Displays

Atmospheric rivers (ARs) are narrow, elongated structures, transporting large amounts of water vapor from the tropics towards polar regions. These synoptic scale features play an important role in the global hydrological cycle and for extreme precipitation events. To study how ARs will change in response to greenhouse warming we use a series of century-long fully coupled ultra-high-resolution simulations conducted with CESM 1.2.2 with an approximate horizontal resolution of ~25 km in the atmosphere and 10 km in the ocean. The simulations were carried out for present-day, 2xCO2 and 4xCO2 conditions. In this high atmospheric resolution, we obtain a much more realistic representation of complex orographic features (such as the Rocky Mountains), which can greatly influence the extreme precipitation often associated with ARs. Results from the present-day simulation are compared with ERA-Interim data to validate the model's fidelity in representing ARs. Our analysis focuses on future greenhouse-warming induced changes in AR frequency, geometry, landfalling latitude and strength. We find a global increase in the frequency of ARs amounting to ~0.5% for 2xCO2 and 0.9% for 4xCO2 respectively. In subtropical areas, such as the southwestern part of the United States AR frequencies increase by up to 7%. The presentation will further document the underlying processes for this increase.

How to cite: Nellikkattil, A., Guan, B., Lee, J.-Y., Timmermann, A., Lee, S.-S., Waliser, D., and Chu, J.-E.: Ultra-high-resolution future coupled model projections of atmospheric rivers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12010, https://doi.org/10.5194/egusphere-egu2020-12010, 2020.

      Midlatitude oceanic fronts play an important role in the air-sea coupled weather and climate system. Created by the confluence of warm and cool oceanic western boundary currents, the strong sea-surface temperature (SST) gradient is maintained throughout the year. The climatological mean turbulent air-sea heat exchange maximizes along these SST fronts and collocates with the major atmospheric storm tracks. A recent study identified that the air-sea heat exchange along the SST front mainly occurs on sub-weekly time scales, associated with synoptic atmospheric disturbances. This implies a crucial role of air-sea moisture exchange along the SST fronts on the atmospheric water cycle through the intensification of atmospheric cyclones and the associated precipitation.  

      In this study, we investigate this influence of the SST front on the atmospheric water cycle by analyzing the atmospheric response to different prescribed SST in the Atmospheric general circulation model For the Earth Simulator (AFES). Changing the latitude of the prescribed zonally symmetric SST in aqua-planet configuration, we find a distinctive response in convective and large-scale precipitation, surface latent and sensible heat fluxes, as well as diabatic heating and moistening with respect to the latitude of SST front. Upward surface latent heat flux and convective precipitation always maximize along the equatorward flank of SST front. On the other hand, large-scale precipitation is always located on the poleward flank of the SST front, in correspondence with the maximum atmospheric moisture flux convergence. The moisture flux convergence is mainly associated with midlatitude eddies and not with the time mean transport. This highlights the influence of mid-latitude SST fronts on the atmospheric water cycle through the organization of atmospheric storm track.

How to cite: Ogawa, F. and Spengler, T.: Influence of mid-latitude oceanic fronts on the atmospheric water cycle , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10022, https://doi.org/10.5194/egusphere-egu2020-10022, 2020.

Stable water isotopes are employed as hydrological tracers to quantify the diverse implications of atmospheric moisture for climate. In a recent study based on several years of in-situ isotope measurements in water vapour of the marine boundary layer it was shown that the isotope signal during evaporation is not modulated by wind speed, contrary to the commonly used theory, but controlled by relative humidity and sea surface temperature, only (Bonne et al., 2019). In sea ice covered regions, the sublimation of deposited snow on sea ice was found as another key process controlling the local water vapour isotopic composition. Here, we evaluate how these new findings will impact the stable water isotope signal both in vapour and precipitation on a global scale. For this purpose, the newly suggested parametrisations are included in two versions of the isotope-enabled atmospheric model ECHAM-wiso (Werner et al., 2016; Cauquoin et al., 2019) and a set of simulations is performed to disentangle the effects of the various controlling factors. Model results are evaluated against a compilation of short-term measurements of the isotopic composition in the marine boundary layer (Benetti et al., 2017), as well as data sets from several coastal stations (Steen-Larsen et al., 2014; 2015; 2017). In addition, the implications of the suggested parameterization changes for the interpretation of various isotope records in paleo-records will be discussed.

How to cite: Werner, M., Bonne, J.-L., Cauquoin, A., and Steen-Larsen, H. C.: Key controls of water vapour isotopes during oceanic evaporation and their global impact, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13131, https://doi.org/10.5194/egusphere-egu2020-13131, 2020.

How to cite: Dahinden, F., Aemisegger, F., Barthlott, S., Christner, E., Dyroff, C., Hase, F., Knippertz, P., Wernli, H., Schneider, M., and Pfahl, S.: Stable water isotopes as a tool to investigate tropospheric moisture transport pathways over the eastern subtropical North Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10979, https://doi.org/10.5194/egusphere-egu2020-10979, 2020.

River floods are the most common and devastating natural hazard in Europe. In this study, we focus on a specific flood type which is associated with so-called Vb-cyclones. These extratropical cyclones are defined by their pathway from the western Mediterranean Sea north-eastward over northern Italy along the eastern fringe of the Alps towards Central Europe. Prominent examples of Vb-floods are the July 1954 and the August 2002 floods in the Elbe and Danube catchments as well as the Odra flooding during May/June 2010.

Only a few Vb-cyclones cause extreme flooding in Central Europe, even though about 2-5 follow the Vb pathway on average per year. The processes which intensify these flood triggering Vb-cyclones are only partly understood. One potential mechanism could be the soil-precipitation feedback over the continent. Moreover, the resulting latent heat release could re-enforce the atmospheric blocking conditions, e.g., over eastern Europe, that foster cyclones to follow the Vb-like pathway.

Our study aims to increase knowledge about potential feedback mechanisms by quantifying the role of specific moisture sources. We analysed the moisture uptake for selected extreme events in the 20^th century based on backward trajectories in dynamically downscaled ERA-20C reanalysis. The downscaling was performed over Europe with a high-resolution and interactively coupled atmosphere-ocean model setup (COSMO-CLM+NEMO). The Mediterranean Sea contributed to rainfall in the affected river catchments often at the event start. Throughout the events, other main moisture uptake regions were the European continent pointing towards an important role of the soil-moisture precipitation feedback, but also other oceanic sources such as the North Atlantic, the North Sea, and the Baltic Sea were identified. The large variety of the identified sources highlights the complex dynamical interplay of different airmasses leading to convergence of moisture during particularly severe flood producing heavy precipitation events.

How to cite: Krug, A., Aemisegger, F., Sprenger, M., Primo, C., and Ahrens, B.: Moisture sources of extreme Vb-floods in Central Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2315, https://doi.org/10.5194/egusphere-egu2020-2315, 2020.

