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The increase in the number and quality of numerical moisture tracking tools has greatly improved our understanding of the hydrological cycle in recent years. However, the lack of observations has prevented a direct validation of these tools, and it is common to find large discrepancies among the results produced by them, especially between Eulerian and Lagrangian methodologies. Here, we evaluate two diagnostic tools for moisture tracking, WaterSip and UTrack, using simulations from the Lagrangian model FLEXPART. We assess their performance against the Weather Research and Forecasting (WRF) model with Eulerian Water Vapor Tracers (WRF-WVTs). Assuming WRF-WVTs results as a proxy for reality, we explore the discrepancies between the Eulerian and Lagrangian approaches for five precipitation events associated with atmospheric rivers and propose some physics-based adjustments to the Lagrangian tools. Our findings reveal that UTrack, constrained by evaporation and precipitable water data, has a slightly better agreement with WRF-WVTs than WaterSip, constrained by specific humidity data. As in previous studies, we find a negative bias in the contribution of remote sources, such as tropical ones, and an overestimation of local contributions. Quantitatively, the root-mean-square-error (RMSE) for contributions from selected source regions is 5.55 for WaterSip and 4.64 for UTrack, highlighting UTrack's narrowly superior performance. Implementing our simple and logical corrections leads to a significant improvement in both methodologies, effectively reducing the RMSE by over 50 % and bridging the gap between Eulerian and Lagrangian outcomes. Our results suggest that the major discrepancies between the different methodologies were not rooted in their inherently different nature, but in the obviation of basic physical considerations that may be easily straightened out. [ABSTRACT FROM AUTHOR]

In recent years, science has hardened the discourse on the emergency of global warming, pointing out that the next decades will be decisive to maintaining the stability of the climate system and, thus, avoiding a cascade effect of events that increase the average temperature above safe limits. The scientific community warns that there are different tipping points that could produce a chain reaction in the global climate. One of them is related to the jet stream. However, despite the importance of this air current in atmospheric dynamics in the Northern Hemisphere and the changes it is experiencing in the context of global warming, the public is still not familiar with this kind of physical concept nor with other much simpler concepts. As concerns about the climate crisis rise, climate literacy remains stagnant. To advance the learning of the science of climate change, in general, and of concepts such as the jet stream, in particular, specific scientific communication formats are required that can successfully tackle the difficult task of explaining such complex problems to the general public. These formats should be included in the media, as the characteristics of the formats (daily section, scientific dissemination, historic perspective, teleconnections and specialization) make them well suited to taking on the challenge of explaining the complexity of climate science. In this article, we present a communication proposal existent in a newspaper published in Spain. We argue that this communication format represents a good model to disseminate climate science, educate readers and even to make physical concepts such as the jet stream accessible. We believe that this format conforms to and complies with the enunciation of Article 12 of the Paris Agreement, which calls on the signatory countries to promote education and training on climate change. [ABSTRACT FROM AUTHOR]

Concern about heavy precipitation events has increasingly grown in the last years in southern Europe, especially in the Mediterranean region. These occasional episodes can result in more than 200 mm of rainfall in less than 24 h, producing flash floods with very high social and economic losses. To better understand these phenomena, a correct identification of the origin of moisture must be found. However, the contribution of the different sources is very difficult to estimate from observational data; thus numerical models are usually employed to this end. Here, we present a comparison between two methodologies for the quantification of the moisture sources in two flooding episodes that occurred during October and November 1982 in the western Mediterranean area. A previous study, using the online Eulerian Weather Research and Forecasting (WRF) Model with water vapor tracer (WRF-WVT) model, determined the contributions to precipitation from moisture evaporated over four different sources: (1) the western Mediterranean, (2) the central Mediterranean, (3) the North Atlantic Ocean and (4) the tropical and subtropical Atlantic and tropical Africa. In this work we use the offline Lagrangian FLEXPART-WRF model to quantify the role played by these same sources. Considering the results provided by WRF-WVT as "ground truth", we validated the performance of the FLEXPART-WRF. Results show that this Lagrangian method has an acceptable skill in identifying local (western Mediterranean) and medium-distance (central Mediterranean and North Atlantic) sources. However, remote moisture sources, like tropical and subtropical areas, are underestimated by it. Notably, for the October event, the tropical and subtropical area reported a relative contribution 6 times lower than with the WRF-WVT. In contrast, FLEXPART-WRF overestimates the contribution of some sources, especially from North Africa. These over- and underestimates should be taken into account by other authors when drawing conclusions from this widely used Lagrangian offline analysis. [ABSTRACT FROM AUTHOR]

