Halogen Occultation Experiment Research Articles

New studies of SAGE II and HALOE ozone profile and long‐term change comparisons

A comparison of the ozone data sets from the Stratospheric Aerosol and Gas Experiment II (SAGE II) (version 6.1) and the Halogen Occultation Experiment (HALOE) (version 19) is discussed for the period 1991–2000. A more accurate approach than those used in previous studies is applied for the comparisons, which takes into account measurement uncertainties and produces improved results. This approach gave closer agreement between the two measurement systems in terms of absolute values and differences in long‐term changes, that is, trends determined from each data set. In our work the analysis of weighted mean differences in coincident measurements by the two experiments shows approximately a 5% bias over the altitude range 20.5–50.5 km in the majority of latitude bands, with some exceptions. The SAGE II values are typically larger than the HALOE values. We calculate the slopes of the time series of differences between the two instruments and analyze ozone trends. This analysis, which covers a longer time period than was studied before, indicates little to no long‐term drift in either measurement system. The improved trend analysis methods used in this study show more statistically significant trends than were reported before. Ozone trend comparisons from these instruments show trend differences on the order of less than 0.3% per year in a majority of latitude bands at 25, 35, 45, and 55 km altitudes. Those differences are smaller than reported in other studies, thus revealing remarkable agreement between SAGE II and HALOE trend results.

Retrieval of stratospheric NOx from 5.3 and 6.2 μm nonlocal thermodynamic equilibrium emissions measured by Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat

We present the first global observations of stratospheric NOx(= NO + NO2) from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat during 24 July, 18 to 27 September, and 11 to 13 October 2002. Volume mixing ratio profiles of both NOx species were derived from MIPAS limb emission spectra by means of an innovative retrieval scheme under consideration of nonlocal thermodynamic equilibrium (non‐LTE) effects. In the quiescent atmosphere, the estimated accuracy of retrieved NOx at the altitude of its stratospheric mixing ratio maximum at 35–40 km is around 1–2 ppbv, and the vertical resolution is around 3.5–6.5 km at altitudes between 20 and 50 km. In order to correctly consider NO2 non‐LTE effects in the retrievals, the photochemical excitation rate of NO2(v3 > 0) vibrational states was derived from NO2(002→001) emissions and was found to be about 50 times smaller than previously estimated from Limb Infrared Monitor of the Stratosphere (LIMS) measurements. The NOx partitioning of the retrieved data is in excellent agreement with steady state photochemistry, which confirms predicted stratospheric NO(v > 0) non‐LTE population enhancements. The retrieved NOx abundances are also consistent with Halogen Occultation Experiment (HALOE) NOx observations.

Stratospheric water vapor predicted from the Lagrangian temperature history of air entering the stratosphere in the tropics

We present results of Lagrangian troposphere‐to‐stratosphere transport (TST) in the tropics based on trajectory calculations for the period 1979–2001. The trajectories and corresponding temperature histories are calculated from wind and temperature fields provided by the reanalysis data ERA‐40 of the European Centre for Medium‐Range Weather Forecasts (ECMWF). The water vapor mixing ratio of air entering the tropical stratosphere is calculated from the minimum saturation mixing ratio over ice encountered by each trajectory. We show that this Lagrangian approach, which considers the global‐scale to synoptic‐scale dynamics of tropical TST but neglects mesoscale dynamics and details of cloud microphysics, substantially improves estimates of stratospheric humidity compared to calculations based on Eulerian mean tropical tropopause temperatures. For the period 1979–2001 we estimate from the Lagrangian calculation that the mean water mixing ratio of air entering the stratosphere is 3.5 ppmv, which is in good agreement with measurements during the same period, ranging from 3.3 ppmv to 4 ppmv, whereas an estimate based on an Eulerian mean calculation is about 6 ppmv. The amplitude of the annual cycle in water vapor mixing ratio at a potential temperature of 400 K in the tropics estimated from the Lagrangian calculation is compared with measurements of water vapor from the Halogen Occultation Experiment (HALOE). For the period 1992–2001, when HALOE measurements and ERA‐40 data overlap, we calculate a peak‐to‐peak amplitude of ∼1.7 ppmv, in good agreement with ∼1.6 ppmv seen in HALOE data. On average, the Lagrangian calculations have a moist bias of ∼0.2 ppmv, equivalent to a warm bias of the Lagrangian cold point of about 0.5 K. We conclude that the Lagrangian calculation based on synoptic‐scale velocity and temperature fields yields estimates for stratospheric water vapor in good agreement with observations and that mesoscale and cloud microphysical processes need not be invoked, at first order, to explain annual mean and seasonal variation of water vapor mixing ratios in the tropical lower stratosphere.

