Comment on “On the Estimation of Potential Evaporation Under Wet and Dry Conditions” by Z. Tu and Y. Yang

Abstract

Tu and Yang (2022) employ the wet-surface temperature estimation method of Yang and Roderick (2019) in their potential evaporation estimation approach and frequently end up with wet-surface (T ws ) temperatures 10-12 K lower (seen e.g., in their Figure 9) than the actual air temperature (T a ). In their example of Figure 9, their estimates of T ws under dry conditions drop below even the actual wet-bulb temperature, T wb (compare Figure 9 with Figure 1 here). Note that between days 40 and 91 the T ws values of Yang and Roderick (2019) stay predominantly below 285 K, while it never happens with T wb in Figure 1. T wb represents the lowest possible temperature the air can be cooled by evaporation under an isenthalpic (i.e., adiabatic and isobaric) process (Figure 2) and zero net radiation (R n ) at the evaporating surface (i.e., at the wet-bulb of the thermometer). The hypothetical wet land surface however cannot be cooler than T wb (Monteith, 1981;Szilagyi, 2021) since at the land surface R n on a daily basis is almost always positive under typical conditions (and it is definitely so in each single day in Figure 1), raising its temperature above that of the wet-bulb of the thermometer (Monteith, 1981) where R n is zero, achieved by double metal tubing of the for example, aspirated psychrometer (e.g., Stull, 2000). Also, during wet environmental conditions (starting with Day 211 in Figure 9 with rains almost every day and relative humidity values often exceeding 85% in Figure 1) T ws cannot be expected (as seen in Figure 9) to be (significantly) lower than the actual air temperature measured over the wet land and therefore yielding downward sensible heat (H) fluxes. It would contradict the common observation that even over extensive wet surfaces the equilibrium air (potential) temperature profile near the surface is decreasing with elevation facilitating an upward H.Tu and Yang (2022) dismisses the T ws estimation method of Szilagyi and Jozsa ( 2008) that assumes unchanging net radiation (R n ) during drying/wetting of the environment, by arguing that net radiation would increase with decreasing surface temperatures (due to declining thermal radiation of the surface, R lo ) as the surface becomes wetter. However, they did not take into consideration that a weakening incoming shortwave radiation can counteract the effect of dropping R lo on R n as cloudiness and humidity typically increase with a regional wetting of the land surface, thus making it possible to leave R n practically intact (Brutsaert, 1982).In summary, the wet-surface temperature estimation method of Yang and Roderick (2019) as employed by Tu and Yang (2022) appears to significantly underestimate the wet surface temperature leading to physical/thermodynamical contradictions. At the same time the Szilagyi and Jozsa (2008) estimated T ws values always stay above T wb and predominantly above T a (depending on e.g., the degree of saturation of the air) under wet conditions, as demonstrated in Figure 1 and therefore claimed to be a more realistic wet-surf...

Reply to Comment on “Potential Evaporation and the Complementary Relationship” by Jozsef Szilagyi

“…The fact that T23 and YR19 are able to recover the observed T ws has already demonstrated that the T ws in T23 and YR19 is physically attainable. Please see our previous reply to Szilagyi (2022) for further details (Yang et al, 2022).…”

Comment on “Potential Evaporation and the Complementary Relationship” by Tu et al. (2023)

“…Such can happen in monsoon regions of the world where during arrival of the rainy season both R sc and cloudiness increase, with a possible zero net effect on R sn . S17 also avoids physical contradictions such as estimated T ws somehow could be smaller than T wb (and all that when R n is supposed to increase at the surface during wetting of the environment provided R sn is kept constant), something that T23 routinely allows for (Szilagyi, 2022) and thermodynamics strictly forbids. When the decrease in R sn due to the forming clouds is not accounted for during an envisioned wetting phase under an assumed constant R sn value then one way to meet the ensuing bloated R n estimate is via excessively cutting back on the estimated T ws value.…”