Research / Research Highlights

Deviations From Mixed-Layer Similarity in the Moist Convective Boundary Layer

Submitter

Salesky, Scott — University of Oklahoma

Area of Research

Surface Properties

Journal Reference

Greene B and S Salesky 2026. "Deviations from Mixed-Layer Similarity in the Moist Convective Boundary Layer." Journal of the Atmospheric Sciences, 83(7), 10.1175/JAS-D-25-0154.1.

Science

Vertical profiles of dimensionless variances from large eddy simulations of the moist, highly convective atmospheric boundary layer. Evaporative fraction increases from top to bottom; line style is used to denote value of humidity entrainment flux ratio. Adapted from Greene and Salesky, *J. Atmos. Sci.* (2026), doi: 10.1175/JAS-D-25-0154.

We systematically investigated the influence of water vapor fluctuations on turbulence statistics in the convective atmospheric boundary-layer using a series of numerical simulations. Although water vapor is typically assumed to play a passive role in turbulence dynamics, we find that it plays an active role, where it contributes to buoyancy independently of temperature and influences the nature of turbulent transport under humid conditions.

Impact

The classical framework of mixed-layer similarity theory for the convective boundary layer—used in modeling, parameterization, and interpreting experiment results—is insufficient for describing turbulence statistics under humid conditions. Our results demonstrate that under moist conditions, turbulent statistics are strongly determined by vertical moisture transport at Earth’s surface and at the top of the convective boundary layer.

Summary

A classical result in convective boundary layer turbulence is mixed-layer similarity theory, which states that dimensionless turbulence statistics should collapse at any dimensionless height for a given value of the instability parameter. A key assumption underpinning mixed-layer similarity is that water vapor plays a passive role and does not directly influence momentum transport or statistical properties of turbulence.

We performed a suite of 24 large-eddy simulations of the moist convective boundary layer, spanning two values of the instability parameter, three values of the evaporative fraction (which quantifies the ratio of turbulent heat to moisture transport at the ground), and four values of the humidity entrainment flux ratio (which characterizes vertical moisture transport at the bottom and top of the boundary layer). We find that under large evaporative fractions (when vertical moisture transport dominates over heat transport at the ground), water vapor significantly influences buoyancy, vertical transport, and turbulence statistics. In these regimes, predictions of mixed-layer similarity theory break down due to water vapor playing an increasingly active role in turbulence dynamics.