Improvement and Evaluation of the Latest Version of WRF-Lake at a Deep Riverine Reservoir

Abstract

The WRF-lake vertically one-dimensional (1D) water temperature model, as a submodule of the Weather Research and Forecasting (WRF) system, is being widely used to investigate water—atmosphere interactions. But previous applications revealed that it cannot accurately simulate the water temperature in a deep riverine reservoir during a large flow rate period, and whether it can produce sufficiently accurate heat flux through the water surface of deep riverine reservoirs remains uncertain. In this study, the WRF-lake model was improved for applications in large, deep riverine reservoirs by parametric scheme optimization, and the accuracy of heat flux calculation was evaluated compared with the results of a better physically based model, the Delft3D-Flow, which was previously applied to different kinds of reservoirs successfully. The results show: (1) The latest version of WRF-lake can describe the surface water temperature to some extent but performs poorly in the large flow period. We revised WRF-lake by modifying the vertical thermal diffusivity, and then, the water temperature simulation in the large flow period was improved significantly. (2) The latest version of WRF-lake overestimates the reservoir—atmosphere heat exchange throughout the year, mainly because of underestimating the downward energy transfer in the reservoir, resulting in more heat remaining at the surface and returning to the atmosphere. The modification of vertical thermal diffusivity can improve the surface heat flux calculation significantly. (3) The longitudinal temperature variation and the temperature difference between inflow and outflow, which cannot be considered in the 1D WRF-lake, can also affect the water surface heat flux.

摘要

WRF-lake是简化的垂向一维水温模型, 作为WRF气象模式的子模块, 广泛应用于水体与大气间交互过程的模拟预测。已有研究表明, WRF-lake不能准确模拟河道型大深水库在大流量时的水温；WRF-lake能否准确计算河道型大深水库与大气间的热通量, 目前也不清楚。本研究首先对WRF-lake模型的参数化方案进行改进, 提高其在河道型大深水库中的水温模拟精度, 并将其与物理过程更完整且已成功应用于各类水库的Delft3D-Flow模型进行对比, 评估了WRF-lake对河道型大深水库表面热通量的模拟准确性。结果表明：（1）虽然最新版本的WRF-lake模型能够在一定程度上准确模拟河道型大深水库的表面水温, 但在大流量时段表现较差。本研究提出的修正垂向热扩散系数方案, 可显著改善大流量时段的水温模拟结果。（2）最新版本的WRF-lake模型全年都高估了水库表面的热交换量, 这主要由于低估了水库中能量的向下传输, 导致更多热量滞留在水体表面并返回到大气中。本研究对垂向热扩散系数的改进, 提高了表面热通量的计算精度。（3）作为简化的垂向一维模型, WRF-lake不能反映水温的纵向变化和出入流温差, 这两个因素也对水体表面热通量具有一定影响。