EGU2020-15661
https://doi.org/10.5194/egusphere-egu2020-15661
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

What balloon soundings can tell about surface heat flux partitioning

Jasper Denissen1, Hendrik Wouters2, René Orth1, Diego Miralles2, and Ryan Teuling3
Jasper Denissen et al.
  • 1Max Planck Institute for Biogeochemistry, Biogeochemical Integration, Jena, Germany (jdenis@bgc-jena.mpg.de)
  • 2Hydrology and Climate research team, Ghent University, Ghent, Belgium
  • 3Hydrology and Quantitative Water Management Group, Wageningen University, Wageningen, Netherlands

The land surface can influence near-surface weather. This happens, amongst others, through the impact of soil moisture availability on surface heat fluxes: when soil moisture is unavailable in soil moisture-limited conditions, most of the available energy will be used for heating the air above the land surface (sensible heat flux). But as soil moisture increases, evapotranspiration (latent heat flux) increases, affecting the surface heat flux partitioning. At the point that ample soil moisture is available in energy-limited conditions, the surface heat flux partitioning remains unaffected by soil moisture. The atmospheric boundary layer (ABL) responds to changes in surface heat flux partitioning in particular in terms of its temperature and humidity. Based on these mechanisms, observations of boundary layer dynamics should allow to infer the large-scale land surface state.

The goal of this study is to use atmospheric measurements of temperature and humidity to estimate the surface heat flux partitioning. This is achieved by constraining an ABL model (CLASS4GL) with the vertical temperature and humidity profiles as observed by thousands of soundings of hot air balloons across the globe. In CLASS4GL, the initial soil moisture is adjusted to yield matching modelled versus observed vertical temperature and humidity profiles. By doing so, the resulting surface fluxes are inferred exclusively from atmospheric measurements.

We find that ABL’s tend to higher, warmer and drier in water-limited conditions. This largely results from changes in soil moisture availability, which mainly affects the sensible heat flux and consequently, the surface heat flux partitioning. We determine the critical soil moisture, which distinguishes between soil moisture- and energy- limited conditions, using the ratio between the sensible- and latent heat flux and independent satellite surface soil moisture.

This is the first time that balloon soundings are used globally to assess the critical soil moisture. This research will help to further improve our understanding of land-atmosphere feedbacks and foster a correct representation of land surface characteristics in Land Models and subsequently, Climate Models.

How to cite: Denissen, J., Wouters, H., Orth, R., Miralles, D., and Teuling, R.: What balloon soundings can tell about surface heat flux partitioning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15661, https://doi.org/10.5194/egusphere-egu2020-15661, 2020

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Presentation version 1 – uploaded on 03 May 2020
  • CC1: Ballon sounding representation of local flux, Annu Panwar, 05 May 2020

    Hey Jasper,

    It is a very interesting work. Looking forward to seeing more results in this work.

    A general question, how do you assess the representation of local flux in the balloon sounding. Ballons travel horizontally and may not anymore represent the surface energy partitioning. 

    I have observed, even in the SGP (which is comparatively homogeneous site) the boundary layer height increases on dry days, but its relationship to evaporation or sensible heat flux is still questionable. 

     

    • AC1: Reply to CC1, Jasper Denissen, 06 May 2020

      Hey Annu,

      Thanks for your question. It is true that balloons also travel horizontally. But the footprint from the balloon sounding at the boundary layer height from just travelling vertically is potentially already >100*100km^2. The horizontal displacement of the balloon needs to be substantial to get a different footprint. And I agree that balloon soundings do not represent the surface heat flux partitioning locally, they will carry information from a larger footprint. I think this is one of the advantages of this method, since many surface heat flux estimates struggle to include the complexity of the heterogenous land surface, whereas in this case the entire complexity is included in the measurements from balloon soundings.

      I don't know the site that you are mentioning, but why do you think that the relation of boundary layer height to latent/sensible heat flux is questionable?

      • CC2: Reply to AC1, Annu Panwar, 06 May 2020

        Hey Jasper,

        I looked at the PBL height estimated using ballon sounding in Southern Great Plains, Oklahoma. In general boundary layer height decreases on days with higher latent heat flux ( or higher evaporative fraction). But the relationship is not very strong. I think the discrepancy can be also from the eddy flux tower which measures the local fluxes.

        • AC3: Reply to CC2, Jasper Denissen, 06 May 2020

          As the boundary layer (height) is not only influenced by land surface processes, but for example also by the free tropospheric lapse rate, entrainment at the top of the boundary layer and advection, the relationship is not expected to be perfect. 

          I also think this could be a scaling problem. As I mentioned before the footprint of a balloon sounding is much larger than the footprint of an EC tower. The fluxes of the entire area that result in the boundary layer height, could deviate from the surface fluxes as measured from the EC tower, resulting in even more uncertainty in the relationship.

          What method do you use to determine the boundary layer height? There are some different methods to estimate the boundary layer height, but there is quite some variation between the resulting boundary layer heights, so I think also this uncertainty should be accounted for.