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This work analyzes the sources, sinks and stores of moisture that originates as Amazonian evapotranspiration (ET) from daily to annual timescales. To do this, we use the Weather Research and Forecast (WRF) regional meteorological model with the added capability of water vapor tracers to track the local evapotranspired moisture. The tracers reveal a strong diurnal cycle of Amazonian water vapor which had not been previously reported. This signal is related to the diurnal cycle of ET, convective precipitation and advected moisture. ET's contribution to atmospheric moisture increases from early morning into the afternoon. Some of this moisture is rained out through convective storms in the early evening. Later in the night and following morning, strong winds associated with the South American Low Level Jet advect moisture downwind. The beating pattern becomes apparent when visualizing the Amazonian water vapor as an animation.

A combination of Flexpart-WRF simulations forced with ERA5 and the CESM2 model incorporated in the CMIP6 project to infer a series of changes over the present century in the behavior of the landfalling ARs arriving at the Iberian Peninsula was used. Potential changes in the strength and position of their main moisture sources are also studied first in the literature. In general terms, a gradual strengthening in the intensity of these events is expected, observable from an increase in the amount of moisture transported, while no significant changes in the net number of events are observed. A northward shift in the position of the centroids has also been detected. In relation to the moisture sources, an increase in the contribution of these sources to the moisture content is expected, compatible with Clausius-Clapeyron amplification.

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUG This study makes use of the new total column water vapour data record (CDR‐2 (v2)), developed by the European Space Agency (ESA) in coordination with the Satellite Application Facility on Climate Monitoring (CM SAF), to analyse the adequacy of the integrated vertical water vapour column (IWV) data provided by the European Centre for Medium‐Range Weather Forecasts (ECMWF) ERA5 and ERA‐Interim reanalyses in regions of critical interest for moisture transport mechanisms. This information is critical for the initialization of moisture transport models—both Eulerian and Lagrangian—used to study the main mechanisms and predict the future evolution of moisture transport events. In particular, almost 40,000 atmospheric river (AR) and nocturnal low‐level jet (NLLJ) events identified on a global scale between 2002 and 2017 have been used to study the variability between the cited reanalyses and CDR‐2, in terms of both bias in the observed values of IWV during each particular event and daily temporal correlation fields. Although some notable discrepancies are reported in the main tropical rainforest regions, it is observed that, in regions of high interest for both ARs and NLLJs, the degree of agreement between the reanalyses and CDR‐2 is high. The bias observed in the regions of interest is generally low, and the temporal correlation in the IWV fields is above 0.8 in most areas. ERA5 appears to show slightly better performance than ERA‐Interim when resolving the moisture column, and both show greater similarity to CDR‐2 in the midlatitudes compared with tropical regions. The probability density functions constructed on an event‐to‐event basis reinforce these ideas. We conclude that the evaluations presented here using CDR‐2 serve to strengthen avaliable evidence that the ECMWF reanalyses can safely be used in the initializations of Lagrangian dispersion models and Eulerian moisture tracer simulations—commonly used for the analysis of main advection mechanisms—in the vast majority of regions critical to the study of ARs and LLJs. They can also safely be used for the detection of moisture source–sink regions in the study of the global hydrological cycle in these regions. Xunta de Galicia | Ref. ED431C 2017/64 Xunta de Galicia | Ref. ED431C2021/44 Agencia Estatal de Investigación | Ref. RTI2018-095772-B-I00

