Vertical Coordinate System Research Articles

Simulation of a Mesoscale Convective System Using a New Formulation for Advection in a Bin Microphysics: Sensitivity to Advection, Microphysics, and Grid Spacing

Abstract A computationally efficient semi-Lagrangian advection (SLA) scheme designed for microphysical variable advection was implemented into the Weather Research and Forecasting (WRF) Model with spectral bin microphysics (SBM). The primary goal was to reduce the CPU time for advection of the scalar SBM variables and demonstrate its applicability to simulation of a real-data case study in the eta vertical coordinate system. A mesoscale convective system (MCS) in Midlatitude Continental Convective Clouds Experiment (MC3E-0520) was selected for simulations. We compared the SLA and high-order, nonlinear monotonic advection schemes and tested the sensitivity of the simulated radar reflectivity, microphysical, and dynamic properties of the MCS to the choice of microphysical schemes, aerosol concentration, and grid spacing. Simulations using the SLA and monotonic advection schemes were similar, and the differences between them were much smaller than those between the SBM and bulk microphysical schemes tested. Improvement of the grid resolution had a larger impact on the vertical velocity field than did the choice of aerosol concentration. The total computational time in simulations with SLA was about 25% shorter than that with monotonic advection, which resulted from a reduction of more than 50% in computational time required for advection of the microphysical variables. Significance Statement Clouds comprise particles of various sizes and types: aerosols, liquid droplets, ice crystals, hail, and snow. Two major approaches describe the time and spatial evolution of these particle species: solving basic microphysical (bin microphysics) and simplified parameterization (bulk microphysics) equations. Bin microphysics is time-consuming as it operates with size distributions described by multiple mass bins. Among the microphysical processes, the advection of the microphysical variables requires significant CPU time. We tested a new numerical advection scheme that requires several times less computer time than the standard high-order nonlinear schemes being of similar accuracy. Due to the introduced change, the advection of droplet size distributions became less of a factor affecting the total computer time of bin-type schemes. This is an important step toward making the bin approach computationally efficient without losing accuracy. It is shown that the sensitivity of the results to spatial grid spacing and to the choice of microphysical schemes is significant and may be larger than that to variations in the aerosol concentration.

A regional NEMO 4.0 configuration of the subpolar North Atlantic

The dynamics of the North Atlantic subpolar gyre (SPG) largely control the Atlantic Meridional Overturning Circulation (AMOC), thus affecting the long-term climate variability of the global ocean. Many key processes in the North Atlantic SPG, such as Gulf Stream detachment, eddy activity, deep convection and overflows, are frequently incorrectly presented or misrepresented in modern ocean general circulation models. Here, we present a new regional eddy-resolving model of the subpolar North Atlantic based on the NEMO4 configuration and analyze its ability to adequately represent the dynamics of the subpolar North Atlantic. The model performance is assessed using in situ data (cross-section hydrography, Argo and sea gliders), remote sensing data (satellite sea surface temperature and salinity) and the ANDRO and ARMOR3D products based on optimal interpolation products. In the next step, we analyze the effects of high-resolution forcing and several recent developments in ocean modeling to improve the model solution. The implementation of the self-diffusive momentum advection scheme with additional viscosity allows for the simulation of more realistic Deep Western Boundary Current and Irminger Rings lifecycle characteristics. The surface current feedback parameterization implemented in the forcing function and the modification of the kinetic energy budget in the upper ocean, especially in intensively eddying areas, simulate the behavior predicted by coupled models. While the model is sensitive to the implementation of cool skin–warm layer parameterization, this parameterization does not significantly improve the solution compared to the observational data. Regional implementation of the local-sigma vertical coordinate system in cascading areas allows overflow waters to spread over the ocean bottom more realistically. The remote effect of the implementation of the local-sigma coordinate is traced in the vertical thermohaline structure at the western slopes of the Irminger Sea and Iceland Basin as well as in the Deep Western Boundary Current in the Labrador Sea. Compared to z-partial step setting, local-sigma allows for correct representation of continuous densification of the water in the overturning part of the AMOC and thus North Atlantic Deep-Water formation. The potential of the new model configuration for analyzing the subpolar North Atlantic, including convective activity, is discussed.

