# Near Real-Time Freeze Detection over Agricultural Plots Using Sentinel-1 Data
## 1 Introduction  
Frozen grounds are soil where part or all of the pore water is found as ice. Frozen surfaces are mostly found in the northern hemisphere, and make up the majority of land surfaces [1; 2]. In permafrost, the ground remains completely frozen for at least two consecutive years [3]. Seasonal frozen regions have frost persisting for several months. In contrast, short term freeze/thaw (FT) cycles only last from hours to a couple of days. Permafrost make up to 24% of total land surfaces, and 55% of exposed land surfaces undergo short term FT cycles each year [4]. The FT cycles are sensitive to the fluctuation of the isothermal temperature threshold (0\({}^{\circ}\)C) at ground level, and have significant impacts on the climate system, surface runoff, water balance, carbon cycle, and crop growth [2; 4; 5]. The relationship between FT cycles and crop yield are well understood and have been extensively studied [6; 7; 8; 9; 10; 11]. FT cycles havean impact on both the physical and biological aspects of the soil in which the plants grow. For example, freezing and thawing causes expansion and contraction that push plant roots up and sometimes out of soil, which damages them. Moreover, frost damage occurs to all kinds of crops, like cereal grains, flowers and seeds, canola pods, legume pods and seeds, flowers, and vines, and can damage whole plant heads in the case of soil freeze, which causes the stems to also freeze [12]. To lower the effects of frost damage, generally, producers often plant crops in a manner so that plant flowers, leaves and grains develop during warmer temperatures. However, due to climate change, warmer temperatures are causing premature vegetation growth, and as a result, crops will more likely be affected by late frost which occurs during spring. In the spring of 2020, a swathe of freezing temperatures overtook much of Europe, causing severe damage to already flowering crops throughout Northern Italy, much of Poland, and the Ukraine. In 2018, crop damage from spring freezing events has been reported as causing severe economic losses over broad areas in Europe and amounting to hundreds of millions of euros [13]. In 2007, the spring frost event in south-central and south-eastern USA caused agricultural damage in the order of 1.6 billion euros [14]. In order to assess the extent of frost damage, and subsequently the potential loss of yield in agricultural areas, FT maps are therefore required. However, such mapping requires long time series of data, with periodic measurements over large geographical areas, which is impractical to collect in situ. To monitor large areas for FT events, in real-time or near real-time, remotely sensed satellite data are primordial. Satellite microwave radars, radiometers and scatterometers are well suited for such monitoring, and they have been extensively used in the past for the monitoring of surface state parameters, such as freeze/thaw [15; 16; 17; 18; 19; 20]. Radars are active sensors that illuminate a target area and measure the backscattered energy, while radiometers are passive sensors that record natural microwave emissions. The potential of radars, radiometers and scatterometers to detect surface FT is very promising due to the dielectric properties of the soil, which are very sensitive to the amount and state of water contained in the soil. Therefore, any variation in the state or amount of water (e.g., in the case of freezing) causes significant alterations to the dielectric properties of the soil. These alterations are distinctly observable in the radar's backscattered signals [21; 22; 23; 24]. With active microwave sensors, a decrease in the order of 3 dB in the backscattered signal can be observed between frozen and unfrozen surfaces over the same area [20; 22; 23]. For passive microwave sensors, a 10 \({}^{\circ}\)C decrease in clay or silt soil temperatures was observed to increase the sensed temperature brightness by 14 (clay) and 28 \({}^{\circ}\)K (silt) [25].