Atmospheric water vapour residence time (WVRT) is an essential indicator of how atmospheric dynamics and thermodynamics mediate hydrological cycle responses to climate change. WVRT is also important in estimating moisture sources and sinks, linking evaporation and precipitation across spatial scales. In this Review, we outline how WVRT is shaped by the interaction between evaporation and precipitation, and, thus, reflects anthropogenic changes in the hydrological cycle. Estimates of WVRT differ owing to contrasting definitions, but these differences can be reconciled by framing WVRT as a probability density function with a mean of 8-10 days and a median of 4-5 days. WVRT varies spatially and temporally in response to regional, seasonal and synoptic-scale differences in evaporation, precipitation, long-range moisture transport and atmospheric mixing. Theory predicts, and observations confirm, that in most (but not all) regions, anthropogenic warming is increasing atmospheric humidity faster than it is speeding up rates of evaporation and precipitation. Warming is, thus, projected to increase global WVRT by 3-6% K-1, lengthening the distance travelled between evaporation sources and precipitation sinks. Future efforts should focus on data integration, joint measurement initiatives and intercomparisons, and dynamic simulations to provide a formal resolution of WVRT from both Lagrangian and Eulerian perspectives. Universidad de Costa Rica/[805-B9-519]/UCR/Costa Rica Gobierno de España/[RTI2018-095772-B-I00]/LAGRIMA/España Ministerio de Ciencia, Innovación y Universidades/[]//España European Regional Development Fund/[]/ERDF/España Xunta de Galicia/[ED481B 2018/069]//España Xunta de Galicia/[ED413C 2017/64]//España Fulbright Program/[]//Estados Unidos Centro Universitario de la Defensa/[]/CUD/España Netherlands Organization for Scientific Research/[016.Veni.181.015]/NWO/Países Bajos National Science Foundation/[1454089]/NSF/Estados Unidos Research Council of Norway/[262710]/RCN/Noruega European Research Council/[773245]/ERC/Bélgica UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigaciones Geofísicas (CIGEFI) UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Física

One of the most robust signals of climate change is the relentless rise in global mean surface temperature, which is linked closely with the water-holding capacity of the atmosphere. A more humid atmosphere will lead to enhanced moisture transport due to, among other factors, an intensification of atmospheric rivers (ARs) activity, which are an important mechanism of moisture advection from subtropical to extra-tropical regions. Here we show an enhanced evapotranspiration rates in association with landfalling atmospheric river events. These anomalous moisture uptake (AMU) locations are identified on a global scale. The interannual variability of AMU displays a significant increase over the period 1980-2017, close to the Clausius-Clapeyron (CC) scaling, at 7 % per degree of surface temperature rise. These findings are consistent with an intensification of AR predicted by future projections. Our results also reveal generalized significant increases in AMU at the regional scale and an asymmetric supply of oceanic moisture, in which the maximum values are located over the region known as the Western Hemisphere Warm Pool (WHWP) centred on the Gulf of Mexico and the Caribbean Sea., Increasing atmospheric temperatures are expected to have various impacts on the global water cycle. Here, the authors show that there is an intensification of atmospheric rivers, that causes enhanced evapotranspiration and thus atmospheric moisture uptake in many regions of the world.

The study of atmospheric rivers (ARs) and their impacts on extreme precipitation are currently of great research interest in view of their clear socioeconomical implications. However, studies of this type generally contain caveats. The first of these is of a meteorological nature, and is concerned with the diversity of the different meteorological patterns that can be associated in the phenomenological definition, in that there is no guarantee that all so-called ARs follow the same one. The second concern involves the initial definition of an AR, which implicitly assumes the subtropical origin of the atmospheric moisture that feeds it. To date, it has been observed that in many cases of ARs, most of the moisture originates in regions at higher latitudes. The aim of this article is to open a debate on these two aspects by using well-known examples of ARs which fit different meteorological patterns, and showing a climatology of the moisture sources that feed ARs. Financiado para publicación en acceso aberto: Universidade de Vigo/CISUG Agencia Estatal de Investigación | Ref. RTI2018-095772-B-I00 Ministerio de Economía y Competitividad | Ref. CGL2015-65141-R Fundação para a Ciência e a Tecnologia | Ref. PTDC/CTA-MET/29233/2017 Fundação para a Ciência e a Tecnologia | Ref. CEECIND/00027/2017 Xunta de Galicia | Ref. ED431C 2021/44 Xunta de Galicia | Ref. EDB481B 2018/06

