Disentangling the impact of air-sea interaction and boundary layer cloud formation on stable water isotope signals in the warm sector of a Southern Ocean cyclone

<p>Stable water isotopes in marine boundary layer water vapour are strongly influenced by the strength of air-sea moisture fluxes and are thus tracers of air-sea interaction. Air-sea moisture fluxes in the extratropics are modulated by large-scale air advection, for instance the advection of warm and moist air masses in the warm sector of extratropical cyclones. A distinct isotopic composition of water vapour in the latter environment has been observed in near-surface water vapour over the Southern Ocean during the 2016/17 Antarctic Circumnavigation coordinated by the Swiss Polar Institute. Most prominently, the second-order isotope variable d-excess shows negative values in the cyclones’ warm sector. Here, we present three single-process air parcel models, which simulate the evolution of d-excess and specific humidity in an air parcel induced by dew deposition, decreasing ocean evaporation or upstream cloud formation, respectively. The air-parcel models are combined with simulations with the isotope-enabled numerical weather prediction model COSMO<sub>iso</sub> (i) to validate the air parcel models, (ii) to study the extent of non-linear interactions between the different processes, and (iii) to quantify the relevance of the three processes for stable water isotopes in the warm sector of the investigated extratropical cyclone. This analysis reveals that dew deposition and decreasing ocean evaporation lead to the strongest d-excess decrease in near-surface water vapour in the warm sector. Furthermore, COSMO<sub>iso</sub> air parcel trajectories show that the persistent low d-excess observed in the warm sector of extratropical cyclones is not a result of material conservation of low d-excess. Instead the latter feature is sustained by the continuous production of low d-excess values in new air parcels entering the warm sector. We show that with the mechanistic approach of using single-process air parcel models we are able to simulate the evolution of d-excess during the air parcel’s transport. This improves our understanding of the effect of air-sea interaction and boundary layer cloud formation on the stable water isotope variability of marine boundary layer water vapour.</p>

Análise noturna dos mecanismos que controlam os gradientes de CO2 e H2O em uma região costeira

Understanding the water and carbon exchange in the soil-atmosphere system and the mechanisms that control these exchanges are fundamental to have a real and adequate representation of the physical processes involved. Under stable night conditions, carbon transfer processes such as respiration and water transfer processes such as dew deposition are more difficult to study by the eddy covariance method. Thus, the aim of this study is to investigate the mechanisms that control the exchange of water and carbon dioxide near a thermal power plant and the ocean, through the gradients of concentration of these quantities at different levels in the atmosphere. Data from the experimental site located in Linhares - ES - Brazil (19° 31'53 "S, 39° 48'03" W), 4 km from the coast and about 250 m from Linhares Geração SA thermeletric, were used. The concentrations of CO2 and H2O are measured by the AP200-Campbell system, which consists of vertically arranged chambers that pull gas flow at the following levels 1, 2, 5, 9, 15 and 20 m. In this study, a total of 67 nights (from 9 pm to 5 am), from 2017, were selected. The wind direction was from the north so that there was no interference from the thermal plant's feathers in the data analysis.

Drivers of stable water isotope variability in the cold and warm sector of extratropical cyclones from two case studies in the Southern Ocean

<p>Dynamical processes in the atmosphere strongly influence the large temporal and spatial variability of the atmospheric branch of the water cycle. For instance, the advection of air masses by synoptic-scale weather systems induces air-sea moisture fluxes such as evaporation, precipitation and dew deposition. It is important to better investigate and quantify this linkage between dynamical phenomena and details of the atmospheric water cycle. In addition, one of the big challenges in monitoring the atmospheric water cycle is the measurement of turbulent moisture fluxes over the ocean. Stable water isotopes (SWIs) serve as a tool to trace atmospheric processes which shape the atmospheric water cycle and, thus, provide important insights into moist processes associated with weather systems, in particular air-sea fluxes.</p><p>In this study, we investigate the impact of air-sea moisture fluxes on the variability of SWI signals in the marine boundary layer. Measurements of the second-order isotope variable deuterium excess in the marine boundary layer of the Southern Ocean show positive/negative anomalies in the cold/warm sector, respectively, of extra-tropical cyclone due to opposing moisture fluxes and non-equilibrium fractionation processes in the two sectors. The drivers of these contrasting SWI signals are analysed using the isotope-enabled Consortium for Small-Scale Modelling model for two case studies. The simulated isotope signals during the case studies show excellent agreement with ship-based isotope measurements from the Southern Ocean performed during the Antarctic Circumnavigation expedition in January and February 2017.</p><p>The main driver of SWI variability in the cold sector is enhanced ocean evaporation which substantially modifies the advected SWI signal from the Antarctic continent during a cold air outbreak. In the warm sector, dew deposition on the ocean surface and cloud formation are mainly driving the observed negative deuterium excess anomaly, which can be conserved and advected over several 100 km in the warm sector of an extratropical cyclone.</p><p>The results of this study illustrate the strong dependence of the isotopic composition of water vapour in the marine boundary layer on the predominant atmospheric large-scale flow situation. In particular in the storm track regions, the variability of SWIs in marine boundary layer water vapour is largely shaped by the sign and strength of air-sea fluxes induced by the meridional transport of air masses.</p>