How to cite: Cloux González, S., Garaboa Paz, A. D., Insua Costa, D., Perez Muñuzuri, V., and Miguez Macho, G.: Extreme precipitation events in the Mediterranean area. A contrasting Lagrangian and Eulerian approach for moisture evaporation sources identification., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5690, https://doi.org/10.5194/egusphere-egu2020-5690, 2020.

The high-elevation eastern Anatolian plateau, located in eastern Mediterranean basin, is cold and snowy in winter, and functions as a water tower in providing water to Mesopotamia through Euphrates and Tigris rivers. These rivers are snow-fed, as much of their discharges occurs in spring when the seasonal warming melts the snowpack. The anomalous warming over the eastern Anatolia in early March 2004 resulted in unprecedented snowmelt runoff in the Euphrates and Tigris basin together with the accompanying rainfall. This study explores an atmospheric river (AR) leading to the extreme hydrometeorological events in the headwaters regions of the Euphrates and Tigris basins in early March 2004, and its possible linkage to the strength of the East Asian trough (EAT). In the analyses, we used reanalysis data, gridded products of surface temperature and snow cover, river discharge data and satellite imagery. We employed an intensity index for the EAT and a trough displacement index for the Mediterranean trough (MedT) to explore the relationship between the strength of the EAT and the displacement of the MedT at pentad resolution. We show that there is a strong relationship between the strength of the EAT and the zonal displacements of the Mediterranean upper layer trough on the 13th pentad of the year, which corresponds to early days of March. In 2004, which appears to be an extreme year for this phenomenon, the MedT is positioned and deepened in the central Mediterranean (about10−15◦E), and extended towards central Africa during the early days of March. This synoptic pattern provided favorable conditions for the development of AR with a southwest-northeast orientation, carrying warm tropical African air towards the eastern Mediterranean and Anatolian highlands resulting in rapid melting of the snowpack as well as severe precipitation, and thus, flooding events, in the eastern Anatolia. A key finding in our analysis is that the strengthening of the EAT was instrumental to the increased amplitude of the ridge-trough system over the Euro-Mediterranean region in the early days of 2004 late winter. A further analysis is ongoing to provide a basis to analyze past individual AR events over the region, especially those associated with extreme precipitation events and snowmelt.

How to cite: Sen, O. L., Bozkurt, D., and Ezber, Y.: Tropical air intrusions over the eastern Mediterranean and Mesopotamia: An atmospheric river case study and role of the East Asian trough , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11354, https://doi.org/10.5194/egusphere-egu2020-11354, 2020.

The Mediterranean is among the global 'hot-spots' of climate change, where severe consequences of climate change are expected. Changes in the atmospheric water cycle are among the leading causes of the vulnerability of the Mediterranean to greenhouse gas-driven warming. Specifically, precipitation is projected to decrease year-round, which is expected to have major impacts on hydrology, biodiversity, agriculture, hydropower, and further economic sectors that rely on sufficient water supply.

We investigate possible causes of the Mediterranean drying in regional climate simulations. To isolate the influence of multiple large-scale drivers on the drying, we sequentially add the respective drivers from global models to regional climate model simulations. We show that the causes of the Mediterranean drying depend on the season. We will present in detail how the summer drying is driven by the land-ocean warming contrast, lapse-rate and other thermodynamic changes, while it only weakly depends on circulation changes. In contrast, changes in the circulation are the primary driver for the projected winter precipitation decline. Since land-ocean contrast, thermodynamic and lapse-rate changes are more robust in climate simulations than circulation changes, the uncertainty associated with the projected drying should be considered smaller in summer than in winter.

Reference: Brogli, R., S. L. Sørland, N. Kröner, and C. Schär, 2019: Causes of future Mediterranean precipitation decline depend on the season. Environmental Research Letters, 14, 114017, doi:10.1088/1748-9326/ab4438.

How to cite: Brogli, R., Sørland, S. L., Kröner, N., and Schär, C.: Causes of future Mediterranean precipitation decline depend on the season, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4638, https://doi.org/10.5194/egusphere-egu2020-4638, 2020.

Dry intrusion (DI) is the slantwise descent of dry air from the extratropical upper troposphere to the mid/lower troposphere of the lower latitudes. When reaching the tropical regions, DIs substantially change the overall amount of available moisture, ocean surface fluxes into the atmosphere, as well as the atmospheric stability to vertical motion and the 3-dimensional flow and associated dynamics. However, the occurrence of such events has not been quantified systematically. Here, we quantify the climatological occurrence of DIs that extend from the extratropics to tropical regions. Specifically, we focus on events that host subsequent cross-equatorial flow. Using 6-hourly ERA-Interim reanalysis data with a Lagrangian approach, we show that during the summer monsoon season (June to September) DIs enter the tropical region from the southern hemisphere with peaks that exceed 10 % frequency in time. DI arrival into the tropics is associated with dry and cold lower-tropospheric anomalies, and consequently induced ocean evaporation and sensible heat flux into the atmosphere. Although cross-equatorial DIs are rare, a hotspot of such DIs is evident in the Indian Ocean, having a potential role for Indian summer monsoon (ISM) water cycle. The dominance of the ISM for the annual rainfall over India implies that small changes in the evaporation and moisture pathways may influence the ISM precipitation downstream significantly. Indeed, we demonstrate the connection between ISM rainfall and the preceding water-cycle interaction under DI conditions, and further show that DIs entering the Indian subcontinent modify the low-level jets. 

How to cite: Rai, D. and Raveh-Rubin, S.: Cross-equatorial dry intrusions and their impact on Indian summer monsoon-associated water cycle , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4545, https://doi.org/10.5194/egusphere-egu2020-4545, 2020.

Climate models are in need of improved constraints for water vapor transport in the atmosphere and tritium can serve as a powerful tracer in the hydrological cycle. Although general principles of tritium distribution and transfer processes within and between the various hydrological compartments are known, variation on short timescales and aspects of altitude dependence are still under debate. To address questions for tritium sources, sinks and transfer processes, sampling of individual precipitation events in Corte on the island of Corsica in the Mediterranean Sea was performed between April 2017 and April 2018. Tritium concentrations of 46 event samples were compared to their moisture origin and corresponding air mass history. Air mass back-trajectories were generated from the novel high-resolution ERA 5 data set of the ECMWF (European Centre for Medium-Range Weather Forecasts). Geographical source regions of similar tritium concentrations were predefined using generally known tritium distribution patterns, such as a ‘continental effect’, and from data records derived at long-term measurement stations of tritium in precipitation across the working area. Our model-derived source region tritium concentrations agreed well with annual mean station values. Moisture that originated from continental Europe and the Atlantic Ocean was most distinct regarding tritium concentrations with values up to 8.8 TU and near 0 TU, respectively. Seasonality of tritium values ranged from 1.6 TU in January to 10.1 TU in May and exhibited well-known elevated concentrations in spring and early summer due to increased stratosphere-troposphere exchange. However, this pattern was interrupted by extreme events. The average altitude of trajectories correlated with tritium concentrations in precipitation, especially in spring and early summer and if outlier values of extreme tritium concentrations were excluded. However, in combination with the trajectory information, these outlier values proved to be valuable for the understanding of tritium movement in the atmosphere. Our work shows how event-based tritium research can advance the understanding of its distribution in the atmosphere.

