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Deniss J. Martinez
,
Alison M. Meadow
,
Beth Rose Middleton Manning
, and
Julie Maldonado

Abstract

Climate and weather-related disasters in California illustrate the need for immediate climate change action - both mitigation to reduce impacts and adaptation to protect our communities, relatives, and the ecosystems we depend upon. Indigenous frontline communities face even greater threats from climate impacts due to historical and political legacies of environmental injustice. Climate change adaptation actions have proven challenging to implement as communities struggle to access necessary climate data at appropriate scales, identify effective strategies that address community priorities, and obtain resources to act, at a whole-community level. In this paper, we present three examples of Indigenous communities in California that have used a climate justice approach to climate change adaptation. These communities are drawing upon community knowledge and expertise to address the challenges of adaptation planning, and taking actions that center community priorities. The three cases address emergency preparation and response, cultural burning and fire management, and community organizing and social cohesion. Across these spheres, they illustrate the ways in which a community-based and climate justice-focused approach to adaptation can be effective in addressing current threats, while also addressing the legacy of imposed, socially constructed vulnerability and environmental injustices. Because we recognize the need for multiple knowledges and skills in adaptation actions, we include recommendations that have emerged based on what’s been learned through these long-standing and engaged participatory research collaborations for climate scientists who wish to contribute to climate justice-focused adaptation efforts by using scientific data to support – not supplant – community efforts, target funding toward genuine community engagement and adaptation actions, and become aware of the historical and political legacies that created the climate vulnerabilities and injustices evident today.

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Mingze Ding
,
Zhehui Shen
,
Ruochen Huang
,
Ying Liu
, and
Hao Wu

Abstract

Evaluating the accuracy of various precipitation datasets over ungauged or even sparse-gauge areas is a challenging task. Cross-validation methods can evaluate three or more datasets based on the error independence from input data, without relying on ground reference. Here, the triple collocation (TC) method is employed to evaluate multi-source precipitation datasets: gauge-based CGDPA, model-based ERA5, and satellite-derived IMERG-Early, IMERG-Late, GSMaP-NRT, and GSMaP-MVK over the Tibetan Plateau (TP). TC-based results show that ERA5 has better performances than satellite-only precipitation products over mountainous regions with complex terrains. For purely satellite-derived products, IMERG products outperform GSMaP products. Considering the potential existence of error dependency among input datasets, caution should be exercised. Thus, it is necessary to introduce an alternative cross-validation method (generalized Three-Cornered Hat) and explore the applicability of cross-validation from the perspective of error independence. We found that cross-validation methods have high applicability in most TP regions with sparse-gauge density (accounting for about 80.1% of the total area). Additionally, we conducted simulation experiments to discuss the applicability and robustness of TC. The simulation results substantiated that cross-validation can serve as a robust evaluation method over sparse-gauge regions. Although it is generally known that the cross-validation methods can be served in sparse-gauge regions, the application condition of different evaluation methods for precipitation products is identified quantitatively in TP now.

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Milind Sharma
,
Robin L. Tanamachi
, and
Eric C. Bruning

Abstract

The dual-polarization radar characteristics of severe storms are commonly used as indicators to estimate the size and intensity of deep convective updrafts. In this study, we track rapid fluctuations in updraft intensity and size by objectively identifying polarimetric fingerprints such as ZDR and KDP columns, which serve as proxies for mixed-phase updraft strength. We quantify the volume of ZDR and KDP columns to evaluate their utility in diagnosing temporal variability in lightning flash characteristics. Specifically, we analyze three severe storms that developed in environments with low-to-moderate instability and strong 0–6 km wind shear in northern Alabama during the 2016-17 VORTEX-Southeast field campaign. In these three cases (a tornadic supercell embedded in stratiform precipitation, a nontornadic supercell, and a supercell embedded within a quasi-linear convective system), we find that the volume of the KDP columns exhibits a stronger correlation with the total flash rate . The higher covariability of KDP column volume with total flash rate suggests that the overall electrification and precipitation microphysics was dominated by cold cloud processes. The lower covariability with ZDR column volume indicates the presence of nonsteady updrafts or a less prominent role of warm rain processes in graupel growth and subsequent electrification. Furthermore, we observe that the majority of cloud-to-ground (CG) lightning strikes carried negative charge to the ground. In contrast to findings from a tornadic supercell over the Great Plains, lightning flash initiations in the Alabama storms primarily occurred outside the footprint of the ZDR and KDP column objects.