Low-level jets (LLJs) are defined as regions of anomalously high wind speeds occurring within the first kilometre of the troposphere. They are filamentous structures that are usually strongly related to moisture transport. This study analysed the moisture transport associated with LLJs, and the role of LLJs in precipitation events that occasionally lead to extreme precipitation and major floods. A daily climatology dataset of nocturnal LLJs was developed and moisture transport was studied by analysing the source and sink regions. A nocturnal LLJ index based on the vertical structure of temporal wind variations was applied, and a total of 33 nocturnal LLJs were identified and characterised. The FLEXPART Lagrangian model was then used to simulate jet behaviour and to identify major source and sink regions during the period from 1980 to 2016. Differences between days with LLJs and days with no LLJs were subsequently calculated for each LLJ on a global scale with respect to moisture transport for the sources and sinks of moisture. Finally, each LLJ was analysed in terms of its associated socioeconomic impact and relationship with water scarcity.

Low-level jets (LLJs) can be defined as wind corridors of anomalously high wind speed values located within the first kilometre of the troposphere. These structures are one of the major meteorological systems in the meridional transport of moisture on a global scale. In this work, we focus on the southerly Great Plains low-level jet, which plays an important role in the moisture transport balance over the central United States. The Gulf of Mexico is the main moisture source for the Great Plains low-level jet (GPLLJ), which has been identified as a key factor for rainfall modulation over the eastern and central US. The relationship between moisture transport from the Gulf of Mexico to the Great Plains and precipitation has been well documented in previous studies. Nevertheless, a large uncertainty still remains in the quantification of the moisture amount actually carried by the GPLLJ. The main goal of this work is to address this question. For this purpose, a relatively new tool, the regional atmospheric Weather Research and Forecasting Model with 3-D water vapour tracers (WRF-WVT; Insua-Costa and Miguez-Macho, 2018) is used together with the Lagrangian model FLEXPART to estimate the load of precipitable water advected within the GPLLJ. Both models were fed with data from ERA Interim. From a climatology of jet intensity over a 37-year period, which follows a Gaussian distribution, we select five cases for study, representing the mean and 1 and 2 standard deviations above and below it. Results show that the jet is responsible for roughly 70 %–80 % of the moisture transport occurring in the southern Great Plains when a jet event occurs. Furthermore, moisture transport by the GPLLJ extends to the north-east US, accounting for 50 % of the total in areas near the Great Lakes. Vertical distributions show the maximum of moisture advected by the GPLLJ at surface levels and maximum values of moisture flux about 500 m above, in coincidence with the wind speed profile. Iago Algarra was financially supported by the Spanish government (MINECO, CGL2015-65141-R). Jorge Eiras-Barca was financially supported by the EDB481B 2018/069 from the Xunta de Galicia and the Fulbright Commission. This work was also partially supported by Xunta de Galicia under Project ED431C 2017/64-GRC “Programa de Consolidación e Estructuración de Unidades de Investigación Competitivas (Grupos de Referencia Competitiva)” SI

Previous studies have estimated that 25% to 35% of Amazonian precipitation comes from evapotranspiration (ET) within the basin. However, due to simplifying assumptions of traditional models, these ...

