2.0 KiB
2.12.3. Surface Albedo¶
For direct radiation, the albedo a for lakes with ground temperature \({T}_{g}\) (K) above freezing is given by (Pivovarov, 1972)
(2.12.1)¶\[a=\frac{0.5}{\cos z+0.15}\]
where z is the zenith angle. For diffuse radiation, the expression in eq. is integrated over the full sky to yield a = 0.10.
For frozen lakes without resolved snow layers, the albedo at cold temperatures _a_0 is 0.60 for visible and 0.40 for near infrared radiation. As the temperature at the ice surface, \({T}_{g}\), approaches freezing [ \({T}_{f}\) (K) (Table 2.2.7)], the albedo is relaxed towards 0.10 based on Mironov et al. (2010):
(2.12.2)¶\[a=a_{0} \left(1-x\right)+0.10x,x=\exp \left(-95\frac{T_{f} -T_{g} }{T_{f} } \right)\]
where a is restricted to be no less than that given in (2.12.1).
For frozen lakes with resolved snow layers, the reflectance of the ice surface is fixed at _a_0, and the snow reflectance is calculated as over non-vegetated surfaces (Chapter 2.3). These two reflectances are combined to obtain the snow-fraction-weighted albedo as in over non-vegetated surfaces (Chapter 2.3).