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The Hunga volcano violently erupted on 15 January 2022, producing the largest perturbation of the stratospheric aerosol layer since Pinatubo 1991, despite the initially estimated modest injection of SO2. This study presents novel SO2 and sulfate aerosol (SA) co‐retrievals from the Infrared Atmospheric Sounding Interferometer, and uses them to quantify the initial progression of the Hunga plume. These observations are consistent with rapid conversion of SO2 (e‐folding time: 17.1 ± 4.3 days) to SA, with an injected burden of >1.0 Tg SO2. This points at larger SO2 injections than previously thought. A long‐lasting SA plume was observed, with two separate build‐up phases, and with a meridional dispersion of marked anomalies from the tropics to the higher southern hemispheric latitudes. A limited (∼20%) SA removal was observed after 1‐year dispersion. The total injected SA mass burden was estimated at 1.6 ± 0.5 Tg in the total atmospheric column, with a build‐up e‐folding time of about 2 months. Plain Language Summary: The eruption of the submarine Hunga volcano in January 2022 polluted the global stratosphere with a large amount of water vapor and significantly perturbed the stratospheric aerosol layer. In this paper, we present a 1‐year long aftermath study of the stratospheric sulfur pollution from this volcanic eruption using observations from the Infrared Atmospheric Sounding Interferometer (IASI) satellite‐borne instrument. Gaseous and aerosol sulfur emissions are observed simultaneously using the specific potential of this sensor. These observations provide unique capabilities to characterize the aerosol type in the Hunga plume and the sulfur cycle associated with the volcanic emissions. An extremely rapid conversion of gaseous sulfur emissions to aerosols is observed, leading to larger than expected and persistent anomalies of the stratospheric aerosol layer (compared with a consistent long‐term climatology), still noticeable in the Southern Hemisphere after 1 year. The total mass of the emitted sulfur in gas and aerosol state is also simultaneously estimated, for the first time. Key Points: Novel co‐retrieval of SO2 and sulfate aerosol from Infrared Atmospheric Sounding Interferometer (IASI) used to study the dispersion of the Hunga plume over the entire year 2022Rapid conversion of SO2 (2 weeks e‐folding time) and long‐lasting sulfate aerosol observedLarger SO2 injected mass burden (>1.0 Tg) than previously thought and a large sulfate aerosol total burden (1.6 Tg) estimated [ABSTRACT FROM AUTHOR]

The tropopause is an important transition layer and can be a diagnostic of upper-tropospheric and lower-stratospheric structures, exhibiting unique atmospheric thermal and dynamic characteristics. A comprehensive understanding of the evolution of fine tropopause structures is necessary and primary for the further study of complex multi-scale atmospheric physical–chemical coupling processes in the upper troposphere and lower stratosphere. A novel method utilizing the bi-Gaussian function is capable of identifying the characteristic parameters of vertical tropopause structures and providing information on double-tropopause (DT) structures. The new method improves the definition of the cold-point tropopause and detects one (or two) of the most significant local cold points by fitting the temperature profiles to the bi-Gaussian function, which defines the point(s) as the tropopause height(s). The bi-Gaussian function exhibits excellent potential for explicating the variation trends of temperature profiles. The results of the bi-Gaussian method and lapse rate tropopause, as defined by the World Meteorological Organization, are compared in detail for different cases. Results indicate that the bi-Gaussian method is able to more stably and obviously identify the spatial and temporal distribution characteristics of the thermal tropopauses, even in the presence of multiple temperature inversion layers at higher elevations. Moreover, 5 years of historical radiosonde data from China (from 2012 to 2016) revealed that the occurrence frequency and thickness of the DT, as well as the single-tropopause height and the first and second DT heights, displayed significant meridional monotonic variations. The occurrence frequency (thickness) of the DT increased from 1.07 % (1.96 km) to 47.19 % (5.42 km) in the latitude range of 16–50° N. The meridional gradients of tropopause height were relatively large in the latitude range of 30–40° N, essentially corresponding to the climatological locations of the subtropical jet and the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

The behavior of ozone gas (O 3) in the atmosphere varies according to the region of the globe. Its formation occurs mainly in the tropical stratosphere through the photodissociation of molecular oxygen with the aid of the incidence of ultraviolet solar radiation. Still, the highest concentrations of O 3 content are found in high-latitude regions (poles) due to the Brewer–Dobson circulation, a large-scale circulation that takes place from the tropics to the pole in the winter hemisphere. This work presents a multi-instrumental analysis at two Brazilian sites, a subtropical one (Santa Maria – 29.72° S, 53.41° W) and an equatorial one (Natal – 5.4° S, 35.4° W), to investigate ozone distributions in terms of vertical profiles (2002–2020) and total abundance in terms of total columns of ozone (1979–2020). The study is based on the use of ground-based and satellite observations. Ozone profiles over Natal, from the ground up to the mesosphere, are obtained by radiosonde experiments (0–30 km) in the framework of the SHADOZ program and by satellite measurements from the SABER instrument (15–60 km). This enabled the construction of a continuous time series for ozone, including monthly values and climatological trends. There is a good agreement between the two measurements in the common observation layer, mainly for altitudes above 20 km. Below 20 km, SABER ozone profiles showed high variability and overestimated ozone mixing ratios by over 50 %. Dynamic and photochemical effects can interfere with O 3 formation and distribution along higher latitudes through the Brewer–Dobson circulation. The measurements of the total ozone columns used are in good agreement with each other (TOMS/OMI × Dobson for Natal and TOMS/OMI × Brewer for Santa Maria) in time and space, in line with previous studies for these latitudes. Wavelet analysis was used over 42 years. The investigation revealed a significant annual cycle in both data series for both sites. The study highlighted that the quasi-biennial oscillation (QBO) plays a significant role in the variability of stratospheric ozone at the two study sites – Natal and Santa Maria. The QBO's contribution was found to be stronger at the Equator (Natal) than at the subtropics (Santa Maria). Additionally, the study showed that the 11-year solar cycle also has a significant impact on ozone variability at both locations. Given the study latitudes, the ozone variations observed at the two sites showed different patterns and amounts. Only a limited number of studies have been conducted on stratospheric ozone in South America, particularly in the region between the Equator and the subtropics. The primary aim of this work is to investigate the behavior of stratospheric ozone at various altitudes and latitudes using ground-based and satellite measurements in terms of vertical profiles and total columns of ozone. [ABSTRACT FROM AUTHOR]

Water vapour in the upper troposphere and lower stratosphere (UTLS) is a key radiative agent and a crucial factor in the Earth's climate system. Here, we investigate a common regional moist bias in the Pacific UTLS during Northern Hemisphere summer in state-of-the-art climate models. We demonstrate, through a combination of climate model experiments and satellite observations, that the Pacific moist bias amplifies local long-wave cooling, which ultimately impacts regional circulation systems in the UTLS. Related impacts involve a strengthening of isentropic potential vorticity gradients, strengthened westerlies in the Pacific westerly duct region, and a zonally displaced anticyclonic monsoon circulation. Furthermore, we show that the regional Pacific moist bias can be significantly reduced by applying a Lagrangian, less-diffusive transport scheme and that such a model improvement could be important for improving the simulation of regional circulation systems, in particular in the Asian monsoon and Pacific region. [ABSTRACT FROM AUTHOR]

