Aura Microwave Limb Sounder Research Articles

Long‐Term Observations of Upper‐Tropospheric Cloud Ice From the MLS

AbstractUpper tropospheric cloud ice varies across different timescales and plays an important role in regulating Earth's climate, but knowledge of the abundances and the variability of cloud ice has been limited for decades. The Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument onboard the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) provides an unprecedented record of cloud ice measurements since its launch in April 2006. However, CALIPSO has left the A‐Train in September 2018 and entered the C‐Train orbit which follows a slightly different ground track with a different local crossing time. This orbit change challenges the continuation of a long‐term record of cloud ice since ice is subject to stronger diurnal cycle. Fortunately, the Aura Microwave Limb Sounder (MLS), still as a member of the A‐Train, has a consistent local crossing time and has measured high quality radiance since launch in 2004, and will probably continue beyond 2024. We present the use of MLS 640‐GHz cloud‐induced radiance (Tcir) to build a robust upper tropospheric partial ice water path (pIWP) product due to its high dynamical range to different sizes of ice particles. The MLS rebuilt pIWP, which extends nearly two decades, captures the spatial and temporal variabilities of upper tropospheric cloud ice that CALIOP is capable of measuring. This provides valuable alternative for studying the upper tropospheric cloud ice and possibly provides a more consistent input to climate studies.

How Atmospheric Chemistry and Transport Drive Surface Variability of N2O and CFC‐11

AbstractNitrous oxide (N2O) is a long‐lived greenhouse gas that affects atmospheric chemistry and climate. In this work, we use satellite measurements of N2O, ozone (O3), and temperature from the Aura Microwave Limb Sounder (MLS) instrument to calculate stratospheric loss of N2O, and thus its atmospheric lifetime. Using three chemistry transport models simulating the Aura period 2005–2017, we verify the stratospheric sink using MLS data and follow that loss signal down to the surface and compare with surface observations. Stratospheric loss has a strong seasonal cycle and is further modulated by the Quasi‐Biennial Oscillation (QBO); these cycles are seen equally in both observations and the models. When filtered for interannual variability, the modeled surface signal is QBO‐caused, and it reproduces the observed pattern, highlighting the potential role of the QBO in tropospheric chemistry and composition, as well as in model evaluation. The observed annual surface signal in the northern hemisphere matches well with the models run without emissions, indicating the annual cycle is driven mostly by stratosphere‐troposphere exchange (STE) flux of N2O‐depleted air and not surface N2O emissions. In the southern hemisphere (SH), all three models disagree and thus provide no guidance, except for indicating that modeling annual STE in the SH remains a major model uncertainty. Parallel model simulations of CFCl3, which has greater stratospheric loss that N2O and possibly surreptitious emissions, show that its interannual variations parallel those of N2O, and thus the observed N2O variability can identify the stratospheric component of the observed CFCl3variability.

Structure, dynamics, and trace gas variability within the Asian summer monsoon anticyclone in the extreme El Niño of 2015–2016

Abstract. A weak El Niño during 2014–2015 boreal winter developed as a strong boreal summer event in 2015 which continued and even enhanced during the following winter. In this work, the detailed changes in the structure, dynamics, and trace gases within the Asian summer monsoon anticyclone (ASMA) during the extreme El Niño of 2015–2016 is delineated by using Aura Microwave Limb Sounder (MLS) measurements, COSMIC radio occultation (RO) temperature, and National Centers for Environmental Prediction (NCEP) reanalysis products. Our analysis concentrates only on the summer months of July and August 2015 when the Niño 3.4 index started to exceed values of 1.5. The results show that the ASMA structure was quite different in summer 2015 as compared to the long-term (2005–2014) mean. In July, the spatial extension of the ASMA is greater than the long-term mean in all the regions except over northeastern Asia, where it exhibits a strong southward shift in its position. The ASMA splits into two, and the western Pacific mode is evident in August. Interestingly, the subtropical westerly jet (STJ) shifted southward from its normal position over northeastern Asia, and as a result midlatitude air moved southward in 2015. Intense Rossby wave breaking events along with STJ are also found in July 2015. Due to these dynamical changes in the ASMA, pronounced changes in the ASMA tracers are noticed in 2015 compared to the long-term mean. A 30 % (20 %) decrease in carbon monoxide (water vapor) at 100 hPa is observed in July over most of the ASMA region, whereas in August the drop is strongly concentrated at the edges of the ASMA. A prominent increase in O3 (> 40 %) at 100 hPa is clearly evident within the ASMA in July, whereas in August the increase is strongly located (even at 121 hPa) over the western edges of the ASMA. Further, the temperature around the tropopause shows significant positive anomalies (∼ 5 K) within the ASMA in 2015. The present results clearly reveal the El-Niño-induced dynamical changes caused significant changes in the trace gases within the ASMA in summer 2015.