Concern about heavy precipitation events has increasingly grown in the last years in Southern Europe, especially in the Mediterranean region. These occasional episodes can result in more than 200 mm of rainfall in less than 24 h, producing flash floods with very high social and economic losses. To the better understanding of this phenomena, a correct identification of the origin of the moisture must be found. However, the contribution of the different sources is very difficult to estimate from observational data, so numerical models are usually used to this end. Here, we present a comparison between two complex methodologies for the quantification of the moisture sources in two infamous events occurred during October and November 1982 in the Western Mediterranean area. In a previous study, using an Eulerian approach it was determined the contributions of moisture evaporated in: 1) Western Mediterranean; 2) Central Mediterranean; 3) North Atlantic ocean and 4) tropical and subtropical Atlantic and tropical Africa. Now, we use the Lagrangian model FLEXPART-WRF to quantify the role played by these sources. Considering the results provided by the Eulerian analysis as the virtual reality, we validated the performance of the Lagrangian model. Results show that the Lagrangian method has an acceptable performance in identifying local (Western Mediterranean) and medium-distance (Central Mediterranean and North Atlantic) sources. However, remote moisture sources, like tropical and subtropical areas, are underestimated by the Lagrangian approach. Notably, for the October event, the tropical and subtropical area reported a relative contribution six times below than the Eulerian method. In contrast, the FLEXPART-WRF overestimates the contribution of some sources, especially from the Sahara. We argue that such an inconsistent contribution is associated with the fact that the Lagrangian method does not consider moisture phase changes. These overand underestimates should be taken into account by other authors when drawing conclusions from the Lagrangian analysis. [ABSTRACT FROM AUTHOR]

Spectral nudging allows forcing a selected part of the spectrum of a model's solution with the equivalent part in a reference dataset, such as an analysis, reanalysis or another model. This constrains the evolution in certain scales, typically the synoptic ones, while allowing the others to evolve freely. In a limited area model (LAM) setting, spectral nudging is commonly used to impose the large-scale circulation in the interior of the domain, so that the high resolution features in the LAM's simulation are consistent with the global circulation patterns. In a previous study developed over a Mid-Latitude domain, we investigated two parameters of spectral nudging that are often overlooked despite having a significant impact on the model solution. First, the cut-off wave number, which is the parameter determining the scales that are nudged and has a critical impact on the spatial structure of the model solution. Second, the spin-up time, which is the time required to balance the nudging force with the model internal climate and roughly indicates the starting point from when the results of the simulation contain useful information. The question remains if our conclusions for Mid-Latitudes are applicable to other areas of the planet. Tropical Latitudes offer an interesting testbed as its atmospheric dynamics has unique characteristics with respect to that further North and yet it is the result of the same underlying physical principles. We study the impact of these two parameters in a domain centred in the Gulf of Mexico, with a particular aim to evaluate their performance related to hurricane modelling. We perform 4-day simulations along 6 monthly periods between 2010 and 2015, testing several spectral nudging configurations. Our results indicate that the optimal cut off wavenumber lies between 1000 Km and 1500 Km depending on the studied variable and that the spin-up time required is at least 72 h to 96 h, which is consistent with our previous work. We evaluate our findings in four hurricane cases, allowing for at least 96 h of spin-up time before the system becomes a tropical storm. Results confirm that the experiments with cut-off wavenumbers near the Rossby Radius of Deformation perform best. We also propose a novel approach in which a different cut-off wavenumber is used for each variable. Our tests in the hurricane cases show that the latter set up is able to outperform all of the other spectral nudging experiments when compared to observations. [ABSTRACT FROM AUTHOR]

Shallow groundwater in the Prairie Pothole Region (PPR) is predominantly recharged by snowmelt in the spring and supplies water for evapotranspiration through the summer and fall. This two-way exchange is underrepresented in current land surface models. Furthermore, the impacts of climate change on the groundwater recharge rates are uncertain. In this paper, we use a coupled land–groundwater model to investigate the hydrological cycle of shallow groundwater in the PPR and study its response to climate change at the end of the 21st century. The results show that the model does a reasonably good job of simulating the timing of recharge. The mean water table depth (WTD) is well simulated, except for the fact that the model predicts a deep WTD in northwestern Alberta. The most significant change under future climate conditions occurs in the winter, when warmer temperatures change the rain/snow partitioning, delaying the time for snow accumulation/soil freezing while advancing early melting/thawing. Such changes lead to an earlier start to a longer recharge season but with lower recharge rates. Different signals are shown in the eastern and western PPR in the future summer, with reduced precipitation and drier soils in the east but little change in the west. The annual recharge increased by 25 % and 50 % in the eastern and western PPR, respectively. Additionally, we found that the mean and seasonal variation of the simulated WTD are sensitive to soil properties; thus, fine-scale soil information is needed to improve groundwater simulation on the regional scale. [ABSTRACT FROM AUTHOR]