This manuscript applies the recently-created atmospheric river intensity and impacts scale (AR Scale) to the European continent. The AR Scale uses an Eulerian perspective based solely upon the time series of integrated vapor transport (IVT) over a given geographic location (often represented by a model or reanalysis “grid cell”). The scale assigns events with persistent, strong IVT at that location to one of five levels (AR1 to AR5), or if the IVT is too weak or short-lived it is determined not to be an AR. AR1 events are primarily beneficial, AR2, 3 and 4 include a mix of beneficial and hazardous impacts, while AR5s are primarily hazardous. The frequency of occurrence, the associated probability of anomalous precipitation and the amount of precipitation explained by each AR rank are provided across Europe for the extended winter season (from October through March). AR1 and AR2 events are the most frequent and explain most of the observed precipitation, but they are associated with a low probability of extreme rainfall. Although AR3, AR4 and AR5 events are much less frequent, and normally provide a smaller fraction of annual precipitation, they are associated with a high probability of extreme rainfall. These results show remarkable variability among the different regions of the European continent. This manuscript also provides an AR detection catalog over Europe for the period 1980–2019, and a simplified version of the algorithm used to rank the events from AR1 to AR5 Financiado para publicación en acceso aberto: Universidade de Vigo/CISUG Xunta de Galicia | Ref. ED481B 2018/069 Xunta de Galicia | Ref. ED481B 2018/062 Ministerio de Economía y Competitividad | Ref. CGL2015-65141-R Xunta de Galicia | Ref. ED431C 2017/64-GRC Fundação para a Ciência e Tecnologia | Ref. CEECIND / 00027/2017 Fundação para a Ciência e Tecnologia | Ref. PTDC / CTA-MET / 29233/2017 Agencia Estatal de Investigación | Ref. RTI2018-095772-B-I00

The spatial distribution of different clouds has a significant effect on the supply of water resources, especially in countries with water shortages. Eight-years CloudSat 2B-CLDCLASS data for extended winters (October to March) from 2010 to 2017 has been used here to analyze the characteristics of the cloud cover over Iran. The results show that cirrus-type clouds are the most abundant, with a presence of 28%, followed by altostratus, with a presence of 22.5%. High variability in spatial distribution has also been observed. The most frequent type of cloudiness associated with each region of the country is detailed in this article. The average height of each type of cloudiness observed is also analyzed, being, in the case of the two most frequent types, 10.47 km for cirrus and 7.36 for altostratus. The greatest contribution to rainfall was, however, made by the nimbostratus, with a rate close to 45%. Behind them, stratocumulus, altocumulus, and clouds associated with deep convection show rates of 23.8%, 9.8%, and 8.33%.

This brief research report is aimed to make a first approach to the study of the type of precipitation associated with a set of atmospheric river (AR) events over the Atlantic region of Galicia. Fifteen ARs that made landfall in the Spanish region of Galicia have been analyzed using the 2B-GEOPROF and 2C-PRECIP-COLUMN from the CloudSat cloud profiling radar (CPR). An estimation of the relative ratio between warm and cold precipitation associated with each event is provided. Broadly speaking, cold precipitation accounts for 80% of the total. This value is slightly higher than the already stated for Pacific AR events. However, similar mean rain rates (0.35 mm/h for the warm precipitation and 1.16 mm/h for the cold counterpart) to those reported by the literature have been obtained. In the absence of a more comprehensive and conclusive statistic, it seems that cold precipitation is predominant along the central axis of a well-developed AR. In this central core of the AR, the bulk of the moisture remains in the lower levels, and the freezing level (FL) is low. According to these results, the interaction between the warm conveyor and the cold conveyor belt may eventually raise the FL to upper levels, leading the warm fraction to play a more important —even though still secondary— role.

The Amazon rainforest is the largest and most biologically diverse ecosystem in the planet. Moisture-laden tropical winds from the Atlantic Ocean enter the Amazon, bringing precipitation to the forest; the dense canopy then then transpires and contributes water for precipitation downwind. In fact, it has been estimated that between 20% and 40% of Amazonian rain comes from transpiration and evaporation (ET) from the forest itself. However, only recently have we been able to quantify the contribution of Amazonian ET to precipitation. Using the state-of-the art Weather Research and Forecast (WRF) atmospheric model with water tracers, we are able to trace the moisture that originates from the Amazon rainforest. This is analogous to putting dye in the model’s hydrologic cycle. By visualizing the output of this model we were able to show, for the first time, that the diurnal cycle of transpiration provides a clear diurnal signal to the overlying atmospheric water vapor and can be visualized as a “beating” over the Amazon. The rain and the winds also have a clear diurnal cycle, and shows how the land’s signature can be seen in the hydroclimatology above.