Cirrus clouds play an important role in the radiation budget of the Earth; nonetheless, the radiative effect of ultra-thin cirrus clouds in the tropopause region and in the lowermost stratosphere remains poorly constrained. These clouds have a small vertical extent and optical depth and are frequently neither observed even by sensitive sensors nor considered in climate model simulations. In addition, their short-wave (cooling) and long-wave (warming) radiative effects are often in approximate balance, and their net effect strongly depends on the shape and size of the cirrus particles. However, the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere instrument (CRISTA-2) allows ultra-thin cirrus clouds to be detected. Here we use CRISTA-2 observations in summer 1997 in the Northern Hemisphere midlatitudes together with the Suite Of Community RAdiative Transfer codes based on Edwards and Slingo (SOCRATES) radiative transfer model to calculate the radiative effect of observed ultra-thin cirrus. Using sensitivity simulations with different ice effective particle size and shape, we provide an estimate of the uncertainty in the radiative effect of ultra-thin cirrus in the extratropical lowermost stratosphere and tropopause region during summer and – by extrapolation of the summer results – for winter. Cloud top height and ice water content are based on CRISTA-2 measurements, while the cloud vertical thickness was predefined to be 0.5 or 2 km. Our results indicate that if the ice crystals of these thin cirrus clouds are assumed to be spherical, their net cloud radiative effect is generally positive (warming). In contrast, assuming aggregates or a hexagonal shape, their net radiative effect is generally negative (cooling) during summer months and very likely positive (warming) during winter. The radiative effect is in the order of ± (0.1–0.01) Wm-2 for a realistic global cloud coverage of 10 %, similar to the magnitude of the contrail cirrus radiative forcing (of ∼ 0.1 Wm-2). The radiative effect is also dependent on the cloud vertical extent and consequently the optically thickness and effective radius of the particle size distribution (e.g. effective radius increase from 5 to 30 µm results in a factor ∼ 6 smaller long- and short-wave effects, respectively). The properties of ultra-thin cirrus clouds in the lowermost stratosphere and tropopause region need to be better observed, and ultra-thin cirrus clouds need to be evaluated in climate model simulations. [ABSTRACT FROM AUTHOR]

Water vapour in the upper troposphere and lower stratosphere is a key radiative agent and crucial factor in the Earth's climate system. The largest observed moisture anomaly in the lower stratosphere occurs in boreal summer in the Asian monsoon region, but global climate models face problems with simulating this moisture pattern and show a common regional moist bias above the Pacific. We demonstrate from combination of climate model experiments and atmospheric observations that the enhanced moisture in the Pacific lower stratosphere critically impacts regional circulation systems by inducing local longwave cooling. Related impacts involve a strengthening of isentropic potential vorticity gradients, strengthened westerlies in the Pacific westerly duct region, and a zonally extended anticyclonic monsoon circulation. Hence, improving regional biases in climate model simulated stratospheric water vapour appears to be an important factor for improving simulation of regional circulation systems, in particular in the Asian monsoon and Pacific region. [ABSTRACT FROM AUTHOR]

Tropical weather phenomena—including tropical cyclones (TCs) and equatorial waves—are influenced by planetary‐to‐convective‐scale processes; yet, existing data sets and tools can only capture a subset of those processes. This study introduces a convection‐permitting aquaplanet simulation that can be used as a laboratory to study TCs, equatorial waves, and their interactions. The simulation was produced with the Model for Prediction Across Scales‐Atmosphere (MPAS‐A) using a variable resolution mesh with convection‐permitting resolution (i.e., 3‐km cell spacing) between 10°S and 30°N. The underlying sea‐surface temperature is given by a zonally symmetric profile with a peak at 10°N, which allows for the formation of TCs. A comparison between the simulation and satellite, reanalysis, and airborne dropsonde data is presented to determine the realism of the simulated phenomena. The simulation captures a realistic TC intensity distribution, including major hurricanes, but their lifetime maximum intensities may be limited by the stronger vertical wind shear in the simulation compared to the observed tropical Pacific region. The simulation also captures convectively coupled equatorial waves, including Kelvin waves and easterly waves. Despite the idealization of the aquaplanet setup, the simulated three‐dimensional structure of both groups of waves is consistent with their observed structure as deduced from satellite and reanalysis data. Easterly waves, however, have peak rotation and meridional winds at a slightly higher altitude than in the reanalysis. Future studies may use this simulation to understand how convectively coupled equatorial waves influence the multi‐scale processes leading to tropical cyclogenesis. Plain Language Summary: Despite many advancements in the science and prediction of tropical cyclones (TCs), scientists are still trying to explain the most important processes governing the evolution and characteristics of TCs. An emerging area of focus is how atmospheric oscillations, known as Kelvin waves, may increase the likelihood that a disturbance (often times referred to as an easterly wave) may become a TC. However, available atmospheric data sets are unable to capture all the fine details of TCs, disturbances, and Kelvin waves. To alleviate this challenge, this study presents a computer simulation of an Earth‐like atmosphere except without continents or seasons. The simplicity of the simulation allows scientists to study the full life cycle of TCs—from their formation to their evolution into powerful hurricanes. Results of this study show that the simulated TCs, Kelvin waves, and easterly waves resemble those that happen in nature. Therefore, the simulation can be used to advance our understanding of how TCs form and of how multi‐scale phenomena, such as Kelvin waves, affect the chances of TC formation at a particular location and time. Key Points: An aquaplanet simulation with convection‐permitting resolution in the tropics is presented as a tool to study tropical weather phenomenaThe structure of tropical cyclones (TCs) and equatorial waves is realistically captured despite the idealized configurationThe simulation may be used for fundamental process‐based studies of TCs, equatorial waves, and their interactions [ABSTRACT FROM AUTHOR]

