Nested Grid Research Articles

The impact of ecosystem functional type changes on the La Plata Basin climate

In this paper, the effects of land cover changes on the climate of the La Plata Basin in southern South America are investigated using the Weather and Research Forecasting (WRF) Model configured on a 30/10-km two-way interactive nested grid. To assess the regional climate changes resulting from land surface changes, the standard land cover types are replaced by time-varying Ecosystem Functional Types (EFTs), which is a newly devised land-cover classification that characterizes the spatial and interannual variability of surface vegetation dynamics. These variations indicate that natural and anthropogenic activities have caused changes in the surface physical parameters of the basin, such as albedo and roughness length, that contributed to regional climate changes. EFTs are obtained from functional attributes of vegetation computed from properties of the Normalized Difference Vegetation Index (NDVI) to represent patches of the land surface with homogeneous energy and gas exchanges with the atmosphere. Four simulations are conducted, each experimental period ranging from September to November in two contrasting years, 1988 and 1998. The influence of an identical EFT change on the surface heat fluxes, 2-m temperature and humidity, 10-m winds, convective instabilities and large-scale moisture fluxes and precipitation are explored for 1988 (a dry year) and 1998 (a wet year). Results show that the surface and atmospheric climate has a larger response to the same EFT changes in a dry year for 2-m temperature and 10-m wind; the response is larger in a wet year for 2-m water vapor mixing ratio, convective available potential energy, vertically integrated moisture fluxes and surface precipitation. For EFTs with high productivity and a weak seasonal cycle, the nearsurface temperature during the spring of 1988 and 1998 increased by as much as 1°C in the central and western portions of La Plata Basin. Additionally, for higher productivity EFTs, precipitation differences were generally positive in both dry and wet years, although the patterns are not uniform and exhibit certain patchiness with drier conditions.

Short-range forecast of heavy precipitation and strong wind using the convection-allowing WRF models

The results are stated of estimation of short-range forecasting of heavy precipitation (P ≥ 10 mm/12 hours) and strong wind at the height of 10 m (V ≥ 10 m/s) using three nonhydrostatic models from the WRF family: ARW, ARW Glob, and NMM. The forecasts on the basis of all three models were carried out using two grids. The horizontal resolution of external grids varied from 9 to 16.5 km and that of the nested grids, from 3 to 5.5 km. For the ARW and NMM models, the values at side boundaries of external nested grids were taken from the forecasts on the basis of the global GFS NCEP model and for the ARW Glob model, from the global forecasts based on this model. The convection parameterization was turned off at nested grids for all models. The forecasts of heavy precipitation and strong wind at nested grids over the European territory of Russia were estimated from the radar and station measurements in summer 2008. It is obtained that all three models reproduce well enough the mesoscale convective systems and associated areas of heavy precipitation and strong wind but they have common shortcomings: they overestimate the amount and area of heavy precipitation and underestimate the strong wind speed. To a lesser degree, these shortcomings are typical of the ARW model. The possible reasons for systematic errors in the forecasts are discussed.

Developing an air quality modeling system for Qatar

Abstract The Qatar Air Quality Modelling System (QAQM) was developed by Qatar Petroleum (QP) and Total Research Centre - Qatar (TRC-Q) to elucidate the causes of the high ozone levels in Qatar's lower atmosphere. Ozone formation is a complex phenomenon involving primary pollutants (nitrogen oxides and volatile organic compounds), solar radiation and local meteorological conditions. The pollutant's long lifetimes underpin the need to consider their impact at regional, as well as local, scales. Meteorological simulations are obtained using well-known MM5 software. Using nested grids, simulations are first run at a large scale with coarse resolution and then refined over the Gulf and Qatar scales in successive iterations. This approach takes into account trans-boundary dispersion of pollutants along the Gulf, as well as local phenomena with a higher resolution (sea-breeze, local turbulence, etc.). A comprehensive emission inventory of ozone precursors has also been constructed at the regional and local scales. Various methodologies have been employed covering all activity sectors and scales (direct knowledge of industrial processes, statistical data combined with emission factors and road traffic emissions models). This emission inventory coupled with Geographical Information Systems produces a geo-referenced database of primary pollutants which serves as an input for the modelling platform. The CHIMERE photochemical model uses the dispersion and emission inventory previously generated as inputs, and then simulates the chemical reactions between pollutants at all steps, in space and time. The outputs from the model simulations were validated against meteorology and air quality data collected from various monitoring stations in Qatar. They showed a global acceptable agreement. This study has also improved the scientific knowledge on processes involved in the ozone cycle in the Gulf region. The QP QAQM platform can be used as a planning tool to assess the environmental impact of new industrial activities. It is also a suitable basis for further developments in the field of air quality monitoring, forecasts and mitigation. TRC-Q is currently building on this successful experience to develop new technologies as an answer to the environmental challenges resulting from the extraordinary growth currently enjoyed by Qatar.

