Air Parcels Research Articles

On the Local Available Potential Energy Perspective of Baroclinic Wave Development

Abstract Extratropical cyclones convert available potential energy (APE) to kinetic energy. However, our current understanding of APE conversion on synoptic scales is limited, as the well-established Lorenz APE framework is only applicable in a global, volume-integrated sense. Here, we employ a recently developed local APE framework to investigate APE and its tendencies in a highly idealized, dispersive baroclinic wave, which leads to the formation of a primary and a downstream cyclone. By utilizing a Lagrangian approach, we demonstrate that locally the downstream cyclone not only consumes APE but also generates it. Initially, APE is transported from both poleward and equatorward reservoirs into the baroclinic zone, where it is then consumed by the vertical displacement of air parcels associated with the developing cyclone. To a lesser extent, APE is also created within the cyclone when air parcels overshoot their reference state; i.e., air colder than its reference state is lifted and air warmer than its reference state is lowered. The volume integral of the APE tendency is dominated by slow vertical displacements of large air masses, whereas the dry intrusion (DI) and warm conveyor belt (WCB) of the cyclone are responsible for the largest local APE tendencies. Diabatic effects within the DI and WCB contribute to the generation of APE in regions where it is consumed adiabatically, thereby enhancing baroclinic conversion in situ. Our findings provide a comprehensive and mechanistic understanding of the local APE tendency on synoptic scales within an idealized setting and complement existing frameworks explaining the energetics of cyclone intensification.

Why does tropical cyclone intensify faster under weaker parameterized horizontal diffusion in three-dimensional numerical simulations?

The mechanisms by which the parameterized horizontal diffusion impacts the tropical cyclone (TC) intensification are investigated via a series of numerical simulations of Supertyphoon Lekima (2019) with different horizontal mixing lengths (Lh). Consistent with previous studies, the TC intensifies faster and attains a higher peak intensity as Lh decreases, which is attributed to a more compact wind structure and the stronger eyewall convective heating. The azimuthal-mean tangential wind and vertical momentum budgets are conducted to explain the differences in TC size and convective strength between the simulations. It is found that when horizontal diffusion weakens, the eddy mixing of tangential momentum becomes larger inside the radius of maximum wind, therefore promoting size contraction. The stronger eddy mixing is attributed to the more active eyewall mesovortices whose growth is suppressed when horizontal diffusion strengthens. The vertical momentum budget results indicate that the air parcels in the simulation with smaller Lh experience stronger upward buoyancy force in the lower-to-mid troposphere (3–7 km) than those in the larger-Lh simulation, leading to more vigorous eyewall convection. It is further revealed that the reduced horizontal diffusion helps maintain the strength of virtual potential temperature perturbations and therefore the buoyancy in the eyewall updrafts. The findings from this study imply that the horizontal diffusion plays an important role in regulating the mesoscale and convective-scale processes in the inner-core region and improvements in its parameterization are urged for improving TC intensity forecast.

Considering intentional stratospheric dehydration for climate benefits.

We introduce a climate intervention strategy focused on decreasing water vapor (WV) concentrations near the tropopause and in the stratosphere to increase outbound longwave radiation. The mechanism is the targeted injection of ice-nucleating particles (INP) in air supersaturated with respect to ice at high altitudes in the tropical entryway to the stratosphere. Ice formation in this region is a critical control of stratospheric WV. Recent airborne in situ data indicate that targeting only a small fraction of air parcels in the region would be sufficient to achieve substantial removal of water. This "intentional stratospheric dehydration" (ISD) strategy would not counteract a large fraction of the forcing from carbon dioxide but may contribute to a portfolio of climate interventions by acting with different time and length scales of impact and risk than other interventions that are already under consideration. We outline the idea, its plausibility, technical hurdles, and side effects to be considered.

Influence of temperature perturbation on moisture dynamics

We analyzed the perturbation effect of temperature on the moisture content in the atmosphere [], considering the saturation phenomenon. The air parcels of a saturated atmosphere are made of dry air, vapor, cloud, and rain water that flow on a rotating Earth, with a temperature perturbations induced at 9°1′48″ N, 38°44′24″E and 6.324 km above the Earth’s surface. The fundamental atmospheric parameter of this saturated moist air was treated as temperature-dependent, and the pressure force exerted on it by the precipitation and cloud water is not considered in the numerical computation. In this proposed research, the initial value of wind speed along zonal, meridional, and vertical directions varies with elevation, and the interaction of the atmosphere with the Earth’s surface is not neglected. The result of the investigation revealed that a single tropospheric disturbance of an atmospheric variable is a major factor in the spread of different long period waves patterns (350 hours) and this atmospheric oscillations play a crucial role in transferring energy, momentum, and mass across different parts of the atmosphere, impacting global climate systems.

