How to observe the real values of surface fluxes over complex terrains

The interaction between land and atmosphere is a much important process for the earth system. We need accurate values of surface fluxes of momentum, heat, water vapor and carbon dioxide to understand the land-atmosphere exchange process and to evaluate the numerical model performance. It has been a long time that observations of surface fluxes are carried out over different land surfaces in the world. For example, the FLUXNET has been worked more than twenty years. The observations were conventionally carried out by the eddy correlation system at a height close to the ground. But the problem is whether the surface fluxes observed in this manner are representative in local scale over complex terrains. Previous studies have not paid enough attention to this problem. The latest study shows that the surface flexes observed near the ground over complex terrains are not representative in local scale.

The research article is titled “Observational study of land-atmosphere turbulent flux exchange over complex underlying surfaces in urban and suburban areas”，and published recently in SCIENCE CHINA Earth Sciences. The authors analyzed the data of turbulent fluxes of momentum, sensible heat and latent heat observed at different heights above the ground over complex terrains. They revealed the characteristics of vertical change of turbulent fluxes in urban roughness sublayer and the significant difference in turbulent fluxes between a near-surface level and a relatively higher level over complex suburban terrain.

For the flows over complex terrains, it is important to obtain the real values of surface fluxes in local scale by observations. The representative surface fluxes can help us to understand the processes of exchange of mass and energy between land and atmosphere, and then to develop the parameterization schemes that are more suitable for describing these processes over the complex terrains. The representative surface fluxes can also help us to reduce the uncertainties in the observational data that are used to evaluate the performance of numerical models.

The results in this study show that in the roughness sublayer above the urban canopy, the momentum flux, sensible heat flux, and latent heat flux increase with height, and the observation value of the surface albedo increases with height. However, the observation value of the net radiation decreases with height, thus resulting in a change in the urban surface energy budget with height. At the SORPES station in the Xianlin Campus of Nanjing University located in a hilly and partially urbanized area, the fluxes of momentum, sensible heat and latent heat measured in the observation yard significantly differ from those measured at two heights on the tower, while the fluxes at the two heights on the tower are nearly equal. These results indicate that the surface fluxes with local-scale representativeness can be observed at a relatively higher altitude (say, in the internal sublayer) rather than at the near-surface height over the complex terrains.

For the observation of turbulent fluxes in the surface layer over complex underlying surfaces, a key goal is to obtain the real values of surface fluxes. That is, the observed fluxes should be representative in local scale over the complex terrains. This study suggests that, to achieve this goal, multilevel observations on a tower should be made so that the representative surface fluxes can be derived.

This research was supported by the National Key R&D Program of China (Grant No. 2016YFC0200500) and the Youth Project of National Natural Science Foundation of China (Grant No. 41805007).

 

See the article:

Sha J, Zou J, Sun J. 2021. Observational study of land-atmosphere turbulent flux exchange over complex underlying surfaces in urban and suburban areas. Science China Earth Sciences, 64(7): 1050-1064, https://doi.org/10.1007/s11430-020-9783-2

https://www.sciengine.com/publisher/scp/journal/SCES/64/7/10.1007/s11430-020-9783-2?slug=fulltext

---

---