A Two-Dimensional Wave Prediction Model Along the Best Track of Typhoon Linda 1997

Problem statement: A two-dimensional wave prediction model along the best track of Typhoon Linda 1997 was interested to study the impact of typhoon wind-wave characteristics. The dynamical wave model with deep water condition was used to predict the wave height (Hs) of Typhoon Linda before and after entering into the Gulf of Thailand (GoT). Approach: The standard one-way nested grid for a regional scale of the third generation WAve Model Cycle 4 (WAMC4) is scrutinized in the present study. This model is enabled to solve the spectral energy balance equation on a coarse resolution grid in order to produce boundary conditions for a small area by the nested grid technique along the best track of typhoon. The model takes full advantage of the fine resolution wind fields in space and time produced by the available US Navy Operational Global Atmospheric Prediction System (NOGAPS) model with 1° resolution. The nested grid application was developed in order to gradually increase the resolution from the open ocean towards the South China Sea (SCS) and the Gulf of Thailand (GoT) respectively. Results: The model results were predicted at five stations which were before and during the typhoon entering into the GoT. The wind speeds of the stations 1-5 were in ranges of 5.14-29.81, 4.11-28.27, 0.51-24.67, 0.51-31.35 and 0.51-33.41 m sec-1, respectively. While the Hs of these stations were found in ranges of 0.54-2.99, 0.68-2.85, 0.11-1.57, 0.12-2.92 and 0.09-2.76 m, respectively. The model results were compared with buoy observations at Ko-Chang and Rayong locations in the GoT which were obtained from the Seawatch project. The comparison of those results at Ko-Chang and Rayong showed the percentage errors of 11.20 and 15.12% respectively. Conclusion: The model results presented the relationship of typhoon wind-induced ocean wave at five stations along the best track. The tendency of the Hs from the model in the spherical coordinate propagation with deep water condition in the fine grid domain was in good agreement with the Hs from the observations.

Numerical Dispersion of Gravity Waves

Abstract When atmospheric gravity waves are simulated in numerical models, they are not only dispersive for physical but also for numerical reasons. Their wave properties (e.g., damping or propagation speed and direction) can depend on grid spacing as well as on the numerical schemes. In this work numerical dispersion relations for atmospheric gravity waves are theoretically derived as well as experimentally measured using the anelastic Mesoscale Transport and Stream model (METRAS). Both the theoretical solution and the numerical model show a retardation of gravity waves with decreasing grid resolution. Furthermore, the influence of a Shapiro seven-point filter is analyzed. The Shapiro seven-point filter causes damping of the shorter waves. Therefore, shorter waves can better be simulated without the seven-point filter. The influence of different advection schemes is analyzed by prescribing a background wind. A first-order upstream scheme and second- and third-order flux integrated essentially nonoscillatory (FIENO) schemes are used. As expected, the damping is the smaller the higher the order of the scheme. The numerical dispersion has severe consequences, when nonuniform grid spacing is used. Waves moving from the fine grid to the coarse are reflected because of numerical dispersion if they are only poorly resolved on the coarse grid. In tests with different refinement factors and wave lengths the reflection is found to be the larger the greater the refinement factor. The results show that refinement factors larger than 3 should not be used with nonuniform grid spacing or two-way nested grids.

