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.
Highlights
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
In this study we analyze the horizontal spatial structure of stratified atmospheric boundary layer (SBL) regime occupation and regime transitions across a small network of towers at the Los Alamos National Laboratory
Three statistical aspects of the spatial dependence of SBL statistics are investigated: probabilities of shared regime occupation, of shared very persistent nights, and of contemporaneous transitions
Summary
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 To calculate the SBL regime occupation sequence with the HMM analysis at each tower site we use a three-dimensional state variable vector consisting of vertically-averaged wind speed (between 46 and 11.5 m), wind speed shear (between 46 and 11.5 m), and potential temperature difference (between 46 and 1.2 m). All towers share these common measurement heights, which are appropriate to estimate SBL regime sequences accurately at this location (cf AM19a). Wind speeds below 2 m s−1 are much more frequent and occur in 25.3 %, 19.5 %, and 22.5 % at respectively TA-6, TA-49, and TA-53
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