Validation of stratospheric temperatures measured by Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard the Envisat satellite provides temperature and various gas profiles from limb‐viewing midinfrared emission measurements. The stratospheric temperatures retrieved at the Institut für Meteorologie und Klimaforschung (IMK) for September/October 2002 and October/November 2003 are compared with a number of reference data sets, including global radiosonde (RS) observations, radio occultation (RO) measurements of Global Positioning System (GPS) on German Challenging Minisatellite Payload (CHAMP) and Argentinean Satelite de Aplicaciones Cientificas‐C (SAC‐C) satellite, Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS), and the analyses of European Centre for Medium‐Range Weather Forecasts (ECMWF) and Met Office (METO), United Kingdom. The data sets show a good general agreement. Between 10 and 30 km altitude the mean differences are within ±0.5 K for the averages over the height interval and within ±(1–1.5) K at individual levels for comparisons with RS, GPS‐RO/CHAMP, and SAC‐C, ECMWF, and METO. Between 30 and 45 km the MIPAS mean temperatures, averaged over the height region, are higher than ECMWF but lower than METO by ∼1.5 K, while they differ by ±0.5 K with respect to HALOE, with maximum discrepancies of ∼2.5 K peaking around 35 km. Between 45 and 50 km, MIPAS temperatures show a low bias compared to HALOE, ECMWF, and METO with mean differences of −1 to −3 K and with a better agreement with HALOE. The large discrepancies between MIPAS and the analyses above 30 km likely suggest deficiency in the underlying general circulation models. The standard deviations vary between 2.5 and 3.5 K for individual data sets, with more than 70% being contributed from the expected variability of the atmosphere. Retrieved temperatures with accuracy of ∼0.5–1 K after removing the atmospheric variability provide highly accurate knowledge to characterize our environment.

Interannual, solar cycle, and trend terms in middle atmospheric temperature time series from HALOE

Temperature versus pressure or T(p) time series from the Halogen Occultation Experiment (HALOE) have been generated and analyzed for the period of 1991–2004 and for the mesosphere and upper stratosphere for latitude zones from 40N to 40S. Multiple linear regression (MLR) techniques were used for the analysis of the seasonal and the significant interannual and solar cycle (or decadal‐scale) terms. An 11‐yr solar cycle (SC) term of amplitude 0.5 to 1.7 K was found for the middle to upper mesosphere; its phase was determined by a Fourier fit to the de‐seasonalized residual. This SC term is largest and has a lag of several years for northern hemisphere middle latitudes of the middle mesosphere, perhaps due to the interfering effects of wintertime wave dissipation. The SC response from the MLR models is weaker but essentially in‐phase at low latitudes and in the southern hemisphere. An in‐phase SC response term is also significant near the tropical stratopause with an amplitude of about 0.4 to 0.6 K, which is somewhat less than predicted from models. Both sub‐biennial (688‐dy) and QBO (800‐dy) terms are resolved for the mid to upper stratosphere along with a decadal‐scale term that is presumed to have a 13.5‐yr period due to their predicted modulation. This decadal‐scale term is out‐of‐phase with the SC during 1991–2004. However, the true nature and source of this term is still uncertain, especially at 5 hPa. Significant linear cooling trends ranging from −0.3 K to −1.1 K per decade were found in the tropical upper stratosphere and subtropical mesosphere. Trends have not emerged so far for the tropical mesosphere, so it is concluded that the cooling rates that have been resolved for the subtropics are likely upper limits. As HALOE‐like measurements continue and their time series lengthen, it is anticipated that better accuracy can be achieved for these interannual, SC, and trend terms.