Simulation of Atmospheric Circulation over Tahiti and of Local Effects on the Transport of 210Pb

Abstract Atmospheric transport of the natural radionuclide 210Pb is simulated by a general circulation model (GCM) and calculated surface concentrations are compared with those recorded at the Tahiti station on a daily scale. Numerical results for 2006 show the underestimation of concentrations for most recorded peaks. The purpose of this paper is to explain the observed discrepancies, to evaluate the GCM physical parameterizations, and to determine by numerical means the concentrations at Tahiti for a pollutant circulating across the South Pacific Ocean. Three meteorological situations in 2006 are further analyzed. Circulation over Tahiti for these periods is simulated by a mesoscale meteorological model using four nested grids with resolutions ranging from 27 to 1 km. The calculated wind fields are validated by those observed at two stations on the northwest coast of Tahiti, which is exposed both to topography-induced vortices and to thermally driven local breezes. Atmospheric dispersion of an offshore plume is then calculated by a particle Lagrangian transport model, driven by the mesoscale model at 1- and 81-km resolutions, representing local and global circulations, respectively. Simulations at 1-km resolution show the complex atmospheric circulation over Tahiti, which results in a large spatial and temporal variability of 210Pb surface concentrations on an hourly scale. The impact of local circulation is, however, limited when daily averaged concentrations at the station are considered. Under the studied regimes, transport simulations at the two resolutions lead to similar daily averaged concentrations. The deficiencies of the GCM in simulating daily averaged 210Pb concentrations could be attributable to the deep convection parameterization.

Mesoscale Atmospheric Modeling Using a High Horizontal Grid Resolution over a Complex Coastal Terrain and a Wine Region of South Africa

Abstract The Regional Atmospheric Modeling System (RAMS) was used to assess local air circulation patterns over the wine-producing Stellenbosch region of South Africa. Numerical simulations using four nested grids (25, 5, and 1 km, and 200 m of horizontal resolution) were performed for each day of February 2000 (during the grape-ripening period) over southern Western Cape Province. Modeled hourly data were extracted from the analysis files and used to produce mean hourly fields (temperature, relative humidity, wind speed, and radiation). Three runs with increasing horizontal resolutions for the finer grid were performed (run 1 with two nested grids of 25 and 5 km; run 2 with three nested grids of 25, 5, and 1 km; run 3 with four nested grids of 25, 5, and 1 km, and 200 m). For each event, the simulations of 1-km and 200-m resolution were superior to the 5-km-resolution simulation, especially in reproducing the local air circulations (sea and slopes breezes) because of a better representation of the local terrain (topography and vegetation cover). The use of a high-resolution grid (200 m) may be of greater value in the identification of potential terrain for viticulture.

A Modeling Study of Typhoon Nari (2001) at Landfall. Part I: Topographic Effects

Although there have been many observational and modeling studies of tropical cyclones, understanding of their intensity and structural changes after landfall is rather limited. In this study, several 84-h cloud-resolving simulations of Typhoon Nari (2001), a typhoon that produced torrential rainfall of more than 1400 mm over Taiwan, are carried out using a quadruply nested–grid mesoscale model whose finest grid size was 2 km. It is shown that the model reproduces reasonably well Nari’s kinematic and precipitation features as well as structural changes, as verified against radar and rain gauge observations. These include the storm track, the contraction and sizes of the eye and eyewall, the spiral rainbands, the rapid pressure rise (∼1.67 hPa h−1) during landfall, and the nearly constant intensity after landfall. In addition, the model captures the horizontal rainfall distribution and some local rainfall maxima associated with Taiwan’s orography. A series of sensitivity experiments are performed in which Taiwan’s topography is reduced to examine the topographic effects on Nari’s track, intensity, rainfall distribution, and amount. Results show that the impact of island terrain on Nari’s intensity is nearly linear, with stronger storm intensity but less rainfall in lower-terrain runs. In contrast, changing the terrain heights produces nonlinear tracks with circular shapes and variable movements associated with different degrees of blocking effects. Parameter and diagnostic analyses reveal that the nonlinear track dependence on terrain heights results from the complex interactions between the environmental steering flow, Nari’s intensity, and Taiwan’s topography, whereas the terrain-induced damping effects balance the intensifying effects of latent heat release associated with the torrential rainfall in maintaining the near-constant storm intensity after landfall.

