Feed aggregator

Abstract

A holistic review is given of the Southern Ocean dynamic system, in the context of the crucial role it plays in the global climate and the profound changes it is experiencing. The review focuses on connections between different components of the Southern Ocean dynamic system, drawing together contemporary perspectives from different research communities, with the objective of closing loops in our understanding of the complex network of feedbacks in the overall system. The review is targeted at researchers in Southern Ocean physical science with the ambition of broadening their knowledge beyond their specific field, and aims at facilitating better-informed interdisciplinary collaborations. For the purposes of this review, the Southern Ocean dynamic system is divided into four main components: large-scale circulation; cryosphere; turbulence; and gravity waves. Overviews are given of the key dynamical phenomena for each component, before describing the linkages between the components. The reviews are complemented by an overview of observed Southern Ocean trends and future climate projections. Priority research areas are identified to close remaining loops in our understanding of the Southern Ocean system.

Abstract

Electron cyclotron harmonic (ECH) waves are electrostatic emissions with frequencies between the harmonics of the electron gyrofrequencies. Their frequency properties provide clues for understanding their generation and are keys to evaluating their scattering efficiency. Based on Magnetospheric Multiscale satellite observations, we explored the statistical frequency properties of first-harmonic band ECH waves in the outer magnetosphere. The frequencies at the peak power of ECH waves are found to be day-night and dawn-dusk asymmetries, with higher values in the regions from dawn to post-noon, and these asymmetries are more evident during weaker geomagnetic activity. Furthermore, the frequencies at the peak power of ECH waves decrease gradually with increasing |MLAT| and are positively correlated with their amplitudes at each magnetic local time or |MLAT|. Information on the frequency properties of ECH waves presented in this study can be crucial for future modeling of their contributions to magnetospheric electron dynamics.

Abstract

Detached subauroral proton arcs are commonly observed during the recovery phase of geomagnetic storms, and have been extensively investigated. However, there is limited study on their occurrence during the main phase of storms. This study investigated nightside detached auroras (NDAs) observed by the far-ultraviolet imager onboard the Defense Meteorological Satellite Program spacecraft. The NDAs occurred in the nightside sector, separated from the equatorward boundary of the auroral oval, and were observed during the main and recovery phases of the geomagnetic storm on 02 October 2013. The occurrence of the NDAs appears to correlate with the expanding auroral oval toward lower latitudes, and is independent of the polarity change in the interplanetary magnetic field Bz component. Particle measurements indicate that the NDAs were generated by energetic protons, primarily above 10 keV, originating from the ring current. These precipitating proton fluxes, predominantly anisotropic, were observed to be detached from the isotropic boundary within the auroral oval. Analysis of Pc1 data obtained by ground stations suggests that electromagnetic ion cyclotron waves account for the generation of the NDAs. The limited latitudinal distribution of the NDAs indicates the wave activity in the magnetospheric source region within a narrow L-shell region. The observations presented in this study would contribute to our understanding of the coupling processes between the magnetosphere and ionosphere within the subauroral region.

Abstract

This study focuses on the poorly known effect of polar cap patches (PCPs) on the ion-neutral coupling in the F-region. The PCPs were identified by total electron content measurements from the Global Navigation Satellite System (GNSS) and the ionospheric parameters from the Defense Meteorological Satellite Program spacecraft. The EISCAT incoherent scatter radars on Svalbard and at Tromsø, Norway observed that PCPs entered the nightside auroral oval from the polar cap and became plasma blobs. The ionospheric convection further transported the plasma blobs to the duskside. Simultaneously, long-lasting strong upper thermospheric winds were detected in the duskside auroral oval by a Fabry-Perot Interferometer (FPI) at Tromsø and in the polar cap by the Gravity Recovery and Climate Experiment satellite. Using EISCAT ion velocities and plasma parameters as well as FPI winds, the ion drag acting on neutrals and the time constant for the ion drag could be estimated. Due to the arrival of PCPs/blobs and the accompanied increase in the F-region electron densities, the ion drag is enhanced between about 220 and 500 km altitudes. At the F peak altitudes near 300 km, the median ion drag acceleration affecting neutrals more than doubled and the associated median e-folding time decreased from 4.4 to 2 hr. The strong neutral wind was found to be driven primarily by the ion drag force due to large-scale ionospheric convection. Our results provide a new insight into ionosphere-thermosphere coupling in the presence of PCPs/blobs.

