Non-hydrostatic Model Research Articles

Deep Convection as Inferred From the C2OMODO Concept of a Tandem of Microwave Radiometers

Probing the atmosphere from space using radiometers is a challenging way to improve our knowledge of deep convection. Exploration of water absorption bands in the 183, 325, and 448 GHz range is promising because of the radiation scattered by icy hydrometeors produced by deep convection. We investigate what properties of deep convection could be inferred from the Convective Core Observations through MicrOwave Derivatives in the trOpics (C2OMODO) concept of a tandem of microwave radiometers separated by several tens of seconds. Two tropical deep convective events (Hector the Convector and a radiative-convective equilibrium case) are simulated with the Meso-NH non-hydrostatic numerical model, the outputs of which are used to compute brightness temperatures (Tbs) using the Radiative Transfer for the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (RTTOV) code. We find different relationships between the ice water path, vertical ice momentum, vertical ice velocity, and the time derivative dTb/dt. They depend on where they are probed after separating the growing convective cores from their surrounding environment with a radiometric point of view. Tb and dTb/dt are highly dependent on the ice water path that depends on horizontal and vertical ice advection and microphysical processes. Looking at deep convection in general, we find that the ice water path increases linearly with decreasing dTb/dt. In the specific case of the core of growing convective cells, the vertical ice momentum and the vertical ice velocity are related to dTb/dt. However, such a relationship breaks down in the anvil because horizontal ice advection can dominate microphysical processes. These results are robust to horizontal resolution and time delay.

Mesoscale Convective Systems Simulated by a High‐Resolution Global Nonhydrostatic Model Over the United States and China

AbstractMesoscale convective systems (MCSs) contribute a large fraction of warm‐season precipitation and generate hazardous weather with substantial socioeconomic impacts. Uncertainties in convection parameterizations in climate models limit our understanding of MCS characteristics and reliability of future projection. We examine MCSs simulated by the global 14 km Nonhydrostatic ICosahedral Atmospheric Model (NICAM) without cumulus parameterization against satellite observation from Global Precipitation Measurement during 2001–2008. We focus on MCSs over the central United States and eastern China where MCSs are prevalent from March to August. A process‐oriented tracking method incorporating both cloud and precipitation criteria is used to identify and track MCSs. About 140/100 MCSs initiate in the central United States/eastern China per warm season and most of them initiate in the east of high mountains and in coastal regions. The frequency distribution of MCS lifetime is well captured by NICAM. However, the simulated MCSs have stronger precipitation, smaller precipitation area, and larger cold cloud system than that observed in both regions, which may be caused by weak entrainment as it is not well resolved at 14 km resolution. The simulated MCS number is also underestimated in summer. By examining the climatological and MCS large‐scale environments, the significant underestimation of MCS number in summer over the central United States may be attributed to the large climatological dry bias in the atmosphere. For China, mean moisture in summer is well simulated but deficiency in capturing the dynamic condition related to the coastal topography for triggering convection may have contributed to underestimation of MCS even in a sufficiently moist environment.

Effects of Different Slope Limiters on Stratified Shear Flow Simulation in a Non-hydrostatic Model

To simulate the dynamical structures of stratified shear flows, the high-resolution Total Variation Diminishing (TVD) method is necessary and widely-used due to its high-order spatial accuracy, oscillation control, and ability to capture the well-defined structures of vortices. Lack of understanding the TVD slope limiters usually results in inaccurate numerical simulation on stratified shear flows in terms of shear instability and spatiotemporal variations of mixing. In this study, the performances of four typical TVD slope limiters, namely the minmod, van Leer, Monotonized Central (MC), and superbee limiters, were investigated on modelling stratified shear flows based on the open-source non-hydrostatic model, NHWAVE. The four slope limiters are all commonly-used and have the typical numerical characteristics. All the limiters were respectively applied in two classical test cases, namely, shear instability and lock-exchange problem. The simulation results showed that the effects of slope limiters were correlated with their characteristics of numerical dissipation (or anti-dissipation), which can influence notably the model predictions of the generation of shear instability, the development of interfacial structures, and the mixing process. In the test cases, MC limiter’s performance was the best, because it could simulate the well-defined structures of instability while not introducing noticeable error. Minmod has an excessively large dissipation, which introduced noticeable numerical errors that can influence the model accuracy and can even suppress or omit the generation of interfacial vortices. Superbee limiter, the most anti-dissipative one, usually over-predicted the instability and mixing effects in time and space domain, and was likely to cause computational instability in some cases. The performances of van Leer and MC were similar, but their predictions of the evolutions of interfacial structures and mixing could be significantly different. Besides, the co-effects of grid resolution and slope limiters were also investigated; it was found that the refinement of grids may not help to reproduce a higher-quality result with a specific slope limiter.