How to cite: Juhlke, T., Sültenfuß, J., Trachte, K., Huneau, F., Garel, E., Santoni, S., Barth, J. A. C., and van Geldern, R.: Combined event-based tritium and air mass back-trajectory analysis of Mediterranean precipitation events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8343, https://doi.org/10.5194/egusphere-egu2020-8343, 2020.

Northeast Greenland is predicted to be one of the most sensitive terrestrial areas of the Arctic to anthropogenic climate change, resulting in an increase in temperature that is much greater than the global average. Associated with this temperature rise, precipitation is also expected to increase as a result of increased evaporation from an ice-free Arctic Ocean. In recent years, numerous palaeoclimate projects have begun working in the region with the aim of improving our understanding of how this highly-sensitive region responds to a warmer world. However, a lack of meteorological stations within the area makes it difficult to place the palaeoclimate records in the context of modern climate.

This study aims to improve our understanding of precipitation and moisture source dynamics over a small arid region located at 80 °N in Northeast Greenland. This region hosts many speleothem-containing caves that are being studied in the framework of the Greenland Caves Project (greenlandcavesproject.org). The origin of water vapour for precipitation over the study site is detected by a Lagrangian moisture source diagnostic, which is applied to reanalysis data from the European Centre for Medium-Range Weather Forecasts (ERA-Interim) from 1979 to 2017.

While precipitation amounts are relatively constant during the year, the regional moisture sources display a strong seasonality. The most dominant winter moisture sources are the ice-free North Atlantic ocean above 45 °N, while in summer the patterns shift towards more local and North Eurasian continental sources. During positive North-Atlantic Oscillation (NAO) phases evaporation and moisture transport from the Norwegian Sea is stronger, resulting in larger and more variable precipitation amounts. Although the annual mean temperature in the study region has increased by 0.7 °C dec ^-1 (95% confidence interval [0.4, 1.0] °C dec ^-1 ) according to ERA-Interim data, we do not detect any change in the amount of precipitation with the exception of autumn where precipitation increases by 8.2 [0.8, 15.5] mm dec ^-1 over the period. This increase is consistent with future predicted Arctic precipitation change.

How to cite: Schuster, L., Maussion, F., Langhamer, L., and Moseley, G. E.: Lagrangian detection of moisture sources for an arid region in Northeast Greenland: relations to the North-Atlantic Oscillation and temporal trends from 1979 to 2017, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11507, https://doi.org/10.5194/egusphere-egu2020-11507, 2020.

Studying the drivers of the water stable isotopic composition at coastal Antarctic sites is useful for the interpretation of coastal ice-core signal or the analysis of recent mass balance evolution. In addition to the classical fingerprint of temperature, the water isotopic composition in water vapor and snow carries a fingerprint of the humidity transport pathway, from evaporation to precipitation. Blowing snow in very windy coastal regions is also expected to carry a particular signature affected by snow-air exchanges as observed for surface snow on the East Antarctic plateau.

Since November 2018, we have been continuously measuring the water stable isotopic composition of vapor at Dumont D’Urville station (Adélie Land) using laser spectrometers in addition to isotopic composition of precipitation, blowing snow and surface snow samples. We present here the full 2019 data series. We focus on the two main weather regimes, in summer and winter, that affect the local hydrological cycle. In summer, temperature and specific humidity signals are characterized by large diurnal cycles due to katabatic winds. Winter variability is largely influenced by large scale synoptic events. We also investigate a few specific cases of precipitation/sublimation and compare two summer periods with opposite sea-ice conditions.

How to cite: Leroy-Dos Santos, C., Landais, A., Fourre, E., Agosta, C., Casado, M., Orsi, A., Aufresne, G., Jossoud, O., and Prié, F.: What drives the isotopic composition of vapor, precipitation and surface snow in a coastal site of East Anrctica: Adelie Land, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20146, https://doi.org/10.5194/egusphere-egu2020-20146, 2020.

How to cite: Penchenat, T., Vimeux, F., Daux, V., Cattani, O., Viale, M., Villalba, R., Srur, A., and Outrequin, C.: Isotopic equilibrium between precipitation and water vapor in Northern Patagonia and its consequences on δ18Ocellulose estimate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22376, https://doi.org/10.5194/egusphere-egu2020-22376, 2020.

The strong coupling between atmospheric circulation, moisture pathways and atmospheric diabatic heating is a great challenge in atmospheric research since this coupling is responsible for most climate feedback mechanisms and controls the evolution of severe weather events. Although diabatic heating rates are the major driving force of atmospheric circulation on weather and climate time scales, the diabatic heating rates obtained from current meteorological reanalyses show significant inconsistencies. This is mainly indebted to the fact that diabatic heating rates cannot be directly observed. Isotopologue observations assimilated into meteorological reanalyses can make an invaluable contribution since the isotopologue composition depends on the history of phase transition. Therefore, isotopologue observations can provide information that is closely linked to latent heating processes. Here, we analyse idealized experiments performed with the isotopes-incorporated General Spectral Model (IsoGSM) to investigate whether the additional assimilation of isotopologue observations can improve the diabatic heating rates. To do so, we use a Local Transform Ensemble Kalman Filter (LETKF) for data assimilation, and mock the high-density isotopologue MUSICA IASI observational data. The MUSICA IASI data apply the retrieval recipe of MUSICA (MUlti-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water) to the thermal nadir spectra recorded by the IASI (Infrared Atmospheric Sounding Interferometer) satellite instrument. The mocked isotopologue observations are then assimilated into the model in addition to temperature, humidity and wind profiles obtained from radiosonde and satellite data. By comparing the ensemble runs with and without the additional assimilation of the isotopologue data we can reveal the potential of MUSICA IASI isotopologue data for constraining uncertainties in diabatic heating rates.

How to cite: Khosrawi, F., Toride, K., Yoshimura, K., Diekmann, C., Ertl, B., and Schneider, M.: Testing isotopologues as diabatic heating proxy for atmospheric data analyses , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4527, https://doi.org/10.5194/egusphere-egu2020-4527, 2020.

