Free convection analytical form of approaches based on surface renewal theory to estimate sensible and latent heat fluxes

Authors: CASTELLVI, F - Universitat De Lleida; RUNKLE, B.R.K. - University Of Arkansas; SUVOCAREV, K. - University Of Arkansas; Reba, Michele

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2020
Publication Date: 4/2/2020
Citation: Castellvi, F., Runkle, B., Suvocarev, K., Reba, M.L. 2020. Free convection analytical form of approaches based on surface renewal theory to estimate sensible and latent heat fluxes. Journal of Hydrology. 586:124917. https://doi.org/10.1016/j.jhydrol.2020.124917.
DOI: https://doi.org/10.1016/j.jhydrol.2020.124917

Interpretive Summary: Micrometeorologic measurements are expensive to establish, maintain, and process the data generated from them, but they are extremely important for land surface modeling. A new approach, Surface Renewal of the Smallest Eddies (SRSE), is described that allows for an estimate of eddy fluxes without taking measurements of mean wind speed. Using eddy covariance (EC) measurements as a reference against the new approach found that the magnitudes of sensible and latent heat were dependent on sampling frequency with 10 Hz resulting in the best surface energy balance closure. Generally the SRSE approach was unbiased and was determined to perform reliably as an independent method. The use of SRSE is less expensive than using EC and could expand the number of locations where eddy flux measurements are being made on the landscape. More measurements at varied locations will help to improve our understanding of sensible and latent heat exchange.

Technical Abstract: The formulation based on surface renewal theory and analysis of the smallest eddies (SRSE) to estimate the sensible heat flux (H) when measurements were taken in the inertial sublayer was extended to estimate the latent heat flux (LE) with the aim to derive a free convection limit (FCL) approach for SRSE (SRSE-FCL). This new approach allows estimating eddy fluxes without taking measurements of the mean wind speed and, therefore, is well-suited for calm conditions. For FCL sensible heat flux estimates (HSRSE-FCL), the input requirements are traces of the fast-response (such as 10 to 20 Hz) air temperature and the zero-plane displacement. For FCL latent heat flux estimates (LESRSE-FCL), input requirement are fast response traces of water vapor density, mean temperature of the air, the available net surface energy (Rn-G, where Rn and G are the net radiation and soil heat flux, respectively) and the zero-plane displacement. Taking eddy covariance (EC) as a reference method (operating at 20 Hz), the performance of the SRSE-FCL method was tested over a growing cotton field which involved three contrasting surfaces: partly mulched bare soil, a sparse canopy and a homogeneous canopy. The magnitude of HSRSE-FCL and LESRSE-FCL had unclear dependency on the sampling frequency. HSRSE-FCL overestimated and underestimated sensible heat fluxes compared to EC flux (HEC) using scalar traces at 10 and 20 Hz, respectively. Regardless of the sampling frequency, in general, LESRSE-FCL tended to slightly underestimate LEEC. However, the SRSE based formulation was, in practice, unbiased. The surface energy balance closure show that (HEC + LEEC) underestimated (Rn-G) in a range of 19% (homogeneous canopy) and 8% (sparse canopy). Given the EC technique’s lack of closure it was difficult to recommend the frequency that best suits the SRSE-based formulation. Regardless of the frequency, (HSRSE-FCL + LESRSE-FCL) performed close to (Rn-G) and (HEC + LEEC). Consistently, (HSRSE-FCL + LESRSE-FCL) operating at 10 Hz had the best closure. Therefore, SRSE-FCL performed reliably as an independent method.