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# Cross Section Data

This icon derives (and returns) a vertical cross section data unit of upper air fields along a specified arbitrary transect line. For each upper air field, point values are interpolated along the transect line, with a spacing consistent with the resolution of the input GRIB data.

The cross section data can be plotted (using a default visualisation and with the plot area based on the range of data values) or saved as a NetCDF data file.

If an orography is plotted it can be customised by applying a Graph Plotting visual definition icon.

If access to the output computed values is not required, or for more control of the plotting, use the Cross Section View icon.

The macro language equivalent is mcross_sect().

## The Cross Section Data Editor

### Data

Specifies the data (GRIB icon) from which to derive the cross-section profile. The input GRIB icon must specify a multi-level meteorological variable, in a latitude-longitude or Gaussian grid. The following vertical coordinates are supported:

• pressure levels
• model levels (hybrid levels used by the IFS). In this case you must include parameter LNSP (logarithm of surface pressure) should you want the orography and the vertical axis of the plot in pressure levels rather than model levels when visualising the output.
• general coordinates (defined by parameter Vertical Coordinates)

If wind arrows are to be plotted, then the input data should include three-dimensional wind data, i.e. the u/v/w wind components should all be present.

If more than one time and/or forecast step is contained in the GRIB icon, it returns a set of cross sections.

### Line

Specifies the coordinates of a transect line along which the cross-section is calculated in lat1/lon1/lat2/lon2 format.

The cross section is calculated from a set of geographical points taken along the input transect line. The point selection takes into consideration the resolution of the data and assures that a minimum of 64 points will be used.

Note that it is possible to define a line through either pole by describing the line’s coordinates as follows:

• First, when specifying the latitudes of the two points, imagine that the latitude values go above 90 when you cross the North Pole and below -90 when you cross the South Pole.
• Next, if you wish a straight line, ensure that the two longitude values are the same as each other.

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

When this option is On, the wind components are projected onto the cross section plane. 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

When this option is On, the wind components are projected onto the normal vector of the cross section plane. The result is 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

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

Valid values are On/Off.

### Lnsp Param

Specifies the ecCodes paramId  of the LNSP data. The default value is 152 (as used by ECMWF).

### U Wind Param

Specifies the ecCodes paramId  of the U wind component data. The default value is 131 (as used by ECMWF).

### V Wind Param

Specifies the ecCodes paramId  of the V wind component data. The default value is 132 (as used by ECMWF).

### W Wind Param

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 level) where the values of the grid points represent the heights of their locations. Valid values are Default and User.The default value is Default.

### Vertical Coordinate Param

Specifies the ecCodes paramId of the general height-based coordinates if Vertical Coordinates is set to User.

### W Wind Scaling Factor Mode

Specifies the representation of the vertical wind component (defined as W Wind Param). The valid values are 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.
• 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:
$w = - \frac{\omega R T}{p g}$

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:

• From Data (default)
• compute the absolute bottom pressure level from the data
• for each model level, compute the average pressure along the cross section line and then use this mean pressure as the vertical pressure co-ordinate for that level
• compute extra levels at the bottom by adding an offset (10 hPa) until it reaches the bottom pressure level, computed previously. This will avoid blank areas in the plot near the orography line.
• Count
• calculate the output pressure levels by taking into account the bottom and top pressure levels (Bottom Level and Top Level) and the given number of levels (Level Count). The computed levels will be evenly spaced on either a linear or a logarithmic scale depending on the value of Vertical Scaling.
• Level List
• use the given list of pressure levels (Level List)

### Level List

Specifies a list of output pressure levels separated by a “/”. Only available if Level Selection Type is set to Level List.

### Level Count

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.

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