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

Stable water isotopes in marine boundary layer water vapour are strongly influenced by the strength of air–sea fluxes. Air–sea fluxes in the extratropics are modulated by the large-scale atmospheric flow, for instance by the advection of warm and moist air masses in the warm sector of extratropical cyclones. A distinct isotopic composition of the water vapour in the latter environment has been observed over the Southern Ocean during the 2016/2017 Antarctic Circumnavigation Expedition (ACE). Most prominently, the secondary isotope variable deuterium excess (d=δ2H–8⋅δ18O) shows negative values in the cyclones’ warm sector. In this study, three mechanisms are proposed and evaluated to explain these observed negative d values. We present three single-process air parcel models, which simulate the evolution of δ2H, δ18O, d and specific humidity in an air parcel induced by decreasing ocean evaporation, dew deposition and upstream cloud formation. Simulations with the isotope-enabled numerical weather prediction model COSMOiso, which have previously been validated using observations from the ACE campaign, are used to (i) validate the air parcel models, (ii) quantify the relevance of the three processes for stable water isotopes in the warm sector of the investigated extratropical cyclone and (iii) study the extent of non-linear interactions between the different processes. This analysis shows that we are able to simulate the evolution of d during the air parcel's transport in a realistic way with the mechanistic approach of using single-process air parcel models. Most importantly, we find that decreasing ocean evaporation and dew deposition lead to the strongest d decrease in near-surface water vapour in the warm sector and that upstream cloud formation plays a minor role. By analysing COSMOiso backward trajectories we show that the persistent low d values observed in the warm sector of extratropical cyclones are not a result of material conservation of low d. Instead, the latter Eulerian feature is sustained by the continuous production of low d values due to air–sea interactions in new air parcels entering the warm sector. These results improve our understanding of the relative importance of air–sea interaction and boundary layer cloud formation on the stable water isotope variability of near-surface marine boundary layer water vapour. To elucidate the role of hydrometeor–vapour interactions for the stable water isotope variability in the upper parts of the marine boundary layer, future studies should focus on high-resolution vertical isotope profiles.publishedVersio

Unravelling the transport of moisture into the Saharan Air Layer using passive tracers and isotopes

The subtropical free troposphere plays a critical role in the radiative balance of the Earth. However, the complex interactions controlling moisture in this sensitive region and, in particular, the relative importance of long-range transport compared to lower-tropospheric mixing, remain unclear. This study uses the regional COSMO model equipped with stable water isotopes and passive water tracers to quantify the contributions of different evaporative sources to the moisture and its stable isotope signals in the eastern subtropical North Atlantic free troposphere. In summer, this region is characterized by two alternating large-scale circulation regimes: (i) dry, isotopically depleted air from the upper-level extratropics, and (ii) humid, enriched air advected from Northern Africa within the Saharan Air Layer (SAL) consisting of a mixture of moisture of diverse origin (tropical and extratropical North Atlantic, Africa, Europe, the Mediterranean). This diversity of moisture sources in regime (ii) arises from the convergent inflow at low levels of air from different neighbouring regions into the Saharan heat low (SHL), where it is mixed and injected by convective plumes into the large-scale flow aloft, and thereafter expelled to the North Atlantic within the SAL. Remarkably, this regime is associated with a large contribution of moisture that evaporated from the North Atlantic, which makes a detour through the SHL and eventually reaches the 850–550 hPa layer above the Canaries. Moisture transport from Europe via the SHL to the same layer leads to the strongest enrichment in heavy isotopes (δ2H correlates most strongly with this tracer). The vertical profiles over the North Atlantic show increased humidity and δ2H and reduced static stability in the 850–550 hPa layer, and smaller cloud fraction in the boundary layer in regime (ii) compared to regime (i), highlighting the key role of moisture transport through the SHL in modulating the radiative balance in this region

Effect of Oil-in-Water Emulsions on 5-Aminolevulinic Acid Uptake and Metabolism to PpIX in Cultured MCF-7 Cells

No Heading: Purpose.: To identify the optimal vehicle for fast and efficient cellular production of the photosensitizer, protoporphyrin IX (PpIX), upon administration of 5-aminolevulinic acid (ALA). Methods.: ALA in various oil/water o/w emulsions was applied to the human mammary epithelial cell line (MCF-7) cultured in microplates. Upon incubation for 1-4 h, the accumulated amount of PpIX was determined by fluorescence spectroscopy. Variables such as the pH and concentration of the emulsions, the temperature and duration of incubation were examined along with the importance of ALA concentration and the presence of endocytosis inhibitors. Results.: An increase in the amount of produced PpIX was observed with an increase in extracellular pH, incubation temperature, and ALA concentration. A saturable mechanism of PpIX accumulation was evident, mainly as a result of the uptake mechanism for ALA. Some of the o/w emulsions increased the amount of intracellular PpIX, and the results indicated that this was not due to an increased km of the extracellular ALA to intracellular PpIX conversion, but to the increased endocytotic uptake in the presence of the emulsions. In general, the increase in PpIX in the presence of emulsions relative to the control was more pronounced after 1 h as compared to after 2-4 h. Conclusions.: The formation of PpIX in MCF-7 cells exposed to ALA is improved by the presence of certain o/w emulsions, which could be explained by endocytosi

