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An example demonstrates this. Say you wanted to defined a straight-line cross-section from 60S/25E to 60S/155W. This would be specified as -60/25/-120/25. The fact that one of the latitude values is below -90 indicates to Metview that a cross-section going through the South Pole is desired. Once this has been established, the fact that the two longitude values are identical tells Metview to use a straight line through the pole. If this is the intent, then only one unique longitude value is required, as the other one can be deduced. Giving Metview two different longitude values will cause a cross-section consisting of two curves to be produced.
Wind Parallel
Setting When this option to On will produce a cross section plot of the projection of the wind is On, the wind components are projected onto the cross section plane (going from a 3D wind field to a 2D wind field projection). This is plotted using wind arrows. Valid . For 2D wind the result is a signed scalar data (a contour plot). When 3D wind data are available a vector plot is produced with the vertical component scaled/computed as specified in parameter W Wind Scaling Factor Mode. Valid values are On/Off.
Wind Perpendicular
Setting When this option to On will produce a cross section plot showing the projection of the horizontal wind components onto the direction perpendicular to is On, the wind components are projected onto the normal vector of the cross section plane. The result is a one-dimensional quantity and is plotted with contour lines. Also produced is another cross section plot of the W component of the wind. a signed scalar data (a contour plot). Valid values are On/Off. This cannot be set to On if Wind Parallel is also On.
Wind Intensity
Setting When this option is On the result depends on other settings:
- When both Wind Parallel and Wind Perpendicular are Off, the result is the length of the 2D/3D wind vector at the cross section plane
- When Wind Parralel is On, the result is the absolute value of the projected wind onto the cross section plane
- When Wind Perpendicular is On, the result is the absolute value of the wind projected onto the normal vector of the cross section plane
to On will produce a cross section plot of the scalar wind intensity, plotted with contour lines. If one of the previous two parameters is On, then the intensity is that of the specified projection. Also produced is another cross section plot of the W component of the wind. Valid values are On/Off.
Lnsp Param
Specifies the ecCodes paramId of the LNSP data. The default value is 152 (as used by ECMWF).
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Specifies the ecCodes paramId of the vertical wind component data. The default value is 135 i.e. pressure velocity (as used by ECMWF).
T Param
Specifies the ecCodes paramId of the temperature data used in the vertical wind computations when W Wind Scaling Factor Mode is set to Compute. The default value is 130 (as used by ECMWF).
Horizontal Point Mode
Specifies how the geographical points along the input transect line will be computed. Valid values are Interpolate and Nearest Gridpoint. Setting this option to Interpolate will create a regular set of interpolated geographical points along the transect line. Setting this option to Nearest Gridpoint will instead select the nearest points from the data.
Vertical Coordinates
Setting this option to User will enable the use of general height-based coordinates. In this mode, additional GRIB fields should be supplied (one per model level) where the values of the grid points represent the heights of their locations. Valid values are Default andUser.The default value is Default.
Vertical
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Coordinate Param
Setting this option to User will enable the use of Specifies the ecCodes paramId of the general height-based coordinates . In this mode, additional GRIB fields should be supplied (one per level) where the values of the grid points represent the heights of their locations. Valid values are Default andUserif Vertical Coordinates is set to User.
Vertical Coordinate Extrapolate
When it is set to On the data is extrapolated onto extra levels below and above the actual height range. The possible values are On and Off. The default value is Default.
Vertical Coordinate Param
Specifies the ecCodes paramId of the general height-based coordinates if Vertical Off. This parameter is only enabled when Vertical Coordinates is set to User.
W Wind Scaling Factor Mode
Specifies the algorithm to compute the scaling factor for representation of the vertical wind component . Valid (defined as W Wind Param). The valid values are Automatic and User. Setting this option to Automaticwill compute a scaling factor taking into consideration as follows:
- Automatic: the values are scaled by a factor based on the geographical area, the top/bottom pressure levels and the size of the plot window
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- User: the values are scaled by the factor defined via parameter W Wind Scaling Factor.
- Compute: in this mode, supposing that W Wind Param defines the the pressure velocity, the vertical wind component in m/s is computed using the following hydrostatic formula:
| Mathdisplay |
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w = - \frac{\omega R T}{p g} |
where:
- ω: pressure velocity (Pa/s)
- p : pressure on (Pa)
- T: temperature (K)
- R: gas constant, 287.058 J kg-1 K-1
- g: gravitational acceleration, 9.81 m/s2
To make this formula work, the input data have to be specified either on pressure levels or on model levels together with LNSP. The temperature's paramId is defined by T Param. When temperature is not available, the computations still work but T is replaced by a constant 273.16 K value in the formula. Having computed the vertical wind component, a scaling with the factor defined by W Wind Scaling Factor is still applied to the resulting values.
The default value is Automatic.
W Wind Scaling Factor
Specifies the vertical wind scaling factor if W Wind Scaling Factor Mode is set to User or Compute. The default values is -100.
Level Selection Type
Specifies the method to define the output pressure levels when converting model level data to pressure levels. Options are:
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Specifies the number of output pressure levels if Level Selection Type is set to Count.
Vertical Scaling
Specifies the type of vertical axis - Linear or Log. Only available if Level Selection Type is set to Count.
Bottom Level
Specifies the lower limit of the cross section, i.e. the bottom pressure level (hPa). Only available if Level Selection Type is set to Count.
Top Level
Specifies the upper limit of the cross section, i.e. the top pressure level (hPa). Only available if Level Selection Type is set to Count.