Near‐Global Variability of Stratospheric Water Vapor Observed by SAGE III/ISS

AbstractThe Stratospheric Aerosol and Gas Experiment III instrument on the International Space Station (SAGE III/ISS) has been making high quality solar occultation measurements of stratospheric water vapor since June 2017. Here we evaluate the large‐scale geophysical variability of the SAGE III/ISS water vapor measurements for the first 3 years of observations (2017–2020) as part of data validation for retrieval version 5.1 (v5.1). Detailed comparisons of SAGE III/ISS v5.1 with the Aura Microwave Limb Sounder (MLS) version 5 retrievals show overall excellent agreement in terms of seasonal mean structure and large‐scale variability. SAGE III/ISS data capture the well‐known seasonal variations in water vapor including the vertically propagating “tape recorder” in the tropics and lower stratospheric maxima linked to the NH summer monsoons. The high vertical resolution (∼2 km) measurements from SAGE III/ISS demonstrate contributions of the monsoons to the wet phase of “tape recorder” during the Northern Hemisphere (NH) summer. Interannual variations over the short data record are also consistent between SAGE III/ISS and MLS in the stratosphere between 16 and 30 km. We furthermore evaluate large‐scale variations in relative humidity (RH) derived from the high vertical resolution SAGE III/ISS water vapor measurements, highlighting the detailed seasonal behavior and links to thermal structure near the tropical tropopause. Spatial distributions of RH at the cold point tropopause (CPT) show a close link between high RH and the minimum CPT temperature in both the NH winter and summer, consistent with temperature control of water vapor near the tropopause.

Planetary Wave Spectrum in the Stratosphere–Mesosphere during Sudden Stratospheric Warming 2018

The planetary wave activity in the stratosphere–mesosphere during the Arctic major Sudden Stratospheric Warming (SSW) in February 2018 is discussed on the basis of microwave radiometer (MWR) measurements of carbon monoxide (CO) above Kharkiv, Ukraine (50.0° N, 36.3° E) and the Aura Microwave Limb Sounder (MLS) measurements of CO, temperature and geopotential heights. From the MLS data, eastward and westward migrations of wave 1/wave 2 spectral components were differentiated, to which less attention was paid in previous studies. Abrupt changes in zonal wave spectra occurred with the zonal wind reversal near 10 February 2018. Eastward wave 1 and wave 2 were observed before the SSW onset and disappeared during the SSW event, when westward wave 1 became dominant. Wavelet power spectra of mesospheric CO variations showed statistically significant periods of 20–30 days using both MWR and MLS data. Although westward wave 1 in the mesosphere dominated with the onset of the SSW 2018, it developed independently of stratospheric dynamics. Since the propagation of upward planetary waves was limited in the easterly zonal flow in the stratosphere during SSW, forced planetary waves in the mid-latitude mesosphere may exist due to the instability of the zonal flow.

Validation and Trend Analysis of Stratospheric Ozone Data from Ground-Based Observations at Lauder, New Zealand

Changes in stratospheric ozone have to be assessed continuously to evaluate the effectiveness of the Montreal Protocol. In the southern hemisphere, few ground-based observational datasets exist, making measurements at the Network for the Detection of Atmospheric Composition Change (NDACC) station at Lauder, New Zealand invaluable. Investigating these datasets in detail is essential to derive realistic ozone trends. We compared lidar data and microwave radiometer data with collocated Aura Microwave Limb sounder (MLS) satellite data and ERA5 reanalysis data. The detailed comparison makes it possible to assess inhomogeneities in the data. We find good agreement between the datasets but also some possible biases, especially in the ERA5 data. The data uncertainties and the inhomogeneities were then considered when deriving trends. Using two regression models from the Long-term Ozone Trends and Uncertainties in the Stratosphere (LOTUS) project and from the Karlsruhe Institute of Technology (KIT), we estimated resulting ozone trends. Further, we assessed how trends are affected by data uncertainties and inhomogeneities. We find positive ozone trends throughout the stratosphere between 0% and 5% per decade and show that considering data uncertainties and inhomogeneities in the regression affects the resulting trends.

