amount of ClONO2 formed due to chemistry induced by mixing of the activated vortex air with NOx‐rich midlatitude air does not exceed 3%. The impact of mixing on the accumulated ozone loss is less than 1%. The ClONO2 collar observed during the flight on 11 March can be understood as a result of deactivation of ClOx through the NOx produced owing to the chemical decomposition of HNO3 without a significant contribution of mixing with NOx‐rich midlatitude air.

Lagrangian estimations of ozone loss in the core and edge region of the Arctic polar vortex 1995/1996: Model results and observations

Ozone evolution and diabatic descent in the Arctic polar vortex in winter 1995/1996 was studied with a newly developed diabatic trajectory–chemistry model (DTCM). To study the chemical and dynamic evolution of the species in the polar vortex, 400 diabatic trajectories were calculated in the vortex core and edge region by using three-dimensional (3-D) wind data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). The averaged diabatic descending motion and ozone behavior were obtained for particles started from the core and from the edge region of the vortex. The difference in ozone-loss rates as well as the difference in descending rates between the vortex core and the vortex-edge region was not statistically significant. The average cumulative ozone loss of 65 ± 16% in the vortex core obtained from the model calculations was consistent with the estimates obtained with a different method (Match experiment). The model results for the vortex core were compared with those obtained using trajectories with the vertical winds calculated on the basis of radiative cooling rates as used by the SLIMCAT 3-D chemical transport model. Although the trajectories based on cooling rates exhibited lower descending rates than those based on 3-D analyzed wind data, the ozone behavior was similar for both types of trajectory. Ozonesonde data from two stations (Ny-Alesund in the vortex core and Yakutsk in the vortex edge) were compared with the model results. For Lagrangian estimation of the ozone loss at these stations, the descending rates obtained by the diabatic trajectory calculations were used. Good agreements were obtained between the model results and observations for both the vortex core and edge region. These results suggest that strong ozone depletion occurred not only in the core, but also in the edge region of the vortex, and that air masses from the mid-latitudes did not appreciably affect the degree of ozone depletion in this winter–spring period. The sensitivity of the model to different descending rates and to the presence of large nitric acid trihydrate (NAT) particles was also examined.

Polar vortex concentricity as a controlling factor in Arctic denitrification

Recent in situ observations in the Arctic stratosphere have detected nitric acid–containing particles with sizes up to 10‐μm radius and number concentrations between 10−5 and 10−3 cm−3. Here we quantify the effect of these particles on Arctic denitrification by using a new three‐dimensional (3‐D) model which can couple particle growth and sedimentation with the full dynamics of the Arctic polar vortex. We show that the very long growth times of large nitric acid trihydrate (NAT) particles lead to a highly nonlinear dependence of Arctic denitrification on the growth and evaporation cycles of individual particles, thus making denitrification dependent on the precise meteorological conditions in a given winter. Using 3‐D wind and temperature fields from December 1999, we identify a period that was optimum for denitrification, in which the cold pool and vortex were nearly concentric and in which a large proportion of the particles were able to sediment over about 8 days through the full depth of the cold pool without evaporating. We then show that small departures from concentric conditions can lead to substantial reductions in denitrification. A case is presented in which denitrification was completely shut off even with over half of the cold pool area remaining within the vortex. Under the same conditions, a model in which the particles were assumed to be in continuous equilibrium with the gas phase caused extensive denitrification. Our results show that low Arctic vortex temperatures in themselves are unlikely to be a reliable indicator of potential denitrification if large NAT particles are involved.

A climatology of stratospheric polar vortices and anticyclones

United Kingdom Meteorological Office global analyses from 1991 to 2001 are used to create a global climatology of stratospheric polar vortices and anticyclones. New methodologies are developed that identify vortices in terms of evolving three‐dimensional (3‐D) air masses. A case study illustrates the performance of the identification schemes during February and March of 1999 when a merger of anticyclones led to a stratospheric warming that split the Arctic polar vortex. The 3‐D structure and temporal evolution of the Arctic vortex and identified anticyclones demonstrates the algorithm's ability to capture complicated phenomena. The mean geographical distribution of polar vortex and anticyclone frequency is shown for each season. The frequency distributions illustrate the climatological location and persistence of polar vortices and anticyclones. A counterpart to the Aleutian High is documented in the Southern Hemisphere: the “Australian High.” The temporal evolution of the area occupied by polar vortices and anticyclones in each hemisphere is shown as a function of potential temperature. Large polar vortex area leads to an increase in anticyclone area, which in turn results in a decrease in the size of the polar vortex. During Northern winter and Southern spring, 9 years of daily anticyclone movement are shown on the 1200 K (36 km, 4 hPa) isentropic surface. Preferred locations of anticyclogenesis are related to cross‐equatorial flow and weak inertial stability. Regimes of traveling and stationary anticyclones are discussed.