Observed and Modeled Mountain Waves from the Surface to the Mesosphere near the Drake Passage

Abstract Four state-of-the-science numerical weather prediction (NWP) models were used to perform mountain wave (MW)-resolving hindcasts over the Drake Passage of a 10-day period in 2010 with numerous observed MW cases. The Integrated Forecast System (IFS) and the Icosahedral Nonhydrostatic (ICON) model were run at Δx ≈ 9 and 13 km globally. The Weather Research and Forecasting (WRF) Model and the Met Office Unified Model (UM) were both configured with a Δx = 3-km regional domain. All domains had tops near 1 Pa (z ≈ 80 km). These deep domains allowed quantitative validation against Atmospheric Infrared Sounder (AIRS) observations, accounting for observation time, viewing geometry, and radiative transfer. All models reproduced observed middle-atmosphere MWs with remarkable skill. Increased horizontal resolution improved validations. Still, all models underrepresented observed MW amplitudes, even after accounting for model effective resolution and instrument noise, suggesting even at Δx ≈ 3-km resolution, small-scale MWs are underresolved and/or overdiffused. MW drag parameterizations are still necessary in NWP models at current operational resolutions of Δx ≈ 10 km. Upper GW sponge layers in the operationally configured models significantly, artificially reduced MW amplitudes in the upper stratosphere and mesosphere. In the IFS, parameterized GW drags partly compensated this deficiency, but still, total drags were ≈6 times smaller than that resolved at Δx ≈ 3 km. Meridionally propagating MWs significantly enhance zonal drag over the Drake Passage. Interestingly, drag associated with meridional fluxes of zonal momentum (i.e., ) were important; not accounting for these terms results in a drag in the wrong direction at and below the polar night jet. Significance Statement This study had three purposes: to quantitatively evaluate how well four state-of-the-science weather models could reproduce observed mountain waves (MWs) in the middle atmosphere, to compare the simulated MWs within the models, and to quantitatively evaluate two MW parameterizations in a widely used climate model. These models reproduced observed MWs with remarkable skill. Still, MW parameterizations are necessary in current Δx ≈ 10-km resolution global weather models. Even Δx ≈ 3-km resolution does not appear to be high enough to represent all momentum-fluxing MW scales. Meridionally propagating MWs can significantly influence zonal winds over the Drake Passage. Parameterizations that handle horizontal propagation may need to consider horizontal fluxes of horizontal momentum in order to get the direction of their forcing correct.

Numerical Simulation of Water Tank Circulation and Water Age Under Different Working Conditions

AbstractWith increasing real estate and apartment vacancy rates, the utilization rate of the water in water tanks is decreasing; this leads to an increase in the water age and deteriorates the water quality in water tanks. The general hydrostatic numerical model cannot simulate the flow field and water age of a water tank correctly because the horizontal scale of the water tank is equivalent to the vertical scale, and the vertical hydrostatic approximation assumption is no longer applicable. In this paper, the water age module is added to a three‐dimensional numerical model based on estuarine and coastal oceans, and a nonhydrostatic numerical model is developed to simulate and analyze the three‐dimensional flow field and the temporal and spatial variations in the water age of the water tank under different working conditions. The total water consumption and the effective volume of the water tank are the main factors affecting the water age, followed by the water level change range. The water age can be reduced to a lesser extent by setting the position of the inlet and outlet at a diagonal angle and adopting a larger inflow velocity. The distributions of the circulation and water age are mainly determined by the inlet in the influent process. There is no obvious dead water zone in the tank due to the strong disturbance, advection and diffusion in the secondary water supply process. The water age distribution in the water tank is mainly affected by advection, while diffusion plays a secondary role.

