| 17 Feb 2026
CO2 and Heat exchange across the Nocturnal Canopy–Atmosphere interface in the Amazon rainforest
Abstract. We investigated the characteristics of the nocturnal boundary layer (NBL) above and within the Amazon rainforest canopy during the 2022 dry season. The study aims to determine how NBL dynamics influence nocturnal CO2 and heat exchange across the canopy-atmosphere interface. Utilising observations from the CloudRoots-Amazon22 field campaign conducted at the Amazon Tall Tower Observatory, we distinguished between the strongly and weakly stable regimes to study the effect of radiative cooling and wind shear on CO2 and heat exchange. Our results reveal a distinct, stable layer above the canopy with an average height of 150 to 188 m, which develops due to strong radiative cooling of the canopy top. Below the canopy, the cooling marks the build-up of a well-mixed layer within the canopy. In the weakly stable regime, increased turbulence at the canopy top was observed, leading to a significant observed CO2 flux of 3.82 𝜇mol m−2 s−1 above the canopy. In contrast, in the strongly stable regime, turbulence was almost absent, and the observed flux was only 0.16 𝜇mol m−2 s−1, suggesting a decoupling of the canopy and the roughness sublayer. The decoupling was confirmed by the 2–3 times decreased vertical heat transport in the strongly stable regime. Even though our method includes typical observational uncertainties, our results show significant differences between CO2 and heat exchange between the two regimes, stressing the importance of correctly representing the nocturnal dynamics in tall canopies like the Amazon Rainforest.
The authors aim to understand how the nocturnal boundary layer (NBL) dynamics influences CO2 and heat exchange between the air layers within and above the Amazon rainforest canopy. The field observations within and above the canopy layer are unique, even though the authors focus on a relatively short period of about two weeks during the Amazon dry season.
I am disappointed about the analysis methods and confused by many detailed descriptions of analyses. I encourage the authors to think more carefully about the key factors in driving CO2 and heat exchange and to demonstrate more clearly what the field observations reveal. I believe readers could learn more from the field analysis without the introduction of various unnecessary assumptions.
Major comments:
1. There are two radiative coolings at night: longwave radiative cooling from the surface and the canopy layer, and the cooling associated with vertical radiative divergence within the air layer, either above or within the canopy. For the development of the NBL, radiative cooling below the NBL is generally much more important than vertical radiative divergence, based on previous field observations as far as I understand. It is therefore important to distinguish the two especially when discussing radiative cooling from the soil surface and the canopy surface. I was often confused about which process was discussed in the manuscript.
With cooling occurring below the air layer, whether from the soil, the canopy surface, or both, the development of turbulent mixing under stable conditions is primarily determined by wind shear as demonstrated clearly in Fig. 5.
2. With the above considerations in mind, my major confusion is why the authors used six criteria to classify the observation based on previous investigations. Although the fundamental physics governing the atmospheric boundary layer is the same, the observation heights and canopy structure in this study differ from all the studies listed in Table 1. Why can the authors not analyze their own observations directly and then highlight the unique characteristics of different stability regimes based on what they observe? After reading the abstract, I expected a comparison between the air layers above and within the canopy and kept looking for distinct stability regimes in each layer. Only after reading the manuscript several times, I realized that all six criteria are based on observations above the canopy layer. Thus the distinction between strongly and weakly stable regimes applies only to the air layer above the canopy.
3. Since the study deals with turbulent transfer across the two layers, a brief description of turbulence measurements such as sensor types and data processing methods would help readers understand the data used in the study even if detailed information is available elsewhere.
Because the study concerns the CO2 and heat exchange between the two layers, it is also important know the canopy density, tree types, and whether low vegetation is present near the soil surface.
In addition, because the analysis is based on measurements from two towers, the manuscript should specify how far apart the two towers are and whether they are within the same forest stand.
4. I am particularly puzzled by evaluation of the boundary layer height. From what I understand, the measurement extends only to 298 m, assuming above the ground. With an average tree height of about 20 m, the maximum depth of the air layer above the canopy layer is therefore about 278 m. Considering the development of the stable air layer above a cooling surface, the NBL impacted by the surface can be quite shallow. Therefore, the NBL heigh, h_NBL, might be directly inferred from vertical profiles of turbulent variables measured from the two towers. Furthermore, any coupling between the two layers should be readily observable from the vertical variation of turbulent variables across the two layers under different conditions.