The planetary boundary layer (PBL) height (PBLH) is an important parameter for weather, climate, and air quality models. Radiosonde is one of the most commonly used instruments for PBLH determination and is generally accepted as a standard for other methods. However, mainstream approaches for the estimation of PBLH from radiosonde present some uncertainties and even show disadvantages under some circumstances, and the results need to be visually verified, especially during the transition period of different PBL regimes. To avoid the limitations of individual methods and provide a benchmark estimation of PBLH, we propose an ensemble method based on high-resolution radiosonde data collected in Beijing in 2017. Seven existing methods including four gradient-based methods are combined along with statistical modification. The ensemble method is verified at 08:00, 14:00, and 20:00 Beijing time (BJT = UTC + 8), respectively. The overestimation of PBLH can be effectively eliminated by setting thresholds for gradient-based methods, and the inconsistency between individual methods can be reduced by clustering. Based on the statistics of a 1-year observational analysis, the effectiveness (E) of the ensemble method reaches up to 62.6 %, an increase of 6.5 %–53.0 % compared to the existing methods. Nevertheless, the ensemble method suffers to some extent from uncertainties caused by the consistent overestimation of PBLH, the profiles with a multi-layer structure, and the intermittent turbulence in the stable boundary layer (SBL). Finally, this method has been applied to characterize the diurnal and seasonal variations of different PBL regimes. Particularly, the average convective boundary layer (CBL) height is found to be the highest in summer, and the SBL is lowest in summer with about 200 m. The average PBLH at the transition stage lies around 1100 m except in winter. These findings imply that the ensemble method is reliable and effective. [ABSTRACT FROM AUTHOR]

Cirrus clouds play an important role in the radiation budget of the Earth. Despite recent progress in remote sensing observations of cirrus in general, the radiative impact of thin cirrus clouds in the tropopause region and in the lowermost stratosphere remains poorly constrained. This is due to their small vertical extent and optical depth, which make them very difficult to observe for most instruments. In addition, their shortwave (cooling) and longwave (warming) radiative effects (RE) are often in approximate balance, which together with uncertainties regarding the shape and size of cirrus particles, make their overall impact on climate difficult to quantify. In this study the SOCRATES (Suite Of Community RAdiative Transfer codes based on Edwards and Slingo) radiative transfer model was used to calculate the shortwave and longwave RE for observed thin cirrus during the second space shuttle mission by the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA-2) instrument. Unusual high cloud top heights with respect to the tropopause and rather high occurrence rates have been retrieved in earlier studies. However, the question remained open if these optically ultra thin cirrus clouds (UTC), so far not considered in global model calculation, are of importance for the Earth's radiation budget. Using sensitivity simulations with different ice effective particle size and shape, we provide an uncertainty range for the RE of UTCs in the lowermost stratosphere and tropopause region during both summer and winter months. Cloud top height and ice water content are based on CRISTA-2 retrievals, while the cloud vertical thicknesses were assumed to be 0.5 or 2 km. Our results indicate that if the ice crystals of these thin cirrus clouds are assumed to be spherical, then their net RE is generally positive (warming). In contrast, if they are assumed to be aggregates, a less likely habit for this high altitude cirrus type, then their net RE is generally negative (cooling) during summer months and positive (warming) during winter months. The cooling or warming RE is in the order of ±(0.1–0.01) W/m2 for a realistic global cloud coverage of 10 %, similar to the magnitude of the contrail cirrus radiative forcing best estimate of ~0.1 W/m2. RE is also dependent on the cloud vertical extent and consequently the optically thickness and effective radius (R eff) of the particle size distribution (e.g. R eff increase from 10 to 30 µ m results in a factor ≃ 3 smaller short and longwave effects). We argue that the radiative impact of UTC clouds in the lowermost stratosphere and tropopause region needs to be better addressed in observations and need to be taken into account in climate simulations. [ABSTRACT FROM AUTHOR]

The Tropical West Pacific is recognized as an important region for stratosphere-troposphere exchange, but has been a measurement gap in the global ozone sounding network. The Palau Atmospheric Observatory (PAO) was established to study the atmospheric composition above the remote Tropical West Pacific with a comprehensive instrumental setup. Since 2016, two laboratory containers in Palau host an Fourier-transform infrared spectrometer, a lidar (micro lidar until 2016, cloud and aerosol lidar from 2018), a Pandora 2S photometer and laboratory space for weather balloon soundings with ozone-, water-vapor-, aerosol- and radiosondes. In this analysis, we focus on the continuous, fortnightly ozone sounding program with Electrochemical Concentration Cell (ECC) ozone sondes. The aim of this study is to introduce the PAO and its research potential, present the first observation of the typical seasonal cycle of tropospheric ozone in the Tropical West Pacific based on a multiannual record of in situ observations, and investigate major drivers of variability and seasonal variation from 01/2016 until 12/2021 related to the large scale atmospheric circulation. We present the PAO ozone (O3) volume mixing ratios (VMR) and relative humidity (RH) time series complemented by other observations. The site is exposed to year-round high convective activity reflected in dominating low O3 VMR and high RH. In 2016, the impact of the strong El Niño is evident as a particularly dry, ozone-rich episode. The main modulator of annual tropospheric O3 variability is identified as the movement of the Intertropical Convergence Zone (ITCZ), with lowest O3 VMR in the free troposphere during the ITCZ position north of Palau. An analysis of the relation of O3 and RH for the PAO and selected sites from the Southern Hemispheric ADditional OZonesondes (SHADOZ) network reveals three different regimes. Palau's O3/RH distribution resembles the one in Fiji, Java and American Samoa, but is unique in its seasonality and its comparably narrow Gaussian distribution around low O3 VMR and the evenly distributed RH. A previously found bimodal distribution of O3 VMR and RH could not be seen in the Palau record. Due to its unique remote location, Palau is an ideal atmospheric background site to detect changes in air dynamics imprinted on the chemical composition of the tropospheric column. The efforts to establish, run and maintain the PAO have succeeded to fill an observational gap in the remote Tropical West Pacific and give good prospects for ongoing operations. The ECC sonde record will be integrated into the SHADOZ database in the near future. [ABSTRACT FROM AUTHOR]

The radiative balance of the upper atmosphere is dependent on the magnitude and distribution of greenhouse gases and aerosols in that region. Climate models predict that with increasing surface temperature, the primary mechanism for transporting tropospheric air into the stratosphere (known as the Brewer–Dobson circulation) will strengthen, leading to changes in the distribution of atmospheric water vapor, other greenhouse gases, and aerosols. Stratospheric relationships between greenhouse gases and other long-lived trace gases with various photochemical properties (such as N 2 O, SF 6 , and chlorofluorocarbons) provide a strong constraint for tracking changes in the stratospheric circulation. Therefore, a cost-effective approach is needed to monitor these trace gases in the stratosphere. In the past decade, the balloon-borne AirCore sampler developed at NOAA's Global Monitoring Laboratory has been routinely used to monitor the mole fractions of CO 2 , CH 4 , and CO from the ground to approximately 25 km above mean sea level. Our recent development work adapted a gas chromatograph coupled with an electron capture detector (GC-ECD) to measure a suite of trace gases (N 2 O, SF 6 , CFC-11, CFC-12, H-1211, and CFC-113) in the stratospheric portion of AirCores. This instrument, called the StratoCore-GC-ECD, allows us to retrieve vertical profiles of these molecules at high resolution (5–7 hPa per measurement). We launched four AirCore flights and analyzed the stratospheric air samples for these trace gases. The results showed consistent and expected tracer–tracer relationships and good agreement with recent aircraft campaign measurements. Our work demonstrates that the StratoCore-GC-ECD system provides a low-cost and robust approach to measuring key stratospheric trace gases in AirCore samples and for evaluating changes in the stratospheric circulation. [ABSTRACT FROM AUTHOR]

