Research On Transfer Coefficients Of Momentum, Heat And Water Vapor In Lake Taihu

Lakes influence local and regional climate through water vapor and heat exchange with atmosphere. The bulk transfer relationships are widely used in lake models to calculate the fluxes of momentum, heat and water vapor between water surface and atmosphere, whereas the critical issue is the parameterization of the transfer coefficients for momentum, heat and water vapor. Based on the eddy covariance and micrometeorological observations at the 5 eddy flux sites of the Taihu Eddy Flux Network in 2014, temporal and spatial variations of the transfer coefficients of momentum, heat and water vapor at lake-atmosphere interface were analyzed. The results indicated that:(1) The transfer coefficients for momentum, heat and water vapor were higher in spring and summer, and lower in autumn and winter. Since the monthly mean wind speed in this study (3.9-5.2 m/s) fell in the transitional regime between aerodynamically smooth and rough flow (2.5～5.5 m/s), there was potential uncertainty on the transfer coefficients.(2) Spatial variability was shown for drag coefficient over Lake Taihu:it is high in the western region (the DPK site) under stronger wind and wave regime; medium in the southern (the XLS site) and central region (the PTS site) with open water surface; small in the eastern region (the BFG site) with submerged vegetation, which restrained water flow. Spatial variability was little for the transfer coefficients of heat and water vapor.(3) The transfer coefficients decreased rapidly with increasing wind speed within the weak wind regime (0-4 m/s), reached minimum values within the wind speed range of 5-6 m/s, and approached constant within strong wind regime. Due to the shallow water depth and limited wind fetch, oceanographic parameterizations would underestimate the transfer coefficients at Lake Taihu, especially in the weak wind regime. Transfer coefficients of heat and water vapor increase gradually with increasing atmospheric instability. The transfer coefficients of momentum under unstable and neutral conditions were much higher than those under stable conditions.This study can provide scientific information for other studies on transfer coefficients over large and shallow lakes, and provide data support for lake model and regional climate model.