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

Abstract

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. It also influences the boundary layer profiles of temperature and moisture through 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 information, as they are influenced by both rain evaporation and equilibration (i.e. the exchange of isotopes between raindrops and ambient air). This study elucidates this option by introducing a novel interpretative 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 ΔδΔd-diagram, which shows the isotopic composition (δ2H, 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 since equilibration and evaporation lead to different pathways in the two-dimensional phase space of the ΔδΔd-diagram, as investigated with a series of sensitivity experiments with an idealized below-cloud interaction model. The analysis of isotope measurements for a specific cold front in central Europe 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 characterized by higher relative humidity, which leads to weaker below-cloud evaporation. Additionally, a lower melting layer after the frontal passage reduces time for exchange between vapour and rain and leads to weaker equilibration. Measurements from four cold frontal events reveal a surprisingly similar slope of ΔdΔδ=-0.30 in the phase space, indicating a potentially characteristic signature of below-cloud processes for this type of rain event.

Variability of Isotope Composition of Precipitation in the Southeastern Tibetan Plateau from the Synoptic to Seasonal Time Scale

“…These variations integrate all processes along air mass trajectories (Figure 4, blue dashed square). The third term (Figure 4, magenta dashed square) represents a combination of precipitation‐vapor disequilibrium effects (due to variations in condensation altitude or postcondensation processes such as rain evaporation), variations in the vertical gradients between the surface (where the precipitation achieves its last equilibration, e.g., Graf et al, 2019) and 5 km (where the water vapor δ 2 H is observed by IASI) and small‐scale variations in water vapor δ 2 H (leading to difference between the water vapor δ 2 H observed by IASI and that with which the precipitation locally equilibrates).…”

Meridional and vertical variations of the water vapour isotopic composition in the marine boundary layer over the Atlantic and Southern Ocean

“…The Craig and Gordon (1965) model has been applied and refined in various studies and has been shown to adequately simulate the isotopic composition of the MBL water vapour under evaporative conditions (e.g. Merlivat and Jouzel, 1979;Gat, 2008;Horita et al, 2008;Pfahl and Wernli, 2009;Benetti et al, 2018;Feng et al, 2019).…”

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

“…This effect can be quantified in a model, but it is very difficult to quantify in nature because it is complicated and uncertain to measure η (Rosenfeld and Mintz, 1988), and it is even more complicated to measure or predict α evap . Rain evaporation can have a depleting or enriching effect depending on microphysical details that are too complex to be addressed here (Graf et al, 2019). Neglecting rain evaporation leads to an error of the order of 500 m in regions of low EIS and 250 m in regions of strong EIS (Fig.…”