Influences of sources and weather dynamics on atmospheric deposition of Se species and other trace elements

Abstract. Atmospheric deposition is an important source of the micronutrient selenium for terrestrial ecosystems and food chains. However, the factors determining the total concentrations and chemical forms (speciation) of selenium in atmospheric deposition remain poorly understood. Here, aerosol samples were collected weekly over 5 years at Pic du Midi Observatory (French Pyrenees), alongside highly temporally resolved samples of aerosols, precipitation, and cloud water taken during a 2-month campaign. Firstly, measurements of selenium, other elements, and water isotopes were combined with sophisticated modelling approaches (aerosol–chemistry–climate SOCOL-AERv2 model and air parcel backward trajectories and Lagrangian moisture source analyses). Aerosol selenium measurements agreed well with SOCOL-AERv2-predicted values, and interestingly, higher fluxes of selenium and other elements were associated with deep convective activity during thunderstorms, highlighting the importance of local cloud dynamics in high deposition fluxes. Our results further indicate the coupling of element and water cycles from source to cloud formation, with decoupling during precipitation due to below-cloud scavenging. Secondly, selenium speciation was investigated in relation to sulfur speciation, organic composition, and moisture sources. While in the 5-year aerosol series, selenite (SeIV) was linked to anthropogenic source factors, in wet deposition it was related to pH and Atlantic moisture sources. We also report an organic selenium fraction, tracing it back to a marine biogenic source in both aerosols and wet deposition. With a comprehensive set of observations and model diagnostics, our study underscores the role of weather system dynamics alongside source contributions in explaining the atmospheric supply of trace elements to surface environments.

Modeling SO2 dispersion from future eruptions in the Auckland Volcanic Field, New Zealand

Auckland city (pop. 1.7 M) is Aotearoa New Zealand’s largest city and an important economic hub. The city is built upon the active intraplate basaltic Auckland Volcanic Field (AVF). An AVF eruption would cause considerable impacts. An important component of volcanic risk management is assessing the likely volcanic hazards to help inform emergency planning and other preparedness activities. Previous volcanic hazard assessments for the AVF, particularly those for emergency planning scenarios, have modeled multiple volcanic hazards including lava flows, pyroclastic density currents, ballistic projectiles and tephra fall. Despite volcanic gas being an important and impactful hazard from intraplate basaltic field eruptions, there has been limited consideration of volcanic gas in AVF hazard assessment to date. This project is one of the first to quantitatively assess potential volcanic gas hazards for an explosive eruption scenario. For basaltic volcanism, sulfur dioxide (SO2) gas is typically the most consequential volcanic gas emitted. The aim of this exploratory study was to model SO2 dispersion from a high impact eruption during weather conditions conducive to high ground level pollutant concentrations. Since ground level SO2 concentrations are influenced by complex wind patterns resulting from interactions of locally driven flow circulations and topographically influenced weather, we modeled SO2 dispersion using the HYSPLIT model, a state-of-the art hybrid Eulerian and Lagrangian dispersion model widely used for volcanic gases, using high-resolution meteorological forcing fields given by the Weather Research and Forecasting (WRF) model.Modeled air parcel trajectories and ground level SO2 concentrations illustrate the effect of the converging sea breeze winds on SO2 dispersion. Under worst-case dispersion conditions, extensive areas of up to hundreds of square kilometers to the north and northwest of the eruption location would exceed New Zealand short-term (24 h) air quality standards and guidelines for SO2, indicating heightened health risks to downwind communities. Using this numerical modeling approach, this work presents a methodology for future applications to other AVF eruption scenarios, with a wider range of meteorological conditions that can help in exploring consequences for health services such as anticipated emergency department respiratory admissions.

