Subgrid Scheme Research Articles

The surface downward longwave radiation of the CMIP6 HighResMIP in East Asia offline corrected by the three-dimensional sub-grid terrain longwave radiative effect scheme

Abstract Terrain significantly regulates surface downward longwave radiation (SDLR). The CMIP6 HighResMIP models without the three-dimensional sub-grid terrain longwave radiative effects (3DSTLRE) produce large SDLR biases over complex terrains. This study applies the 3DSTLRE scheme to correct the SDLR simulated by these models in East Asia and assesses the correction’s effectiveness. Results indicate that the CMIP6 HighResMIP models without the 3DSTLRE clearly underestimate the SDLR over the rugged terrains and the underestimation increases with the sub-grid terrain complexity. The offline correction of 3DSTLRE can evidently improve the SDLR simulations in different seasons, and the improvements increase with the sub-grid terrain complexity. The most significant improvements are observed over the Himalayas, the Tibetan Plateau, the Tianshan Mountains, and the Hengduan Mountains. The relative root mean square error of SDLR simulations over the areas with the most complex sub-grid terrains can be decreased by ∼40% due to the offline correction of 3DSTLRE. Considering the 3DSTLRE may be an efficient way to improve the simulations of the SDLR and surface energy balance over the regions with complex sub-grid terrains.

Impacts of Topography‐Based Subgrid Scheme and Downscaling of Atmospheric Forcing on Modeling Land Surface Processes in the Conterminous US

AbstractThe effects of small‐scale topography‐induced land surface heterogeneity are not well represented in current Earth System Models (ESMs). In this study, a new topography‐based subgrid structure referred to as topographic units (TGU) designed to better capture subgrid topographic effects, and methods to downscale atmospheric forcing to the land TGUs have been implemented in the Energy Exascale Earth System Model (E3SM) Land Model (ELM). Effects of the subgrid scheme and downscaling methods on ELM simulated land surface processes are evaluated over the conterminous United States (CONUS). For this purpose, ELM simulations are performed using two configurations without (NoD ELM) and with (D ELM) downscaling, both using TGUs derived for the 0.5‐degree grids and the same land surface parameters. Simulations using the two ELM configurations are compared over the CONUS domain, regional levels, and at observational sites (e.g., SNOTEL). The CONUS‐level results suggest that D ELM simulates more snowfall and snow water equivalent (SWE), higher runoff, and less ET during spring and summer. Regional‐level results suggest more pronounced impacts of downscaling over regions dominated by higher elevation TGUs and regions with maximum precipitation occurring during cool seasons. Results at the SNOTEL sites suggest that D ELM has superior capability of reproducing the observed SWE at 83% of the sites, with more pronounced performance over topographically heterogeneous TGUs with their maximum precipitation occurring during cool seasons. The results highlight the importance of improving representation of small‐scale surface heterogeneity in ESMs and motivate future research to understand their effects on land‐atmosphere interactions, streamflow, and water resources management over mountainous regions.

A High-Precision Sub-Grid Parameterization Scheme for Clear-Sky Direct Solar Radiation in Complex Terrain—Part II: Considering Atmospheric Transparency Differences in Sub-Grid; Pre-Research for Application

Existing sub-grid parameterization schemes for clear-sky direct solar radiation (SPS-CSDSR) assume that the sub-grid cells have the same atmospheric transparency. This study shows that in undulating terrain, significant errors can occur when the sub-grid is in turbid weather or partly above the cloud top. A correction factor was proposed. It can effectively eliminate errors under a cloudless sky and can reduce some errors when part of the sub-grid is above the cloud or fog top. For atmospheric models with high horizontal resolution, example test verification shows that the cast shadowless coverage method can lead to large errors. It should no longer be used based on current computing power. These improvements and the high-precision fast terrain occlusion algorithm in Part I will allow SPS-CSDSR to achieve unprecedented high accuracy. Based on the proposed daily interpolation method, the high-precision SPS-CSDSR is also feasible for regional climate simulation. The analysis pointed out that the sub-grid terrain radiative effect (STRE) is distributed over inclined surfaces with larger areas and at different heights. Existing methods of coupling STRE on one flat surface have certain physical drawbacks. This paper suggests introducing parameterization of STRE at different altitudes and improving the coupling of land–air.