Comprehensive Study of the Climatology of the Quasi‐6‐Day Wave in the MLT Region Based on Aura/MLS Observations and SD‐WACCM‐X Simulations

AbstractThe westward quasi‐6‐day wave (Q6DW) with zonal wavenumber 1 is a prominent and recurrent phenomenon in the mesosphere and lower thermosphere (MLT) and has a significant impact on day‐to‐day ionospheric variability. Geopotential height measurements from Aura Microwave Limb Sounder and Specified Dynamics Whole Atmosphere Community Climate Model eXtended Version simulations during 2005–2019 are utilized to study the climatological variations of Q6DW. The spectral analysis clearly indicates that four typical Q6DW events occur in the MLT region before and after the two equinoxes in 1 year. The wave amplitudes in the summer hemisphere are considerably larger than the amplitudes in the winter hemisphere. The Eliassen‐Palm flux diagnostics show that the wave source of the post‐September equinox event is located in both hemispheres, while the sources of the other three Q6DW events are in the winter hemisphere. The diagnostic analysis results show that the climatological features of Q6DW are primarily due to the seasonal variations of the mean flow, which can determine the Q6DW critical layers, baroclinic/barotropic instability, and waveguides. Specifically, the Q6DW can be amplified in the summer hemisphere mesosphere at high latitudes during pre‐ and postequinox periods when the critical layers penetrate the unstable region. At the two equinoxes, the Q6DW can also propagate into the MLT region with similar amplitudes in both hemispheres due to the weak zonal mean flow but without additional amplification. At the two solstices, the Q6DW is suppressed because the critical layers envelop the whole unstable region, which prevents its amplification through wave‐mean flow interaction.

Extreme Outliers in Lower Stratospheric Water Vapor Over North America Observed by MLS: Relation to Overshooting Convection Diagnosed From Colocated Aqua-MODIS Data.

Convectively injected water vapor (HO) in the North American (NA) summer lowermost stratosphere results in significant outliers in the 100‐hPa HO measurements from the Aura Microwave Limb Sounder (MLS). MLS statistics from 15 years confirm that the NA region contains over 60% of global 100‐hPa HO > 12 ppmv, despite having only 1.8% of all MLS observations. A profile sampled in August 2019 stands out, with H2O=26.3 ppmv, far exceeding the prior record and the median 4.5‐ppmv abundance in NA. This particular outlier is associated with a large overshooting convective event (OCE) that spanned multiple U.S. states and persisted for several hours. Colocation of the MLS data over NA with cloud observations from Aqua's Moderate Resolution Imaging Spectroradiometer (MODIS) reveals the unique character of this case, as only 2.3% of MLS profiles are as close to an OCE and only 0.024% of OCEs cover as large an area within a 500‐km perimeter of a profile.

Australian New Year's PyroCb Impact on Stratospheric Composition

AbstractFollowing the Australian New Year's pyrocumulonimbus complex between 29 December 2019 and 4 January 2020, the Aura Microwave Limb Sounder (MLS) observed a plume with unprecedented enhancements of H2O and biomass‐burning products (CO, HCN, CH3Cl, CH3CN, and CH3OH) in the lower/middle stratosphere, accompanied by depressions in stratospheric species (O3and HNO3). The plume persisted for ∼110 days, circling the globe twice while ascending to 5.62 hPa (∼35 km). Air masses drawn off the main plume moved toward the developing Antarctic polar vortex but do not appear to have penetrated it. Comparison of species in the plume requires consideration of their measurements' spatial resolutions and background abundances. The apparent decay of some long‐lived plume constituents is largely attributable to their coarsening spatial resolution with height, which reduces observed peak values. Differing HCN/H2O signatures indicate that multiple early plumes originated from different stratospheric injection events.