Future changes in North Atlantic winter cyclones in CESM-LE – Part 2: A Lagrangian analysis

Abstract. Future changes in extratropical cyclone structure and dynamics may lead to important impacts but are not yet fully understood. In the first part of this study, we have applied a composite approach together with potential vorticity (PV) inversion to study such changes in the dynamics of North Atlantic cyclones. Here, this is complemented with the help of a Lagrangian perspective, making use of air parcel trajectories to investigate the causes of altered PV anomalies as well as the role that cyclone airstreams play in shaping these changes. Intense cyclones in the extended winter seasons of two periods, 1990–2000 and 2091–2100, are studied in Community Earth System Model Large Ensemble (CESM-LE) simulations, and backward trajectories are calculated from the cyclone area as a basis to construct cyclone-centered composites of Lagrangian tendencies and their projected future changes. Our results show that diabatic processes on a timescale of 24 h shape the cyclones' low-level PV distribution and corroborate that the increasing moisture content along with enhanced ascent in warm conveyor belts leads to amplified latent heat release and larger low- and mid-level PV anomalies near the cyclone center in a warmer climate. In contrast, projected upper-level PV changes are due to a combination of several processes. These processes include cloud diabatic PV changes, anomalous PV advection, and likely also radiative PV generation in the lower stratosphere above the cyclone center. For instance, enhanced poleward advection is the primary reason for a projected decrease in upper-level PV anomalies south of the cyclone center. Warm conveyor belt outflow regions are projected to shift upward, but there is not robust change in the associated upper-level PV anomalies due to compensation between enhanced low-level PV generation and upper-level PV destruction. In summary, our two-part study points to future changes in the relative importance of different processes for the dynamics of intense North Atlantic cyclones in a warming climate, with important consequences for the near-surface wind pattern. In particular, a larger role of cloud diabatic processes is projected, affecting the cyclones through PV production in the lower troposphere. The role of other mechanisms, in particular radiative changes near the tropopause, should be investigated in more detail in future studies.

Spatial variability in stable isotopes from Lesotho surface waters: insights into regional moisture transport

Precipitation in Lesotho is highly spatially variable, a feature of the high altitude and rugged topography. The hydroclimate dynamics, despite being critical to the water security of Lesotho and adjacent South Africa, are poorly understood. Ratios of oxygen and hydrogen isotopes in meteoric water are excellent tracers of hydroclimatic processes. This study presents the first analysis of stable isotopes from surface waters in Lesotho, and an investigation into the moisture sources. Our results demonstrate considerable variability in isotope values. There are statistically significant relationships between both oxygen and hydrogen isotopes and the altitude of the site and source of rivers sampled, and with hydrogen isotopes and longitude. The meteoric water line for the Lesotho samples is most closely aligned with that of the Global Network of Isotopes in Precipitation (GNIP) station at Harare, in Zimbabwe. The meteoric water line for Windhoek is more closely aligned to the Lesotho samples than the more proximate Cape Town or Pretoria meteoric water lines, which would more closely represent the South African winter- and summer-rainfall zones respectively. HYSPLIT back-trajectory air parcel analysis supports these findings, demonstrating a frequent continental anticyclonic track through southern Zimbabwe. Deuterium excess values vary widely, although are most likely related to processes during moisture transport rather than differences in moisture source. These findings are of particular importance in the context of the future water security of both Lesotho and South Africa, especially as the poleward displacement of the westerly moisture corridor has raised concerns for winter precipitation in the region.

Aircraft observations in a tropical supercluster over the equatorial Indian Ocean during MISO-BOB field campaign

The Monsoon Intra-Seasonal Oscillations in the Bay of Bengal (MISO-BOB) field campaign was conducted in the Indian Ocean during the 2018 and 2019 summer monsoon seasons. WC-130J aircraft of the 53rd Weather Reconnaissance Squadron of the US Air Force participated in the campaign in June 2018. The dropsonde observations across a tropical supercluster showed zonal wind variations in association with the structure of the convectively coupled Kelvin wave (CCKW). Within the supercluster, easterlies (westerlies) were observed in the upper (lower) troposphere; this transformation occurred just below the 0∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{\\circ }$$\\end{document} C level. The cold pool had an easterly component throughout, and it was coldest (by 2.5∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{\\circ }$$\\end{document} C) at the center of the supercluster, deepest (∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document}1000m\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1000\\,\\hbox {m}$$\\end{document}) at its rear/western end, and shallowest (∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document} 300 m) at the front/eastern end. The level of free convection (LFC) at the front end was at 897m\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$897\\,\\hbox {m}$$\\end{document} altitude. At the eastern flank of the supercluster, zonal convergence in the lower troposphere occurred between 500-1500 m levels above the surface between the westerlies within the supercluster and opposing ambient easterlies. Thus, the uplifting of conditionally unstable air parcels above LFC to the east of the supercluster was likely to occur due to this convergence rather than the cold pool influence. Conversely, the western flank of the supercluster had low-level zonal divergence. These observations support the notion of ‘self-similarity’ among the mesoscale convective systems and large-scale waves.

