Wave‐induced Lagrangian fluctuations of temperature and vertical velocity in the lower stratosphere are quantified using measurements from superpressure balloons (SPBs). Observations recorded every ...minute along SPB flights allow the whole gravity wave spectrum to be described and provide unprecedented information on both the intrinsic frequency spectrum and the probability distribution function of wave fluctuations. The data set has been collected during two campaigns coordinated by the French Space Agency in 2010, involving 19 balloons over Antarctica and 3 in the deep tropics. In both regions, the vertical velocity distributions depart significantly from a Gaussian behavior. Knowledge on such wave fluctuations is essential for modeling microphysical processes along Lagrangian trajectories. We propose a new simple parameterization that reproduces both the non‐Gaussian distribution of vertical velocities (or heating/cooling rates) and their observed intrinsic frequency spectrum.
Key Points
Long‐duration balloon observations are used to characterize Lagrangian temperature fluctuations
Intrinsic frequency spectra and PDFs are derived for temperature and cooling rates
A parameterization of gravity wave temperature fluctuations in the lower stratosphere is developed
The role of gravity waves on microphysics of tropical cirrus clouds and air-parcel dehydration was studied using the combination of Lagrangian observations of temperature fluctuations and a 1.5D ...model. High-frequency measurements during isopycnal balloon flights were used to resolve the gravity-wave signals with periods ranging from a few days to 10 min. The detailed microphysical simulations with homogeneous freezing, sedimentation, and a crude horizontal mixing represent the slow ascent of air parcels in the tropical tropopause layer (TTL). A reference simulation describes the slow ascent of air parcels in the tropical tropopause layer, with nucleation occurring only below the cold-point tropopause with a small ice-crystal density. The inclusion of the gravity waves drastically modifies the vertical profile of low ice concentration and weak dehydration found during the ascent alone, with the increased ice-crystal number and size distribution agreeing better with observations. Numerous events of nucleation occur below and above the cold-point tropopause, efficiently restoring the relative humidity over ice to equilibrium with respect to the background temperature, as well as increasing the cloud fraction in the vicinity of the cold-point tropopause. The corresponding decrease in water vapor is estimated at 2 ppmv around the cold-point tropopause.
The 15 January 2022 eruption of the Hunga volcano (Tonga) generated a rich spectrum of waves, some of which achieved global propagation. Among numerous platforms monitoring the event, two ...stratospheric balloons flying over the tropical Pacific provided unique observations of infrasonic wave arrivals, detecting five complete revolutions. Combined with ground measurements from the infrasound network of the International Monitoring System, balloon‐borne observations may provide additional constraint on the scenario of the eruption, as suggested by the correlation between bursts of acoustic wave emission and peaks of maximum volcanic plume top height. Balloon records also highlight previously unobserved long‐range propagation of infrasound modes and their dispersion patterns. A comparison between ground‐ and balloon‐based measurements emphasizes superior signal‐to‐noise ratios onboard the balloons and further demonstrates their potential for infrasound studies.
Plain Language Summary
The eruption of the Hunga volcano on 15 January 2022 was one of the most powerful blasts of the last century. This fast and strong perturbation of the atmosphere triggered atmospheric waves which were followed around the world multiple times. Here, we use records of sound waves emitted by the eruption from two balloons flying at about 20 km altitude over the Pacific combined with ground stations around the volcano to help characterize the event and its scenario. Due to weak relative wind and turbulence, the sounds on the balloon are generally clearer than on the ground, demonstrating the potential of high‐altitude measurements for extreme events.
Key Points
Comparison between balloon‐borne and ground‐based observations of infrasound waves triggered by the January 2022 Hunga eruption
Eruption sequence from infrasound in broad agreement with plume top height evolution
Benchmark for long‐range monitoring of infrasound from large explosive sources using stratospheric balloon observations
Tropical gravity wave activity is investigated using measurements of momentum fluxes gathered during Strateole‐2 superpressure balloon flights. The data set consists of eight balloon flights ...performed in the deep tropics from November 2019 to February 2020. The flights lasted for 2–3 months each, and in‐situ meteorological data were collected every 30 s. The relation between gravity waves and deep convection is investigated using geostationary satellite data from the NOAA/NCEP GPM_MERGIR satellite data product, at 1 h resolution. The amplitude of gravity wave momentum fluxes shows a clear dependence on the distance to the nearest convective system, with a strong decay as distance to convection increases. The largest momentum‐flux values (>5mPa) are only found less than 200 km away from deep convection. The sensitivity of the wave flux to distance from convection is stronger for high frequency gravity waves (periods shorter than 60 min). Lower frequency waves tend to a non‐zero, background value away from convection, supporting some background value in gravity‐wave drag parameterizations. On the other hand, the wide range of momentum flux values observed close to the convection emphasizes the intermittent nature of the gravity‐wave source. The large scale variation of gravity‐wave intermittency within the equatorial belt is also studied. The results highlight spatial variations of gravity wave activity, with the highest momentum flux recorded over land.