A Review on the Vertical Coordinate Systems used in Oceanic and Atmospheric Circulation Numerical Model

In a numerical method for the study of the circulation model, various vertical coordinate systems are used to simulate the physical response of the ocean and atmosphere to the increasing greenhouse gas emission. In this study, four types of vertical coordinate systems frequently used in oceanic and atmospheric circulation numerical models, i.e., height, general, pressure, and normalized vertical coordinate systems, respectively are introduced. Finally, the hydrostatic pressure equation, vertical velocity, equation of horizontal motion, and continuity equation expressed in a vertical coordinate system were introduced, and the pros and cons of the vertical coordinate system were summarized to promote the accuracy of numerical model development.

Mixing-layer-height-referenced ozone vertical distribution in the lower troposphere of Chinese megacities: stratification, classification, and meteorological and photochemical mechanisms

Abstract. Traditional tropospheric ozone (O3) climatology uses a simple average substantially smoothed stratification structure in individual O3 profiles, limiting our ability to properly describe and understand how O3 is vertically distributed at the interface between the mixing layer (ML) and free troposphere (FT). In this study, we collected 1897 ozonesonde profiles from two Chinese megacities (Beijing and Hong Kong) over the period 2000–2022 to investigate the climatological vertical heterogeneity of the lower-tropospheric O3 distribution with a mixing-layer-height-referenced (h-referenced) vertical coordinate system. The mixing-layer height (h) was first estimated following an integral method that integrates the information of temperature, humidity, and cloud. After that, a so-called h-referenced vertical distribution of O3 was determined by averaging all individual profiles expressed as a function of z/h rather than z (where z is altitude). We found that the vertical stratification of O3 is distributed heterogeneously in the lower troposphere, with stronger vertical gradients at the surface layer and ML–FT interface. There are low vertical autocorrelations of O3 between the ML and FT but high autocorrelations within each of the two atmospheric compartments. These results suggest that the ML–FT interface acts as a geophysical “barrier” separating air masses of distinct O3 loadings. This barrier effect varies with season and city, with an ML–FT detrainment barrier in summer (autumn) and an FT–ML entrainment barrier in other seasons in Beijing (Hong Kong). Based on a Student's t test, daily h-referenced O3 profiles were further classified into three typical patterns: MLO3-dominated, FTO3-dominated, and uniform distribution. Although the FTO3-dominated pattern occurs most frequently during the whole study period (69 % and 54 % of days in Beijing and Hong Kong, respectively), the MLO3-dominated pattern prevails in the photochemically active season, accounting for 47 % of summer days in Beijing and 54 % of autumn days in Hong Kong. These occurrences of the MLO3-dominated pattern are significantly more frequent than in previously reported results at northern mid-latitudes, indicating intensive photochemical MLO3 production under the high-emission background of a Chinese megacity. From a FTO3-dominated to MLO3-dominated pattern, the O3 precursor CH2O (NO2) experiences a substantial increase (decrease) in Beijing but a slight change in Hong Kong. Vertically, the increment of CH2O is larger in the upper ML, and the decrement of NO2 is larger in the lower ML. Such vertical changes in O3 precursors push O3 production sensitivity away from the VOC-limited regime and facilitate high-efficiency production of O3 via photochemical reactions, particularly in the upper ML of Beijing.

