Freezing of Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 46

2.5 Lake Ice Climatology

As written in Sect. 2.2.4 , freezing lakes can be divided into the following three main
zones: perennial zone, seasonal zone, and ephemeral zone (Table 2.8 ). Considering the
climatology of these zones, two binary variables are introduced (see Jevrejeva 2004): Ice
occurrence I, I(n) = 1 (0) if ice occurs (does not occur) in ice year n, and Open lake
occurrence O, O(n)= 1 (0) if ice year n is ice-free (is not ice-free). In fully seasonal

lakes, I + O = 0, while in ice-free and perennially ice-covered lakes I + O equals

−

− 1 and 1,

respectively. Note that in Sect. 2.1.1 , the ice year was de ned as the period between two

consequent summer maxima of the lake heat content. There are long records of lakes
freezing over in extreme years in relatively warm climate regions. Example Maurer (1924)
reported of cases from southern Germany and Switzerland back to year 1400. In contrast,
ice-free winters would provide information of extremely warm winters in the stable
seasonal lake ice zone, but such records are not known to the author. 9
Lake ice phenology studies the dates of freezing and ice breakup (Futter 2003). Their
annual timing has been recorded for long periods in a large number of lakes due to
practical reasons and due to general interest in nature phenomena (Magnusson et al.
2000). They are most eye-striking events with major impacts on the nature and society.
Also lake ice phenology is a good climatic indicator due to its sensitivity to the climate
conditions and the existence of long records (e.g., Robertson et al. 1992; Livingstone
2000; Duguay et al. 2006; Korhonen 2006; Karetnikov and Naumenko 2008; Bernhardt
et al. 2011). To be precise, some basic de

nitions of ice phenology are rst needed.

first day in an ice season when the observation area has
frozen over. Often this date refers to the freeze-over of the whole lake, but in larger lakes
observations do not necessarily cover the whole water body. The freeze-up date, also called
ice-on date, is used for the

The freezing date is de ned as the

first day of complete coverage of ice in a given season. The ice
break-up date, also called ice-off date, in turn, is the last ice day in a given winter season. The
length of ice season is the time between the freezing and breakup dates, while the number of
ice days is the number of days when ice actually exists. Care should be taken in conducting a
comparative analysis whether the freezing date or freeze-up date has been consistently used.
Their difference can be appreciably long, especially in large and deep lakes.
An example of a phenomenological time series is given in Fig. 2.17 showing typical
features in boreal lakes. In Lake Kallavesi (left) the variability of the freezing date is
2

months, and for the breakup date it is 1 month. For the last 100 years most lake ice
time series show slow trends toward milder winters with high random variability. Toward
the climatological ice margin in Lake Stechlin (right) the variability of the freezing and
breakup date increases and ice-free winters become more common.
Ice formation is generally governed by intense radiative and turbulent heat losses from
the warmer lake surface to the colder atmosphere. The strati

½

cation in the atmospheric
surface layer is unstable and turbulent losses can be large. When the water temperature is

9

In Finland, all lakes have frozen over every year, except possibly some deep basins in 1930.