Enhanced Temperature-Humidity Similarity Caused by Entrainment Processes With Increased Wind Shear

Abstract

A number of studies already suggested that entrainment of warm/dry air from aloft degrades the similarity between air temperature (T) and specific humidity (q) in the atmospheric surface layer (ASL). Less is known about entrainment of cool/dry air on the ASL T-q similarity. Using eddy covariance measurements over a large inland water surface, enhanced T-q similarity with increased friction velocity (u*) was measured during daytime unstable conditions. Explaining this enhancement is the main goal of this work. Cool/dry air masses originating and advecting from land set the upper boundary condition on a quasi-equilibrated internal boundary layer embedding the warm/moist ASL over water. With increased u*, the entrainment velocity (dhI/dt ∝ u*) increases, thereby enhancing entrainment of cool/dry air originating from land, where t is time and hI is the internal boundary layer depth. By analyzing the scale-dependent (spectral) correlation coefficients, the enhanced T-q similarity with increased u* was shown to originate from spectral correlation coefficients in the middle- to low-frequency ranges (large eddies ≫ z, the distance from the surface). With further increases in u*, similarity between T and q within large eddies was further enhanced and phase differences between T and q was reduced. Quadrant analysis showed large increases in flux and time fractions of downward large cool/dry eddies with increasing u*. These results provide evidence that enhanced entrainment of synchronized, large cool/dry eddies was a primary cause for the increased T-q similarity with increasing u*.