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# u2_fao56

Compute 2-meter wind speed (m/s) as described in FAO 56. Available in version 6.4.0 and later.

## Prototype

```load "\$NCARG_ROOT/lib/ncarg/nclscripts/csm/crop.ncl"  ; This library is automatically loaded
; from NCL V6.5.0 onward.
; No need for user to explicitly load.

function u2_fao56 (
uz        : numeric,
z         : integer,
wunit  : integer
)
```

## Arguments

uz

A scalar or array of any dimensionality containing wind speed at a height of z meters. See wunit description.

z

A variable of the same size and shape as uz containing the height(m) of uz above ground surface.

wunit

An array of length 2 which specifies the units of the input uz and the output 2m wind speed:

• wunit(0)=0 ; input uz are m/s
• wunit(0)=1 ; input uz are km/hr
• wunit(0)=2 ; input uz are mph
• wunit(1)=0 ; output u2 are m/s
• wunit(1)=1 ; output u2 are km/hr
• wunit(1)=2 ; output u2 are mph

## Return value

An array with the same dimensionality and type as uz containing estimated 2-meter wind speeds (m/s).

## Description

Compute 2-meter wind speed (m/s) as described in the Food and Agriculture Organization (FAO) Irrigation and Drainage Paper 56 entitled: Crop evapotranspiration - Guidelines for computing crop water requirement . Specifically, see equation 47 of Chapter 3.

If necessary, the input uz will be converted to (m/s). The input uz are not changed.

## Examples

Example 1: Replicate example 14 in Chapter 3 in Chapter 3.

```  uz  =  3.2                        ; m/s
z   = 10.0                        ; m

u2_00 = u2_fao56(uz, z, (/0,0/))   ; 2.39 m/s
u2_01 = u2_fao56(uz, z, (/0,1/))   ; 8.62 km/hr
u2_02 = u2_fao56(uz, z, (/0,2/))   ; 5.35 mph
printVarSummary(u2)

```
The output for 'u2_00' is

```     Variable: u2
Type: float
Total Size: 4 bytes
1 values
Number of Dimensions: 1
Dimensions and sizes:	
Coordinates:
Number Of Attributes: 4
long_name :	Estimated 2-meter wind speed
units :	m/s
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; EQN 47
```

Example 2: Consider UZ(time,lat,lon) and Z(time,lat,lon) with sizes (3,73,144).

```  U2  = u2_fao56(UZ, Z,  (/0,0/))
printVarSummary(U2)
```

The output for 'U2' is

```     Variable: U2
Type: float
Total Size: 126144 bytes
31536 values
Number of Dimensions: 3
Dimensions and sizes:	[time | 3] x [lat | 73] x [lon | 144]
Coordinates:
time: [1..3]
lat: [-90..90]
lon: [ 0..357.5]
Number Of Attributes: 4
long_name :	Estimated 2-meter wind speed
units :	m/s
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; EQN 47
```