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This study presents airborne in situ and satellite remote sensing climatologies of cirrus clouds and humidity. The climatologies serve as a guide to the properties of cirrus clouds, with the new in situ database providing detailed insights into boreal midlatitudes and the tropics, while the satellite-borne data set offers a global overview. To this end, an extensive, quality-checked data archive, the Cirrus Guide II in situ database, is created from airborne in situ measurements during 150 flights in 24 campaigns. The archive contains meteorological parameters, ice water content (IWC), ice crystal number concentration (Nice), ice crystal mean mass radius (Rice), relative humidity with respect to ice (RHice), and water vapor mixing ratio (H2O) for each of the flights. Depending on the parameter, the database has been extended by about a factor of 5–10 compared to earlier studies. As one result of our investigation, we show that the medians of Nice, Rice, and RHice have distinct patterns in the IWC–T parameter space. Lookup tables of these variables as functions of IWC and T can be used to improve global model cirrus representation and remote sensing retrieval methods. Another outcome of our investigation is that across all latitudes, the thicker liquid-origin cirrus predominate at lower altitudes, while at higher altitudes the thinner in situ-origin cirrus prevail. Further, examination of the radiative characteristics of in situ-origin and liquid-origin cirrus shows that the in situ-origin cirrus only slightly warm the atmosphere, while liquid-origin cirrus have a strong cooling effect. An important step in completing the Cirrus Guide II is the provision of the global cirrus Nice climatology, derived by means of the retrieval algorithm DARDAR-Nice from 10 years of cirrus remote sensing observations from satellite. The in situ measurement database has been used to evaluate and improve the satellite observations. We found that the global median Nice from satellite observations is almost 2 times higher than the in situ median and increases slightly with decreasing temperature. Nice medians of the most frequently occurring cirrus sorted by geographical regions are highest in the tropics, followed by austral and boreal midlatitudes, Antarctica, and the Arctic. Since the satellite climatologies enclose the entire spatial and temporal Nice occurrence, we could deduce that half of the cirrus are located in the lowest, warmest (224–242 K) cirrus layer and contain a significant amount of liquid-origin cirrus. A specific highlight of the study is the in situ observations of cirrus and humidity in the Asian monsoon anticyclone and the comparison to the surrounding tropics. In the convectively very active Asian monsoon, peak values of Nice and IWC of 30 cm−3 and 1000 ppmv are detected around the cold point tropopause (CPT). Above the CPT, ice particles that are convectively injected can locally add a significant amount of water available for exchange with the stratosphere. We found IWCs of up to 8 ppmv in the Asian monsoon in comparison to only 2 ppmv in the surrounding tropics. Also, the highest RHice values (120 %–150 %) inside of clouds and in clear sky are observed around and above the CPT. We attribute this to the high H2O mixing ratios (typically 3–5 ppmv) observed in the Asian monsoon compared to 1.5 to 3 ppmv found in the tropics. Above the CPT, supersaturations of 10 %–20 % are observed in regions of weak convective activity and up to about 50 % in the Asian monsoon. This implies that the water available for transport into the stratosphere might be higher than the expected saturation value.

This study investigates the sources and regional attributions of nitrogen oxides (NOx) in the upper troposphere|upper tropospheric (UT) during the Asian Summer Monsoon (ASM). The importance of South Asia (SA) and East Asia (EA) contributions is the subject of main interest. Using artificial tracers in a chemistry‐climate model, simulations with tracers from surface anthropogenic and lightning sources in SA and EA are conducted. Model results are validated with airborne observations from the Asian Summer Monsoon Chemical and Climate Impact Project (ACCLIP) campaign in 2022 over the West Pacific. Good agreement between modeled and observed NOx is found in the UT. The results indicate that within the ASM anticyclone, both SA and EA sources significantly contribute to the UT NOx, with contributions of 41% and 36%, respectively. While in the ACCLIP region during 2022, EA sources play a more important role, accounting for 50% compared to 19% from SA sources. Plain Language Summary Nitrogen oxides (NOx) in the upper troposphere (UT) are involved in various processes that are important for the greenhouse gas budget, with both cooling and warming effects. NOx is also a significant component that produces ozone through chemical smog processes. The ASM is associated with an enhancement of NOx in the UT, as it is an efficient pathway to transport the surface NOx emission upward along with strong lightning NOx production from deep convection. However, the specific contributions from various sources and regions to UT NOx remain unclear. Our study employs model simulations to elucidate the origins of NOx, particularly within the ASM and its eastward shedding region. We find that within the ASM anticyclone, both South Asia and East Asia sources are significant contributors to the UT NOx, with 47% from anthropogenic and 30% from lightning sources, respectively. In the Asian Summer Monsoon Chemical and Climate Impact Project region, EA sources become prominent, accounting for 50% of the contribution, with anthropogenic sources outweighing lightning by approximately 30% in UT NOx contribution. Key Points A chemistry‐climate model with a nitrogen oxides (NOx) tagging mechanism is used to examine upper troposphere|upper tropospheric (UT) NOx origins Both South Asia and East Asia (EA) sources are significant UT NOx contributors within the Asian Summer Monsoon (ASM) anticyclone For the observed NOx during the 2022 Asian Summer Monsoon Chemical and Climate Impact Project campaign, EA contributions are prominent and especially from anthropogenic sources

