M. L. Santee’s research while affiliated with Jet Propulsion Laboratory and other places

With this paper, we aim to shed light on the extent to which nadir-viewing hyperspectral infrared (IR) sounders can support the study of stratospheric chemical processes and ozone loss in the extratropics. We use CLIMCAPS (Community Long-term Infrared Microwave Combined Atmospheric Processing System) retrievals from JPSS-1 (Joint Polar Satellite System) CrIS (Cross-track Infrared Sounder) measurements as the baseline case. CLIMCAPS retrieves a suite of Earth system variables that includes atmospheric temperature (Tair), water vapor (H2Ovap), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), ozone (O3), and nitric acid (HNO3). Unlike the Rodgers (2000) Optimal Estimation (OE) retrieval approach, CLIMCAPS regularizes its Bayesian inverse solution dynamically using singular value decomposition (SVD) at run-time to separate measurement signal from noise. We illustrate how the CLIMCAPS approach enables stable retrievals of stratospheric HNO3 under highly variable conditions, allowing characterization of seasonal patterns. Nitric acid is typically used as an indicator species for heterogeneous chemical processing inside the winter stratospheric polar vortices. This paper summarizes our diagnostic evaluation of CLIMCAPS observing capability during the Northern Hemisphere winter of 2019/2020. We contrast CLIMCAPS HNO3 retrievals with those from the limb-viewing MLS (Microwave Limb Sounder) to illustrate the capability of this retrieval approach and lay the foundation for in-depth validation studies in future.

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. To improve the model’s UTLS transport representation, particularly from the PBL over days to weeks, we derive zonally and seasonally resolved climatologies of CO partial columns (XCO, spanning 147 to 68 hPa) and compare them with Microwave Limb Sounder (MLS) and Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) observations, as well as in situ data. Incorporating a parameterization for unresolved convection significantly improves CO anomaly representation in the UTLS, particularly in capturing seasonal and spatial patterns. While the simulated absolute XCO values align better with ACE-FTS, the model reproduces MLS anomalies more accurately, suggesting MLS better represents CO variability. In situ observations in the boreal polar region generally support lower ACE-FTS CO values, while MLS better represents CO enhancements in air affected by the Asian summer monsoon above 10 km.

The Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS) on SCISAT-1 and Microwave Limb Sounder (MLS) on NASA’s Aura satellite have contributed significantly to understanding the impacts of human activities on the stratospheric ozone layer. The two-decade-long data record from these instruments has allowed quantification of ozone depletion caused by human-released ozone-depleting substances, the effects of extreme natural events like major volcanic eruptions including Hunga in 2022, as well as events amplified by human-caused climate change such as wildfires that inject material into the stratosphere, as happened over Australia in early 2020. Both platforms are nearing the end of their operational lifetimes, and their decommissioning will cause a substantial gap in the measurement of critical atmospheric components, including water vapor, inorganic chlorine species, and tracers of stratospheric transport. This upcoming “data desert” poses significant challenges for monitoring the recovery of the ozone layer and assessing the effects on stratospheric composition of future extreme events, threats posed by increases in space debris from satellite burn-up, and the possible injection of stratospheric aerosol to mitigate global warming. The lack of confirmed future missions that can provide daily near-global profile measurements of stratospheric composition highlights the need for observational strategies to bridge this impending gap. This paper discusses the essential role of ACE-FTS and MLS in advancing our understanding of the stratosphere, the impact of data loss after the cessation of one or both instruments, and the urgency of developing strategies for mitigating the impact of these observational losses at a time marked by dramatic changes in the stratosphere due to human and natural factors.

The continued monitoring of the ozone layer and its long-term evolution leans on comparative studies of merged satellite records. Comparing such records presents unique challenges due to differences in sampling, coverage, and retrieval algorithms between observing platforms, all of which complicate the detection of trends. Here we examine the effects of broad nadir averaging kernels on vertically resolved ozone trends, using one record as an example. We find errors as large as 1 % per decade and displacements in trend profile features by as much as 6 km in altitude due to the vertical redistribution of information by averaging kernels. Furthermore, we show that averaging kernels tend to increase (by 10 %–80 %, depending on the location) the length of the record needed to determine whether trend estimates are distinguishable from natural variability with good statistical confidence. We conclude that trend uncertainties may be underestimated, in part because averaging kernels misrepresent decadal to multidecadal internal variability, and in part because the removal of known modes of variability from the observed record can yield residual errors. The study provides a framework to reconcile differences between observing platforms and highlights the need for caution when using records from instruments with broad averaging kernels to quantify trends and their uncertainties.

