Annika Nordbo 1, Ivan Mammarella, Anne Ojala, Kukka-Maaria Erkkilä, Matti Leppäranta, Heikki Järvinen, Timo Vesala
1 Helsingin yliopisto
Lakes affect regional weather and hydrology, and thus they have recently been included in numerical weather prediction models (NWP) through lake subroutines and have longer been included in hydrological forecasting. The NWP subroutines require external data on water clarity, which is not globally available and thus crudely a constant value is used instead. Water clarity is represented by a diffuse light extinction coefficient (Kd), which describes how the received radiative flux is attenuated in a lake water column. Kd influences directly lake‒atmosphere interaction: the surface temperature of a low-clarity lake (high Kd) is higher, which leads to enhanced evaporation and sensible heat flux, and also has effects on a seasonal scale.
The aim of this study is to assess the sensitivity of two 1-dimensional lake models on Kd. A multilayer model (LAKE) and a model with a parameterized temperature profile (FLake) are used for simulating the water temperature profile and surface fluxes of a lake. The models are forced and validated against direct measurements at Lake Kuivajärvi, Finland: surface energy budget (including eddy-covariance measurements), water temperature profile, Kd and basic meteorology. The models are run for a full year with two key scenarios: (i) a constant time-independent Kd which is varied from run to run; and (ii) using direct measurements of Kd providing a time-dependent evolution throughout the year. Recommendations based on this sensitivity analysis are given: How important is water clarity in lake‒atmosphere interaction modeling and should a global database of it be included in NWP? Is the seasonal variation of Kd substantial enough that a constant value in time cannot be used?