Identifying Nonstationarity in the Atmospheric Surface Layer

The atmospheric boundary layer is inherently nonstationary. quickly influences the wind speed profile. The the transition in sky conditions as cloud layers develop or dissipate rapidly forces the surface temperature just as do sunrise and sunset transitions. Monin-Obukhov similarity theory, which organizes our understanding of the atmospheric boundary layer especially the atmospheric surface layer relies on two assumptions that seem at odds with this depiction of the atmospheric boundary layer: that the atmosphere is statistically stationary and that the surface is horizontally homogeneous. Because clouds are ubiquitous, we speculate that many of the measurements of the Monin-Obukhov similarity functions that have been reported were collected in nonstationary conditions. Such violations of the premises on which Monin-Obukhov similarity rests may explain some of the scatter that still exists in these universal similarity functions despite almost 50 years of measurements to quantify them. We present a method for identifying nonstationarity. Our method has three advantages: It has a theoretical basis, it relies on accepted definitions of what constitutes nonstationarity, and it associates a probability as to whether any nonstationary period it identifies is truly nonstationary. Prepared in collaboration with CHIRES, Inc., San Antonio, TX. Published in preprints of the Conference on Hydrology (21st), San Antonio, TX, 14-18 Jan 2007, American Meteorological Society; proceedings in press. The original document contains color images.