An Observational Study of the Final Breakdown of the Southern Hemisphere Stratospheric Vortex in 2002

Abstract The 2002 Southern Hemisphere final warming occurred early, following an unusually active winter and the first recorded major warming in the Antarctic. The breakdown of the stratospheric polar vortex in October and November 2002 is examined using new satellite observations from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument aboard the European Space Agency (ESA) Environment Satellite (ENVISAT) and meteorological analyses, both high-resolution fields from the European Centre for Medium-Range Weather Forecasts and the coarser Met Office analyses. The results derived from MIPAS observations are compared to measurements and inferences from well-validated solar occultation satellite instruments [Halogen Occultation Experiment (HALOE), Polar Ozone and Aerosol Measurement III (POAM III), and Stratospheric Aerosol and Gas Experiments II and III (SAGE II and III)] and to finescale tracer fields reconstructed by transporting trace gases based on MIPAS or climatological data using a reverse-trajectory method. These comparisons confirm the features in the MIPAS data and the interpretation of the evolution of the flow during the vortex decay revealed by those features. Mapped ozone and water vapor from MIPAS and the analyzed isentropic potential vorticity vividly display the vortex breakdown, which occurred earlier than usual. A large tongue of vortex air was pulled out westward and coiled up in an anticyclone, while the vortex core remnant shrank and drifted eastward and equatorward over the South Atlantic. By roughly mid-November, the vortex remnant at 10 mb had shrunk below scales resolved by the satellite observations, while a vortex core remained in the lower stratosphere.

Ozone Chemistry during the 2002 Antarctic Vortex Split

Abstract In September 2002, the Antarctic polar vortex was disturbed, and it split into two parts caused by an unusually early stratospheric major warming. This study discusses the chemical consequences of this event using the Chemical Lagrangian Model of the Stratosphere (CLaMS). The chemical initialization of the simulation is based on Halogen Occultation Experiment (HALOE) measurements. Because of its Lagrangian nature, CLaMS is well suited for simulating the small-scale filaments that evolve during this period. Filaments of vortex origin in the midlatitudes were observed by HALOE several times in October 2002. The results of the simulation agree well with these HALOE observations. The simulation further indicates a very rapid chlorine deactivation that is triggered by the warming associated with the split of the vortex. Correspondingly, the ozone depletion rates in the polar vortex parts rapidly decrease to zero. Outside the polar vortex, where air masses of midlatitude origin were transported to the polar region, the simulation shows high ozone depletion rates at the 700-K level caused mainly by NOx chemistry. Owing to the major warming in September 2002, ozone-poor air masses were transported into the midlatitudes and caused a decrease of midlatitude ozone by 5%–15%, depending on altitude. Besides this dilution effect, there was no significant additional chemical effect. The net chemical ozone depletion in air masses of vortex origin was low and did not differ significantly from that of midlatitude air, in spite of the different chemical composition of the two types of air masses.

Mixing and Chemical Ozone Loss during and after the Antarctic Polar Vortex Major Warming in September 2002

Abstract The 3D version of the Chemical Lagrangian Model of the Stratosphere (CLAMS) is used to study the transport of CH4 and O3 in the Antarctic stratosphere between 1 September and 30 November 2002, that is, over the time period when unprecedented major stratospheric warming in late September split the polar vortex into two parts. The isentropic and cross-isentropic velocities in CLAMS are derived from ECMWF winds and heating/cooling rates calculated with a radiation module. The irreversible part of transport, that is, mixing, is driven by the local horizontal strain and vertical shear rates with mixing parameters deduced from in situ observations. The CH4 distribution after the vortex split shows a completely different behavior above and below 600 K. Above this potential temperature level, until the beginning of November, a significant part of vortex air is transported into the midlatitudes up to 40°S. The lifetime of the vortex remnants formed after the vortex split decreases with the altitude with values of about 3 and 6 weeks at 900 and 700 K, respectively. Despite this enormous dynamical disturbance of the vortex, the intact part between 400 and 600 K that “survived” the major warming was strongly isolated from the extravortex air until the end of November. According to CLAMS simulations, the air masses within this part of the vortex did not experience any significant dilution with the midlatitude air. By transporting ozone in CLAMS as a passive tracer, the chemical ozone loss was estimated from the difference between the observed [Polar Ozone and Aerosol Measurement III (POAM III) and Halogen Occultation Experiment (HALOE)] and simulated ozone profiles. Starting from 1 September, up to 2.0 ppmv O3 around 480 K and about 70 Dobson units between 450 and 550 K were destroyed until the vortex was split. After the major warming, no additional ozone loss can be derived, but in the intact vortex part between 450 and 550 K, the accumulated ozone loss was “frozen in” until the end of November.

