Refractory black carbon (rBC) is a primary aerosol species, produced through incomplete combustion, that absorbs sunlight and contributes to positive radiative forcing. The overall climate effect of rBC depends on its spatial distribution and atmospheric lifetime, both of which are impacted by the efficiency with which rBC is transported or removed by convective systems. These processes are poorly constrained by observations. It is especially interesting to investigate rBC transport efficiency through the Asian Summer Monsoon (ASM) since this meteorological pattern delivers vast quantities of boundary layer air from Asia, where rBC emissions are high, to the upper troposphere/lower stratosphere (UT/LS) where the lifetime of rBC is expected to be long. Here we present in-situ observations of rBC made during the Asian Summer Monsoon Chemistry and Climate Impact Project (ACCLIP) of summer, 2022. We use observed relationships between rBC and CO in ASM outflow to show that rBC is removed nearly completely (>98%) from uplifted air, and that rBC concentrations in ASM outflow are statistically indistinguishable from the UT/LS background. We compare observed rBC and CO concentrations to those expected based on two chemical transport models and find that the models reproduce CO to within a factor of 2 at all altitudes while rBC is overpredicted by a factor of 20-100 at altitudes associated with ASM outflow. We find that the rBC particles in recently convected air have thinner coatings than those found in the UTLS background, suggesting non-zero transport of rBC number that is not relevant to concentration.

The Asian Summer Monsoon (ASM) has garnered attention in recent years for its impacts on the composition of the upper troposphere and lower stratosphere (UTLS) via deep convection. A recent observational effort into this mechanism, the Asian summer monsoon Chemical and CLimate Impact Project (ACCLIP), sampled the composition of the ASM UTLS over the northwestern Pacific during boreal summer 2022 using two airborne platforms. In this work, we integrate Lagrangian trajectory modeling with convective cloud top observations to diagnose ASM convective transport which contributed to ACCLIP airborne observations. This diagnostic is applied to explore the properties of convective transport associated with prominent ASM sub-systems, revealing that convective transport along the East Asia Subtropical Front generally contained more pollutants than from South Asia, for species ranging in lifetime from days to months. The convective transport diagnostic is used to isolate three convective transport events over eastern Asia which had distinct chemical tracer relationship slopes, indicating the different economical behaviors of the contributing source regions. One of these transport events is explored in greater detail, where a polluted air mass was sampled from convection over the Northeast China Plain. This event was largely confined to 12-15 km altitude, which may be high enough to impact the composition of the stratosphere. Overall, the presented diagnosis of convective transport contribution to ACCLIP airborne sampling indicates a key scientific success of the campaign and enables process studies of the climate interactions from the two ASM sub-system.

During the Asian Summer Monsoon Chemical and Climate Impact Project (ACCLIP), National Aeronautics and Space Administration (NASA) WB-57 high altitude research aircraft observed strong turbulence at an altitude of 15 to 18 km near the outflow of the Super Typhoon Hinnamnor in the Northwest Pacific. Our analyses based on the trial version of the “RRJ-ClimCORE” mesoscale reanalysis have revealed that a thin layer of strong vertical wind shear (VWS) extending just below the tropopause along the upper surface of the outflow from the typhoon eyewall provides favorable conditions for shear instability. Further analyses of the mesoscale background conditions based on RRJ-ClimCORE as well as the geostationary satellite Himawari-8 cloud imagery suggest that the convolution of such small-scale processes as convectively generated concentric gravity waves and shallow convection appearing as radially-banded cirrus clouds may have further increased the potential for turbulence generation. Absolute vorticity analysis suggests the occurrence of inertial instability to the north of the typhoon, which explains the development of radially-banded cirrus clouds as well as the layer of strong VWS around the flight altitudes. The present study thus demonstrates the advantages of the hourly output of RRJ-ClimCORE over 3-hourly output operational mesoscale analysis for investigating source mechanisms of turbulence in the upper-troposphere and lower-stratosphere.

We calculate the climate forcing for the two years after the January 15, 2022, Hunga Tonga-Hunga Ha’apai (Hunga) eruption. We use satellite observations of stratospheric aerosols, trace gases and temperatures to compute the tropopause radiative flux changes relative to climatology. Overall, the net downward radiative flux decreased compared to climatology. Although the Hunga stratospheric water vapor anomaly increases the downward infrared radiative flux, the solar flux reduction due to Hunga aerosol shroud dominates the net flux over most of the two-year period. Decreases in temperature produced by the Hunga stratospheric circulation changes contributes to the decrease in downward flux; however, the Hunga induced decrease in ozone increases the net short-wave downward flux creating small sub-tropical net flux increase in late 2022. Coincident with the aerosols settling out, the water vapor anomaly disperses, and circulation changes disappear so that the contrasting forcings all decrease together. By the end of 2023, most of the Hunga induced radiative forcing changes have disappeared. There is some disagreement in the satellite stratospheric aerosol optical depth (SAOD) which we view as a measure of the uncertainty; however, SAOD uncertainty does not alter our conclusion that, overall, aerosols dominate the radiative flux changes followed by temperature and ozone.

