Seasonality in aerodynamic resistance across a range of North American ecosystems

Authors: YOUNG, A.M. - Northern Arizona University; FRIEDL, M.A. - Boston University; SEYEDNASROLLAH, B. - Northern Arizona University; BEAMESDERFER, E. - Northern Arizona University; CARRILLO, C.M. - Cornell University; XIAOLU, L. - Cornell University; MOON, M. - Boston University; ARAIN, M.A. - McMaster University; BALDOCCHI, D.D. - University Of California; BLANKEN, P.D. - University Of Colorado; BOHRER, G. - The Ohio State University; BURNS, S.P. - University Of Colorado; CHU, H. - Lawrence Berkeley National Laboratory; DESAI, A. - University Of Wisconsin; GRIFFIS, T.J. - University Of Minnesota; HOLLINGER, D.Y. - Us Forest Service (FS); LITVAK, M.E. - University Of New Mexico; NOVICK, K. - University Of Indiana; Scott, Russell - Russ; SUYKER, A.E. - University Of Nebraska; VERFAILLIE, J. - University Of California; WOOD, J.D. - University Of Missouri; RICHARDSON, A.D. - Northern Arizona University

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/15/2021
Publication Date: 9/3/2021
Citation: Young, A., Friedl, M., Seyednasrollah, B., Beamesderfer, E., Carrillo, C., Xiaolu, L., Moon, M., Arain, M., Baldocchi, D., Blanken, P., Bohrer, G., Burns, S., Chu, H., Desai, A., Griffis, T., Hollinger, D., Litvak, M., Novick, K., Scott, R.L., Suyker, A., Verfaillie, J., Wood, J., Richardson, A. 2021. Seasonality in aerodynamic resistance across a range of North American ecosystems. Agricultural and Forest Meteorology. 310. Article 108613. https://doi.org/10.1016/j.agrformet.2021.108613.
DOI: https://doi.org/10.1016/j.agrformet.2021.108613

Interpretive Summary: Phenology, the timing of seasonal vegetation growth and activity, may impact important land-atmosphere interactions through multiple underlying mechanisms, such as changing the amount of energy that is absorbed by the land and its surface temperature, altered surface roughness, or increases in plant transpiration. This work specifically investigates how phenology impacts and controls surface parameters related to the transfer of momentum and heat exchange. To conduct this work, we used about 170 site-years of specialized ecosystem energy exchange data and paired permanent camera data to quantify seasonal vegetation greenness from 23 sites. These sites cover a wide climatic range and include multiple vegetation types. There are two key findings from this work. First, in most ecosystems, a key heat exchange parameter decreases in response to phenological green-up. Second, we found that predictions of land-atmosphere heat exchanges are sensitive to varying assumptions of the seasonal behavior of surface roughness parameters. Accurately capturing seasonal variations in surface roughness significantly reduces prediction errors of heat exchange. Overall, this work provides insight into how phenology governs sensible heat exchange, important for understanding and modeling seasonal variability in land-atmosphere interactions.

Technical Abstract: Surface roughness – a key control on aerodynamic resistance and thereby land-atmosphere exchanges of heat and momentum – differs between dormant and growing seasons. How surface roughness responds to phenology at fine time scales (e.g. days) is not well understood. This study: (1) explores how phenology controls seasonal changes in the aerodynamic resistance; and (2) quantifies the importance of including seasonally changing aerodynamic resistance in model-based predictions of sensible heat flux (H). We evaluated aerodynamic resistance and surface roughness lengths for momentum (z0m) and heat (z0h) using the kB-1 parameter (ln(z0m /z0h)), derived from Monin-Obukhov Similarity Theory. We used AmeriFlux data to obtain surface roughness estimates, and PhenoCam data for phenology estimates. This analysis spanned a continental-scale precipitation and temperature gradient, including 23 sites and ~170 site years from deciduous broadleaf, evergreen needleleaf, woody savanna, agriculture, grassland, and shrubland plant-functional types (PFT). Results indicate clear seasonal patterns in aerodynamic resistance to heat transfer (Rah). For most PFTs, this seasonality was linked to PhenoCam-derived start-of-season green-up, with Rah decreasing in response to this transition. Our findings highlight decreases in kB-1 are an important control over Rah, explaining > 50% of total seasonal variation in Rah across most sites. Decreases in kB-1 during green-up were caused by increasing z0h in response to higher leaf area index and not to seasonal changes in z0m. There was high sensitivity in model-based predictions of H to using constant or seasonally changing kB-1. Specifically, assuming kB-1 to be constant resulted in significant prediction errors of H, displaying strong seasonal patterns. Our findings highlight surface roughness is sensitive to phenology at relatively fine temporal scales, and successfully accounting for phenology is critical for accurately modeling land-atmosphere coupling. More broadly, understanding this variation in aerodynamic resistance can ultimately provide insight into seasonal dynamics of land-atmosphere interactions.