Proglacial Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 195

Fig. 6.7 Radiation balance from December 12, 2010 to January 22, 2011 in Lake Suvivesi. Netrad
net radiation, Surfrad surface radiation balance, and Radin solar radiation penetrating into the lake
body (Lepp ä ranta et al. 2013b)

The lake body has a good transparency. Figure 6.8 shows the quantum scalar irradiance
recorded with a spherical PAR sensor at a depth of 60 cm at 10-min intervals. The
numerical values can be converted to energy flux (in W m − 2 ) approximately by dividing

by 5.0 (see Sect. 2.2 ) and, assuming diffuse irradiance conditions, they need to be further
divided by 4 to obtain the planar downwelling irradiance. The peak values in the

fl

first half

mol m − 2 s − 1 and in the latter part 2,000 mol m − 2 s − 1 .

of the record are about 2,500 ʼ ʼ

mol m − 2 s − 1 . Using the albedometer measure- Daily averages were about 900 1,200 ʼ

-

0.7 m − 1 ments, the estimated attenuation coef ranta et al.

2013b). This corresponds to the transparency of the surface ice and the mixture of liquid
water and slush. In the hard ice beneath the lake, the attenuation is expected to be larger if
there are sediments or gas bubbles, but smaller for clear ice.

cient was ʺ ¼ 0.5 (Lepp ä

-

Fig. 6.8 Quantum scalar irradiance at a depth of 60 cm in Lake Suvivesi in austral summer
2010 - 2011. Date is given as day/month/year (Lepp ä ranta et al. 2013b)