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Forest fires in British Columbia in August 2017 caused a pyrocumulonimbus event that injected a polluted air mass into the lower stratosphere. The Microwave Limb Sounder (MLS) on the Aura satellite first observed the polluted air mass on 14 August 2017 and continued to observe it for 60 d (100 d in water vapour). We estimate the mass of CO injected into the stratosphere to be 2400 Gg. Events in which a fire injects its burning products directly into the stratosphere are rare: this is the third of four such events in the 16 years since the launch of Aura, the second largest of the four events, and the only one in the Northern Hemisphere. The other three events occurred in Australia in December 2006, February 2009 and from December 2019 to January 2020. Unlike the 2006 and 2009 events, but like the 2019–2020 event, the polluted air mass described here had a clearly elevated water vapour content: between 2.5 and 5 times greater than that in the surrounding atmosphere. We describe the evolution of the polluted air mass, showing that it rose to an altitude of about 24 km (31 hPa) and divided into several identifiable parts. In addition to CO and H2O, we observe enhanced amounts of HCN, CH3CN, CH3Cl and CH3OH with mixing ratios in the range to be expected from a variety of measurements in other biomass burning plumes. We use back trajectories and plume-dispersion modelling to demonstrate that the pollutants observed by MLS originated in the British Columbia fires, the likeliest source being at 53.2∘ N, 121.8∘ W at 05:20 UTC on 13 August 2017.

Following the Australian New Year's pyrocumulonimbus complex between 29‐Dec‐2019 and 4‐Jan‐2020, the Aura Microwave Limb Sounder (MLS) observed a plume with unprecedented enhancements of H2O and biomass‐burning products (CO, HCN, CH3Cl, CH3CN, and CH3OH) in the lower/middle stratosphere, accompanied by depressions in stratospheric species (O3 and HNO3). The plume persisted for ⁓110 days, circling the globe twice while ascending to 5.62 hPa (⁓35 km). Air masses drawn off the main plume moved toward the developing Antarctic polar vortex but do not appear to have penetrated it. Comparison of species in the plume requires consideration of their measurements' spatial resolutions and background abundances. The apparent decay of some long‐lived plume constituents is largely attributable to their coarsening spatial resolution with height, which reduces observed peak values. Differing HCN/H2O signatures indicate that multiple early plumes originated from different stratospheric injection events.

This is an open access article.-- This work is distributed under the Creative Commons Attribution 4.0 License., The Sub-Millimetre Radiometer (SMR) on board the Odin satellite performs limb sounding measurements of the middle atmosphere to detect molecular emission from different species. Carbon monoxide (CO) is an important tracer of atmospheric dynamics at these altitudes, due to its long photochemical lifetime and high vertical concentration gradient. In this study, we have successfully recovered over 18 years of SMR observations, providing the only dataset to date being so extended in time and stretching out to the polar regions, with regards to satellite-measured mesospheric CO. This new dataset is part of the Odin/SMR version 3.0 level 2 data. Much of the level 1 dataset - except the October 2003 to October 2004 period - was affected by a malfunctioning of the phase-lock loop (PLL) in the front end used for CO observations. Because of this technical issue, the CO line could be shifted away from its normal frequency location, causing the retrieval to fail or leading to an incorrect estimation of the CO concentration. An algorithm was developed to locate the CO line and shift it to its correct location. Nevertheless, another artefact causing an underestimation of the concentration, i.e. a line broadening, stemmed from the PLL malfunctioning. This was accounted for by using a broader response function. The application of these corrections resulted in the recovery of a large amount of data that was previously being flagged as problematic and therefore not processed. A validation study has been carried out, showing how SMR CO volume mixing ratios are in general in good accordance with the other instruments considered in the study. Overall, the agreement is very good between 60 and 80 km altitude, with relative differences close to zero. A positive bias at low altitudes (50-60 km) up to +20% and a negative bias up to -20% at high altitudes (80-100 km) were found with respect to the comparison instruments. © Author(s) 2020., This research has been supported by the Swedish National Space Agency (grant nos. Dnr 88/14 and Dnr 72/17) and the European Space Agency (Odin reprocessing, MesosphEO grant).

The Sub-Millimetre Radiometer (SMR) on board the Odin satellite performs limb sounding measurements of the middle atmosphere to detect molecular emission from different species. Carbon monoxide (CO) is an important tracer of atmospheric dynamics at these altitudes, due to its long photochemical lifetime and high vertical concentration gradient. In this study, we have successfully recovered over 18 years of SMR observations, providing the only dataset to date being so extended in time and stretching out to the polar regions, with regards to satellite-measured mesospheric CO. This new dataset is part of the Odin/SMR version 3.0 level 2 data. The much of the level 1 dataset – except the October 2003 to October 2004 period – was affected by a malfunctioning of the Phase-Lock Loop (PLL) in the frontend used for CO observations. Because of this technical issue, the CO line could be shifted away from its normal frequency location causing the retrieval to fail or leading to an incorrect estimation of the CO concentration. An algorithm was developed to locate the CO line and shift it to its correct location. Nevertheless, another artifact causing an underestimation of the concentration, i.e. a line broadening, stemmed from the PLL malfunctioning. This was accounted for by using a broader response function. The application of these corrections resulted in the recovery of a large amount of data that was previously being flagged as problematic and therefore not processed. A validation study has been carried out, showing how SMR CO volume mixing ratios are in general in good accordance with the other instruments considered in the study. Overall, the agreement is very good between 60 and 80 km altitude, with relative differences close to zero. A positive bias at low altitudes (50–60 km) up to +20 % and a negative bias up to −20 % at high altitudes (80–100 km) were found with respect to the compared instruments.

In 1985, Scrimger [Nature 318, 647 (1985)] reported measurements of noise levels significantly above the ambient level for snow falling on a quiet freshwater lake. He examined only the time-averaged sound levels and did not report measurements of individual snowflake impacts. Subsequently, the noise produced by individual and multiple snowflake impacts was examined for a number of different snowfalls. The radiated acoustic signals generated by the impact of individual snowflakes upon a body of water have a remarkable similarity to each other and differ principally in the frequency of the emitted sound wave. The acoustic signal of a snowflake impact thus generates a characteristic signature for snowfall that is clearly distinct from other forms of precipitation noise. Various aspects of this signature suggest that the radiated acoustic waveform from a snowflake impacting with water is due to the entrainment of a gas bubble into the liquid, and the subsequent oscillation of this bubble as it establishes its equilibrium state. Various scenarios are presented for bubble entrainment and approximations to the amplitude of the radiated signal and the acoustic waveform are obtained.

