the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Evaluation of a wind tunnel designed to investigate the response of evaporation to changes in the incoming longwave radiation at a water surface. I. Thermodynamic characteristics
Michael L. Roderick
Chathuranga Jayarathne
Angus J. Rummery
Callum J. Shakespeare
Abstract. To investigate the sensitivity of evaporation to changing longwave radiation we developed a new experimental facility that locates a shallow water bath at the base of an insulated wind tunnel with evaporation measured using an accurate digital balance. The new facility has the unique ability to impose variations in the incoming longwave radiation at the water surface whilst holding the air temperature, humidity and wind speed in the wind tunnel at fixed values. The underlying scientific aim is to isolate the effect of a change in the incoming longwave radiation on both evaporation and surface temperature. In this initial paper we describe the configuration and operation of the system and outline the experimental design and approach. We then evaluate the thermodynamic properties of the new system and demonstrate that the evaporation, air temperature, humidity and wind speed are measured with sufficient precision to support the scientific aims. We find that the shallow water bath naturally adopts a steady state temperature that closely approximates the thermodynamic wet bulb temperature.
Michael L. Roderick et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2022-986', Bernd Jähne, 10 Mar 2023
The authors intend to investigate the influence of longwave radiation on evaporation. They claim that because longwave radiation is absorbed in the top 20 µm at the ocean surface this heat source/sink might cause significant deviations of the surface temperature from the underlying water. This effect changes the saturation water vapor pressure at the water surface. Therefore the concentration difference between the water surface and a reference height is influenced and with it the water vapor flux and evaporation rate. They claim that this potentially large effect is completely ignored in bulk formulas for evaporation.
The authors therefore want to study the effect of longwave radiation on evaporation in a wind tunnel specifically designed for this purpose. In the first paper (egusphere-2022-986) they focus on a thermodynamic characterization of the facility, and in a second one (egusphere-2022-986) on the radiative characterization.
The subject of the study is ill-defined and the described facility and instrumentation not really suitable for the intended purpose. Therefore the reviewer recommends rejection of the publication of the two manuscripts.
The claim that “mass transfer formulations for evaporation … not directly consider the langwave radiative fluxes” is simply not correct. The reviewer did not perform a systematic literature search, but quickly found two almost 30 years old papers, dealing with the subject: Zhang and McPhaden 1995 (https://doi.org/10.1175/1520-0442(1995)008<0589:TRBSST>2.0.CO;2 ) and Fairall et al. 1996 (https://doi.org/10.1029/95JC03190). The actual version 3.6 of the COARE algorithm published on zenodo explicity includes longwave irradiation (named there IR flux): Bariteau et al., 2021 (https://doi.org/10.5281/zenodo.5110991)
The authors are obviously not familiar with the extended research work on the difference between the ocean surface temperature and the underlying bulk water (“cool skin”). Much of the pioneering work was done by Katsaros, see, e. g., Katsaros 1980 (https://doi.org/10.1007/BF00117914 ) or Katsaros 1990 (https://doi.org/10.1007/978-94-009-0627-3_9 ). A comprehensive account of the near-surface layer of the ocean is given in the monograph of Soloviev and Lukas 2014 (https://doi.org/10.1007/978-94-007-7621-0).
There are still, of course, open question. Most of them are related to the mechanisms of the transport from the ocean surface down to the bulk water, especially the influence of wind waves. The wind tunnel built by the authors is not suitable to address these questions because of the tiny and shallow water basin. A large wind-wave facility, such as the LASIF at the University of Marseilles (France) would be required for such studies (https://www.osupytheas.fr/?-LASIF-Grande-Soufflerie-air-eau-de-Luminy-&lang=en) and instrumentation and methods to image the water surface temperatures and temperature profiles in the aqueous viscous boundary layer.
Citation: https://doi.org/10.5194/egusphere-2022-986-RC1 - CC1: 'Author Reply on RC1', Michael Roderick, 15 Mar 2023
- AC2: 'Reply on RC1', Callum Shakespeare, 24 Mar 2023
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RC2: 'Comment on egusphere-2022-986', Nathan Laxague, 22 Mar 2023
A review of "Evaluation of a wind tunnel designed to investigate the response of evaporation to changes in the incoming longwave radiation at a water surface. I. Thermodynamic characteristics" by Michael L. Roderick, Chathuranga Jayarathne, Angus J. Rummery, and Callum J. Shakespeare.
In this manuscript, the authors describe an experimental setup for isolating the effect of longwave radiative flux on evaporation from a water surface. In short, the setup includes a wind tunnel containing a small water bath, with embedded sensors for measuring the humidity of the air and the temperature of the air and water. Additionally, incoming longwave radiative flux was measured via pyrgeometer and the water skin temperature was measured via microbolometer. The room containing the setup was described as being "temperature-controlled", though the cooling/ventilation system operated beyond the control of the authors, producing a noticeable oscillation in ambient temperature and humidity.
