title: Two Improvements of an Operational Two-Layer Model for Terrestrial Surface Heat Flux Retrieval.
abstract: In order to make the prediction of land surface heat fluxes more robust, two improvements were made to an operational two-layer model proposed previously by Zhang. These improvements are: 1) a surface energy balance method is used to determine the theoretical boundary lines (namely 'true wet/cool edge' and 'true dry/warm edge' in the trapezoid) in the scatter plot for the surface temperatureversusthe fractional vegetation cover in mixed pixels; 2) a new assumption that the slope of theTm-fcurves is mainly controlled by soil water content is introduced. The variables required by the improved method include near surface vapor pressure, air temperature, surface resistance, aerodynamic resistance, fractional vegetation cover, surface temperature and net radiation. The model predictions from the improved model were assessed in this study byin situmeasurements, which show that the total latent heat flux from the soil and vegetation are in close agreement with thein situmeasurement with an RMSE (Root Mean Square Error) ranging from 30 w/m²~50 w/m²,which is consistent with the site scale measurement of latent heat flux. Because soil evaporation and vegetation transpiration are not measured separately from the field site,in situmeasured CO₂ flux is used to examine the modeledλEveg. Similar trends of seasonal variations of vegetation were found for the canopy transpiration retrievals andin situCO₂ flux measurements. The above differences are mainly caused by 1) the scale disparity between the field measurement and the MODIS observation; 2) the non-closure problem of the surface energy balance from the surface fluxes observations themselves. The improved method was successfully used to predict the component surface heat fluxes from the soil and vegetation and it provides a promising approach to study the canopy transpiration and the soil evaporation quantitatively during the rapid growing season of winter wheat in northern China.
authors.affiliation: Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, P.R. China. zhangrh@igsnrr.ac.cn.
authors.forename: Renhua
authors.initials: R
authors.lastname: Zhang
authors.affiliation: Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, P.R. China. tianj.04b@igsnrr.ac.cn.
authors.forename: Jing
authors.initials: J
authors.lastname: Tian
authors.affiliation: Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, P.R. China. suhb@igsnrr.ac.cn.
authors.forename: Hongbo
authors.initials: H
authors.lastname: Su
authors.affiliation: Synthesis Center of Chinese Ecosystem Research Network, Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, P.R. China. sunxm@igsnrr.ac.cn.
authors.forename: Xiaomin
authors.initials: X
authors.lastname: Sun
authors.affiliation: Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, P.R. China. chensh@igsnrr.ac.cn.
authors.forename: Shaohui
authors.initials: S
authors.lastname: Chen
authors.affiliation: Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing, 100101, P.R. China. xiaj@igsnrr.ac.cn.
authors.forename: Jun
authors.initials: J
authors.lastname: Xia
keywords: Bowen Radio; Two-layer model; surface energy balance; surface evapotranspiration; trapezoid method