Reconstruction and Simulation of Stratospheric Ozone Distributions during the 2002 Austral Winter

Abstract Satellite-based solar occultation measurements during the 2002 austral winter have been used to reconstruct global, three-dimensional ozone distributions. The reconstruction method uses correlations between potential vorticity and ozone to derive “proxy” distributions from the geographically limited occultation observations. Ozone profiles from the Halogen Occultation Experiment (HALOE), the Polar Ozone and Aerosol Measurement III (POAM III), and the Stratospheric Aerosol and Gas Experiment II and III (SAGE II and III) are incorporated into the analysis. Because this is one of the first uses of SAGE III data in a scientific analysis, preliminary validation results are shown. The reconstruction method is described, with particular emphasis on uncertainties caused by noisy and/or multivalued correlations. The evolution of the solar occultation data and proxy ozone fields throughout the winter is described, and differences with respect to previous winters are characterized. The results support the idea that dynamical forcing early in the 2002 winter influenced the morphology of the stratosphere in a significant and unusual manner, possibly setting the stage for the unprecedented major stratospheric warming in late September. The proxy is compared with ozone from mechanistic, primitive equation model simulations of passive ozone tracer fields during the time of the warming. In regions where chemistry is negligible compared to transport, the model and proxy ozone fields agree well. The agreement between, and changes in, the large-scale ozone fields in the model and proxy indicate that transport processes, particularly enhanced poleward transport and mixing, are the primary cause of ozone changes through most of the stratosphere during this unprecedented event. The analysis culminates with the calculation of globally distributed column ozone during the major warming, showing quantitatively how transport of low-latitude air to the polar region in the middle stratosphere led to the diminished ozone hole in 2002.

Using a photochemical model for the validation of NO<sub>2</sub> satellite measurements at different solar zenith angles

Abstract. SCIAMACHY (Scanning Imaging Spectrometer for Atmospheric Chartography) aboard the recently launched Environmental Satellite (ENVISAT) of ESA is measuring solar radiance upwelling from the atmosphere and the extraterrestrial irradiance. Appropriate inversion of the ultraviolet and visible radiance measurements, observed from the atmospheric limb, yields profiles of nitrogen dioxide, NO2, in the stratosphere (SCIAMACHY-IUP NO2 profiles V1). In order to assess their accuracy, the resulting NO2 profiles have been compared with those retrieved from the space borne occultation instruments Halogen Occultation Experiment (HALOE, data version v19) and Stratospheric Aerosol and Gas Experiment II (SAGE II, data version 6.2). As the HALOE and SAGE II measurements are performed during local sunrise or sunset and because NO2 has a significant diurnal variability, the NO2 profiles derived from HALOE and SAGE II have been transformed to those predicted for the solar zenith angles of the SCIAMACHY measurement by using a 1-dimensional photochemical model. The model used to facilitate the comparison of the NO2 profiles from the different satellite sensors is described and a sensitivity ananlysis provided. Comparisons between NO2 profiles from SCIAMACHY and those from HALOE NO2 but transformed to the SCIAMACHY solar zenith angle, for collocations from July to October 2002, show good agreement (within +/-12%) between the altitude range from 22 to 33km. The results from the comparison of all collocated NO2 profiles from SCIAMACHY and those from SAGE II transformed to the SCIAMACHY solar zenith angle show a systematic negative bias of 10 to 35% between 20km to 38km with a small standard deviation between 5 to 14%. These results agree with those of Newchurch and Ayoub (2004), implying that above 20km NO2 profiles from SAGE II sunset are probably somewhat high.

The polar mesospheric cloud mass in the Arctic summer

We infer the polar mesospheric cloud (PMC) mass throughout the Arctic summer using results from two sets of satellite observations and a microphysical model. Solar backscatter ultraviolet (SBUV) PMC observations in July 1999 indicate a burst of activity persisting for ∼8 days after a space shuttle launch and averaging 262 ± 52 t near 4.7 local time. This mass is consistent with the propellant mass available from the shuttle's main engines and accounts for 22% of the total SBUV PMC mass over the season between 65° and 75°N. This is the first evidence that PMCs formed by space shuttle water exhaust can contribute significantly to both the number of observed PMCs and the total PMC mass in a season. In another approach, 11 years of observations by the Halogen Occultation Experiment (HALOE) indicate that on average 90 ± 12 t of water ice is present near local midnight between 65° and 75°N. Using simultaneous HALOE water vapor observations, we find that a one‐dimensional microphysical model reproduces the start and end of the PMC season but overpredicts the ice mass by about a factor of 1.8 when compared with the observations. This overprediction is within the time‐dependent variability of ice formation and the uncertainties of temperature, water vapor, and vertical winds used to initialize the model.

