The Hengduan Mountains (HM) are located on the southeastern edge of the Tibetan Plateau (TP) and feature high mountain ridges (> 6000 m a.s.l.) separated by deep valleys. The HM region also features an exceptionally high biodiversity, believed to have emerged from the topography interacting with the climate. To investigate the role of the HM topography on regional climate, we conduct simulations with the regional climate model COSMO at high horizontal resolutions (at ~12 km and a convection-permitting scale of ~4.4 km) for the present-day climate. We conduct one control simulation with modern topography and two idealised experiments with modified topography, inspired by past geological processes that shaped the mountain range. In the first experiment, we reduce the HM’s elevation by applying a spatially non-uniform scaling to the topography. The results show that, following the uplift of the HM, the local rainy season precipitation increases by ~25%. Precipitation in Indochina and the Bay of Bengal (BoB) also intensifies. Additionally, the cyclonic circulation in the BoB extends eastward, indicating an intensification of the East Asian summer monsoon. In the second experiment, we remove the deep valley by applying an envelope topography to quantify the effects of terrain undulation with high amplitude and frequency on climate. On the western flanks of the HM, precipitation slightly increases, while the remaining fraction of the mountain range experiences ~20% less precipitation. Simulations suggest an overall positive feedback between precipitation, erosion, and valley deepening for this region, which could have influenced the diversification of local organisms.

A document by Shaokun Deng. Click on the document to view its contents.

A document by Luke Conibear. Click on the document to view its contents.

This study uses a unique combination of geostationary and low-Earth orbiting satellite-based observations of lightning and precipitation, respectively, to examine the evolution of deep convection during the tropical cyclone (TC) lifecycle. The study spans the Atlantic Basin hurricane seasons of 2018-2021 and is unique as it provides the first known analysis of total lightning (intra cloud and cloud to ground) observed in TCs through their extratropical transition and post-tropical cyclone (PTC) phases. We consider the TC lifecycle stage, geographic location (e.g., land, coast, ocean), shear strength, and quadrant relative to the storm motion and environmental shear vectors. Total lightning maxima are found in the forward right quadrant relative to storm motion and downshear of the TC center, consistent with previous studies using mainly cloud-to-ground lightning. Increasing environmental shear focuses the lightning maxima to the downshear right quadrant with respect to the shear vector in tropical storm phases. Vertical profiles of radar reflectivity from the Global Precipitation Measurement mission show that super-electrically active convective precipitation features (>100 flashes) within the PTC phase of TCs have deeper mixed phase depths and higher reflectivity at -10°C than other phases, indicating the presence of more intense convection. Differences in the net convective behavior observed throughout TC evolution manifest in both the TC-scale frequency of lightning-producing cells and the intensity variations of amongst individual convective cells. The combination of continuous lightning observations and precipitation snapshots improves our understanding of convective-scale processes in TCs, especially in PTC phases, as they traverse the tropics and mid-latitudes.

A document by Aodhan John Sweeney. Click on the document to view its contents.

During the wintertime 2019-2020 of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, state-of-the-art remote sensing techniques have been used to study the observed dependency of the micro- and macro-physical cloud properties of low level clouds on the presence of sea ice leads in the vicinity of the RV Polarstern. It has been suggested that the water vapor transport (WVT) can be used as a mechanism to relate the cloud properties to the influence of sea ice leads quantified by the magnitude of sea ice lead fraction (LF). Among the findings are that clouds influenced by sea ice leads show a significant increase of liquid water path, cloud thickness and decrease in liquid phase cloud base height, as well as a slight dependency of total ice water content. Moreover, it has been found that the fraction of ice water content within the lowest cloud layer has a substantial difference when depicted as a function of cloud top temperature and segregated by the coupling status to the sea ice leads via the water vapor transport \cite{saavedra_garfias_asymmetries_2023}. This finding, however, considers the ice water fraction including the precipitating part of the cloud down to the surface. Therefore, the present contribution aims to study whether or not the snowfall shows a similar sea ice lead presence dependency when separated by the coupling status of the cloud-WVT-LF system. The extensive and meticulous study on the differences of snowfall estimated during the MOSAiC expedition performed by \citet{matrosov_high_2022a} , is exploited to easily add the different snowfall estimate dataset to the MOSAiC wintertime cloud properties dataset by \citet{garfias2023_datamosaic}.Presented at: 4th International Summer Snowfall Workshop, 11-13 September 2023, Leipzig, Germany.