A High-Precision Sub-Grid Parameterization Scheme for Clear-Sky Direct Solar Radiation in Complex Terrain—Part I: A High-Precision Fast Terrain Occlusion Algorithm

In atmospheric modeling, sub-grid parameterization is an important method for studying the topographic effects of solar radiation using high-resolution Digital Elevation Model (DEM) data. For reducing the amount of computation, some approximate methods that can lead to errors are used in existing sub-grid parameterization schemes for clear-sky direct solar radiation (SPS-CSDSR). The lack of a high-precision fast terrain occlusion algorithm (HPFTOA) remains one of the biggest constraints in this field. This study proposed an HPFTOA. It mainly uses two kinds of acceleration algorithms. One method is to use a dynamic, lossless, and fast occlusion search radius. Another way is to use the rectangular grid for calculations within the accuracy of DEM data to avoid coordinate projection conversions. The test results indicate that the HPFTOA can carry out large-scale computation based on DEM data with a resolution of 90 m. Because it rarely uses approximation algorithms and considers the curvature of the Earth, SPS-CSDSR can achieve unprecedented precision. The HPFTOA can also be used in the fields of mountain solar energy assessment, remote sensing, and telemetry, including terrain-obscuring the probe. As computer performance improves and algorithms and execution code are optimized, the application prospects will be very broad.

Estimating infrasonic noise levels using a high resolution weather model

The presence of turbulence and other wind-induced pressure fluctuations are considered a nuisance in infrasound monitoring. For this reason, wind noise filters are typically in place for suppression. In this study, we establish a relation between microbarometer observations and in situ turbulence measurements at the Cabauw Atmospheric Research Site in The Netherlands. Using this relation, we forecast turbulent pressure levels using forecasted levels of turbulence kinetic energy (TKE) that are computed as part of the high-resolution (2.5 × 2.5 km grid scale) HARMONIE weather model. The estimates are compared with pressure noise levels as a function of frequency. This approach has two foreseen applications: (1) the forecasted turbulence fields could possibly help in site selection for infrasound arrays and (2) microbarometer observations could possibly be of use in the further refining of sub-grid scale turbulence schemes in weather models.

Effects of subgrid‐scale turbulence parametrization on the representation of clear‐air turbulence using kilometre‐ to hectometre‐scale numerical simulations

A turbulence event arising in a jet exit region above the Belgium–Luxembourg area, determined from airliner in‐situ measurements, is reproduced using the meteorological models Application de la Recherche à l'Opérationnel à Méso‐Echelle (AROME) and Meso‐NH at horizontal resolutions of 1.3 km and 260 m. The behaviour of the subgrid turbulence scheme at 1.3 km and its sensitivity to various parameters are analysed, with results being evaluated using measurements. An increase of the vertical resolution around the tropopause levels with m is shown to greatly enhance the turbulence representation. The use of a non‐local formulation of the mixing length in the current parametrization at 1.3 km allows reproduction of a turbulence signal in agreement with the observations. On the contrary, the use of a fully three‐dimensional formulation has no impact on the simulation at this resolution (1.3 km). Using the 260 m runs, this turbulence event is linked to hydrodynamical wind shear instabilities characterized by horizontal wavelength of 4.5 km, sub‐resolved at the operational resolution. At these small grid‐size scales, turbulence evolution and equation budgets reflect an equilibrium between dynamical production and turbulence dissipation, and highlight the importance of horizontal gradients. Subgrid turbulence intensities are assessed to be underestimated by the current parametrization at 1.3 km when compared with this high‐resolution reference simulation. Finally, different tests on the turbulence parametrization illustrate a transfer between resolved and subgrid kinetic energy in the model. This transfer stresses the importance of a trade‐off between mixing intensity and the representation of wind at resolved scales for the upper troposphere.