Meteorological and atmospheric models are being extended up to 80 km altitude but there are very few observing techniques that can measure stratospheric–mesospheric winds at altitudes between 20 and 80 km to verify model datasets. Here we demonstrate the feasibility of horizontal wind profile measurements using ground-based passive millimetre-wave spectroradiometric observations of ozone lines centred at 231.28, 249.79, and 249.96 GHz. Vertical profiles of horizontal winds are retrieved from forward and inverse modelling simulations of the line-of-sight Doppler-shifted atmospheric emission lines above Halley station (75°37′ S, 26°14′ W), Antarctica. For a radiometer with a system temperature of 1400 K and 30 kHz spectral resolution observing the ozone 231.28 GHz line we estimate that 12 h zonal and meridional wind profiles could be determined over the altitude range 25–74 km in winter, and 28–66 km in summer. Height-dependent measurement uncertainties are in the range 3–8 m s−1 and vertical resolution ∼ 8–16 km. Under optimum observing conditions at Halley a temporal resolution of 1.5 h for measuring either zonal or meridional winds is possible, reducing to 0.5 h for a radiometer with a 700 K system temperature. Combining observations of the 231.28 GHz ozone line and the 230.54 GHz carbon monoxide line gives additional altitude coverage at 85 ± 12 km. The effects of clear-sky seasonal mean winter/summer conditions, zenith angle of the received atmospheric emission, and spectrometer frequency resolution on the altitude coverage, measurement uncertainty, and height and time resolution of the retrieved wind profiles have been determined.

We describe the publicly available data from the Global OZone Chemistry And Related trace gas Data records for the Stratosphere (GOZCARDS) project and provide some results, with a focus on hydrogen chloride (HCl), water vapor (H2O), and ozone (O3). This data set is a global long-term stratospheric Earth system data record, consisting of monthly zonal mean time series starting as early as 1979. The data records are based on high-quality measurements from several NASA satellite instruments and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) on SCISAT. We examine consistency aspects between the various data sets. To merge ozone records, the time series are debiased relative to SAGE II (Stratospheric Aerosol and Gas Experiments) values by calculating average offsets versus SAGE II during measurement overlap periods, whereas for other species the merging derives from an averaging procedure during overlap periods. The GOZCARDS files contain mixing ratios on a common pressure–latitude grid, as well as standard errors and other diagnostics; we also present estimates of systematic uncertainties in the merged products. Monthly mean temperatures for GOZCARDS were also produced, based directly on data from the Modern-Era Retrospective analysis for Research and Applications. The GOZCARDS HCl merged product comes from the Halogen Occultation Experiment (HALOE), ACE-FTS and lower-stratospheric Aura Microwave Limb Sounder (MLS) data. After a rapid rise in upper-stratospheric HCl in the early 1990s, the rate of decrease in this region for 1997–2010 was between 0.4 and 0.7 % yr−1. On 6–8-year timescales, the rate of decrease peaked in 2004–2005 at about 1 % yr−1, and it has since levelled off, at ~ 0.5 % yr−1. With a delay of 6–7 years, these changes roughly follow total surface chlorine, whose behavior versus time arises from inhomogeneous changes in the source gases. Since the late 1990s, HCl decreases in the lower stratosphere have occurred with pronounced latitudinal variability at rates sometimes exceeding 1–2 % yr−1. Recent short-term tendencies of lower-stratospheric and column HCl vary substantially, with increases from 2005 to 2010 for northern midlatitudes and deep tropics, but decreases (increases) after 2011 at northern (southern) midlatitudes. For H2O, the GOZCARDS product covers both stratosphere and mesosphere, and the same instruments as for HCl are used, along with Upper Atmosphere Research Satellite (UARS) MLS stratospheric H2O data (1991–1993). We display seasonal to decadal-type variability in H2O from 22 years of data. In the upper mesosphere, the anticorrelation between H2O and solar flux is now clearly visible over two full solar cycles. Lower-stratospheric tropical H2O has exhibited two periods of increasing values, followed by fairly sharp drops (the well-documented 2000–2001 decrease and a recent drop in 2011–2013). Tropical decadal variability peaks just above the tropopause. Between 1991 and 2013, both in the tropics and on a near-global basis, H2O has decreased by ~ 5–10 % in the lower stratosphere, but about a 10 % increase is observed in the upper stratosphere and lower mesosphere. However, such tendencies may not represent longer-term trends. For ozone, we used SAGE I, SAGE II, HALOE, UARS and Aura MLS, and ACE-FTS data to produce a merged record from late 1979 onward, using SAGE II as the primary reference. Unlike the 2 to 3 % increase in near-global column ozone after the late 1990s reported by some, GOZCARDS stratospheric column O3 values do not show a recent upturn of more than 0.5 to 1 %; long-term interannual column ozone variations from GOZCARDS are generally in very good agreement with interannual changes in merged total column ozone (Version 8.6) data from SBUV instruments. A brief mention is also made of other currently available, commonly formatted GOZCARDS satellite data records for stratospheric composition, namely those for N2O and HNO3.

Stratospheric transport and polar processing during the 2014/2015 Arctic winter were strongly influenced by a minor sudden stratospheric warming (SSW) in early January. Disturbances to temperatures and trace gases in the middle and upper stratosphere were similar in character to those associated with major SSWs: The stratopause dropped, and vertical temperature gradients weakened, followed by renewed descent of mesospheric air. The lower stratospheric polar vortex was barely disrupted and remained unusually strong throughout the winter. The SSW did, however, cause lower stratospheric temperatures to rise well above chlorine activation thresholds; trace gas abundances from the Aura Microwave Limb Sounder (MLS) were consequently exceptional. The degree of chlorine activation in January was the smallest, and lower stratospheric ozone values in February were the highest, in the 11 year MLS record. The major role played by a minor SSW highlights the Arctic stratosphere's sensitivity to a spectrum of dynamical variability.

