The role of surface energy fluxes in determining mixing layer heights

Authors: BEAMESDERFER, E - Northern Arizona University; BIRAUD, S. - Lawrence Berkeley National Laboratory; BRUNSELL, N. - University Of Kansas; FRIEDL, M. - Boston University; HELBIG, M. - Dalhousie University; HOLLINGER, D. - Us Forest Service (FS); MILLIMAN, T. - University Of New Hampshire; RAHN, D - Lawrence Berkeley National Laboratory; Scott, Russell; STOY, P.C. - University Of Wisconsin; DIEHL, J. - Northern Arizona University; RICHARDSON, A.D. - Northern Arizona University

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/25/2023
Publication Date: 9/11/2023
Citation: Beamesderfer, E., Biraud, S., Brunsell, N., Friedl, M., Helbig, M., Hollinger, D., Milliman, T., Rahn, D.A., Scott, R.L., Stoy, P., Diehl, J., Richardson, A. 2023. The role of surface energy fluxes in determining mixing layer heights. Agricultural and Forest Meteorology. 342. Article 109687. https://doi.org/10.1016/j.agrformet.2023.109687.
DOI: https://doi.org/10.1016/j.agrformet.2023.109687

Interpretive Summary: The atmospheric boundary layer (ABL), the lowest layer of the atmosphere that is in direct contact with the Earth’s surface, is a critical component of the Earth’s climate system and processes within the ABL control the exchange of energy, aerosols and pollutants between the land surface and the atmosphere. Determining the height of the ABL is important to understanding its dynamics. This study utilized new, continuous measurements of cloud heights and boundary layer heights and compared these data to neighboring land and atmospheric observations in an attempt to identify the surface influence of seasonal height dynamics at select sites across the United States. We found significant agreement between our measurements and traditional ones at two sites with co-located observations. The presence of clouds was shown to impact the growth and seasonality of boundary layer heights across all sites. Moreover, exchanges of water and heat at the Earth’s surface were found to have the largest relative strength in influencing afternoon heights. This study therefore presents an overview of an approach that examines the complicated land-atmosphere interactions that exist across ecosystems spanning a unique climatological gradient.

Technical Abstract: The atmospheric mixing layer height (MLH) is a critical variable for understanding and constraining ecosystem a nd climate dynamics. Past MLH estimation efforts have largely relied on data with low temporal (radiosondes) o r spatial (reanalysis) resolutions. This study is unique in that it utilized continuous point-based ceilometer- and r adiosonde-derived measurements of MLH at surface flux tower sites to identify the surface influence on MLH d ynamics. We found a strong correlation (R 2 = 0.73-0.91) between radiosonde MLH and ceilometer MLH at two sites with co-located observations. Seasonally, mean MLH was the highest at all sites during the summer, while t he highest annual mean MLH was found at the warm and dry sites, dominated by high sensible heat fluxes. At d aily time scales, surface fluxes of sensible heat, latent heat, and vapor pressure deficit had the largest influence o n afternoon MLH. However, at best, the identified forcing variables and surface fluxes only accounted for ~38- 6 5% of the variability in MLH under all sky conditions, and ~53-76% of the variability under clear skies. These r esults highlight the difficulty in using single-point observations to explain MLH dynamics but should encourage t he use of ceilometers or similar atmospheric measurements at surface flux sites in future studies.