Synoptic to mesoscale processes affecting the water vapor isotopic daily cycle over a coastal lagoon
Introduction
Synoptic and mesoscale flows are relevant to studies of atmospheric chemistry and dynamics, with applications from local weather forecasting to studies of pollutant dispersal. While synoptic conditions drive the long-range transport of air masses, the thin atmospheric layer where the human activities occur is strongly affected by surface fluxes and by local circulation. The Planetary Boundary Layer (PBL) is the part of the atmosphere that is directly influenced by the Earth's surface within a timescale of an hour or less, it is also where turbulent mixing dominates (Feng et al., 2016; Stull, 2012). Therefore, the development of the diurnal PBL is linked to the mixing height, and ultimately, determines the volume available for the dispersion of pollutants (Seibert et al., 2000). In addition to vertical mixing, horizontal transport due to surface winds acts on pollutant dispersion. Coastal areas are characterized by special circulation systems like sea breeze circulation, created by the daytime differential heating of land and sea (Miller, 2003; Simpson, 1994). The sea breeze can affect air quality in coastal areas (Mavrakou et al., 2012; Papanastasiou and Melas, 2009; Tsai et al., 2011), providing moisture for fog formation (Dias and Machado, 1997) as well as trigger thunderstorms (Bhate et al., 2016). All the processes mentioned above involve the exchange of moisture between atmospheric reservoirs. For example, the free atmosphere water vapor pool interacts with the PBL pool during vertical mixing, while, during land/sea breeze circulation, marine water vapor interacts with the land water vapor. Hydrogen and oxygen stable isotopes can be used as tracers of the water cycle, since they provide information about water transport, mixing and changes of phase in the atmosphere. For these reasons the isotopic composition of water vapor represents an ideal tool for studying atmospheric processes (Galewsky et al., 2016). It is usual to refer to the isotopic composition of water as a relative difference expressed in permille [‰] from a common standard, as given in equation (1):where R is the isotopic ratio of the sample and of the standard water (Vienna Standard Mean Ocean Water, VSMOW), for D/H (δD) and 18O/16O (δ18O).
Recent advances in laser spectroscopy have allowed a large increase in studies where the isotopic composition of water vapor is used. For instance, water vapor isotopes were used to trace atmospheric moisture transport (Bonne et al., 2015), to reveal moisture sources in marine (Steen-Larsen et al., 2015) and continental (Lai and Ehleringer, 2011) settings, to detect sea breeze penetration (Kopec et al., 2016) and to quantify the input of stratospheric air into the middle troposphere (Galewsky and Samuels-Crow, 2014). Several studies agree that water vapor isotope observations can give important information for weather forecasts and studies related to the future water cycle (Farlin et al., 2013; Yoshimura, 2015). These latter applications involve the use of isotope enabled general circulation models (iso-GCM) and water vapor observations. Steen-Larsen et al. (2016) benchmarked several iso-GCM outputs with water vapor observations to this effect.
In this study we present 21 days of continuous monitoring of the water vapor isotopic composition at the inland shoreline of a coastal lagoon in northern Italy, using a commercial Cavity Ring-Down Spectroscopy (CRDS) analyzer. The study period, from the 8th to 29th of March 2017, was chosen due to the large frequency of sea breeze circulation buildups that occur in the spring and summer seasons (Camuffo, 1982; Masiol et al., 2010). Water vapor isotopes represent an ideal tool to elucidate atmospheric processes, such as the land and marine air mass interactions. In this work, the atmospheric water vapor isotopic signal is used to:
- 1.
Determine how synoptic conditions influence the isotopic composition of water vapor on a weekly time scale.
- 2.
Determine the spatial extent of the control on the isotopic composition of water vapor, particularly the d-excess parameter defined as d-excess = δD-8*δ18O [‰] (Dansgaard, 1964).
- 3.
Represent the water vapor cycle using hydrogen and oxygen stable isotopes on an hourly time scale and quantify how the magnitude of vertical mixing and local sea breeze circulation impacts in the study area.