Lunar occultation with SCIAMACHY: First retrieval results

Scanning imaging absorption spectrometer for atmospheric chartography (SCIAMACHY) is a moderate resolution imaging spectrometer on board the environmental satellite (ENVISAT) launched in March 2002. SCIAMACHY has eight channels, covering a spectral range from 240 to 2380 nm and observes the Earth’s atmosphere in nadir, limb, and occultation geometries. From SCIAMACHY lunar occultation measurements, nighttime vertical profiles of O 3 and NO 2 have been retrieved over the southern hemisphere (60°–90°S) using the optimal estimation method. The first preliminary validation of retrieved O 3 profiles with halogen occultation experiment and comparisons with stratospheric aerosol and gas experiment III (SAGE III), and Michelson interferometer for passive atmospheric sounding (MIPAS) O 3 profiles were carried out. In addition, the retrieved NO 2 profiles were compared to SAGE III and MIPAS results. The results of these preliminary validation and comparisons give confidence that reasonable scientific data products (trace gas profiles) can be derived from SCIAMACHY spectroscopic lunar occultation data.

Seasonal and Diurnal Ozone Variations: Observations and Modeling

Ozone mixing ratios observed by the Bordeaux microwave radiometer between 1995 and 2002 in an altitude range 25–75 km show diurnal variations in the mesosphere and seasonal variations in terms of annual and semi-annual oscillations (SAO) in the stratosphere and in the mesosphere. The observations with 10–15 km altitude resolution are presented and compared to photochemical and transport model results. Diurnal ozone variations are analyzed by averaging the years 1995−1997 for four representative months and six altitude levels. The photochemical models show a good agreement with the observations for altitudes higher than 50 km. Seasonal ozone variations mainly appear as an annual cycle in the middle and upper stratosphere and a semi-annual cycle in the mesosphere with amplitude and phase depending on altitude. Higher resolution (2 km) HALOE (halogen occultation experiment) ozone observations show a phase reversal of the SAO between 44 and 64 km. In HALOE data, a tendancy for an opposite water vapour cycle can be identified in the altitude range 40–60 km. Generally, the relative variations at all altitudes are well explained by the transport model (up to 54 km) and the photochemical models. Only a newly developed photochemical model (1-D) with improved time-dependent treatment of water vapour profiles and solar flux manages to reproduce fairly well the absolute values.

The Winter Ozone Minimum over the Subtropical Northwestern Pacific

Over the subtropical northwestern Pacific, a prominent ozone minimum of less than 235 Dobson Units (DU) is observed in winter, with extremely low values of less than 200 DU occurring occasionally over this region for short time periods (days to weeks). In this study the vertical structure of this ozone minimum is examined using a 9 year (1991‐2000) average of the total ozone mapping spectrometer (TOMS) and Halogen Occultation Experiment (HALOE) data. It is found that the ozone minimum is mainly stratospheric origin with two distinct low-ozone layers in the stratosphere, one in the middle stratosphere of 10‐15 hPa, and the other in the lower stratosphere of 40‐60 hPa. The mid-stratospheric low-ozone layer is attributed to southward transport of high-latitude low-ozone air by atmospheric circulation in wintertime associated with the Aleutian High, which is simulated successfully using a simple chemical transport model. The extremely low ozone was observed in December 2001 over the subtropical northwestern Pacific. This case is also examined by ozonesonde observations at Naha (26 � 12 0 N, 127 � 41 0 E) and the model. Both the results from the 9 year climatological field, and the December 2001 field, show that the wintertime total ozone minimum over the subtropical northwestern Pacific is substantially dynamical in origin.