Abstract

Grasping the physical interactions when two tropical cyclones (TCs) (TC) are in proximity is essential for boosting the accuracy of TC forecasts. This study dissects an uncommon scenario wherein the merging with Tropical Depression 13 W significantly hastened the rapid weakening of Super Typhoon Hinnamnor (2022), utilizing comparative experiments with and without 13 W in simulation's initial field. The findings reveal strong correlations between the merger, amplified environmental vertical wind shear (VWS), and Hinnamnor's consecutive weakening, unfolding in two stages— “top-down” (Stage 1) and “bottom-up” weakening (Stage 2) stage. In Stage 1, 13 W led to downdrafts from upper level, hindering the eyewall updrafts and weakening the warm core. In Stage 2, 13 W merged into Hinnamnor's outer rainband, introduced low-entropy air into the boundary layer and also vied with the eyewall for energy. This research emphasizes that even minor, less-intense vortices can have profound impacts on the rapid intensity change in TCs.

No abstract is available for this article.

Abstract

Vegetation optical depth (VOD) satellite microwave retrievals provide significant insights into vegetation water content and responses to hydroclimatic changes. While VOD variations are commonly linked to dry biomass and live fuel moisture content (LFMC), the impact of canopy temperature (T c ) remains overlooked in large-scale studies. Here, we investigated the impact of T c on L-band (1.4 GHz) and X-band (10.7 GHz) VOD at diurnal and seasonal timescales. Synthetic benchmark VOD was created using realistic fields of T c , LFMC, and biomass in an electromagnetic model. Perturbation experiments revealed that T c strongly affects diurnal VOD variations at both L-band and X-band. Seasonally, while biomass emerges as the largest contributor to VOD variations in 70% (at X-band) and 90% (at L-band) of our study region, T c and LFMC still play substantial roles. The findings stress the importance of refining retrieval algorithms to distinguish T c , LFMC, and biomass effects for future VOD applications in ecohydrology.

Abstract

In recent decades, an increase in rainfall has been observed on the northwestern edge of the Indian summer monsoon (ISM; NWEISM). However, no studies have focused on model performances over NWEISM, which calls for an urgent evaluation of models. Here, we utilize historical simulations from 24 CMIP6 models to demonstrate that current models tend to underestimate the observed increasing rainfall over NWEISM, with only ∼30% of the observed intensity. The models broadly capture the spring Middle East land warming, which is the main driver of increased rainfall over NWEISM. Unfortunately, most models fail to reproduce the associated significant decrease in sea level pressure over the surrounding landmasses. This deficiency results in an ineffective trigger of cross-equatorial southwesterly winds, impeding the accurate simulation of the poleward shift of the summer low-level jet (LLJ). Consequently, it leads to a weaker link from the Middle East warming to rainfall enhancement over NWEISM.

Abstract

Juno conducted two close Io flybys on 30 December 2023 and 03 February 2024, both at a minimum altitude of 1,500 km. Filamentary structures in the electric and magnetic field spectra indicate Juno crossed the Alfvén wing, the magnetic structure connecting Io to Jupiter's polar ionosphere. We show that the first pass took Juno diametrically through the northern Alfvén wing, while the second pass had Juno graze the southern Alfvén wing boundary, enabling extended measurements of the transition region between Io's vicinity and the Jovian magnetosphere. Of note, evidence of local electron beams is inferred from whistler-mode emissions. We demonstrate that their energies are sub-keV, are sourced from Io's ionosphere or local torus, and are part of a distributed current system connecting Io to Jupiter. Finally, upper hybrid resonances indicate electron densities are significantly elevated in Io's polar region (∼28,000 cm−3) compared to the local Io torus (∼2,000 cm−3).

No abstract is available for this article.

Abstract

This study uses Mars Atmosphere and Volatile EvolutioN observations of electron density and magnetic field for a period of four Martian years (MYs 33–36) (∼8 Earth years) to investigate the effects of Martian crustal magnetic fields on the distribution and variability of Mars' ionosphere. The results show a clear enhancement in electron density in the southern hemisphere in the region where the strong crustal magnetic fields are present with the longitudes between 120° and 240° (i.e., the central longitude), which is in agreement with previous studies. On the contrary, the corresponding northern hemisphere region in the central longitudes shows an exactly opposite behavior that the electron density is lower compared to the surrounding longitude regions. These effects are found to be primarily dayside phenomena. As opposed to dayside, the nightside electron density in the central longitudes are slightly reduced at altitudes below 200 km, compared to longitudes on its western and eastern sides. Above 200 km, the nightside effects are not very clear. Significant hemispheric asymmetry is observed in the longitude regions of enhanced crustal magnetic fields compared to other longitude regions during the daytime. This dayside south-north asymmetry in the central longitude region is observed to be a constant feature across all seasons. However, on the nightside, the south-north asymmetry remains more or less similar across all longitude regions, during all seasons implying a weakened control of the crustal fields over the nightside ionosphere. Even then, the southern hemisphere retains a stronger nightside ionosphere during all seasons except summer.