Comparison of Major Sudden Stratospheric Warming Impacts on the Mid-Latitude Mesosphere Based on Local Microwave Radiometer CO Observations in 2018 and 2019

In this paper, a comparison of the impact of major sudden stratospheric warmings (SSWs) in the Arctic in February 2018 (SSW1) and January 2019 (SSW2) on the mid-latitude mesosphere is given. The mesospheric carbon monoxide (CO) and zonal wind in these two major SSW events were observed at altitudes of 70–85 km using a microwave radiometer (MWR) at Kharkiv, Ukraine (50.0°N, 36.3°E). Data from ERA-Interim and MERRA-2 reanalyses and Aura Microwave Limb Sounder measurements were also used. It is shown that: (i) The differences between SSW1 and SSW2, in terms of local variability in zonal wind, temperature, and CO in the stratosphere and mesosphere, were clearly defined by the polar vortex (westerly in cyclonic circulation) and mid-latitude anticyclone (easterly) migrating over the MWR station, therefore; (ii) mesospheric intrusions of CO-rich air into the stratosphere over the Kharkiv region occurred only occasionally, (iii) the larger zonal wave 1–3 amplitudes before SSW1 were followed by weaker polar vortex recovery than that after SSW2, (iv) the strong vortex recovery after SSW2 was supported by earlier event timing (midwinter) favoring vortex cooling due to low solar irradiance and enhanced zonal circulation, and (v) vortex strengthening after SSW2 was accompanied by wave 1–3 amplification in March 2019, which was absent after SSW1. Finally, the influence of the large-scale circulation structures formed in individual major SSW events on the locally recorded characteristics of the atmosphere is discussed.

Climatological impact of the Brewer–Dobson circulation on the N<sub>2</sub>O budget in WACCM, a chemical reanalysis and a CTM driven by four dynamical reanalyses

Abstract. The Brewer–Dobson circulation (BDC) is a stratospheric circulation characterized by upwelling of tropospheric air in the tropics, poleward flow in the stratosphere, and downwelling at mid and high latitudes, with important implications for chemical tracer distributions, stratospheric heat and momentum budgets, and mass exchange with the troposphere. As the photochemical losses of nitrous oxide (N2O) are well known, model differences in its rate of change are due to transport processes that can be separated into the mean residual advection and the isentropic mixing terms in the transformed Eulerian mean (TEM) framework. Here, the climatological impact of the stratospheric BDC on the long-lived tracer N2O is evaluated through a comparison of its TEM budget in the Whole Atmosphere Community Climate Model (WACCM), in a chemical reanalysis of the Aura Microwave Limb Sounder version 2 (BRAM2) and in a chemistry transport model (CTM) driven by four modern reanalyses: the European Centre for Medium-Range Weather Forecasts Interim reanalysis (ERA-Interim; Dee et al., 2011), the Japanese 55-year Reanalysis (JRA-55; Kobayashi et al., 2015), and the Modern-Era Retrospective analysis for Research and Applications version 1 (MERRA; Rienecker et al., 2011) and version 2 (MERRA-2; Gelaro et al., 2017). The effects of stratospheric transport on the N2O rate of change, as depicted in this study, have not been compared before across this variety of datasets and have never been investigated in a modern chemical reanalysis. We focus on the seasonal means and climatological annual cycles of the two main contributions to the N2O TEM budget: the vertical residual advection and the horizontal mixing terms. The N2O mixing ratio in the CTM experiments has a spread of approximately ∼20 % in the middle stratosphere, reflecting the large diversity in the mean age of air obtained with the same CTM experiments in a previous study. In all datasets, the TEM budget is closed well; the agreement between the vertical advection terms is qualitatively very good in the Northern Hemisphere, and it is good in the Southern Hemisphere except above the Antarctic region. The datasets do not agree as well with respect to the horizontal mixing term, especially in the Northern Hemisphere where horizontal mixing has a smaller contribution in WACCM than in the reanalyses. WACCM is investigated through three model realizations and a sensitivity test using the previous version of the gravity wave parameterization. The internal variability of the horizontal mixing in WACCM is large in the polar regions and is comparable to the differences between the dynamical reanalyses. The sensitivity test has a relatively small impact on the horizontal mixing term, but it significantly changes the vertical advection term and produces a less realistic N2O annual cycle above the Antarctic. In this region, all reanalyses show a large wintertime N2O decrease, which is mainly due to horizontal mixing. This is not seen with WACCM, where the horizontal mixing term barely contributes to the TEM budget. While we must use caution in the interpretation of the differences in this region (where the reanalyses show large residuals of the TEM budget), they could be due to the fact that the polar jet is stronger and is not tilted equatorward in WACCM compared with the reanalyses. We also compare the interannual variability in the horizontal mixing and the vertical advection terms between the different datasets. As expected, the horizontal mixing term presents a large variability during austral fall and boreal winter in the polar regions. In the tropics, the interannual variability of the vertical advection term is much smaller in WACCM and JRA-55 than in the other experiments. The large residual in the reanalyses and the disagreement between WACCM and the reanalyses in the Antarctic region highlight the need for further investigations on the modeling of transport in this region of the stratosphere.