Simulating cumulus clouds based on self-organized criticality

Recently it was shown that self-organized criticality is an important component of the dynamics of cumulus clouds (Najafi et al., 2021). Here we introduce a new algorithm to simulate cumulus clouds in two-dimensional square lattices, based on two important facts: the cohesive energy of wet air parcels and a sand-pile-type diffusion of cloud segments. The latter is realized by considering the evaporation/condensation of air parcels in various regions of the cloud, which enables them to diffuse to the neighboring regions. The results stemming from this model are in excellent agreement with observations reported in the above-cited paper, in which the exponents were determined for two-dimensional earth-to-sky RGB cloud images. The exponents obtained at the lowest condensation level in our model are consistent with the exponents observed in nature. We find that the cloud fields obtained from our model are fractal, with the outer perimeter having a fractal dimension of Df=1.25±0.01. Furthermore, the distributions of the radius of gyration and the loop length follow a power-law function with exponents τr=2.3±0.1 and τl=2.1±0.1, respectively. The loop Green function is found to be logarithmic with the radius of gyration of the loops following the observational results. The winding angle statistic of the external perimeter of the cloud field is also analyzed, showing an exponent in agreement with the fractal dimension, which may serve as the conformal invariance of the system.

Characterizing Isotopic Composition and Trajectories of Atmospheric River Events

Landfalling atmospheric rivers (LARs) are important drivers of mid-latitude climate; however, our understanding of the water vapour sources, storm trajectories, and receiving waters of ARs is limited. This study aims to characterize LARs in southwest British Columbia by their isotopic composition and storm track trajectories and to better understand how AR-derived precipitation is manifested in watershed waters. ARs were depleted (−11.71‰ δ18O, −85.80‰ δ2H, n = 19) compared to non-ARs (−9.47‰ δ18O, −69.58‰ δ2H, n = 32) (p = 0.03); however, the difference is minimal. LAR storm tracks did not show any obvious correlation to their isotopic composition, despite the large variability in their source regions across the Pacific Ocean. The lack of correlation is attributed to mixing air parcels, thereby incorporating moisture with different isotopic compositions into the main transport mechanism. D-excess values for ARs and non-ARs were statistically similar, although seasonal differences were observed. ARs with higher d-excess were sourced from the central Pacific, whereas ARs with lower d-excess had storm tracks through the northern Pacific. Watershed water d-excess values (mean = 8.58 ± 2.97‰) were more similar to winter precipitation (mean = 10.1 ± 5.1‰), compared to summer (mean = 2.8 ± 4.3‰), likely due to their source of winter precipitation at high elevation. A greater range in AR d-excess winter values relative to summer values (3.6–16.6‰, −0.3–6.0‰, respectively) is attributed to storm track variability.

Environmental ingredients that lead to tornado outbreak and tornado failure: A comparison between two similar recurving tropical cyclones