It is an old question whether tropospheric water vapor at different levels changes consistently in response to the enhanced greenhouse gas in the atmosphere. Earlier studies using older versions of climate models and available data revealed a significant difference between models and observations. Water vapor changes in the interior of the tropical troposphere have been found to be more strongly coupled to changes at the surface in climate models than in observations. We reexamine this issue using four leading CMIP5 models (CCSM4, HadGEM2-A, GFDL-CM3 and MPI-ESM-MR) and more updated observational datasets (ERA-Interim and NCEP reanalysis). Focusing on the Tropics, we have calculated the correlations between interannual variation of specific humidity in all levels of the troposphere with that at the surface. It is found that the previously noted biases in the strength of the coupling between water vapor changes in the interior of the troposphere and those at the surface still exist in the updated models—the change in the tropical averaged tropospheric water vapor is more strongly correlated with the change in the surface, especially in the middle troposphere. It is argued that the vertical profile of water vapor correlations in observations is more consistent with the “hot tower” concept for tropical convections. Zonal mean correlation results and those from the moisture regime sorting method are consistent with each other, both of which indicate the role of deep convection as a mechanism to couple the middle tropospheric water vapor and that in the surface and that an inaccurate representation of deep convection as a possible cause for the discrepancies between models and observations in the coupling between middle tropospheric water vapor and those at the surface.

How to cite: Yang, M., Sun, D.-Z., and Zhang, G. J.: Relationship between surface and tropospheric water vapor variation on interannual timescale: A revisit, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22688, https://doi.org/10.5194/egusphere-egu2020-22688, 2020.

Understanding vulnerabilities of continental precipitation to changing climatic conditions is of critical importance to society at large. Terrestrial precipitation is fed by moisture originating as evaporation from oceans and from recycling of water evaporated from continental sources. In this study, continental precipitation and evaporation recycling processes in the Earth system model GFDL-ESM2G are shown to be consistent with estimates from two different reanalysis products. The GFDL-ESM2G simulations of historical and future climate also show that values of continental moisture recycling ratios were systematically higher in the past and will be lower in the future.

Global mean recycling ratios decrease 2%–3% with each degree of temperature increase, indicating the increased importance of oceanic evaporation for continental precipitation. Theoretical arguments for recycling changes stem from increasing atmospheric temperatures and evaporative demand that drive increases in evaporation over oceans that are more rapid than those over land as a result of terrestrial soil moisture limitations. Simulated recycling changes are demonstrated to be consistent with these theoretical arguments. A simple prototype describing this theory effectively captures the zonal mean behavior of GFDL-ESM2G.

Key sources of terrestrial evaporation, notably the interior of the Amazon basin and parts of the Ganges-Brahmaputra and Indus River basins, may experience reductions in moisture recycling. This has implications for key sink regions of terrestrial recycled precipitation, especially in rain-fed agricultural regions where crop yields will become increasingly soil moisture limited, such as the La Plata River basin, the corn producing regions of North America, southern Africa and the Sahel.

The results presented here have been published last year in Journal of Climate dx.doi.org/10.1175/JCLI-D-19-0145.1

How to cite: Findell, K., Keys, P., van der Ent, R., Lintner, B., Berg, A., and Krasting, J.: Rising Temperatures Increase Importance of Oceanic Evaporation as a Source for Continental Precipitation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10401, https://doi.org/10.5194/egusphere-egu2020-10401, 2020.

How to cite: Do, H. and Kim, J.: Long-term change of warm-season precipitation climatology in South Korea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20963, https://doi.org/10.5194/egusphere-egu2020-20963, 2020.

There has been growing interest in studying precipitation recycling and identifying relationships between moisture sources and receptors. The network built upon the relationships is crucial for the knowledge of the atmospheric water cycle, weather prediction, and adaptation to hydroclimatic disasters. This study aims to provide an interesting perspective of a Source-to-Receptor (SR) network to study the dynamics of the East Asian Summer Monsoon (EASM). By prescribing 24 sources and 6 EASM subregions, the SR network during the wet season is quantified using the two-dimensional physically-based Dynamical Recycling Model (DRM). Results reveal that in addition to oceanic sources, land sources including the often-overlooked plateau regions play an important role in supplying moisture to most EASM subregions. A seesaw relationship of the Indian Ocean/South Asia sector from April to June and the Pacific Ocean/East Asia sector from July to September is evidenced in the intraseasonal variation of the SR network for EASM subregions including South China coast and Taiwan, Yangtze River basin, South Japan and Korean Peninsula. Conversely, weaker intraseasonal variation is seen in the SR network for the Yellow River basin and North China. During heavy rainfall days, the zonal oscillation of western North Pacific Subtropical High (WNPSH) is deemed crucial to modulate the SR network through enhanced contributions from Bay of Bengal, Indochina, Indian subcontinent and Southwest China (the Philippine Sea and western North Pacific) during the positive (negative) phase. Coupled circulations such as two distinct pressure dipoles and coherent upper-level wave trains from mid-latitudes are responsible for bridging the moisture routes. Lastly, preceding winter/springtime El Niño is likely associated with the enhanced (weakened) moisture supply from the southwesterly (Pacific Ocean) sources. Longer-term variabilities such as the Pacific Decadal Oscillation is also considered influential to the SR network. We believe that the attributable atmospheric bridges and the SR network itself can offer insights to the current understanding of EASM and model simulations of the monsoon systems and the water cycles.

How to cite: Cheng, T. F. and Lu, M.: Moisture Source-to-Receptor Network for East Asian Summer Monsoon and the Associated Atmospheric Bridges, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-375, https://doi.org/10.5194/egusphere-egu2020-375, 2020.

The identification, climatic modulation and hydrological impact of Atmospheric Rivers (ARs) is an emergent scientific topic in recent years. ARs are important and yet understudied for East Asia (EA). We use our new AR identification algorithm (Pan & Lu, 2019), to build up a comprehensive AR catalog for this region for the first time.  Interesting patterns are found: (1) there is a dominant AR route, originating from the Arabian Sea, crossing over the Bay of Bengal and Indochina, South China Sea (SCS) and Southeast China (SEC), and terminating in the western North Pacific; and (2) a nine-stage annual pattern in the climatological frequency is revealed.  Stage 1: mid-Mar to mid-May, the formation of Western North Pacific Subtropical Height (WNPSH) near the SCS steers and confines AR in its northwest flank over SEC.  Stages 2-5: during the monsoon season from mid-May to late-Aug, the evolution of AR follow the intra-seasonal progression of Asia-Pacific monsoon (including South Asian monsoon, East Asian monsoon and western North Pacific monsoon. Stages 6-9: late-Aug to mid-Mar, ARs leave EA and only occur over the North Pacific. Over all stages, we find the contribution of AR grows significantly with more extreme rainfall (i.e., from the annual rainfall, heavy rainfall, persistent heavy rainfall to large spatial extent persistent heavy rainfall), especially in spring and early-monsoon season. This emphasizes ARs’ significant role in extreme or catastrophic rainfall events. Intriguingly, divergence of AR trajectories (also in their characteristics) occurs along the extratropical direction, and such divergent features have spatially heterogenous dependence on the leading modes of a collection of steering atmospheric and regulating climatic signals. Large divergence indicates high sensitivity of AR to transient steering; while small divergence promises high predictability of ARs, thus their associated hydrological impacts.