How model uncertainties influence tropical humidity in global storm-resolving simulations

We conduct a series of eight 45-day experiments with a global storm-resolving model (GSRM) to test the sensitivity of relative humidity R in the tropics to changes in model resolution and parameterizations. These changes include changes in horizontal and vertical grid spacing as well as in the parameterizations of microphysics and turbulence, and are chosen to capture currently existing differences among GSRMs. To link the R distribution in the tropical free troposphere with processes in the deep convective regions, we adopt a trajectory-based assessment of the last-saturation paradigm. The perturbations we apply to the model result in tropical mean R changes ranging from 0.5 to 8 (absolute) in the mid troposphere. The generated R spread is similar to that in a multi-model ensemble of GSRMs and smaller than the spread across conventional general circulation models, supporting that an explicit representation of deep convection reduces the uncertainty in tropical R. The largest R changes result from changes in parameterizations, suggesting that model physics represent a major source of humidity spread across GSRMs. The R in the moist tropical regions is disproportionately sensitive to vertical mixing processes within the tropics, which impact R through their effect on the last-saturation temperature rather than their effect on the evolution of the humidity since last-saturation. In our analysis the R of the dry tropical regions strongly depends on the exchange with the extra-tropics. The interaction between tropics and extratropics could change with warming and presage changes in the radiatively sensitive dry regions

Assessing the Sampling Quality of a Low-Tech Low-Budget Volume-Based Rainfall Sampler for Stable Isotope Analysis

To better understand the small-scale variability of rainfall and its isotopic composition it is advantageous to utilize rain samplers which are at the same time low-cost, low-tech, robust, and precise with respect to the collected rainwater isotopic composition. We assessed whether a self-built version of the Kennedy sampler is able to collect rainwater consistently without mixing with antecedent collected water. We called the self-built sampler made from honey jars and silicon tubing the Zurich sequential sampler. Two laboratory experiments show that high rainfall intensities can be sampled and that the volume of water in a water sample originating from a different bottle was generally less than 1 ml. Rainwater was collected in 5 mm increments for stable isotope analysis using three (year 2011) and five (years 2015 and 2016) rain samplers in Zurich (Switzerland) during eleven rainfall events. The standard deviation of the total rainfall amounts between the different rain gauges was <1%. The standard deviation of δ18O and δ2H among the different sequential sampler bottles filled at the same time was generally <0.3‰ for δ18O and <2‰ for δ2H (8 out of 11 events). Larger standard deviations could be explained by leaking bottle(s) with subsequent mixing of water with different isotopic composition of at least one out of the five samplers. Our assessment shows that low-cost, low-tech rain samplers, when well maintained, can be used to collect sequential samples of rainfall for stable isotope analysis and are therefore suitable to study the spatio-temporal variability of the isotopic composition of rainfall.publishedVersio

First data set of H₂O/HDO columns from the Tropospheric Monitoring Instrument (TROPOMI)

This paper presents a new data set of vertical column densities of the water vapour isotopologues H2O and HDO retrieved from short-wave infrared (2.3 μm) reflectance measurements by the Tropospheric Monitoring Instrument (TROPOMI) aboard the Sentinel-5 Precursor satellite. TROPOMI features daily global coverage with a spatial resolution of up to 7 km × 7 km. The retrieval utilises a profile-scaling approach. The forward model neglects scattering, thus strict cloud filtering is necessary. For validation, recent ground-based water vapour isotopologue measurements by the Total Carbon Column Observing Network (TCCON) are employed. A comparison of TCCON δD with measurements by the project Multi-platform remote Sensing of Isotopologues for investigating the Cycle of Atmospheric water (MUSICA) for data prior to 2014 (where MUSICA data is available) shows a bias in TCCON δD estimates. As TCCON HDO is currently not validated, an overall correction of recent TCCON HDO data is derived based on this finding. The agreement between the corrected TCCON measurements and collocated TROPOMI observations is good with an average bias of (0.02 ± 2) · 1021 molec cm−2 in H2O and (−0.3 ± 7) · 1017 molec cm−2 in HDO, which corresponds to a bias of (−12 ± 17) ‰ in a posteriori δD. The use of the data set is demonstrated with a case study of a blocking anticyclone in northwestern Europe in July 2018 using single overpass data

A Lagrangian perspective on stable water isotopes during the West African Monsoon

