Importance of Parameterizing Lake Surface and Internal Thermal Processes in WRF for Simulating Freeze Onset of an Alpine Deep Lake

Abstract The alpine lakes widely distributed over the Tibetan Plateau (TP) are not only highly sensitive to climate but also regulate the regional climate. However, the lack of TP alpine lake observations limits the understanding of the lake‐atmosphere interactions. Here, we show the relative importance of parameterizing lake surface and internal thermal processes in describing the lake energy budget and lake‐atmosphere interactions. Based on the in situ observations at Lake Nam Co, a large and deep lake in the TP, we employed the coupled Weather Research and Forecasting with lake (WRF‐Lake) model to clarify their relative roles. Results show that the original model produces cold biases and too early timing of lake freeze onset. The adjustments of parameterizations of lake internal (temperature of maximum water density, extinction coefficient) and lake surface (roughness length) processes significantly improve the ability of the WRF‐Lake in simulating the lake thermal features and the freeze onset timing. The different parameterizations of lake internal thermal processes affect the onset timing of lake freeze mainly by altering the release of turbulent heat fluxes in summer, but the adjustments of lake surface roughness lengths change the turbulent heat fluxes during the unfrozen season from summer through early winter. Accordingly, the simulated lake freeze onset is much more sensitive to the surface roughness length schemes than to the internal thermal processes schemes. As the lake‐atmosphere interactions are very sensitive to the lake freeze onset, our results are critical for understanding and simulating the impact of TP lakes on the regional climate.