Synoptic Control of Mesoscale Precipitating Systems in the Pacific Northwest

AbstractThis research examines whether an adequate representation of flow features on the synoptic scale allows for the skillful inference of mesoscale precipitating systems. The focus is on the specific problem of landfalling systems on the west coast of the United States for a variety of synoptic types that lead to significant rainfall. The methodology emphasizes rigorous hypothesis testing within a controlled hindcast setting to quantify the significance of the results. The role of lateral boundary conditions is explicitly accounted for by the study.The hypotheses that (a) uncertainty in the large-scale analysis and (b) upstream buffer size have no impact on the skill of precipitation simulations are each rejected at a high level of confidence, with the results showing that mean precipitation skill is higher where low analysis uncertainty exists and for small nested grids. This indicates that an important connection exists between the quality of the synoptic information and predictability at the mesoscale in this environment, despite the absence of such information in the initialization or boundary conditions. Further, the flow-through of synoptic information strongly constrains the evolution of the mesoscale such that a small upstream buffer produces superior results consistent with the higher quality of the information crossing the boundary. Some preliminary evidence that synoptic type has an influence on precipitation skill is also found. The implications of these results for data assimilation, forecasting, and climate modeling are discussed.

Binary Formation in Star-forming Clouds with Various Metallicities

Cloud evolution for various metallicities is investigated by three-dimensional nested grid simulations, in which the initial ratio of rotational to gravitational energy of the host cloud \beta_0 (=10^-1 - 10^-6) and cloud metallicity Z (=0 - Z_\odot) are parameters. Starting from a central number density of n = 10^4 cm^-3, cloud evolution for 48 models is calculated until the protostar is formed (n \simeq 10^23 cm^-3) or fragmentation occurs. The fragmentation condition depends both on the initial rotational energy and cloud metallicity. Cloud rotation promotes fragmentation, while fragmentation tends to be suppressed in clouds with higher metallicity. Fragmentation occurs when \beta_0 > 10^-3 in clouds with solar metallicity, while fragmentation occurs when \beta_0 > 10^-5 in the primordial gas cloud. Clouds with lower metallicity have larger probability of fragmentation, which indicates that the binary frequency is a decreasing function of cloud metallicity. Thus, the binary frequency at the early universe (or lower metallicity environment) is higher than at present day (or higher metallicity environment). In addition, binary stars born from low-metallicity clouds have shorter orbital periods than those from high-metallicity clouds. These trends are explained in terms of the thermal history of the collapsing cloud.

A High-Resolution Modeling Study of the 24 May 2002 Dryline Case during IHOP. Part II: Horizontal Convective Rolls and Convective Initiation

Abstract In Part I of this paper, the timing and location of convective initiation along a dryline on 24 May 2002 were accurately predicted, using a large 1-km-resolution nested grid. A detailed analysis of the convective initiation processes, which involve the interaction of the dryline with horizontal convective rolls, is presented here. Horizontal convective rolls (HCRs) with aspect ratios (the ratio of roll spacing to depth) between 3 and 7 develop in the model on both sides of the dryline, with those on the west side being more intense and their updrafts reaching several meters per second. The main HCRs that interact with the primary dryline convergence boundary (PDCB) are those from the west side, and they are aligned at an acute angle with the dryline. They intercept the PDCB and create strong moisture convergence bands at the surface and force the PDCB into a wavy pattern. The downdrafts of HCRs and the associated surface divergence play an important role in creating localized maxima of surface convergence that trigger convection. The downward transport of westerly, southwesterly, or northwesterly momentum by the HCR downdrafts creates asymmetric surface divergence patterns that modulate the exact location of maximum convergence. Most of the HCRs have a partially cellular structure at their mature stage. The surface divergence flows help concentrate the background vertical vorticity and the vorticity created by tilting of environmental horizontal vorticity into vortex centers or misocyclones, and such concentration is often further helped by cross-boundary shear instability. The misocyclones, however, do not in general collocate with the maximum updrafts and, therefore, the points of convective initiation, but can help enhance surface convergence to their south and north. Sequences of convective cells develop at the locations of persistent maximum surface convergence, then move away from the source with the midlevel winds. When the initial clouds propagate along the convergence bands that trigger them, they grow faster and become more intense. While the mesoscale convergence of dryline circulation preconditions the boundary layer by deepening the mixed layer and lifting moist air parcels to their LCL, it is the localized forcing by the HCR circulation that determines the exact locations of convective initiation. A conceptual model summarizing the findings is proposed.

