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28

All numerical atmospheric models are built around calculations derived from primitive equations that describe atmospheric flow. Vilhelm Bjerknes discovered the relationships and thereby became the father of numerical weather prediction. Conceptually, the equations can be thought of as describing how a parcel of air would move in relationship to its ...


20

Weather models (or, as they are more commonly called in the field, atmospheric models) are computer programs that read in input data (initial conditions) and solve partial differential equations to produce a future state of the atmosphere. Although @JonEricson provides an overall good but anecdotal summary of what models do, here I describe the exact steps ...


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High resolution means that the grid mesh the model uses is fine, or put another way, $dx$, $dy$, $dz$, and $dt$ are small numbers. A model like the HRRR uses a horizontal resolution of 3 km, whereas some of the older regional models used horizontal resolutions of 30 km (though even the NAM is using 5 km these days). Comparing resolutions to GFS and ECMWF ...


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For very quick visual comparison I would use Cube Browser or ncview together with a command line tool like the Climate Data Operators. For quick production of nice looking graphics (and animations) Panoply really makes good job. For further analysis or special graphics keep following your approach and script with things like MATLAB, Python (e.g. with Iris), ...


18

The major differences between weather and climate models are many. At their core lie the same set of primitive equations, but from here there are many differences. A weather model only (skillfully) predicts about 10 days into the future, while a climate model integrates forward in time for hundreds of years. The main difference here is that in a weather ...


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, ...


18

Theoretically, I agree that there should be one instable equilibrium point between the two stable equilibrium points when we have a 2D current field. However, there are some additional components of the problem: Ocean currents are not 2D but 3D currents. Considering only the sea surface might lets field lines disappear. The wind has an additional forcing. ...


18

Yes, you're right, there should be (at least one) saddle point in the middle of the loop. And indeed, if you search for illustrations of Pacific ocean circulation, you can see the saddle point in some of them, like this one: Source: Introduction to Tropical Meteorology, 2nd ed., chapter 3.3.1, fig. 3.20. Now, in a lot of other maps, the saddle region is ...


17

The writers of netCDF, UNIDATA, maintain a pretty extensive list of visualisation software on the netCDF website. It even mentions an Excel add-in, for the masochistic, presumably. Over the years, I've found Ferret to be reliable with CF compliant files (and non-compliant ones, for that matter) and useful for interactive quick looks and simple ...


15

This answer is not complete, but it is a start. One of the most significant differences is: Weather models use measurements, whereas climate models do not Put another way: a weather model is an initial value problem. The initial values that go in are of essential importance for the result to be correct. A climate model solves what is primarily a ...


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 ...


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 ...


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

Your premise is incorrect. Numerical models normally have: A full temperature profile A specific field for the 2 metre temperature A specific field for the skin temperature For example, see ERA-40 daily fields, which both 2 metre temperature and Skin temperature, or CFS reanalysis 6-hour fields, which has Air temperature, Skin temperature, and Surface air ...


13

Yes. Spectral wave models cannot model storm surge because the wave energy balance equation that they integrate does not describe the physical processes associated with storm surge. Wave models solve the wave energy balance equation: $$ \dfrac{\partial E}{\partial t} + \dfrac{\partial (c_gE)}{\partial x} + \dfrac{\partial (\dot{k}E)}{\partial k} + \dfrac{...


13

To understand why the nesting ratio of 3 is preferred to the nesting ratio of 2, it is important to understand the following two features of WRF: 1) Grids are Arakawa C-staggered: mass points are at cell centers, u-velocities are at east-west cell edges, v-velocities are at north-south cell edges. See Mesinger and Arakawa 1976, Chapter 4 for a good ...


12

Almost all finite methods that use forward time models adhere to the Courant-Friedrichs-Lewy law which calculates a courant number and compares it to a $C_{max}$, which is what determines stability, so for 2-D: $$C = \frac {V_xdt}{dx} + \frac{V_ydt}{dy} \ge C_{max}$$ Where $C$ is the courant number, $V_i$ is the velocity in subscripted direction, $dx$ or $...


12

I second ncview for taking a quick look at NetCDF files. I would also recommend trying Unidata's Integrated Data Viewer (IDV). It is great for overlaying geophysical fields in 3-D from different sources. Besides NetCDF, it supports many other formats. It also comes with a pre-loaded listing of various observational and model data repositories through ...


12

This is not a complete answer. One aspect of weather models consists of Data assimilation or 4D-var. I agree that they are amazing, and the question how do they work is too broad to be answered. So I recommend you read up on data assimilation and in particular 4D-Var. Concepts are somewhat similar in inverse theory, but of much higher dimensionality. In ...


12

The main difference between online and offline trajectory calculation is that the online is implemented as part of the model, and the trajectories are part of the model output. Offline trajectory calculation is a separate piece of software that takes Eulerian model output fields as input and calculates the trajectories. The main advantage of online particle ...


12

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

Just to add my point of view; Using ncks you can do many things, i.e. differentiate, get ratio, extract some vars, slice on the dimension etc.. If you want to make some binary operations on netcdf files consider ncbo. For huge files I prefer to cut down what I want at the first place, it does opendap remote as well. Hate matlab so I moved to NCL (NCAR) ...


11

Disclaimer: This is a partial answer given that my background is modeling of the ocean. I hope that some mantle convection modelers can complement this answer. The question is good but the answer is complex. The short answer is: No, they are not the same. Simply because computationally it would not make sense. I will try my best to break it apart and make ...


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Normally I use the following softwares for a quick view of my NetCDF files: NASA Panoply (Panoply netCDF, HDF and GRIB Data Viewer): java based, very good in opening HDF, NetCDF, GRIBs... CDO functions as shaded, contour, etc. (very basic but useful from command-line) MATLAB and R with their basic functions


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 ...


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&...


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The Barotropic vorticity equation is insufficient to model rain. Part of this is due to the word 'Barotropic,' which indicates a lack of thermal gradient. The other reason is that while vorticity is a useful quantity, it lacks explicit information on the atmospheric state. The primitive equations are a system of partial differential equations that are used ...


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