Localized General Vertical Coordinates for Quasi‐Eulerian Ocean Models: The Nordic Overflows Test‐Case

AbstractA generalized methodology to deploy different types of vertical coordinate system in arbitrarily defined time‐invariant local areas of quasi‐Eulerian numerical ocean models is presented. After detailing its characteristics, we show how the general localization method can be used to improve the representation of the Nordic Seas overflows in the UK Met Office NEMO‐based eddy‐permitting global ocean configuration. Three z*‐levels with partial steps configurations localizing different types of hybrid geopotential/terrain‐following vertical coordinates in the proximity of the Greenland‐Scotland ridge are implemented and compared against a control configuration. Experiments include a series of idealized and realistic numerical simulations where the skill of the models in computing pressure forces, reducing spurious diapycnal mixing and reproducing observed properties of the Nordic Seas overflows are assessed. Numerical results prove that the localization approach proposed here can be successfully used to embed terrain‐following levels in a global geopotential levels‐based configuration, provided that the localized vertical coordinate chosen is flexible enough to allow a smooth transition between the two. In addition, our experiments show that deploying localized terrain‐following levels via the multi‐envelope method allows the crucial reduction of spurious cross‐isopycnal mixing when modeling bottom intensified buoyancy driven currents, significantly improving the realism of the Nordic Seas overflows simulations in comparison to the other configurations. Important hydrographic biases are found to similarly affect all the realistic experiments and a discussion on how their interaction with the type of localized vertical coordinate affects the realism of the simulated overflows is provided.

A Moving-Wave Implementation in WRF to Study the Impact of Surface Water Waves on the Atmospheric Boundary Layer

Abstract While numerous modeling studies have focused on the interaction of ocean surface waves with the atmospheric boundary layer, most employ idealized waves that are either monochromatic or synthetically generated from a theoretical wave spectrum, and the atmospheric solvers are typically incompressible. To study wind–wave coupling in real-world scenarios, a model that can simulate both realistic meteorological and wave conditions is necessary. In this paper we describe the implementation of a moving bottom boundary condition into the Weather Research and Forecasting Model for large-eddy simulation applications. We first describe the moving bottom boundary conditions within WRF’s pressure-based vertical coordinate system. We then validate our code with idealized test cases that have analytical solutions, including flow over a monochromatic wave with and without viscosity. Finally, we present results from turbulent flows over a moving monochromatic wave with different wave ages, and demonstrate satisfactory agreement of the wave growth rate with results from the literature. We also compare atmospheric stress and wind parameters from two physically equivalent cases. The first specifies a wind moving in the same direction as a propagating wave, while the second involves a stationary wave with the wind adjusted such that the wind relative to the wave is the same as in the first case. Results indicate that the velocity and Reynolds stress profiles for the two cases match, further validating the moving bottom implementation.

Fireworks: A Potential Artificial Source for Imaging Near-Surface Structures

Abstract Seismic waves induced by incident acoustic waves from air disturbances can be used to image near-surface structures. In this article, we analyze seismic waveforms recorded by a dense array on the Xishancun landside in Li County, Sichuan Province, southwest China during the Lunar New Year’s Eve (27 January 2017). A total of eight event clusters have been identified as a result of firework explosions. For each cluster, which comprises dozens of individual events with high similarity, we manually pick arrival times of the first event recorded by the array and locate it with a grid-search method. We then rotate three-component waveforms of all events from the east, north, and vertical coordinate system to the local LQT coordinates (L, positive direction perpendicular to the landslide surface and pointing downwards; Q, positive direction is from the launch location of firework to the station along the landslide surface; T, perpendicular to the plane formed by the L and Q directions, and the selected positive direction of the T axis makes LQT form the left-hand coordinate system), and stack the LQT components for those events with cross-correlation values CC ≥ 0.8 with respect to the first event. Characteristics of the stacked LQT components are also examined. The particle motions at each station are retrograde ellipse in the frequency range of ∼5–50 Hz, suggesting air-coupled Rayleigh waves generated by the firework explosions. Spectrograms of the Rayleigh waves also show clear dispersions, which might be used to image near-surface velocity structures. Although we cannot directly extract the phase velocities due to the limitation of the seismic array, our study shows that the fireworks might provide a low-cost and easy-to-use seismic source for imaging near-surface structures.

