The stable isotopic composition of water vapour above Corsica during the HyMeX SOP1 campaign: insight into vertical mixing processes from lower-tropospheric survey flights

Sodemann, Harald; Aemisegger, Franziska; Pfahl, Stephan; Bitter, Mark; Corsmeier, U.; Feuerle, Thomas; Graf, Pascal; Hankers, Rolf; Hsiao, G.; Schulz, Helmut; Wieser, Andreas; Wernli, Heini

doi:10.5194/acp-17-6125-2017

Cited by 66 publications

(94 citation statements)

References 60 publications

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“…The observed effects in Figure 3 are larger than typical measurement uncertainties for deuterium excess in vapor (between 1‰ and 10‰; Aemisegger et al, 2012;Bailey et al, 2015;Sodemann et al, 2017;Steen-Larsen et al, 2017). However, in comparison to the -scale effect the nonequilibrium and temperature effects only have a minor impact on the linear deuterium excess, in particular, for low 2 H 0 .…”

Section: Isotopic Fractionationmentioning

confidence: 69%

“…They usually have opposite signs and compensate each other to some extent, maintaining the linear relationship of the GMWL at cold temperatures (Craig, 1961;Gat, 1996). Nevertheless, very high deuterium excess values measured, for example, in the Tien Shan mountains (Kreutz et al, 2003), in the Andes (Samuels-Crow et al, 2014), in the tropical tropopause layer (Webster & Heymsfield, 2003), and above the Mediterranean (Sodemann et al, 2017) indicate the influence of the -scale effect in regions where air parcels have experienced strong rainout, that is, typically at high altitudes or latitudes. In these regions, the traditional linear definition of the deuterium excess may therefore lose some of its explanatory power.…”

Section: Introductionmentioning

confidence: 99%

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The Impact of Nonequilibrium and Equilibrium Fractionation on Two Different Deuterium Excess Definitions

Dütsch

Sodemann

2017

JGR Atmospheres

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The deuterium excess (d) is a useful measure for nonequilibrium effects of isotopic fractionation and can therefore provide information about the meteorological conditions in evaporation regions or during ice cloud formation. In addition to nonequilibrium fractionation, two other effects can change d during phase transitions. The first is the dependence of the equilibrium fractionation factors on temperature, and the second is the nonlinearity of the δ scale on which d is defined. The second effect can be avoided by using an alternative definition that is based on the logarithmic scale. However, in this case d is not conserved when air parcels mix, which can lead to changes without phase transitions. Here we provide a systematic analysis of the benefits and limitations of both deuterium excess definitions by separately quantifying the impact of the nonequilibrium effect, the temperature effect, the δ‐scale effect, and the mixing effect in a simple Rayleigh model simulating the isotopic composition of air parcels during moist adiabatic ascent. The δ‐scale effect is important in depleted air parcels, for which it can change the sign of the traditional deuterium excess in the remaining vapor from negative to positive. The alternative definition mainly reflects the nonequilibrium and temperature effect, while the mixing effect is about 2 orders of magnitude smaller. Thus, the alternative deuterium excess definition appears to be a more accurate measure for nonequilibrium effects in situations where moisture is depleted and the δ‐scale effect is large, for instance, at high latitudes or altitudes.

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Section: Isotopic Fractionationmentioning

confidence: 69%

Section: Introductionmentioning

confidence: 99%

The Impact of Nonequilibrium and Equilibrium Fractionation on Two Different Deuterium Excess Definitions

Dütsch

Sodemann

2017

JGR Atmospheres

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“…The initial ∆d of precipitation is uncertain, but assumed to be small. The increase of d v with height (e.g., Bony et al, 2008;Sodemann et al, 2017) and non-equilibrium effects during precipitation formation in mixed-phase clouds, both increase d p,eq (and ∆d). This increase is however counteracted by a decrease of d p,eq during the vapour-solid transition and by a decrease due to the non-linearity of the δ-scale (Dütsch et al, 2017).…”

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confidence: 99%

A new interpretative framework for below-cloud effects on stable water isotopes in vapour and rain

