4
DERIVATION OF AIR SEA FLUXES BY PARAMETERISATIONS
Table 1 Cloud reduction of the shortwave radiation
II'
Total cloud cover nt
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
5
1.00 1.02 1.02 1.00 0.95 0.91 0.85 0.78 0.70 0.57 0.37
10 1.00 1.02 1.02 1.00 0.96 0.92 0.85 0.78 0.69 0.56 0.36
20 1.00 1.01 1.01 0.99 0.96 0.92 0.86 0.78 0.68 0.54 0.35
30 1.00 1.00 1.00 0.98 0.96 0.93 0.87 0.80 0.70 0.57 0.38
40 1.00 1.00 0.99 0.98 0.96 0.94 0.88 0.81 0.72 0.60 0.42
50 1.00 0.99 0.98 0.97 0.96 0.94 0.89 0.83 0.75 0.64 0.45
60 1.00 0.98 0.97 0.97 0.96 0.94 0.90 0.84 0.77 0.68 0.49
70 1.00 0.97 0.96 0.96 0.96 0.95 0.92 0.86 0.80 0.71 0.52
80 1.00 0.96 0.96 0.96 0.96 0.96 0.93 0.89 0.83 0.75 0.55
90 1.00 0.96 0.96 0.96 0.96 0.96 0.95 0.91 0.86 0.78 0.59
In a second step the effects of clouds are taken into account by
(8)
where the factor of cloud reduction F depends on total cloud cover nt and solar altitude 'Y
(fig.3). This factor was tabulated by MALEVSKII (1992) and is reproduced as table 1.
The MALEVSKII scheme may be compared
to other parameterisations of shortwave radiation, e.g. REED (1977). He introduced an often applied formula, which has been modified by
ISEMER & HASSE (1987). They proposed to
. increase the cloud coefficient en from 0.620 to
0.637 and to decrease the transmissivity of the
atmosphere from 0.70 to 0.69. Compared to the
results of ISEMER & HASSE, MALEVSKII's
scheme yields more radiation in high latitudes
and smaller values in low latitudes (fig. 4). But
the differences are in general less than 5 W m - 2 .
Averaged over the Atlantic between 700N and
f-------;;;;;.------'---f,;;;--<:....---.;;;------,,;;----'-----i 2SOS, which is the southern limit of definition
C=C=t==C~r::::.5 .
•• ,O .
• • , 5 for REED's formula, both parameterisations give
Figure 4: Annual mean difference of
about 183 Wm- 2 and differ less than 1 Wm- 2 •
shortwave radiation (Malevskii minus
Isemer & Hasse)
The parameterisation of MALEVSKII yields
hourly means of the radiation. Since the merchant ship observations are carried out at fixed
hours referred to GMT, the local time of ship reports is a function of geographic longitude.
To avoid errors due to this effect, for each individual report a fictitious daily mean radiation
is computed by averaging over all solar altitudes of the individual day and position, assuming
the same cloud cover throughout the day.
DERIVATION OF AIR SEA FLUXES BY PARAMETERISATIONS
Table 1 Cloud reduction of the shortwave radiation
II'
Total cloud cover nt
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
5
1.00 1.02 1.02 1.00 0.95 0.91 0.85 0.78 0.70 0.57 0.37
10 1.00 1.02 1.02 1.00 0.96 0.92 0.85 0.78 0.69 0.56 0.36
20 1.00 1.01 1.01 0.99 0.96 0.92 0.86 0.78 0.68 0.54 0.35
30 1.00 1.00 1.00 0.98 0.96 0.93 0.87 0.80 0.70 0.57 0.38
40 1.00 1.00 0.99 0.98 0.96 0.94 0.88 0.81 0.72 0.60 0.42
50 1.00 0.99 0.98 0.97 0.96 0.94 0.89 0.83 0.75 0.64 0.45
60 1.00 0.98 0.97 0.97 0.96 0.94 0.90 0.84 0.77 0.68 0.49
70 1.00 0.97 0.96 0.96 0.96 0.95 0.92 0.86 0.80 0.71 0.52
80 1.00 0.96 0.96 0.96 0.96 0.96 0.93 0.89 0.83 0.75 0.55
90 1.00 0.96 0.96 0.96 0.96 0.96 0.95 0.91 0.86 0.78 0.59
In a second step the effects of clouds are taken into account by
(8)
where the factor of cloud reduction F depends on total cloud cover nt and solar altitude 'Y
(fig.3). This factor was tabulated by MALEVSKII (1992) and is reproduced as table 1.
The MALEVSKII scheme may be compared
to other parameterisations of shortwave radiation, e.g. REED (1977). He introduced an often applied formula, which has been modified by
ISEMER & HASSE (1987). They proposed to
. increase the cloud coefficient en from 0.620 to
0.637 and to decrease the transmissivity of the
atmosphere from 0.70 to 0.69. Compared to the
results of ISEMER & HASSE, MALEVSKII's
scheme yields more radiation in high latitudes
and smaller values in low latitudes (fig. 4). But
the differences are in general less than 5 W m - 2 .
Averaged over the Atlantic between 700N and
f-------;;;;;.------'---f,;;;--<:....---.;;;------,,;;----'-----i 2SOS, which is the southern limit of definition
C=C=t==C~r::::.5 .
•• ,O .
• • , 5 for REED's formula, both parameterisations give
Figure 4: Annual mean difference of
about 183 Wm- 2 and differ less than 1 Wm- 2 •
shortwave radiation (Malevskii minus
Isemer & Hasse)
The parameterisation of MALEVSKII yields
hourly means of the radiation. Since the merchant ship observations are carried out at fixed
hours referred to GMT, the local time of ship reports is a function of geographic longitude.
To avoid errors due to this effect, for each individual report a fictitious daily mean radiation
is computed by averaging over all solar altitudes of the individual day and position, assuming
the same cloud cover throughout the day.