Evaluation of the CLM4 Lake Model at a Large and Shallow Freshwater Lake*

Abstract

Models of lake physical processes provide the lower flux boundary conditions for numerical predictions of weather and climate in lake basins. So far, there have been few studies on evaluating lake model performance at the diurnal time scale and against flux observations. The goal of this paper is to evaluate the National Center for Atmospheric Research Community Land Model version 4–Lake, Ice, Snow and Sediment Simulator using the eddy covariance and water temperature data obtained at a subtropical freshwater lake, Lake Taihu, in China. Both observations and model simulations reveal that convective overturning was commonplace at night and timed when water switched from being statically stable to being unstable. By reducing the water thermal diffusivity to 2% of the value calculated with the Henderson–Sellers parameterization, the model was able to reproduce the observed diurnal variations in water surface temperature and in sensible and latent heat fluxes. The small diffusivity suggests that the drag force of the sediment layer in this large (2500 km2) and shallow (2-m depth) lake may be strong, preventing unresolved vertical motions and suppressing wind-induced turbulence. Model results show that a large fraction of the incoming solar radiation energy was stored in the water during the daytime, and the stored energy was diffused upward at night to sustain sensible and latent heat fluxes to the atmosphere. Such a lake–atmosphere energy exchange modulated the local climate at the daily scale in this shallow lake, which is not seen in deep lakes where dominant lake–atmosphere interactions often occur at the seasonal scale.

The Taihu Eddy Flux Network: An Observational Program on Energy, Water, and Greenhouse Gas Fluxes of a Large Freshwater Lake

“…The data collected at Lake Taihu have been used to improve the lake model in NCAR's Community Land Model (CLM) system. In contrast, we found that for Lake Taihu the diffusivity should be adjusted downward by 92% in order for the model to reproduce successfully the observed diurnal variations in the lake surface temperature (Deng et al 2013). Tests of the model for deep lakes reveal that the eddy thermal diffusivity given by the model is 1-2 orders of magnitude too low (Martynov et al 2010;Subin et al 2012).…”

“…To date, some progress has been made toward answering the first science question, using data from a subset of the eddy flux sites (Deng et al 2013;Xiao et al 2013;Wang et al 2014). To date, some progress has been made toward answering the first science question, using data from a subset of the eddy flux sites (Deng et al 2013;Xiao et al 2013;Wang et al 2014).…”

Trends in evaporation of a large subtropical lake

“…According to the Priestley-Taylor model (Priestley and Taylor 1972), open water evaporation is controlled by the available energy and temperature and is independent of wind speed. Parameter analysis with the CLM4-LISSS lake model indicates that the surface temperature of Lake Taihu is insensitive to wind (Deng et al 2013). In the present study, increasing the MERRA wind speed by 10 % changed the mean evaporation rate only slightly, by 0.4 % to 981 mm from the original mean of 977 mm.…”

“…The main forcing variables are net shortwave radiation flux, downward longwave radiation flux, wind speed at the 10-m height, and specific humidity and air temperature at the 2-m height. Recently, our group (Deng et al 2013) evaluated the model against the eddy covariance observations at Lake Taihu. We found that the model does a good job simulating the eddy fluxes and the water temperature after an adjustment has been made to the water thermal diffusivity parameterization.…”

“…We found that the model does a good job simulating the eddy fluxes and the water temperature after an adjustment has been made to the water thermal diffusivity parameterization. In this study, we used the version tuned by Deng et al (2013).…”