Vertically resolved distributions of sulfur dioxide (SO2) with global coverage in the height region from the upper troposphere to ~ 20 km altitude have been derived from observations by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat for the period July 2002 to April 2012. Retrieved volume mixing ratio profiles representing single measurements are characterized by typical errors in the range of 70-100 pptv and by a vertical resolution ranging from 3-5 km. Comparison with ACE-FTS observations revealed a slightly varying bias with altitude of -20 to 50 pptv for the MIPAS dataset in case of volcanically enhanced concentrations. For background concentrations the comparison showed a systematic difference between the two major MIPAS observation periods. After debiasing, the difference could be reduced to biases within -10 to 20 pptv in the altitude range of 10-20 km with respect to ACE-FTS. Further comparisons of the debiased MIPAS dataset with in-situ measurements from various aircraft campaigns showed no obvious inconsistencies within a range of around ±50 pptv. The SO2 emissions of more than thirty volcanic eruptions could be identified in the upper troposphere and lower stratosphere (UTLS). Emitted SO2 masses and lifetimes within different altitude ranges in the UTLS have been derived for a large part of these eruptions. Masses are in most cases within estimations derived from other instruments. From three of the major eruptions within the MIPAS measurement period - Kasatochi in August 2008, Sarychev in June 2009 and Nabro in June 2011 - derived lifetimes of SO2 for the altitude ranges 10-14, 14-18, and 18-22 km are 13.3±2.1, 23.6±1.2, and 32.3±5.5 d, respectively. By omitting periods with obvious volcanic influence we have derived background mixing ratio distributions of SO2. At 10 km altitude these indicate an annual cycle at northern mid- and high latitudes with maximum values in summer and an amplitude of about 30 pptv. At higher altitudes of about 16-18 km enhanced mixing ratios of SO2 can be found in the region of the Asian and the North-American monsoon in summer - a possible connection to an aerosol layer discovered by Vernier et al. (2011b) in that region., H. Schlager, A. Roiger and K. Wissmuller acknowledge support by the DLR projects ESMVal and VolcATS. This work was supported by the European Community within the StratoClim project (grant no. 603557).

It is known from ground-based measurements made during the 1982-83 and 1997-98 El Niño events that atmosphericHCN tends to be higher than usual during such years. The Microwave Limb Sounder (MLS) on Aura has been measuring HCNmixing ratios since launch in 2004; the measurements are ongoing at the time of writing. The winter of 2015-16 has seen thelargest El Niño event since 1997-98. We present MLS measurements of HCN in the lower stratosphere for the Aura missionto date, comparing the 2015-16 5 El Niño period to the rest of the mission. HCN in 2015-16 is higher than at any other timeduring the mission, but ground based measurements suggest that it may have been even more elevated in 1997-98. As the MLSHCN data are essentially un-validated, we show them alongside data from the MIPAS and ACE-FTS instruments; the threeinstruments agree reasonably well in the tropical lower stratosphere. Global HCN emissions calculated from the GFED (V4.1)database are very much greater during large El Niño events and are greater in 1997-98 than in 2015-16, thereby showing good10 qualitative agreement with the measurements. Correlation between ENSO indices, measured HCN and GFED HCN emissionsis less clear away from the 2015-16 event. In particular, the 2009-10 winter had fairly strong El Niño conditions and fairlylarge GFED HCN emissions, but very little effect is observed in the MLS HCN.

A new, computationally efficient coupled stratosphere-troposphere chemistry-climate model (S/T-CCM) has been developed based on three well-documented components: a 64-level general circulation model from the UK Met Office Unified Model, the tropospheric chemistry transport model (STOCHEM), and the UMSLIMCAT stratospheric chemistry module. This newly developed S/T-CCM has been evaluated with various observations, and it shows good performance in simulating important chemical species and their interdependence in both the troposphere and stratosphere. The modeled total column ozone agrees well with Total Ozone Mapping Spectrometer observations. Modeled ozone profiles in the upper troposphere and lower stratosphere are significantly improved compared to runs with the stratospheric chemistry and tropospheric chemistry models alone, and they are in good agreement with Michelson Interferometer for Passive Atmospheric Sounding satellite ozone profiles. The observed CO tape recorder is also successfully captured by the new CCM, and ozone-CO correlations are in accordance with Atmospheric Chemistry Experiment observations. However, because of limitations in vertical resolution, intrusion of CO-rich air in the stratosphere from the mesosphere could not be simulated in the current version of S/T-CCM. Additionally, the simulated stratosphere-to-troposphere ozone flux, which controls upper tropospheric OH and O3 concentrations, is found to be more realistic in the new coupled model compared to STOCHEM. © 2010 by the American Geophysical Union.

A space shuttle launch deposits 700 tonnes of water in the atmosphere. Some of this water is released into the upper mesosphere and lower thermosphere where it may be directly detected by a limb sounding satellite instrument. We report measurements of water vapour plumes from shuttle launches made by the Microwave Limb Sounder (MLS) on the Aura satellite. Approximately 50%–65% of shuttle launches are detected by MLS. The signal appears at a similar level across the upper 10 km of the MLS limb scan, suggesting that the bulk of the observed water is above the top of the scan. Only a small fraction at best of smaller launches (Ariane 5, Proton) are detected. We conclude that the sensitivity of MLS is only just great enough to detect a shuttle sized launch, but that a suitably designed instrument of the same general type could detect the exhausts from a large proportion of heavy-lift launches.

This paper presents an evaluation of a new linear parameterization valid for the troposphere and the stratosphere, based on a first order approximation of the carbon monoxide (CO) continuity equation. This linear scheme (hereinafter noted LINCO) has been implemented in the 3-D Chemical Transport Model (CTM) MOCAGE of Météo-France. On the one hand, a one and a half years of LINCO simulation has been compared to output obtained from a detailed chemical scheme output. In spite of small differences, the seasonal and global CO distributions obtained by both schemes present similar general characteristics. The mean differences between both schemes remain small within about ±25 ppbv (part per billion by volume) in the troposphere and ±15 ppbv in the stratosphere. On the other hand, LINCO has been compared to diverse observations from satellite instruments covering the troposphere (Measurements Of Pollution In The Troposphere: MOPITT) and the stratosphere (Microwave Limb Sounder: MLS) and also from aircraft (Measurements of ozone and water vapour by Airbus in-service aircraft: MOZAIC programme) mostly flying in the upper troposphere and lower stratosphere. A good agreement is generally found in the troposphere and the lower stratosphere. In the troposphere, the LINCO seasonal variations as well as the vertical and horizontal distributions are quite close to MOPITT CO observations. However, a bias of ~−40 ppbv is observed at 700 hPa between LINCO and MOPITT which is probably caused by too low emission values. In the stratosphere, MLS and LINCO present similar large-scale patterns, except over the poles where the CO concentration is underestimated by the model. We suggest that the underestimation of CO at polar latitudes is not related to the linear scheme but is induced by a too rapid transport by the meridional circulation. In the UTLS (Upper Troposphere Lower Stratosphere), LINCO tends to slightly overestimate the MOZAIC aircraft observations, with general small biases less than 2%. LINCO is a simple parameterization compared to a detailed chemical scheme, allowing very fast calculations and thus making possible long reanalyses of MOPITT CO data. The computational cost just corresponds to the transport of an additional passive tracer. For this, we used a variational 3-D-FGAT (First Guess at Appropriate Time) method in conjunction with MOCAGE for a long run of one and a half years. The data assimilation greatly improves the vertical CO distribution in the troposphere from 700 to 350 hPa compared to independent MOZAIC profiles. At 146 hPa, the assimilated CO 2-D distribution is improved compared to MLS observations by reducing the bias up to a factor of 2 in the tropics. At extratropical latitudes, the assimilated fields tend to underestimate the CO concentrations resulting from an excessive equator to pole circulation. This study confirms that the linear scheme is able to simulate reasonably well the CO distribution in the troposphere and in the lower stratosphere. Therefore, the low computing cost of the linear scheme opens new perspectives to make free runs and CO data assimilation runs at high resolution and over periods of several years.