Groundwater plays an important role in the terrestrial water cycle, interacting with the land surface via vertical fluxes through the water table and distributing water resources spatially via gravity-driven lateral transport. It is therefore essential to have a correct representation of groundwater processes in land surface models, as land–atmosphere coupling is a key factor in climate research. Here we use the LEAFHYDRO land surface and groundwater model to study the groundwater influence on soil moisture distribution and memory, and evapotranspiration (ET) fluxes in the Iberian Peninsula over a 10-year period. We validate our results with time series of observed water table depth from 623 stations covering different regions of the Iberian Peninsula, showing that the model produces a realistic water table, shallower in valleys and deeper under hilltops. We find patterns of shallow water table and strong groundwater–land surface coupling over extended interior semi-arid regions and river valleys. We show a strong seasonal and interannual persistence of the water table, which induces bimodal memory in the soil moisture fields; soil moisture "remembers" past wet conditions, buffering drought effects, and also past dry conditions, causing a delay in drought recovery. The effects on land–atmosphere fluxes are found to be significant: on average over the region, ET is 17.4 % higher when compared with a baseline simulation with LEAFHYDRO's groundwater scheme deactivated. The maximum ET increase occurs in summer (34.9 %; 0.54 mm d -1). The ET enhancement is larger over the drier southern basins, where ET is water limited (e.g. the Guadalquivir basin and the Mediterranean Segura basin), than in the northern Miño/Minho basin, where ET is more energy limited than water limited. In terms of river flow, we show how dry season baseflow is sustained by groundwater originating from accumulated recharge during the wet season, improving significantly on a free-drain approach, where baseflow comes from water draining through the top soil, resulting in rivers drying out in summer. Convective precipitation enhancement through local moisture recycling over the semi-arid interior regions and summer cooling are potential implications of these groundwater effects on climate over the Iberian Peninsula. Fully coupled land surface and climate model simulations are needed to elucidate this question. [ABSTRACT FROM AUTHOR]

Floods and flash floods are frequent in the south of Europe resulting from heavy rainfall events that often produce more than 200 mm in less than 24 h. Even though the meteorological conditions favourable for these situations have been widely studied, there is a lingering question that still arises: what humidity sources could explain so much precipitation? To answer this question, the regional atmospheric Weather Research and Forecasting (WRF) model with a recently implemented moisture tagging capability has been used to analyse the main moisture sources for two catastrophic flood events that occurred during the autumn of 1982 (October and November) in the western Mediterranean area, which is regularly affected by these types of adverse weather episodes. The procedure consists in selecting a priori potential moisture source regions for the extreme event under consideration, and then performing simulations using the tagging technique to quantify the relative contribution of each selected source to total precipitation. For these events we study the influence of four possible potential sources: (1) evaporation in the western Mediterranean; (2) evaporation in the central Mediterranean; (3) evaporation in the North Atlantic; and (4) advection from the tropical and subtropical Atlantic and Africa. Results show that these four moisture sources explain most of the accumulated precipitation, with the tropical and subtropical input being the most relevant in both cases. In the October event, evaporation in the western and central Mediterranean and in the North Atlantic also had an important contribution. However, in the November episode tropical and subtropical moisture accounted for more than half of the total accumulated rainfall, while evaporation in the western Mediterranean and North Atlantic played a secondary role and the contribution of the central Mediterranean was almost negligible. Therefore, remote sources were crucial: in the October event they played a similar role to local sources, whereas in the November case they were clearly dominant. In both episodes, long-distance moisture transport from the tropics and subtropics mostly occurred in mid-tropospheric layers, via well-defined moisture plumes with maximum mixing ratios at medium levels. [ABSTRACT FROM AUTHOR]