How to cite: Pan, M., Lu, M., Lall, U., and Dong, Q.: Spatio-Temporal Dynamics of East Asia Atmospheric Rivers and their Atmospheric Steering and Climatic Regulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-257, https://doi.org/10.5194/egusphere-egu2020-257, 2020.

Atmospheric river (AR), which is defined as long, narrow and transient corridor with enhanced moisture transport, received more and more scientific attention because of its crucial roles in the global water cycle, water resource management and hydrometeorological extremes. In recent years, dozens of AR identification algorithms are proposed to detect and quantify ARs. However, limitations still exist. In this study, a novel global AR identification algorithm is developed to address some limitations among all the state-of-the-art AR algorithms. First, in the AR pathway detection, a coupled quantile and Gaussian kernel smoothing technique is implemented to define the IVT threshold to make a balance in capturing the spatiotemporal variation of IVT climatology and avoiding largely biased estimation. Second, in spite of the variety of AR shape, orientation and curvature, more reliable AR metrics (e.g., length and width) can be determined based on the smooth AR trajectory, which is generated by modifying and integrating the concept of local regression and K-nearest-neighbors. Third, a robust and resilient criterion is developed to filter the tropical moisture swell. Four, an exquisite metric (turning angle series) is proposed, which is helpful to distinguish the tropical cyclone-like (TC-like) features and quantify the AR curvature which may bridge the ARs to the atmospheric circulation system. Last but not least, another novel metric () is developed to measure the localized IVT coherence on the AR pathway. For each grid, the  is defined as the inter-decile range of the IVT direction of its neighbor grids. The IVT coherent/discordant segments on the AR pathway are extracted by an image segmentation algorithm according to their spatial pattern and  values. The coherent segments are more likely to carry on long-distance moisture transport,  governed by the persistent and large-scale circulation system and related to hydrometeorological extreme, while discordant segments are more likely to be corresponding to the localized turbulence, low pressure system or TC-like features. So, flagging the segments into different categories will be significant in the study of the climatic modulation of AR occurrence, intensity, spatial pattern and the associated rainfall predictability. We believe that this algorithm with various metric will facilitate further quantitative investigations by the AR research community in terms of water resource management, hydrometeorological extreme predictability and climate change projections.

How to cite: Lu, M. and Pan, M.: A novel global AR identification algorithm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-240, https://doi.org/10.5194/egusphere-egu2020-240, 2020.

Warm conveyor belts (WCBs) are important airstreams in extratropical
cyclones, leading to the formation of intense precipitation
and the transport of substantial amounts of water vapour upward and
poleward. This study presents a scenario of a WCB that ascended from
western Europe towards the Baltic Sea using aircraft, lidar and
radar observations from the field experiments HyMeX and
T-NAWDEX-Falcon in October 2012.
Trajectories based on the ensemble data assimilation
system of the ECMWF are used to quantify probabilistically
the occurrence of the WCB and Lagrangian matches
between different observations. Despite severe limitations
for research flights over Europe, the DLR Falcon successfully
sampled WCB air masses during different phases of
the ascent. The overall picture of the WCB trajectories revealed
measurements in several WCB branches: trajectories
that ascended from the East Atlantic over northern France
while others had their inflow in the western Mediterranean
region and passed across the Alps. For the latter ones, Lagrangian
matches coincidentally occurred between lidar water
vapour measurements in the inflow of the WCB south,
radar measurements during the ascent at and its outflow
north of the Alps during a mid-tropospheric flight leg over
Germany.
The comparison of observations and ensemble analyses
reveals a moist bias of the analyses in parts of the WCB inflow
and an underestimation of cloud water species in the
WCB during ascent. In between, the radar instrument measured
strongly precipitating WCB air mass with embedded
linking trajectories directly above the melting layer while
orographically ascending at the southern slops of the Alps.
An inert tracer air mass could confirm the long pathway
of WCB air from the inflow in the marine boundary layer
until the outflow in the upper troposhpere near the Baltic
sea several hours later. This case study illustrates the complexity
of the interaction of WCBs with the Alpine topography,
which leads to (i) various pathways over and around
the Alpine crest and (ii) locally steep WCB ascent with increased
cloud content that might result in enhancement
of precipitation where the WCB flows over the Alps. The
combination of observational data and detailed ensemble-based
trajectory calculations reveals important aspects of
orographically-modified WCBs.

How to cite: Boettcher, M., Schäfler, A., Sodemann, H., Sprenger, M., Kaufmann, S., Voigt, C., Schlager, H., Summa, D., Di Girolamo, P., Nerini, D., Germann, U., and Wernli, H.: A portrayal of an orographic Warm Conveyor Belt using observations from aircraft, lidar and radar, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18794, https://doi.org/10.5194/egusphere-egu2020-18794, 2020.

The vertical structure of the water vapor field in the lower troposphere is only sparsely documented in mountainous regions and particularly above Alpine lakes. This may in part due to the complexity of the system, being intimately linked to the orography surrounding the lakes and the forcing of the topography-induced winds. The question arises as to how the vertical extent of evaporation processes over the lakes and how these are influenced by larger scale forcing, in particularly with regard to the vertical dimension.

In order to gain understanding on the vertical structure of atmospheric water vapour above mountain lakes, the L-WAIVE (Lacustrine-Water vApor Isotope inVentory Experiment) field campaign was conducted in the Annecy valley in the French Alps in June 2019. This campaign was based on a synergy between ground-based lidar measurements and ship-borne as well as airborne observations. Two ultra-light aircraft (ULA) were equipped with remote sensing and in-situ instruments to characterize the vertical distribution of the main water vapour isotopes. One ULA embarked a backscatter lidar to monitor the horizontal evolution of the vertical structure of the lower troposphere above and around the lake, and the other one carried an L2130-i Picarro isotope analyser for the in-situ measurement of the H₂¹⁶O, H₂¹⁸O and HDO concentrations, an iMet probe for the measurement of thermodynamic properties (T, RH, p), as well as a pre-cleaned Caltech Active Strand Cloud Water Collector which was modified to efficiently collect cloud water at the speed of the ULA. Offset calibration of the Picarro analyser was carried out for each flight before take-off and after landing. Three-dimensional explorations of the lake environment up to 4 km above the mean sea level (~3.5 km above the ground level) were conducted with the ULAs. Simultaneous vertical profiles of water vapour, temperature, aerosols and winds were acquired from two co-located ground-based lidars installed on the shore of the southern part of the Annecy Lake named “petit lac”, in the commune of Lathuile (45°47' N, 6°12' E). Finally, ship-borne profile measurements of the lake water temperature, pH, conductivity and dissolved O₂ as well as water sampling for isotopic analyses were accrued out across the lake of Annecy.

