太湖湍流动能耗散率的廓线分布及其可能机制

Profile features of dissipation rates of turbulence in Lake Taihu and possible mechanism

湍流不仅是导致物质、动量等在水气交界面的交换、水柱内部输送的关键过程,也是促进浅水湖泊中底泥再悬浮及水生生态系统演变的驱动力;其中湍流动能耗散率(ε)不仅是描述湍流动能变化的关键物理量,也是刻画水体中湍流产生机制的关键过程.基于2017年10月29日—11月2日位于太湖梅梁湾采集的高频三维流速廓线和水温廓线观测资料,结合东山气象站同期的风场和辐射等气象资料,探讨太湖中ε的分布特征及其变化的可能机制.结果表明,0.7 m深度以下水平方向上ε介于10^-8~10^-7 m^2/s^3之间,比垂直方向大一个数量级左右.尽管大型浅水湖泊几何深度浅,但其ε的深度廓线依然存在典型的3层:ε随深度递减的风浪直接作用层,深度从水面至1.0 m左右;ε基本不随深度变化的常数层,分布区间为水深1.0~1.9 m;随后是ε随深度递减的底边界混合层.热力分层的强弱(垂向温差)、位置对常数层和底部边界中ε的贡献显著,甚至造成ε的常数层起始深度下移.本研究有利于进一步理解大型浅水湖泊的动力学过程及其对水生生态系统演变的驱动效应.

Turbulence is a key process regulating the exchange of matter and momentum not only across the water-gas interface,but also within water columns.It is also a driving force for the resuspension of sediments and the evolution of shallow lake ecosystems.The dissipation rate of the turbulent kinetic energy (ε) is a key physical quantity which depicts the evolution of the turbulent kinetic energy,and discriminates turbulence production rate from mechanisms in water bodies.Based on observations for profiles of three-dimensional currents using Acoustic Doppler Current Profiles (ADCP ) and records on water temperature and the wind field in Lake Taihu from October 29,2017 to November 2,2017,the profile features of ε in this large-scale shallow lake were explored.The horizontal ε is about 10^-8-10^-7 m^2/s^3 below 0.7 m water depth,about one order of magnitude larger than the vertical ε.Despite the shallowness of the lake,there are still three typical layers of ε for u′,v′,and w′: the wind-wave layer,where the water depth is shallower than 1.0 m,exhibits that ε decreases with depth due to wind forcing,Langmuir turbulence and wave breaking;constant layer,in the range of 1.0 m to 1.9 m or so,shows that ε is little variation with the depth;bottom boundary mixed layer,below the constant layer,exhibits that ε decreases with depth.In Lake Taihu,the strength and location of stratification significantly contribute to ε in the constant layer and the bottom boundary,and even cause the initial depth of the constant layer of ε to move downward.This study will help to further understand the kinetic process of large shallow lakes and their evolutionary effects on aquatic ecosystems.