Tehuantepecers or Tehuanos are extreme winds produced in the Isthmus of Tehuantepec, blowing south through Chivela Pass, the mountain gap across the isthmus, from the Gulf of Mexico into the Pacific Ocean. They are the result of the complex interaction between large-scale meteorological conditions and local orographic forcings around Chivela Pass, and occur mainly in winter months due to cold air damming in the wake of cold fronts that reach as far south as the Bay of Campeche. Even though the name refers mostly to the intense mountain gap outflow, Tehuantepecer episodes can also generate other localized extreme wind conditions across the region, such as downslope windstorms and hydraulic jumps, which are strong turbulent flows that have a direct effect on the Pacific side of the isthmus and the Gulf of Tehuantepec. This study focuses on investigating these phenomena using high horizontal and vertical resolution WRF (Weather Research and Forecasting) model simulations. In particular, we employ a 4-nested grid configuration with up to 444 m horizontal spacing in the innermost domain and 70 hybrid-sigma vertical levels, 8 of which lie within the first 200 m above ground. We select one 36 h period in December 2013, when favorable conditions for a strong gap wind situation were observed. The high-resolution WRF experiment reveals a significant fine-scale structure in the strong Tehuano wind flow, beyond the well known gap jet. Depending on the Froude number upstream of the topographic barrier, different downslope windstorm conditions and hydraulic jumps with rotor circulations develop simultaneously at different locations east of Chivela Pass with varied crest height. A comparison with observations suggests that the model accurately represents the spatially heterogeneous intense downslope windstorm and the formation of mountain wave clouds for several hours, with low errors in wind speed, wind direction and temperature. [ABSTRACT FROM AUTHOR]

We investigate the interannual and interdecadal hydrological changes in the Amazon River basin and its sub-basins during the 1980–2015 period using GRACE satellite data and a physically based, 2 km grid continental-scale hydrological model (LEAF-Hydro-Flood) that includes a prognostic groundwater scheme and accounts for the effects of land use–land cover (LULC) change. The analyses focus on the dominant mechanisms that modulate terrestrial water storage (TWS) variations and droughts. We find that (1) the model simulates the basin-averaged TWS variations remarkably well; however, disagreements are observed in spatial patterns of temporal trends, especially for the post-2008 period. (2) The 2010s is the driest period since 1980, characterized by a major shift in the decadal mean compared to the 2000s caused by increased drought frequency. (3) Long-term trends in TWS suggest that the Amazon overall is getting wetter (1.13 mm yr -1), but its southern and southeastern sub-basins are undergoing significant negative TWS changes, caused primarily by intensified LULC changes. (4) Increasing divergence between dry-season total water deficit and TWS release suggests a strengthening dry season, especially in the southern and southeastern sub-basins. (5) The sub-surface storage regulates the propagation of meteorological droughts into hydrological droughts by strongly modulating TWS release with respect to its storage preceding the drought condition. Our simulations provide crucial insight into the importance of sub-surface storage in alleviating surface water deficit across Amazon and open pathways for improving prediction and mitigation of extreme droughts under changing climate and increasing hydrologic alterations due to human activities (e.g., LULC change). [ABSTRACT FROM AUTHOR]

Shallow groundwater in the Prairie Pothole Region (PPR) is recharged predominantly by snowmelt in the spring and may supply water for evapotranspiration through the summer/fall. This two-way exchange is underrepresented in land-surface models. Furthermore, the impacts of climate change on the groundwater recharge are uncertain. In this paper, we use a coupled land and groundwater model to investigate the hydrologic cycle of shallow groundwater in the PPR and study its response to climate change at the end of the 21st century. The results show that the model reasonably simulates the water table depth (WTD) and the timing of recharge processes, but underestimates the seasonal variation of WTD, due to mismatches of the soil types between observations and the model. The most significant change under future climate occurs in the winter, when the warmer temperature changes the rain/snow partitioning, delay the time for snow accumulation/soil freezing while bringing forward early melting/thawing. Such changes lead to an earlier start to a longer recharge season, but with lower recharge rates. Different signals are shown in the eastern and western PPR in the future summer, with reduced precipitation and drier soils in the east but little change in the west. The annual recharge increased by 25% and 50% in the eastern and western PPR, respectively. Additionally, we found the mean and seasonal variation of the simulated WTD are sensitive to soil properties and fine-scale soil information is needed to improve groundwater simulation on a regional scale. [ABSTRACT FROM AUTHOR]