Convective rolls contribute largely to the exchange of momentum, sensible heat and moisture in the boundary layer. They have been shown to reinforce air-sea interaction under strong wind conditions. This raises the question of how surface turbulent fluxes can, in turn, affect the rolls. Representing the air-sea exchanges during extreme wind conditions is a major challenge in weather prediction and can lead to large uncertainties in surface wind speed. The sensitivity of rolls to different representations of surface fluxes is investigated using Large Eddy Simulations. The study focuses on the Mediterranean windstorm Adrian, where convective rolls resulting from thermal and dynamical instabilities are responsible for the transport of strong winds to the surface. Considering sea spray in the parameterization of surface fluxes significantly influences roll morphology. Sea spray increases heat fluxes and favors convection. With this more pronounced thermal instability, the rolls are 30\% narrower and extend over a greater height, and the downward transport of momentum is intensified by 40\%, resulting in higher wind speeds at the surface. Convective rolls vanish within a few minutes in the absence of momentum fluxes, which maintain the wind shear necessary for their organization. They also quickly weaken without sensible heat fluxes, which feed the thermal instability required for their development, while latent heat fluxes play minor role. These findings emphasize the necessity of precisely representing the processes occurring at the air-sea interface, as they not only affect the thermodynamic surface conditions but also the vertical transport of momentum within the windstorm.

Moisture transport in summer induces annual precipitation peak over the Tibetan Plateau (TP) thus being one crucial sustentation of water cycle between the TP and its surrounding areas. Simulating moisture transport accurately over the TP remains uncertain for current numerical models with one important influencing factor as horizontal resolution. In this study, in order to investigate the difference in moisture transport at resolutions from hydrostatic to non-hydrostatic scales, three experiments are conducted for summer of 2015 using a global variable-resolution model, including one with a globally quasi-uniform resolution of 60 km (U60km) and two with regional refinements over the TP at resolutions of 16 km (V16km) and 4 km (V4km), respectively. The differences in moisture transport among three simulations are significantly influenced by the changes in wind fields through the Himalayas and eastern TP at two layers, 700~600 and 600~400 hPa, which is largely modulated by their difference in large-scale circulations particularly monsoon depression. At hydrostatic scale (from 60 km to 16 km), the monsoon depression is slightly stronger and shifts northward along with the mid-latitude westerlies, which is due to the combination of the sensitivity of convection scheme to integrating timestep and different extents of resolved dynamical processes at different resolutions. With horizontal resolution increasing to convection-permitting scale (from 16 km to 4 km), the resolved moist convection along with its associated less latent heat leads to weaker monsoon depression over the south of TP, which is much larger than the resolution induced difference at hydrostatic scale.

Observations suggest tropical convection intensifies when aerosol concentrations enhance, but quantitative estimations of this effect remain highly uncertain. Leading theories for explaining the influence of aerosol concentrations on tropical convection are based on the dynamical response of convection to changes in cloud microphysics, neglecting possible changes in the environment. In recent years, global convection-permitting models (GCPM) have been developed to circumvent problems arising from imposing artificial scale separation on physical processes associated with deep convection. Here, we use a GCPM to investigate how enhanced concentrations of aerosols that act as cloud condensate nuclei (CCN) impact tropical convection features by modulating the convection-circulation interaction. Results from a pair of idealized non-rotating radiative-convective equilibrium simulations show that the enhanced CCN concentration leads to weaker large-scale circulation, the closeness of deep convective systems to the moist cluster edges, and more mid-level cloud water at an equilibrium state in which convective self-aggregation occurred. Correspondingly, the enhanced CCN concentration modulates how the diabatic processes that support or oppose convective aggregation maintain the aggregated state at equilibrium. Overall, the enhanced CCN concentration facilitates the development of deep convection in a drier environment but reduces the large-scale instability and the convection intensity. Our results emphasize the importance of allowing atmospheric phenomena to evolve continuously across spatial and temporal scales in simulations when investigating the response of tropical convection to changes in cloud microphysics.

