Home About us Contact
|
Home About us Contact | ||
|
|
||
Convection Schemes (convection + scheme)
Selected AbstractsA combination of implicit and adaptative upwind tools for the numerical solution of incompressible free surface flowsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 6 2007V. G. Ferreira Abstract This paper is concerned with the numerical solutions of time dependent two-dimensional incompressible flows. By using the primitive variables of velocity and pressure, the Navier,Stokes and mass conservation equations are solved by a semi-implicit finite difference projection method. A new bounded higher order upwind convection scheme is employed to deal with the non-linear (advective) terms. The procedure is an adaptation of the GENSMAC (J. Comput. Phys. 1994; 110:171,186) methodology for calculating confined and free surface fluid flows at both low and high Reynolds numbers. The calculations were performed by using the 2D version of the Freeflow simulation system (J. Comp. Visual. Science 2000; 2:199,210). In order to demonstrate the capabilities of the numerical method, various test cases are presented. These are the fully developed flow in a channel, the flow over a backward facing step, the die-swell problem, the broken dam flow, and an impinging jet onto a flat plate. The numerical results compare favourably with the experimental data and the analytical solutions. Copyright © 2006 John Wiley & Sons, Ltd. [source] A convection scheme for data assimilation: Description and initial testsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 606 2005Philippe Lopez Abstract A new simplified parametrization of subgrid-scale convective processes has been developed and tested in the framework of the ECMWF Integrated Forecasting System for the purpose of variational data assimilation, singular vector calculations and adjoint sensitivity experiments. Its formulation is based on the full nonlinear convection scheme used in ECMWF forecasts, but a set of simplifications has been applied to substantially improve its linear behaviour. These include the specification of a single closure assumption based on convective available potential energy, the uncoupling of the equations for the convective mass flux and updraught characteristics and a unified formulation of the entrainment and detrainment rates. Simplified representations of downdraughts and momentum transport are also included in the new scheme. Despite these simplifications, the forecasting ability of the new convective parametrization is shown to remain satisfactory even in seasonal integrations. A detailed study of its Jacobians and the validity of the linear hypothesis is presented. The new scheme is also tested in combination with the new simplified parametrization of large-scale clouds and precipitation recently developed at ECMWF. In contrast with the simplified convective parametrization currently used in ECMWF's operational 4D-Var, its tangent-linear and adjoint versions account for perturbations of all convective quantities including convective mass flux, updraught characteristics and precipitation fluxes. Therefore the new scheme is expected to be beneficial when combined with radiative calculations that are directly affected by condensation and precipitation. Examples are presented of applications of the new moist physics in 1D-Var retrievals using microwave brightness temperature measurements and in adjoint sensitivity experiments. Copyright © 2005 Royal Meteorological Society. [source] The diurnal cycle of shallow cumulus clouds over land: A single-column model intercomparison studyTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 604 2004Geert Lenderink Abstract An intercomparison study for single-column models (SCMs) of the diurnal cycle of shallow cumulus convection is reported. The case, based on measurements at the Atmospheric Radiation Measurement program Southern Great Plains site on 21 June 1997, has been used in a large-eddy simulation intercomparison study before. Results of the SCMs reveal the following general deficiencies: too large values of cloud cover and cloud liquid water, unrealistic thermodynamic profiles, and high amounts of numerical noise. Results are also strongly dependent on vertical resolution. These results are analysed in terms of the behaviour of the different parametrization schemes involved: the convection scheme, the turbulence scheme, and the cloud scheme. In general the behaviour of the SCMs can be grouped in two different classes: one class with too strong mixing by the turbulence scheme, the other class with too strong activity by the convection scheme. The coupling between (subcloud) turbulence and the convection scheme plays a crucial role. Finally, (in part) motivated by these results several models have been successfully updated with new parametrization schemes and/or their present schemes have been successfully modified. © Royal Meteorological Society, 2004. S. Irons's contribution is Crown copyright [source] Uncertainties in future projections of extreme precipitation in the Indian monsoon regionATMOSPHERIC SCIENCE LETTERS, Issue 3 2009A. G. Turner Abstract Uncertainties in changes to the spatial distribution and magnitude of the heaviest extremes of daily monsoon rainfall over India are assessed in the doubled CO2 climate change scenarios in the IPCC Fourth Assessment Report. Results show diverse changes to the spatial pattern of the 95th and 99th subseasonal percentiles, which are strongly tied to the mean precipitation change during boreal summer. In some models, the projected increase in heaviest rainfall over India at CO2 doubling is entirely predictable based upon the surface warming and the Clausius,Clapeyron relation, a result which may depend upon the choice of convection scheme. Copyright © 2009 Royal Meteorological Society and Crown Copyright [source] Higher-resolution convection schemes for flow in porous media on highly distorted unstructured gridsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 