It has long since been known that soil moisture controls the partitioning of energy fluxes at the land surface, and that this partitioning affects the humidity and temperature of the air. A global synthesis of how soil moisture impacts the properties of the atmospheric boundary layer (ABL), however, was missing. A new study by Denissen and co-workers reveals a clear impact of soil moisture conditions on several boundary layers properties from observations around the globe.
There are not many ways in which the impact of land conditions on the ABL can be studied globally from observations. Weather balloons are one of the most promising observations in this regard, because these observations give information on the vertical structure of the atmosphere, and they are available across the world in an archive dating back numerous decades. Such data can be complimented by satellite records of soil moisture, and an inverse modeling framework.
The use of balloon soundings in combination with a simplified 1-dimensional model (CLASS) of the atmospheric boundary-layer to study the impact of soil moisture droughts was pioneered by Miralles et al. (2014) in a study on the 2003 European heatwave and the 2010 Russian heatwave. Using the recent global implementation of the same 1-dimensional boundary layer model CLASS4GL by Wouters et al. (2019), Denissen et al. (2021) were able to analyze for the first time globally how soil moisture conditions control the atmospheric boundary layer, and thus near-surface weather.
The analysis by Denissen et al. reveals that even at larger scales (balloon soundings reflect soil moisture conditions in a large region upwind), the relation between soil moisture and energy flux partitioning is nonlinear. The distinct water and energy limited regimes show on average different ABL conditions, with afternoon ABLs during dry conditions being on average warmer (≈3 K), higher (≈400 m) and drier (≈1 kPa) than during wet conditions. This shows that the results from previous studies on heatwaves can be extended globally, and that drought conditions cannot be seen in isolation from their impact on the atmosphere.
Further reading
Denissen, J. M. C.; R. Orth; H. Wouters; D. G. Miralles; C. C. van Heerwaarden; J. Vilà-Guerau de Arellano & A. J. Teuling (2021), Soil moisture signature in global weather balloon soundings. npj Climate and Atmospheric Science, 4, 13, doi:10.1038/s41612-021-00167-w.
Miralles, D. G.; A. J. Teuling; C. C. van Heerwaarden & J. Vilà-Guerau de Arellano (2014), Mega-heatwave temperatures due to combined soil desiccation and atmospheric heat accumulation. Nature Geoscience, 7(5), 345–349, doi:10.1038/ngeo2141.
Wouters, H.; I. Y. Petrova; C. C. van Heerwaarden; J. Vilà-Guerau de Arellano; A. J. Teuling; V. Meulenberg; J. A. Santanello & D. G. Miralles (2019), Atmospheric boundary layer dynamics from balloon soundings worldwide: CLASS4GL v1.0. Geoscientific Model Development, 12, 2139–2153, doi:10.5194/gmd-12-2139-2019.