Abstract

The interplay between compaction-driven fluid flow and plastic yielding within porous media is investigated through numerical modeling. We establish a framework for understanding the dynamics of fluid flow in deforming porous materials that corresponds to the equations describing solitary porosity wave propagation. A concise derivation of the coupled fluid flow and poro-viscoelastoplastic matrix behavior is presented, revealing a connection to Biot's equations of poroelasticity and Gassmann's theory in the elastic limit. Our findings demonstrate that fluid overpressure resulting from channelized fluid flow initiates the formation of new shear zones. Through three-dimensional simulations, we observe that the newly formed shear zones exhibit a parabolic shape. Furthermore, plasticity exerts a significant influence on both the velocity of fluid flow and the shape of fluid channels. Importantly, our study highlights the potential of spontaneous channeling of porous fluids to trigger seismic events by activating both new and pre-existing faults.

Evaluation of optimized flux chamber design for measurement of ammonia emission after field application of slurry with full-scale farm machinery
Johanna Pedersen, Sasha D. Hafner, Andreas Pacholski, Valthor I. Karlsson, Li Rong, Rodrigo Labouriau, and Jesper N. Kamp
Atmos. Meas. Tech., 17, 4493–4505, https://doi.org/10.5194/amt-17-4493-2024, 2024
Field-applied animal slurry is a significant source of NH3 emission. A new system of dynamic flux chambers for NH3 measurements was developed and validated using three field trials in order to assess the variability after application with a trailing hose at different scales: manual (handheld) application,  a 3 m slurry boom, and a 30 m slurry boom. The system facilitates NH3 emission measurement with replication after both manual and farm-scale slurry application with relatively high precision.

Abstract

To understand future summer precipitation changes over the Great Lakes Region (GLR), we performed an ensemble of regional climate simulations through the Pseudo-Global Warming (PGW) approach. We found that different types of convective precipitation respond differently to the PGW signal. Isolated deep convection (IDC), usually concentrated in the southern domain, shows an increase in precipitation to the north of the GLR. Mesoscale convective systems (MCSs), usually concentrated upwind of the GLR, shift to the downwind region with increased precipitation. Thermodynamic variables such as convective available potential energy (CAPE) and convective inhibition energy (CIN) are found to increase across almost the entire studied domain, creating a potential environment more favorable for stronger convection systems and less favorable for weaker ones. Meanwhile, changes in the lifting condensation level (LCL) and level of free convection (LFC) show a strong correlation with variations in convective precipitation, highlighting the significance of these thermodynamic factors in controlling precipitation over the domain. Our results indicate that the decrease in LCL and LCF in areas with increased convective precipitation is mainly due to increased atmospheric moisture. In response to the prescribed warming perturbation, MCSs occur more frequently downwind, while localized IDCs exhibit more intense rain rates, longer durations, and larger rainfall area.

Abstract

Juno performed two close flybys of Io and found enhanced field-aligned proton fluxes are absorbed by Io. These protons are absorbed at mass input rates comparable to previous estimates for hydrogen losses from Io, hence Jupiter is likely the source of hydrogen at Io. The conditions necessary for this to occur are: (a) formation of Alfvén waves at Io, (b) wave-particle coupling to energize protons, (c) anti-planetward transport of ions due to the magnetic mirror force and/or parallel acceleration, and (d) strong sub-Alfvénic interaction slowing the flow connected to Io's fluxtube allowing for sufficient travel time for energized ions to transit to Io. The derived slowdown of ≤12% the upstream value is linked to filamentation within the Alfvén wing. This mechanism is likely operating at all strongly interacting satellites and provides an avenue to transfer material from a planetary body to its satellites, including exoplanets and brown dwarfs.