Assimilation of crowd‐sourced surface observations over Germany in a regional weather prediction system

AbstractNear‐surface temperature and humidity observations over Germany, coming on the one hand from the citizen weather station's network Netatmo and on the other hand from synoptic weather stations, were assimilated into the limited are mode of the Icosahedral Nonhydrostatic Model with 2‐km resolution (ICON‐D2). For that we use the Kilometre‐Scale Ensemble Data Assimilation (KENDA) system and a bias‐correction approach that improves the assimilation of the observations by taking into account the diurnal cycle of temperature and humidity variables. Our results show that the assimilation of bias‐corrected observations from Netatmo stations reduces the forecast error considerably; meanwhile, the assimilation of Netatmo observations without bias correction leads to a strong warm bias with a negative impact on forecast performance. In contrast, for the assimilation of synoptic observations the usage of our bias‐correction approach does not lead to any significant decrease in the forecast error, yet reduces the bias for the diurnal cycle of synoptic stations. Overall, it can be concluded that the forecast quality can gain from assimilating Netatmo data, provided an effective bias‐correction approach is applied.

Corrigendum to “Non-hydrostatic modeling of drag, inertia and porous effects in wave propagation over dense vegetation fields” [Coast. Eng. 149 (July 2019) 49–64

Corrigendum to “Non-hydrostatic modeling of drag, inertia and porous effects in wave propagation over dense vegetation fields” [Coast. Eng. 149 (July 2019) 49–64

Numerical study of coastal wave profiles at the sandy beaches of Nowshahr (Southern Caspian Sea)

This study aimed to investigate the capability of the one-dimensional (1D) mode of the Simulating WAves till SHore (SWASH), as a non-hydrostatic wave-flow model with six vertical layers, to reproduce the cross-shore wave evolution. For this purpose, the given model was initially calibrated for wave energy and the outputs were then verified with the field data measured at the Southern Caspian Sea. The calibration coefficients obtained for wave breaking are significantly less than the ones which have been mostly reported in previous studies for the two-dimensional (2D) mode of the SWASH. Although the reproduced wave height parameters are generally in good accordance with the field observations, the period parameters and the number of waves are overestimated and underestimated by the model, respectively. Moreover, the inaccuracies at the shallow stations are worse than at the transitional depths. The overestimation in both the reproduced energy of infragravity waves (IG) and their wavelength along with the underestimation in the wind-wave energy content are also among the factors responsible for the model deficiencies. The findings have revealed that the overestimation of the reproduced IG waves is the main reason for the underestimation of the breaking dissipation rate for irregular wave trains in the 1D mode. Therefore, more intensive breaking dissipation via selecting lower coefficient values is necessary to exhaust a certain energy content from longer waves in the 1D mode. This approach ultimately induces an over-dissipation of short wind-waves.

Impacts of Topographic Complexity on Modeling Moisture Transport and Precipitation over the Tibetan Plateau in Summer

The non-hydrostatic global variable resolution model (MPAS-atmosphere) is used to conduct the simulations for the South Asian Summer monsoon season (June, July, and August) in 2015 with a refinement over the Tibetan Plateau (TP) at the convection-permitting scale (4 km). Two experiments with different topographical datasets, complex (4-km) and smooth (60-km) topography, are designed to investigate the impacts of topographical complexity on moisture transport and precipitation. Compared with the observations and reanalysis data, the simulation can successfully capture the general features of key meteorological fields over the TP despite slightly underestimating the inflow through the southern TP. The results indicate that the complex topography can decrease the inward and outward moisture transport, ultimately increasing the total net moisture transport into the TP by ∼11%. The impacts of complex topography on precipitation are negligible over the TP, but the spatial distributions of precipitation over the Himalayas are significantly modulated. With the inclusion of complex topography, the sharper southern slopes of the Himalayas shift the lifted airflow and hence precipitation northward compared to the smooth topography. In addition, more small-scale valleys are resolved by the inclusion of complex topography, which serve as channels for moisture transport across the Himalayas, further favoring a northward shift of precipitation. Overall, the difference between the two experiments with different topography datasets is mainly attributed to their differing representation of the degree of the southern slopes of the Himalayas and the extent to which the valleys are resolved.