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Arthur Coquereau
,
Florian Sévellec
,
Thierry Huck
,
Joël J.-M. Hirschi
, and
Antoine Hochet

Abstract

As well as having an impact on the background state of the climate, global warming due to human activities could affect its natural oscillations and internal variability. In this study, we use four initial-condition ensembles from the CMIP6 framework to investigate the potential evolution of internal climate variability under different warming pathways for the 21st century. Our results suggest significant changes in natural climate variability, and point to two distinct regimes driving these changes. First, a decrease of internal variability of surface air temperature at high latitudes and all frequencies, associated with a poleward shift and the gradual disappearance of sea-ice edges, which we show to be an important component of internal variability. Second, an intensification of the interannual variability of surface air temperature and precipitation at low latitudes, which appears to be associated with the El Niño–Southern Oscillation (ENSO). This second regime is particularly alarming because it may contribute to making the climate more unstable and less predictable, with a significant impact on human societies and ecosystems.

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Wojciech W. Grabowski
,
Yongjoon Kim
, and
Seong Soo Yum

Abstract

Numerical simulations of turbulent moist Rayleigh–Bénard convection driving CCN activation and droplet growth in the laboratory Pi chamber are discussed. Supersaturation fluctuations come from isobaric mixing of warm and humid air rising from the lower boundary with colder air featuring lower water vapor concentrations descending from the upper boundary. Lagrangian particle-based microphysics is used to represent growth of haze CCN and cloud droplets with kinetic, solute, and surface tension effects included. Dry CCN spectra in the range between 2 to 200 nm radius from field observations are considered. Increasing the total CCN concentration from pristine to polluted conditions results in the increase of the droplet concentration and reductions of the mean droplet radius and spectral width. These are in agreement with Pi chamber observations and numerical simulations, as well as with numerous past studies of CCN cloud-base activation in natural clouds. The key result is that a relatively small fraction of the available CCN is activated in the Pi chamber fluctuating supersaturations, from about a half in pristine case to only a tenth in the polluted case. The activation fraction as a function of the dry CCN radius is similar in all simulations, close to zero at the CCN small end, increasing to a maximum at CCN radius around 50 nm, and decreasing to close to zero at the large CCN end. This is explained as too small supersaturations to activate small CCN as in natural clouds, and insufficient time to allow large CCN reaching the critical radius.

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Thomas C. Pagano
,
Barbara Casati
,
Stephanie Landman
,
Nicholas Loveday
,
Robert Taggart
,
Elizabeth E. Ebert
,
Mohammadreza Khanarmuei
,
Tara L. Jensen
,
Marion Mittermaier
,
Helen Roberts
,
Steve Willington
,
Nigel Roberts
,
Mike Sowko
,
Gordon Strassberg
,
Charles Kluepfel
,
Timothy A. Bullock
,
David D. Turner
,
Florian Pappenberger
,
Neal Osborne
, and
Chris Noble

Abstract

Operational agencies face significant challenges related to the verification and evaluation of weather forecasts. These challenges were investigated in a series of online workshops and polls engaging operational personnel from six countries. Five key themes emerged: inadequate verification approaches for both existing and emerging products; incomplete and uncertain observations; difficulties in accurately capturing users’ real-world experiences using simplified metrics; poor communication and understanding of forecasts and complex verification information; and institutional factors such as limited resources, evolving meteorologist roles, and concerns over reputational damage. We identify nearly 50 operationally relevant scientific questions and suggest calls to action. Addressing these needs includes designing forecast systems with verification as a central consideration, enhancing the availability of observations, and developing and adopting community software systems. Additionally, we propose the establishment of an international community comprising environmental and social science researchers, statisticians, verification practitioners, and users to provide sustained support for this collective endeavor.