We present a Lagrangian framework for identifying mechanisms that control the isotopic composition of mid-tropospheric water vapor in the Sahel region during the West African Monsoon 2016. In this region mixing between contrasting air masses, strong convective activity, as well as surface and rain evaporation lead to high variability in the distribution of stable water isotopologues. Using backward trajectories based on high-resolution isotope-enabled model data, we obtain information not only about the source regions of Sahelian air masses, but also about the evolution of H$_{2}$O and its isotopologue HDO (expressed as δD) along the pathways of individual air parcels. We sort the full trajectory ensemble into groups with similar transport pathways and hydro-meteorological properties, such as precipitation and relative humidity, and investigate the evolution of the corresponding paired {H$_{2}$O, δD} distributions. The use of idealized process curves in the {H$_{2}$O, δD} phase space allows us to attribute isotopic changes to contributions from (1) air mass mixing, (2) Rayleigh condensation during convection, and (3) microphysical processes depleting the vapor beyond the Rayleigh prediction, i.e., partial rain evaporation in unsaturated and isotopic equilibration δin saturated conditions. Different combinations of these processes along the trajectory ensembles are found to determine the final isotopic composition in the Sahelian troposphere during the monsoon. The presented Lagrangian framework is a powerful tool for interpreting tropospheric water vapor distributions. In the future, it will be applied to satellite observations of H$_{2}$O, δD} over Africa and other regions in order to better quantify characteristics of the hydrological cycle

The role of radiative cooling and leaf wetting in air–leaf water exchange during dew and radiation fog events in a temperate grassland

During prolonged dry periods, non-rainfall water (NRW) plays a vital role as water input into temperate grasslands, affecting the leaf surface water balance and plant water status. Previous chamber and laboratory experiments investigated air–leaf water exchange during dew deposition, but overlooked the importance of radiative cooling on air–leaf water exchange because the chamber is a heat trap, preventing radiative cooling. To complement these previous studies, we conducted a field study, in which we investigated the effect of radiatively-induced NRW inputs on leaf water isotope signals and air–leaf water exchange in a temperate grassland during the dry-hot summers of 2018 and 2019. We carried out field measurements of the isotope composition of atmospheric water vapor, NRW droplets on foliage, leaf water, xylem water of root crown, and soil water, combined with meteorological and plant physiological measurements. We combined radiation measurements with thermal imaging to estimate leaf temperatures using different methods, and computed the corresponding leaf conductance and air–leaf water exchange. Our results indicate that radiative cooling and leaf wetting induced a switch of direction in the net water vapor exchange from leaf-to-air to air-to-leaf. The leaf conductance and air–leaf water exchange varied by species due to the species-specific biophysical controls. Our results highlight the ecological relevance of radiative cooling and leaf wetting in natural temperate grasslands, a process which is expected to influence land surface water budgets and may impact plant survival in many regions in a drier climate

Disentangling different moisture transport pathways over the eastern subtropical North Atlantic using multi-platform isotope observations and high-resolution numerical modelling

Due to its dryness, the subtropical free troposphere plays a critical role in the radiative balance of the Earth’s climate system. But the complex interactions of the dynamical and physical processes controlling the variability in the moisture budget of this sensitive region of the subtropical atmosphere are still not fully understood. Stable water isotopes can provide important information about several of the latter processes, namely subsidence drying, turbulent mixing, dry and moist convective moistening. In this study, we use high-resolution simulations of the isotope-enabled version of the regional weather and climate prediction model of the Consortium for Small-Scale Modelling (COSMO$_{iso}$) to investigate predominant moisture transport pathways in the Canary Islands region in the eastern subtropical North Atlantic. Comparison of the simulated isotope signals with multi-platform isotope observations (aircraft-based in situ measurements, ground-based and space-based remote sensing observations) from a field campaign in summer 2013 shows that COSMO$_{iso}$ can reproduce the observed variability of stable water vapour isotopes on time scales of hours to days, and thus allows studying the mechanisms that control the subtropical free-tropospheric humidity. Changes of isotopic signals along backward trajectories from the Canary Islands region reveal the physical processes behind the short-term isotope variability. We identify four predominant moisture transport pathways of mid-tropospheric air, each with distinct isotopic signatures: (1) Air parcels originating from the convective boundary layer of the Saharan heat low (SHL). These are characterised by a homogenous isotopic composition with a particularly high δD (median mid-tropospheric δD = −122 ‰), which results from dry convective mixing of low-level moisture of diverse origin advected into the SHL. (2) Air parcels originating from the free troposphere above the SHL. Although experiencing the largest changes in humidity and δD during their subsidence over West Africa, these air parcels typically have lower δD values (median δD = −148 ‰) than air parcels originating from the boundary layer of the SHL. (3) Air parcels originating from outside the SHL region, typically descending from tropical upper levels south of the SHL, which are often affected by moist convective injections from mesoscale convective systems in the Sahel. Their isotopic composition is much less enriched in heavy isotopes (median δD = −175 ‰) than those from the SHL region. (4) Air parcels subsiding from the upper-level extratropical North Atlantic. This pathway leads to the driest and most depleted conditions (median δD = −255 ‰) in the middle troposphere near the Canary Islands. The alternation of these transport pathways explains to a large degree the observed high variability in humidity and δD on synoptic time scales. We further show that the four different transport pathways are related to specific large scale-flow conditions. In particular, distinct differences in the location of the North African mid-level anticyclone and of extratropical Rossby wave patterns occur between the four transport pathways. Overall, this study demonstrates that the adopted Lagrangian isotope perspective enhances our understanding of air mass transport and mixing and offers a sound interpretation of the free-tropospheric variability of specific humidity and isotope composition on time scales of hours to days in contrasting atmospheric conditions over the eastern subtropical North Atlantic