Key Points
Tropical gravity wave activity is characterized from long‐duration superpressure balloon flights
Gravity‐wave momentum fluxes exhibit a clear dependence to the distance to the nearest convective system
Observed wave intermittency results from the time‐ and spatially‐varying distribution of the source
A new class of vortices has been observed in the stratosphere following extreme wildfires (Canada 2017, Australia 2020) and volcanic eruptions (Raikoke 2019). These vortices are long‐lived mesoscale ...anticyclones (hundreds to 1,000 km in diameter) trapping plumes of aerosols and combustion/volcanic compounds. Owing to their unusual composition, these anticyclonically trapped plumes (ATPs) are associated with a significant radiative heating, which fuels their ascent through the stratosphere. This article investigates the dynamics of ATPs using two complementary approaches: analytically, in a potential vorticity (PV) perspective, and through idealised numerical simulations with the Weather Research and Forecasting (WRF) model. In both cases, we consider the vortical flow forced by a heating Lagrangian tracer. By reformulating the problem in the potential radius–potential temperature coordinate system introduced for tropical cyclones, we first clarify that ATP formation is concomitant with the injection of air into the stratosphere at extratropical latitudes. Then, we derive a set of simplified one‐dimensional equations describing the subsequent evolution of the flow after the injection. The equation obtained for the tracer is a variant of the classical Burgers' equation. In qualitative agreement with the three‐dimensional WRF simulations, this theoretical model predicts that ATPs develop an upper tracer front associated with sustained near‐zero anticyclonic PV, followed by a smooth tracer tail of cyclonic PV. Radiative relaxation of the temperature perturbations induced by the anticyclone and the presence of an initial PV anomaly tend to stabilise ATPs during their ascent. Finally, we note that the theory predicts a similar relationship between the plume and anticyclonic PV for cooled ATPs, which is supported by three‐dimensional simulations and may apply to the 2022 Hunga volcanic plume.
Satellite and reanalysis data have revealed that some stratospheric plumes from extreme wildfires and volcanic eruptions self‐organise into mesoscale anticyclones ascending through the stratosphere. Previous research identified radiative heating from sunlight‐absorbing aerosols as a key driver of this phenomenon. Here, we use potential vorticity theory and idealised numerical simulations to investigate the flow forced by a heating Lagrangian tracer representing the aerosols. The study provides new insights into the formation, evolution, and maintenance of anticyclonic plumes ascending in the stratosphere.
Tropical tropopause layer (TTL) clouds have a significant impact on the Earth's radiative budget and regulate the amount of water vapor entering the stratosphere. Estimating the total coverage of ...tropical cirrus clouds is challenging, since the range of their optical depth spans several orders of magnitude, from thick opaque cirrus detrained from convection to sub-visible clouds just below the stratosphere. During the Strateole-2 observation campaign, three microlidars were flown on board stratospheric superpressure balloons from October 2021 to late January 2022, slowly drifting only a few kilometers above the TTL. These measurements have unprecedented sensitivity to thin cirrus and provide a fine-scale description of cloudy structures both in time and in space. Case studies of collocated observations with the spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) show very good agreement between the instruments and highlight the Balloon-borne Cirrus and convective overshOOt Lidar's (BeCOOL) higher detection sensitivity. Indeed, the microlidar is able to detect optically very thin clouds (optical depth τ<2×10-3) that are undetected by CALIOP. Statistics on cloud occurrence show that TTL cirrus appear in about 50 % of the microlidar profiles and have a mean geometrical depth of 1 km. Ultrathin TTL cirrus (τ<2×10-3) have a significant coverage (23 % of the profiles), and their mean geometrical depth is 0.5 km.
Due to their increasing spatial resolution, numerical weather prediction (NWP) models and the associated analyses resolve a growing fraction of the gravity wave (GW) spectrum. However, it is unclear ...how well this "resolved" part of the spectrum truly compares to the actual atmospheric variability. In particular, the Lagrangian variability, relevant, for example, to atmospheric dispersion and to microphysical modeling in the upper troposphere-lower stratosphere (UTLS), has not yet been documented in recent products.