Shelf slope, estuarine dynamics and river plumes in a z* vertical coordinate, unstructured grid model

The finite element, unstructured grid model SHYFEM has been used to carry out some fundamental studies on the quality of a specific type of Generalized Vertical Coordinate (GVC) system, the so-called z∗ coordinates versus z−geopotential coordinates. Idealized and realistic test cases are shown to assess the impact of the vertical coordinates on key physical processes, i.e. the internal tide generation at a continental slope, a river plume shape and intensity and the salt water intrusion.The results showed that the z∗ coordinates produce a stronger water column stratification than the z−geopotential ones and together with the unstructured grid modeling they can better solve the coastal sea dynamics.An idealized twin experiment configuration of the model was carried out to verify the proper implementation of the GVC z∗ by simulating the production of internal tides at the continental slope. The results confirmed that the z∗ coordinate model reproduces the expected internal tide fields better than z−geopotential model. A set of realistic experiments in the Po river delta coastal region, demonstrated that the representation of the dynamical processes at the coastal scales is conditioned by the high resolution (25 cm layers), which was only possible by the z∗ formulation in the upper water column.

Embedded heat pipe SiC interposer for distributed power devices

Passive heat dissipation was a prospective solution to gratify the thermal management requirements of the continuously compact and miniaturized package. In here, we proposed an embedded heat pipe SiC interposer for the distributed power devices. The thermal effects of wick shape and filling ratio were investigated through the established VOF (volume of fluid) models. The hotspots' arrangement was identified by comparing thermal performance with hotspots' placing at 22.5°, 45°, and 90° in the vertical coordinate system. The 45° arrangement was settled by analyzing the effects on refilling path between the evaporation and condensation areas. Profited from the SiC substrate transparency, the embedded thermography visualization platform was deployed to verify the two-phase flow behaviors inside the SiC interposer with various heat fluxes. Through the Infrared imager and non–contact Raman–based thermometry, the heat transfer coefficient (90 kW/m2·°C @ 160 W/cm2), maximum temperature difference (within 2.6 °C @ 160 W/cm2), and the distributed radial and axial thermal resistances (0.167–0.178 °C/W and 0.157–0.163 °C/W) of the SiC interposer were ascertained, respectively. Towards the distributed power devices embedded heat pipe interposer, the substrate material, the wick geometry, the filling ratio, and hotspots’ arrangement need to be comprehensively optimized.

GULF18, a high-resolution NEMO-based tidal ocean model of the Arabian/Persian Gulf

Abstract. The sensitivity of a shelf sea model of the Gulf area to changes in the bathymetry, lateral and vertical resolution, vertical coordinates and river and atmospheric forcing is explored. Two new Gulf models with a resolution of 1.8 km, named GULF18-3.6 and GULF18-4.0, differing only in the vertical coordinate system and the NEMO code base employed (NEMO-3.6 and NEMO-4.0.4, respectively) are introduced. We compare them against the existing 4 km PGM4 model, which is based on NEMO-3.4 and is developed and used by the Met Office. PGM4 and GULF18-3.6 use similar types of quasi-terrain-following vertical levels, while GULF18-4.0 employs the multi-envelope method to discretise the model domain in the vertical direction. Our assessment compares non-assimilative hindcast simulations of the three Gulf models for the period 2014–2017 against available observations of tides, hydrography and surface currents. Numerical results indicate that both high-resolution models have higher skill than PGM4 in representing the sea surface temperature and the water column stratification on the shelf. In addition, in the proximity of the shelf break and the deep part of the domain, GULF18-4.0 generally presents the highest accuracy, demonstrating the benefit of optimising the vertical grid for the leading physical processes. For the surface currents, the three models give comparable results on the shelf, while the higher-resolution models might be prone to the double-penalty effect in deeper areas. For the tides, PGM4 has a better skill than GULF18 models, and our tidal harmonic analysis suggests that future work may be needed in order to get real benefit from using a more realistic bottom topography, as in the case of the GULF18 models.

