HS1.2.6

The U.S. Great Plains low-level jet (LLJ) is active on 26% and 62% of May-September days in the northern and southern Plains, respectively. Characterized by a diurnally-oscillating low-level wind maximum below 700 hPa, large vertical wind shear, and enhanced atmospheric moisture convergence, LLJs have been shown to fuel extreme wind- and precipitation generating mesoscale convective systems. Overall, they explain 30-50% of May-September precipitation in the Plains. The considerable societal impacts of LLJs, which span agriculture, severe weather, and wind energy, have long motivated meteorologist-led investigations into their dynamics and predictability. The sensitivity of LLJs to regional soil moisture gradients was established over thirty years ago. However, it was only recently that our work provided the first estimates of the added-value of satellite soil moisture data assimilation (DA) to LLJ forecasts.

In this presentation, we review and expand upon our previous analysis of 75 NASA Unified WRF LLJ case studies simulated with- and without weakly-coupled NASA Soil Moisture Active Passive (SMAP) soil moisture DA. Of the 75-jet cases, 43 are uncoupled LLJs and 32 are coupled LLJs. Their dynamical classification corresponds with the probable efficacy of land data assimilation. Cyclone-induced coupled LLJs, found in the warm sector of frontal systems, are strongly driven by synoptics and less likely to be influenced by land forcing. Conversely, uncoupled LLJs that occur during quiescent conditions of an anticyclonic high pressure ridge system are likely to be more strongly affected by terrain and soil moisture gradient-induced circulations.

It is shown that SMAP DA is generally more effective in uncoupled LLJ cases. However, significant SMAP DA-induced wind speed differences are noted for both LLJ types at their core and exit regions. Notably, the range of SMAP DA-induced wind speed differences between LLJs of the same class (i.e., uncoupled LLJs) is comparable to the range of differences between LLJs of different classes (i.e., coupled vs. uncoupled). Follow-on analyses presented here address the question of what differs between LLJs with small and large SMAP DA effects. Specifically, we explore attribution of event-scale differences in the added-value of SMAP DA to factors including SMAP spatial coverage, antecedent soil moisture, and the strength of synoptic forcing. Finally, a closer look is given to the verification of jet exit region SMAP DA-induced wind speed shifts using Rapid Refresh.

How to cite: Ferguson, C. R., Agrawal, S., and Bosart, L. F.: The importance of satellite soil moisture assimilation for low-level jet forecasts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2862, https://doi.org/10.5194/egusphere-egu21-2862, 2021.

Land surface models are important tools to improve our understanding of interacting ecosystem processes, but their predictions are associated with uncertainties related to model forcings, parameters and process simplifications. As high-quality observations become more and more available, they can be used to constrain the uncertainty of land surface model predictions. In this study, we use data assimilation for the fusion of data into the Community Land Model 5.0 (CLM5). CLM5 simulates a broad variety of important land surface processes including moisture and energy partitioning, surface runoff, subsurface runoff, photosynthesis and carbon and nitrogen storage in vegetation and soil. Here, we focus on water movement in soils and related soil hydraulic parameters and assimilate in-situ soil moisture data into CLM5 to improve the estimate of model states and soil hydraulic parameters. To do this, we have coupled the Parallel Data Assimilation Framework (PDAF) with CLM5. This coupling is based on the online variant of PDAF, i.e., data assimilation occurs during simulation runtime in the main memory and not via input/output files. Online coupling requires modification of the model source code, but we aim to keep the modifications to the CLM5 code minimal so that maintenance of the ongoing CLM5 developments remains straightforward. To this end, our approach reuses the existing CLM5 ensemble mode with only necessary adjustments to connect the PDAF parallel communicators. Furthermore, we developed the coupling in the framework of the Terrestrial System Modeling Platform (TSMP). TSMP is a highly modular modeling system for the fully integrated soil-vegetation-atmosphere system. To illustrate the potential of this coupling, we use the ensemble Kalman Filter to perform simultaneous state and parameter updates in a forest headwater catchment.

How to cite: Strebel, L., Bogena, H., Vereecken, H., and Hendricks Franssen, H.-J.: Coupling the Community Land Model 5.0 with the Parallel Data Assimilation Framework, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11805, https://doi.org/10.5194/egusphere-egu21-11805, 2021.