5. The description of the data used in this study is confusing. See my detailed comments below.
Detailed comments
Figure 1: Is this figure intended as a schematic illustrating the five criteria? Also are NLLJs frequently observed under the strongly stable conditions?
L.112: What is h_theta?
L 119: How do the authors justify the assumption of U=U_g? It seems to me that this is an unnecessary assumption, especially if the goal is simply to classify the strongly and weakly stable regimes.
L 123: If “sufficient turbulence above the canopy is required for the application of the criteria”, why is it necessary to separate the strongly and the weakly stable stratification?
L 126: At which measurement level is TKE assumed to be 0.2 TKE_0?
Eq. (2). Ri_can is not actually an in-canopy layer Richardson number as stated in L. 133. Instead, it represents a Ri across the canopy top.
L. 157: Do the authors mean horizontal advection of TKE?
L. 158: is theta_v derived from sonic anemometer temperature measurements?
L. 168: Assuming latent heat release to be zero at night is questionable. Even under very stable conditions with weak turbulent mixing, evaporation generally continues throughout the night.
L. 180: What do the authors mean by “sufficient to get a realistic estimate of radiative cooling”? See Hoch et al. (2007 JAMC, 46, 1469).
L. 187: There is no molecular viscosity in the CO2 balance.
L. 196: With such a valuable two-weeks observational dataset, only eight hours of the data are used for the study? That corresponds to less than 6% of the nighttime observations. What characterizes the excluded hours ? Where they associated with Intermittent turbulent mixing? Since clouds are mentioned here, where the selected hours limited to cloud-free conditions? By the end of the manuscript, it appears that Scu clouds were frequently present even during the dry season. Is there a way to identify impacts of cloud cover on surface radiative cooling and the subsequent development of the NBL? How representative are the selected data for this study overall? How many nights were ultimately included in the analyses?
L. 200: Should this refer to Fig. 2 instead of Fig.5?
L. 238: Only 8 hrs data were used for the study. How many independent data points can be obtained from a two-hour average?
Fig. 3: Are these profiles derived from the 8-hours observations mentioned earlier?
L.243: “…, a shallow stable daytime layer within the canopy was present”. Based on Fig.3, both the entire NBL and the within-canopy layer appear stable. Is Fig. 3 constructed from the three nights listed in Table 2? If ambient wind and temperature vary from night to night, then Fig. 3 does not represent the temporal variation of theta and CO2 during any individual night. What exactly does the figure represent then?
L. 250: S2,…S4 and W3 are not introduced here without prior explanation. I eventually fount them in Table 2, and they are only discussed later in L 294. Does this mean that all analyses are based on the three nights listed in Table 2? On 13 August, the NBL apparently transitioned from weakly stable to strongly stable conditions. It would be interesting to see how this transition occurred.
L. 253: A case study illustrating the evening transition would be particularly informative.
L. 255. “Black error bars”? All the horizontal bars in the figure appear red. How were the error bars calculated? Do they represent variations between nights?
L. 257: The two proposed mechanisms sound good. Which one is more important for the present study?
L. 273: “from 11 nights”? I am totally lost here. The “eight hours” mentioned at the beginning of section 3 gave the impression that the entire section was based on that limited dataset. Does the observational dataset vary between subsections of section 3? As Table 2 does not cover the full period from 18:00 to 06:00, please specify the data used for each figure and analysis.
Figure 5. I find this figure is the most informative although I still don’t know which nights are represented.
L. 313: “the four selected periods”, where were they described?
Figures 6, 7, 8: What do the shaded areas represent?
L. 448: Clouds. Yes, it would be interesting to see their impact on the NBL development.
L. 486: The study focuses on nighttime conditions during which stable layers are present throughout. Is the discussion here referring to longwave radiation emitted from the canopy top or to radiative divergence cooling within the atmosphere?
Fig. B1: Interesting! Whenever winds are strong over the canopy layer (W1-W4), turbulent mixing in the air layer above the canopy can penetrate into the canopy layer resulting in enhanced heat transfer. When winds above the canopy layer are weak (S1-S4), the positive heat flux below the canopy crown (presumably) in the nearly neutral canopy layer (Fig. 5a) may develop because shear-generated turbulent mixing transports warm air downward from above the canopy while turbulent development near the soil surface remains constrained.