Dew Can Prolong Photosynthesis and Water Status During Drought in Some Epiphytic Bromeliads From a Seasonally Dry Tropical Forest

Dew can represent an alternate water source in epiphytic bromeliads. However, the physiological relevance of dew to withstand the dry season, within seasonal forests, is not fully understood. To study the effect of dew deposition in the physiological response of four Tillandsia species with contrasting morphologies, we performed an experiment in the tropical dry deciduous forest of Dzibilchaltún, Mexico, during the transition from the wet to the dry season. Half of the individuals were covered every night with a plastic tarp to prevent dew deposition. Environmental variables were monitored, and physiological variables (relative water content, leaf succulence, nocturnal tissue acidification and electron transport rate) were measured at the beginning and end of the experiment. We found that throughout the drought, there was consistent nighttime dew formation for >4 h. Both the time the leaves spent at a temperature below dew point of the air and the effect on water and carbon metabolism was species -specific, as evidenced by the comparison among the exposed and covered (dew -deprived) plants. Tillandsia elongata and Tillandsia brachycaulos had longer times of dew formation and showed higher water content at the end of the experiment when exposed to dew, the latter species also had a significant effect of dew on nocturnal acidity. In contrast, neither Tillandsia yucatana nor Tillandsia fasciculata seemed to be using dew as a relevant source of water during the dry period. We discuss the species’ morphoanatomical traits that may be related to the differences in dew formation and use.

Dew-induced transpiration suppression impacts the water and isotope balances of Colocasia leaves

Foliar uptake of water from the surface of leaves is common when rainfall is scarce and non-meteoric water such as dew or fog is more abundant. However, many species in more mesic environments have hydrophobic leaves that do not allow the plant to uptake water. Unlike foliar uptake, all species can benefit from dew- or fog-induced transpiration suppression, but despite its ubiquity, transpiration suppression has so far never been quantified. Here, we investigate the effect of dew-induced transpiration suppression on the water balance and the isotope composition of leaves via a series of experiments. Characteristically hydrophobic leaves of a tropical plant, Colocasia esculenta, are misted with isotopically enriched water to reproduce dew deposition. This species does not uptake water from the surface of its leaves. We measure leaf water isotopes and water potential and find that misted leaves exhibit a higher water potential (p < 0.05) and a more depleted water isotope composition than dry leaves (p < 0.001), suggesting a ~30% decrease in transpiration rate (p < 0.001) compared to control leaves. We propose three possible mechanisms governing the interaction of water droplets with leaf energy balance: increase in albedo from the presence of dew droplets, decrease in leaf temperature from the evaporation of dew, and local decrease in vapor pressure deficit. Comparing previous studies on foliar uptake to our results, we conclude that transpiration suppression has an effect of similar amplitude, yet opposite sign to foliar uptake on leaf water isotopes.

On the apparent CO₂ absorption by alkaline soils

Alkaline soils in the Gubantonggut Desert were recently demonstrated socking away large quantities of CO2 in an abiotic form. This demands a better understanding of abiotic CO2 exchange in alkaline sites. Reaction of CO2 with the moisture or dew in the soil was conjectured as a potential mechanism. The main goal of this study is to determine the extent to which the dew deposition modulates Land–Atmosphere CO2 exchange at highly alkaline sites (pH ~ 10). Experiments were conducted at the most barren sites (canopy coverage < 5%) to cut down uncertainty. Dew quantities and soil CO2 fluxes were measured using a micro-lysimeters and an automated flux system (LI-COR, Lincoln, Nebraska, USA), respectively. There is an evident increase of dew deposition in nocturnal colder temperatures and decrease in diurnal warmer temperatures. Variations of soil CO2 flux are almost contrary, but the increase in diurnal warmer temperatures is obscure. It was shown that the accumulation and evaporation of dew in the soil motivates the apparent absorption and release of CO2. It was demonstrated that dew amounts in the soil has an exponential relation with the part in Fc beyond explanations of the worldwide utilized Q10 model. Therefore dew deposition in highly alkaline soils exerted a potential CO2 sink and can partly explain the apparent CO2 absorption. This implied a crucial component in the net ecosystem carbon balance (NECB) at alkaline sites which occupies approximately 5% of the Earth's land surface (7 million km). Further explorations for its mechanisms and representativeness over other arid climate systems have comprehensive perspectives in the quaternary research.