We describe a novel scanning microwave limb sounder (SMLS) instrument that performs rapid and broad azimuth conical scans of Earth's limb while simultaneously scanning the limb in the vertical. This azimuthal scanning capability gives dramatic improvement in temporal and spatial coverage over that of previous limb sounding instruments. In a 1500-kilometer altitude, 52°-inclination Earth orbit, SMLS provides 6–8 vertical profile measurements separated by 1.9 hours every 24 hours everywhere between ±65° latitude, and 2–4 such measurements everywhere between ±(65–82°). Horizontal resolution is ∼50×50 km. Vertical resolution is ∼2 km for water vapor and cloud ice and ∼1–3 km for chemical species. In an equatorial orbit, emphasizing the tropics and subtropics, SMLS produces profile measurements every 1.9 hours everywhere between ±35° latitude. SMLS measurements address scientific issues of relevance to the upper troposphere, stratosphere, mesosphere, and lower thermosphere regions of the atmosphere (heights from ∼10 km to ∼100 km).

Plain Language Summary The 2022 Hunga eruption injected an unprecedented amount of water vapor directly into the very dry stratosphere. This abrupt increase in water vapor from Hunga occurred at a time when the stratosphere was already gradually becoming moister. Using measurements from the Microwave Limb Sounder (MLS) on NASA's Aura satellite, we show that stratospheric water vapor remained elevated, essentially unchanged, from the time of the eruption until at least early 2024. MLS data further reveal that, in 2023, one of the main mechanisms for drying the stratosphere—permanent removal of water vapor by formation and settling of ice polar stratospheric cloud particles over Antarctica—was substantially more effective than usual, boosted by the excess water vapor from Hunga. Projections indicate that the return to moisture levels that would have been expected in the absence of the eruption depends on how humid the stratosphere continues to get. Considering the ongoing moistening trend and the water vapor injected by Hunga, the stratosphere could remain unusually humid for a considerable period.

The continued monitoring of the ozone layer and its long-term evolution leans on comparative studies of merged satellite records. Such records present unique challenges due to differences in sampling, coverage, and retrieval algorithms between observing platforms, leading to discrepancies in trend calculations. Here we examine the effects of optimal estimation retrieval algorithms on vertically resolved ozone trends, using one merged record as an example. We find errors as large as 1 % per decade and displacements in trend profile features of as much as 6 km altitude due to the vertical redistribution of information by averaging kernels. Furthermore, we show that averaging kernels tend to increase the length of record needed to determine whether vertically resolved trend estimates are distinguishable from natural variability with good statistical confidence. We conclude that trend uncertainties may be underestimated, in part because averaging kernels misrepresent decadal to multi-decadal internal variability, and in part because the removal of known modes of variability from the observed record can yield residual errors. The study provides a framework to reconcile differences between observing platforms, and highlights the need for caution when using merged satellite records to quantify trends and their uncertainties.

We use measurements of trace gases from the Microwave Limb Sounder and polar stratospheric clouds (PSCs) from the Cloud‐Aerosol Lidar with Orthogonal Polarization to investigate how the extraordinary stratospheric water vapor enhancement from the 2022 Hunga eruption affected polar processing during the 2023 Antarctic winter. Although the dynamical characteristics of the vortex itself were generally unexceptional, the excess moisture initially raised PSC formation threshold temperatures above typical values. Cold conditions, especially in early July, prompted ice PSC formation and unusually severe irreversible dehydration at higher levels (500–700 K), while atypical hydration occurred at lower levels (380–460 K). Heterogeneous chemical processing was more extensive, both vertically (up to 750–800 K) and temporally (earlier in the season), than in prior Antarctic winters. The resultant HCl depletion and ClO enhancement redefined their previously observed ranges at and above 600 K. Albeit unmatched in the satellite record, the early‐winter upper‐level chlorine activation was insufficient to induce substantial ozone loss. Chlorine activation, denitrification, and dehydration processes ran to completion by July/August, with trace gas evolution mostly following the climatological mean thereafter, but with chlorine deactivation starting slightly later than usual. While cumulative ozone losses at 410–550 K were relatively large, probably because of the delayed chlorine deactivation, they were not unprecedented. Thus, ozone depletion was unremarkable throughout the lower stratosphere. Although Hunga enhanced PSC formation and chemical processing in early winter, saturation of lower stratospheric denitrification, dehydration, and chlorine activation (as is typical in the Antarctic) prevented an exceptionally severe ozone hole in 2023.

The 2022 Hunga eruption caused unprecedented stratospheric hydration. Aura Microwave Limb Sounder (MLS) measurements show that the stratospheric water vapor mass remains essentially unchanged as of early 2024 and that the Hunga hydration occurred atop a robust (possibly accelerating) moistening trend in the stratosphere. Enhanced by the excess Hunga water vapor, dehydration via polar stratospheric cloud (PSC) sedimentation in the 2023 Antarctic vortex exceeded climatological values by ~50%. Simple projections, based solely on Antarctic dehydration, illustrate that the timing of the return to humidity levels that would have been expected absent the Hunga hydration depends on the ongoing stratospheric water vapor trend. For strong moistening, the influx of water entering the stratosphere could offset the enhanced PSC dehydration, resulting in a new, more humid ‘equilibrium’ stratospheric state. With the Hunga hydration compounding an underlying moistening trend, the stratosphere could remain anomalously humid for an extended period.