This study develops a physics-informed neural network (PINN) model to efficiently generate high-fidelity local circulation patterns in Taiwan while preserving an accurate representation of the physical relationship between generated local turbulence and upstream synoptic flow regimes. Large ensemble semi-realistic simulations were conducted using a high-resolution (2 km) TaiwanVVM model, in which the critical characteristics of the various synoptic flow regimes are carefully selected to concentrate on the effects of local circulation variations. A variational autoencoder (VAE) was constructed to capture essential representations of local circulation scenarios associated with the lee vortices by training on the ensemble dataset. The VAE’s latent space effectively encapsulated synoptic flow regimes as controlling factors, aligning with the physical understanding of Taiwan’s local circulation. The critical transition of flow regimes under the southeasterly synoptic flow regimes is also well represented in the VAE’s latent space, indicating that the VAE can learn the nonlinear characteristics of the multiscale interaction of the lee vortex. We further construct a reduced-order model for local circulation predictions under diverse synoptic conditions. This physics-informed VAE ensures the accurate prediction of the nonlinear characteristics of multiscale interactions between synoptic flows and the turbulence induced by topography, thereby accelerating the assessment of the local circulation responses under various climate change scenarios.

In this study, we explore the relationship between anvil cloud fraction and horizontal model resolution in small domain radiative-convective equilibrium (RCE) simulations, building on the findings of \citeA{jeevanjee22}. Using the System of Atmosphere Modeling (SAM) model, we find that finer resolutions yield higher anvil cloud fractions due to larger convective updrafts mass flux and increased mass detrainment at anvil levels. Employing two different microphysics schemes, we illustrate that finer resolution can enhance mass flux through either stronger cloud evaporation or weaker upper-troposphere stability, as the consequence of enhanced horizontal mixing. Moreover, we refine an analytical zero-buoyancy plume model to investigate the effects of adjusting entrainment rate and evaporation rate on vertical atmosphere profiles in a simple theoretical framework. Our solutions of the zero-buoyancy plume model suggest that stronger horizontal mixing can lead to larger convective updraft mass flux, consistent with the analysis in numerical simulations. We also observe the likelihood of atmospheric profiles converging at a grid size of approximately 100m, potentially as a result of converging entrainment rate and mixing strength. These insights have implications for global storm-resolving simulations, implying a possible convergence of high cloud and deep convection properties as the horizontal resolution approaches around 100m.

A drift-diffusion model is used to simulate the low-altitude electron distribution, accounting for azimuthal drift, pitch angle diffusion, and atmospheric backscattering effects during a rapid electron dropout event on August 21st, 2013, at L=4.5. Additional external loss effects are introduced during times when the low-altitude electron distribution cannot be reproduced by diffusion alone. The model utilizes low-altitude electron count rate data from five POES/MetOp satellites to quantify pitch angle diffusion rates. Low-altitude data provides critical constraint on the model because it includes the drift loss cone region where the electron distribution in longitude is highly dependent on the balance between azimuthal drift and pitch angle diffusion. Furthermore, a newly derived angular response function for the detectors onboard POES/MetOp is employed to accurately incorporate the bounce loss cone measurements, which have been previously contaminated by electrons from outside the nominal field-of-view. While constrained by low-altitude data, the model also shows reasonable agreement with high-altitude data. Pitch angle diffusion rates during the event are quantified and are faster at lower energies. Precipitation is determined to account for all of the total loss observed for 350 keV electrons, 76% for 600 keV and 45% for 900 keV. Predictions made in the MeV range are deemed unreliable as the integral energy channels E3 and P6 fail to provide the necessary constraint at relativistic energies.

