## Calculate the long-wave back radiation

INTERFACE:

```   subroutine back_radiation(method,dlat,tw,ta,cloud,qb)
```
DESCRIPTION:

Here, the long-wave back radiation is calculated by means of one out of four methods, which depend on the value given to the parameter method: method=1: Clark et al. (1974), method=2: Hastenrath and Lamb (1978), method=3: Bignami et al. (1995), method=4: Berliand and Berliand (1952). It should ne noted that the latitude must here be given in degrees.

USES:

```   use airsea_variables, only: emiss,bolz
use airsea_variables, only: ea,qa
IMPLICIT NONE
```
INPUT PARAMETERS:
```   integer, intent(in)                 :: method
REALTYPE, intent(in)                :: dlat,tw,ta,cloud
```
OUTPUT PARAMETERS:
```   REALTYPE, intent(out)               :: qb
```
REVISION HISTORY:
```   Original author(s): Adols Stips, Hans Burchard & Karsten Bolding
```
LOCAL VARIABLES:
```
integer, parameter   :: clark=1        Clark et al, 1974
integer, parameter   :: hastenrath=2   Hastenrath and Lamb, 1978
integer, parameter   :: bignami=3      Bignami et al., 1995 - Medsea
integer, parameter   :: berliand=4     Berliand and Berliand, 1952 - ROMS

REALTYPE, parameter, dimension(91)  :: cloud_correction_factor = (/ &
0.497202,     0.501885,     0.506568,     0.511250,     0.515933, &
0.520616,     0.525299,     0.529982,     0.534665,     0.539348, &
0.544031,     0.548714,     0.553397,     0.558080,     0.562763, &
0.567446,     0.572129,     0.576812,     0.581495,     0.586178, &
0.590861,     0.595544,     0.600227,     0.604910,     0.609593, &
0.614276,     0.618959,     0.623641,     0.628324,     0.633007, &
0.637690,     0.642373,     0.647056,     0.651739,     0.656422, &
0.661105,     0.665788,     0.670471,     0.675154,     0.679837, &
0.684520,     0.689203,     0.693886,     0.698569,     0.703252, &
0.707935,     0.712618,     0.717301,     0.721984,     0.726667, &
0.731350,     0.736032,     0.740715,     0.745398,     0.750081, &
0.754764,     0.759447,     0.764130,     0.768813,     0.773496, &
0.778179,     0.782862,     0.787545,     0.792228,     0.796911, &
0.801594,     0.806277,     0.810960,     0.815643,     0.820326, &
0.825009,     0.829692,     0.834375,     0.839058,     0.843741, &
0.848423,     0.853106,     0.857789,     0.862472,     0.867155, &
0.871838,     0.876521,     0.881204,     0.885887,     0.890570, &
0.895253,     0.899936,     0.904619,     0.909302,     0.913985, &
0.918668 /)

REALTYPE                  :: ccf
REALTYPE                  :: x1,x2,x3
```

Karsten Bolding 2012-12-28