Abstract

Sub6 GHz non-line-of-sight signals are a potential opportunistic source of rainfall information that promises to improve the current urgent need regarding near-surface rainfall detection, but the complex mechanisms in which these signals are impacted by rainfall have hindered further development in this area. In this study, we focus on four types of microwave propagation processes to explore the theoretical basis for Sub6 GHz signal sensitivity to rainfall. We also investigate how these signals change during rainy conditions using a cellphone signal recording experiment. The results demonstrate that the indirect effect of rainfall-induced changes in the interfacial water film may significantly affect the Sub6 GHz signal, making it an opportunity to reflect rainfall information. Finally, we offer a comprehensive overview of the potential challenges, benefits, and drawbacks of low-frequency non-line-of-sight links in the context of rainfall inversion.

Abstract

Improving tropical cyclone (TC) rainfall prediction is vital as climate change has led to an increase in TC rainfall rates. Enhanced reliability in predicting TC tracks has paved the way for statistical methodologies to make use of them in estimating current TC rainfall, achieved by identifying similar past TC tracks and obtaining their corresponding rainfall data. While the Fuzzy C Means (FCM) clustering algorithm is widely used, it has limitations stemming from its clustering-centric design, hindering its ability to pinpoint the most appropriate similar TCs. Our study introduces the Sinkhorn distance, a novel similarity metric that measures the cost of transforming one set of data to another, for assessing TC similarity in rainfall prediction. Our findings indicate that utilizing Sinkhorn distance enhances the accuracy of TC rainfall predictions across the Western North Pacific region. When compared to the conventional approach using FCM, our Sinkhorn distance-based methodology yields slightly better yet statistically significant results. The improvement is due to better identification of similar TCs, characterized by closer proximity of similar TC tracks to the target TC track, facilitated by Sinkhorn distance. This underscores how minor differences in TC track can alter rainfall distribution, emphasizing the critical importance of accurate track prediction in rainfall prediction and the need to reconsider how we categorize TCs together, which can have implications for climate and atmospheric sciences. Collectively, the inclusion of Sinkhorn distance stands as a valuable addition to our toolkit for discerning similar TC tracks, thus elevating the accuracy of TC rainfall predictions.

A novel, balloon-borne UV–Vis spectrometer for direct sun measurements of stratospheric bromine
Karolin Voss, Philip Holzbeck, Klaus Pfeilsticker, Ralph Kleinschek, Gerald Wetzel, Blanca Fuentes Andrade, Michael Höpfner, Jörn Ungermann, Björn-Martin Sinnhuber, and André Butz
Atmos. Meas. Tech., 17, 4507–4528, https://doi.org/10.5194/amt-17-4507-2024, 2024
A novel balloon-borne instrument for direct sun and solar occultation measurements of several UV–Vis absorbing gases (e.g. O3, NO2, BrO, IO, and HONO) is described. Its major design features and performance during two stratospheric deployments are discussed. From the measured overhead BrO concentration and a suitable photochemical correction, total stratospheric bromine is inferred to (17.5 ± 2.2) ppt in air masses which entered the stratosphere around early 2017 ± 1 year.

On Path Length, Beam Divergence, and Retroreflector Array Size in Open-Path FTIR Spectroscopy
Cameron E. N. Power and Aldona Wiacek
Atmos. Meas. Tech. Discuss., https://doi.org/10.5194/amt-2024-97,2024
Preprint under review for AMT (discussion: open, 0 comments)
The choice of path length and retroreflector array size in open-path FTIR spectroscopy must be made with care.  Longer paths increase target gas absorption (lowering detection limits) and larger retroreflector arrays improve the SNR of spectra by increasing the return signal (improving retrieved concentration precision), but there are limitations to both.  An optimum array size and path combination exists in each specific observational environment and application, as explored in this work.

Abstract

Volcanic eruptions carry essential information on the dynamics of volcanic systems. Studies have documented variable eruption styles and eruptive surface deformation. However, co-eruptive subsurface structural changes remain poorly understood. Here we characterize the seismic velocity changes from July 2019 to July 2023 at Great Sitkin Volcano in the central Aleutian volcanic arc, using single-station ambient noise interferometry at five three-component seismic stations. Coincident with the lava effusion since late July 2021, about two months after the explosive eruption on 26 May 2021, we observe a sustained velocity increase, most prominently to the northwest of the caldera. We attribute this velocity increase to the structural changes with magma extrusion, with the spatial variation controlled by the geometry of the magma system or the property of shallow volcaniclastics. Our findings offer insights into understanding co-eruptive structural modifications at active volcanoes.