Gravity waves (GWs) play an important role in the dynamics and energetics of the mesosphere. Geomagnetic activity is a known source of GWs in the upper atmosphere. However, how deep the effects of geomagnetic activity induced GWs penetrate into the mesosphere remains an open question. We use temperature measurements from the SABER/TIMED instrument between 2002 - 2018 to study the variations of mesospheric GW activity following intense geomagnetic disturbances identified by AE and Dst indices. By considering several case studies, we show for the first time that the GWs forced by geomagnetic activity can propagate down to about 80 km in the high latitude mesosphere. Only regions above 55° latitudes show a clear response. The fraction of cases in which there is an unambiguous enhancement in GW activity following the onset of geomagnetic disturbance is smaller during summer than other seasons. Only about half of the events show an unambiguous increase in GW activity during non-summer periods and about one quarter of the events in summer show an enhancement in GWs. In addition, we also find that the high latitude mesopause is seen to descend in altitude following onset of geomagnetic activity in the non-summer high latitude region.

The objective of this study was to examine both the climatology of the residual mean circulation, and the roles of resolved wave (RW) and unresolved wave (UW) forcings over four Mars years, based on the transformed Eulerian mean equation system using the EMARS reanalysis dataset. While RW forcing was estimated directly as Eliassen–Palm flux divergence, the forcing by UWs, including subgrid-scale gravity waves, was estimated indirectly using the zonal momentum equation. This indirect method, devised originally for study of Earth’s middle atmosphere, is applicable to latitudinal regions having angular momentum isopleths connected from the surface to the top of the atmosphere, which are usually mid- and high-latitude regions. In low latitudes of the winter hemisphere, a strong residual mean poleward flow is observed at an altitude range of 40–80 km, where the latitudinal gradient of the absolute angular momentum is small. The strong poleward flow crosses the isopleths of angular momentum in the regions of its northern and southern ends, indicating the necessity of the wave forcing. Our results suggest that the structure of the residual mean circulation at mid- and high-latitude regions is largely determined by UW forcing, particularly above the altitude of 60 km, whereas the RW contribution is also large below the altitude of 60 km.

This study assesses less-explored Southern Ocean sea-ice parameters, namely Sea-ice Thickness and Volume, through a comprehensive comparison of 26 CMIP6 models with reanalyses and satellite observations. Findings indicate that models replicate the mean seasonal cycle and spatial patterns of sea-ice thickness, particularly during its maxima in February. However, some models simulate implausible historical mean states compared to satellite observations, leading to large inter-model spread. September sea-ice thickness is consistently biased low across the models. Our results show a positive relationship between modeled mean sea-ice area and thickness in September (i.e., models with more area tend to have thicker ice); in February this relationship becomes negative. While CMIP6 models demonstrate proficiency in simulating Area, thickness accuracy remains a challenge. This study, therefore, highlights the need for improved representation of Antarctic sea-ice processes in models for accurate projections of thickness and volume changes.

We examine tropical rainfall from Geophysical Fluid Dynamics Laboratory’s Atmosphere Model version 4 (GFDL AM4) at three horizontal resolutions of 100 km, 50 km, and 25 km. The model produces more intense rainfall at finer resolutions, but a large discrepancy still exists between the simulated and the observed frequency distribution. We use a theoretical precipitation scaling diagnostic to examine the frequency distribution of the simulated rainfall. The scaling accurately produces the frequency distribution at moderate-to-high intensity (≥10 mm day-1). Intense tropical rainfall at finer resolutions is produced primarily from the increased contribution of resolved precipitation and enhanced updrafts. The model becomes more sensitive to the grid-scale updrafts than local thermodynamics at high rain rates as the contribution from the resolved precipitation increases. On the contrary, the observed tropical precipitation extremes do not show a strong sensitivity to the grid-scale updrafts.