Tehuantepecers or Tehuanos are extreme winds produced in the Isthmus of Tehuantepec, blowing south through the Chivela pass, the mountain gap across the isthmus, from the Gulf of Mexico into the Pacific Ocean. They are the result of the complex interaction between large-scale meteorological conditions and local orographic forcings around Chivela pass and occur mainly in winter months, due to cold air damming in the wake of cold fronts that reach as far south. They can generate localized extreme events, such as downslope windstorms and hydraulic jumps, strong turbulent flows that have a direct effect on the Pacific side of the isthmus and the Gulf of Tehuantepec. This study focuses on investigating these phenomena using high horizontal and vertical resolution WRF (Weather Research and Forecasting) model simulations. In particular, we employ a four nested grid configuration, with up to 444 m horizontal spacing in the innermost domain and 70 hybrid-sigma vertical levels, 8 of which lie within the first 200 m above ground. We select one 36 hour period in December 2013, when favorable conditions for a strong gap wind situation were observed. The high-resolution WRF experiment reveals a significant fine-scale structure in the strong Tehuano wind flow, beyond the well known gap jet. Depending on the Froude number upstream the topographic barrier, different downslope windstorm conditions and hydraulic jumps with rotor circulations develop simultaneously at different locations east of Chivela pass with varied crest height. A comparison with observations suggests that the model accurately represents the spatially heterogeneous intense downslope windstorm and the formation of mountain wave clouds for several hours, with low errors in wind speed, wind direction, and temperature. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Groundwater plays an important role in the terrestrial water cycle, interacting with the land surface via vertical fluxes through the water table and distributing water resources spatially via gravity-driven lateral transport. It is therefore essential to have a correct representation of groundwater processes in land surface models, as land-atmosphere coupling is a key factor in climate research. Here we use the Land Surface and Groundwater Model LEAFHYDRO to study the groundwater influence on soil moisture distribution and memory, and evapotranspiration (ET) fluxes in the Iberian Peninsula over a 10-year period. We validate our results with time series of observed water table depth from 623 stations covering different regions of the Iberian Peninsula, showing that the model produces a realistic water table, shallower in valleys and deeper under hilltops. We find patterns of shallow water table and strong groundwater-land surface coupling over extended interior semi-arid regions and river valleys. We show a strong seasonal and interannual persistence of the water table, which induces bimodal memory in the soil moisture fields; soil moisture "remembers" past wet conditions, buffering drought effects, and also past dry conditions, causing a delay in drought recovery. The effects on land-atmosphere fluxes are found to be significant, on average over the region, ET is 17.4% higher when compared with a baseline simulation with LEAFHYDRO's groundwater scheme deactivated. The maximum ET increase occurs in summer (34.9%; 0.54 mm day-1). The ET enhancement is larger over the drier southern basins, where ET is water limited (e.g. the Guadalquivir basin and the Mediterranean Segura basin), than in the northern Miño/Minho basin, where ET is more energy limited than water limited. In terms of river flow, we show how dry season baseflow is sustained by groundwater originating from accumulated recharge during the wet season, improving significantly on a free-drain approach, where baseflow comes from water draining through the top soil, resulting in rivers drying out in summer. Convective precipitation enhancement through local moisture recycling over the semiarid interior regions and summer cooling are potential implications of these groundwater effects on climate over the Iberian Peninsula. Fully coupled land surface and climate model simulations are needed to elucidate this question. [ABSTRACT FROM AUTHOR]

