Abstract
Abstract The evolution of profiles of meteorological state variables during nights with and without transitions in the nocturnal stably stratified boundary layer (SBL) between weakly stable (wSBL) and very stable (vSBL) regimes, as classified by a hidden Markov model, is examined at nine different tower sites. During wSBL-to-vSBL transitions, inversion strengths increase, near-surface winds decelerate, and atmospheric layers vertically decouple. Turbulence kinetic energy (TKE) steadily decreases before wSBL-to-vSBL transitions and fluctuations of the vertical velocity become weak. In contrast to land-based sites where wSBL-to-vSBL transitions are normally caused by surface cooling, at sea-based stations the transitions generally are initiated by advection of warm air aloft. The vSBL-to-wSBL transition is characterized by a fast breakdown of the inversion strength, acceleration of wind profiles, and a restored vertical coupling of the atmospheric flow. TKE recovers on time scales of minutes first in atmospheric levels between 50 and 100 m. Profiles of state variables for the two different regimes during very persistent nights (nights without SBL regime transitions) are clearly separated and similar to structures during nights with transitions away from transition times. During very persistent nights the wind conditions stay relatively steady. Similarly, the temperature is steady after an initial adjustment time at sunset (wSBL) or shortly after sunset (vSBL). Even though nights with and without transitions are a common feature of the SBL, there is no clear indicator in Reynolds-averaged mean variables that distinguishes very persistent nights from nights with transitions.
Highlights
On the basis of Reynolds-averaged mean data, observations of the nocturnal stably stratified boundary layer (SBL) are often classified into two distinct regimes (e.g., Mahrt 1998a; Acevedo and Fitzjarrald 2003; Mahrt 2014; van Hooijdonk et al 2015; Monahan et al 2015; Vercauteren and Klein 2015; Acevedo et al 2016; Vignon et al 2017b; Abraham and Monahan 2019a,b, hereafter AM19a and AM19b)
Composite wind and stratification profiles across times of transitions are qualitatively similar for all land-based tower sites (Fig. 1; note that the scale of the vertical axes differ between the different sites and that the stratification is calculated as the potential temperature difference between each height and the measurement nearest the surface)
Regime sequences obtained from a hidden Markov model (HMM) allow for systematic characterization of the climatological mean behavior of meteorological state variables during regime transitions in the nocturnal stably stratified boundary layer (SBL) and in nights without transitions (‘‘very persistent nights’’) at nine different tower sites
Summary
On the basis of Reynolds-averaged mean data, observations of the nocturnal stably stratified boundary layer (SBL) are often classified into two distinct regimes (e.g., Mahrt 1998a; Acevedo and Fitzjarrald 2003; Mahrt 2014; van Hooijdonk et al 2015; Monahan et al 2015; Vercauteren and Klein 2015; Acevedo et al 2016; Vignon et al 2017b; Abraham and Monahan 2019a,b, hereafter AM19a and AM19b). Composite wind and stratification profiles (which include all state variables used in the HMM analyses) across times of transitions are qualitatively similar for all land-based tower sites (Fig. 1; note that the scale of the vertical axes differ between the different sites and that the stratification is calculated as the potential temperature difference between each height and the measurement nearest the surface).
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