We report here on a system developed to automatically measure the flow rate characteristics (i.e., the pump efficiency) of pumps on ozonesondes under various pressure levels simulating upper-air conditions. The system consists of a flow measurement unit incorporating a polyethylene airbag, a pressure control unit that reproduces low-pressure environmental conditions, and a control unit that integrates and controls these elements to enable fully automatic measurement. The Japan Meteorological Agency (JMA) has operationally measured pump efficiency for electrochemical concentration cell (ECC) ozonesondes using the system since 2009, resulting in a significant amount of related data. Extensive measurements collected for the same ozonesonde pump over a period of around 12 years indicate the long-term stability of the system's performance. These long-term data also show that ozonesonde pump flow characteristics differed among production lots. Evaluation of the impacts of variance in these characteristics on observed ozone concentration data indicated that the influence on total ozone estimation was up to approximately 4 %, the standard deviation per lot was approximately 1 %, and the standard deviation among lots was approximately 0.6 %. [ABSTRACT FROM AUTHOR]

We analyze cirrus cloud measurements from two dual-instrument cloud spectrometers, two hygrometers and a backscattersonde in view to connect cirrus optical parameters usually accessible by remote sensing with microphysical size resolved and bulk properties accessible in situ. Specifically, we compare the particle backscattering coefficient and depolarization ratio to the particle size distribution, effective and mean radius, surface area density, particle aspherical fraction and ice water content. Data have been acquired by instruments on board the M55 Geophysica research aircraft during July and August 2017 during the Asian Monsoon campaign based in Kathmandu, Nepal, in the framework of the StratoClim (Stratospheric and upper tropospheric processes for better climate predictions) project. Cirrus have been observed over the Hymalaian region between 10 km and the tropopause, situated at 17–18 km. The observed particle number densities varied between 10 and 10-4 cm -3 in the dimensional range from 1.5 to 468.5 μ m in radius. Correspondingly, backscatter ratios from one tenth up to 50 have been observed. Optical scattering theory has been used to compare the backscattering coefficient computed from measured particle size distribution with those directly observed by the backscattersonde. The aspect ratio of the particles, modeled as spheroids for the T-matrix approach, was left as a free parameter to match the calculations to the optical measures. The computed backscattering coefficient can be set in good agreement with the observed one, but the match between simulated and determined depolarization ratios is insufficient, however. Relationships between ice particle concentration, mean and effective radius, surface area density and ice water content with the measured backscattering coefficient are investigated for an estimate of the bulk microphysical parameters of cirrus clouds from remote sensing lidar data. The comparison between particle depolarization and aspherical fraction as measured by one of the cloud spectrometers equipped with a detector for polarization, represents a novelty since it was the first time the two instruments are operated simultaneously on aircraft. The analysis shows the difficulty of establishing an univocal link between depolarization values and the presence and amount of aspherical scatterers. This suggests the need of further investigation that could take into consideration not only the fraction of aspheric particles but also their predominant morphology. [ABSTRACT FROM AUTHOR]

The radiative balance of the upper atmosphere is dependent on the magnitude and distribution of greenhouse gases and aerosols in that region of the atmosphere. Climate models predict that with increasing surface temperature, the primary mechanism for transporting tropospheric air into the stratosphere (known as the Brewer-Dobson Circulation) will strengthen, leading to changes in the distribution of atmospheric water vapor, other greenhouse gases, and aerosols in this region. Stratospheric relationships between greenhouse gases and other long-lived trace gases with various photochemical properties (such as N2O, SF6, and chlorofluorocarbons) provide a strong constraint for tracking changes in the stratospheric circulation. Therefore, a cost-effective approach is needed to monitor these trace gases in the stratosphere. In the past decade, the balloon-borne AirCore sampler developed at NOAA/GML has been routinely used to monitor the mole fractions of CO2, CH4, and CO from ground to approximately 25 km above mean sea level. Our recent development work adapted a gas chromatograph coupled with an electron capture detector (GC-ECD) to measure a suite of trace gases (N2O, SF6, CFC-11, CFC-12, H-1211, and CFC-113) in the stratospheric portion of AirCores. This instrument, called the StratoCore-GC-ECD, allows us to retrieve vertical profiles of these molecules at high resolution (5–7 hPa per measurement). We then launched four AirCore flights and analyzed the stratospheric air samples for these trace gases. The results showed consistent and expected tracer-tracer relationships and good agreement with recent aircraft campaign measurements. Our work demonstrates that the StratoCore-GC-ECD system provides a low-cost and robust approach to measuring key stratospheric trace gases in AirCore samples and for evaluating changes in the stratospheric circulation. [ABSTRACT FROM AUTHOR]

Hydrogen chloride (HCl) is the main reservoir species of chlorine and chemical decomposition of nitrous oxide (N2O) is the primary source of NOx (=NO + NO2) in the stratosphere. Changes in stratospheric HCl and N2O play a critical role in modulating variations in stratospheric ozone. Thus, long-term trends in stratospheric HCl and N2O have been investigated in many studies, whereas short-term changes have not received enough attention. Here, using satellite observations and a chemical transport model, we found that two extreme change events for HCl and N2O in the Northern Hemisphere mid-latitude middle and lower stratosphere have occurred over past decades, which are characterized by a sharp increase in HCl and a decrease in N2O over several months; for example, HCl increased (and N2O decreased) by 0.135 ppbv (−33.352 ppbv) in 1987/1988 and by 0.196 ppbv (−28.553 ppbv) in 2010/2011. Further analysis shows that the extreme change events of stratospheric HCl and N2O in these two periods are closely related to anomalous residual circulation caused by the joint effects of the strong easterly phase of the semi-annual oscillation and the strong polar vortex. [ABSTRACT FROM AUTHOR]

We describe and validate a method of calculating convective cloud top altitudes and potential temperatures up to 50° latitude at high spatial (0.25°) and temporal (3 hr) resolution. The approach uses the statistics of the CloudSat cloud radar deep convective cloud classification product coupled with nighttime CALIOP lidar measurements to effectively "calibrate" the high frequency, high resolution global rainfall and brightness temperature data that is used to derive convective cloud top altitudes. Thus, our product agrees well with the statistics of the CloudSat/CALIOP convective cloud tops, especially in the tropics and over oceanic regions. Agreement is reasonable, but not as good, for land‐based convection. The estimated uncertainty in cloud top altitudes in our product is 0.5–1.0 km over land areas, with smaller uncertainties over the oceans. The diurnal cycle of the new convection data set is in good agreement with precipitation radar convective climatology. Plain Language Summary: Convection modifies the water vapor concentration and injects trace gases into the upper troposphere and the lower stratosphere. To quantify the impact of convection, we have developed a data set of convective cloud top altitudes. Our algorithm uses lidar and cloud radar data to calibrate a combination of rainfall and geostationary satellite brightness temperature data to produce a database of convective cloud top altitudes. The statistics of our high‐resolution convective cloud top altitudes agree well with the statistics of other more limited convective cloud top data sets, especially over the oceans and in the tropics. Key Points: A high resolution data set of convective cloud top altitudes is calculated based on rainfall and brightness temperature observationsThe frequency statistics and diurnal cycle of this data set show good agreement with other observationsThe uncertainty in the data set of convective cloud tops is 0.5–1.0 km, or 5–15 K potential temperature [ABSTRACT FROM AUTHOR]

