Rainfall Effects on Atmospheric Turbulence and Near-Surface Similarities in the Stable Boundary Layer

Near-surface similarities and atmospheric turbulence characteristics have a large impact on numerical weather prediction models. However, the validity of these similarities is unclear during precipitation. This study investigates the modulations in atmospheric boundary layer turbulence and the variations of the near-surface scaling similarities caused by rainfall. Here we present our field observations on the effects of rainfall on the near-surface similarities and atmospheric turbulence in the stable boundary layer using a Parsivel(2) disdrometer and a 3D ultrasonic anemometer at our outdoor rainfall laboratory in San Antonio, Texas, USA. During moderate to heavy rainfall conditions, higher turbulent energy was observed than those in non-rainy conditions when the turbulence intensity and the wind speeds were relatively low. On the contrary, when the turbulence intensity and the wind speeds were relatively high, the turbulence energy in the stable boundary layer were dampened due to the raindrops. Raindrops with high particle Reynolds numbers ( Re-p = D(m)v(t)/theta; D-m-mean volume diameter, v(t) -terminal raindrop fall speed, and theta-kinematic viscosity of the surrounding air) can act as either a source or a sink of turbulent kinetic energy depending on the turbulence intensity of the atmosphere. Our field observations showed that near-surface similarities deviated from the scaled similarities under the influence of rainfall. The normalized standard deviations of the streamwise and vertical velocity components and the dissipation rate were higher during rainy than non-rainy times. Rainfall effects on turbulence modulations and near-surface scaling parameters of the stable boundary layer are discussed with considerations of the relevant mechanisms.