Comparative Study on Water Temperature Stratified Flow under Different Vertical Coordinate Systems in Delft3D

Reservoirs often suffer from water blooms, which are likely related to the hydrodynamic and water temperature characteristics of the tributary bays. To obtain the detailed changing process of hydrodynamics and water temperature stratification, it is necessary to choose a suitable vertical coordinate system in order to achieve the required precision. Based on a physical model experiment of cold water flowing into the Generalized Reservoir Hydraulics (GRH) flume, both the σ-coordinate system model and the z-coordinate system model are built for comparison. For the z-coordinate system model, the influences of different grid resolutions and different bottom slopes on the simulation accuracy are also analyzed. The results show that the σ-coordinate system model can simulate cold-water underflow in a reservoir better than the z-coordinate system model, and the numerical errors of the z-coordinate system model can be reduced but not eliminated by increasing the horizontal grid resolution. When the bottom slope of the reservoir is less than 18‰, the z-coordinate system model can also be used to simulate cold-water underflow in a reservoir. The conclusions about vertical coordinate systems can be applied to the development of a three-dimensional hydrodynamic and water temperature model of reservoirs.

Applications of generalized vertical coordinates in ocean circulation models

Two types of generalized vertical coordinates are introduced to reduce model errors in calculating the baroclinic pressure gradient (BPG), advection term and horizontal mixing in tracer equations in a three-dimensional (3D) ocean circulation model. These two types of vertical coordinates are defined as the generalized terrain-following coordinate (GTC) and generalized piecewise terrain-following coordinate (GPTC). The main feature of the GTC is the use of a series of constant-depth levels in the water column above the sea bottom. The GPTC differs from the GTC in which the depth differences between two adjacent grids are reduced, especially in the bottom layer over steep topography. Performances of these two vertical coordinate systems are assessed first in two types of idealized numerical experiments with a tall seamount and continuously varying bottom slope respectively. The model results in the tall seamount experiment demonstrate that both the GTC and GPTC perform well in reducing the model errors in the BPG over the steep topography, and greatly alleviate the spurious diapycnal mixing in terrain-following coordinate models. In the continuously varying bottom slope experiment, both the GTC and GPTC perform better than the z-level coordinate in simulating the 3D currents. The performances of the GTC and GPTC are further assessed by examining prognostic simulations of 3D currents and hydrography in the Yellow and East China Seas with realistic external forcing and bottom topography. The model results in the third experiment demonstrate that the GPTC restrains the overestimated upward climbing of the bottom cold water in comparison with other coordinates. The GPTC also performs better than the GTC and the conventional sigma-layer coordinate in simulating the shoreward intrusion of relatively high-salinity subsurface waters, intermediate low-salinity waters and the Yellow Sea bottom cold waters. By comparison, the GTC simulates the Kuroshio, the Slope Counter-Current and their seasonal variability significantly better than other coordinates.

Analysis of baroclinic vertical structures around the Korean marginal seas with a primary focus on the model configuration

Determining the appropriate spatial resolution is crucial for configuring regional ocean models. The first baroclinic Rossby radius of deformation serves as an objective length-scale for ensuring horizontal grid spacing, while little equivalent quantifiable standard exists for the vertical coordinate system. Many models determine vertical grids based on the analysis of baroclinic modes commensurate with the theoretical capabilities of a given horizontal grid. However, under this approach, most of the focus is on the global ocean or ideal cases. Therefore, we analyzed the baroclinic vertical modes around the Korean marginal seas using serial observation data provided by the National Fisheries Research and Development Institute's Korean Oceanographic Data Center. Using the deformation radius-model horizontal grid ratio (Lm/Δ˜) and baroclinic mode analysis, we suggest the optimal level to configure a high-resolution model. The results indicate that a model with a 1/33° horizontal resolution can reproduce up to the first baroclinic mode in the Yellow Sea, the second mode in the East China Sea, and the third mode in the East/Japan Sea. From the zero-crossing depth distribution analysis, we also provided the recommended minimum number of vertical levels to satisfy each baroclinic mode in the sub-regions of the Korean marginal seas. This study clearly highlights that more attention should be paid to configure the vertical coordinate system when designing regional modeling, especially for the sub-areas of the Korean marginal seas, because these values are considerably different from each other with a significant interannual variability.

