Abstract

An investigation on vertical variations of the mixing lengths for momentum and heat under neutral and stable conditions was conduced using the data collected from the Cooperative Atmosphere-Surface Exchange Study in 1999 (CASES99). By comparing κz with the mixing lengths under neutral conditions ... Show moreAn investigation on vertical variations of the mixing lengths for momentum and heat under neutral and stable conditions was conduced using the data collected from the Cooperative Atmosphere-Surface Exchange Study in 1999 (CASES99). By comparing κz with the mixing lengths under neutral conditions calculated using the observations from CASES99, the vertical layer where Monin-Obukhov similarity theory (MOST) is was identified. Here κ is the von Kármán constant and z is the height above the ground. On average, MOST is approximately valid between 0.5 m and 10 m. Above the layer, the observed mixing lengths under neutral conditions are smaller than the MOST κz and can be approximately described by Blackadar's mixing length, κz/(1+ κz/l∞), with l∞ = 15 m for up to z ~ 20 m for the mixing length for momentum, and up to the highest observation height for the mixing length for heat. Above ~ 20 m, the mixing length for momentum approaches a constant. Both MOST κz and Blackadar's formula systematically overestimate the mixing length for momentum above ~ 20 m, leading to overestimates of turbulence. Show less