Spatial Dependence of Stably Stratified Nocturnal Boundary-Layer Regimes in Complex Terrain
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Spatial Dependence of Stably Stratified Nocturnal Boundary-Layer Regimes in Complex Terrain Carsten Abraham1
· Adam H. Monahan1
Received: 26 October 2019 / Accepted: 19 May 2020 © Springer Nature B.V. 2020
Abstract The stably stratified atmospheric boundary layer (SBL) has been found in previous studies to display distinct regimes of behaviour. In particular, a contrast is often drawn between the weakly (wSBL) and very (vSBL) stable boundary layers. Time series of SBL regime affiliation obtained from hidden Markov model analyses of data from three different towers at the Los Alamos National Laboratory are used to investigate the spatial dependence of SBL regime occupation and SBL transitions. The local topography influences the flow such that south-west to north-east flow prevails, for which wSBL and vSBL conditions respectively are more likely to occur. Joint probabilities of shared regime occupation at the three towers (with and without conditioning on wind direction) are much larger than would be expected from statistically independent regime sequences at the different locations. Very persistent wSBL nights (without any transitions to the vSBL) have a higher probability of occurring across the entire tower network domain than very persistent vSBL nights. Many regime transitions occur within a narrow time window between the different towers; occurrence probabilities of such events are much higher than would be expected from statistically independent regime transitions. Of such events, transitions occurring at exactly the same time across the tower network occur most often. Many co-occurring turbulence recovery events can be associated with high-intensity intermittent turbulence events. Our results imply that the scale on which the SBL regime occupation and transitions are dependent exceeds 10 km in this region of complex terrain. Keywords Clustering · Spatial structure · Spatial variability · Stable boundary layer · Statistics
1 Introduction On the basis of Reynolds-averaged mean data, the stably stratified nocturnal boundary layer (SBL) is often classified into two distinct regimes denoted the weakly and very stable boundary layers (respectively, wSBL and vSBL, e.g., Mahrt 1998; Mahrt et al. 2013; Mahrt 2014;
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Carsten Abraham [email protected] School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3065 STN CSC, Victoria, BC V8P 5C2, Canada
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C. Abraham, A. H. Monahan
Mahrt et al. 2015; Acevedo and Fitzjarrald 2003; Bonin et al. 2015; van Hooijdonk et al. 2015; Monahan et al. 2015; Acevedo et al. 2016, 2019; Vignon et al. 2017; Abraham and Monahan 2019a, b, c, hereafter AM19a, AM19b, and AM19c) sometimes also denoted nearly neutral and stable regimes (Sun et al. 2012, 2016). In this classification scheme the wSBL is characterized by weak stratification, strong winds, and sustained turbulence. The vSBL, on the other hand, is characterized by strong stratification, low wind speeds, weak to collapsed turbulence, and a weak vertical coupling of the atmospheric layers. In this study we anal
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