Mediterranean coastal areas are prone to hydrometeorological extremes. Their complex orography often enhances the severity of high impact events and, at the same time, makes forecasts more challenging, particularly in the medium range. Nevertheless, global operational forecasts significantly improved their accuracy in the last decades, while several novelties in mesoscale modelling are emerging, such as the atmospheric-hydrological fully coupled approach, which explicitly describes the complex interactions between the Planetary Boundary Layer (PBL) and land surface including terrestrial lateral water transport. Overall, several clues open new perspectives to define new standards in medium-range forecast performances in the Mediterranean basin.

This study investigates the skills of the Advanced Research WRF (ARW) mesoscale model both one-way and two-way coupled with the hydrological extension WRF-Hydro in providing a medium-range (7 days) forecast of a severe event hitting the Calabrian peninsula (southern Italy) in November 2019. Such event was simulated in a classical ensemble approach, using the European Center for Medium-Range Weather Forecasting (ECMWF) ensemble product (Ensemble Prediction System – EPS), which consists of 50 members providing the initial and boundary conditions to the mesoscale model. WRF model was applied in two one-way nested domains with 10 km and 2 km horizontal resolutions, encompassing most of the Mediterranean basin. WRF-Hydro was applied in the innermost domain, with NOAH-MP as Land Surface Model. Surface and subsurface routing was performed adopting 200 m as horizontal resolution.

Results highlighted that the fully coupled approach increased soil moisture and latent heat flux from land in an increasing way in the days preceding the event. Such an increase partially affected the lower PBL layers. However, when shoreward moisture transport from surrounding sea rapidly increased becoming the dominant process, only a weak signature of moisture contribution from land to the atmosphere could be detected, resulting in only slightly higher precipitation forecast and slightly increased hydrological response. Overall, the proof-of-concept carried out in this study highlighted a remarkable performance of the medium-range ensemble forecasts, suggesting a profitable use of the fully coupled approach in the selected study area for forecasting purposes in circumstances in which soil moisture dynamics and exchanges with the atmosphere are of particular interest.

How to cite: Furnari, L., Magnusson, L., Mendicino, G., and Senatore, A.: Impact of the atmospheric-hydrological fully coupled approach in a high-resolution medium-range forecasts: a case study in the Mediterranean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10883, https://doi.org/10.5194/egusphere-egu21-10883, 2021.

Land surface characteristics and processes may have complex interactions with the physical and dynamical processes of the atmosphere. However, adequate methods for systemically understanding individual processes of the nonlinearly coupled land-atmosphere continuum are still rare. Therefore, in this study, the age-weighted evaporation tagging approach of Wei et al. (2016) and the three-dimensional online atmospheric water budget analysis of Arnault et al. (2016) were implemented into the Weather Research and Forecast (WRF) model. In addition to the total and tagged atmospheric water states of matter, the latter approach was further extended for age-weighted tagged atmospheric water states of matter, thereby providing a prognostic equation of the residence time of state variables in the atmospheric water cycle. This extension allows to systematically quantify the fate of evaporated and transpired water in terms of magnitude, location, composition, and residence time. The extended WRF model was tested for a land use and land cover change study for the Poyang Lake basin, the largest freshwater lake in China. Two hypothetical scenarios, i.e., a dried-up lake and a forest restoration scenario, were simulated and then compared to a real-case control simulation using the original land-use data. The results of the basin-scale precipitation recycling in the context of evapotranspiration partitioning and the modified atmospheric water cycle due to both scenarios will be presented and discussed. We conclude that our newly developed modelling framework and the proposed analysis strategy have the potential to be applied for better understanding and quantifying the nonlinearly intertwined processes between the land and the atmosphere.

References:

Arnault, J., Knoche, R., Wei, J., & Kunstmann, H. (2016). Evaporation tagging and atmospheric water budget analysis with WRF: A regional precipitation recycling study for West Africa. Water Resources Research, 52(3), 1544–1567. https://doi.org/10.1002/2015WR017704

Wei, J., Knoche, R., & Kunstmann, H. (2016). Atmospheric residence times from transpiration and evaporation to precipitation: An age-weighted regional evaporation tagging approach. Journal of Geophysical Research: Atmospheres, 121(12), 6841–6862. https://doi.org/10.1002/2015JD024650

How to cite: Wei, J., Arnault, J., Zhang, Z., Laux, P., Fersch, B., Wagner, S., Dong, N., Yang, Q., Yang, C., Yu, Z., and Kunstmann, H.: How is the atmospheric residence time of evapotranspired water altered with a dried-up lake or a forest restoration scenario and what is the impact on precipiation?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7368, https://doi.org/10.5194/egusphere-egu21-7368, 2021.