The Organization of Tropical East Pacific Convection (OTREC) field campaign, conducted August through October 2019, focuses on studying convection in the eastern Pacific and the Caribbean. An unprecedented number of dropsondes were deployed (648) during 22 missions to study the region of strong sea surface temperature (SST) gradients in the eastern Pacific region, the region just off the coast of Columbia, and in the uniform SST region in the southwestern Caribbean. The dropsondes were assimilated in the European Centre for Medium-Range Weather Forecasts (ECMWF) model. This study quantifies departures, observed minus the model value of a variable, in dropsonde denial experiments and studies time series of convective variables, saturation fraction which measures moisture and instability index and deep convective inhibition which quantify atmospheric stability and boundary layer stability to convection, respectively. Departures are small whether dropsondes are assimilated or not, except in a special case of developing convection and organization prior to Tropical Storm Ivo where wind departures are significantly larger when dropsondes are not assimilated. Departures are larger in cloudy regions compared to cloud-free regions when comparing a vertically integrated departure with a cloudiness estimation. Abovementioned variables are all well represented by the model when compared to observations, with some systematic deviations in and above the boundary layer. Time series of these variables show artificial convective activity in the model, in the eastern Pacific region off the coast of Costa Rica, which we hypothesize occurs due to the overestimation of moisture content in that region. [ABSTRACT FROM AUTHOR]

The relative importance of two processes that help control the concentrations of stratospheric water vapor, the Quasi-Biennial Oscillation (QBO) and El Niño–Southern Oscillation (ENSO), are evaluated in observations and in comprehensive coupled ocean–atmosphere-chemistry models. The possibility of nonlinear interactions between these two is evaluated both using multiple linear regression (MLR) and three additional advanced machine learning techniques. The QBO is found to be more important than ENSO; however nonlinear interactions are nonnegligible, and even when ENSO, the QBO, and potential nonlinearities are included, the fraction of entry water vapor variability explained is still substantially less than what is accounted for by cold-point temperatures. While the advanced machine learning techniques perform better than an MLR in which nonlinearities are suppressed, adding nonlinear predictors to the MLR mostly closes the gap in performance with the advanced machine learning techniques. Comprehensive models suffer from too weak a connection between entry water and the QBO; however a notable improvement is found relative to previous generations of comprehensive models. Models with a stronger QBO in the lower stratosphere systematically simulate a more realistic connection with entry water. [ABSTRACT FROM AUTHOR]

Sub-millimeter (200–1000 GHz) wavelengths contribute a unique capability to fill in the sensitivity gap between operational visible–infrared (VIS–IR) and microwave (MW) remote sensing for atmospheric cloud ice and snow. Being able to penetrate clouds to measure cloud ice mass and microphysical properties in the middle to upper troposphere, a critical spectrum range, is necessary for us to understand the connection between cloud ice and precipitation processes. As the first spaceborne 883 GHz radiometer, the IceCube mission was NASA's latest spaceflight demonstration of commercial sub-millimeter radiometer technology. Successfully launched from the International Space Station, IceCube is essentially a free-running radiometer and collected valuable 15-month measurements of atmosphere and cloud ice. This paper describes the detailed procedures for Level 1 (L1) data calibration, processing and validation. The scientific quality and value of IceCube data are then discussed, including radiative transfer model validation and evaluation, as well as the unique spatial distribution and diurnal cycle of cloud ice that are revealed for the first time on a quasi-global scale at this frequency. IceCube Level 1 dataset is publicly available at (10.25966/3d2p-f515). [ABSTRACT FROM AUTHOR]

The Stratosphere-troposphere Processes and their Role in Climate (SPARC) Data Initiative (SPARC, 2017) performed the first comprehensive assessment of currently available stratospheric composition measurements obtained from an international suite of space-based limb sounders. The initiative's main objectives were (1) to assess the state of data availability, (2) to compile time series of vertically resolved, zonal monthly mean trace gas and aerosol fields, and (3) to perform a detailed intercomparison of these time series, summarizing useful information and highlighting differences among datasets. The datasets extend over the region from the upper troposphere to the lower mesosphere (300–0.1 hPa) and are provided on a common latitude–pressure grid. They cover 26 different atmospheric constituents including the stratospheric trace gases of primary interest, ozone (O3) and water vapor (H2O), major long-lived trace gases (SF6 , N2O , HF , CCl3F , CCl2F2 , NOy), trace gases with intermediate lifetimes (HCl , CH4 , CO , HNO3), and shorter-lived trace gases important to stratospheric chemistry including nitrogen-containing species (NO , NO2 , NOx , N2O5 , HNO4), halogens (BrO , ClO , ClONO2 , HOCl), and other minor species (OH , HO2 , CH2O , CH3CN), and aerosol. This overview of the SPARC Data Initiative introduces the updated versions of the SPARC Data Initiative time series for the extended time period 1979–2018 and provides information on the satellite instruments included in the assessment: LIMS, SAGE I/II/III, HALOE, UARS-MLS, POAM II/III, OSIRIS, SMR, MIPAS, GOMOS, SCIAMACHY, ACE-FTS, ACE-MAESTRO, Aura-MLS, HIRDLS, SMILES, and OMPS-LP. It describes the Data Initiative's top-down climatological validation approach to compare stratospheric composition measurements based on zonal monthly mean fields, which provides upper bounds to relative inter-instrument biases and an assessment of how well the instruments are able to capture geophysical features of the stratosphere. An update to previously published evaluations of O3 and H2O monthly mean time series is provided. In addition, example trace gas evaluations of methane (CH4), carbon monoxide (CO), a set of nitrogen species (NO , NO2 , and HNO3), the reactive nitrogen family (NOy), and hydroperoxyl (HO2) are presented. The results highlight the quality, strengths and weaknesses, and representativeness of the different datasets. As a summary, the current state of our knowledge of stratospheric composition and variability is provided based on the overall consistency between the datasets. As such, the SPARC Data Initiative datasets and evaluations can serve as an atlas or reference of stratospheric composition and variability during the "golden age" of atmospheric limb sounding. The updated SPARC Data Initiative zonal monthly mean time series for each instrument are publicly available and accessible via the Zenodo data archive (Hegglin et al., 2020). [ABSTRACT FROM AUTHOR]