8 2008Sadok Lamine Abstract Higher-resolution schemes are presented for convective flow approximation on highly distorted unstructured grids. The schemes are coupled with continuous full-tensor Darcy-flux approximations. A sequence of non-uniform and distorted grid formulations are developed and compared for a range of unstructured meshes with variable grid spacing. The higher-order schemes are constructed using non-uniform grid slope limiters such that they are stable with a local maximum principle, ensuring that solutions are free of spurious oscillations. Benefits of the resulting schemes are demonstrated for classical test problems in reservoir simulation including cases with full-tensor permeability fields. The test cases involve a range of unstructured grids with variations in grid spacing, orientation and permeability that lead to flow fields that are poorly resolved by standard simulation methods. The higher-order formulations are shown to effectively reduce numerical diffusion, leading to improved resolution of concentration and saturation fronts. Copyright © 2008 John Wiley & Sons, Ltd. [source] A simple strategy for constructing bounded convection schemes for unstructured gridsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 10 2004Peter L. Woodfield Abstract Due to the great geometrical flexibility, popularity for unstructured grid methods in fluid dynamics has been increasing in recent years. In parallel with this interest there is a need for bounded second or higher order convection schemes which can be implemented easily in the unstructured setting. In the present work a simple strategy for achieving convective boundedness in the context of a vertex-centered unstructured finite volume algorithm is demonstrated. Testing is carried out on an inviscid oblique step problem using both structured and unstructured grid arrangements. Further testing for numerical diffusion is done using a distorted grid in a two dimensional channel. The proposed scheme is straightforward to implement and is found to perform well for the cases considered. The overall algorithm converges well and the limiter appears to introduce little extra numerical diffusion beyond that inherently present in the base scheme. Copyright © 2004 John Wiley & Sons, Ltd. [source] Sensitivity of the regional climate of East/Southeast Asia to convective parameterizations in the RegCM3 modelling system.INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 14 2008Part 1: Focus on the Korean peninsula Abstract This study investigates the capability of the regional climate model, RegCM3, to simulate fine-scale regional climate over a narrow peninsula or archipelago. The model is run in one-way double-nested mode with one mother domain and two nested domains. The mother domain encompasses the eastern and southern regions of Asia and adjacent oceans with a grid spacing of 60 km. The first nested domain focuses on the Korean peninsula and the second one covers the Philippine archipelago with a grid spacing of 20 km. The simulation spans a period of 5 years and 1 month, from November 2000 to December 2004. The sensitivity of the two convection schemes, namely, the Grell scheme (Grell) and the MIT-Emanuel scheme (EMU), is studied. Model results obtained with both the Grell and EMU show reasonable performance in capturing the seasonal variation and the spatial characteristics of the East Asian monsoon. However, the Grell simulation appears to have persistent cold and dry biases in the summer season. There is a definite improvement in these model deficiencies by the implementation of EMU. Although the temperature fields in the Grell and EMU simulations are essentially the same in terms of the spatial distribution, the EMU simulation is quantitatively in better agreement with the observed estimates, indicating a substantial reduction in the cold bias. Further, in comparison with the Grell simulation, the EMU simulation shows an improvement in the timing and amplitude of the rain band propagating northward. The spatial distributions of precipitation also have good quality, capturing the localized maxima over Korea. The frequency distributions of daily temperature and precipitation simulated by EMU are closer to observations than those of the Grell simulation. It is found that the convective precipitation derived from different convection parameterizations is a major contributor to the performance of the model in summer. Copyright © 2008 Royal Meteorological Society [source] Sensitivity of moist convection to environmental humidityTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 604 2004S. H. Derbyshire Abstract As part of the EUROCS (EUROpean Cloud Systems study) project, cloud-resolving model (CRM) simulations and parallel single-column model (SCM) tests of the sensitivity of moist atmospheric convection to midtropospheric humidity are presented. This sensitivity is broadly supported by observations and some previous model studies, but is still poorly quantified. Mixing between clouds and environment is a key mechanism, central to many of the fundamental differences between convection schemes. Here, we define an idealized quasi-steady ,testbed', in which the large-scale environment is assumed to adjust the local mean profiles on a timescale of one hour. We then test sensitivity to the target profiles at heights above 2 km. Two independent CRMs agree reasonably well in their response to the different background profiles and both show strong deep precipitating convection in the more moist cases, but only shallow convection in the driest case. The CRM results also appear to be numerically robust. All the SCMs, most of which are one-dimensional versions of global climate models (GCMs), show sensitivity to humidity but differ in various ways from the CRMs. Some of the SCMs are improved in the light of these comparisons, with GCM improvements documented elsewhere. © Crown copyright, 2004. [source] | |||||||||||||