We use the GEOS-Chem global 3-D chemistry transport model to investigate the relative importance of chemical and physical processes that determine observed variability of hydrogen cyanide (HCN) in the troposphere and lower stratosphere. Consequently, we reconcile ground-based FTIR column measurements of HCN, which show annual and semi-annual variations, with recent space-borne measurements of HCN mixing ratio in the tropical lower stratosphere, which show a large two-year variation. We find that the observed column variability over the ground-based stations is determined by a superposition of HCN from several regional burning sources, with GEOS-Chem reproducing these column data with a positive bias of 5%. GEOS-Chem reproduces the observed HCN mixing ratio from the Microwave Limb Sounder and the Atmospheric Chemistry Experiment satellite instruments with a mean negative bias of 20%, and the observed HCN variability with a mean negative bias of 7%. We show that tropical biomass burning emissions explain most of the observed HCN variations in the upper troposphere and lower stratosphere (UTLS), with the remainder due to atmospheric transport and HCN chemistry. In the mid and upper stratosphere, atmospheric dynamics progressively exerts more influence on HCN variations. The extent of temporal overlap between African and other continental burning seasons is key in establishing the apparent bienniel cycle in the UTLS. Similar analysis of other, shorter-lived trace gases have not observed the transition between annual and bienniel cycles in the UTLS probably because the signal of inter-annual variations from surface emission has been diluted before arriving at the lower stratosphere (LS), due to shorter atmospheric lifetimes.

Meteorological and atmospheric models are being extended up to 80 km altitude but there are very few observing techniques that can measure stratospheric-mesospheric winds at altitudes between 20 km and 80 km to verify model data-sets. Here we demonstrate the feasibility of horizontal wind profile measurements using ground-based passive millimetre-wave spectroradiometric observations of ozone lines centred at 231.28 GHz, 249.79 GHz, and 249.96 GHz. Vertical profiles of horizontal winds are retrieved from forward and inverse modelling simulations of the line-of-sight Doppler-shifted atmospheric emission lines above Halley station (75°37’S, 26°14’W), Antarctica. For a radiometer with a system temperature of 1400 K and 30 kHz spectral resolution observing the ozone 231.28 GHz line we estimate that 12hr zonal and meridional wind profiles could be determined over the altitude range 25–74 km in winter, and 28–66 km in summer. Height-dependent measurement uncertainties are in the range 3–8 m s–1 and vertical resolution ~8–16 km. Under optimum observing conditions at Halley a temporal resolution of 1.5hrs for measuring either zonal or meridional winds is possible, reducing to 0.5hr for a radiometer with a 700 K system temperature. Combining observations of the 231.28 GHz ozone line and the 230.54 GHz carbon monoxide line gives additional altitude coverage at 85±12 km. The effects of clear-sky seasonal mean winter/summer conditions, zenith angle of the received atmospheric emission, and spectrometer frequency resolution on the altitude coverage, measurement uncertainty, and height and time resolution of the retrieved wind profiles have been determined.

Limb sounding instruments provide high vertical resolution data on the temperature and composition of the atmosphere. Their data are, therefore, valuable for assimilating into general circulation models of the atmosphere. Direct assimilation of radiances from limb sounders is more complex in practice than from nadir sounders due to the need to know the tangent pressures of the measurements. This paper discusses the practical implications of tangent pressures in direct radiance assimilation of limb sounding radiances and demonstrates that a neural network can be used to find these tangent pressures for the Earth Observing System Microwave Limb Sounder with a root mean-square error of sigma=50 m, which is comparable with that in traditional retrieval techniques

We describe the publicly available dataset from the Global OZone Chemistry And Related Datasets for the Stratosphere (GOZCARDS) project, and provide some results, with a~focus on hydrogen chloride (HCl), water vapor (H2O), and ozone (O3). This dataset is a global long-term stratospheric Earth System Data Record (ESDR), consisting of monthly zonal mean time series starting as early as 1979. The data records are based on high quality measurements from several NASA satellite instruments and ACE-FTS on SCISAT. We examine consistency aspects between the various datasets. To merge ozone records, the time series are debiased by calculating average offsets with respect to SAGE II during periods of measurement overlap, whereas for other species, the merging derives from an averaging procedure based on overlap periods. The GOZCARDS files contain mixing ratios on a common pressure/latitude grid, as well as standard errors and other diagnostics; we also present estimates of systematic uncertainties in the merged products. Monthly mean temperatures for GOZCARDS were also produced, based directly on data from the Modern-Era Retrospective analysis for Research and Applications (MERRA). The GOZCARDS HCl merged product comes from HALOE, ACE-FTS and (for the lower stratosphere) Aura MLS data. After a~rapid rise in upper stratospheric HCl in the early 1990s, the rate of decrease in this region for 1997–2010 was between 0.4 and 0.7% yr−1. On shorter timescales (6 to 8 years), the rate of decrease peaked in 2004–2005 at about 1% yr−1, and has since levelled off, at ~0.5 yr−1. With a delay of 6–7 years, these changes roughly follow total surface chlorine, whose behavior vs. time arises from inhomogeneous changes in the source gases. Since the late 1990s, HCl decreases in the lower stratosphere have occurred with pronounced latitudinal variability at rates sometimes exceeding 1–2 yr−1. There has been a significant reversal in the changes of lower stratospheric HCl abundances and columns for 2005–2010, in particular at northern midlatitudes and in the deep tropics, where short-term increases are observed. However, lower stratospheric HCl tendencies appear to be reversing after about 2011, with (short-term) decreases at northern midlatitudes and some increasing tendencies at southern midlatitudes. For GOZCARDS H2O, covering the stratosphere and mesosphere, the same instruments as for HCl are used, along with UARS MLS stratospheric H2O data (1991–1993). We display seasonal to decadal-type variability in H2O from 22 years of data. In the upper mesosphere, the anti-correlation between H2O and solar flux is now clearly visible over two full solar cycles. Lower stratospheric tropical H2O has exhibited two periods of increasing values, followed by fairly sharp drops, the well-documented 2000–2001 decrease, and another recent decrease in 2011–2013. Tropical decadal variability peaks just above the tropopause. Between 1991 and 2013, both in the tropics and on a near-global basis, H2O has decreased by ~ 5–10% in the lower stratosphere, but about a 10% increase is observed in the upper stratosphere and lower mesosphere. However, recent tendencies may not hold for the long-term, and the addition of a few years of data can significantly modify trend results. For ozone, we used SAGE I, SAGE II, HALOE, UARS and Aura MLS, and ACE-FTS data to produce a~merged record from late 1979 onward, using SAGE II as the primary reference for aligning (debiasing) the other datasets. Other adjustments were needed in the upper stratosphere to circumvent temporal drifts in SAGE II O3 after June 2000, as a result of the (temperature-dependent) data conversion from a density/altitude to a mixing ratio/pressure grid. Unlike the 2 to 3% increase in near-global column ozone after the late 1990s reported by some, GOZCARDS stratospheric column O3 values do not show a recent upturn of more than 0.5 to 1%; continuing studies of changes in global ozone profiles, as well as ozone columns, are warranted. A brief mention is also made of other currently available, commonly-formatted GOZCARDS satellite data records for stratospheric composition, namely those for N2O and HNO3.