Collapse and fragmentation of rotating magnetized clouds - II. Binary formation and fragmentation of first cores

Subsequent to Paper I, the evolution and fragmentation of a rotating magnetized cloud are studied with use of three-dimensional magnetohydrodynamic nested grid simulations. After the isothermal runaway collapse, an adiabatic gas forms a protostellar first core at the centre of the cloud. When the isothermal gas is stable for fragmentation in a contracting disc, the adiabatic core often breaks into several fragments. Conditions for fragmentation and binary formation are studied. All the cores which show fragmentation are geometrically thin, as the diameter-to-thickness ratio is larger than 3. Two patterns of fragmentation are found. (1) When a thin disc is supported by centrifugal force, the disc fragments into a ring configuration (ring fragmentation). This is realized in a rapidly rotating adiabatic core as Ω > 0.2t−1ff, where Ω and tff represent the angular rotation speed and the free-fall time of the core, respectively. (2) On the other hand, the disc is deformed to an elongated bar in the isothermal stage for a strongly magnetized or rapidly rotating cloud. The bar breaks into 2–4 fragments (bar fragmentation). Even if a disc is thin, the disc dominated by the magnetic force or thermal pressure is stable and forms a single compact body. In either ring or bar fragmentation mode, the fragments contract and a pair of outflows is ejected from the vicinities of the compact cores. The orbital angular momentum is larger than the spin angular momentum in the ring fragmentation. On the other hand, fragments often quickly merge in the bar fragmentation, since the orbital angular momentum is smaller than the spin angular momentum in this case. Comparison with observations is also shown.

Interaction of binary tropical cyclones in a coupled tropical cyclone‐ocean model

The motion and evolution of binary tropical cyclones was investigated using a coupled tropical cyclone‐ocean movable nested grid model. The model consists of eight‐layer atmospheric and seven‐layer ocean primitive equation models. Several regimes of binary storm interaction have been identified, depending on the initial separation distance (d) and differences in storm strengths. At d less than a few hundred kilometers, interacting storms experienced complete merger (CM) or partial merger (PM). At larger d (between about 600 km and 1000 km), three regimes of storm interaction have been found: PM, straining out (SO), characterized by complete disintegration of the weaker storm, and mutual straining out (MSO), characterized by weakening and dissipation of both storms. SO occurred when the interacting storms had substantially different intensities and strengths. MSO was observed when the interacting storms were comparable in size and intensity. In the latter case the storms were unable to approach each other at distances smaller than a certain minimum distance (of about 450–500 km) without being mutually stretched out. Moreover, initial attraction of the storms in this regime was replaced by repulsion, as frequently observed in the western Pacific. At d exceeding about 1000 km, elastic interaction (EI) was found, when the storms interact without any significant changes in their intensity and structure. In additional experiments with a conditional instability of the second kind (CISK) type parameterization of convective heating the storm interaction was very different: The storms were nearly axisymmetric and very compact, and they continued approaching each other until they merged. Thus more realistic simulations of binary storm interaction can be achieved by using a physically more reasonable convective parameterization.

The spatial and temporal distribution of the grain aphid Sitobion avenae in winter wheat

AbstractThe spatial and temporal distribution of the grain aphid Sitobion avenae F. (Homoptera: Aphididae) was studied within a field of winter wheat during the summer of 1996. Sampling was done using four nested grids comprising 133 locations. Analysis by Taylor's power law gave results typical for insect populations. Analysis by SADIE (Spatial Analysis by Distance Indices) showed spatial pattern due to edge effects and sampling scale, and positive but mild spatial association, although spatial patterns were ephemeral. Reasons for these findings and the implications for integrated crop management are discussed.

A Comparison of Precipitable Water Vapor Estimates by an NWP Simulation and GPS Observations

Abstract Simulated time series of the total precipitable water (PW) vapor from a limited area numerical weather prediction model are compared to estimates derived from observations done with ground-based Global Positioning System (GPS) receivers. The model data examined are from the delayed-mode High Resolution Limited Area Model (HIRLAM) data assimilation (reanalysis) and the short-range forecasts on double nested grids. The observational data are derived from GPS measurements at 25 sites in Sweden and Finland over a 4-month period, August–November 1995. In general, the HIRLAM reanalysis system demonstrates considerable skill in reproducing the spatial and temporal evolution of the PW as depicted by the GPS estimations. Using a 0.2° horizontal resolution and 31 vertical levels, the HIRLAM reanalysis generates a PW time series that has, in comparison to that of the GPS estimates, an average offset of −0.1 mm and a root-mean-square difference of 2.4 mm. The average correlation between the PW time series fr...