Initial Results and Diurnal Variations Measured by a New Microwave Stratospheric ClO Instrument at Mauna Kea

AbstractWe present the first year of ClO and O3 measurements from a new 278 GHz Chlorine monOxide Experiment (ChlOE) microwave instrument on Mauna Kea. We make use of comparisons with the Aura Microwave Limb Sounder (MLS) O3 measurements to recalibrate the ground‐based instrument and find that the recalibrated ground‐based O3 retrievals remain stable relative to MLS throughout the year. The MLS measurements provide daily daytime (~1330 local solar time) and nighttime (~0200 local solar time) ClO retrievals, and we find that the monthly ChlOE ClO retrievals show mixing ratios slightly smaller than those from MLS in the vicinity of Mauna Kea, with daytime values near the peak ~6–8% lower at 30–45 km for the average of the 12 months in this study. The nighttime monthly ClO values are almost all within ±0.03 ppbv over most of this altitude range, with very little overall bias. We have also made detailed hourly diurnal O3 comparisons with a multiyear analysis of previous retrievals from the much stronger 110 GHz O3 emission line being measured by our instrument at Mauna Loa and find that the hour‐to‐hour differences are within ±2% at 40 km, which indicates that the overall calibration of the ChlOE instrument is stable relative to time of day. We show hourly diurnal variations of ClO and compare these to 2010 measurements from the Submillimeter‐Wave Limb‐Emission Sounder and to a model.

Effect of deep convection on the tropical tropopause layer composition over the southwest Indian Ocean during austral summer

Abstract. Balloon-borne measurements of cryogenic frost-point hygrometer (CFH) water vapor, ozone and temperature and water vapor lidar measurements from the Maïdo Observatory on Réunion Island in the southwest Indian Ocean (SWIO) were used to study tropical cyclones' influence on tropical tropopause layer (TTL) composition. The balloon launches were specifically planned using a Lagrangian model and Meteosat-7 infrared images to sample the convective outflow from tropical storm (TS) Corentin on 25 January 2016 and tropical cyclone (TC) Enawo on 3 March 2017. Comparing the CFH profile to Aura's Microwave Limb Sounder's (MLS) monthly climatologies, water vapor anomalies were identified. Positive anomalies of water vapor and temperature, and negative anomalies of ozone between 12 and 15 km in altitude (247 to 121 hPa), originated from convectively active regions of TS Corentin and TC Enawo 1 d before the planned balloon launches according to the Lagrangian trajectories. Near the tropopause region, air masses on 25 January 2016 were anomalously dry around 100 hPa and were traced back to TS Corentin's active convective region where cirrus clouds and deep convective clouds may have dried the layer. An anomalously wet layer around 68 hPa was traced back to the southeast Indian Ocean where a monthly water vapor anomaly of 0.5 ppmv was observed. In contrast, no water vapor anomaly was found near or above the tropopause region on 3 March 2017 over Maïdo as the tropopause region was not downwind of TC Enawo. This study compares and contrasts the impact of two tropical cyclones on the humidification of the TTL over the SWIO. It also demonstrates the need for accurate balloon-borne measurements of water vapor, ozone and aerosols in regions where TTL in situ observations are sparse.

Assessment of Observational Evidence for Direct Convective Hydration of the Lower Stratosphere

AbstractIn situ and remote sensing observations of water vapor are analyzed to assess the evidence for direct convective hydration of the lower stratosphere. We have examined several hundred balloon‐borne and airborne in situ measurements of lower stratospheric humidity in the tropics and northern midlatitudes. We find that the tropical lower stratospheric H2O enhancements above the background occur quite infrequently, and the height of the enhancements is within about 1 km of the cold‐point tropopause. Following Schwartz et al. (2013, https://doi.org/10.1002/grl.50421), we examine the anomalously high (above 8 ppmv) water vapor mixing ratios retrieved by the Aura Microwave Limb Sounder (MLS) at 100‐ and 82‐hPa pressure levels, and we determine their vertical location relative to the local tropopause based on both Global Forecast System (GFS) operational analysis and the ERA5 reanalysis temperature data. We find that essentially all of the >8‐ppmv MLS water vapor measurements over the extratropical North American monsoon region are above the relatively low lapse‐rate tropopause in the region, and most are above the local cold‐point tropopause. Over the Asian monsoon region, most (80/90%) of the high H2O values occur below the relatively high‐altitude local lapse‐rate/cold‐point tropopause. Anomalously high MLS water vapor retrievals at 100 and 82 hPa almost never occur in the deep tropics. We show that this result is consistent with the in situ observations given the broad vertical averaging kernel of the MLS measurement. The available evidence suggests that direct hydration of the lower stratosphere is important over North America during the monsoon season but likely has limited impact in the tropics.

