Synoptic to mesoscale processes affecting the water vapor isotopic daily cycle over a coastal lagoon

Highlights

• Synoptic and mesoscale processes highlighted by the water vapor isotopic composition.

• Trend of water vapor isotopic signal related to SLP field at the regional scale.

• Daily depletion of water vapor isotopes strongly related to PBL height.

• Sea breeze imprints a clear pattern to water vapor d-excess.

• Daily component of isotopic signal dominated by entrainment and mixing.

Abstract

Processes on both synoptic scale and mesoscale contribute to variations in the atmospheric water vapor isotopic signal in the lower troposphere. The drivers of the isotopic composition of water vapor at these temporal scales is a key topic of discussion. The aim of this paper is to decouple mesoscale from synoptic processes that affect the water vapor isotopic composition over a coastal lagoon and to quantify the vertical mixing and sea breeze contributions to the daily water vapor cycle. We monitored the water vapor isotopic composition at the beginning of the 2017 spring season for 21 days, at the inland boundary of the Venetian Lagoon (Italy) with a commercially available cavity ring-down spectroscopy analyzer. Mean atmospheric pressure fields and back trajectories are used to investigate the long-range transport of water vapor to the study area. Moisture source areas and d-excess correlation maps show that the control on the d-excess parameter is dominated by the mesoscale contribution. The daily cycle of water vapor is studied in detail, showing that the intrusion of air from the free atmosphere represents the main process that occurs in the morning. Under those circumstances, the morning variability of the water vapor isotopic composition can be used as a proxy for the Planetary Boundary Layer height. The study period was characterized by a constant development of the sea breeze circulation that conserved the marine signature of water vapor until the afternoon. However, the water vapor isotopic signal tends to stabilize when atmospheric conditions such as high relative humidity and a weak gradient in the atmospheric pressure field occur, leading to the establishment of an isotopic equilibrium between the atmospheric water vapor and surface waters.

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):

$$\delta =\left(\frac{R_{Sample}}{R_{Standard}}-1\right){10}^3$$

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:

• Determine how synoptic conditions influence the isotopic composition of water vapor on a weekly time scale.

• 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).

• 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.