We present results of early validation studies using retrieved atmospheric profiles from the Earth Observing System Microwave Limb Sounder (MLS) instrument on the Aura satellite. "Global" results are presented for MLS measurements of atmospheric temperature, ozone, water vapor, hydrogen chloride, nitrous oxide, nitric acid, and carbon monoxide, with a focus on the January-March 2005 time period. These global comparisons are made using long-standing global satellites and meteorological datasets, as well as some measurements from more recently launched satellites. Comparisons of MLS data with measurements from the Ft. Sumner, NM, September 2004 balloon flights are also presented. Overall, good agreement is obtained, often within 5% to 10%, but we point out certain issues to resolve and some larger systematic differences; some artifacts in the first publicly released MLS (version 1.5) dataset are noted. We comment briefly on future plans for validation and software improvements.

The Earth Observing System Microwave Limb Sounder measures several atmospheric chemical species (OH, HO/sub 2/, H/sub 2/O, O/sub 3/, HCl, ClO, HOCl, BrO, HNO/sub 3/, N/sub 2/O, CO, HCN, CH/sub 3/CN, volcanic SO/sub 2/), cloud ice, temperature, and geopotential height to improve our understanding of stratospheric ozone chemistry, the interaction of composition and climate, and pollution in the upper troposphere. All measurements are made simultaneously and continuously, during both day and night. The instrument uses heterodyne radiometers that observe thermal emission from the atmospheric limb in broad spectral regions centered near 118, 190, 240, and 640 GHz, and 2.5 THz. It was launched July 15, 2004 on the National Aeronautics and Space Administration's Aura satellite and started full-up science operations on August 13, 2004. An atmospheric limb scan and radiometric calibration for all bands are performed routinely every 25 s. Vertical profiles are retrieved every 165 km along the suborbital track, covering 82/spl deg/S to 82/spl deg/N latitudes on each orbit. Instrument performance to date has been excellent; data have been made publicly available; and initial science results have been obtained.

Thermal and humidity structures near the tropical tropopause are studied using microwave satellite retrievals of water vapor, along with contemporaneous dynamical structure in ECMWF analyses and cold clouds in high-resolution global cloud imagery. Examined during November 1991–February 1992, these fields all vary coherently with the outflow from convective centers—in the upper troposphere as well as in the lowermost stratosphere. The outbreak of deep convection is accompanied by diabatic heating below a level between 250 and 150 mb but by diabatic cooling at higher levels. The reversal from heating to cooling is broadly consistent with cumulus detrainment. Through irreversible mixing, that process serves as a heat source for the environment below the level of neutral buoyancy (LNB) but as a heat sink at higher levels. Calculations, inclusive of entrainment, place the LNB very near the observed reversal from heating to cooling. The outbreak of convection is also accompanied by humidification below...

Lightning is an important natural source of nitrogen oxide especially in the middle and upper troposphere. Hence, it is essential to represent lightning in chemistry transport and coupled chemistry–climate models. Using ERA-Interim meteorological reanalysis data we compare the lightning flash density distributions produced using several existing lightning parametrisations, as well as a new parametrisation developed on the basis of upward cloud ice flux at 440 hPa. The use of ice flux forms a link to the non-inductive charging mechanism of thunderstorms. Spatial and temporal distributions of lightning flash density are compared to tropical and subtropical observations for 2007–2011 from the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite. The well-used lightning flash parametrisation based on cloud-top height has large biases but the derived annual total flash density has a better spatial correlation with the LIS observations than other existing parametrisations. A comparison of flash density simulated by the different schemes shows that the cloud-top height parametrisation has many more instances of moderate flash densities and fewer low and high extremes compared to the other parametrisations. Other studies in the literature have shown that this feature of the cloud-top height parametrisation is in contrast to lightning observations over certain regions. Our new ice flux parametrisation shows a clear improvement over all the existing parametrisations with lower root mean square errors (RMSEs) and better spatial correlations with the observations for distributions of annual total, and seasonal and interannual variations. The greatest improvement with the new parametrisation is a more realistic representation of the zonal distribution with a better balance between tropical and subtropical lightning flash estimates. The new parametrisation is appropriate for testing in chemistry transport and chemistry–climate models that use a lightning parametrisation.

Satellite observations of water vapor and aerosol extinction along with temperature trajectory calculations are analyzed for the Southern Hemisphere winter of 1992 in order to determine the onset, extent, and duration of dehydration within the polar vortex. Our investigation uses measurements of water vapor from the Microwave Limb Sounder (MLS) and aerosol extinction from the Cryogenic Limb Array Etalon Spectrometer (CLAES), both on board the Upper Atmosphere Research Satellite (UARS). Evidence of persistent ice cloud formation, supported by temperature statistics obtained from air parcel trajectories, suggests that the onset of the dehydration process occurs between late June and early July. By late August-early September water vapor depleted areas within the vortex no longer coincide with high aerosol extinctions, indicating that severe dehydration has occurred with the irreversible removal of water vapor over vast areas. Areas with depleted levels of water vapor, below the prewinter values, persist well into November. Evidence for dehydration is found on potential temperature surfaces from 420 K (the lower limit of the MLS measurements) to 520 K (approximately 16 to 22 km). The horizontal extent of the dehydrated area at 465 K encompasses up to 35% of the total vortex area equatorward of 80°S. A comparison of CLAES aerosol extinction measurements and model calculations of aerosol extinction suggests an average ice particle number concentration and size of 10−2–10−3 cm−3 and 10–30 μm, respectively. We show that the difference between the timing of the onset of dehydration found here and that in a recent analysis of Polar Ozone and Aerosol Measurement III (POAM) observations can be explained by the latitudinal sampling pattern of the POAM instrument.