Binary neutron star merger simulations with neutrino transport and turbulent viscosity: impact of different schemes and grid resolution

ABSTRACT We present a systematic numerical relativity study of the impact of different physics input and grid resolution in binary neutron star mergers. We compare simulations employing a neutrino leakage scheme, leakage plus M0 scheme, the M1 transport scheme, and pure hydrodynamics. Additionally, we examine the effect of a sub-grid scheme for turbulent viscosity. We find that the overall dynamics and thermodynamics of the remnant core are robust, implying that the maximum remnant density could be inferred from gravitational wave observations. Black hole collapse instead depends significantly on viscosity and grid resolution. Differently from recent work, we identify possible signatures of neutrino effects in the gravitational waves only at the highest resolutions considered; new high-resolution simulations will be thus required to build accurate gravitational wave templates to observe these effects. Different neutrino transport schemes impact significantly mass, geometry, and composition of the remnant’s disc and ejecta; M1 simulations show systematically larger proton fractions, reaching maximum values larger than 0.4. r-process nucleosynthesis yields reflect the different ejecta compositions; they are in agreement and reproduce residual solar abundances only if M0 or M1 neutrino transport schemes are adopted. We compute kilonova light curves using spherically-symmetric radiation-hydrodynamics evolutions up to 15 d post-merger, finding that they are mostly sensitive to the ejecta mass and electron fraction; accounting for multiple ejecta components appears necessary for reliable light curve predictions. We conclude that advanced neutrino schemes and resolutions higher than current standards are essential for robust long-term evolutions and detailed astrophysical predictions.

Calibration of cloud and aerosol related parameters for solar irradiance forecasts in WRF-solar

Model parameters are one of the sources of uncertainties in numerical weather prediction. Recently, the Weather Research and Forecasting model with Solar extensions (WRF-Solar) has been upgraded by enhancing the treatment of sub-grid scale cloud and aerosols with augmentations of a sub-grid scale cloud scheme (CLD3) and an upgraded aerosol-aware Thompson-Eidhammer scheme (TE14). However, the value of model parameters associated with these parameterizations are assigned based on limited measurements or theoretical calculations. Calibrating the sensitive parameters has the potential to improve solar irradiance predictions. In this work, we adopted a multi-objective surrogate-based optimization (SBO) framework to calibrate nine parameters used in CLD3 and TE14 that lead to the largest sensitivity in simulated irradiance. The normalized mean-absolute-error (NMAE) of global horizontal irradiance (GHI) and direct normal irradiance (DNI) are minimized by calibrating WRF-Solar over two regions including the Southern Great Plains (SGP) and Central California. We selected two cloudy cases, one over less-polluted SGP and another over Central California with high aerosol loading associated with wildfire events. The results show that generalized linear model (GLM)-based surrogate models approximate physical models well, particularly when the third order and three-way interaction terms are considered. The SBO framework efficiently searches the parameter space for optimal solutions with less computational costs than directly calibrating the physical model. We first calibrate CLD3 parameters over the less-polluted SGP region. Optimized CLD3 parameters alone result in NMAE reduction by 14% for the site-mean and up to 33% for individual cases over the SGP region. With further calibration of TE14 parameters over the Central California during active fire periods, the optimized parameters lead to over 20% reductions of NMAE.

Sensitivity of solar irradiance to model parameters in cloud and aerosol treatments of WRF-solar

We investigate the parametric sensitivity of solar irradiance to a set of parameters within the sub-grid cloud scheme (CLD3) and an upgraded aerosol-aware Thompson-Eidhammer scheme (TE14) in an upcoming enhanced version of the Weather Research and Forecasting-Solar model. We conduct ensemble simulations over the Southern Great Plains and Hanford, California, focusing on the parametric sensitivity under cloudy conditions with different aerosol loading. We adopt the Quasi-Monte Carlo sampling approach to explore the high-dimensional parameter space and apply the generalized linear model to quantify the relative contribution of individual parameters to the total variance. We find that CLD3 parameters related to entrainment and the cloud condensation threshold contribute to most of the variance in the ensemble simulations under less-polluted conditions. Larger entrainment and a higher condensation threshold produce more solar irradiance via decreasing cloud fraction and cloud optical depth. As the sensitivities of cloud fraction and cloud optical properties to perturbed parameters vary with cloud cover, the relative contributions of parameters to irradiance variance also change with cloud cover. As a contrast, under the heavily-polluted wildfire conditions, the model sensitivities to the parameters within the enhanced TE14 scheme become pronounced, especially those related to water-friendly aerosol emission rate and the modal radius of black carbon. The variations of those aerosol parameters cause significant changes in simulated irradiance, especially over the clear sky, suggesting the importance of accurate representation of emission source for aerosols and their precursors.

