**Z**

(dZ/dlon) is the zonal gradient (dZ/dlat) is the meridional gradient

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The gradient is a physical quantity that describes the rate at which a quantity
changes. Meridional and zonal gradients represent rates of change in the
meridional (latitudinal) and zonal (longitudinal) directions.
Mathematically, these quantities are obtained by applying the del operator to
a variable which is a function of position. Specifically, for variable **Z**

(dZ/dlon) is the zonal gradient (dZ/dlat) is the meridional gradient

NCL **6.4.0** is not released. However, the 6.4.0 beta 'contributed.ncl' library which
contains the most up-to-date functions can be downloaded from
**here**.
This contains the **grad_latlon_cfd** function used in Example 1.

gradient_1.ncl:
Using a global gaussian rectilinear grid, calculate the meridional and zonal gradients
using two independent methods. The first uses the "highly accurate"
spherical harmonic procedure **gradsg**. The second method uses
simple centered finite differences
(**grad_latlon_cfd**).

The 2-meter temperature variable was used because it has tight gradients and can be a bit noisy. These characteristics will amplify the differences beween the two approaches. If (say) a 500 hPa variable had been chosen the differences would be smaller.

Differences between the two approaches are explained because the
centered finite differences are less accurate. For example, the 2nd plot on the left
displays the meridional gradients derived from using
the **gradsg** procedure (top; labeled 'TGY: grads') and the
**grad_latlon_cfd** function (bottom; labeled 'TGY: cfd').
The visual results from **gradsg** show more detail as seen by the deeper colors.
The **grad_latlon_cfd** are smoother and lack the small scale detail.
(Of course, if the small scale detail are the result of noise, do you want them?)
The rightmost plot displays the actual numerical differences
of the TGX (top) and TGY (bottom) gradients.