AbstractRecurving tropical cyclones (TCs) can sometimes produce tornado outbreaks, while some TCs with similar tracks and intensities may produce none of tornado, which makes it challenging to assess tornado risk within recurving TCs. This study investigates two recurving TCs, Typhoon Yagi (2018) and Typhoon In‐Fa (2021), that made landfall in eastern China. Despite the similar recurving tracks and intensities, Yagi produced 11 tornadoes while In‐Fa produced none. Results show that both TCs were characterized by similar large‐scale conditions that were dynamically favourable for tornadoes during the recurvature process. The non‐tornadic In‐Fa even featured a higher shear and helicity environment in its northeast sector than did the tornado‐productive Yagi. The greatest difference between Yagi and In‐Fa is the thermodynamic instability owing to the different lower–middle‐tropospheric lapse rates that are attributable to the differences in air trajectories at low levels. In‐Fa featured marginal instability due to the cooler air at low levels because almost all of the air parcels came from the Pacific Ocean while most air parcels for Yagi came from the warm land. The cooler low‐level air tends to create higher relative humidity in In‐Fa's interior and thus leads to widespread precipitation which in turn also contributes to the low‐level cooling. The different air trajectories are demonstrated related to the TC's translation speed, size and synoptic characteristics days before TC's landfall. Numerical simulations suggest that the upward motions within the widespread precipitation regions of In‐Fa are overall weaker than those of Yagi due to the limited instability in the former. These findings suggest that even though two TCs were characterized by similar tracks, intensities and large‐scale forcings, their different low‐level air pathways may have significant influence on priming the mesoscale environment for supercell or tornado formation.

Steady-State Supersaturation Distributions for Clouds under Turbulent Forcing

Abstract The supersaturation equation for a vertically moving adiabatic cloud parcel is analyzed. The effects of turbulent updrafts are incorporated in the shape of a stochastic Lagrangian model, with spatial and time correlations expressed in terms of turbulent kinetic energy. Using the Fokker–Planck equation, the steady-state probability distributions of supersaturation are analytically computed for a number of approximations involving the time-scale separation between updraft fluctuations and phase relaxation, and droplet or ice particle size fluctuations. While the analytical results are presented in general for single-phase clouds, the calculated distributions are used to compute mixed-phase cloud properties—mixed fraction and mean liquid water content in an initially icy cloud—and are argued to be useful for generalizing and constructing new parameterization schemes. Significance Statement Supersaturation is the fuel for the development of clouds in the atmosphere. In this paper, our goal is to better understand the supersaturation budget of clouds embedded in a turbulent environment by analyzing the basic equations of cloud microphysics. It is found that the turbulent characteristics of an air parcel substantially affect the cloud’s supersaturation budget and hence its life cycle. This is also shown in the context of mixed-phase clouds where, depending on the turbulent regime, different liquid-to-ice ratios are found. Consequently, the theoretical approach of this paper is crucial to develop tools to parameterize small-scale atmospheric features, like clouds, into global circulation models to improve climate projections for the future.

Deep Learning Parameterization of Vertical Wind Velocity Variability via Constrained Adversarial Training

Abstract Atmospheric models with typical resolution in the tenths of kilometers cannot resolve the dynamics of air parcel ascent, which varies on scales ranging from tens to hundreds of meters. Small-scale wind fluctuations are thus characterized by a subgrid distribution of vertical wind velocity W with standard deviation σW. The parameterization of σW is fundamental to the representation of aerosol–cloud interactions, yet it is poorly constrained. Using a novel deep learning technique, this work develops a new parameterization for σW merging data from global storm-resolving model simulations, high-frequency retrievals of W, and climate reanalysis products. The parameterization reproduces the observed statistics of σW and leverages learned physical relations from the model simulations to guide extrapolation beyond the observed domain. Incorporating observational data during the training phase was found to be critical for its performance. The parameterization can be applied online within large-scale atmospheric models, or offline using output from weather forecasting and reanalysis products. Significance Statement Vertical air motion plays a crucial role in several atmospheric processes, such as cloud droplet and ice crystal formation. However, it often occurs at scales smaller than those resolved by standard atmospheric models, leading to uncertainties in climate predictions. To address this, we present a novel deep learning approach that synthesizes data from various sources, providing a representation of small-scale vertical wind velocity suitable for integration into atmospheric models. Our method demonstrates high accuracy when compared to observation-based retrievals, offering potential to mitigate uncertainties and enhance climate forecasting.