An ensemble Kalman filter data assimilation system for the whole neutral atmosphere

Abstract. A data assimilation system with a four-dimensional local ensemble transform Kalman filter (4D-LETKF) is developed to make a new analysis dataset for the atmosphere up to the lower thermosphere using the Japanese Atmospherics General Circulation model for Upper Atmosphere Research. The time period from 10 January to 20 February 2017, when an international radar network observation campaign was performed, is focused on. The model resolution is T42L124, which can resolve phenomena at synoptic and larger scales. A conventional observation dataset provided by the National Centers for Environmental Prediction, PREPBUFR, and satellite temperature data from the Aura Microwave Limb Sounder (MLS) for the stratosphere and mesosphere are assimilated. First, the performance of the forecast model is improved by modifying the vertical profile of the horizontal diffusion coefficient and modifying the source intensity in the non-orographic gravity wave parameterization by comparing it with radar wind observations in the mesosphere. Second, the MLS observational bias is estimated as a function of the month and latitude and removed before the data assimilation. Third, data assimilation parameters, such as the degree of gross error check, localization length, inflation factor, and assimilation window, are optimized based on a series of sensitivity tests. The effect of increasing the ensemble member size is also examined. The obtained global data are evaluated by comparison with the Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) reanalysis data covering pressure levels up to 0.1 hPa and by the radar mesospheric observations, which are not assimilated.

Whole Atmosphere Model Simulations of Ultrafast Kelvin Wave Effects in the Ionosphere and Thermosphere

AbstractThis paper examines the response of the upper atmosphere to equatorial Kelvin waves with a period of ∼3 days, also known as ultrafast Kelvin waves (UFKWs). The whole atmosphere model Ground‐to‐topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) is used to simulate the UFKW events in the late summer of 2010 and 2011 as well as in the boreal winter of 2012/2013. When the lower layers of the model below 30‐km altitude are constrained with meteorological data, GAIA is able to reproduce salient features of the UFKW in the mesosphere and lower thermosphere as observed by the Aura Microwave Limb Sounder. The model also reproduces ionospheric response, as validated through comparisons with total electron content data from the Gravity field and steady‐state Ocean Circulation Explorer satellite as well as with earlier observations. Model results suggest that the UFKW produces eastward‐propagating ∼3‐day variations with zonal wavenumber 1 in the equatorial zonal electric field and F region plasma density. Model results also suggest that for a ground observer, identifying ionospheric signatures of the UFKW is a challenge because of ∼3‐day variations due to other sources. This issue can be overcome by combining ground‐based measurements from different longitudes. As a demonstration, we analyze ground‐based magnetometer data from equatorial stations during the 2011 event. It is shown that wavelet spectra of the magnetic data at different longitudes are only in partial agreement, with or without a ∼3‐day peak, but a spectrum analysis based on multipoint observations reveals the presence of the UFKW.

Erythemal Radiation, Column Ozone, and the North American Monsoon

AbstractRecently, Anderson et al. (2012, https://doi.org/10.1126/science.1222978, 2017, https://doi.org/10.1073/pnas.1619318114) and Anderson and Clapp (2018, https://doi.org/10.1039/C7CP08331A) proposed that summertime convectively injected water vapor over North America could lead to stratospheric ozone depletion through halogenic catalytic reactions. Such ozone loss would reduce the ozone column and increase erythemal daily dose (EDD). Using 10 years of observations over the North American monsoon region from the Aura Ozone Monitoring Instrument, we find that the column ozone and EDD has a ~0.8–0.9 spatial correlation with lower stratospheric water vapor measured by the Aura Microwave Limb Sounder. We show that this correlation appears to be due to the elevation of the monsoonal tropopause and associated monsoonal convection. The increase in tropopause altitude reduces the ozone column and increases EDD. We see no apparent evidence of substantial heterogeneous chemical ozone loss in lower stratospheric ozone coincident with the stratospheric monsoonal water vapor enhancement.