Establishing a non-hydrostatic global atmospheric modeling system at 3-km horizontal resolution with aerosol feedbacks on the Sunway supercomputer of China

During the era of global warming and highly urbanized development, extreme and high impact weather as well as air pollution incidents influence everyday life and might even cause the incalculable loss of life and property. Despite the vast development of atmospheric models, there still exist substantial numerical forecast biases objectively. To accurately predict extreme weather, severe air pollution, and abrupt climate change, numerical atmospheric model requires not only to simulate meteorology and atmospheric compositions simultaneously involving many sophisticated physical and chemical processes but also at high spatiotemporal resolution. Global integrated atmospheric simulation at spatial resolutions of a few kilometers remains challenging due to its intensive computational and input/output (I/O) requirement. Through multi-dimension-parallelism structuring, aggressive and finer-grained optimizing, manual vectorizing, and parallelized I/O fragmenting, an integrated Atmospheric Model Across Scales (iAMAS) was established on the new Sunway supercomputer platform to significantly increase the computational efficiency and reduce the I/O cost. The global 3-km atmospheric simulation for meteorology with online integrated aerosol feedbacks with iAMAS was scaled to 39,000,000 processor cores and achieved the speed of 0.82 simulation day per hour (SDPH) with routine I/O, which enabled us to perform 5-day global weather forecast at 3-km horizontal resolution with online natural aerosol impacts. The results demonstrate the promising future that the increasing of spatial resolution to a few kilometers with online integrated aerosol feedbacks may significantly improve the global weather forecast.

Infragravity wave amplification by isolated topography: Field observations and semi-analytical modeling

Infragravity waves (IGWs) play a crucial role in nearshore hydrodynamics and are the primary drivers of coastal hazards such as harbor resonance and overtopping over seawalls. Field observations show that during five storm events in Fall 2013, the IGW heights at the landward side were significantly larger than those at the seaward side of an isolated shoal near Cape Canaveral, Florida, USA. The concurrent offshore incident waves were energetic with directions nearly perpendicular to the shoal. The maximum wave height-to-depth ratio was below 0.3, indicating no wave breaking. Here, we propose a semi-analytical model in combination with field observations to investigate the shoal-induced increase in the cross-shoal IGW energy flux. The model was proposed based on the theory of spatial evolution of nonlinear energy transfer from primary waves to IGWs over an idealized trapezoidal topography under nonbreaking conditions, and was calibrated using a numerical dataset of 21 test cases generated by the non-hydrostatic numerical wave model Simulating Waves till SHore (SWASH). The predicted increase in the cross-shoal IGW energy fluxes are in reasonable agreement with field observations, with 78% data variability explained by the model. Also, the IGW energy flux increase calculated from the observed IGW height increase was found to coincide with the observed IGW energy flux increase in the cross-shoal direction. The results strongly indicate that the observed amplification of IGWs near Cape Canaveral in Fall 2013 is largely attributed to the topography-induced nonlinear energy transfers from sea-swells to IGWs.

Vortex shedding and evolution induced by the interactions between a solitary wave and a submerged horizontal plate

This paper presents results of a solitary wave propagating over a submerged horizontal plate based on a non-hydrostatic model. A high-resolution advection scheme is employed based on a second-order flux-limiter method. The model is first validated by comparing numerical results with recently obtained measured data. Then, a series of simulations are carried out to investigate the effect of the plate length on the characteristics of vortex shedding and evolution. With the increase in the length of the plate, more dominant vortical structures induced by the solitary wave with the same wave height can be identified near the lee side of the plate. There are two separate positive vortex shedding from the lower corner of the plate near the lee side, which are not presented in experiments. The secondary positive vortex leads to the water surface deformation for the case that is close to the limit when wave breaking occurs.