Carbonyl sulfide (OCS) is an important atmospheric sulfur species that plays a dominant role in the formation of (non-volcanic) stratospheric sulfate aerosol in the middle stratosphere. Major uncertainties in surface sources and sinks and inconsistent model representation of vertical transport limit understanding of OCS distribution, particularly in the sparsely sampled upper atmosphere. During the 2022 Asian Summer Monsoon Chemical & CLimate Impact Project (ACCLIP) campaign in-situ measurements of OCS in the Upper Troposphere and Lower Stratosphere (UTLS) at the eastern edge of the Asian Summer Monsoon anticyclone (ASM), showed significant OCS enhancements (>750 ppt) near the tropopause from convectively influenced air parcels. Here we compare these novel Asian UTLS measurements with long term satellite observations and regional measurements to broaden understanding of OCS trends and its transport by the ASM. Trajectory analysis identifies the main source regions for deep convective lofting and demonstrates ASM entrainment of OCS enriched parcels in the UTLS, allowing evaluation of global model predictions for OCS’s stratospheric influence. The ACCLIP dataset provides vital in-situ validation of limited vertically resolved OCS data in a region of significant anthropogenic emissions, which serves to enhance our understanding of the global sulfur budget.

On Jan. 15, 2022, the Hunga Tonga-Hunga Ha’apai (HT) eruption injected SO2 and water into the middle stratosphere. Shortly after the eruption, the water vapor anomaly moved northward toward and across the equator. This northward movement appears to be due to a Rossby wave forced by the excessive IR water vapor cooling. Following the early eruption stage, persistent mid-stratospheric water vapor and aerosol layers were mostly confined to Southern Hemisphere (SH) tropics (Eq. to 30°S). However, during the spring of 2022, the westerly phase of the tropical quasi-biennial oscillation (QBO) descended through the tropics. The HT water vapor and aerosol anomalies were observed to again split across the equator coincident with the descent of the QBO shear zone. This split occurred because of the enhanced meridional transport circulation associated with the QBO. Neither transport event can be reproduced using MERRA2 assimilated winds.

Water vapor in the stratosphere is primarily controlled by temperatures in the tropical upper troposphere and lower stratosphere. However, the direct impact of deep convection on the global lower stratospheric water vapor budget is still an actively debated issue. Two complementary modeling approaches are used to investigate the convective impact in boreal winter and summer. Backward trajectory model simulations coupled with a detailed treatment of cloud microphysical processes indicate that convection moistens the global lower stratosphere by approximately 0.3 ppmv (~10% increase) in boreal winter and summer 2010. The diurnal peak in convection is responsible for about half of the total convective moistening during boreal winter and nearly all of the convective moistening during boreal summer. Deep convective cloud tops overshooting the local tropopause have relatively minor effect on global lower stratospheric water vapor (~1% increase). A forward trajectory model coupled with a simplified cloud module is used to esimate the relative magnitude of the interannual variability of the convective impact during 2006-2016. Combing the results from the two models, we find that the convective impact on the global lower stratospheric water vapor during 2006-2016 is approximately 0.3 ppmv with year-to-year variations of up to 0.1 ppmv. The dominant mechanism of convective hydration of the lower stratosphere is via the detrainment of saturated air and ice into the tropical uppermost troposphere. Convection shifts the relative humidity distribution of subsaturated air parcels in the upper troposphere toward higher relative humidity values, thereby increasing the water vapor in the stratosphere.

On Jan. 15, 2022, the Hunga Tonga-Hunga Ha’apai eruption injected SO2 and H2O into the middle stratosphere. The eruption produced a persistent mid-stratospheric sulfate aerosol layer, mostly below 26 km, confined to Southern Hemisphere (SH) tropics. Coincident with, and slightly above the aerosol layer, an enhanced H2O layer is also observed. The SH tropical confinement is simulated using a trajectory model. Measurements over several months following the eruption show that the H2O layer is slowly rising while the aerosol layer is descending. The H2O layer upward movement is consistent with the vertical velocity at these altitudes. Gravitationally settling explains the descent of the aerosol layer. A cold anomaly coincident with the H2O enhancement is observed and is caused by thermal adjustment to the additional H2O IR cooling. A simple model of volcanic water injection at the time of the eruption simulates the observed H2O.

We describe our Solar Aerosol and Gas Experiment (SAGE) III/ISS cloud detection algorithm. As in previous SAGE II/III studies this algorithm uses the extinction at 1022 nm and the extinction color ratio 520nm/1022nm to separate aerosols and clouds. We identify three types of clouds: visible cirrus (extinction coefficient > 3x10-2 km-1, subvisible cirrus (extinction < 3x10-2 km-1 and >10-3 km-1), and very low extinction cloud-aerosol mixtures (extinction < 10-3 km-1). Visible cirrus cannot be quantitatively measured by SAGE because of its high extinction, but we infer the presence of cirrus through the solar attenuation of the SAGE vertical scan. We then assume that cirrus layers extend 0.5 km below the scan termination height. SAGE cirrus cloud fraction estimated this way is in qualitative agreement with CALIPSOmeasurements. Analyzing three years of SAGE III/ISS data, we find that visible cirrus and subvisible cirrus have nearly equal abundance in the tropical upper troposphere and the average cloud fraction is about 25%. At 16 km, the highest concentration visible cirrus and subvisible cirrus is over the Tropical West Pacific, central Africa and central South America during winter. Latitudinal gaps in zonal mean cloud fraction and average aerosol extinction apparent in the subtropical transition region are aligned with descending branch of the residual mean circulation. We also identify four anomalous aerosol extinction periods that can be tentatively assigned to significant volcanic or fire events. Using tropopause relative coordinates, we show that maximum cloud top heights are consistently restricted to a narrow region near the tropopause.