Secondary flows in statistically unstable turbulent boundary layers with spanwise heterogeneous roughness

•Large-scale secondary motions induced by spanwise roughness heterogeneity are studied under the effect of thermally unstable stratification.•The strength of secondary motions weakens by increasing the intensity of unstable stratification, and they are replaced by stronger secondary motions rotating in opposite directions.•The new secondary rolls are created by the dominance of the buoyancy effect, and they are stronger than roughness-induced secondary rolls.