The campaign period included several cases of weather events leading to variability between dry and humid conditions, cloudy and cloud-free conditions, and regimes dominated by weak and strong winds. Flight patterns have been repeated at several times in the day to capture the diurnal evolution as well as variation between different weather regimes. Additional flights have been conducted to map the spatial variability of the water vapour isotope composition with regard to the lake and topography. The scientific strategy of the experiment will be presented, and the first observational results will be described with emphasis on the vertical structure of the lower troposphere and its relationship to orography, including the characterisation of the water vapour isotopologues variability in, above and around the Annecy lake.

How to cite: Chazette, P., Dieudonné, E., Monod, A., Sodemann, H., Totems, J., Baron, A., Diana, C., Doira, P., Durand, A., Maignan, F., Ravier, S., Seidl, A., and Flamant, C.: The L-WAIVE campaign over the Annecy lake: An analysis of water vapor variability in complex terrain, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3665, https://doi.org/10.5194/egusphere-egu2020-3665, 2020.

The water cycle in atmospheric and coupled models is a major contributor to model uncertainty, in particular at high-latitudes, where contrasts between ice-covered regions and the open ocean fuel intense heat fluxes. However, observed atmospheric vapour concentrations do not allow us to disentangle the contributions of different processes, such as evaporation, mixing, and cloud microphysics, to the overall moisture budget. As a natural tracer, stable water isotopes provide access to the moisture sources and phase change history of atmospheric water vapour and precipitation.

Here we present a unique dataset of stable isotope measurements in water vapour and precipitation from the IGP (Iceland Greenland Seas Project) field campaign that took place during February and March 2018. The dataset includes simultaneous measurements from three platforms (a land-station at Husavik, Iceland, the R/V Alliance, and a Twin Otter aircraft) during winter conditions in the Arctic region. Precipitation was collected on an event basis on the research ship, and along two north-south transects in Northern Iceland, and analysed at two stable isotope laboratories. Airborne vapour isotope data was obtained from 10 flights covering a large geographic range (64 °N to 72 °N). Careful data treatment was applied to all stable isotope measurements to ensure sufficient data quality in a challenging measurement environment with predominantly cold and dry conditions, and characterised by strong isotope and humidity gradients. Data quality was confirmed by inter-comparison of the vapour isotope measurements both between ship and aircraft, and between the aircraft and Husavik station.

We exemplify the value of the observations from the analysis of several flights dedicated to the study of the atmosphere-ocean interactions, from low-levels legs and vertical sections across the boundary layer during Cold Air Outbreak (CAO) conditions. The precipitation in Northern Iceland collected at the precipitation sampling network shows clear co-variation with the upstream water vapour measurements at Husavik station, indicative of the wider spatial representativeness of the isotope signals. The land-based snow and vapour measurements are furthermore consistent with the isotope composition in upstream ocean regions sampled by the research vessel, and as linked from aircraft measurements.

How to cite: Sodemann, H., Touzeau, A., Barrell, C., Burkhart, J. F., Elvidge, A., Jónsson, Þ., Lachlan-Cope, T. A., Lacour, J.-L., Lanzky, M., Midtgarden Golid, H., Ólafsdóttir, R., Papritz, L., Renfrew, I. A., Steen-Larsen, H. C., Sveinsbjörnsdóttir, Á., and Weng, Y.: Water vapour and precipitation isotope measurements from different platforms during the IGP campaign, Iceland, in 2018 connect evaporation sources to precipitation sinks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17782, https://doi.org/10.5194/egusphere-egu2020-17782, 2020.

Seasonal snow cover is a crucial resource for hydropower in Norway. Understanding water sources and processes related to inter-annual snow cover variability is therefore of fundamental societal relevance. The stable water isotope composition of precipitation provides a natural, integrated tracer of the condensation history during atmospheric water transport. The main parameters dD and d18O along with the secondary quantity d-excess give information about the origin and transport history of moisture from its source to its sink. When snow falls and deposits on the ground as a sediment, it creates a record in the form of the seasonal snow pack.

Here we utilize data acquired during a field campaign in the winter season of 2018-2019 at the Finse Alpine Research Station Center (1222m, 60.6N, 7.5E) in Norway, in order to investigate the transfer of the isotopic signal of source and transport conditions from vapour to snowfall, and to the snow pack.

Over a main period of two months, snowfall was sampled daily, while the water vapour was continuously measured from ambient air guided through a heated inlet to a Picarro L2130i infrared spectrometer, with daily calibration runs. During five periods with intense snowfall, we carried out higher frequency sampling down to 15 minute intervals. Covering the entire winter season, five snowpits were sampled for isotopic analysis as well as detailed stratigraphy. In total more than 400 snow samples where taken and analysed for their isotopic composition, accompanied by routine meteorological observations over the winter season at the site. In addition, we compare the variations in the observed isotope signal at Finse with one derived from moisture source analysis using the Lagrangian diagnostic WaterSip, based on the FLEXPART model and ERA Interim reanalysis data.

To investigate to what degree moisture source information is archived in the snow pack, and how it evolves during the season, we compare snow observations at different time resolution (daily and high frequency snowfall samples) with the record of the snow pack, aided by the snow model CROCUS. The meteorological observations supply context for understanding the snow formation conditions. In particular, deviations from isotopic equilibrium between vapour and precipitation at ambient temperature conditions provide insight into the dominant condensation regime during different intense observation periods.

How to cite: Lanzky, M., Touzeau, A., Burkhart, J. F., Filhol, S., Weng, Y., and Sodemann, H.: Links between the moisture origin and isotopic signature in water vapour, snowfall and snow pack at Finse Alpine Research Center (1222m) in Southern Noway, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18719, https://doi.org/10.5194/egusphere-egu2020-18719, 2020.

