Analysis of the surface energy budget during supercooling in rivers

In northern rivers, heat loss from the water surface is the key driver of supercooling in rivers and the subsequent generation of river ice. The ability to estimate the different surface heat components is crucial to accurately model supercooling and the various ice formation processes. To calibrate these models, concurrent water temperature and local meteorological data are needed, which can be a challenging task. Therefore, it is important to understand the relative importance of the different heat components on supercooling of water. For this purpose, the properties of 190 supercooling events observed during the 2016–2017 season on two regulated rivers in Alberta, Canada were analyzed together with the calculated surface heat budget using weather data from local weather stations. Longwave radiation was found to be the dominant negative heat flux for 80.0% of all events. During supercooling events, the longwave radiation and sensible components had average values of −65.7 and −46.6 W/m², respectively. The evaporative heat flux component was found to be negligible with an average value −4.52 W/m². Sensible heat flux tended to be the dominant cooling heat flux when the air temperature was approximately -15 °C or colder. The shortwave radiation component was the dominant warming heat flux for 97.4% of all events with an average value 52.6 W/m². The diurnal cycling of the net heat flux due to shortwave radiation was found to be the most significant factor in determining the start and end of supercooling events.