A Supercomputer-Based Modeling System for Short-Term Prediction of Urban Surface Air Quality

This paper proposes a mathematical model and an effective supercomputer-based numerical method for short-term prediction of extreme meteorological conditions and atmospheric air quality over limited stretches of land encompassing large population centers. The mathematical model includes a pollutant transport model with a reduced chemical mechanism and a non-hydrostatic mesoscale meteorological model with a modern moisture microphysics parametrization scheme. The numerical method relies on the use of the finite volume method and semi-implicit difference schemes of the second order of approximation, which are solved using the TDMA method with a linear dependence of the number of arithmetic operations on the size of the grid. This property of the numerical method ensures high efficiency when parallelized: not less than 70% when using up to 256 computing cores with a horizontal grid size of 0.5–1.0 km. Development of parallel programs was carried out using the Message Passing Interface parallel programming technology, two-dimensional decomposition of the grid area along horizontal (west to east and south to north) directions, and introduction of additional fictitious grid nodes along the perimeter of the decomposition subdomains.

Importance of aerosols and shape of the cloud droplet size distribution for convective clouds and precipitation

Abstract. The predictability of deep moist convection is subject to large uncertainties resulting from inaccurate initial and boundary data, the incomplete description of physical processes, or microphysical uncertainties. In this study, we investigate the response of convective clouds and precipitation over central Europe to varying cloud condensation nuclei (CCN) concentrations and different shape parameters of the cloud droplet size distribution (CDSD), both of which are not well constrained by observations. We systematically evaluate the relative impact of these uncertainties in realistic convection-resolving simulations for multiple cases with different synoptic controls using the new icosahedral non-hydrostatic ICON model. The results show a large systematic increase in total cloud water content with increasing CCN concentrations and narrower CDSDs, together with a reduction in the total rain water content. This is related to a suppressed warm-rain formation due to a less efficient collision–coalescence process. It is shown that the evaporation at lower levels is responsible for diminishing these impacts on surface precipitation, which lies between +13 % and −16 % compared to a reference run with continental aerosol assumption. In general, the precipitation response was larger for weakly forced cases. We also find that the overall timing of convection is not sensitive to the microphysical uncertainties applied, indicating that different rain intensities are responsible for changing precipitation totals at the ground. Furthermore, weaker rain intensities in the developing phase of convective clouds can allow for a higher convective instability at later times, which can lead to a turning point with larger rain intensities later on. The existence of such a turning point and its location in time can have a major impact on precipitation totals. In general, we find that an increase in the shape parameter can produce almost as large a variation in precipitation as a CCN increase from maritime to polluted conditions. The narrowing of the CDSD not only decreases the absolute values of autoconversion and accretion but also decreases the relative role of the warm-rain formation in general, independent of the prevailing weather regime. We further find that increasing CCN concentrations reduce the effective radius of cloud droplets in a stronger manner than larger shape parameters. The cloud optical depth, however, reveals a similarly large increase with larger shape parameters when changing the aerosol load from maritime to polluted. By the frequency of updrafts as a function of height, we show a negative aerosol effect on updraft strength, leading to an enervation of deep convection. These findings demonstrate that both the CCN assumptions and the CDSD shape parameter are important for quantitative precipitation forecasting and should be carefully chosen if double-moment schemes are used for modeling aerosol–cloud interactions.

An efficient 3D non-hydrostatic model for predicting nonlinear wave interactions with fixed floating structures

This paper presents a three-dimensional (3D) non-hydrostatic model for the prediction of the interaction between nonlinear waves and fixed floating structures. The model solves the incompressible Euler equation by use of a semi-implicit, fractional step algorithm. The water surface elevation is treated as a single-valued function of horizontal position. In order to deal with floating structures, a new numerical algorithm is proposed which combines the immersed boundary method and the global continuity equation in the pressurized region (flow region under the structure). This new algorithm holds the symmetry of the Poisson equation and therefore results in an efficient model. The developed model is validated with the data of two test cases involving 3D nonlinear wave interactions with a floating structure. The model results are compared with experimental data or results of other models. The proposed model exhibits generally good agreement with experimental data and/or other model results, demonstrating its accuracy in resolving 3D nonlinear wave interaction with floating structures.