Diurnal cycle of precipitation over the tropics and central United States: intercomparison of general circulation models

AbstractDiurnal precipitation is a fundamental mode of variability that climate models have difficulty in accurately simulating. Here the diurnal cycle of precipitation (DCP) in participating climate models from the Global Energy and Water Exchanges' DCP project is evaluated over the tropics and central United States. Common model biases such as excessive precipitation over the tropics, too frequent light‐to‐moderate rain, and the failure to capture propagating convection in the central United States still exist. Over the central United States, the issues of too weak rainfall intensity in climate runs is well improved in their hindcast runs with initial conditions from numerical weather prediction analyses. But the improvement is minimal over the central Amazon. Incorporating the role of the large‐scale environment in convective triggering processes helps resolve the phase‐locking issue in many models where precipitation often incorrectly peaks near noon due to maximum insolation over land. Allowing air parcels to be lifted above the boundary layer improves the simulation of nocturnal precipitation which is often associated with the propagation of mesoscale systems. Including convective memory in cumulus parameterizations acts to suppress light‐to‐moderate rain and promote intense rainfall; however, it also weakens the diurnal variability. Simply increasing model resolution (with cumulus parameterizations still used) cannot fully resolve the biases of low‐resolution climate models in DCP. The hierarchy modeling framework from this study is useful for identifying the missing physics in models and testing new development of model convective processes over different convective regimes.

The meteorology and impacts of the September 2020 Western United States extreme weather event

In September 2020, Western North America was impacted by a highly anomalous meteorological event. Over the Pacific Northwest, strong and dry easterly winds exceeded historically observed values for the time of year and contributed to the rapid spread of several large wildfires. Nine lives were lost and over 5000 homes and businesses were destroyed in Oregon. The smoke from the fires enveloped the region for nearly two weeks after the event. Concurrently, the same weather system brought record-breaking cold, dramatic 24-h temperature falls, and early-season snowfall to parts of the Rocky Mountains. Here we use synoptic analysis and air parcel backward trajectories to build a process-based understanding of this extreme event and to put it in a climatological context. The primary atmospheric driver was the rapid development of a highly amplified 500 hPa tropospheric wave pattern that persisted for several days. A record-breaking ridge of high pressure characterized the western side of the wave pattern with a record-breaking trough of low pressure to the east. A notable anticyclonic Rossby wave breaking event occurred as the wave train amplified. Air parcel backward trajectories show that dry air over the Pacific Northwest, which exacerbated the fire danger, originated in the mid-troposphere and descended through subsidence to the surface. At the same time, dramatic temperature falls were recorded along the east side of the Rocky Mountains, driven by strong transport of high-latitude air near the surface.

Incorporating an Interactive Fire Plume‐Rise Model in the DOE's Energy Exascale Earth System Model Version 1 (E3SMv1) and Examining Aerosol Radiative Effect

AbstractThe vertical distribution of biomass burning aerosol (BBA) is important in regulating their impacts on weather and climate. The plume‐rise process affects the injection height of BBA and interacts with the air parcel lifting and cloud processes. However, these processes are not represented in most global climate models. In this study, we replaced the fixed vertical profiles of monthly BBA emissions in the Department of Energy's Energy Exascale Earth System Model version 1 (E3SMv1) with an interactive fire plume‐rise model. The vertical distribution of BBA emissions was calculated as a function of ambient thermodynamic conditions from the host E3SMv1, with distributions of fire sizes and sensible heat fluxes derived from the observations. The maximum fire radiative power (FRP) technique was used to determine the fire size. Scaling‐FRP technique is used to calculate the wildfire heat release. Daily BBA emission, superimposed with a fire diurnal cycle retrieved from the satellite observation, was included in model simulations. The model shows improved agreement with satellite retrievals and in situ measurement during the National Oceanic and Atmospheric Administration Wildfire Experiment for Cloud chemistry, Aerosol absorption, and Nitrogen campaign. The model‐observation comparison demonstrates the importance of the plume‐rise model and fire diurnal cycle assumption in determining the BBA fields. We also find that E3SMv1 with new features produces a larger carbonaceous aerosol burden, leading to 0.13 W m−2 warming at the top of atmosphere compared to the default E3SMv1. This highlights the importance of accurately representing the BBA injection height and indicates a no‐linear nature in the BBA‐induced radiative effect.

