Numerical Experiment, Assessment And Improvement Of The Canopy Interception From Three Versions Of NCAR Community Land Model

Surface water cycle is an important part and main driver of material and energy cycle of climate system. Surface water cycle and its mechanism is complex, while the vegetation makes it become more complex. Water is the material basis of the plant activities and determines the type and distribution of vegetation. Conversely, the vegetation has changed the water balance and water cycle to some extent. Regulation of vegetation on the water cycle is mainly manifested in the canopy interception, water retention and redistribution of precipitation. Canopy interception is the first step of the vegetation affect precipitation. Therefore, canopy interception has important hydrological effects, and to a certain extent affects the climate.Impacts of canopy interception on land surface water and energy balance are investigated through numerical experiment by using the NCAR Community Land Model (CLM4.0). By contrast of numerical experiments with and without canopy interception, the possible impacts are discussed.(1) For the water balance, it is shown that results without canopy interception, the total water content of soil, surface runoff, sub-surface runoff, surface evaporation and plant transpiration will increase, while the evapotranspiration will decrease. Spatial distribution indicates that, there is a high distribution of difference between the two experiments at low latitudes with the seasonal north-south oscillation. It is also shown that the high value area expansion in spring, heyday in summer, and retreat in autumn and winter at northern mid-high latitudes. The main reason of the decrease of evapotranspiration is canopy evaporation disappeared when canopy interception disappeared.(2) For the energy balance, it is shown that results without canopy interception, sensible heat flux will increase, while latent flux will decrease. The value of increase in the canopy sensible heat is grater than the value of reduction in the surface sensible heat. The spatial distribution of energy balance is similar to the water balance components. There are significant differences between the responses of different vegetation types when the canopy interception is neglected. Factors such as leaf area index, precipitation, and deciduous coverage are possible reasons for these differences.Then this paper assesses the canopy interception evaporation analog performance of CLM series mode. The results show that CLM3.5and CLM4.0can simulate the spatial distribution and seasonal variation of global canopy evaporation exactly. Overall, the root mean square error simulated by CLM4.0is smallest among three, and the spatial correlation coefficient simulated by CLM3.5is highest. CLM3.0simulates worst. For most vegetation types, CLM3.0simulation error is significantly greater than CLM3.5and CLM4.0through assessing the results of the simulation of different vegetation types. There is little difference between the root mean square error of CLM3.5and CLM4.0. In addition, CLM4.0simulates best.Based on the assessment, a new Logistic canopy interception parameterization scheme is introduced. The results show that, compared to the simulation results of the CLM4.0original program, the new program has been improved to some extent. The simulation of Logistic scheme is better than CLM4.0program for most vegetation types. Meanwhile, the simulation errors have obvious seasonal changes. Errors of the rainy season are greater than the dry season.