# Tag Info

## Hot answers tagged numerical-modelling

18

Relatively important, depending on the basic principles of the modeling you are interested in, and to what extent you want to get yourself involved in it. In many areas of computational geophysics, e.g. atmospheric, oceanic, hydrological modeling etc., there are modelers, modelers, and modelers. First group of modelers get model output from somebody else, ...

15

I've been a WRF user for almost 5 years now, and contributed code to a recent public release. I am not aware that WPS (WRF Preprocessing System) has such a tool that takes in the grid and point coordinates and returns the appropriate index. However, it is very straightforward to do so yourself. Some suggest using an external library, I think that may be an ...

15

The hydrostatic approximation begins with the full 3-D momentum equation (Navier-Stokes) and through scale analysis the vertical momentum equation reduces to: $$\dfrac{\partial p}{\partial z} = -\rho g$$ This is a balance between the vertical pressure gradient force and gravity with no net acceleration. This tends to hold for atmospheric phenomena that ...

14

The biggest difference between LES and RANS is that, contrary to LES, RANS assumes that $\overline{u'_i} = 0$ (see the Reynolds-averaged Navier–Stokes equations). In LES the filter is spatially based and acts to reduce the amplitude of the scales of motion, whereas in RANS the time filter removes ALL scales of motion with timescales less than the filter ...

13

You are looking for the drag force imposed by the fluid flow on an object: $$F = \dfrac{1}{2} \rho v^{2} C_D A$$ where $\rho$ is the density of the fluid, $v$ is the speed of the flow, $C_D$ is the drag coefficient depending on the shape of the object, and $A$ is the surface area normal to the fluid flow. Let's plug in some characteristic numbers for ...

13

For fundamentals on numerical methods applied in atmosphere and ocean modeling, see the GARP publication by Mesinger and Arakawa: Numerical Methods Used in Atmospheric Models. This old, but certainly not outdated text provides the basics on time and space discretization and stability, with particular focus on solving advection equations, and gravity-inertia ...

12

The stability correction factor ASF is related to the effects of atmospheric stability (function of buoyancy and shear) on wave growth, and has been implemented in Wavewatch3 in the Tolman and Chalikov 1996 input source term. The code where the correction happens can be found in w3updtmd.ftn: ! 5. Stability correction ( !/STAB2 ) ! Original settings : !...

11

Yes, it makes a difference. In this case the grid-registered dataset is higher fidelity. However, the cell-registered is easier to handle. As you see, both types of data are available on the dataset's page. The page states: The grid-registered is the authoritative registration. The page also points out: The cell-registered is derived from the grid-...

11

A model is a simplified representation of a system. Rather than try to model everything down to the microscopic level, or as vectors, we often aggregate phenomena across geographic areas, to simplify the computation a lot, and to lose only a little accuracy and precision. So if I want to model the UK, which roughly spans 8°W - 2°E, 50°N - 60°N, that's an ...

11

This is my favorite example of the difference between a hydrostatic and a non-hydrostatic code. The simulation depicts a lock exchange which you can picture as opening your window if you live in a cold place and it is winter. Inside your house, presumably, the air is warmer than the outside. So when you open the window, the cooler (heavier) air will ...

10

Yes, there were many model coupling projects in the past, as well as many ongoing coupling projects in the present and near future. The main motivation behind model coupling is the need for the interactive feedback processes between two or more separate physical systems, for example, atmosphere and ocean. Historically, these models have been developed ...

10

Coupler software (e.g. OASIS, MCT, C-Coupler) is frequently used to combine these different components of earth system models. Of course interfacing models with each other using these couplers or other methods always needs some technical work and sometimes also some algorithmic work. Therefore the number of model combinations will always be limited.

9

This question might be better suited for GIS stack exchange... but I'll take a shot: dx and dy are the distance per grid cell in the projected grid units, which is part of your grid definition. ref_lat and ref_lon are part of your projection definition. i_parent_start and j_parent_start are the horizontal grid cell numbers of the parent grid, where your ...

8

In Earth-system models, the term "coupling" is often used in two ways. The first is "offline" coupling, where output from one model is passed to another model for computation of some variable. The second is "online" coupling, where the feedbacks are allowed to pass between the two models. Online coupling is synonymous with "fully coupled". A good ...

8

I am posting my code that calculates i_parent_start and j_parent_start. It is in Java and as people are aware Java does not have functions such as minloc or Python's unravel_index. So I divided my program into three steps. Read in the xlat_m and xlong_m arrays using java netcdf. Subtracting desired lat and desired lon from each value of the 2-dimensional ...