The topic as described by the authors is certainly of interest to the readership of AMT, and the manuscript was written with clear language. However, I have major concerns with some core elements of the laboratory setup. Furthermore, I found it difficult to assess the importance of this manuscript as an independent piece of research. The whole project is motivated by a desire to investigate the specific impact of longwave radiative flux on evaporation, but the details of the radiative component are left to the (as of yet unseen) part 2. I don't know that part 1 stands on its own as a meaningful contribution- is the result that bulk water temperature is sometimes close to the wet bulb temperature of the air above it? In any case, I believe that the work the authors have done can contribute the the body of knowledge, but I strongly recommend that they significantly revise this manuscript. Without seeing the second manuscript, I can only recommend that the revision to part 1 should include more details regarding the radiometric measurements (and results related to the total heat budget calculations). It may be that such a revision would combine the two parts- or that new radiative measurements need to be made with higher quality instrumentation, but it's impossible to say having only seen part 1 of the work.
Major Concerns
The room that was described as "temperature-controlled" showed a regular oscillation of ambient air temperature and specific humidity (approximately 1000 second period). This has a meaningful impact on what should be a sensitive measurement. To this point, the oscillation in incoming longwave radiative flux appears to track (in both shape and phase) the oscillation in humidity- not temperature. Does this mean that radiative flux sensed by the pyrgeometer is representative of more volumetric (path) absorption/emission in conditions of elevated humidity? In any case- unless this periodic behavior can be leveraged as an asset in the heat flux balance calculations (i.e., the longwave radiative flux is the only oscillating flux, allowing the authors to parse its effect on the evaporation), I believe it will be a major liability in the calculations.
Since the thin film-covered window only occupies a small segment of the hemisphere above the pool, how will the authors account for the difference between radiative flux emanating from the walls of the room and the flux emanating from the inside of the wind tunnel? Would a view factor (or some sort of solid angle accommodation) be helpful in accounting for this difference? Regardless, I'm not certain that a hemispheric pyrgeometer is the ideal sensor for this type of indoor, spatially heterogeneous measurement.
I mentioned the need for more information about the radiometry. It strikes me that measurements of the skin temperature are missing here. But even if they were included- can the authors make a case that the FLIR E50 is up to the challenge of providing high-quality radiometric measurements? It appears to be a handheld system that is optimized for qualitative evaluation of heat sources in industrial/construction use cases. If the authors are able to provide the results of blackbody calibration to establish the instrument's accuracy, stability, and low noise, that might put these fears to rest. But non research-grade microbolometers are notorious for being poor in those categories and are exceptionally prone to drift (which might be the worst type of error one could have when making the "steady state dis-equilibrium" measurements described here). A cooled single-point infrared radiation thermometer is usually regarded as the superior instrument for these sensitive measurements.
Minor Comments
- For most parameters, variability is represented via standard deviation (or 95% confidence intervals). However, several quantities (ambient air temperature, ambient air humidity, incoming longwave radiative flux) oscillate with the room's cooling system. Perhaps the authors could report the amplitude of oscillation for these quantities?
- The variation in enthalpy (Figure 9) is computed via temperature difference between the beginning and end of steady-state period. What could be learned from performing multiple short-window linear least-squares regressions during the steady state period, thereby obtaining a LHF estimate for each subwindow?
- The Kipp & Zonen radiometer is said to be located 'in the laboratory (but outside the tunnel)' during evaporation experiments. Could this be pointed out in Figure 2?
- It is a bit taxing to jump between Figure 3 and the body of the manuscript to find the definitions for the state variables. I recommend adding descriptive labels on the figure or more content to the caption.
- For Figure 3, I think that a hashmark or dot pattern along the solid regions of the tunnel setup would aid the reader in interpreting the setup. Furthermore, arrows or streamlines in the tunnel portion would be helpful.
- Figure 11 is effective, but the extra information presented in Figure 12 is difficult to digest. Perhaps the authors could establish the concept in Figure 11 (as already done), then replacing Figure 12 with scatter plots that relate Tw, TB (or better, Tskin), and the inferred psychrometer constants.
Citation: https://doi.org/10.5194/egusphere-2022-986-RC2 - AC1: 'Reply on RC2', Callum Shakespeare, 24 Mar 2023
Michael L. Roderick et al.
Data sets
Wind tunnel data Roderick, Jayarathne, Rummery, Shakespeare https://doi.org/10.5281/zenodo.7111987
Michael L. Roderick et al.
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