We investigated how lake thermal processes responded to whole lake warming manipulation in an arctic lake through observations and numerical modeling. The warming manipulation was conducted by artificially heating the epilimnion as a proxy for climate warming. We performed numerical modeling with an improved lake scheme based on the Community Land Model (CLM). We simulated a control run (CTL) without warming and a warming manipulation simulation (WARM). Results indicated WARM accurately captured observed temperatures where water stratification was extended in time, and water stability was strengthened. Two additional sensitivity tests with different warming onset dates and of the same warming duration showed that earlier warming onsets are predicted to make the water column more stable and less easily mixed relative to a later onset of warming. The results provide a more complete understanding of lake thermal processes in arctic freshwater lake systems and how they will respond to predicted future warming.