8

The answer by @IsopycnalOscillation explains some implications of using explicit and implicit schemes and in what applications one is preferred over the other. Here, I describe how these two methods actually work, and how does the implicit scheme uses knowledge of the later state. Take the ordinary differential equation: $$\dfrac{\partial u}{\partial t} = ... 7 Implicit and explicit methods have the same differences no matter what context. The building blocks from which these methods are constructed are the same, they all use Taylor series expansion of a function. Of course, there are many different numerical methods, explicit and implicit, with different degrees of numerical accuracy, consistency and stability. ... 7 Presume that the atmosphere can be modeled with waves of different frequencies. Therefore, the atmosphere can be written as a series of sine and cosine functions with different frequencies and coefficients. Consider the linear advection equation:$$\frac{\partial u}{\partial t}= -c \frac{\partial u}{\partial x} Consider the assumption that $u$ can be ...

6

This is a big question, but there are benefits and difficulties with all representations, and the efficacy of any coordinate will also depend upon other numerical choices such as the calculation of horizontal pressure gradients [Zängl 2012]. There are broadly two categories: terrain following layers and Cartesian systems such as step terrain and cut cell ...

6

There are lots of studies considering Lagrangian dispersion. One critical aspect is the addition of random displacement to the deterministic particle track in order to consider subgrid-scale processes. Your measure of dispersion seems acceptable except for the fact that it might be dependent on the resolution of your flow field. In order to compare your ...

6

Try NCL. It has a package for WRF-related functions, including the bidirectional grid-coordinates conversion routine. To convert coordinate pairs to grid indices, follow the documentation here: wrf_user_ll_to_ij. A backward (i.e. gird to coordinate) routine can be found here: wrf_user_ij_to_ll. They are quite handy. For your scenario, you may load your ...

6

Coupling refers to communication and interchange of information between models. For example a typical standalone atmospheric model may be initialized with information about sea surface temperature. Depending on the model the ocean may be treated as a heat reservoir and may or may not affect any change to the SST. The ocean itself is not modeled. ...

6

First download the esmf tar file from http://www.earthsystemmodeling.org/esmf_releases/public/ESMF_7_1_0r/reg/ESMF_Framework_Down.html Then install netcdf library and gfortran compiler using sudo apt-get install git tcsh pkg-config sudo apt-get install gfortran sudo apt-get install netcdf-bin libnetcdf-dev libnetcdff-dev sudo apt-get install openmpi-bin ...

5

Coupling is used in the context of feedback between various processes. It can be weak or strong. E.g, if you heat a rod it expands mechanically due to thermoelastic effects (strong coupling) but when you mechanically compress it the temperature rise is insignificant (weak coupling). On the other hand for a poroelastic material the coupling is strong both ...

5

Specifically the term the values are archived on 2.5° grid means that the values that are being reported are being saved and are available at that resolution for historical values. The higher resolutions (1°x1°, 1/4°x1/4°) mean exactly what they say: the values are reported on a 2-dimensional grid every degree, or quarter degree, across the area of ...

5

To add a bit more mathematical conception to the idea of a grid: think about a 2-D grid(X,Y) as a matrix: this matrix represents a solution to the system of equations you are trying to solve at grid point x,y (individual points on X,Y respectively). Why is it important to make this distinction? Because knowing the resolution, span, regularity and structure ...

4

The hydroxyl radical chemistry is not modeled explicitly in air quality models, so numerical stability is not an issue. Instead, OH is held in pseudo-steady-state. For instance, see CMAQ's documentation which states: In CMAQ's gas phase, OH is assumed to be in pseudo-steady state, and it is not a transported species. This is because hydroxyl radical ...

4

I did my undergraduate capstone on observational moisture changes around cities. It's an interesting field, though it certainly seems fairly challenging to draw out and explain changes given the incredible spatial and temporal variability. So that's definitely one of the challenges to it. Another is the current observational density, as there are very few ...

3

I got in touch with the author of the dataset with my remaining concerns, who responded surprisingly quickly. Here is the essence of everything I learned from people I talked to, including kwinkunks, Simon W, and the author of the dataset, and is intended to augment the existing answers. For both the grid registration and the cell registration, the ...

3

Disclaimer: I have no special knowledge of this data set. But from reading the linked page, which I agree is not very clear, here's my understanding: As you have stated, the data points actually represent cells with a finite area rather than points. The difference between the two approaches is whether the location of the cell is identified by giving the ...

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