The assessment of sources of moisture is key to the understanding of the hydrological cycle at different time scales, because it enables the establishment of source-sink relationships and the identification of the main moisture transport conveyors and associated processes, the result of which is precipitation. Gimeno et al. (2012) provided a comprehensive review of the state-of-the-art in the assessment of moisture source-sinks and how different approaches can contribute to improving our knowledge of this component of the Earth’s Climate System. Since then, a variety of studies have focused on more specific aspects of the moisture budget and the source-sink distribution across the globe by integrating observations, satellite-derived products, physical tracers and numerical modelling. Here, we summarise the main advances in the field related to the impact of the moisture source-sink relationship on rainfall distribution, and add to the scientific debate on the question of the residence time of water vapour. We also revisit some of the recent advances in the role of the major mechanisms of moisture transport at a global scale, mainly Atmospheric Rivers and Low-Level Jet systems (Gimeno et al., 2016), in terms of their effects on precipitation extremes. Finally, we set out some of the main challenges for future research. Gobierno de España/[LAGRIMA (RTI2018-095772-B-I00)]//España Gobierno de España/[EVOCAR (CGL2015-65141-R)]//España Xunta de Galicia/[ED481B 2018/062]//España Xunta de Galicia/[ED481B 2018/069]//España Xunta de Galicia/[ED481B 2019/070]//España Ministerio de Asuntos Económicos y Transformación Digital/[CGL2015-65141-R]/MINECO/España Fundação para a Ciência e a Tecnologia/[PTDC/CTA-MET/29233/2017]/FCT/Portugal Fundação para a Ciência e a Tecnologia/[FCT, CEECIND/00027/2017]/FCT/Portugal National Science Foundation/[Award AGS 1454089]/NSF/Estados Unidos Xunta de Galicia/[ED431C2017/64-GRC]//España UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigaciones Geofísicas (CIGEFI) UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Física

The explosive cyclogenesis of extratropical cyclones and the occurrence of atmospheric rivers are characteristic features of a baroclinic atmosphere, and are both closely related to extreme hydrometeorological events in the mid-latitudes, particularly on coastal areas on the western side of the continents. The potential role of atmospheric rivers in the explosive cyclone deepening has been previously analysed for selected case studies, but a general assessment from the climatological perspective is still missing. Using ERA-Interim reanalysis data for 1979–2011, we analyse the concurrence of atmospheric rivers and explosive cyclogenesis over the North Atlantic and North Pacific basins for the extended winter months (ONDJFM). Atmospheric rivers are identified for almost 80% of explosive deepening cyclones. For non-explosive cyclones, atmospheric rivers are found only in roughly 40% of the cases. The analysis of the time evolution of the high values of water vapour flux associated with the atmospheric river during the cyclone development phase leads us to hypothesize that the identified relationship is the fingerprint of a mechanism that raises the odds of an explosive cyclogenesis occurrence and not merely a statistical relationship. These new insights on the relationship between explosive cyclones and atmospheric rivers may be helpful to a better understanding of the associated high-impact weather events. Jorge Eiras-Barca was financially supported by the Spanish government (MINECO) and Xunta de Galicia (CGL2013-45932-R, GPC2015/014 – ERDF), and contributions by the COST action MP1305 and CRETUS Strategic Partnership (AGRUP2015/02). Alexandre M. Ramos was supported through a postdoctoral grant (SFRH/BPD/84328/2012) from the Portuguese Science Foundation (Fundação para a Ciência e a Tecnologia, FCT). Alexandre M. Ramos and Ricardo M. Trigo were supported by the project IMDROFLOOD – Improving Drought and Flood Early Warning, Forecasting and Mitigation using real-time hydroclimatic indicators (WaterJPI/0004/2014), funded by Fundação para a Ciência e a Tecnologia, Portugal (FCT). Joaquim G. Pinto thanks AXA Research Fund for support SI