The accuracy of horizontal winds and temperature in the equatorial lower stratosphere is evaluated in different (re)analyses (European Centre for Medium‐Range Weather Forecasts (ECMWF) operational ...analysis, ERA Interim, and Modern‐Era Retrospective Analysis for Research and Applications) using an independent data set collected at low latitudes during long‐duration balloon flights in early 2010. The three analyzed wind products are found significantly less accurate than in the extratropics, with periods of ≳10m/sdisagreement with the observations lasting several days. To highlight the dynamical context in which the major disagreement events occur, case studies are carried out. The events are shown to be related to an improper representation of large‐scale equatorial Kelvin and Yanai wave packets with vertical wavelengths smaller than 5 km. Such events can induce large errors on trajectories computed with analyzed winds relatively to the actual (balloon) trajectory: 4000 km separation after 5 days of calculation. Reasons for analyses inaccuracy are discussed. The vertical resolution of the underlying model likely plays a role, but the main factor responsible for deficiencies appears to be the lack of wind observations. Indeed, errors in analyzed winds during the campaign have a strong longitudinal structure, with root‐mean‐square errors twice as large over the Indian Ocean and western Pacific, poorly covered by radiosounding stations, as over the Maritime Continent or South America. For the ECMWF analysis, this structure mirrors that of the analysis increments, which have largest amplitudes over observed regions. We argue that the reported events are more likely to happen during maximum shear phases of the quasi‐biennial oscillation.
Key PointsIn situ wind observations are used to assess the realism of (re)analysesErrors on UTLS winds can exceed 10 m/s for 10 consecutive daysMisrepresented equatorial waves in poorly observed regions cause these errors
A new constellation of radio occultation satellites called COSMIC‐2 (Constellation Observing System for Meteorology, Ionosphere, and Climate‐2) is providing unprecedented dense measurements of the ...tropical atmosphere, with on average more than 4,000 high quality observations per day over 40°N–S. We use these data to evaluate large‐ and small‐scale thermal variability in the tropical lower stratosphere during October 2019 – April 2020. Space‐time spectral analysis of 6‐hourly gridded COSMIC‐2 data reveals a rich spectrum of traveling planetary‐scale waves, including Kelvin waves, mixed Rossby‐gravity waves and inertia gravity waves, in addition to propagating diurnal tides. These coherent modes show enhanced amplitudes from the tropical tropopause through the lower stratosphere (∼17–25 km). Characteristics of small‐scale temperature variances, calculated as deviations from the gridded fields, reveal systematic spatial patterns including time average maxima over Africa and South America overlying frequent deep convection. Small‐scale variances also exhibit transient maxima in the equatorial lower stratosphere tied to local variations in static stability, associated with large‐scale Kelvin waves. The new COSMIC‐2 observations provide novel details on the rich spectrum of large‐ and small‐scale waves near and above the tropical tropopause.
Plain Language Summary
A new constellation of radio occultation satellites called COSMIC‐2 (Constellation Observing System for Meteorology, Ionosphere and Climate‐2) is providing unprecedented dense measurements of the tropical atmosphere, with on average more than 4,000 high quality observations per day over 40°N–S. We use these data to provide novel understanding of temperature variability near the tropical tropopause and lower stratosphere (∼10–30 km). COSMIC‐2 data reveal a rich spectrum of large‐ and small‐scale waves, including eastward‐ and westward‐propagating planetary‐scale equatorial waves and diurnal tides. The measurements also identify localized regions of enhanced temperature variability tied to small‐scale gravity waves. These new measurements are valuable for constraining global models and understanding water vapor and high‐level clouds in the tropics.
Key Points
Tropical temperature variability is quantified using dense COSMIC‐2 radio occultation measurements
COSMIC‐2 data reveal a rich spectrum of equatorial planetary waves and diurnal tides in the tropical lower stratosphere
Small‐scale temperature variances reveal coherent geophysical structures tied to convection and stratospheric Kelvin waves
Water vapour (H.sub.2 O) in the upper troposphere and lower stratosphere (UTLS) has a significant role for global radiation. A realistic representation of H.sub.2 O is therefore critical for accurate ...climate model predictions of future climate change. In this paper we investigate the effects of current uncertainties in tropopause temperature, horizontal transport and small-scale mixing on simulated H.sub.2 O in the lower stratosphere (LS).
PDF
