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

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.