On Determining Nonstationary Events during Extended Data-Acquisition Episodes: Eliminating Prescribed Analysis Periods

Abstract:

Current parameterizations of turbulent motions within atmospheric models assume statistically stationary and horizontally homogeneous flow conditions. The development and evaluation of turbulence parameterizations typically hinge on analysis of field observations of turbulence responding to nonstationary forcing at wide-ranging scales. Therefore, determining the occurrence, rate, and driving force of nonstationary transitions in the observations is critical to accurately quantify model uncertainties associated with the assumption of statistically stationary turbulence. Stationarity analysis of time series has become a key aspect of surface-layer-turbulence data-quality-control techniques, but traditional approaches relying upon prescribed analysis periods cannot provide comprehensive evaluation of nonstationary events. Specifically, the prescribed analysis periods (e.g., 30-min periods starting from whole and half hours) are only a small subset of periods with all possible lengths and starting points during extended data-acquisition episodes. The recently developed multi-sensor stationarity analysis technique version 2 (MSATv2) enables interrogation of all possible periods, which provides objective and robust evaluation of nonstationarity at any time within the series. In this work, we apply MSATv2 to the Canopy Horizontal Array Turbulent Study (CHATS) data acquired in a walnut orchard field in Dixon, CA. Located in the Sacramento Valley, which is primarily south-north oriented, the dirunal evolution of near-surface flows is characterized by switches between south- and north-wind episodes. MSATv2 results will be used to quantify the occurrence and rate of nonstationary transitions in near-surface flows driven by the diurnal evolution of regional circulations as well as the local atmospheric stability. The influence of regional circulations on the turbulence response to the local atmospheric stability is of particular interest.