A vortex‐scale simulation of the growth and sedimentation of large nitric acid hydrate particles

Nitric acid–containing particles with diameters of 10–20 μm were detected inside the Arctic polar vortex in the period January to March 2000. We present the results of a unique three‐dimensional microphysical simulation of these large HNO3‐containing particles covering the entire Arctic vortex. The model describes the simultaneous growth, evaporation, sedimentation, and advection of several thousand individual nitric acid hydrate particles over their complete lifetime. We compare modeled and observed particle size distributions as a test of different particle nucleation mechanisms. The model is able to produce particles with sizes typical of those observed and broadly reproduces the change in particle characteristics through the winter assuming nitric acid trihydrate (NAT) particle growth. The possibility that the observed large nitric acid–containing particles were composed of nitric acid dihydrate (NAD) cannot be excluded within the uncertainty of the HNO3 field above the aircraft. The formation of nitric acid hydrate particles on synoptic ice clouds may be a source of some of the observed large nitric acid–containing particles. However, a direct, but highly selective, nucleation of NAT or NAD particles over wide regions appears to be necessary to explain the observations.

Photochemical ozone loss in the Arctic as determined by MSX/UVISI stellar occultation observations during the 1999/2000 winter

The combined SAGE III Ozone Loss and Validation Experiment and Third European Stratospheric Experiment on Ozone 2000 (SOLVE/THESEO 2000) campaign during winter 1999/2000 sought, in part, to quantify ozone loss within the Arctic polar vortex using a variety of aircraft‐, balloon‐, ground‐, and space‐based instrument platforms. The Midcourse Space Experiment/Ultraviolet and Visible Imagers and Spectrographic Imagers (MSX/UVISI) suite of instruments performed 31 stellar occultation observations from 23 January through 4 March 2000 in and near the Arctic polar vortex. Using a newly developed combined extinctive‐refractive algorithm, ozone mixing ratio profiles, along with profiles of total density, pressure, and temperature, were retrieved. Retrieved temperature and ozone profiles are shown to agree well with other measurements. Diabatic trajectory calculations are used to remove the effects of subsidence within the vortex, allowing photochemical ozone loss to be inferred from these occultation measurements. A maximum ozone loss of about 1 ppmv is found at 400–500 K (∼16–21 km) during the 41‐day period for which occultation data are available, in agreement with several other ozone loss analyses of the campaign. This corresponds to an average daily loss rate of ∼0.024 ppmv/day. The combined extinctive‐refractive stellar occultation technique is demonstrated to accurately measure stratospheric ozone loss during polar winter.

In situ observations of gravity waves and comparisons with numerical simulations during the SOLVE/THESEO 2000 campaign

This article presents observational and numerical results on gravity waves in the Arctic polar vortex during the SAGE III—Ozone Loss and Validation Experiment and Third European Stratospheric Experiment on Ozone 2000 (SOLVE/THESEO) campaign. Long‐duration balloons that were launched from Kiruna, Sweden, on 18 February 2000 provided in situ meteorological measurements for several weeks in the lower stratosphere. A strong gravity wave event was observed above southern Scandinavia on 2 March 2000. The main characteristics (amplitude of disturbances, frequency, and wavelengths) are reported, and it is shown that the wave induced mesoscale temperature fluctuations were large (18 K peak to peak). Furthermore, it is found that the gravity wave was most likely generated by flow across the Norwegian mountains. The observations are compared with results of numerical simulations. In particular, the mesoscale and ray‐tracing simulations reproduced some features of the observed wave packet. However, the fluctuations induced by the wave were significantly underestimated in the general circulation model of the European Center for Medium‐Range Weather Forecasting. Finally, the overall gravity wave activity during the flight is analyzed and is found to be relatively small.

In situ observations of HO2 and OH obtained on the NASA ER‐2 in the high‐ClO conditions of the 1999/2000 Arctic polar vortex

Extensive observations of OH and HO2 obtained aboard the NASA ER‐2 inside the Arctic polar vortex during the SAGE III Ozone loss and Validation Experiment (SOLVE) provide the opportunity to identify and test interferences during the measurement of HO2 in the presence of high concentrations of ClO. In‐flight calibrations are consistent with small interferences from CH3O2 (8%) and from ClO (3%) to the total signal on average inside the vortex. Measurements of HO2, OH, ClO, and O3 are used with rate constants derived from laboratory measurements to test the consistency of the reactions that control HO2/OH. Inside the vortex the calculated HO2/OH under predicts the observed HO2/OH by 7% with the JPL‐00 rate constants and over‐predicts the observations by 23% when recent laboratory rates are used. Measurements of HO2 and ClO are used to calculate ozone loss rates and concentrations of HOCl with uncertainties determined from the analysis of HO2/OH. The calculated concentration of HOCl is 52−28+50 pptv, or 0.07 × ClO on average in the vortex. The ozone loss rate from the two cycles controlled by HO2 + ClO is ∼1.5−0.5+1.2 ppbv d−1 in early March.