Comparisons of MIPAS/ENVISAT ozone profiles with SMR/ODIN and HALOE/UARS observations

Ozone volume mixing ratio (VMR) profiles are measured by the Michelson Interferometer for passive atmospheric sounding (MIPAS) on ENVISAT. The data sets produced by the science data processor at Institut für Meteorologie und Klimaforschung (IMK), Germany are compared with those obtained by halogen occultation experiment (HALOE) on UARS and by sub-millimetre radiometer (SMR) on ODIN. For the stratospheric measurements taken during September/October 2002, the three instruments show reasonable agreement, with global mean differences within 0.1–0.3 ppmv. The typical zonal mean differences are of 0.4 ppmv for HALOE and 0.6 ppmv for SMR (4–6%) in the ozone VMR peak region at 25–30 km near the equator, though larger differences of 0.8–1 ppmv (8–10%) are also observed in a small latitude–altitude region in the tropic. A positive bias of about 0.2–0.4 ppmv in the MIPAS data in the 35–40 km region has also been found. Further studies are under way to explain these differences.

On the distribution of ozone in stratospheric anticyclones

Twelve years (1991–2003) of satellite occultation ozone data are combined with a climatology of stratospheric anticyclones and polar vortices to quantify the climatological ozone differences between air mass types. Ozone data from the Stratospheric Aerosol and Gas Experiment (SAGE) II, the Halogen Occultation Experiment (HALOE), the Polar Ozone and Aerosol Measurement (POAM) II and III missions, and the Improved Limb Atmospheric Spectrometer (ILAS) sensor are used in this study. Daily ozone measurements in the 400–1600 K altitude range are categorized as being either (1) in an anticyclone, (2) in a polar vortex, or (3) in neither (hereinafter referred to as “ambient”). Monthly mean ozone is then calculated in each air mass category from data combined over all years. This study focuses on ozone differences between anticyclones and the ambient environment. A composite annual cycle of ozone in ambient regions is compared to ozone in anticyclones as a function of altitude and latitude. Ozone differences result from both anomalous transport and photochemistry in the vicinity of stratospheric anticyclones. The relative importance of transport versus chemistry is inferred from the long‐term ozone anomalies observed here. In summer, ozone gradients between different air mass types are small. In other seasons, results indicate three distinct vertical regimes at high latitudes. Below ≈600 K, anticyclones coincide with regions where ozone is reduced by 10–50% during the winter. Between 600 and 900 K from fall to midwinter, anticyclones are associated with ≈10% more ozone than ambient air masses. Positive ozone anomalies are also observed in low‐latitude anticyclones within this vertical layer; however, they are half the size of their high‐latitude counterparts and peak several months later. Above 900 K the abundance of ozone in anticyclones is ≈10% less than in the surrounding air. This upper stratospheric regime results from the formation of “low‐ozone pockets” (LOPs) in stratospheric anticyclones. All LOPs that formed between November 1991 and November 2003 and were observed by a satellite used in this study (108 pockets) are documented here in the most comprehensive catalog of LOPs to date.

Intercomparison of ground-based microwave remote sensing measurements of stratospheric ozone over the Mendoza region, Argentina with HALOE data

The Tropospheric Water Vapour and Stratospheric Ozone (TROPWA) project has measured ground-based stratospheric ozone by means of millimetre wave radiometry tuned at 142 GHz from 1993 to 2000 in Mendoza, Argentina. Additionally, tropospheric water vapour was measured using a 92-GHz radiometer. This paper presents the theoretical error analysis used to characterize the ozone instrument, and a comparative study of the retrieved profiles with the coincident measurements taken with different instruments. To evaluate and validate the retrieved stratospheric ozone profiles, we have used a set of ozone profiles measured with the Halogen Occultation Experiment (HALOE); while the water vapour data was calibrated against a set of 3-year-radiosounding-balloon data taken by the Argentine National Weather Service. This study also includes a comparison of individual ozone profiles measured using a second ground-based millimetre wave radiometer–spectrometer tuned at 276 GHz from the Max-Planck-Institut für Aeronomie (MPAE), Germany. During this particular campaign carried out in November 1994, the ground-based measurements were contrasted with two space-born experiments: the Millimetre Wave Atmospheric Sounder (MAS), flown in the NASA-ATLAS 3 mission and the above-mentioned HALOE. From the error analysis and the comparison tests, it follows that between 20 to 40 km the TROPWA instrument is able to retrieve ozone profiles with absolute errors varying from 10% to 20%, relative errors less than 5%, and with a height resolution, calculated as full width at half maximum (FWHM), varying from 5 to 11 km depending on the altitude. The major discrepancies between the different set of profiles are about +8% to −10% (+0.4 to −0.8 ppmv), mainly due to the coarser height resolution of our instrument.