Stratospheric intrusions are an important source of ozone (O3) in the troposphere. In this study, 17 years of O3 sounding data from the Hong Kong Observatory with a weekly sampling frequency are analyzed to identify stratospheric intrusions and quantify their impact on O3 enhancement events in springtime from 2004 to 2020. 24.7% of O3 enhancement events are related to stratospheric intrusion whereas 31.7% of intrusions lead to the enhancement of O3 in the lower troposphere. Occurrences of stratospheric intrusion in springtime are closely related to tropopause folding and tropospheric convective activities in subtropical regions. The Weather Research and Forecasting model coupled with Chemistry simulations are conducted with the integrated process analysis to quantify the impact of stratospheric intrusions on the enhancement of O3 in the lower free troposphere as well as near the surface under different synoptic patterns. Synoptic patterns associated with stratospheric intrusion‐driving O3 enhancements are classified into two categories: Saddle Point ("S") and Cold Front ("F"). While downward transport of O3‐enriched air from the enhancement layer exerts an important impact on surface O3 for synoptic pattern "S," the impact on surface O3 is limited for surface pattern "F." The impact of stratospheric intrusions together with entrainment on surface O3 may have a strong indication on the development of effective emission control strategies on surface O3 reduction in Hong Kong. Key Points: Stratospheric intrusion is an important factor to the enhancement of ozone (O3) in the lower troposphere observed over Hong Kong during springtimeOccurrence of stratospheric intrusion in the subtropical region is attributed to the large‐scale subsidence associated with the upper‐level subtropical jetThe impact of stratospheric intrusion can be extended to the atmospheric boundary layer, which may lead to the increase of surface O3 [ABSTRACT FROM AUTHOR]

From 27 July to 10 August 2017, the airborne StratoClim mission took place in Kathmandu, Nepal, where eight mission flights were conducted with the M-55 Geophysica up to altitudes of 20 km. New particle formation (NPF) was identified by the abundant presence of nucleation-mode aerosols, with particle diameters dp smaller than 15 nm, which were in-situ-detected by means of condensation nuclei (CN) counter techniques. NPF fields in clear skies as well as in the presence of cloud ice particles (dp > 3 µ m) were encountered at upper troposphere–lowermost stratosphere (UTLS) levels and within the Asian monsoon anticyclone (AMA). NPF-generated nucleation-mode particles in elevated concentrations (Nnm) were frequently found together with cloud ice (in number concentrations Nice of up to 3 cm -3) at heights between ∼ 11 and 16 km. From a total measurement time of ∼ 22.5 h above 10 km altitude, in-cloud NPF was in sum detected over ∼ 1.3 h (∼ 50 % of all NPF records throughout StratoClim). Maximum Nnm of up to ∼ 11 000 cm -3 was detected coincidently with intermediate ice particle concentrations Nice of 0.05–0.1 cm -3 at comparatively moderate carbon monoxide (CO) contents of ∼ 90–100 nmol mol -1. Neither under clear-sky nor during in-cloud NPF do the highest Nnm concentrations correlate with the highest CO mixing ratios, suggesting that an elevated pollutant load is not a prerequisite for NPF. Under clear-air conditions, NPF with elevated Nnm (> 8000 cm -3) occurred slightly less often than within clouds. In the presence of cloud ice, NPF with Nnm between 1500–4000 cm -3 was observed about twice as often as under clear-air conditions. NPF was not found when ice water contents exceeded 1000 µ mol mol -1 in very cold air (< 195 K) at tropopause levels. This indicates a reduction in NPF once deep convection is prevalent together with the presence of mainly liquid-origin ice particles. Within in situ cirrus near the cold point tropopause, recent NPF or intense events with mixing ration nnm larger than 5000 mg -1 were observed only in about 6 % of the in-cloud NPF data. In determining whether the cloud-internal NPF is attenuated or prevented by the microphysical properties of cloud elements, the integral radius (IR) of the ice cloud population turned out to be indicative. Neither the number of ice particles nor the free distance between the ice particles is clearly related to the NPF rate detected. While the increase in ice particles' mass per time dmdt is proportional to the IR and mainly due to the condensation of water vapour, additional condensation of NPF precursors proceeds at the expense of the NPF rate as the precursor's saturation ratio declines. Numerical simulations show the impact of the IR on the supersaturation of a condensable vapour, such as sulfuric acid, and furthermore illustrate that the IR of the cloud ice determines the effective limitation of NPF rates. [ABSTRACT FROM AUTHOR]

Increasing the temperature of the tropical cold-point region through heating by volcanic aerosols results in increases in the entry value of stratospheric water vapor (SWV) and subsequent changes in the atmospheric energy budget. We analyze tropical volcanic eruptions of different strengths with sulfur (S) injections ranging from 2.5 Tg S up to 40 Tg S using EVAens, the 100-member ensemble of the Max Planck Institute – Earth System Model in its low-resolution configuration (MPI-ESM-LR) with artificial volcanic forcing generated by the Easy Volcanic Aerosol (EVA) tool. Significant increases in SWV are found for the mean over all ensemble members from 2.5 Tg S onward ranging between [5, 160] %. However, for single ensemble members, the standard deviation between the control run members (0 Tg S) is larger than SWV increase of single ensemble members for eruption strengths up to 20 Tg S. A historical simulation using observation-based forcing files of the Mt. Pinatubo eruption, which was estimated to have emitted (7.5±2.5) Tg S, returns SWV increases slightly higher than the 10 Tg S EVAens simulations due to differences in the aerosol profile shape. An additional amplification of the tape recorder signal is also apparent, which is not present in the 10 Tg S run. These differences underline that it is not only the eruption volume but also the aerosol layer shape and location with respect to the cold point that have to be considered for post-eruption SWV increases. The additional tropical clear-sky SWV forcing for the different eruption strengths amounts to [0.02, 0.65] Wm-2 , ranging between [2.5, 4] % of the aerosol radiative forcing in the 10 Tg S scenario. The monthly cold-point temperature increases leading to the SWV increase are not linear with respect to aerosol optical depth (AOD) nor is the corresponding SWV forcing, among others, due to hysteresis effects, seasonal dependencies, aerosol profile heights and feedbacks. However, knowledge of the cold-point temperature increase allows for an estimation of SWV increases of 12 % per Kelvin increase in mean cold-point temperature. For yearly averages, power functions are fitted to the cold-point warming and SWV forcing with increasing AOD. [ABSTRACT FROM AUTHOR]

