ALBERT

Description: This paper introduces a newly compiled set of feature-based climatologies identified from ERA-Interim (1979–2014). Two categories of flow features are considered: (i) Eulerian climatologies of jet streams, tropopause folds, surface fronts, cyclones and anticyclones, blocks, and potential vorticity streamers and cutoffs and (ii) Lagrangian climatologies, based on a large ensemble of air parcel trajectories, of stratosphere–troposphere exchange, warm conveyor belts, and tropical moisture exports. Monthly means of these feature climatologies are openly available at the ETH Zürich web page (http://eraiclim.ethz.ch) and are annually updated. Datasets at higher resolution can be obtained from the authors on request. These feature climatologies allow studying the frequency, variability, and trend of atmospheric phenomena and their interrelationships across temporal scales. To illustrate the potential of this dataset, boreal winter climatologies of selected features are presented and, as a first application, the very unusual Northern Hemispheric winter of 2009/10 is identified as the season when most of the considered features show maximum deviations from climatology. The second application considers dry winters in the western United States and reveals fairly localized anomalies in the eastern North Pacific of enhanced blocking and surface anticyclones and reduced cyclones.

Description: Stable water isotopologues (SWIs) are useful tracers of moist diabatic processes in the atmospheric water cycle. They provide a framework to analyse moist processes on a range of timescales from large-scale moisture transport to cloud formation, precipitation and small-scale turbulent mixing. Laser spectrometric measurements on research vessels produce high-resolution time series of the variability of the water vapour isotopic composition in the marine boundary layer. In this study, we present a 5-month continuous time series of such ship-based measurements of δ2H and δ18O from the Antarctic Circumnavigation Expedition (ACE) in the Atlantic and the Southern Ocean in the time period from November 2016 to April 2017. We analyse the drivers of meridional SWI variations in the marine boundary layer across diverse climate zones in the Atlantic and Southern Ocean using Lagrangian moisture source diagnostics and relate vertical SWI differences to near-surface wind speed and ocean surface state. The median values of δ18O, δ2H and deuterium excess during ACE decrease continuously from low to high latitudes. These meridional SWI distributions reflect climatic conditions at the measurement and moisture source locations, such as air temperature, specific humidity and relative humidity with respect to sea surface temperature. The SWI variability at a given latitude is highest in the extratropics and polar regions with decreasing values equatorwards. This meridional distribution of SWI variability is explained by the variability in moisture source locations and its associated environmental conditions as well as transport processes. The westward-located moisture sources of water vapour in the extratropics are highly variable in extent and latitude due to the frequent passage of cyclones and thus widen the range of encountered SWI values in the marine boundary layer. Moisture loss during transport further contributes to the high SWI variability in the extratropics. In the subtropics and tropics, persistent anticyclones lead to well-confined narrow easterly moisture source regions, which is reflected in the weak SWI variability in these regions. Thus, the expected range of SWI signals at a given latitude strongly depends on the large-scale circulation. Furthermore, the ACE SWI time series recorded at 8.0 and 13.5 m above the ocean surface provide estimates of vertical SWI gradients in the lowermost marine boundary layer. On average, the vertical gradients with height found during ACE are -0.1‰m-1 for δ18O, -0.5‰m-1 for δ2H and 0.3 ‰ m−1 for deuterium excess. Careful calibration and post-processing of the SWI data and a detailed uncertainty analysis provide a solid basis for the presented gradients. Using sea spray concentrations and sea state conditions, we show that the vertical SWI gradients are particularly large during high wind speed conditions with increased contribution of sea spray evaporation or during low wind speed conditions due to weak vertical turbulent mixing. Although further SWI measurements at a higher vertical resolution are required to validate these findings, the simultaneous SWI measurements at several heights during ACE show the potential of SWIs as tracers for vertical mixing and sea spray evaporation in the lowermost marine boundary layer.

Description: Stable isotopes of water vapour are powerful indicators of meteorological processes on a broad range of scales, reflecting evaporation, condensation, and air mass mixing processes. With the recent advent of fast laser-based spectroscopic methods, it has become possible to measure the stable isotopic composition of atmospheric water vapour in situ at a high temporal resolution. Here we present results from such comprehensive airborne spectroscopic isotope measurements in water vapour over the western Mediterranean at a high spatial and temporal resolution. Measurements have been acquired by a customized Picarro L2130-i cavity-ring down spectrometer deployed onboard the Dornier 128 D-IBUF aircraft together with a meteorological flux measurement package during the HyMeX SOP1 (Hydrological cycle in Mediterranean Experiment special observation period 1) field campaign in Corsica, France, during September and October 2012. Taking into account memory effects of the air inlet pipe, the typical time resolution of the measurements was about 15–30 s, resulting in an average horizontal resolution of about 1–2 km. Cross-calibration of the water vapour measurements from all humidity sensors showed good agreement under most flight conditions but the most turbulent ones. In total 21 successful stable isotope flights with 59 flight hours have been performed. Our data provide quasi-climatological autumn average conditions and vertical profiles of the stable isotope parameters δD, δ18O, and d-excess during the study period. A d-excess minimum in the overall average profile is reached in the region of the boundary-layer top, possibly caused by precipitation evaporation. This minimum is bracketed by higher d-excess values near the surface caused by non-equilibrium fractionation, and a maximum above the boundary layer related to the increasing d-excess in very depleted and dry high-altitude air masses. Repeated flights along the same pattern reveal pronounced day-to-day variability due to changes in the large-scale circulation. During a period marked by a strong inversion at the top of the marine boundary layer, vertical gradients in stable isotopes reached up to 25.4 ‰ 100 m−1 for δD and 24.0 ‰ 100 m−1 for the d-excess.

Description: 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.

Description: Stable water isotopes are powerful indicators of meteorological processes on a broad range of scales, reflecting evaporation, condensation, and airmass mixing processes. With the recent advent of fast laser-based spectroscopic methods it has become possible to measure the stable isotopic composition of atmospheric water vapour in situ at high temporal resolution, enabling to tremendously extend the measurement data base in space and time. Here we present the first set of airborne spectroscopic stable water isotopes measurements over the western Mediterranean. Measurements have been acquired by a customised Picarro L2130-i cavity-ring down spectrometer deployed onboard of the Dornier 128 D-IBUF aircraft together with a meteorological flux measurement package during the HyMeX SOP1 field campaign in Corsica, France during September and October 2012. Taking into account memory effects of the air inlet pipe, the typical time resolution of the measurements was about 15–30 s, resulting in an average horizontal resolution of about 1–2 km. Cross-calibration of the water vapour measurements from all humidity sensors showed good agreement in most flight conditions but the most turbulent ones. In total 21 successful stable isotope flights with 59 flight hours have been performed. Our data provide quasi-climatological autumn average conditions of the stable isotope parameters δD, δ18O and d-excess during the study period. A time-averaged perspective of the vertical stable isotope composition reveals for the first time the mean vertical structure of stable water isotopes over the Mediterranean at high resolution. A d-excess minimum in the overall average profile is reached in the region of the boundary layer top due to precipitation evaporation, bracketed by higher d-excess values near the surface due to non-equilibrium fractionation and above the boundary layer due to the non-linearity of the d-excess definition. Repeated flights along the same pattern reveals pronounced day-to-day variability due to changes in the large-scale circulation. During a period marked by a strong inversion at the top of the marine boundary layer, vertical gradients in stable isotopes reached up to 25.4 ‰ 100 m−1 for δD.

Description: 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 information, 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 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. 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 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 events.