Three-dimensional non-hydrostatic model for dam-break flows

A three-dimensional (3D) non-hydrostatic model is presented for the simulation of dam-break flows. The model solves the Reynolds-averaged Navier–Stokes equations using the projection method. 3D computational grids are constructed from a two-dimensional horizontal unstructured mesh by adding horizontal layers in the vertical direction. Based on the horizontal unstructured grid system, horizontal advection terms are discretized by a momentum conservative scheme. The proposed model is validated with several physical experiments. The agreement between the model results and experimental data is generally good, which demonstrates the capability of the proposed model to resolve dam-break flows over flat and uneven bottoms with complex geometries. Moreover, the efficiency of the model is evaluated with 3D dam-break flow experiments. Comparisons between the non-hydrostatic model and the corresponding quasi-3D shallow water model are also performed, which confirm the role of non-hydrostatic effects in dam-break flows.

Smoothed Particle Hydrodynamics simulations of reef surf zone processes driven by plunging irregular waves

As waves interact with the slopes of coral reefs and other steep bathymetry profiles, plunging breaking usually occurs where the free surface overturns and violent water motion is triggered. Resolving these surf zone processes pose significant challenges for conventional mesh-based hydrodynamic models, due to the rapidly-deforming nature of the free surface and associated flows. Yet the accurate prediction of these surf zone hydrodynamics is critical for predicting a wide range of nearshore processes driven by wave breaking (e.g., wave dissipation and energy transfers; mean water levels and currents; and wave runup). In this study we assess the ability of the mesh-free, Lagrangian particle-based numerical modelling approach Smoothed Particle Hydrodynamics (SPH) based on DualSPHysics, to simulate the fine-scale hydrodynamic processes driven by irregular wave transformation over a fringing reef profile, by comparing results against detailed experimental observations from a physical modelling study. To greatly improve the computational efficiency, the SPH model was coupled to the mesh-based multi-layer nonhydrostatic wave-flow model SWASH. With this coupled approach, SWASH was used to efficiently simulate the evolution of non-breaking waves from the wavemaker up to the fore reef slope, with the SPH model then used to simulate the detailed hydrodynamic processes over the reef from just offshore of the breakpoint to the shoreline. The SPH model was able to accurately reproduce the complex free surface deformations during plunging breaking, the spectral evolution of waves across the reef flat (including nonlinear wave shape), the mean water levels and currents, and wave runup at the shoreline. Using the long duration simulations (>400 wave periods), the model was able to reproduce the full range of wave motions over the reef (from sea-swell to infragravity frequencies), including the increasing dominance of low frequency waves towards the shoreline and the large cross-reef standing wave motions excited by the reef geometry.

Validation and inter-comparison of models for landslide tsunami generation

The Mapping and Modeling Subcommittee of the US National Tsunami Hazard Mitigation Program convened a workshop in January 2017 to evaluate the present state of numerical models for the simulation of tsunamis generated by submarine or subaerial landslides. A range of benchmark tests were provided to participants, with three tests emphasized: (i) a laboratory submarine solid slide in a 2D horizontal tank, (ii) a laboratory submarine granular slide in a 1D flume, and (iii) a field case based on submarine slides which occurred in Port Valdez, AK during the 1964 Alaska earthquake. Nine landslide tsunami models configured with 21 different combinations of physical options were benchmarked, including: (1) hydrostatic models with no frequency dispersion, which include the nonlinear shallow equation models traditionally used for modeling coseismic tsunamis; (2) Boussinesq or one-layer weakly dispersive models; (3) Multi-layer or non-hydrostatic (i.e., dispersive) models; and (4) Full Navier–Stokes models. Model/data comparison indicates that the inclusion of frequency dispersion in model formulations is critical to obtaining physically reasonable results for the test cases considered. Because the importance of dispersive effects is unknown a priori for any given simulated event, the central recommendation from this work is that a model with at minimum a leading-order representation of frequency dispersion effects be used whenever possible for landslide tsunami simulations.