This study mainly focuses on the d-excess parameter. This non-equilibrium index was historically considered a tracer of humidity conditions over the oceans, but recent studies suggest that evapotranspiration occurring within the PBL exerts a strong control on its behavior in water vapor (Aemisegger et al., 2014; Bastrikov et al., 2014; Huang and Wen, 2014; Zhao et al., 2014). Whether the water vapor d-excess measured at ground level on land can be considered a tracer of ocean evaporation is still under debate. A frequently observed feature of water vapor d-excess is its daily oscillation with the maximum centered around midday (Delattre et al., 2015; Welp et al., 2012). In section 3 we will present the results of the study ranging from synoptic conditions to mesoscale circulation, including the influence of the tide and sea breeze circulation on the dampening or intensifying of the d-excess daily oscillation. Section 4 focuses on the water vapor isotopic composition daily cycle and its relationship to local meteorological conditions and mesoscale circulation. Vertical mixing between boundary layer air and dryer air from the free atmosphere is considered the main driver of the isotopic signal in the morning, while the horizontal transport of moist air from the sea is assumed to be the main driver in the afternoon.
Section snippets
Study area
The study area is on the inland boundary of the Venetian Lagoon (northern Italy), a shallow marine lagoon connected to the Adriatic Sea by three main inlets (Bocche di Porto), as showed in Fig. 1. With a surface area of ∼550 km2, the Venetian Lagoon is the largest Italian lagoon and is one of the largest in the Mediterranean Sea (Gačić et al., 2002; Goffredo and Zvy, 2013). About 80% of the lagoon surface consists of tidal shallows and salt marshes that are strongly affected by the tide, in
Influence of synoptic conditions on water vapor isotopic composition
The study period was characterized by large daily oscillations of mixing ratio and of water vapor isotopic composition (oscillation amplitude was on average 3000 ppmv for mixing ratio, 4‰ for δ18O, 15‰ for d-excess). As shown in Fig. 3, daily oscillations of d-excess are easily detectable while δ18O and δD display abrupt variation (depletion) in the morning, especially from the 8th to 19th of March and from 25th to 29th March. Although large oscillations are also visible for air temperatures
The daily cycle of water vapor isotopic composition
The variability in synoptic scale pressure systems and atmospheric transport explain quite well the underlying trends of mixing ratio and water vapor isotopic composition in Venice. However, the local humidity conditions seem to explain a large part of the measured d-excess (R2 = 0.77). Therefore, other processes on different temporal and spatial scales coexist with the underlying trend of isotopic signal, contributing to the daily oscillations. The synoptic (∼28 days) and daily component of δ18
Conclusions
Water vapor isotopes can be successfully used to improve understanding of synoptic and local scale flows. Atmospheric processes involve moisture exchange between water vapor pools. For instance, the free atmosphere interacts with the PBL during vertical mixing, while, for the case of land/sea breeze circulation, marine water vapor interplays with land moisture. In this study we present 21 days of continuous monitoring of water vapor isotopic composition at a coastal lagoon of the Mediterranean
Acknowledgements
This work is part of the PhD project of Daniele Zannoni, with a scholarship from Ca’ Foscari University. We are grateful to Prof. Xuhui Lee and Prof. Kei Yoshimura for sharing isotope data of their field measurements (available at: https://vapor-isotope.yale.edu/). We are grateful to Prof. Warren Cairns and Dr. Ilaria Crotti for their English language support.
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The atmospheric water cycle of a coastal lagoon: An isotope study of the interactions between water vapor, precipitation and surface waters
2019, Journal of HydrologyCitation Excerpt :This effect can be recognized as a relationship between the regression parameters and the wind direction. Similarly, Zannoni et al. (2019) showed that a daily increase in water vapor d-excess is due to advection of marine air in the study area during the spring season. This translates in a wind direction-dependence of water vapor d-excess, clearly detectable during the developing of the sea-breeze circulation.
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