Optimal coupling heights improve surface-atmosphere modeling

Weather, climate and hydrometeorology forecasts require accurate surface-atmosphere coupled modeling. This requires the use of proper coupling heights in computing surface turbulent fluxes, or the exchanges of heat, moisture and momentum between the surface of the Earth and the near-surface thin layer of air called the surface layer.

Traditionally, Earth systems models (ESMs) have used an arbitrarily set height in this layer as the reference height, usually between 10 and 50 m, to compute surface fluxes. However, the impact of reference heights on the surface-atmosphere interactions has not received much attention, and the optimal height is unknown. Recently, a team of atmospheric scientists from Sun Yat-sen University and The University of Arizona addressed this issue and found that more representative reference heights provide better estimates of surface turbulent fluxes and thus better ESM predictions.

The study is featured on the cover of the latest issue of Advances in Atmospheric Sciences.

“We wanted to know if there are optimal reference heights for the purpose of surface flux estimation using current surface layer theory,” said Shaofeng Liu, associate professor in the School of Atmospheric Sciences at Sun Yat-sen University and first author of the research paper.

To do so, the team determined how much the wind gradient, computed using a given reference height, differs from the actual gradient. The deviation was then used to estimate a new reference height until the deviation was close to zero.

This iterative process also enabled the team to obtain the optimal height and quantify the dependence of surface flux estimations on the reference height. The results were robust whether the researchers used the classical or their own revised surface layer theory.

“Computation of land surface fluxes in Earth system models is dependent on the reference height. This height is usually set as the lowest level in the atmosphere, varying from a few meters to several tens of meters. We find that reference heights near the top of the surface layer tend to provide the best estimate of surface fluxes,” said Liu.

The momentum flux, which reflects the rate at which momentum, or the product of wind speed and air mass, is transferred between the Earth’s atmosphere and surface per unit time, was particularly affected. The computation of heat flux, representing the transfer of heat between the Earth’s surface and the atmosphere, was also improved.

“We hope this study will help provide guidelines for setting the proper reference heights for Earth system modeling,” said co-author Yongjiu Dai, professor in the School of Atmospheric Sciences at Sun Yat-sen University.

“These results also need to be further verified using observational data from land–atmosphere feedback observatories,” added co-author Xubin Zeng, professor at The University of Arizona.

In the future, the team also hopes to optimize reference height estimations for the latest modeling applications, including high-resolution Earth system modeling that includes multiple grid levels in the surface layer.