Effects of water clarity on lake stratification and lake‐atmosphere heat exchange

Abstract

Recent progress of including lake subroutines in numerical weather prediction (NWP) models has led to more accurate forecasts. In lake models, one essential parameter is water clarity, parameterized via the light extinction coefficient, K d , for which a global constant value is usually used. We used direct eddy covariance fluxes and basic meteorological measurements coupled with lake water temperature and clarity measurements from a boreal lake to estimate the performance of two lake models, LAKE and FLake. These models represent two 1-D modeling frameworks broadly used in NWP. The results show that the lake models are very sensitive to changes in K d when it is lower than 0.5 m À1. The progress of thermal stratification depended strongly on K d . In dark-water simulations the mixed layer was shallower, longwave and turbulent heat losses higher, and therefore the average water column temperatures lower than in clear-water simulations. Thus, changes in water clarity can also affect the onset of ice cover. The more complex LAKE modeled the seasonal thermocline deepening, whereas it remained virtually constant during summer in the FLake model. Both models overestimated the surface water temperatures by about 1°C and latent heat flux by >30%, but the variations in heat storage and sensible heat flux were adequately simulated. Our results suggest that, at least for humic lakes, a lake-specific, but not time-depending, constant value for K d can be used and that a global mapping of K d would be most beneficial in regions with relatively clear lakes, e.g., in lakes at high altitudes.

Satellite-derived light extinction coefficient and its impact on thermal structure simulations in a 1-D lake model

“…SDD is a suitable characteristic to describe water transparency for small values of K d . For high values of K d (ranging above 4 m −1 ), Arst et al (2008) and Heiskanen et al (2015) suggested that SDD is unable to describe any changes in K d . Figure 3 also shows that SDD cannot describe the scatter of K d for values above 4 m −1 .…”

“…The study found a critical threshold for K d (0.5 m −1 ) in 1-D lake models. Heiskanen et al (2015) concluded that for too-clear waters (K d < 0.5 m −1 ), the model is much more sensitive to K d . The study recommends a global mapping of K d to run the FLake model for regions with clear waters (K d < 0.5 m −1 ) for future use in NWP models.…”

“…This variable is considered as an apparent optical property and is parameterized using the light extinction coefficient (K d ) to describe the absorption of shortwave radiation within the water body as a function of depth (Heiskanen et al, 2015). A global constant value of K d is usually used to run lake models, including FLake.…”

“…Martynov, personal communication, 2015) for all North American lakes, including Lake Erie, for the years [2005][2006][2007]. Heiskanen et al (2015) evaluated the sensitivity of two 1-D lake models, LAKE and FLake, to seasonal variations and the general level of K d for simulating water temperature profiles and turbulent fluxes of heat and momentum in a small boreal Finnish lake. Modeled values were compared to those measured for the lake during the ice-free period of 2013.…”

Transparency, Geomorphology and Mixing Regime Explain Variability in Trends in Lake Temperature and Stratification across Northeastern North America (1975–2014)

“…Similarly, milder winters, shortening ice cover and less snow cover on ice occurring in NENA and around the northern latitudes [50,51] will all contribute to lowered lake albedo, earlier stratification and longer duration of energy absorption, resulting in intensified summer warming [11,12,52]. Changes in wind speed [53,54], water transparency [55,56] and hydrologic inflows [54] can all alter lake warming rates.…”