Graf¹,

Wernli²,

Sodemann³

2018

Preprint

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Abstract. Raindrops interact with water vapour in ambient air while sedimenting from the cloud base to the ground. They constantly exchange water molecules with the environment and, in sub-saturated air, they evaporate partially or entirely. The latter of these below-cloud processes is important for predicting the resulting surface rainfall amount and it influences the boundary layer profiles of temperature and moisture through to evaporative latent cooling and humidity changes. However, despite its importance, it is very difficult to quantify this process from observations. Stable water isotopes provide such infor-5 mation, as they are influenced by both rain evaporation and equilibration. This study elucidates this option by introducing a novel interpretation framework for stable water isotope measurements performed simultaneously at high temporal resolution in both near-surface vapour and rain. We refer to this viewing device as the ∆δ∆d-diagram, which shows the isotopic composition (δ 2 H, d-excess) of equilibrium vapour from precipitation samples relative to the ambient vapour. It is shown that this diagram facilitates the diagnosis of below-cloud processes and their effects on the isotopic composition of vapour and rain 10 since equilibration and evaporation lead to different pathways in the two-dimensional phase space of the ∆δ∆d-diagram. For a specific cold front in Central Europe, the analysis shows that below-cloud processes lead to distinct and temporally variable imprints on the isotope signal in surface rain. The influence of evaporation on this signal is particularly strong during periods with a weak precipitation rate. After the frontal passage, the near-surface atmospheric layer is characterised by higher relative humidity and a lower melting layer, leading to weaker below-cloud evaporation and equilibration. Measurements from four 15 cold frontal events reveal a surprisingly similar slope of ∆d ∆δ = −0.30 in the phase space, indicating a potentially characteristic signature of below-cloud processes for this type of rain events.

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“…We do not aim at capturing the second-order parameter d-excess because our model requires some knowledge about free-tropospheric vertical profiles of isotopic composition. While δD is known to decrease with altitude (Ehhalt, 1974;Ehhalt et al, 2005;Sodemann et al, 2017), vertical profiles of d-excess are more diverse and less well understood (Sodemann et al, 2017). In addition, there is more need for an extension of MJ79 for δD than for d-excess since the effect of convective mixing is larger on δD than on d-excess (Risi et al, 2010d;Benetti et al, 2014).…”

Section: Goal Of the Articlementioning

confidence: 99%

Controls on the water vapor isotopic composition near the surface of tropical oceans and role of boundary layer mixing processes

et al. 2019

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Understanding what controls the water vapor isotopic composition of the sub-cloud layer (SCL) over tropical oceans (δD 0 ) is a first step towards understanding the water vapor isotopic composition everywhere in the troposphere. We propose an analytical model to predict δD 0 motivated by the hypothesis that the altitude from which the free tropospheric air originates (z orig ) is an important factor: when the air mixing into the SCL is lower in altitude, it is generally moister, and thus it depletes the SCL more efficiently. We extend previous simple box models of the SCL by prescribing the shape of δD vertical profiles as a function of humidity profiles and by accounting for rain evaporation and horizontal advection effects. The model relies on the assumption that δD profiles are steeper than mixing lines, and that the SCL is at steady state, restricting its applications to timescales longer than daily. In the model, δD 0 is expressed as a function of z orig , humidity and temperature profiles, surface conditions, a parameter describing the steepness of the δD vertical gradient, and a few parameters describing rain evaporation and horizontal advection effects. We show that δD 0 does not depend on the intensity of entrainment, in contrast to several previous studies that had hoped that δD 0 measurements could help estimate this quantity.Based on an isotope-enabled general circulation model simulation, we show that δD 0 variations are mainly controlled by mid-tropospheric depletion and rain evaporation in ascending regions and by sea surface temperature and z orig in subsiding regions. In turn, could δD 0 measurements help estimate z orig and thus discriminate between different mixing processes? For such isotope-based estimates of z orig to be useful, we would need a precision of a few hundred meters in deep convective regions and smaller than 20 m in stratocumulus regions. To reach this target, we would need daily measurements of δD in the mid-troposphere and accurate measurements of δD 0 (accuracy down to 0.1 ‰ in the case of stratocumulus clouds, which is currently difficult to obtain). We would also need information on the horizontal distribution of δD to account for horizontal advection effects, and full δD profiles to quantify the uncertainty associated with the assumed shape for δD profiles. Finally, rain evaporation is an issue in all regimes, even in stratocumulus clouds. Innovative techniques would need to be developed to quantify this effect from observations.

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The stable isotopic composition of water vapour above Corsica during the HyMeX SOP1 campaign: insight into vertical mixing processes from lower-tropospheric survey flights

Cited by 66 publications

References 60 publications

The Impact of Nonequilibrium and Equilibrium Fractionation on Two Different Deuterium Excess Definitions

The Impact of Nonequilibrium and Equilibrium Fractionation on Two Different Deuterium Excess Definitions

A new interpretative framework for below-cloud effects on stable water isotopes in vapour and rain

Controls on the water vapor isotopic composition near the surface of tropical oceans and role of boundary layer mixing processes

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