In the year 2018, Central Europe experienced a meteorological drought and a heatwave, which led to a subsequent evolution of a hydrological drought that is still detectable in subsurface water storage anomalies today. Most likely, the drought also led to significant changes in the energy balance between solar radiation, latent and sensible heat fluxes. In conjunction with water scarcity in the subsurface, these changes may lead to feedbacks that mitigate or enforce drought conditions in the context of land-atmosphere coupling. Understanding these feedbacks is of great interest, especially under various large-scale weather patterns that strongly influence the water and energy budgets over Europe at the interannual time scale. We improve our understanding by applying the Terrestrial Systems Modeling Platform (TSMP) over the 12km resolution pan-European CORDEX model domain simulating the water and energy cycles from the groundwater to the top of the atmosphere. TSMP couples a hydrological, land-surface and atmospheric model, facilitating studies of feedbacks between total water storage anomalies, the energy budget and atmospheric processes. To investigate the feedbacks, we performed TSMP ensemble simulations of three anomalously dry water years (September to August) over Central Europe. The ensembles were initialized with the surface and subsurface states of the end of August of the drought years 2011, 2018 and 2019 from an ERA-Interim driven climatology simulated continuously with TSMP from 1989 to 2019. Every ensemble consists of 22 members, each representing a full subsequent water year, sampled from ERA-Interim reanalysis meteorological boundary conditions from 1996 to 2019, thereby simulating the influence of drought conditions over a wide range of large-scale weather patterns that occurred in Europe since 1996. In addition, to illustrate the potential range of feedbacks we also ran idealized experiments with a completely dry or wet subsurface. The results show that drought conditions may have a significant impact on cloud water and solar radiation at interannual timescales. Effects in winter are negligible, while in summer, an impact of the drought conditions of the previous year on cloud water and solar radiation is detectable in all three ensembles. The results suggest that positive feedbacks between dry subsurface water storage anomalies and atmosphere processes are not negligible and may intensify drought conditions also at the interannual time scale.

How to cite: Hartick, C., Furusho-Percot, C., Goergen, K., and Kollet, S.: Exploring the mechanism behind successive droughts: Intensification of continental droughts due to positive feedbacks between subsurface water storage anomalies and atmospheric process, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8625, https://doi.org/10.5194/egusphere-egu21-8625, 2021.

Many observational and modelling studies have highlighted the important role that irrigation plays in the terrestrial hydrological and energy cycle. Land surface models are a key tool to study these interactions, underlining the importance of an accurate representation of irrigation in these models. However, most land surface models either ignore irrigation or represent it in a crude way. Here we improve and evaluate the implementation of irrigation in the Community Terrestrial Systems Model (CTSM), the land component of the Community Earth System Model (CESM). In this improvement, we consider three irrigation techniques (flood, sprinkler and drip), which differ in the amount and way of water applied. By combining global maps of the area equipped for irrigation with the distribution of different irrigation techniques, we represent the transient spatial distribution of irrigation techniques. Subsequently, we evaluate the performance of CTSM with the improved irrigation module. Three experiments are conducted: one with irrigation switched off, the second with the original irrigation module and the third with the improved irrigation module implemented. All three outputs are evaluated against observed or remotely sensed land surface energy fluxes and near-surface climate datasets. We anticipate that the results will reveal how our new irrigation schemes improve or reduce the performance of the land surface model.

How to cite: Yao, Y., Swenson, S., Lawrence, D., Lombardozzi, D., Vanderkelen, I., and Thiery, W.: Evaluation of the new irrigation implementation in CTSM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4253, https://doi.org/10.5194/egusphere-egu21-4253, 2021.

Floodplains are flat regions close to rivers which are temporarily or permanently flooded. When they are next to large streamflow, their flooding is mainly related to the river overflow and, thus, to the precipitation occurring in the upstream regions. Large floodplains are important for the regional water cycle, the hydrological resources, the ecological services they provide and, when they are located in tropical regions, for their interaction with the atmosphere. Large tropical floodplains exist in the Amazon, the Mississippi, the Congo, the Paraguay and the Nile basins. 