Identifying the pathways of extreme rainfall in South Africa using storm trajectory analysis and unsupervised machine learning techniques

Abstract This study has utilised National Oceanic and Atmospheric Administration (NOAA) NCEP/NCAR Reanalysis 1 project meteorological data and the HYSPLIT model to extract the air parcel trajectories for selected historical extreme rainfall events in South Africa. The k-means unsupervised machine learning algorithm has been used to cluster the resulting trajectories, and from this, the spatial origin of moisture for each of the rainfall events has been determined. It has been demonstrated that rainfall events on the east coast with moisture originating from the Indian Ocean have distinctly larger average maximum daily rainfall magnitudes (279 mm) compared to those that occur on the west coast with Atlantic Ocean influences (149 mm) and those events occurring in the central plateau (150 mm) where moisture has been continentally recirculated. Further, this study has suggested new metrics by which the HYSPLIT trajectories may be assessed and demonstrated the applicability of trajectory clustering in a region not previously studied. This insight may in future facilitate improved early warning systems based on monitoring of atmospheric systems, and an understanding of rainfall magnitudes and origins can be used to improve the prediction of design floods for infrastructure design.

Towards a more reliable forecast of ice supersaturation: concept of a one-moment ice-cloud scheme that avoids saturation adjustment

Abstract. A significant share of aviation's climate impact is due to persistent contrails. Thus, avoiding the creation of contrails that exert a warming impact is a crucial step in approaching the goal of sustainable air transportation. For this purpose, a reliable forecast of when and where persistent contrails are expected to form is needed (i.e. a reliable prediction of ice supersaturation). With such a forecast at hand, it would be possible to plan aircraft routes on which the formation of persistent contrails can be avoided. One problem on the way to these forecasts is the current systematic underestimation of the frequency and degree of ice supersaturation at cruise altitudes in numerical weather prediction due to the practice of “saturation adjustment”. In this common parameterisation, the air inside cirrus clouds is assumed to be exactly at ice saturation, while measurement studies have found cirrus clouds to be quite often out of equilibrium. In this study, we propose a new ice-cloud scheme that overcomes saturation adjustment by explicitly modelling the decay of the in-cloud humidity after nucleation, thereby allowing for both in-cloud super- and subsaturation. To achieve this, we introduce the in-cloud humidity as a new prognostic variable and derive the humidity distribution in newly generated cloud parts from a stochastic box model that divides a model grid box into a large number of air parcels and treats them individually. The new scheme is then tested against a parameterisation that uses saturation adjustment, where the stochastic box model serves as a benchmark. It is shown that saturation adjustment underestimates humidity, both shortly after nucleation, when the actual cloud is still highly supersaturated, and also in aged cirrus if the temperature keeps decreasing, as the actual cloud remains in a slightly supersaturated state in this case. The new parameterisation, on the other hand, closely follows the behaviour of the stochastic box model in any considered case. The improvement in comparison with saturation adjustment is largest if slow updraughts occur in relatively clean air in models with a high spatial and temporal resolution. We conclude that our parameterisation is promising but needs further testing in more realistic frameworks.

Pollution status and source resolution of atmospheric mercury in the intersectional region of Northern Philippines, southern Taiwan, and northern South China Sea

A four-season sampling of speciated atmospheric mercury (SAM) was employed in East Asia. Three mercury species including gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate mercury (PHg) were measured at three background sites. This study explored the temporal variation, spatial distribution, gas-solid partition, transport routes, and source resolution of SAM. The yearly average concentrations of GEM, GOM, and PHg were 2.76 ± 0.36 ng/m3, 23.14 ± 8.67 pg/m3, and 0.23 ± 0.058 ng/m3. The highest GEM and PHg concentrations occurred at Laoag (northern Philippines) in spring, while that of GOM was observed at Laoag but in summer. The lowest GEM and PHg concentrations always occurred at Dongsha Islands (northern South China Sea) in summer, while that of GOM was also observed at Dongsha Islands but in fall. SAM was partition as showed 92.64–96.72% of total gas-phase mercury (TGM) and 3.28–7.36% of solid phase mercury. Eight clustered transport routes were resolved by a HYSPLIT model with polluted air parcels originating from Central/North China. Routes from Indochina Peninsula and South China showed the highest GEM concentration. Moreover, air parcels transported from West Pacific Ocean and South China Sea (SCS) had the lowest concentration of GEM. High concentrations of SAM in the region were mainly attributed to long-range transport (LRT) from Asian continental outflows (ACOs). We further revealed that GEM was positively correlated to SO2, CO, NOx, and PM2.5, but negatively correlated to O3. GOM was positively correlated to SO2, O3, NOx, and PM2.5, but negatively correlated to CO, While, PHg was positively correlated to SO2, CO, O3, NOx, and PM2.5.