Lagrangian modeling of transport, as implemented in the Chemical Lagrangian Model of the Stratosphere (CLaMS), connects the advective (reversible) component of transport along 3D trajectories with mixing, the irreversible component. Here, we investigate the interplay between strongly localized convective uplifts and large-scale flow dynamics in the upper troposphere and lower stratosphere (UTLS). We revisit the Lagrangian formulation of convection in CLaMS-3.0/MESSy, driven by ECMWF's ERA5 reanalysis, and further develop the model. These developments include refining spatial resolution in the Planetary Boundary Layer (PBL) and decoupling the frequency of the adaptive grid procedure - which captures isentropic mixing and redefines Lagrangian air parcels - from the parameterization of convection.

The Asian summer monsoon (ASM) anticyclone circulation system is recognized to be a significant transport pathway for water vapor and pollutants to enter the stratosphere. The observational evidence, however, is largely based on satellite retrievals. We report the first coincident in situ measurements of water vapor and ozone within the ASM anticyclone. The combined water vapor and ozonesondes were launched from Kunming, China in August 2009 and Lhasa, China in August 2010. In total, 11 and 12 sondes were launched in Kunming and Lhasa, respectively. We present the key characteristics of these measurements, and provide a comparison to similar measurements from an equatorial tropical location, during the Tropical Composition, Cloud and Climate Coupling (TC4) campaign in July and August of 2007. Results show that the ASM anticyclone region has higher water vapor and lower ozone concentrations in the upper troposphere and lower stratosphere than the TC4 observations. The results also show that the cold point tropopause in the ASM region has a higher average height and potential temperature. The in situ observations therefore support the satellite‐based conclusion that the ASM is an effective transport pathway for water vapor to enter stratosphere. Key Points First in situ measurements of water vapor and ozone within the ASM anticyclone ASM anticyclone has higher water vapor and lower ozone in the UTLS ASM region has a higher cold point tropopause level

Airborne observations from ACCLIP on 2 August 2022, combined with Lagrangian particle dispersion model back trajectories, reveal that SO2 mixing ratios at 14–16 km were enhanced by a factor of 4–6 in regions influenced by tropical cyclones (TCs). These enhancements are linked to rapid lofting of marine dimethyl sulfide (DMS) into the upper troposphere (UT). GEOS‐Chem simulations indicated that on 31 July 2022, TC‐scale circulation injected DMS into the UT within hours, with a mean flux of 9.4 kg hr−1 across 0.5–12 km and 8.4% of emissions penetrating above 12 km, consistent with observations of elevated DMS at the same altitudes. Because of its low solubility and longer UT lifetime (59.2 vs. 5.7 hr at the surface), DMS sustains SO2 production that is largely resistant to wet scavenging. This TC‐driven pathway provides a significant natural SO2 source in the UT, with implications for aerosol–cloud–climate interactions.

We present data and analysis of a set of balloon‐borne sounding profiles, which includes co‐located O3, CO, CH4, and particles, over the northern Tibetan Plateau during an Asian summer monsoon (ASM) season. These novel measurements shed light on the ASM transport behavior near the northern edge of the anticyclone. Joint analyses of these species with the temperature and wind profiles and supported by back trajectory modeling identify three distinct transport processes that dominate the vertical chemical structure in the middle troposphere, upper troposphere (UT), and the tropopause region. The correlated changes in profile structures in the middle troposphere highlight the influence of the strong westerly jet. Elevated constituent concentrations in the UT identify the main level of convective transport at the upstream source regions. Observed higher altitude maxima for CH4 characterize the airmasses' continued ascent following convection. These data complement constituent observations from other parts of the ASM anticyclone. Plain Language Summary Asian summer monsoon deep convection transports surface pollutants to the stratosphere. Although satellite data have provided clear evidence of this transport, in situ measurements are critical for characterizing how monsoon is vertically re‐distributing the regional emissions. We report new balloon‐borne measurements over the Tibetan Plateau that provide a unique data set on the northern edge of the anticyclone, complementing other observations. Key Points A novel set of in‐situ profile measurements of O3, CO, CH4 and particles from Tibetan Plateau during Asian summer monsoon are presented Joint analyses of the profiles provide insights into transport processes controlling the northern edge of the Asian monsoon anticyclone Observed CO profile maxima at 13–14 km (∼360–370 K) identify the level of convective transport at the upstream source regions

