An Observational and Modeling Study of Inverse‐Temperature Layer and Water Surface Heat Flux

An “inverse‐temperature layer” (ITL) of water temperature increasing with depth is predicted based on physical principles and confirmed by in situ observations. Water temperature and other meteorological data were collected from a fixed platform in the middle of a shallow inland lake. The ITL persists year‐around with its depth on the order of one m varying diurnally and seasonally and shallower during daytimes than nighttimes. Water surface heat flux derived from the ITL temperature distribution follows the diurnal cycle of solar radiation up to 300 W m−2 during daytime and down to 50 W m−2 during nighttime. Solar radiation attenuation in water strongly influences the ITL dynamics and water surface heat flux. Water surface heat flux simulated by two non‐gradient models independent of temperature gradient, wind speed and surface roughness using the data of surface temperature and solar radiation is in close agreement with the ITL based estimates. Plain Language Summary Heat stored in water bodies resulting from the absorption of solar radiation is the energy supply of evaporation and sensible heat flux into the atmosphere from water surface. Transfer of the thermal energy from water body into the atmosphere is only possible when water temperature increasing with depth within the top water layer referred to as the “inverse temperature layer (ITL).” The existence and persistence of the theoretically predicted phenomenon are demonstrated by the field observations of water temperature profile at an inland lake. The ITL depth is found to be comparable to the penetration depth of solar radiation with evident diurnal and seasonal cycles following closely those of solar radiation. Further understanding and analysis of the ITL process require higher resolution data of water temperature and solar radiation profiles within the top‐layer than those commonly collected in previous field experiments. Key Points Inverse temperature layer (ITL) allows transfer of heat from water into atmosphere ITL has prounced diurnal seasonal cycles persisting year‐around Water surface heat flux is simulated using non‐gradient models