Modeling three-dimensional underwater acoustic propagation over multi-layered fluid seabeds using the equivalent source method.

This paper develops an efficient three-dimensional (3D) underwater acoustic propagation model with multi-layered fluid seabeds based on the equivalent source method (ESM). It solves the Helmholtz equation exactly by a superposition of fields generated by equivalent sources. A linear system coupling ESM equations is derived by imposing boundary conditions and solved iteratively using the generalized minimum residual method. Unlike a direct ESM solver, matrix-vector products in each iteration are evaluated by a pre-corrected fast Fourier transformation (PFFT), significantly reducing the numerical cost and enabling efficient solution of 3D large-scale propagation. Moreover, sound speed profiles can be taken into account by dividing the water column into sub-layers, each of which requires an individual PFFT procedure using an FFT subgrid scheme. Simulations of propagation over a Gaussian canyon validate the PFFT-accelerated ESM (PFFT-ESM). The capability of the PFFT-ESM for 3D scattering problems is demonstrated by further presenting the Gaussian canyon simulations with corrugated surface waves.

Disentangling the Impacts of Anthropogenic Aerosols on Terrestrial Carbon Cycle During 1850-2014.

Aerosols have a dimming and cooling effect and change hydrological regimes, thus affecting carbon fluxes, which are sensitive to climate. Aerosols also scatter sunlight, which increases the fraction of diffuse radiation, increasing photosynthesis. There remains no clear conclusion whether the impact of aerosols on land carbon fluxes is larger through diffuse radiation change than through changes in other climate variables. In this study, we quantified the overall physical impacts of anthropogenic aerosols on land C fluxes and explored the contribution from each factor using a set of factorial simulations driven by climate and aerosol data from the IPSL‐CM6A‐LR experiments during 1850–2014. A newly developed land surface model which distinguishes diffuse and direct radiation in canopy radiation transmission, ORCHIDEE_DF, was used. Specifically, a subgrid scheme was developed to distinguish the cloudy and clear sky conditions. We found that anthropogenic aerosol emissions since 1850 cumulatively enhanced the land C sink by 22.6 PgC. Seventy‐eight percent of this C sink enhancement is contributed by aerosol‐induced increase in the diffuse radiation fraction, much larger than the effect of the aerosol‐induced dimming. The cooling of anthropogenic aerosols has different impacts in different latitudes but overall increases the global land C sink. The dominant role of diffuse radiation changes found in this study implies that future aerosol emissions may have a much stronger impacts on the C cycle through changing radiation quality than through changing climate alone. Earth system models need to consider the diffuse radiation fertilization effect to better evaluate the impacts of climate change mitigation scenarios.

Evaluation of the near-surface climate of the recent global atmospheric reanalysis for Qilian Mountains, Qinghai-Tibet Plateau

In this study, temperature, air pressure, wind speed and precipitation from 19 basic stations over Qilian Mountains are used to evaluate the applicability for reanalysis datasets (ERA5, JRA-55, ERAI and HAR) during 1980–2018. The four global reanalysis products describe the state of the near-surface climate with different performances. The results show that (1) The four reanalysis products perform pretty satisfactorily for temperature and air pressure while the precipitation with the lowest quality; (2) ERA5 outperforms for most variables in correlation coefficients, especially for wind speed, but not significantly improved than EARI of other variables; (3) Overestimate of wind speed issues in HAR were also identified as the sub-grid orographic drag scheme is not included in WRF; (4) Despite some discrepancy, a warming trend is showed with all these four products; in addition, the decade from 1990 to 1999 has the fastest warming rate.