Continued deforestation in the Amazon forest can alter the subsurface/surface and atmospheric branches of the hydrologic cycle. The sign and magnitude of these changes depend on the complex interactions between the water, energy, and momentum budgets. To understand these changes, we use the weather research and forecasting (WRF) model with improved representation of groundwater dynamics and the added feature of Amazonian moisture tracers. The control simulation uses moderate resolution imaging spectroradiometer (MODIS) based observations of land use, and the deforestation simulations use a "business-as-usual" scenario projected for 2040-2050. Our results show that deforestation leads to changes that are seasonally very different. During the dry season, deforestation results in increased albedo and less available net radiation. This change, together with reduced leaf area, results in decreased evapotranspiration (ET), less atmospheric moisture of Amazonian origin, and an increase in temperature. However, we find no changes in precipitation over the basin. Conversely, during the wet season, surface winds increase significantly due to decreased surface roughness. Vapor transport increases throughout the deforested region and leads to an increase in easterly moisture export, and significant decrease in precipitation within the deforested regions of Eastern Amazon. Contrary to expectations, the moisture tracers in WRF show no evidence that precipitation decreases are due to recycling or changes in stability.

Two Lagrangian tracer tools are evaluated for studies on atmospheric moisture sources and pathways. In these methods, a moisture volume is assigned to each particle, which is then advected by the wind flow. Usual Lagrangian methods consider this volume to remain constant and the particle to follow flow path lines exactly. In a different approach, the initial moisture volume can be considered to depend on time as it is advected by the flow due to thermodynamic processes. In this case, the tracer volume drag must be taken into account. Equations have been implemented and moisture convection was taken into account for both Lagrangian and inertial models. We apply these methods to evaluate the intense atmospheric rivers that devastated (i) the Pacific Northwest region of the US and (ii) the western Iberian Peninsula with flooding rains and intense winds in early November 2006 and 20 May 1994, respectively. We note that the usual Lagrangian method underestimates moisture availability in the continent, while active tracers achieve more realistic results ERA-Interim data were supported by the ECMWF. This work was financially supported by Ministerio de Economía, Industria y Competitividad (CGL2017-89859-Rand CGL2013-45932-R), with contributions by the COST Action MP1305 and CRETUS strategic partnership (AGRUP2015/02). All these programs are co-funded by the ERDF (EU) SI

Atmospheric rivers (ARs) – long and narrow structures of anomalously high water vapor flux located in the warm sector of extratropical cyclones – have been shown to be closely related to extreme precipitation and flooding. In this paper we analyze the connection between ARs and flooding in the northwestern Spanish region of Galicia under a variety of synoptic conditions represented by the so-called “weather types”, a classification of daily sea-level pressure patterns obtained by means of a simple scheme that adopts the subjective procedure of Lamb. Flood events are identified from official reports conducted by the Spanish emergency management agency (Protección Civil) from 1979 to 2010. Our results suggest that, although most flood events in Galicia do not coincide with the presence of an overhead AR, ARs are present in the majority of severe cases, particularly in coastal areas. Flood events associated with ARs are connected to cyclonic weather types with westerly and southwesterly flows, which occur mostly in winter months. The link between ARs and severe flooding is not very apparent in inland areas or during summer months, in which case heavy precipitation is usually not frontal in nature but rather convective. Nevertheless, our results show that, in general, the amount of precipitation in flood events in Galicia more than doubles when an AR is present The authors acknowledge fruitful discussions with Swen Brands and MeteoGalicia. Jorge Eiras-Barca is funded by the “Ministerio Español de Economía y Competitividad” and FEDER (EDRF) (CGL2013-45932-R). This work was partially supported by Xunta de Galicia under project “gts0005 ED431C 2017/64-GRC” Programa de Consolidación e Estruturación de Unidades de Investigación Competitivas (Grupos de Referencia Competitiva) SI