Modelling study of the IOP2 cold front of the FRONTS 87 experiment

On 11–12 November 1987, a frontal system associated with an intense narrow cold frontal rainband was observed over the Channel between England and France. This case constitutes intensive observational period 2 (IOP2) of the FRONTS 87 experiment. It was an ideal example of upper-level and low-level jet coupling with substantial ascending motions and precipitation. The frontal system is analysed, in this paper, using the mesoscale non-hydrostatic model CSU/RAMS. The model uses a twoway interactive nested grid structure and full cloud microphysics parameterisation at various levels of complexity. The model results are compared with previous numerical investigations of this case as well as with observations obtained from the experimental network deployed during the campaign. The model succeeded in representing the frontal structure at meso- and convective scales, taking into account the large scale forcing; thus it is an important tool for the study of secondary cyclogenesis observed during the FASTEX campaign. Copyright © 1998 Royal Meteorological Society

A nested grid refinement technique for magnetohydrodynamical flows

A grid refinement technique for high resolution magnetohydrodynamical flows is described. It is based on the use of multiple nested grids having successively higher resolution. The numerical algorithm is formulated in such a way that the advection part of the hydrodynamical equations are solved in conservative form on the whoel integration domain. In particular, the divergence free constraint for the magnetic field, ▿ · B = 0 , is fulfilled during the simulation. Several numerical tests are presented demonstrating the applicability of the nested grid code to many astrophysical problems including phenomena in which steep gradients such as shocks or contact discontinuities appear.

Use of multiply nested grids for the solution of flux-limited radiation diffusion and hydrodynamics

Local grid refinement techniques allow improved spatial resolution for a large class of astrophysical problems involving radiation hydrodynamics. We describe here the implementation of multiply nested grids in a 2D radiation hydrodynamic code. The basic hydrodynamic code is explicit (except for the equation of energy conservation), spatially second-order, invokes artificial viscosity for the treatment of shocks, and through the use of operator-splitting is greater than first-order accurate in time. Radiation transfer is treated in the flux-limited diffusion approximation and solved implicitly in a separate substep. Tests and the following astrophysical applications are presented and briefly discussed: (a) expansion of a blast wave through several nested grids; (b) protostellar collapse with the formation of a circumstellar disk around a central source; (c) the interaction of circumstellar disk with a hot, hydrogen-ionizing star with a stellar wind.

Simulation and Analysis of Tornado Development and Decay within a Three-Dimensional Supercell Thunderstorm

Abstract A three-dimensional numerical simulation using a two-way interactive nested grid is to study tornado-genesis within a supercell. During a 40-minute period, two tornadoes grow and decay within the storm's mesocyclone. The tornadoes have life spans of approximately 10 minutes. Maximum ground-relative surface wind speeds exceed 60 m s−1 during both tornadoes, and horizontal pressure gradients reach 18 hPa km−1 during the second tornado. Comparison of the simulated storm evolution with Doppler and field observations of supercells and tornadoes shows many similar features. Vertical vorticity in the mesocyclone and the tornado vortex at low levels is initially created by the tilting of the environmental vorticity and baroclinically generated vorticity along the forward gland gust front of the storm. Tornadogenesis is initiated when mesocyclone rotation increase above cloud base. The increased rotation generates lower pressure in the mesocyclone, increasing the upward pressure gradient forces. The upwar...

Large eddy simulation of microburst winds flowing around a building

A large eddy simulation of a microburst producing thunderstorm is carried out. The thunderstorm is initiated when a thermal within a developing mixed layer reaches the lifting condensation level and strong latent heating occurs. A microburst is subsequently produced as condensate from the thunderstorm falls beneath the melting level. In this study, the viability of using two-way interactive multiple nested grids to investigate the interaction of the outflow from the microburst with a much smaller scale architectural structure is investigated. The relationship of the fluctuating winds around the building to the microburst structure is described.