We show the signature of the tropical intraseasonal oscillation (TIO) in upper tropospheric moisture and dynamical fields, roughly between 200 and 100 hPa. Relationships among these fields are examined using lag-correlation analysis and using multivariate extended empirical orthogonal functions (MEEOFs), which maximize the shared explained variance among several fields for both spatial and temporal variations. The MEEOFs show that all of the fields respond to the TIO and that the TIO is the dominant factor influencing each of the fields on these timescales. Convection associated with the TIO moistens the upper troposphere up to about 150 hPa, as expected; the behavior at 100 hPa is more complex. Over the longitude range where the TIO is associated with convection, roughly 60o-180oE, 100-hPa temperature and water vapor tend to be reduced above convection on TIO timescales. East of 180 o, though, the temperature and water vapor variations at 100 hPa become decoupled. The water vapor variations, like those of 200-hPa velocity potential, appear to speed up at about 180oE.

We present zonal mean time series of water vapor in the lower stratosphere as measured by UARS MLS. The data are taken from a development version known as V104; we demonstrate that this version behaves in a physically realistic manner at 100 and 68 hPa, lower altitudes than are useful in previous versions of the data. The 100 hPa data show that the annual cycle in tropical lower stratosphere water vapor is driven by the northern hemisphere. Both low and high mixing ratios spread from the northern hemisphere tropics into the southern hemisphere tropics and into the mid-latitudes with transport out of the tropics being greater at 100 hPa than at 68 hPa. Transport into mid-latitudes is greater in the northern hemisphere and greater in spring than in fall. At 46 hPa, the tropical annual cycle is strongly confined to low latitudes.

Microwave limb sounding is an important new technique for measuring the temperature of the middle atmosphere and the concentration of various trace species in it. The spectral resolution of the instruments used is such that the measurements are usually made at a number of frequencies across the width of a single spectral line. It is therefore important to model the line shape and the instrument response accurately as inaccurate modelling will lead to a poor match between the measured radiances and those re-calculated from the retrieved atmosphere. In this paper, we consider the 183.3 GHz rotational transition of the water molecule and the 184.4 GHz transition of ozone. These transitions have been used to measure water vapour and ozone in the middle atmosphere by two orbiting instruments: the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) and the Millimetre-wave Atmospheric Sounder (MAS) which was flown on the ATLAS platform on board the space shuttle. Both instruments have had similar problems matching measured and re-calculated 183 GHz radiances. We show that by allowing the retrieval algorithm to fit certain spectral and instrumental parameters in addition to the mixing ratio profiles, we can improve both the quality of the retrieved profiles and our knowledge of certain spectral parameters, in particular the pressure shift of the line.

The UARS Microwave Limb Sounder (MLS) measured water vapor in the middle atmosphere between September 1991 and April 1993. The current version of the data, version 4, and its predecessor, version 3, have been used in many scientific studies. As part of the process of developing the next version of UARS MLS data, a new prototype retrieval for the stratosphere/mesosphere water vapor product was developed at the University of Edinburgh. The main improvements made were (1) corrections for systematic errors and (2) doubling of the vertical resolution of the retrieval grid. Initial results were sufficiently encouraging that the code was used to produce data for all UARS days on which the MLS 183 GHz radiometer was operational. This paper describes these data and examines their quality. Comparisons are made with the available correlative data and it emerges that the prototype agrees better with the correlative data than does version 4. Agreement with frost point hygrometers is excellent in the lower stratosphere and agreement with a ground-based microwave is satisfactory in the upper stratosphere and mesosphere. Agreement with two solar occultation instruments, Atmospheric Trace Molecular Spectroscopy Experiment (ATMOS) and Halogen Occultation Experiment (HALOE), is better than it was for earlier MLS versions but the prototype data are uniformly drier than both of these instruments. We compare the data with version 4, showing several features which are more clearly visible in the new prototype. These include seasonal cycles in the upper mesosphere and equatorial lower stratosphere.

A complete monthly record of the annual variation of Na and temperature in the upper mesosphere has been obtained from 3 years of nighttime lidar observations at two midlatitude sites, Urbana-Champaign, Illinois (40°N), and Fort Collins, Colorado (41°N). The Na density exhibits a strong annual variation at all heights between 81 and 107 km, with the column abundance of the layer peaking in early winter and then decreasing by nearly a factor of 4 to a midsummer minimum. There are also significant semiannual components to the variations in the centroid height and thickness of the layer. The nighttime temperature profile between 81 and 105 km exhibits a high winter mesopause at about 101 km and a summer mesopause at about 85 km. During spring and autumn, the mesopause oscillates apparently randomly between these states. A seasonal model of the Na layer was then constructed incorporating recent laboratory studies of the pertinent neutral and ionic reactions of the metal. The background atmospheric composition was provided from three off-line models, as well as from UARS/Microwave Limb Sounder satellite measurements of H 2 O. With a small number of permitted adjustable parameters, the model is able to reproduce many observed features of the Na layer remarkably well, including the monthly variation in column abundance and layer shape. The biggest discrepancy is during midsummer, when the modeled layer is displaced 2-3 km above that observed, although a factor contributing to this is that the lidar observing period during summer was relatively short and the effect of the diurnal tide could have been incompletely sampled. Both the observations and the model show that Na density and temperature are highly correlated below 96 km (correlation coefficient equal to 0.8-0.95), mostly as a result of the influence of odd oxygen/hydrogen chemistry on the partitioning of sodium between atomic Na and its principal reservoir species, NaHCO 3 . Above 96 km, a weak negative correlation (-0.2) is explained by the dominance of ion-molecule chemistry. Finally, it was shown that if the eddy diffusion coefficient in the middle mesosphere is significantly smaller or if the global meteoric influx is much larger than the values used in the present model, then processes for permanently removing gas-phase Na species in the mesosphere, such as polymerisation and deposition onto dust particles, will need to be included.

The Microwave Limb Sounder (MLS) experiments obtain measurements of atmospheric composition, temperature, and pressure by observations of millimeter- and submillimeter-wavelength thermal emission as the instrument field of view is scanned through the atmospheric limb. Features of the measurement technique include the ability to measure many atmospheric gases as well as temperature and pressure, to obtain measurements even in the presence of dense aerosol and cirrus, and to provide near-global coverage on a daily basis at all times of day and night from an orbiting platform. The composition measurements are relatively insensitive to uncertainties in atmospheric temperature. An accurate spectroscopic database is available, and the instrument calibration is also very accurate and stable. The first MLS experiment in space, launched on the (NASA) Upper Atmosphere Research Satellite (UARS) in September 1991, was designed primarily to measure stratospheric profiles of ClO, O3, H2O, and atmospheric press...

