Vertical Variations of Mixing Lengths under Neutral and Stable Conditions during CASES-99

Abstract

n investigation on vertical variations of the mixing lengths for momentum and heat under neutral and stable conditions was conducted using the data collected from the Cooperative Atmosphere?Surface Exchange Study in 1999 (CASES-99). By comparing ?z with the mixing lengths under neutral conditions calculated using the observations from CASES-99, the vertical layer where the Monin?Obukhov similarity theory (MOST) is valid was identified. Here ? is the von Kármán constant and z is the height above the ground. On average, MOST is approximately valid between 0.5 and 10 m. Above the layer, the observed mixing lengths under neutral conditions are smaller than the MOST ?z and can be approximately described by Blackadar?s mixing length, ?z/[1 + (?z/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 ?z and Blackadar?s formula systematically overestimate the mixing length for momentum above ~20 m, leading to overestimates of turbulence.