# Mapping Atmospheric Exposure of the Intertidal Zone with Sentinel-1 CSAR in Northern Norway
## 1 Introduction  
\"The intertidal zone (ITZ) is the area where the ocean meets the land between high and low tides\" [1] and is, thus, defined as the area which is exposed to air at low tide and covered by water at high tide. It is a vulnerable coastal ecosystem with high biodiversity providing important ecosystem services such as coastal protection, coast stabilization, recreation, food production [2], and climate mitigation by carbon sequestration [3], coming in various forms of many different types: mudflats, sandy, stone, and rock beaches, steep cliffs, different types of saltmarshes, etc. Mudflats have a particular high biodiversity and are important habitats for shore- and seabirds, as well as for invertebrates, insects, seaweeds, and algae [4]. The ITZ is under strong pressure due to, e.g., climate change, sea-level rise, erosion, changes in marine use and aquaculture, and changes in land use, e.g., construction and contamination [5, 6, 7]. Norway has a long coastline with locally extensive intertidal zones. According to [8], Norway has the second longest coastline in the world after Canada. With a length of 100,915 km including all the islands, it is of vital importance. The coast stretches across 14 deg latitude, from 58 deg to 71 degN and, therefore encompasses a range of climatic conditions, nature types, and biodiversity [9]. However, there are little data on the extent and type of coastal wetlands that exist in Nordic countries [10]. Tides, which define and condition the ITZ, are caused by the gravitational effects of the sun and moon and the rotation of the earth [11]. Tidal levels are additionally influenced by meteorological conditions, particularly by atmospheric pressure, as well as by the bathymetry, the coastline, and currents. They can, therefore, vary geographically and temporally at relatively short distances. The largest constituent is the principal lunar semidiurnal (M2) tidal constituent with a period of 12 h 25.2 min. The maximum tidal range is called spring tide and occurs when the tidal forces of the sun and the moon reinforce each other at full or new moon; on the other hand, the minimum tidal range is called near tide and occurs when the sun's tidal force partially cancels the moon's tidal force. There is about a 7 day interval between spring and neap tides. Tidal ranges vary from less than 50 cm in southern Norway to more than 250 cm in Northern Norway [12]. The Norwegian Biodiversity Information Center [13] has developed the NiN system (Nature in Norway) [14] to classify the Norwegian nature into landscapes, i.e., land and water systems, and to describe them further with complex environmental variables. Among those variables is atmospheric exposure (\"Terrleggingsvarighet\" in Norwegian), which is the gradient between full and no water coverage, i.e., the percentage of time an area is not covered by water. Because of its highly dynamic nature and being temporarily under water, the ITZ is a challenging area to map, and the only possibility to observe the atmospheric exposure over the whole tidal range is to temporally sample it with dense time series. Locally, this can be done using video cameras [15; 16] or marine radars [17; 18], preferably mounted on towers or high elevations along the shoreline. Airborne LiDAR (light detection and ranging) can map the bathymetry on a larger scale when flown at low tides [19]. On a regional, national, or even continental scale, the only cost-effective method is using satellite imagery. The European Copernicus program with its four high-resolution sun-synchronous satellites, Sentinel-1 (S1A&B) and Sentinel-2 (S2 A&B), provides C-band synthetic aperture radar (CSAR) and optical observations at 10 m resolution [20], respectively. As a result of the increased availability of satellite images in recent years, there has been an increased focus on national- and global-scale satellite-based mapping of wetlands and coastal areas [2; 21; 22; 23]. Using a 33 year time series of Landsat images, [PERSON] et al.