Upper Atmosphere Research Satellite tracer data and isentropic transport calculations using U.K. Meteorological Office winds initialized with these data show evidence of eastward-traveling waves in the polar upper stratosphere in late austral winter 1992. Microwave Limb Sounder (MLS) H2O from prototype iterative retrievals shows a 4-day wave signal at levels from ∼1.5 to 0.1 hPa; a 4-day wave signal was not obvious in production retrievals of MLS H2O. At 1800 K, the 4-day wave signal in MLS H2O has a double-peaked structure in latitude, which is reproduced in isentropic transport calculations. The time evolution, amplitude, and phase of the 4-day wave in the transport calculations agree well with observations at high latitudes; the position and shape of the polar vortex and of H2O drawn up around the vortex are reproduced by the transport calculations. Spectral analyses of the Cryogen Limb Array Etalon Spectrometer (CLAES) CH4 are dominated by more slowly eastward-moving waves (∼6–10 days), but a...

Observations of water vapor with high tempo- ral and spatial resolution and good horizontal coverage just above the tropical tropopause have been scarce, but a pre- liminary version of such data has been developed using radi- ance measurements of the Microwave Limb Sounder. These data reveal distinct variations with periods in the ranges 10{ 25 days and 30{70 days, consistent with (respectively) slow Kelvin waves and the tropical intraseasonal oscillation.

ARSTRACT The Microwave Limb Sounder on the UARS satellite measures water vapour concentration in the stratosphere and mesosphere. Water vapour profiles are retrieved from radiance measurements using a version of the optimal estimation algorithm. This requires an a priori profile which is obtained from a climatology. The MLS retrieval currently uses the standard IJARS pre-launch climatology, which conbains wa.ter vapou~ based on a 2-D model constrained to LIMS data in the stratosphere. This climatology ha,s several defects which affect the ability of MLS to retrieve water vapour. This paper presents a new climatology constructed from the HALOE (HALogen Occultation Experiment) and SAGE II (Stratospheric Aerosol a.nd Gas Experiment) data, which have become available recently. The new climatology is more realistic ilr several ways, particularly in the mesosphere and near the tropopause. It is proving to be an improvemerit, as an II prioTj for MLS retrievals and might also have other uses. The pa.per will present the climatology, show how it was const.ructed, and compase ii, t.o t,he UARS prelaunch climatology and to various other data. As it stands, this climatology is not suitable for a reference atmosphere, but it is an improvement on ClRA part III in some wa,ys simply because iI, contains morf\ accurate data and shows a number of new features. Suggestions are made for constructing an improvc!d reference climatology for middle atmosphere water vapour. 01998 COSPAR. Published hy E:iscvier

The Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) measures water vapour concentration in the stratosphere and mesosphere. The version 4 algorithm used at present to retrieve the concentration from the radiance measurements has a number of limitations. We describe work which attempts to circumvent some of these limitations. The two main limitations of the current algorithm are that (1) The retrieval assumes linearity and (2) The concentration is retrieved onto a grid with a 5.3 km spacing. The theoretical vertical resolution of the instrument is better than this. To overcome limitation (1) we use a non-linear iterative retrieval; we overcome limitation (2) by using a vertical grid with a 2.6 km spacing. We show that both these measures result in a useful improvement. These improvements are planned to be incorporated in version 5 algorithms now under development for MLS.

Variations in the mixing ratio of trace gases of tropospheric origin entering the stratosphere in the tropics are of interest for assessing both troposphere to stratosphere transport fluxes in the tropics and the impact of these transport fluxes on the composition of the tropical lower stratosphere. Anomaly patterns of carbon monoxide (CO) and long-lived tracers in the lower tropical stratosphere allow conclusions about the rate and the variability of tropical upwelling to be drawn. Here, we present a simplified chemistry scheme for the Chemical Lagrangian Model of the Stratosphere (CLaMS) for the simulation, at comparatively low numerical cost, of CO, ozone, and long-lived trace substances (CH4, N2O, CCl3F (CFC-11), CCl2F2 (CFC-12), and CO2) in the lower tropical stratosphere. For the long-lived trace substances, the boundary conditions at the surface are prescribed based on ground-based measurements in the lowest model level. The boundary condition for CO in the lower troposphere (below about 4 km) is deduced from MOPITT measurements. Due to the lack of a specific representation of mixing and convective uplift in the troposphere in this model version, enhanced CO values, in particular those resulting from convective outflow are underestimated. However, in the tropical tropopause layer and the lower tropical stratosphere, there is relatively good agreement of simulated CO with in situ measurements (with the exception of the TROCCINOX campaign, where CO in the simulation is biased low ≈10–15 ppbv). Further, the model results (and therefore also the ERA-Interim winds, on which the transport in the model is based) are of sufficient quality to describe large scale anomaly patterns of CO in the lower stratosphere. In particular, the zonally averaged tropical CO anomaly patterns (the so called "tape recorder" patterns) simulated by this model version of CLaMS are in good agreement with observations, although the simulations show a too rapid upwelling compared to observations as a consequence of the overestimated vertical velocities in the ERA-Interim reanalysis data set. Moreover, the simulated tropical anomaly patterns of N2O are in good agreement with observations. In the simulations, anomaly patterns of CH4 and CFC-11 were found to be very similar to those of N2O; for all long-lived tracers, positive anomalies are simulated because of the enhanced tropical upwelling in the easterly shear phase of the quasi-biennial oscillation.

We compare water vapor measurements from the Naval Research Laboratory ground-based Water Vapor Millimeter-wave Spectrometer (WVMS) instruments with measurements taken by five space-based instruments. For coincident measurements the retrievals from all of the instruments show qualitatively similar altitude profiles. The retrieved mixing ratios from most instruments generally differ from an average calculated using retrievals from all of the instruments by

Until recently, measurements of water vapour in the upper mesosphere were scarce. Instruments on the UARS platform have made the first global measurements of upper mesospheric H2O; the Microwave Limb Sounder is one of these instruments. It has also measured upper mesospheric ozone. So far, various random and systematic errors have prevented accurate retrievals of either species in the upper mesosphere but much can be learned from the raw radiance measurements. In this paper we present these measurements and show how they can provide information on the dynamics of the mesosphere.

The Upper Atmosphere Research Satellite (UARS) microwave limb sounder (MLS) makes measurements of thermal emission at 183.3 GHz which are used to infer the concentration of water vapor over a pressure range of 46-0.2hPa (approximately 20-60 km). We provide a validation of MLS H2O by analyzing the integrity of the measurements, by providing an error characterization, and by comparison with data from other instruments. It is estimated that version 3 MLS H2O retrievals are accurate to within 20-25% in the lower stratosphere and to within 8-13% in the upper stratosphere and lower mesosphere. The precision of a single profile is estimated to be approximately 0.15 parts per million by volume (ppmv) in the midstratosphere and 0.2 ppmv in the lower and upper stratosphere. In the lower mesosphere the estimate of a single profile precision is 0.25-0.45 ppmv. During polar winter conditions, H2O retrievals at 46 hPa can have a substantial contribution from climatology. The vertical resolution of MLS H2O retrievals is approximately 5 km.

