Role of independent stratification mechanisms on stable open-channel flows*

Long-lived or static stable stratification of the atmosphere is a common feature in high-latitude regions due to the absence of a neutral residual layer that serves as a memory of the daytime convective mixing. To study this problem, we perform a series of direct numerical simulations of turbulent open-channel flows, serving as a simple model for the atmospheric surface layer over a flat terrain. In this model problem, a neutrally stratfied turbulent flow is stabilized simultaneously by a constant negative surface buoyancy flux as well as an independent ambient stratification characterized by its own Brunt-Väisälä frequency N. When both types of stratification mechanisms are active, marked departures from MOST profiles are possible despite the overall weakly stable state of the bulk flow. We show that the degree of deviation from the neutral dimensionless shear as a function of the vertical coordinate can be used as a qualitative measure of the strength of stable stratification. Furthermore, an extended version of MOST, originally formulated by Zilintinkevich and Calanca in 2000, shows promise in capturing the dimensionless shear, but the extended similarity theory is less accurate in predicting the dimensionless gradients of scalar quantities.