Modelling of short-term variations of currents, temperature, salinity and sea level in the Strait of Dardanelles

The Strait of Dardanelles is a long and shallow strongly stratified strait connecting the Aegean and Marmara Seas and characterized by a complicated time-varying oceanographic regime. Despite that hydrodynamic conditions are critical for navigation and hydraulic structures in the Strait, only a few observation and simulation studies were carried out. For this high-resolution study, the model SCHISM based on unstructured grids and a vertical coordinate system LSC2 was used. The numerical experiments were focused on the period September 2008–January 2009 when measurement data were available. The simulation agrees with observed profiles of velocity and discharges in upper and lower layers. Simulated and observed features include flow reversals in the Strait during strong storm events (November 2008) and high-frequency variability of currents and elevation in the Strait and the Marmara Sea. Simulations have shown the persistent hydraulic jump, mixing, and complex flow structure in the narrowest and bending Strait part. The results of simulation for locations of two pylons (“European” and “Anatolian”) of the bridge “1915 Çanakkale” showed strong temporal variability of currents on a daily scale caused by fluctuations of sea level in connecting basins, flow from the Bosphorus Strait, direct wind forcing, and tidal forcing. • High-resolution modeling study of short-term variations of the Strait of Dardanelles was carried out. • Simulated features include detailed study of flow reversals in the Strait during strong storm events (21–23 November 2008). • It was found strong temporal variability of currents on a daily scale in locations of two pylons of bridge “1915 Çanakkale”.

Уточнение глобальной модели геоида по данным GPS и нивелирования для территории Ферганской долины

The Republic of Uzbekistan currently uses the Baltic normal system of heights associated with the quasi-geoid surface. The introduction of innovative technologies of global navigation satellite systems (GNSS) made it possible to determine with high accuracy the geometric heights related to the Earth model — an ellipsoid. For a complex representation of spatial data and analysis of information about the territory, natural resources, man-made objects, their dynamics in the National Geographic Information System (NGIS), first of all, it is necessary to solve the problem of refining the reference vertical coordinate system and ensuring its connection with the global ones, namely, to ensure adjustment of these two height systems. The task is complicated by the fact that for the territory of the country there is no information about the quasi-geoid determined by modern methods, which are necessary for the transition from geometric ellipsoidal heights obtained using GPS measurements to normal heights which are used for solving a wide range of practical problems in various fields of environmental sciences. In recent years, global geopotential models of the Earth (GGM) calculated using various satellite missions has become an alternative solution to the problem. The study considers two high order GGMs (EGM2008 and GECO) to create a system of normal heights for the territory of the Fergana Valley. The height anomaly values for them reach the maximum for the region — about −49 m. The method of constructing parametric models (or corrective surfaces) using leveling and GPS measurements at “common points” was used to refine them. The range of corrections made was from −0.28 m to 0.29 m. Moreover, the GECO model was significantly improved compared to EGM2008 in the flatland part of the study area, which has large geoid anomalies. The Krasovsky ellipsoid and the corresponding Baltic height system were chosen as a reference surface to create in the future a grid for converting heights between local and geocentric coordinate systems. The results of a comprehensive statistical analysis made it possible to reveal that GECO gives a more accurate representation of the region’s relief and is recommended for solving practical problems in the Fergana Valley.

Modelling seasonal and intraseasonal variations of circulation,temperature, salinity and sea level in the Bellingshausen Seaand on the Antarctic Peninsula shelf