Mesospheric temperature inversions over the Indian tropical region

Abstract. To study the mesospheric temperature inversion, daily temperature profiles obtained from the Halogen Occultation Experiment (HALOE) aboard the Upper Atmospheric Research Satellite (UARS) during the period 1991-2001 over the Indian tropical region (0-30° N, 60-100° E) have been analyzed for the altitude range 34-86km. The frequency of occurrence of inversion is found to be 67% over this period, which shows a strong semiannual cycle, with a maximum occurring one month after equinoxes (May and November). Amplitude of inversion is found to be as high as 40K. Variation of monthly mean peak and bottom heights along with amplitude of inversions also show the semiannual cycle. The inversion layer is detected most frequently in the altitude range of 70-85km, with peak height ranging from 80 to 83km and that of the bottom height from 72 to 74km. A comparison of frequency of temperature inversion with that obtained from Rayleigh lidar observations over Gadanki (13.5° N, 60-100° E) is found to be reasonable. The seasonal variation of amplitude and frequency of occurrence of temperature inversion indicates a good correlation with seasonal variation of average ozone concentration over the altitude range of the inversion layer.

Retrieval of nitrogen dioxide stratospheric profiles from ground-based zenith-sky UV-visible observations: validation of the technique through correlative comparisons

Abstract. A retrieval algorithm based on the Optimal Estimation Method (OEM) has been developed in order to provide vertical distributions of NO2 in the stratosphere from ground-based (GB) zenith-sky UV-visible observations. It has been applied to observational data sets from the NDSC (Network for Detection of Stratospheric Change) stations of Harestua (60° N, 10° E) and Andøya (69° N, 16° E) in Norway. The information content and retrieval errors have been analyzed following a formalism used for characterizing ozone profiles retrieved from solar infrared absorption spectra. In order to validate the technique, the retrieved NO2 vertical profiles and columns have been compared to correlative balloon and satellite observations. Such extensive validation of the profile and column retrievals was not reported in previously published work on the profiling from GB UV-visible measurements. A good agreement - generally better than 25% - has been found with the SAOZ (Système d'Analyse par Observations Zénithales) and DOAS (Differential Optical Absorption Spectroscopy) balloons. A similar agreement has been reached with correlative satellite data from the HALogen Occultation Experiment (HALOE) and Polar Ozone and Aerosol Measurement (POAM) III instruments above 25km of altitude. Below 25km, a systematic underestimation - by up to 40% in some cases - of both HALOE and POAM III profiles by our GB profile retrievals has been observed, pointing out more likely a limitation of both satellite instruments at these altitudes. We have concluded that our study strengthens our confidence in the reliability of the retrieval of vertical distribution information from GB UV-visible observations and offers new perspectives in the use of GB UV-visible network data for validation purposes.

Rayleigh lidar observation of a warm stratopause over a tropical site, Gadanki (13.5° N; 79.2° E)

Abstract. The first Rayleigh lidar observation of a stratopause warming over a tropical site, Gadanki (13.5° N; 79.2° E), is presented in this paper. The warming event was observed on 22-23 February 2001, and found to occur in the stratopause region (~45km). The magnitude of the warming was found to be ~18K with respect to the winter-mean temperature profile derived from the lidar data collected over March 1998 to July 2001. The event observed by the lidar has also been seen in data from the Halogen Occultation Experiment (HALOE) on board the UARS satellite. The zonal-mean temperature at 80° N and the zonal-mean zonal wind at 60° N from the National Centre for Environmental Prediction (NCEP) reanalysis and the European Centre for Medium-Range Weather Forecasts (ECMWF) analysis indicate that a major warming episode took place in the northern polar hemisphere a week before the day of the observation over Gadanki. Eliassen-Palm (E-P) flux calculations from ECMWF analysis show evidence of propagation of planetary-wave activity from high and mid- to low latitudes subsequent to the major warming episode over the pole. Our results support the view that the most likely source mechanism for the observed stratopause warming is the increase in planetary-wave activity.