Turbulent transport of energy across a forest and a semiarid shrubland

Abstract

Abstract. The role of secondary circulations has recently been studied in the context of well-defined surface heterogeneity in a semiarid ecosystem where it was found that energy balance closure over a desert-forest system and the structure of the boundary layer was impacted by advection and flux divergence. As a part of the CliFF ("Climate feedbacks and benefits of semi-arid forests", a collaboration between KIT, Germany, and the Weizmann Institute, Israel) campaign, we studied the boundary layer dynamics and turbulent transport of energy corresponding to this effect in Yatir Forest situated in the Negev Desert in Israel. The forest surrounded by small shrubs presents a distinct feature of surface heterogeneity, allowing us to study the differences between their interactions with the atmosphere above by conducting measurements with two eddy covariance (EC) stations and two Doppler lidars. As expected, the turbulence intensity and vertical fluxes of momentum and sensible heat are found to be higher above the forest compared to the shrubland. Turbulent statistics indicative of nonlocal motions are also found to differ over the forest and shrubland and also display a strong diurnal cycle. The production of turbulent kinetic energy (TKE) over the forest is strongly mechanical, while buoyancy effects generate most of the TKE over the shrubland. Overall TKE production is much higher above the forest compared to the shrubland. The forest is also found to be more efficient in dissipating TKE. The TKE budget appears to be balanced on average both for the forest and shrubland, although the imbalance of the TKE budget, which includes the role of TKE transport, is found to be quite different in terms of diurnal cycles for the forest and shrubland. The difference in turbulent quantities and the relationships between the components of TKE budget are used to infer the characteristics of the turbulent transport of energy between the desert and the forest.

Multi-variable turbulence characteristics of the daytime atmospheric boundary layer over an arid region using measurements from a 200-m tall tower

“…Additionally, there are similarities in the sign of S w (z) being negative below 10-m in the results from Dupont and Patton (2012) with the S w (z) presented here. Further resemblance is found with the sign of the triple correlation of w' in Banerjee et al 2018 and the sign of S w (z) found in our vertical velocity analysis. There exist some differences, however, with the sign of S w (z).…”

“…Results from the 2 u (z) shown in Figure 4a are similar in magnitude to 2 u (z) calculated in Banerjee et al 2018. The maximum 2 u (z) of 10.56 m 2 s -2 reported here, however, is much greater than the maximum 2 u (z) (~4 m 2 s -2 ) found by Banerjee et al 2018.…”

“…The 2 w (z) results have both similarities and differences with previous literature. Banerjee et al (2018) conducted turbulence studies over a semi-arid shrubland using meteorological tower measurements at the heights of 9.0 and 16.0 m. Comparing their results with the respective measurement levels on the 200-m RTC tower (i.e., 10.1 and 16.8 m) highlights similarities in the magnitude of the 2 w (z) between both studies. Not only is the magnitude of 2 w (z) similar (i.e., 2 w (z) ~0.30 -0.50 m 2 s -2 ), the diurnal cycle of 2 w (z) is also strikingly alike.…”

“…Another impact that could explain the differences in results (Figure 3) with previous literature is the difference in sampling rates and response time. The sampling rate of the sonic anemometers deployed on the RTC 200-m tower is 50 Hz while the sampling rates from previous literature for the vertical velocity component varied (i.e., 60 Hz in Dupont and Patton (2012); 20 Hz in Banerjee et al 2018;and 10 Hz in Martins et al 2021). An instrument with a faster sampling rate (e.g., Dupont and Patton (2012)) and lower response time would help resolve turbulent eddies more accurately than with the sensors with slower sampling rate and higher response time as reported in Banerjee et al 2018.…”

“…The sampling rate of the sonic anemometers deployed on the RTC 200-m tower is 50 Hz while the sampling rates from previous literature for the vertical velocity component varied (i.e., 60 Hz in Dupont and Patton (2012); 20 Hz in Banerjee et al 2018;and 10 Hz in Martins et al 2021). An instrument with a faster sampling rate (e.g., Dupont and Patton (2012)) and lower response time would help resolve turbulent eddies more accurately than with the sensors with slower sampling rate and higher response time as reported in Banerjee et al 2018. Moreover, this might result in higher 2 w (z) with an instrument using a higher sampling rate and/or having a lower response time.…”