Many studies underline the importance of groundwater assessment at the larger, i.e. global, scale. The groundwater models used for these assessments are dedicated to the global scale and therefore not often applied for studies in smaller areas, e.g. catchments, because of their simplifying assumptions. In New Zealand, advanced numerical groundwater flow models have been applied in several catchments. However, that application is piecemeal: only for a limited amount of aquifers and through a variety of groundwater model suites, formats, and developers. Additionally, there are large areas where groundwater models and data are sparse. Hence, an inter-catchment, inter-regional, or nationwide overview of important groundwater information, such as the water table, does not exist. The investment needed to adequately cover New Zealand with high-resolution groundwater models in a consistent approach would be significant and is therefore not considered possible at this stage. This study proposes a solution that obtains a nationwide overview of groundwater that bridges the gap between the (too-)expensive advanced local models and the (too-)simple global-scale models. We apply an existing, global-scale, groundwater flow model and improve it by feeding in national input data of New Zealand terrain, geology, and recharge, and by slight adjustment of model parametrisation and model testing. The resulting nationwide maps of hydraulic head and water table depths show that the model points out the main alluvial aquifers with fine spatial detail (200 m grid resolution). The national input data and finer spatial detail result in better and more realistic variations of water table depth than the original, global-scale, model outputs. In two regional case studies in New Zealand, the hydraulic head shows excellent correlation with the available groundwater level data. Sensitivity and other analyses of our nationwide water tables show that the model is mostly driven by recharge, model resolution, and elevation (gravity), and impeded by the geology (permeability). The use of this first dedicated New Zealand-wide model can aid in provision of water table estimates in data-sparse regions. The national model can also be used to solve inconsistency of models in areas of trans-boundary aquifers, i.e. aquifers that cover more than one region in New Zealand. Comparison of the models, i.e. the national application (National Water Table model: NWT) with the global model (Equilibrium Water Table model: EWT), shows that most improvement is achieved by feeding in better and higher-resolution input data. The NWT model still has a bias towards shallow water tables (but less than the EWT model because of the finer model resolution), which could only be solved by feeding in a very high resolution terrain model that incorporates drainage features. Although this is a model shortcoming, it can also be viewed as a valuable indicator of the pre-human water table, i.e. before 90 % of wetlands were drained for agriculture since European settlement in New Zealand. Calibration to ground-observed water level improves model results but can of course only work where there are such data available. Future research should therefore focus on both model improvements and more data-driven, improved estimation of hydraulic conductivity, recharge, and the digital elevation model. We further surmise that the findings of this study, i.e. successful application of a global-scale model at smaller scales, will lead to subsequent improvement of the global-scale model equations. [ABSTRACT FROM AUTHOR]

Floods and flash floods are frequent in the South of Europe resulting from heavy rainfall events that often produce more than 200 mm in less than 24 h. Even though the meteorological conditions favorable for these situations have been widely studied, there is a lingering question that still arises: which are the sources of humidity that could explain so much precipitation? To answer this question, the regional atmospheric Weather Research and Forecasting (WRF) Model with a recently implemented moisture tagging capability has been used to analyze the main moisture sources in two famous flood events occurred during the autumn of 1982 (October and November) in the Western Mediterranean area, which is regularly affected by this type of adverse weather episodes. The procedure consists in selecting a priori potential moisture source regions for the considered extreme event, and then performing simulations with the tagging technique to quantify the relative contribution of each selected source to total precipitation. For these events we study the influence of four possible potential sources: 1) evaporation in the Western Mediterranean; 2) evaporation in the Central Mediterranean; 3) evaporation in the North Atlantic; 4) advection from the tropical and subtropical Atlantic and Africa. Results show that these four moisture sources explain most of the accumulated precipitation, with the tropical and subtropical input being the most relevant in both cases. In the October event, evaporation in the Western and Central Mediterranean and in the North Atlantic also had an important contribution. In the November episode, however, tropical and subtropical moisture accounted for more than half of the total accumulated rainfall, while evaporation in the Western Mediterranean and North Atlantic played a secondary role and the contribution of the Central Mediterranean was almost negligible. Remote sources were therefore crucial: in the October event they played a similar role to local sources while in the November case they were clearly dominant. In both episodes, long distance moisture transport from the tropics and subtropics occurred mostly in mid tropospheric layers, through well-defined moisture plumes with maximum mixing ratios at medium levels. [ABSTRACT FROM AUTHOR]

