Monitoring Change in the Arctic - Satellite-based Applications on Climate Change - page 138

Fig. 9.9 Annual cycle of net all-wave cloud forcing for six Arctic regions averaged over the
period 1982-1999 on a local solar time of 14:00

forcing.” Cloud forcing is calculated from net shortwave and longwave radiative
fluxes at the surface or TOA. It is defined as

Z

A c

@ F λ ; z
@a

CF λ ;z ¼ d a ¼ F λ ;z ðA c ÞF λ ;z ð

Þ

0 (9.1)

0

where F λ , z is the net flux (W/m 2 ) in shortwave or longwave radiation at the surface
or TOA,

is the wavelength, z is the altitude above the surface, and A c is the cloud
fraction in the scene. The net flux is equal to the downwelling minus upwelling
fluxes. The all-wave cloud forcing can be calculated by

λ

CF z ¼ CF shortwave þ CF longwave (9.2)

Figure 9.9 shows the annual cycle of the shortwave, longwave, and all-wave
cloud forcing at the surface. As discussed above, the shortwave cloud forcing is
always negative, while the longwave cloud forcing is always positive and follows
the annual cycle of cloud fraction. In the cold season (October-March), the all-
wave cloud forcing is positive, implying a warming effect of the clouds on the
surface. In the warm season (April-September), the all-wave cloud forcing is
negative, indicating a cooling effect of the clouds on the surface. On annual average