WRF-LES Simulation of the Boundary Layer Turbulent Processes during the BLLAST Campaign

A real case long-term nested large eddy simulation (LES) of 25-day duration is performed using the WRF-LES modelling system, with a maximum horizontal grid resolution of 111 m, in order to explore the ability of the model to reproduce the turbulence magnitudes within the first tens of metres of the boundary layer. Sonic anemometer measurements from a 60-m tower installed during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign are used for verification, which is focused on the turbulent magnitudes in order to assess the success and limitations in resolving turbulent flow characteristics. The mesoscale and LES simulations reproduce the wind speed and direction fairly well, but only LES is able to reproduce the energy of eddies with lifetimes shorter than a few hours. The turbulent kinetic energy in LES simulation is generally underestimated during the daytime, mainly due to a vertical velocity standard deviation that is too low. The turbulent heat flux is misrepresented in the model, probably due to the inaccuracy of the sub-grid scheme.

Resolution Dependence of Turbulent Structures in Convective Boundary Layer Simulations

Large-eddy simulations are performed using the U.K. Met Office Large Eddy Model to study the effects of resolution on turbulent structures in a convective boundary layer. A standard Smagorinsky subgrid scheme is used. As the grid length is increased, the diagnosed height of the boundary layer increases, and the horizontally- and temporally-averaged temperature near the surface and in the inversion layer increase. At the highest resolution, quadrant analysis shows that the majority of events in the lower boundary layer are associated with cold descending air, followed by warm ascending air. The largest contribution to the total heat flux is made by warm ascending air, with associated strong thermals. At lower resolutions, the contribution to the heat flux from cold descending air is increased, and that from cold ascending air is reduced in the lower boundary layer; around the inversion layer, however, the contribution from cold ascending air is increased. Calculations of the heating rate show that the differences in cold ascending air are responsible for the warm bias below the boundary layer top in the low resolution simulations. Correlation length and time scales for coherent resolved structures increase with increasing grid coarseness. The results overall suggest that differences in the simulations are due to weaker mixing between thermals and their environment at lower resolutions. Some simple numerical experiments are performed to increase the mixing in the lower resolution simulations and to investigate backscatter. Such simulations are successful at reducing the contribution of cold ascending air to the heat flux just below the inversion, although the effects in the lower boundary layer are weaker.

Does Subgrid Routing Information Matter for Urban Flood Forecasting? A Multiscenario Analysis at the Land Parcel Scale

AbstractThe accessibility of high-resolution surface data enables fine distributed modeling for urban flooding. However, the surface routing processes between nonhomogeneous land cover components remain in most grid units, due to the high spatial heterogeneity of urban surfaces. Limited by the great difficulty in the acquisition, subgrid routing information (SRI) is always ignored in high-resolution urban flood modeling, and more importantly, the potential impacts of missing SRI on flood forecasting are still less understood. In this study, 54 urban-oriented scenarios of subgrid routing schemes are designed at an isolated grid, including three types of land parcels, two routing directions, and nine routing percents. The impacts of missing SRI are evaluated comprehensively under 60 different rainfall scenarios, in terms of the peak runoff (PR) and the runoff coefficient (RC). Furthermore, the influence mechanism is revealed as well to explain the discrepancy of the impacts under different conditions. Results show the missing of the routing process from impervious to pervious areas leads to significant impacts on the simulation of both PR and RC. Overestimated RC is detected, however, the impacts on PR are bidirectional depending on the rainfall intensity. Overestimation of PR due to missing SRI is observed in light rainfall events, but the opposite effect is identified under heavy rainfall conditions. This study highlights the importance of incorporating the SRI for urban flood forecasting to avoid underestimating the hazard risk in heavy rainfall. Simultaneously, it identifies that blindly utilizing infiltration-based green infrastructure is not feasible in urban stormwater management, due to the possible increase in peak runoff.