This study presents a detailed statistical analysis on the relationship of precipitation water origin and its stable hydrogen and oxygen isotope compositions for six sites in Hungary. We carried out a moisture source diagnostic by analyzing backward trajectories as it has become a common method for identifying moisture uptake locations. For providing 96 hours long precipitation-event based backward trajectories, we used the NOAA HYSPLIT model on daily basis for six sites of three elevation, 500 m, 1500 m and 3000 m. The moisture uptake regions were determined by calculating specific humidity along the trajectories. Five possible moisture source regions for precipitation were defined: Atlantic Ocean, North European Seas, Mediterranean Sea, Black Sea, Carpathian Basin and European continental areas excluding the Carpathian Basin. The main water vapor source areas are in order the continental regions following by the Mediterranean Sea and the Atlantic Ocean. However, there are spatial differences among the sampling sites reflecting the importance of the geographical locations. Principal component analysis based on the d-excess value of precipitation events showed that source regions such as the Carpathian Basin, the Atlantic Ocean and Mediterranean Sea are separated on the plain determined by the first two principal components. In order to evaluate the impact of the moisture source region on the d-excess value of precipitation events, we carried out ANOVA on the precipitation-event based macrosynoptic classification (Hess-Brezowsky and Péczely). Our results suggest that there are significant differences between amount-weighted d-excess values belonging to different macrosynoptic patterns and these types are related to precipitation events from different moisture source regions. Cluster analysis confirmed the differences in precipitation stable isotope values according to the moisture sources. The observations (precipitation events) were projected on the plain outspreaded by the first two principal components. The coordinates of the observations in this coordinate-system are separated according to the three main moisture source regions. Cluster analysis was also carried out based on d-excess values. The investigation showed that lower d-excess values are related to the Atlantic Ocean, while higher values to the Mediterranean Sea. Thus, we can conclude that the moisture source has strong impact on the stable isotope composition of precipitation water even relative far from the marine regions. The research was supported by the ÚNKP-19-3 New National Excellence Program of the Ministry for Innovation and Technology, the National Research, Development and Innovation Office (project No. OTKA NK 101664, PD 121387) and the AgroMo project (GINOP-2.3.2-15-2016-00028).

How to cite: Bottyán, E., Kristóf, E., Kármán, K., Haszpra, L., Weidinger, T., and Czuppon, G.: Statistical relationship between the air moisture source and stable isotope composition of precipitation in Hungary, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11413, https://doi.org/10.5194/egusphere-egu2020-11413, 2020.

Stable isotope ratios (δ¹⁸O and δ²H) in precipitation (_P) and atmospheric water vapor (_V) can provide mechanistic information about water cycle processes such as moisture evaporation, transport and recycling dynamics. Such insight is valuable in the Arctic where declining sea ice is amplifying atmospheric temperature and humidity, leading to complex seasonal patterns of synoptic climate and atmospheric moisture transport. Here, we present two years of continuous water vapor isotope data from Pallas-Yllästunturi National Park, northern Finland, to investigate moisture source and transport processes in the Barents Region of the Arctic. High-resolution (1-sec) measurements obtained between December 2017 and December 2019 are coupled with on-site automated weather station data – including air temperature, humidity, solar flux, wind speed and direction – as well as event-based precipitation sampling and stable isotope data over the same interval. Over the two-years, mean vapor δ¹⁸O_V, δ²H_V and d-excess_V values are -24.50‰, -181.49‰ and 14.49‰, respectively. These values are strongly correlated and define a local vapor line for Pallas where δ²H_V = 7.6 x δ¹⁸O_V + 5.9 (R²=0.98). We observe a mean offset of 10.9 ‰ between Pallas δ¹⁸O_V and δ¹⁸O_P, and d-excess is -4.8 ‰ lower in δ¹⁸O_P. There is a larger offset between vapor and precipitation d-excess during summer (-8.4‰) compared to winter (0.1‰) that may reflect varying fractionation coefficients between solid and liquid cloud-precipitation phases. The timeseries exhibits strong seasonality characterized by lower δ¹⁸O_V/δ²H_V and higher d-excess during winter, and the reverse during summer. In winter these broad patterns are primarily driven by synoptic-scale processes that influence the source and transport pathway of atmospheric moisture, and three dominant oceanic evaporative source regions are identified: the Barents, Norwegian, and Baltic Seas. Yet on diurnal timescales we observe distinct summer diel cycles that correlate with local fluctuations in specific humidity (q). These seasonal relationships are explored in context of spatial-temporal patterns in sea ice and snow cover distribution, as well as evapotranspiration processes across northern Eurasia. Finally, to better understand how current changes in the Arctic hydrologic cycle relate to inherent variability of the polar jet stream and related synoptic-scale weather, our isotope data are examined in context of dynamic circulation modes of the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO).

How to cite: Bailey, H., Mustonen, K.-R., Klein, E., Akers, P., Kopec, B., Mellat, M., Hubbard, A., Causey, D., and Welker, J.: Multi-year water vapor isotopes (δ18O/ δ2H) reveal dynamic drivers of moisture source and transport in the Barents Region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15241, https://doi.org/10.5194/egusphere-egu2020-15241, 2020.

Many palaeoclimate reconstructions are based on the fact that stable water isotopes are conserved in different highly resolved paleo-archives such as ice cores or calcium carbonates. Stable water isotopes are tracers of moisture in the atmosphere because they record information about evaporation and condensation processes during the transport of air parcels. These processes cause isotopic fractionation that leads to isotopic enrichment or depletion. The isotopic composition of precipitation is strongly correlated with altitude above sea level, distance to the coast and local surface air temperature. Knowledge on the source and transport of moisture is thus crucial for the interpretation of stable isotopes in precipitation and in palaeo-archives.
Studies analysing the linkage between stable water isotope measurements and moisture sources in southern Africa are scarce. Yet, as changes in the transport pattern can influence precipitation patterns and amounts, in a semi-arid region like southern Africa that is threatened by droughts, this knowledge is of particular interest. Thus, the aims of this study are (1) to reveal the principal moisture source areas and transport routes of specific target areas in southern Africa, (2) to assess the influence of different transport patterns on the isotopic composition of precipitation and by this (3) to create a modern analogue for palaeoclimate studies in this region.
About 200 water samples, mainly from headwaters of rivers, but also from precipitation events, springs and lakes, were collected throughout southern Africa and the stable water isotope composition (δ²H and δ¹⁸O) was analysed. To detect moisture sources for this set of isotope measurements, backward air parcel trajectories were calculated from the sample location, using the LAGRANTO tool based on ERA5 reanalysis data. Variations in specific humidity along the trajectories were then used to detect moisture uptake.
The analysis reveals main transport patterns related to the Intertropical Convergence Zone and easterly winds as well as the effects of topographical forcing, which is, for example, very pronounced above Lesotho. The results provide detailed insights into the relationships between atmospheric circulation and δ²H and δ¹⁸O values of precipitation over southern Africa, which is a prerequisite for the interpretation of isotopic records that are used for palaeoclimatic reconstructions.

How to cite: Geppert, M., Pfahl, S., Struck, U., Kirchner, I., Shemang, E., Hartmann, K., and Riedel, F.: Identification of source-sink relationships in southern Africa by stable water isotopes analysis and Lagrangian moisture source diagnostics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16042, https://doi.org/10.5194/egusphere-egu2020-16042, 2020.

Dynamical processes in the atmosphere strongly influence the large temporal and spatial variability of the atmospheric branch of the water cycle. For instance, the advection of air masses by synoptic-scale weather systems induces air-sea moisture fluxes such as evaporation, precipitation and dew deposition. It is important to better investigate and quantify this linkage between dynamical phenomena and details of the atmospheric water cycle. In addition, one of the big challenges in monitoring the atmospheric water cycle is the measurement of turbulent moisture fluxes over the ocean. Stable water isotopes (SWIs) serve as a tool to trace atmospheric processes which shape the atmospheric water cycle and, thus, provide important insights into moist processes associated with weather systems, in particular air-sea fluxes.