The objective of the study is to simulate using numerical methods the seasonal and intraseasonal variations of circula-tion, sea level, temperature and salinity in the Bellingshausen Sea and on the shelf of the western part of the Antarctic Penin-sula (WAP). The Semi-implicit Cross-scale Hydroscience Integrated System Model with an unstructured triangular horizontal grid and a vertical local sigma coordinate system and ice dynamic-thermodynamic Finite-Element Sea Ice Model were applied. Heat, momentum and salt fluxes were set on the ocean surface using the ERA5 reanalysis data. At the open boundaries, the vertical distribution of temperature and salinity was determined according to the COPERNICUS reanalysis. At the western open boundary of the computational domain, the vertical distribution of velocity of currents from COPERNICUS was also specified, whereas at the open eastern boundary the level deviations were specified. Time variability analysis of sea level wasperformed using wavelet analysis. The results of modelling of the sea level, temperature, and salinity fields for 2014–2015 were compared with the available observational data on the shelf of the WAP, including data from the Ukrainian Antarctic Expedition. The simulated horizontal and vertical distributions of the subsurface layer with minimum of potential temperature Tmin (Winter Water) are given. The depth of the Tmin varies in the range of 10–100 m increasing to the north. The values of minimum of po-tential temperature Tmin also increase to the north from –1.8 to 1.2 °C. The intraseasonal oscillations of sea level computed by the model for 2014—2015 were analysed together with data of observations at the tidal stations Faraday/Akademik Vernadsky and Rothera located at the coast of WAP. In the range 1–150 days the largest amplitudes of the level scalegrams were found for a period of approximately 100 days in 2014 and 120 days in 2015 at both stations. The largest amplitudes of modelled level scale-grams were observed with a period of approximately 88 days in 2014 and 80 days in 2015 at both stations. The largest amplitudesof scalegrams for Antarctic Oscillation (AAO) were found for a period of 105 days in 2014 and 123 days in 2015. The corresponding correlation coefficients between observed sea level scalegrams and AAO for 2014 were 0.84 and 0.86, respectively, whereas for 2015 they were 0.87 and 0.90, respectively. It was concluded that the relationship between intraseasonal processes in the ocean in West Antarctica and AAO existed at a time scale of about 100 days.

Improved Prediction of Cold-Air Pools in the Weather Research and Forecasting Model Using a Truly Horizontal Diffusion Scheme for Potential Temperature

AbstractThe terrain-following vertical coordinate system used by many atmospheric models, including the Weather Research and Forecasting (WRF) Model, is prone to errors in regions of complex terrain. These errors stem, in part, from the calculation of horizontal gradients within the diffusion term of the momentum or scalar evolution equations. In WRF, such gradients can be calculated along coordinate surfaces, or using metric terms that help account for grid skewness. However, neither of these options ensures a truly horizontal gradient calculation, especially if a grid cell is skewed enough that the heights of the neighboring grid points used in the calculation fall outside the vertical range of the cell. In this work, an improved scheme that uses Taylor series approximations to vertically interpolate variables to the level necessary for a truly horizontal gradient calculation is implemented in WRF for the diffusion of potential temperature. The scheme is validated using an atmosphere-at-rest configuration, in which spurious flows develop only as a result of numerical errors and can thus be used as a proxy for model performance. Following validation, the method is applied to the simulation of cold-air pools (CAPs), which occur in regions of complex terrain and are characterized by strong near-surface temperature gradients. Using the truly horizontal scheme, idealized simulations demonstrate reduced numerical mixing in a quiescent CAP, and a realistic case study in the Columbia River basin shows a reduction in positive wind speed bias by up to roughly 20% compared to observations from the Second Wind Forecast Improvement Project.

Seismic imaging of S-wave structures of shallow sediments in the East China Sea using OBN multicomponent Scholte-wave data