Low-Level Jets (LLJs) can be defined as filamentous wind corridors of anomalously high wind speed values located within the first km of the troposphere. These structures, together with atmospheric rivers (ARs), are the major meteorological systems in the meridional transport of moisture on a global scale. In this work, we focus on the 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 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 is 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 3D water vapour tracers (WRF-TT, 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. From a climatology of jet intensity over a 37-year period (Rife et al., 2010), which follows a Gaussian distribution, we select for study 5 cases representing the mean, and one and two 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 northeast 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A new moisture tagging tool, usually known as water vapor tracer (WVT) method or online Eulerian method, has been implemented into the Weather Research and Forecasting (WRF) regional meteorological model, enabling it for precise studies on atmospheric moisture sources and pathways. We present here the method and its formulation, along with details of the implementation into WRF. We perform an in-depth validation with a 1-month long simulation over North America at 20 km resolution, tagging all possible moisture sources: lateral boundaries, continental, maritime or lake surfaces and initial atmospheric conditions. We estimate errors as the moisture or precipitation amounts that cannot be traced back to any source. Validation results indicate that the method exhibits high precision, with errors considerably lower than 1% during the entire simulation period, for both precipitation and total precipitable water. We apply the method to the Great Lake-effect snowstorm of November 2014, aiming at quantifying the contribution of lake evaporation to the large snow accumulations observed in the event. We perform simulations in a nested domain at 5 km resolution with the tagging technique, demonstrating that about 30-50% of precipitation in the regions immediately downwind, originated from evaporated moisture in the Great Lakes. This contribution increases to between 50 and 60% of the snow water equivalent in the most severely affected areas, which suggests that evaporative fluxes from the lakes have a fundamental role in producing the most extreme accumulations in these episodes, resulting in the highest socioeconomic impacts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A new moisture-tagging tool, usually known as water vapor tracer (WVT) method or online Eulerian method, has been implemented into the Weather Research and Forecasting (WRF) regional meteorological model, enabling it for precise studies on atmospheric moisture sources and pathways. We present here the method and its formulation, along with details of the implementation into WRF. We perform an in-depth validation with monthly long simulations over North America at 20 km resolution, tagging all possible moisture sources: lateral boundaries, continental, maritime or lake surfaces and initial atmospheric conditions. We estimate errors as the moisture or precipitation amounts that cannot be traced back to any source. Validation results indicate that the method exhibits high precision, with errors considerably lower than 1 % during the entire simulation period, for both precipitation and total precipitable water. We apply the method to the Great Lake-effect snowstorm of November 2014, aiming at quantifying the contribution of lake evaporation to the large snow accumulations observed in the event. We perform simulations in a nested domain at 5 km resolution with the tagging technique, demonstrating that about 30-50 % of precipitation in the regions immediately downwind, originated from evaporated moisture in the Great Lakes. This contribution increases to between 50-60 % of the snow water equivalent in the most severely affected areas, which suggests that evaporative fluxes from the lakes have a fundamental role in producing the most extreme accumulations in these episodes, resulting in the highest socio-economic impacts. [ABSTRACT FROM AUTHOR]

The explosive cyclogenesis of extra-tropical cyclones and the occurrence of atmospheric rivers are characteristic features of baroclinic atmospheres, 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. This insight can be helpful for the predictability of high impact weather associated with explosive cyclones and atmospheric rivers. [ABSTRACT FROM AUTHOR]

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. This paper analyzes the connection between ARs and floods in the Spanish region of Galicia under different 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 of the Spanish Emergency Agency from 1979 to 2010. Results suggest that although most flood events in Galicia do not coincide with the presence of an overhead atmospheric river, the latter are present in the majority of severe cases, particularly in coastal areas. Flood events associated with ARs are connected to cyclonic weather types with west and southwesterly flow and occur mostly in winter months. The link between ARs and severe flooding is not so apparent in inland areas or summer months, in which cases heavy precipitation is usually not of frontal nature but 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. [ABSTRACT FROM AUTHOR]

Offline Land-Surface Model (LSM) simulations are useful for studying the continental hydrological cycle. Because of the nonlinearities in the models, the results are very sensitive to the quality of the meteorological forcing; thus, high-quality gridded datasets of screen-level meteorological variables are needed. Precipitation datasets are particularly difficult to produce due to the inherent spatial and temporal heterogeneity of that variable. They do, however, have a large impact on the simulations, and it is thus necessary to carefully evaluate their quality in great detail. This paper reports the quality of two high-resolution precipitation datasets for Spain at the daily time scale: the new SAFRAN-based dataset and Spain02. SAFRAN is a meteorological analysis system that was designed to force LSMs and has recently been extended to the entirety of Spain for a long period of time (1979/80-2013/14). Spain02 is a daily precipitation dataset for Spain and was created mainly to validate Regional Climate Models. In addition, ERA-Interim is included in the comparison to show the differences between local high-resolution and global low-resolution products. The study compares the different precipitation analyses with rain gauge data and assesses their temporal and spatial similarities to the observations. The results show that SAFRAN and Spain02 have very similar skill scores, although the later has better scores in general. As expected, SAFRAN and Spain02 perform better than ERA-Interim, which has difficulty capturing the effects of the relief on precipitation due to its low resolution. However, ERA-Interim reproduces spells remarkably well, in contrast to the low skill shown by the high-resolution products. The high-resolution gridded products overestimate the number of precipitation days, which is a problem that affects SAFRAN more than Spain02 and is likely caused by the interpolation method. Both SAFRAN and Spain02 underestimate high precipitation events, but SAFRAN does so more than Spain02. The overestimation of low precipitation events and the underestimation of intense episodes will probably have hydrological consequences once the data are used to force a land surface or hydrological model. [ABSTRACT FROM AUTHOR]