The Asian summer monsoon (ASM) creates a hemispheric-scale signature in trace-gas distributions in the upper troposphere and lower stratosphere (UTLS). Data from satellite retrievals are the best source of information for characterizing these large-scale signatures. Measurements from the Microwave Limb Sounder (MLS), a limb-viewing satellite sensor, have been the most widely used retrieval products for these types of studies. This work explores the information for the ASM influence on UTLS chemical distribution from two nadir-viewing sensors, the Infrared Atmospheric Sounding Interferometer (IASI) and the Ozone Monitoring Instrument (OMI), together with the MLS. Day-to-day changes in carbon monoxide (CO) and ozone (O3) tracer distributions in response to dynamical variability are examined to assess how well the data from different sensors provide useful information for studying the impact of sub-seasonal-scale dynamics on chemical fields. Our results, using June–August 2008 data, show that although the MLS provides relatively sparse horizontal sampling on daily timescales, interpolated daily CO distributions show a high degree of dynamical consistency with the synoptic-scale structure of and variability in the anticyclone. Our analysis also shows that the IASI CO retrieval has sufficient sensitivity to produce upper tropospheric (UT) CO with variabilities independent from the lower to middle tropospheric CO. The consistency of IASI CO field with the synoptic-scale anticyclone dynamical variability demonstrates that the IASI UT CO product is a physically meaningful dataset. Furthermore, IASI CO vertical cross sections combined with the daily maps provide the first observational evidence for a model analyses-based hypothesis on the preferred ASM vertical transport location and the subsequent horizontal redistribution via east–west eddy shedding. Similarly, the OMI O3 profile product is shown to be capable of distinguishing the tropospheric-dominated air mass in the anticyclone from the stratospheric-dominated background on a daily timescale, providing consistent and complementary information to the MLS. These results not only highlight the complementary information between nadir and limb sensors but also demonstrate the value of “process-based” retrieval evaluation for characterizing satellite data information content.

Isentropic mixing across and above the subtropical jet is a significant mechanism for stratosphere–troposphere exchange. In this work, we show new observational evidence on the role of this process in moistening the lowermost stratosphere. The new measurement, obtained from the Spatial Heterodyne Observations of Water (SHOW) instrument during a demonstration flight on the NASA's ER-2 high-altitude research aircraft, captured an event of poleward water vapour transport, including a fine-scale (vertically <∼1 km) moist filament above the local tropopause in a high-spatial-resolution two-dimensional cross section of the water vapour distribution. Analysis of these measurements combined with ERA5 reanalysis data reveals that this poleward mixing of air with enhanced water vapour occurred in the region of a double tropopause following a large Rossby wave-breaking event. These new observations highlight the importance of high-resolution measurements in resolving processes that are important to the lowermost-stratosphere water vapour budget.

1. Overview The upper troposphere-lower stratosphere （UTLS） of the Asian summer monsoon （ASM） region is characterized by a continental-scale anticyclonic circulation, which is dynami- cally active and coupled to monsoonal convection. The monsoon anticyclone exhibits anomalous chemical and aerosol characteristics, linked to the outflow of deep convection and the large-scale circulation, and strongly influences the global UTLS composition during boreal summer.

We evaluate ozone profile retrievals from the Atmospheric Infrared Sounder (AIRS), the Infrared Atmospheric Sounding Interferometer (IASI), and the Ozone Monitoring Instrument (OMI) using in situ measurements collected on board the NSF/NCAR Gulfstream‐V aircraft during the Stratosphere‐Troposphere Analyses of Regional Transport 2008 (START08) experiment. The focus of this study is to examine how well the satellite retrieval products capture the ozone gradients and variability in the extratropical upper troposphere lower stratosphere (UTLS). The AIRS retrieval examined is version 5, while IASI and OMI retrievals are research products. All satellite instruments show excellent ability in capturing synoptic‐scale ozone gradients associated with strong potential vorticity (PV) gradients. The positive ozone‐PV correlation near the tropopause is also well represented in the satellite data in comparison to collocated aircraft measurements. During aircraft cruise legs, more than 90% of collocated satellite retrievals agree with aircraft measurements within ±50% for ozone mixing ratios greater than 200 ppbv. Below 200 ppbv, AIRS and IASI retrievals show significant positive biases, while OMI shows both positive and negative biases. Ozone gradients across the tropopause are well‐captured, with median values within 30% (positive for AIRS and IASI, negative for OMI) and variances within ±50%. Ozone variability in the UTLS is captured by the satellite retrievals at the 80% level. In the presence of high clouds, however, the infrared retrievals show the largest positive biases. Despite the limited vertical information content, the high horizontal coverage and long‐term data availability make these satellite data sets a valuable asset for UTLS research.