Cold pools formed by precipitating convective clouds are an important source of mesoscale temperature variability. However, their sub-mesoscale (100 m to 10 km) structure has not been studied, which impedes validation of numerical models and understanding of their atmospheric and societal impacts. We quantify temperature variability in observed and simulated cold pools using variograms calculated from dense network observations collected during a field experiment and in high-resolution case study and idealized simulations. The temperature variance in cold pools is enhanced for spatial scales between ~5-15 km compared to pre-cold pool conditions, but the magnitude varies strongly with cold pool evolution and environment. Simulations capture the overall cold pool variogram shape well but underestimate the magnitude of the variability, irrespective of model resolution. Temperature variograms outside of cold pool periods are represented by the range of simulations evaluated here, suggesting that models misrepresent cold pool formation and/or dissipation processes.

The escalating climate crisis requires rapid action to reduce the concentrations of atmospheric greenhouse gases and lower global surface temperatures. Methane will play a critical role in near-term warming due to its high radiative forcing and short atmospheric lifetime. Methane emissions have accelerated in recent years and there is significant risk and uncertainty associated with the future growth in natural emissions. The largest natural sink of methane occurs through oxidation reactions with atmospheric hydroxyl and chlorine radicals. Enhanced atmospheric oxidation could be a potential approach to remove atmospheric methane. One method proposes the addition of iron salt aerosols (ISA) to the atmosphere, mimicking a natural process that is proposed to occur when mineral dust mixes with chloride from sea spray to form iron chlorides, which are photolyzed by sunlight to produce chlorine radicals. Under the right conditions, lofting ISA into the atmosphere could potentially reduce atmospheric methane concentrations and lower global surface temperatures. Recognizing that potential atmospheric methane removal must only be considered as an additive measure-in addition to, not replacing, crucial anthropogenic greenhouse gas emission reductions and carbon dioxide removal-roadmaps can be a valuable tool to organize and streamline interdisciplinary and multifaceted research to efficiently move towards an understanding of whether an approach may be viable and socially acceptable, or if it is nonviable and further research should be deprioritized. Here we present an example of a five-year research roadmap to explore whether ISA enhancement of the chlorine radical sink could be a viable and socially acceptable atmospheric methane removal approach.

Hurricane Nicholas was classified as a Category 1 tropical cyclone (TC) at 0000 UTC on 14 September 2021 and made landfall along the upper Texas Gulf Coast at 0530 UTC. The sustained maximum wind speed increased from a low-end estimate of 13 m s-1 (0000 UTC 13 September) to 33 m s-1 (0000 UTC 14 September) indicating rapid intensification. Lightning activity, monitored by the Houston Lightning Mapping Array (HLMA), developed in the rainband at 1700 UTC on 13 September, diminished by 2030 UTC, and re-intensified after 2200 UTC. At 2004 UTC (13 September), a curved megaflash (~220 km) was observed in the outer rainband’s stratiform precipitation region. Convection developed and intensified in the eastern eyewall region by 0130 UTC on 14 September. Several transient luminous events (TLEs) were observed in the western eyewall region between 0230-0300 UTC with VHF source points exceeding 40 km during a decline in lightning activity. The TLEs occurred during a period of strong cloud top divergence resulting from complex interactions between southwesterly low-level and westerly deep layer wind shear. Charge analysis of Nicholas revealed an overall normal dipole structure, while the megaflash and TLE cases exhibited inverted charge structures. The upper-level screening and primary charge layer heights of the TLEs heavily influenced the VHF source altitudes. Interestingly, a surface wind gust of 42 m2 s-2 was observed near the time of the first TLE, suggesting a second period of brief intensification. Future investigations of TC evolution and behavior may benefit from charge analyses.

The Kuroshio-Oyashio Extension and Gulf Stream oceanic frontal zones with sharp sea-surface temperature gradients are characterized by enhanced activity of synoptic-scale cyclones and anticyclones and vigorous air-sea exchange of heat and moisture in the cold season. However, the air-sea exchanges attributed separately to cyclones and anticyclones have not been assessed. Here we quantify cyclonic and anticyclonic contributions around the oceanic frontal zones to surface turbulent heat fluxes, precipitation, and the associated hydrological cycle. The evaluation reveals that precipitation exceeds evaporation climatologically within cyclonic domains while evaporation dominates within anticyclonic domains. These features as well as the net moisture transport from anticyclonic to cyclonic domains are all enhanced in the presence of the frontal zones. Oceanic frontal zones thus climatologically act to strengthen the hydrological cycle through increasing low-level storm-track activity and specific humidity. These findings aid our understanding of the relationship between midlatitude air-sea interactions on synoptic- and longer-time scales.