We describe observations of tropical stratospheric water vapor q that show clear evidence of large-scale upward advection of the signal from annual fluctuations in the effective "entry mixing ratio" qE of air entering the tropical stratosphere. In other words, air is "marked," on emergence above the highest cloud tops, like a signal recorded on an upward moving magnetic tape. We define qE as the mean water vapor mixing ratio, at the tropical tropopause, of air that will subsequently rise and enter the stratospheric "overworld" at about 400 K. The observations show a systematic phase lag, increasing with altitude, between the annual cycle in qE and the annual cycle in q at higher altitudes. The observed phase lag agrees with the phase lag calculated assuming advection by the transformed Eulerian-mean vertical velocity of a qE crudely estimated from 100-hPa temperatures, which we use as a convenient proxy for tropopause temperatures. The phase agreement confirms the overall robustness of the calculation and strongly supports the tape recorder hypothesis. Establishing a quantitative link between qE and observed tropopause temperatures, however, proves difficult because the process of marking the tape depends subtly on both small- and large-scale processes. The tape speed, or large-scale upward advection speed, has a substantial annual variation and a smaller variation due to the quasi-biennial oscillation, which delays or accelerates the arrival of the signal by a month or two in the middle stratosphere. As the tape moves upward, the signal is attenuated with an e-folding time of about 7 to 9 months between 100 and 50 hPa and about 15 to 18 months between 50 and 20 hPa, constraining possible orders of magnitude both of vertica.1 diffusion Kand of ra.tes of mixing in from the extratropics. For instance, if there were no mixing in, then Kwould be in the range 0.03-0.09 ms-; this is an upper bound on Ifs.

We used the GEOS-Chem chemistry-transport model to investigate impacts of surface emissions and dynamical processes on the spatial and temporal patterns of CO observed by the Microwave Limb Sounder (MLS) in the upper troposphere (UT) and lower stratosphere (LS). Model simulations driven by GEOS-4 and GEOS-5 assimilated fields present many features of the seasonal and inter-annual variation of CO in the upper troposphere and lower stratosphere. Both model simulations and the MLS data show a transition from semi-annual variations in the UT to annual variations in the LS. Tagged CO simulations indicate that the semi-annual variation of CO in the UT is determined mainly by the temporal overlapping of surface biomass burning from different continents as well as the north-south shifts of deep convection. Both GEOS-4 and GEOS-5 have maximum upward transport in April and May with a minimum in July to September. The CO peaks from the Northern Hemisphere (NH) fires propagate faster to the LS than do those from the Southern Hemisphere (SH) fires. Thus the transition from a semi-annual to an annual cycle around 80 hPa is induced by a combination of the CO signal at the tropopause and the annual cycle of the Brewer-Dobson circulation. In GEOS-5, the shift to an annual cycle occurs at a lower altitude than in MLS CO, a result of inadequate upward transport. We deduce vertical velocities from MLS CO, and use them to evaluate the velocities derived from the archived GEOS meteorological fields. We find that GEOS-4 velocities are similar to those from MLS CO between 215 hPa and 125 hPa, while the velocities in GEOS-5 are too low in spring and summer. The mean tropical vertical velocities from both models are lower than those inferred from MLS CO above 100 hPa, particularly in GEOS-5, with mean downward, rather than upward motion in boreal summer. Thus the models' CO maxima from SH burning are transported less effectively than those in MLS CO above 147 hPa and almost disappear by 100 hPa. The strongest peaks in the CO tape-recorder are in late 2004, 2006, and 2010, with the first two resulting from major fires in Indonesia and the last from severe burning in South America, all associated with intense droughts.

A recent paper [J. A. Nystuen, L. H. Ostwald, and H. Medwin, ‘‘The Hydroacoustics of a raindrop impact,’’ J. Acoust. Soc. Am. 92, 1017–1021 (1992)] describes the pressure pulse measured by placing a hydrophone in the water below an impacting raindrop. The form of the pulse as measured at a range of about 0.1 m is undoubtedly correct, consisting of a spike of about 60 μs in width followed by a ‘‘plateau,’’ which remains approximately constant over a time scale of about half a millisecond. Nystuen et al. interpret the spike as the near‐field acoustic pressure, i.e., the term that reduces to a solution of Laplace’s equation as the sound speed tends to infinity. They infer that the far‐field pressure will be its time derivative, a single‐cycle pulse. The plateau they describe as being quite unconnected with the acoustic field. The purpose of this Letter is to point out that this is not the only possible interpretation. The alternative is that the plateau is the near‐field pulse and that the spike is the far‐field pulse, being proportional to the time derivative of the plateau.

The vertical structure and evolution of the wintertime annular modes are studied using 6 years of geopotential height, carbon monoxide (CO), and water vapor (H2O) data from Aura Microwave Limb Sounder. The Northern Hemisphere annular mode (NAM) and the Southern Hemisphere annular mode (SAM) reveal a strong coupling of the dynamics in the stratosphere and mesosphere between 316 hPa (similar to 9 km) and 0.002 hPa (similar to 90 km). CO is a good tracer throughout the middle atmosphere, while variable vertical gradients of H2O limit the regions where it is useful as a dynamical tracer. The maximum of the CO NAM and SAM (CNAM and CSAM) indices is used to monitor and characterize the evolution of wintertime polar dynamics as a function of time and height. The CNAM analysis reveals reformation of a stronger mesospheric polar vortex after significant stratospheric sudden warmings in 2006, 2009, and 2010. There is a significant anticorrelation between the mesospheric and stratospheric CNAM indices during 2005-2010 winters, supporting the hypothesis of mesosphere-stratosphere coupling through planetary-gravity wave interactions.

The large bush fires which occurred in southeast Australia in February 2009 were unusually destructive. However, they were also unusual in the amounts of various combustion products which were injected directly into the stratosphere. We report the observations by the Microwave Limb Sounder (MLS) instrument on the Aura satellite of some of these combustion products. The highest quality observations are of CO; these clearly show a large region of enhanced mixing ratios to the north of New Zealand which remains in that region for about ten days before drifting westwards and finally dissipating over the Atlantic about a month after the fire. The region of enhanced CO ascends from the tropopause to 46 hPa during this period. Back trajectories run from the points where MLS observes enhanced CO pass close to the site of the fire. The MLS observations of CH3CN and HCN resemble those of CO except for their poorer vertical resolution and more limited vertical range. An apparent enhancement in ClO is also observed by MLS, but detailed analysis of the measured radiances reveals this feature to be a signature of CH3OH, which is not currently retrieved by the MLS data processing system. The fires of February 2009 are the only event of this type and magnitude in the 7-yr MLS record.