One Eulerian and two Lagrangian tracers’ tools are evaluated for studies on atmospheric moisture sources and pathways. The first method has been recently implemented online into the Weather Research and Forecasting (WRF) mesoscale model (Insua-Costa and Miguez-Macho, 2017), while the Lagrangian methods are described here. In these methods, a moisture volume is assigned to each particle which is then advected by the wind flow. Usual Lagrangian methods consider this volume to remain constant and the particle follows exactly the stream lines of the flow (Stohl and James, 2004). On the other hand, the initial moisture volume can be thought to depend on time as the flow is advected due to thermodynamic processes (for example, pressure, and temperature changes). In this case, the drag on the tracer volume must be taken into account. Equations have been implemented and moisture convection (Forster et al., 2007) was taken into account for both Lagrangian models. We apply these methods to evaluate the intense atmospheric river (AR; i.e., a narrow plume of strong water vapor flux) that devastated the Pacific North Western America with flooding rains and intense winds in early November 2006 (Neiman et al., 2008). We note that the usual Lagrangian method underestimate moisture availability on the continent while the active tracers (both Eulerian and Lagrangian) achieve better results. Forster, C., Stohl, A. and Seibert, P. J. Applied Meteo. and Climatology. 46, 403 (2007). Insua-Costa, D. and Miguez-Macho, G. Eath Syst. Dyn. (2017). Neiman, P.J. et al. Monthly Wea. Rev. 136, 4398 (2008). Stohl, A. and James, P. J. Hidrometeor. 5, 656 (2004).

Large-scale tropospheric mixing and Lagrangian transport properties have been analyzed for the long-term period 1979–2014 in terms of the finite-time Lyapunov exponents (FTLEs).Wind field reanalyses from the European Centre for Medium-Range Weather Forecasts were used to calculate the Lagrangian trajectories of large ensembles of particles. Larger values of the interannual and intra-annual mixing variabilities highlight the El Niño Southern Oscillation, the storm track, or the Intertropical Convergence Zone among other largescale structures. The mean baroclinic instability growth rate and the mean atmospheric river occurrence show large correlation values with the FTLE climatology as an indication of their influence on tropospheric mixing in the midlatitudes. As a case study, the role that land-falling atmospheric rivers have on large-scale tropospheric mixing and the precipitation rates observed in Saharan Morocco and the British Isles has been analyzed. The atmospheric river contribution to tropospheric mixing is found to decrease from 15% in Saharan Morocco to less than 5% for the UK and Ireland regions, in agreement with their contribution to precipitation that is 40% larger in the former than in the latter region. ERA-Interim data were supported by ECMWF. This work was financially supported by Ministerio de Economía y Competitividad and Xunta de Galicia (CGL2013- 45932-R, GPC2015/014) and contributions by the COST Action MP1305 and CRETUS Strategic Partnership (AGRUP2015/02). All these programs are co-funded by ERDF (EU) SI

Large-scale tropospheric mixing and Lagrangian transport properties have been analyzed for a long-term period 1979–2014 in terms of the finite-time Lyapunov exponents (FTLE). Wind fields reanalysis from the European Centre for Medium-Range Weather Forecasts were used to calculate Lagrangian trajectories of large ensembles of particles. The FTLE climatology shows large correlation values with the baroclinic instability growth rate. Larger values of the inter and intra-annual mixing variabilities highlight El Niño Southern Oscillation, the storm track or the Intertropical Convergence Zone among other large-scale structures. As a case study, the role that atmospheric rivers have on the large-scale atmospheric mixing and the precipitation rates observed in the Sahara-Morocco and British Isles regions have been analyzed. Atmospheric rivers contribution to tropospheric mixing is found to decrease from 15 % in Sahara-Morocco to less than 5 % for UK-Ireland regions, in agreement to their contribution to precipitation that is 40 % larger in the former than for the latter region.