The shear-wave (S-wave) structures of shallow marine sediments are important for offshore geotechnical studies, deep crustal S-wave imaging, multicomponent seismic exploration, and underwater acoustics studies. We have applied the multicomponent Scholte-wave analysis technique to an active-source shallow marine seismic profile in the East China Sea. Scholte waves have been excited by shots from a 5450 inch3air-gun array and their recordings have been conducted at the seafloor using ocean bottom nodes (OBNs). First, we extract the common-receiver gathers (CRGs) and correct for the time drift simultaneously using a forward and inverse fast Fourier transform resampling algorithm. Three CRGs of seismic sensors are used for Scholte-wave analysis. Raw sensor CRGs are rotated to the inline, crossline, and vertical coordinate system. The rotated tilt and roll angle are directed using the inner electric compass log value, and the shot inline azimuth is estimated using the particle motion method. Then, the velocity spectra are calculated from the inline and vertical components using the phase-shift method. Higher Scholte-wave modes dominate on the horizontal components, whereas the stronger fundamental mode dominates on the vertical components. The multicomponent velocity spectrum stacking method is adopted to produce the final dispersion energy image. Up to four modes of dispersion curves are retrieved within the 1.1–4.3 Hz frequency band. The multimode dispersion curve inversion is constructed for imaging the shallow sediments. The results suggest a low [Formula: see text] of 180–650 m/s and few lateral variations within the top 0.5 km of shallow marine sediments in the East China Sea. This model can provide an important reference for offshore geotechnical investigations, especially for OBN multicomponent seismic exploration data processing. The use of OBNs has high feasibility in [Formula: see text] imaging for shallow marine sediments when combined with the Scholte-wave dispersion-curve inversion.

The worldwide physical height datum project

The definition of a common global vertical coordinate system is nowadays one of the key points in Geodesy. With the advent of GNSS, a coherent global height has been made available to users. The ellipsoidal height can be obtained with respect to a given geocentric ellipsoid in a fast and precise way using GNSS techniques. On the other hand, the traditional orthometric height is not coherent at global scale. Spirit levelling allows the estimation of height increments so that orthometric heights of surveyed points can be obtained starting from a benchmark of known orthometric heights. As it is well known, this vertical coordinate refers to the geoid, which is assumed to be coincident to the mean sea level. By means of a tide gauge, the mean sea level is estimated and thus a point of known orthometric height is defined. This assumption, which was acceptable in the past, became obsolete given the level of precision which is now required. Based on the altimetry observation, one can precisely quantify the existing discrepancy between geoid and mean sea level that can amount to 1 ÷ 2 m at global scale. Therefore, different tide gauges provide biased estimates of the geoid, given the discrepancy between this equipotential surface and the mean sea level. Also, in the last years, another vertical coordinate was used, the normal height that was introduced in the context of the Molodensky theory. In this paper, a review of the existing different height systems is given and the relationships among them are revised. Furthermore, an approach for unifying normal height referring to different tide gauges is presented and applied to the Italian test case. Finally, a method for defining a physical height system that is globally coherent is discussed in the context of the definition of the International Height Reference System/Frame, a project supported by the Global Geodetic Observing System of the International Association of Geodesy (IAG). This project was established in 2015 during the XXVI IAG General Assembly in Prague as described in IAG Resolution no. 1 that was presented and adopted there.

Three dimensional numerical modeling using a multi-level nesting system for identifying a Water layer suitable for scallop farming in Tongyeong, Korea

Three-dimensional modeling was performed in Tongyeong, South Korea to identify a space suitable for Yesso scallop farming that may not be successful if seawater temperature remains above 22 °C. In the research area, an underwater basin was found at a depth of 70 m; the depth outside the basin was not deeper than 30 m. It was expected that the water temperature in the interior of the basin might be low enough for farming during summer. The present study was then designed to investigate the temperature distribution in this basin area to verify if a water layer with consistently low temperature existed below the surface layer, using a MOHID (MOdelo HIDrodinámico) three-dimensional numerical model. The model applied mixed vertical coordinates so that the difficulties in modeling near the basin, where water depth sharply changes, could be overcome. In addition, a multi-level nesting system was developed based on the 72 -h forecasting system of the sea status around South Korea, which increased the modeling accuracy using updated boundary conditions. The model was validated by the measured tide and temperature data. The results showed that a strong thermocline was formed at depth of 15 m and a sub-surface space existed at depths deeper than 30 m, where seawater temperature was lower than 22 °C even in summer, which indicates the possibility of successful farming in the research area. The measurements also supported this result because dissolved oxygen and chlorophyll-a were not scarce in the sub-surface space probably due to the strong tidal mixing that could bring the surface seawater properties down below the thermocline. It was also found that the model was effective in resolving the thin thermocline and the sub-surface space below it due to the mixed vertical coordinate system.