Here we ask the following questions: (1) What is the depth of soil water uptake across the vast Amazon ecosystem and across the wet and dry seasons or wet and dry years? Reports of 18m deep roots in terra-firme trees and 0.2m deep roots in lowland swamp forests suggest the influence of local drainage conditions. (2) Is the uptake source the recent rain reaching shallow soils, or past wet-season rain stored in deep soils, or past rain that reached the water table which sends the water back up through capillary flux, or past rain that flowed down the topographic gradient from ridges to valleys (i.e., upland to lowland subsidy)? We address these questions through a synthesis of observations compiled from the literature, and a dynamic and hillslope-resolving model informed by the synthesis, which explicitly couples surface-groundwater and root uptake, driven by reanalysis atmosphere and observed leaf area. Our findings shed critical lights on the depth and origin of the water supporting Amazon photosynthesis, hence their resilience or vulnerability to seasonal-interannual droughts across the Amazon. [ABSTRACT FROM AUTHOR]

The Western Mediterranean Region (WMR) is characterized by a high frequency in the occurrence of torrential rainfall episodes and floods that cause severe damages, with a very high social and economic impact. Several particularities make this region prone to extreme rains; the mild sea waters or its orography are some examples. Most cases occur in autumn, when the combination of a still warm sea surface temperature (after a peak in late summer), and a southward displacement of the jet stream (which usually favors the appearance of Atlantic lows or cut-off-lows affecting the WMR), make this season the most favourable for the development of these adverse episodes.Here, we developed a new WMR extreme precipitation events database for the period 1980-2015. The events' detection is based on the MESCAN precipitation analysis dataset (recently available in the ECMWF MARS archive at 5.5 km) and the HYMEX flood database. Then, by applying the k-means method of cluster analysis, the identified episodes are classified into different groups based on the synoptic pattern of each event. The procedure yields a dataset consisting of the main episodes, their associated floods (if any) and the referral atmospheric circulation pattern associated with each event. In the future, this archive could be useful in many other different studies of WMR torrential rains. For example, we plan to use this database for the study of moisture sources, trying to associate some recurrent sources to each event cluster created. [ABSTRACT FROM AUTHOR]

Wildfires are a common natural hazard in the west of the Iberian Peninsula during the warm season, having devastating effects on the environment and also affecting the region economically. It is well known that atmospheric conditions (wind, temperature, humidity, etc.), together with topography and vegetation characteristics, strongly influence the propagation of fires. These three factors are considered in the WRF-FIRE model, which couples a mesoscale atmospheric model (WRF) with a semi-empirical fire spread model, and thus can be used as a fire-spread modelling tool.We present here preliminary simulations with WRF-FIRE for Galician wildfires. There are some characteristics in this region that turn wildfire simulation more complicated, like generalized arson and the heterogeneous forest distribution. As a result, the accuracy in the location of the ignition points, which are often multiple, and the adequacy of the fuel characterization may play an important role. We assess the influence of these factors with simulations of past wildfires and compare results with satellite images.A correct implementation of the model in this region can be useful to improve the mapping of high-risk fire areas or to obtain operational fire spread predictions that can help firefighting. [ABSTRACT FROM AUTHOR]

Ecosystem productivity and resilience to environmental stress are tightly linked to soil water availability and plant rooting depth. We explore here the controls on plant root uptake, imposed by climate and groundwater accessibility, underlying the spatial and temporal patterns of rain-fed and groundwater-fed ecosystems at the large and catchment scale. We estimate root water-uptake depths with an inverse model, based on observed productivity and atmosphere, at 30″ (∼1-km) global grids to capture the topography critical to soil hydrology. Results reveal strong sensitivities of rooting depth to local soil water profiles determined by precipitation infiltration depth from the top (reflecting climate and soil), and groundwater table depth from below (reflecting topography-driven land drainage). They indicate highly variable uptake-depth across seasonal and local hydrologic gradients, and a far more common occurrence of deep (>5m) uptake than previously thought. The resulting patterns of plant rooting depth bear a strong topographic and hydrologic signature at catchment to global scales and explain the contrasting rooting depths observed under the same climate for the same species but at distinct topographic positions. These results highlight the role of groundwater accesibility as one of the main organizing principles shaping ecosystem patterns and stress response at the catchment scale. [ABSTRACT FROM AUTHOR]