In this study, we investigate the impact of air-sea moisture fluxes on the variability of SWI signals in the marine boundary layer. Measurements of the second-order isotope variable deuterium excess in the marine boundary layer of the Southern Ocean show positive/negative anomalies in the cold/warm sector, respectively, of extra-tropical cyclone due to opposing moisture fluxes and non-equilibrium fractionation processes in the two sectors. The drivers of these contrasting SWI signals are analysed using the isotope-enabled Consortium for Small-Scale Modelling model for two case studies. The simulated isotope signals during the case studies show excellent agreement with ship-based isotope measurements from the Southern Ocean performed during the Antarctic Circumnavigation expedition in January and February 2017.

The main driver of SWI variability in the cold sector is enhanced ocean evaporation which substantially modifies the advected SWI signal from the Antarctic continent during a cold air outbreak. In the warm sector, dew deposition on the ocean surface and cloud formation are mainly driving the observed negative deuterium excess anomaly, which can be conserved and advected over several 100 km in the warm sector of an extratropical cyclone.

The results of this study illustrate the strong dependence of the isotopic composition of water vapour in the marine boundary layer on the predominant atmospheric large-scale flow situation. In particular in the storm track regions, the variability of SWIs in marine boundary layer water vapour is largely shaped by the sign and strength of air-sea fluxes induced by the meridional transport of air masses.

How to cite: Thurnherr, I., Aemisegger, F., Jansing, L., Hartmuth, K., Gehring, J., Pfahl, S., Böttcher, M., Berne, A., and Wernli, H.: Drivers of stable water isotope variability in the cold and warm sector of extratropical cyclones from two case studies in the Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13001, https://doi.org/10.5194/egusphere-egu2020-13001, 2020.

The Iberian Peninsula has experienced on recent years an increasing number of high impact cyclones (e.g. Klaus, 23-24 January 2009 and Xynthia, 27-28 February 2010; Liberato et al. 2011; 2013) associated with extreme precipitation events, flooding and damage to infrastructure. Recent examples are cyclones Elsa and Fabien, on December 2019, which forced more than 250 people to be evacuated from their homes in Mondego region villages, in central Portugal, due to rising river waters and infrastructure disruption .

However until now not enough evidence has been gathered to confirm a general and significant increase in the frequency and intensity of these events in the north-eastern Atlantic. In fact, according to Karremann et al. (2016) the maximum in recent years is comparable to other stormy periods in the 1960s and 1980s, suggesting that their frequency of occurrence undergoes strong multi-decadal variability.

In this study a high impact extratropical cyclones dataset developed in the framework of project “WEx-Atlantic - Weather Extremes in the Euro Atlantic Region: Assessment and Impacts” is used to assess the variability in frequency and intensity of these events over the last decades in the Iberian Peninsula. A ranking of daily precipitation days for the Iberian Peninsula taking into account not only the area affected but also its average intensity (Ramos et al. 2014) is also used. Additionally, a spatio-temporal variability of sea surface temperature (SST) is performed in the North Atlantic, using ECMWF ERA5 reanalysis data for the period 1979-2019. Finally the relevance of the North Atlantic SST variability on the intensity of these extreme events affecting the Iberian Peninsula on recent winter seasons is discussed.  

Acknowledgements

The authors would like to acknowledge the financial support by Fundação para a Ciência e a Tecnologia, Portugal (FCT), through projects PTDC/CTA-MET/29233/2017 and UIDB/50019/2020 – IDL. A.M. Ramos is supported by Scientific Employment Stimulus 2017 from FCT (CEECIND/00027/2017).

References

Karremann et al. (2016) Atmos. Sci. Let., 17: 354-361 DOI: 10.1002/asl.665

Liberato et al. (2011) Weather, 66: 330-334 DOI: 10.1002/wea.755

Liberato et al. (2013) Nat. Hazards Earth Syst. Sci., 13: 2239-2251 DOI: 10.5194/nhess-13-2239-2013

Ramos et al. (2014) Atmos. Sci. Let., 15: 328–334, DOI: 10.1002/asl2.507

How to cite: Ferreira, F., Liberato, M. L. R., Ramos, A. M., and Nieto, R.: North Atlantic SST variability and high impact storms affecting the Iberian Peninsula, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21964, https://doi.org/10.5194/egusphere-egu2020-21964, 2020.

The moisture recycling is defined as the fraction of precipitation over a delimited region that comes from the evaporation over that region. Its importance lies in the fact that it is an approximated measurement of a regional feedback between the atmosphere and the surface. Thus, this study estimates the spatio-temporal distribution of moisture recycling over the Iberian Peninsula (IP), and focuses on the impact of the use of 3DVAR data assimilation during the modeling stage.

For that purpose, two different simulations were run using the Weather and Research Forecasting (WRF) model with a horizontal resolution of 15 km over the IP. The first simulation (WRF N) was nested inside ERA-Interim as usual in numerical downscaling exercises, with information passed to the domain through the boundaries. The second run (WRF D) presents the same configuration as WRF N, but it also includes 3DVAR data assimilation step every six hours (at 00, 06, 12 and 18 UTC). Sea surface temperature was updated daily, and observations in PREPBUFR format included in the NCEP ADP Global Upper Air and Surface Weather Observations dataset were used for the data assimilation step. Only those inside a 120-minute window centered at the analysis times were assimilated. Both simulations cover the period 2010-2014, but the experiment WRF D was extended later until 2018.

The lowest values of moisture recycling (around 3 %) are obtained from November to February, while the most remarkable values are observed in spring (around 16 %) in both simulations. The moisture recycling is confined to the southeastern corner of the IP during winter. However, during spring and summer, a gradient of higher values towards the northeastern corner of the IP are observed in both simulations. The differences between simulations, associated to the dryness of the soil in the model, are highlighted during summer and autumn. WRF D presents a lower bias and produces more reliable results because of a better representation of the atmospheric moisture.

A Cross-Correlation Function (CCF) based analysis was performed for each combination of moisture recycling, accumulated precipitation and mean soil moisture over the IP. For the common period (2010-2014), the results show that the WRF D experiment extends the lifespan of moisture over the IP. The CCF analysis for soil moisture against precipitation also shows an unphysical negative lag (-1 month) for WRF N, whilst for WRF D both variables are simultaneous. For the extended WRF D simulation (2010-2018), it was found that the delay between precipitation and moisture recycling over the IP is five months.

How to cite: González-Rojí, S. J., Sáenz, J., Díaz de Argandoña, J., and Ibarra-Berastegi, G.: Moisture recycling over the Iberian Peninsula. The impact of 3DVAR data assimilation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1354, https://doi.org/10.5194/egusphere-egu2020-1354, 2020.