Role of Intra-Seasonal Variability in the Indian Summer Monsoon on the Hydration and Dehydration of the Upper Troposphere

Seven years of high-resolution measurements of upper tropospheric humidity (UTH) from the Indian Geostationary satellite, Kalpana-1, are used to understand the hydration and dehydration processes during the active and break phases of the Indian summer monsoon (ISM). The active and break phases are identified using the TRMM-3B42 daily rainfall data over the Central Indian region. The most significant and new observation is that the upper troposphere is dry during the active and wet during the break phases of the ISM. Satellite measurements of humidity from Megha-Tropiques SAPHIR and water vapor mixing ratio from Aura microwave limb sounder also show dryness in the upper troposphere during the active and wetness during the break phases of ISM. The analysis illustrates that dehydration dominates during the active and hydration during the break phases. Cloud-aerosol lidar with orthogonal polarization data shows that cirrus (sub-categorized as sub-visible, thin, and thick) occurrence is more during the active phase as compared with the break phase. The dehydration mechanism during the active phase of ISM is attributed to the presence of cirrus. This study provides evidence that the upper troposphere shows dehydration during the active and hydration during the break phase of the ISM, thereby exhibiting intra-seasonal variability in the UTH.

Dehydration and low ozone in the tropopause layer over the Asian monsoon caused by tropical cyclones: Lagrangian transport calculations using ERA-Interim and ERA5 reanalysis data

Abstract. Low ozone and high water vapour mixing ratios are common features in the Asian summer monsoon (ASM) anticyclone; however, low ozone and low water vapour values were observed near the tropopause over Kunming, China, within the ASM using balloon-borne measurements performed during the SWOP (sounding water vapour, ozone, and particle) campaign in August 2009 and 2015. Here, we investigate low ozone and water vapour signatures in the upper troposphere and lower stratosphere (UTLS) using FengYun-2D, FengYun-2G, and Aura Microwave Limb Sounder (MLS) satellite measurements and backward trajectory calculations. Trajectories with kinematic and diabatic vertical velocities were calculated using the Chemical Lagrangian Model of the Stratosphere (CLaMS) trajectory module driven by both ERA-Interim and ERA5 reanalysis data. All trajectory calculations show that air parcels with low ozone and low water vapour values in the UTLS over Kunming measured by balloon-borne instruments originate from the western Pacific boundary layer. Deep convection associated with tropical cyclones over the western Pacific transports ozone-poor air from the marine boundary layer to the cold tropopause region. Subsequently, these air parcels are mixed into the strong easterlies on the southern side of the Asian summer monsoon anticyclone. Air parcels are dehydrated when passing the lowest temperature region (< 190 K) at the convective outflow of tropical cyclones. However, trajectory calculations show different vertical transport via deep convection depending on the employed reanalysis data (ERA-Interim, ERA5) and vertical velocities (diabatic, kinematic). Both the kinematic and the diabatic trajectory calculations using ERA5 data show much faster and stronger vertical transport than ERA-Interim primarily because of ERA5's better spatial and temporal resolution, which likely resolves convective events more accurately. Our findings show that the interplay between the ASM anticyclone and tropical cyclones has a significant impact on the chemical composition of the UTLS during summer.

Solar Cycle Modulation of Nighttime Ozone Near the Mesopause as Observed by MLS

AbstractEleven‐year solar cycle variations of nighttime ozone near the secondary ozone maximum layer in the mesosphere are analyzed with Aura Microwave Limb Sounder (MLS) observations since 2004, fully covering solar cycle 24. Produced primarily from the recombination of molecular oxygen (O2) with atomic oxygen (O) transported from the lower thermosphere, the mesospheric nighttime ozone concentration is proportional to atomic oxygen density [O], which itself is modulated by ultraviolet (UV) solar cycle variations. MLS nighttime ozone data and UV data at 240 nm from Solar Radiation and Climate Experiment (SORCE) Solar‐Stellar Irradiance Comparison Experiment (SOLSTICE) show a positive correlation over the solar cycle. Nighttime O3 and nighttime carbon monoxide (CO) distributions are highly correlated with each other with similar seasonal and solar cycle variations, because both [O3] and [CO] depend strongly on UV photolysis and are modulated by Eddy diffusion in this region. Nighttime ozone correlates strongly with temperature, with a generally positive correlation, except at high latitudes during boreal winter.