Case study of a moisture intrusion over the Arctic with the ICOsahedral Non-hydrostatic (ICON) model: resolution dependence of its representation

Abstract. The Arctic is warming faster than the global average and any other region of a similar size. One important factor in this is the poleward atmospheric transport of heat and moisture, which contributes directly to the surface and air warming. In this case study, the atmospheric circulation and spatio-temporal structure of a moisture intrusion event is assessed, which occurred from 5 to 7 June 2017 over the Nordic seas during an intensive measurement campaign over Svalbard. This analysis focuses on high-spatial-resolution simulations with the ICON (ICOsahedral Non-hydrostatic) model which is put in context with coarser-resolution runs as well the ERA5 reanalysis. A variety of observations including passive microwave satellite measurements is used for evaluation. The global operational ICON forecasts from the Deutscher Wetterdienst (DWD) at 13 km horizontal resolution are used to drive high-resolution Limited-Area Mode (LAM) ICON simulations over the Arctic with 6 and 3 km horizontal resolutions. The results show the skilful capacity of the ICON-LAM model to represent the observed spatio-temporal structure of the selected moisture intrusion event and its signature in the temperature, humidity and wind profiles, and surface radiation. In several aspects, the high-resolution simulations offer a higher accuracy than the global simulations and the ERA5 reanalysis when evaluated against observations. One feature where the high-resolution simulations demonstrated an advanced skill is the representation of the changing vertical structure of specific humidity and wind associated with the moisture intrusion passing Ny-Ålesund (western Svalbard); the humidity increase at 1–2 km height topped by a dry layer and the development of a low-level wind jet are best represented by the 3 km simulation. The study also demonstrates that such moisture intrusions can have a strong impact on the radiative and turbulent heat fluxes at the surface. A drastic decrease in downward shortwave radiation by ca. 500 W m−2 as well as an increase in downward longwave radiation by ca. 100 W m−2 within 3 h have been determined. These results highlight the importance of both moisture and clouds associated with this event for the surface energy budget.

Nonhydrostatic Numerical Modeling of Fixed and Mobile Barred Beaches: Limitations of Depth-Averaged Wave Resolving Models around Sandbars

Along sandy coastlines, submerged, shore-parallel sandbars play an essential role in shoreline morphology by dissipating wave energy through depth-induced wave breaking. While wave breaking and sediment transport around sandbars are complex three-dimensional (3D) processes, shoreline morphology is typically simulated with depth-averaged models that feature lower computational demand than do 3D models. In this context, this study examines the implications of depth-averaging the flow field and approximating the breaking process in nonhydrostatic models (e.g., XBeach nonhydrostatic) for the hydro- and morphodynamic processes around sandbars. The implications are drawn based on reproducing large-scale experiments of a barred beach profile using the single-layer (XBNH) and the reduced two-layer (XBNH+) modes of XBeach. While hydrodynamic processes were predicted with high accuracy on the sandbar’s seaward side, wave heights were overpredicted on the bar’s landward side. The overestimation was due to the simplified reproduction of the complex breaking process near the sandbar’s peak, particularly in terms of the generated turbulence in the water column. Moreover, the velocity profile with a strong undertow could only be represented in a simplified way even using the two-layer mode XBNH+, thus resulting in inaccurate predictions of sediment loads around the sandbar. A parametric study is performed, and it revealed which model parameters control the simulation of the wave-breaking process. Thus, wave height predictions could be improved by tuning the energy-dissipation parameters. However, flow velocities and morphodynamic predictions could not be improved accordingly. Thus, this study identifies possible hydrodynamic model improvements, such as incorporating a roller dissipation model. Moreover, it improves understanding of key drivers and processes that should be included in nonhydrostatic depth-averaged models to simulate morphological changes around sandbars more efficiently.