We are reporting the discovery of a new radio analysis for studying plasma magnetic fields. In the high frequency regime, wherein the electromagnetic wave frequency exceeds the plasma and cyclotron frequencies, propagating electromagnetic waves assume four different plasma wave modes: (1) the ordinary “O-mode” (present with/without a magnetic field), (2) the right-handed circularly polarized “RCP-mode” (parallel to magnetic field), (3) the left-handed circularly polarized “LCP-mode” (parallel to magnetic field), and (4) the extra-ordinary “X-mode”. The X-mode occurs when the magnetic field is perpendicular to the electromagnetic wave propagation direction. Until now, the X-mode has not been studied. For applications such as total electron content measurements, the X-mode is a source of error. We show how to isolate this effect, eliminating the error as well as enabling the magnetic field to be studied. This also enables the remote measurement of total magnetic field strength when used in combination with Faraday rotation.

Understanding surface temperature is important for habitability. Recent work on Mars has found that the dependence of surface temperature on elevation (surface lapse rate) converges to zero in the limit of a thin $\mathrm{CO_2}$ atmosphere. However, the mechanisms that control the surface lapse rate are still not fully understood. It remains unclear how the surface lapse rate depends on both greenhouse effect and surface pressure. Here, we use climate models to study when and why “mountaintops are cold”. We find the tropical surface lapse rate increases with the greenhouse effect and with surface pressure. The greenhouse effect dominates the surface lapse rate transition and is robust across latitudes. The pressure effect is important at low latitudes in moderately opaque ($\tau \sim 0.1$) atmospheres. A simple model provides insights into the mechanisms of the transition. Our results suggest that topographic cold-trapping may be important for the climate of arid planets.

In the context of space weather forecasting, solar EUV irradiance specification is needed on multiple time scales, with associated uncertainty quantification for determining the accuracy of downstream parameters. Empirical models of irradiance often rely on parametric fits between irradiance in several bands and various solar indices. We build upon these empirical models by using Generalized Additive Models (GAMs) to represent solar irradiance. We apply the GAM approach in two steps: (1) A GAM is between FISM2 irradiance and solar indices F10.7, Revised Sunspot Number, and the Lyman-alpha solar index. (2) A second GAM is fit to model the residuals of the first GAM with respect to TIMED/SEE irradiance. We evaluate the performance of this approach during Solar Cycle 24 and compare it with FISM2, and show that mean relative irradiance error for integrated solar EUV irradiance is decreased from -3.459% for FISM2 to 0.04% using GAMs driven by known solar indices with a small increase in variance. We demonstrate negligible dependence of performance on solar cycle and season, and we assess the efficacy of the GAM approach across different wavelengths.

The influence of climate feedbacks on regional hydrological changes under warming is poorly understood. Here, a moist energy balance model (MEBM) with a Hadley Cell parameterization is used to isolate the influence of climate feedbacks on changes in zonal-mean precipitation-minus-evaporation (P-E) under greenhouse-gas forcing. It is shown that cloud feedbacks act to narrow bands of tropical P-E and increase P-E in the deep tropics. The surface-albedo feedback shifts the location of maximum tropical P-E and increases P-E in the polar regions. The intermodel spread in the P-E changes associated with feedbacks arises mainly from cloud feedbacks, with the lapse-rate and surface-albedo feedbacks playing important roles in the polar regions. The P-E change associated with cloud feedback locking in the MEBM is similar to that of a climate model with inactive cloud feedbacks. This work highlights the unique role that climate feedbacks play in causing deviations from the “wet-gets-wetter, dry-gets-drier” paradigm.