On the one hand, floodplains are regions with scarce ground observations which lead to difficulties to assess the accuracy of the satellite products that limits their calibration. One the other hand, the dynamic of the floodplains is usually not integrated in Land Surface Models and even less in Earth System Models although they may be important for land-atmosphere interactions. There is a need to develop numerical schemes in order to be able to represent the impact of the floodplains on the water cycle. These schemes will also allow us to better understand the hydrological dynamics in these regions. 

The Land Surface component of the IPSL Earth System Model, ORCHIDEE (CMIP6 version) includes a river routing scheme with a floodplains scheme at a resolution of 0.5°. This scheme allows the water from the precipitation over the upstream region to flood and evaporate over the floodplains. Recent developments in ORCHIDEE driven by the need for a higher resolution routing scheme, based on sub-grid hydrological units, allowed us to implement a floodplain scheme which improves the representation of the overbank flow and the spatial distribution of ponded water with respect to the CMIP6 version of ORCHIDEE. 

This study focuses on the Pantanal region which is the world’s largest tropical floodplains and is located in the La Plata Basin, in the Upper Paraguay River (South America). ORCHIDEE’s sensitivity to the activation of floodplain schemes has been assessed through simulations performed at various resolutions. These simulations have shown the importance of representing floodplains to simulate the water cycle in the area. Combining these simulations and observations, we estimated the evapotranspiration loss by models when the floodplains scheme is deactivated to 90 mm/year over the Pantanal. The higher resolution scheme shows realistic simulations of the river discharge over the floodplains and is expected to improve the spatial distribution of the flooded area and, thus, the representation of evapotranspiration.

How to cite: Schrapffer, A., Polcher, J., Sörensson, A., and Fita, L.: Floodplains representation in Land Surface Model : toward higher resolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9735, https://doi.org/10.5194/egusphere-egu21-9735, 2021.

By now, humans have constructed more than 45 000 large reservoirs across the globe, increasing the global lake area with 8%. These reservoirs have large impacts on freshwater processes and resources by impounding continental runoff and altering river flows. So far, the impact of reservoirs on the climate remains largely unknown, as they are typically not represented in current Earth System Models (ESMs). This is remarkable, as two-way interactions between reservoirs and climate are likely to alter hydrological extremes and impact future water availability.

Here we present the implementation of the role of reservoirs in the Community Terrestrial Systems Model (CTSM), a land surface model, by accounting for the increase in open water surfaces due to reservoir construction throughout the 20^th century. To this end, we allow lake area to expand in the model, while ensuring that the surface energy and mass balances remain closed. We use reservoir and lake extent  from the state-of-the-art Global Reservoir and Dams (GRanD) and HydroLAKES data sets.

By conducting both land-only and coupled simulations with CTSM and the Community Earth System Model (CESM), we assess the added value of accounting for reservoir expansion in the land surface model performance and investigate their impacts on the mean climate and extremes. Globally, the effect of reservoirs on temperatures and the surface energy balance is small, but  responses can be substantial locally, in particular for grid cells where reservoirs make up a large fraction. Our results show that reservoirs reduce temperature extremes and moderate the seasonal temperature cycle, by up to -1.5 K (for reservoirs covering > 15% of the grid cell).

This study is an important step towards incorporating human water management in ESMs.

How to cite: Vanderkelen, I., van Lipzig, N. P. M., Sacks, W. J., Lawrence, D. M., Clark, M., Mizukami, N., Pokhrel, Y., and Thiery, W.: The impact of global reservoir expansion on the present-day climate , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-723, https://doi.org/10.5194/egusphere-egu21-723, 2021.

Three-dimensional distributions of atmospheric moisture with high spatial and temporal variability arise from the complex interaction of the various branches in the hydrological cycle. By studying abundances of stable water isotopes, one can retrieve fundamental information about the physical processes acting in these branches. Differences in the molecular structure lead to characteristic responses of each water isotope to phase change processes, which reflects on characteristic ratios of water isotopes in different branches of the hydrological cycle.

In this study, we use tropospheric distributions of H₂O and HDO (denoted as δD) to identify dominant processes in the hydrological cycle during the wet phase of the West African Monsoon in boreal summer. Here, large gradients in water vapor, strong convective activity and continental recycling lead to high variability of tropospheric moisture and its isotopic composition. This complexity makes a direct attribution of observed water vapor signals to underlying processes challenging.

