Figure 11: (a) Monthly sensible heat flux (red), latent heat flux (blue) and net radiation (black) from June 2018 to December 2021. Bars indicate monthly minimum and maximum average values. (b) Monthly Bowen ratio. The error bars indicate the maximum and minimum values observed for each month.
The Bowen ratios exhibited high negative values during the first seven months of each year due to low LE values. After the onset of the heat release period (end of July), it stayed around zero before gradually increasing to about 1.5 in December. This indicates that the reservoir was mostly releasing heat through sensible heat flux, as evaporation was constrained by the small vapour pressure deficit induced by the cold air temperatures.
The energy state of the reservoir is closely tied to the stability regime of the overlying atmosphere. During heat release (August to early January), the overlying atmosphere was unstable, while it remained mostly stable during the heat storage (mid-May to August) and ice-cover periods. Figure 12 explores the relationship between daily LE and atmospheric stability ζ for heat storage and release. The figure confirms that larger daily evaporation occurs under near-neutral (ζ ≈ 0) and unstable conditions (ζ < 0), and that stable conditions (ζ > 0) are related to low evaporation rates. Note that condensation occurred primarily under near-neutral and stable conditions.