Cirrus clouds contribute to the general radiation budget of the Earth and play an important role in climate projections. Of special interest are optically thin cirrus clouds close to the tropopause due to the fact that their impact is not yet well understood. Measuring these clouds is challenging as both high spatial resolution as well as a very high detection sensitivity are needed. These criteria are fulfilled by the infrared limb sounder GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere). This study presents a characterization of observed cirrus clouds using the data obtained by GLORIA aboard the German research aircraft HALO during the WISE (Wave-driven ISentropic Exchange) campaign in September and October 2017. We developed an optimized cloud detection method based on the cloud index and the extinction coefficient retrieved at the microwindow 832.4–834.4 cm-1. We derived macro-physical characteristics of the detected cirrus clouds such as cloud top height, cloud bottom height, vertical extent and cloud top position with respect to the tropopause. The fraction of cirrus clouds detected above the tropopause is on the order of 13 % to 27 %. In general, good agreement with the clouds predicted by the ERA5 reanalysis dataset is obtained. However, cloud occurrence is ≈ 50 % higher in the observations for the region close to and above the tropopause. Cloud bottom heights are also detected above the tropopause. However, considering the uncertainties, we cannot confirm the formation of unattached cirrus layers above the tropopause. [ABSTRACT FROM AUTHOR]

According to Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) satellite precipitation composites, a broad maritime area over the far eastern tropical Pacific and western Colombia houses one of the rainiest spots on Earth. This study aims to present a suite of mechanistic drivers that help create such a world‐record‐breaking rainy spot. Previous research has shown that this oceanic and nearly continental precipitation maximum has a strong early morning precipitation peak and a high density of mesoscale convective systems. We examined new and unique observational evidence highlighting the role of both dynamical and thermodynamical drivers in the activation and duration of organized convection. Results showed the existence of a rather large combination of mechanisms, including: (1) dynamics of the Choco (ChocoJet) and Caribbean Low‐Level Jets along their confluence zone, including the Panama semi‐permanent low; (2) ChocoJet deceleration offshore is favored by land breeze, enhancing the nighttime and early morning low‐level convergence; (3) a wind sheared environment that conforms to the long‐lived squall line theory; (4) action of mid‐level gravity waves, which further support the strong diurnal variability; and (5) mesoscale convective vortices related to subsidence in the stratiform region and top‐heavy mass flux profiles. This study emphasizes the multiscale circulation and thermodynamics mechanisms associated with the formation of one of the rainiest spots on Earth and showcases new observations gathered during the Organization of Tropical East Pacific Convection field campaign (OTREC; August–September, 2019) that support the outlined mechanisms. Key Points: One of the rainiest spots on Earth is located offshore in the far eastern tropical Pacific and linked to Mesoscale Convective SystemsMesoscale Convective Systems are related to days with enhanced Choco low‐level jetThe Organization of Tropical East Pacific Convection field campaign helped elucidate the role of local circulation on Mesoscale Convective Systems development [ABSTRACT FROM AUTHOR]

The cloud particle concentration, size, and shape data from optical array probes (OAPs) are routinely used to parameterise cloud properties and constrain remote sensing retrievals. This paper characterises the optical response of OAPs using a combination of modelling, laboratory, and field experiments. Significant uncertainties are found to exist with such probes for ice crystal measurements. We describe and test two independent methods to constrain a probe's sample volume that remove the most severely mis-sized particles: (1) greyscale image analysis and (2) co-location using stereoscopic imaging. These methods are tested using field measurements from three research flights in cirrus. For these cases, the new methodologies significantly improve agreement with a holographic imaging probe compared to conventional data-processing protocols, either removing or significantly reducing the concentration of small ice crystals (< 200 µ m) in certain conditions. This work suggests that the observational evidence for a ubiquitous mode of small ice particles in ice clouds is likely due to a systematic instrument bias. Size distribution parameterisations based on OAP measurements need to be revisited using these improved methodologies. [ABSTRACT FROM AUTHOR]

Data from recent field programs studying deep convection may be useful in constraining cumulus parameterizations. To this end, gridded dropsonde analyses are made using data from the OTREC (Organization of Tropical East Pacific Convection) and PREDICT (PreDepression Investigation of Cloud‐Systems in the Tropics) projects to characterize the mesoscale properties of tropical oceanic convection in terms of selected thermodynamic parameters computable from the explicit grids of large‐scale models. In particular, saturation fraction, lower tropospheric moist convective instability, and convective inhibition appear to govern column‐integrated moisture convergence, while sea surface temperature is related to the top‐heaviness of mass flux profiles and the integrated entropy divergence. Local (as opposed to global) surface heat and moisture fluxes and convective available potential energy correlate weakly with these quantities. Recommendations to improve cumulus parameterizations are enumerated. Plain Language Summary: Observations of the atmosphere from two field programs over tropical oceans are used to determine the characteristics of rain‐producing clouds in these regions. In particular, we are interested in what types of temperature and humidity profiles in the atmosphere promote rainfall and which types suppress it. This information is useful in designing and testing of treatments of clouds and rain in global weather and climate models. As a result of this research, we present a set of recommendations to modelers involved in this work. Key Points: Observations of emergent properties of convection from the OTREC and PREDICT field programs are reportedThree environmental parameters govern integrated moisture convergenceThe top‐heaviness of mass flux profiles and moist entropy divergence increase with increasing sea surface temperature [ABSTRACT FROM AUTHOR]

The amount of ice injected into the tropical tropopause layer has a strong radiative impact on climate. A companion paper (Part 1) used the amplitude of the diurnal cycle of ice water content (IWC) as an estimate of ice injection by deep convection, showed that the Maritime Continent (MariCont) region provides the largest injection to the upper troposphere (UT; 146 hPa) and to the tropopause level (TL; 100 hPa). This study focuses on the MariCont region and extends that approach to assess the processes, the areas and the diurnal amount and duration of ice injected over islands and over seas during the austral convective season. The model presented in the companion paper is again used to estimate the amount of ice injected (Δ IWC) by combining ice water content (IWC) measured twice a day by the Microwave Limb Sounder (MLS; Version 4.2) from 2004 to 2017 and precipitation (Prec) measurements from the Tropical Rainfall Measurement Mission (TRMM; Version 007) binned at high temporal resolution (1 h). The horizontal distribution of Δ IWC estimated from Prec (Δ IWC Prec) is presented at 2 ∘×2∘ horizontal resolution over the MariCont. Δ IWC is also evaluated by using the number of lightning events (Flash) from the TRMM-LIS instrument (Lightning Imaging Sensor, from 2004 to 2015 at 1 h and 0.25 ∘ × 0.25 ∘ resolution). Δ IWC Prec and Δ IWC estimated from Flash (Δ IWC Flash) are compared to Δ IWC estimated from the ERA5 reanalyses (Δ IWC ERA5) with the vertical resolution degraded to that of MLS observations ΔIWCERA5. Our study shows that the diurnal cycles of Prec and Flash are consistent with each other in phase over land but different over offshore and coastal areas of the MariCont. The observational Δ IWC range between Δ IWC Prec and Δ IWC Flash , interpreted as the uncertainty of our model in estimating the amount of ice injected, is smaller over land (where Δ IWC Prec and Δ IWC Flash agree to within 22 %) than over ocean (where differences are up to 71 %) in the UT and TL. The impact of the MLS vertical resolution on the estimation of Δ IWC is greater in the TL (difference between Δ IWC ERA5 and ΔIWCERA5 of 32 % to 139 %, depending on the study zone) than in the UT (difference of 9 % to 33 %). Considering all the methods, in the UT, estimates of Δ IWC span 4.2 to 10.0 mg m -3 over land and 0.4 to 4.4 mg m -3 over sea, and in the TL estimates of Δ IWC span 0.5 to 3.9 mg m -3 over land and 0.1 to 0.7 mg m -3 over sea. Finally, based on IWC from MLS and ERA5, Prec and Flash, this study highlights that (1) at both levels, Δ IWC estimated over land can be more than twice that estimated over sea and (2) small islands with high topography present the largest Δ IWC (e.g., island of Java). [ABSTRACT FROM AUTHOR]