To address this challenge, we use remotely sensed {H₂O, δD} – pair data retrieved from spectra of the thermal infrared satellite sensor IASI, which are available daily and globally from October 2014 to June 2019. For an improved understanding of the IASI data, we add high-resolution model data from the regional isotope-enabled model COSMO-iso and generate Lagrangian backward trajectories for the Sahelian troposphere. This provides valuable insights into geometrical and moisture pathways along the history of Sahelian air masses that were observed from IASI. Further, after applying a retrieval simulator on the COSMO-iso data, we can conduct direct satellite-to-model comparisons.
By drawing these datasets together, we document and analyze the characteristic variability of the {H₂O, δD} – pairs for the Sahelian troposphere on interannual, seasonal and convective scales. We identify distinct effects on {H₂O, δD} – pairs of (1) synoptic-scale and boundary air mass mixing, (2) rain condensation during convection and (3) partial evaporation and isotope equilibration of rain drops during convection.

This study reveals the potential of using MUSICA IASI {H₂O, δD} – pair data together with high-resolution modeling for investigating the tropospheric hydrological cycle. This approach is promising for understanding the relative importance of large-scale dynamics against microphysical phase transitions during convection on the tropical moisture distribution.

How to cite: Diekmann, C., Schneider, M., Aemisegger, F., Dahinden, F., Ertl, B., Hase, F., Khosrawi, F., Pfahl, S., de Vries, A., Wernli, H., Knippertz, P., and Braesicke, P.: Analysis of stable water isotopes in tropospheric moisture during the West African Monsoon, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10121, https://doi.org/10.5194/egusphere-egu21-10121, 2021.

The distribution of tracers in the atmosphere results from the presence and emission of gaseous and particulate matter, as well as their transport, sedimentation and (photo-)chemical transformations. Understanding and quantifying these processes in the atmosphere can be addressed through the use of global-scale or regional-scale chemistry-transport numerical models such as CHIMERE (Mailler et al., 2016).

 While possible in principle, it is impractical to use this model to represent long-range transport of dense plumes of gas and aerosols, resulting for instance from massive emissions by volcanic eruptions, forest fires and desertic aerosol tempests. Indeed such studies requiring both large domains and high resolution have a prohibitive numerical cost due to the formulation of CHIMERE on a regular Cartesian mesh. This limitation is shared by all currently operational chemistry-transport models. Additionally, traditional Cartesian meshes pose a numerical singularity at the poles, where the longitude lines converge.

 These limitations may be lifted by replacing CHIMERE’s Cartesian mesh by a fully unstructured mesh. This would allow modelers to vary resolution in space, and hence to focus computational resources in key regions with sharp variations (e.g. volcanic eruptions) where high spatial and temporal resolution is required.

 As a first step in this direction, we compare the numerical performance of transport schemes formulated on Cartesian meshes and schemes formulated on unstructured meshes (Dubey et al., 2015). To focus on differences due to numerics, the unstructured mesh is a quasi-uniform icosahedral mesh such as the one used by global dynamical core DYNAMICO (Dubos et al., 2015). Spatial and temporal coupled and de-coupled schemes of various order are implemented in each mesh framework. A suite of test cases is used to evaluate different properties of the mesh-scheme pairings.To avoid the Cartesian pole singularity, the Cartesian mesh covers a limited domain excluding the poles. Analytical wind fields adapted to this limited domain are used. Metrics are evaluated using the quantities obtained in the simulations, such as convergence using root mean square errors, shape preservation using non-linear tracer relations, and diffusion using total entropy. The stability and monotonicity of the used schemes are also numerically validated.

 We find that a scheme of the Van Leer family on the unstructured mesh has a performance slightly inferior to a similar scheme on a Cartesian mesh. However, since this loss in quality remains moderate, it should be possible to more than compensate for it with a variable resolution. We are currently investigating this question and will present variable-resolution results if this ongoing work is timely completed. If successful, fully unstructured meshes would be a significant step forward in the modeling of scale interactions in atmospheric chemistry, and would potentially allow breakthrough for the understanding of such interactions.

How to cite: Takache, S., Dubos, T., and Mailler, S.: 2D tracer transport on Cartesian and icosahedral grids: scheme comparisons for idealized test cases, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15082, https://doi.org/10.5194/egusphere-egu21-15082, 2021.