Open access
Eun-Pa Lim
,
Harry H. Hendon
,
Amy H. Butler
,
David W. J. Thompson
,
Zachary D. Lawrence
,
Adam A. Scaife
,
Theodore G. Shepherd
,
Inna Polichtchouk
,
Hisashi Nakamura
,
Chiaki Kobayashi
,
Ruth Comer
,
Lawrence Coy
,
Andrew Dowdy
,
Rene D. Garreaud
,
Paul A. Newman
, and
Guomin Wang
Open access
Gabor Vali
and
Russell C. Schnell

Abstract

An overview is given of the path of research that led from asking how hailstones originate to the discovery that ice nucleation can be initiated by bacteria and other microorganisms at temperatures as high as −2°C. The major steps along that path were finding exceptionally effective ice nucleators in soils with a high content of decayed vegetative matter, then in decaying tree leaves, and then in plankton-laden ocean water. Eventually, it was shown that Pseudomonas syringae bacteria were responsible for most of the observed activity. That identification coincided with the demonstration that the same bacteria cause frost damage on plants. Ice nucleation by bacteria meant an unexpected turn in the understanding of ice nucleation and of ice formation in the atmosphere. Subsequent research confirmed the unique effectiveness of ice nucleating particles (INP) of biological origin, referred to as bio-INPs, so that bio-INPs are now considered to be important elements of lower-tropospheric cloud processes. Nonetheless, some of the questions which originally motivated the research are still unresolved, so that revisiting the early work may be helpful to current endeavors. Part I of this manuscript summarizes how the discovery progressed. Part II (Schnell and Vali) shows the relationship between bio-INPs in soils and in precipitation with climate and other findings. The online supplemental material contains a bibliography of recent work about bio-INPs.

Open access
Verónica Martín-Gómez
,
Belén Rodríguez-Fonseca
,
Irene Polo
, and
Marta Martín-Rey

Abstract

In the last decades, many efforts have been made to understand how different tropical oceanic basins are able to impact El Niño Southern Oscillation (ENSO). However, the collective connectivity among the tropical oceans and their associated influence on ENSO is less understood. Using a complex network methodology, the degree of collective connectivity among the tropical oceans is analyzed focusing on the detection of periods when the tropical basins collectively interact and Atlantic and Indian basins influence the equatorial Pacific sea surface temperatures (SST). The background state for the periods of strong collective connectivity is also investigated.

Our results show a marked multidecadal variability in the tropical interbasin connection, with periods of stronger and weaker collective connectivity. These changes seem to be modulated by changes in the North Atlantic ocean mean state a decade in advance. In particular, strong connectivity occurs in periods with colder than average tropical north Atlantic surface ocean. Associated with this cooling an anomalous convergence of the vertical integral of total energy flux (VIEF) takes place over the tropical north-west Atlantic, associated with anomalous divergence of VIEF over the equatorial eastern Pacific. In turn, an anomalous zonal surface pressure gradient over the tropical Pacific weakens the trades over the western equatorial Pacific. Consequently, a shallower thermocline emerges over the western equatorial Pacific, which can enhance thermocline feedbacks, the triggering of ENSO events, and therefore, ENSO variability. By construction, our results put forward opposite conditions for periods of weak tropical basins connectivity. These results have important implications for seasonal to decadal predictions.

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Xavier Chartrand
,
Louis-Philippe Nadeau
, and
Antoine Venaille

Abstract

The Quasi-Biennial Oscillation (QBO) is understood to result from wave-mean-flow interactions, but the reasons for its relative stability remain a subject of ongoing debate. In addition, consensus has yet to be reached regarding the respective roles of different equatorial wave types in shaping the QBO’s characteristics. Here we employ Holton-Lindzen-Plumb’s quasilinear model to shed light on the robustness of periodic behavior in the presence of multiple wave forcings. A comprehensive examination of the various dynamical regimes in this model reveals that increased vertical wave propagation at higher altitudes favors periodicity. In the case of single standing wave forcing, enhanced vertical propagation is controlled by the wave attenuation length scale. The occurrence of non-periodic states at high forcing amplitudes is explained by the excitation of high vertical unstable modes. Increasing the attenuation length scale prevents the emergence of such modes. When multiple wave forcing is considered, the mean flow generated by a dominant primary wave facilitates greater vertical propagation of a perturbation wave. Raising the altitude where most of the wave damping occurs favors periodicity by preventing the development of secondary jets responsible for the aperiodic behavior. This mechanism underscores the potential role of internal gravity waves in supporting the periodicity of a QBO primarily driven by planetary waves.

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