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Stratospheric water vapor (SWV) is recognized as a potentially important positive feedback in global warming. The SWV change induces significant downward radiative flux perturbation at the tropopause and therefore is hypothesized to substantially amplify the surface warming. To test this hypothesis, we use a global climate model to quantify the surface warming contributed by the SWV change in the context of the quadrupled CO2. By prescribing the SWV increase as an external forcing, we find that SWV only accounts for 0.42 K surface warming, making up merely 5.4% of the total CO2‐caused surface warming (7.7 K). The efficacy of the stratosphere‐adjusted SWV forcing is small (38%), where the efficacy is defined as the ratio of the global temperature response per unit radiative forcing relative to that of the CO2 forcing. With the aid of a series of auxiliary experiments, we find that although the stratosphere‐adjusted SWV forcing at the top of atmosphere (TOA) is significant (1.13 W m−2), more than half of the forcing is offset by a high‐cloud decrease and an upper tropospheric warming in the tropospheric adjustment. The direct radiative impact of the SWV increase on surface temperature is negligible, and the SWV‐induced surface temperature change is a result of interactions between the radiative and nonradiative processes. Plain Language Summary: What role does the stratospheric water vapor play in global warming? Previous works demonstrated that stratospheric water vapor increase exerts significant downward radiative flux perturbation at the tropopause. This paper investigates whether this SWV radiative perturbation can substantially amplify the surface warming. Our simulation shows a negative answer to this question. The surface warming attributed to the stratospheric water vapor change is small, due to the effect of the tropospheric adjustment. The stratospheric water vapor increase causes a decrease in the high clouds and an increase in the upper tropospheric temperature, which offsets more than half of the stratospheric water vapor‐induced radiative perturbation. Key Points: The surface warming efficacy of the stratosphere‐adjusted SWV forcing is low (38%)SWV increase accounts for about 5% of total warming in the context of CO2‐driven global warmingMore than half of the stratosphere‐adjusted SWV forcing is offset by the high cloud decrease and the upper tropospheric warming in the tropospheric forcing adjustment [ABSTRACT FROM AUTHOR]

As knowledge about the cirrus clouds in the lower stratosphere is limited, reliable long-term measurements are needed to assess their characteristics, radiative impact and important role in upper troposphere and lower stratosphere (UTLS) chemistry. We used 6 years (2006–2012) of Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) measurements to investigate the global and seasonal distribution of stratospheric cirrus clouds and compared the MIPAS results with results derived from the latest version (V4.x) of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data. For the identification of stratospheric cirrus clouds, precise information on both the cloud top height (CTH) and the tropopause height is crucial. Here, we used lapse rate tropopause heights estimated from the ERA-Interim global reanalysis. Considering the uncertainties of the tropopause heights and the vertical sampling grid, we define CTHs more than 0.5 km above the tropopause as stratospheric for CALIPSO data. For MIPAS data, we took into account the coarser vertical sampling grid and the broad field of view so that we considered cirrus CTHs detected more than 0.75 km above the tropopause as stratospheric. Further sensitivity tests were conducted to rule out sampling artefacts in MIPAS data. The global distribution of stratospheric cirrus clouds was derived from night-time measurements because of the higher detection sensitivity of CALIPSO. In both data sets, MIPAS and CALIPSO, the stratospheric cirrus cloud occurrence frequencies are significantly higher in the tropics than in the extra-tropics. Tropical hotspots of stratospheric cirrus clouds associated with deep convection are located over equatorial Africa, South and Southeast Asia, the western Pacific, and South America. Stratospheric cirrus clouds were more often detected in December–February (15 %) than June–August (8 %) in the tropics (±20 ∘). At northern and southern middle latitudes (40–60 ∘), MIPAS observed about twice as many stratospheric cirrus clouds (occurrence frequencies of 4 %–5 % for MIPAS rather than about 2 % for CALIPSO). We attribute more frequent observations of stratospheric cirrus clouds with MIPAS to the higher detection sensitivity of the instrument to optically thin clouds. In contrast to the difference between daytime and night-time occurrence frequencies of stratospheric cirrus clouds by a factor of about 2 in zonal means in the tropics (4 % and 10 %, respectively) and at middle latitudes for CALIPSO data, there is little diurnal cycle in MIPAS data, in which the difference of occurrence frequencies in the tropics is about 1 percentage point in zonal mean and about 0.5 percentage point at middle latitudes. The difference between CALIPSO day and night measurements can also be attributed to their differences in detection sensitivity. Future work should focus on better understanding the origin of the stratospheric cirrus clouds and their impact on radiative forcing and climate. [ABSTRACT FROM AUTHOR]

Stratospheric water vapour (SWV), in spite of its low concentration in the stratosphere as compared to the troposphere, contributes significantly to the surface energy budget and can have an influence on the surface climate. This study investigates the dynamical processes that determine SWV on interannual to decadal time‐scales. First, we evaluate two SWV reanalysis products and show that SWV is better represented in a new‐generation reanalysis product, ERA5, than in its predecessor, ERA‐Interim. In particular, it is shown that SWV in ERA5 is highly consistent with observational data obtained from the SPARC Data Initiative Multi‐Instrument Mean (SDI MIM). Second, we investigate the variability of tropical SWV and its relationship to dynamical stratospheric variables. The analyses show that the interannual variability in the tropical lower‐stratospheric water vapour is closely linked to the tropical Quasi‐Biennial Oscillation (QBO). When westerlies occupy the middle stratosphere and easterlies the lower stratosphere, a decrease is observed in lower‐stratospheric water vapour due to a colder tropical tropopause and a QBO‐induced enhanced residual circulation. On decadal time‐scales, the composite analysis of the boreal winter in two typical periods shows that less SWV is related to a warm anomaly in the North Atlantic sea‐surface temperature, which leads to stronger upward propagation of planetary wave activity at high latitudes, a weaker polar vortex and an enhanced residual circulation. The opposite occurs during periods with higher concentrations of SWV. [ABSTRACT FROM AUTHOR]