A new 3-D tracer tool is coupled to the WRF model to analyze the origin of the moisture in two extreme atmospheric river (AR) events: the so-called Great Coastal Gale of 2007 in the Pacific Ocean and the Great Storm of 1987 in the North Atlantic. Results show that between 80 and 90 % of moisture advected by the ARs, and a high percentage of the total precipitation produced by the systems have a tropical origin. The tropical contribution to precipitation is in general above 50 % and largely exceeds this value in the most affected areas. Local convergence transport is responsible for the remaining moisture and precipitation. The ratio of tropical moisture to total moisture is maximized as the cold front arrives on land. Vertical cross sections of the moisture content suggest that the maximum in tropical humidity does not necessarily coincide with the low-level jet (LLJ) of the extratropical cyclone. Instead, the amount of tropical humidity is maximized in the lowest atmospheric level in southern latitudes and can be located above, below or ahead of the LLJ in northern latitudes in both analyzed cases This work has been founded by the Ministerio de Economía y Competitivad (CGL2013-45932-R) from the Spanish Government and its mobility grants for pre-doc researchers. Funding for Dominguez and Hu comes from NASA grant NNX14AD77G SI

In recent years, the Arctic has become a subject of special interest due to the drastic effect of climate change over the region. Despite that there are several mechanisms that influence the Arctic region, some recent studies have suggested significant influences of moisture transport over the observed loss of sea ice. Moisture transport can affect the region in different ways: direct precipitation over the region, radiative effect from the cloud cover and through the release of latent heat. Atmospheric rivers (ARs) represent one of the main events involved in moisture transport from the tropics to the mid-latitudes and despite having been shown especially relevant on the northward advection, their effect over the Arctic has not been deeply investigated. The aim of this work was to establish the groundwork for future studies about the effect of ARs linked to moisture transport over the Arctic region. For this purpose, an automated algorithm was used to identify regions of maximum AR occurrence over the Arctic. This was done by analysing the number of AR detections every month over a band of 10° of latitude centred on 60° N. The Lagrangian model FLEXPART was used to find the areas where the ARs take their moisture to the Arctic. Using this model, the anomalous moisture contribution to these baroclinic structures was analysed taking into account only the dates of AR occurrence. From the results, it appears that the main moisture sources for AR events extend over the North Atlantic and North Pacific oceans, moreover, the local input of moisture over the region of maximum AR occurrence seems to be especially relevant. In general terms, moisture comes from major evaporative areas over the western part of the oceanic regions in the band between 30° and 40° N for most months in the year, showing a continental origin in the summer months. This behaviour agrees with the climatological moisture transport into the Arctic determined in previous studies. However, in special association with AR events, an intensification of local moisture uptake is observed over the area of maximum AR activity and nearby. The study of the origin of this moisture and associated anomalies for Arctic ARs is an important step in the analysis of the effect of these structures on the Arctic environment.

We show that filamentous Atmospheric Rivers (ARs) over the Northern Atlantic Ocean are closely linked to attracting Lagrangian Coherent Structures (LCSs) in the large scale wind field. The detected LCSs represent lines of attraction in the evolving flow with a significant impact on all passive tracers. Using Finite-Time Lyapunov Exponents, we extract LCSs from a two-dimensional flow derived from water vapor flux of atmospheric reanalysis data and compare them to the three-dimensional LCS obtained from the wind flow. We correlate the typical filamentous water vapor patterns of ARs with LCSs and find that LCSs bound the filaments on the back side. Passive advective transport of water vapor in the AR from tropical latitudes is potentially possible.

Financiado para publicación en acceso aberto: Universidade de Vigo/CISUG The effect of increased populations concentrated in urban areas, coupled with the ongoing threat of climate change, means that society is becoming increasingly vulnerable to the effects of extreme precipitation. The study of these events is therefore a key topic in climate research, in their physical basis, in the study of their impacts, and in our adaptation to them. From a meteorological perspective, the main questions are related to the definition of extreme events, changes in their distribution and intensity both globally and regionally, the dependence on large-scale phenomena including the role of moisture transport, and changes in their behavior due to anthropogenic pressures. In this review article, we address all these points and propose a set of challenges for future research. Agencia Estatal de Investigación | Ref. RTI2018‐095772‐B‐I00 Xunta Galicia | Ref. D431C 2021/44