New Trends in Ensemble Forecast Strategy: Uncertainty Quantification for Coarse-Grid Computational Fluid Dynamics

Numerical simulations of industrial and geophysical fluid flows cannot usually solve the exact Navier–Stokes equations. Accordingly, they encompass strong local errors. For some applications—like coupling models and measurements—these errors need to be accurately quantified, and ensemble forecast is a way to achieve this goal. This paper reviews the different approaches that have been proposed in this direction. A particular attention is given to the models under location uncertainty and stochastic advection by Lie transport. Besides, this paper introduces a new energy-budget-based stochastic subgrid scheme and a new way of parameterizing models under location uncertainty. Finally, new ensemble forecast simulations are presented. The skills of that new stochastic parameterization are compared to that of the dynamics under location uncertainty and of randomized-initial-condition methods.

A full Stokes subgrid scheme in two dimensions for simulation of grounding line migration in ice sheets using Elmer/ICE (v8.3)

Abstract. The flow of large ice sheets and glaciers can be simulated by solving the full Stokes equations using a finite element method. The simulation is particularly sensitive to the discretization of the grounding line, which separates ice resting on bedrock and ice floating on water, and is moving with time. The boundary conditions at the ice base are enforced by Nitsche's method and a subgrid treatment of the grounding line element. Simulations with the method in two dimensions for an advancing and a retreating grounding line illustrate the performance of the method. The computed grounding line position is compared to previously published data with a fine mesh, showing that similar accuracy is obtained using subgrid modeling with more than 20-times-coarser meshes. This subgrid scheme is implemented in the two-dimensional version of the open-source code Elmer/ICE.

Stability enhancement of planar transport solution based whole-core calculation employing augmented axial method of characteristics

The planar Method of Characteristic (MOC) solution based direct whole core calculation method is stabilized and extended by employing an axial one-dimensional (1D) MOC solver with several augmentations. In order to resolve the inherent instabilities involved in the coarse mesh finite difference formulation employing a nodal simplified P3 (SP3) axial solver, the factors causing negative fluxes are identified and eliminated. The augmentations consist of (1) a subgrid scheme, (2) transverse leakage splitting, and (3) PL scattering and limited transport correction that are selectively applied depending on regional characteristics. Axial domain decomposition is employed for parallel solution of 1D MOC. Various core calculation results demonstrate that the augmented 1D MOC solver considerably enhances stability while retaining comparable accuracy with the SP3 nodal kernel and not causing significant computational overhead.

A comparative study of bounce-back and immersed boundary method in LBM for turbulent flow simulation

In this study, we compared the modified bounce-back (BB) method and immersed boundary (IB) method for the treatments of no-slip boundary condition based on the Lattice Boltzmann Method (LBM) for turbulent flow. The benchmark case of flow past a square cylinder confined in a rectangular duct has been simulated with two schemes at Re d = 3000. The turbulent flow was modeled using Large Eddy Simulation (LES). The conventional Smagorinsky subgrid scheme was used to resolve the small-scale motions. The proposed algorithm is parallelized to run on NVIDIA Tesla P100 Graphics Processing Unit (GPU) using Compute Unified Device Architecture (CUDA) programming language. The results obtained from the bounce-back and immersed boundary method has been validated with the experimental and LES data reported in the literature for different turbulent statistics including time-averaged velocities, root-mean-square velocity fluctuations, and Reynolds shear stress. Later, the comparison between the two approaches discussed in terms of accurate predictions for the drag coefficient. The computational efficiency on GPU for both methods has also been reported. [copyright information to be updated in production process]

Algorithmic differentiation for cloud schemes (IFS Cy43r3) using CoDiPack (v1.8.1)

Abstract. Numerical models in atmospheric sciences not only need to approximate the flow equations on a suitable computational grid, they also need to include subgrid effects of many non-resolved physical processes. Among others, the formation and evolution of cloud particles is an example of such subgrid processes. Moreover, to date there is no universal mathematical description of a cloud, hence many cloud schemes have been proposed and these schemes typically contain several uncertain parameters. In this study, we propose the use of algorithmic differentiation (AD) as a method to identify parameters within the cloud scheme, to which the output of the cloud scheme is most sensitive. We illustrate the methodology by analyzing a scheme for liquid clouds, incorporated into a parcel model framework. Since the occurrence of uncertain parameters is not limited to cloud schemes, the AD methodology may help to identify the most sensitive uncertain parameters in any subgrid scheme and therefore help limiting the application of uncertainty quantification to the most crucial parameters.