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Adam H. Monahan, Tim Rees, Yanping He, and Norman McFarlane

necessarily) drive transitions from strong to weak stable stratification. Based on the observational results discussed above, the SBL is often classified into weakly and very stably stratified regimes (respectively denoted the wSBL and vSBL; cf. Okamoto and Webb 1970 ; Kondo et al. 1978 ; Mahrt 1998 ; Acevedo and Fitzjarrald 2003 ; Shravan Kumar et al. 2012 ; Liang et al. 2014 ; Mahrt 2014 ; van Hooijdonk et al. 2015 ; Vercauteren and Klein 2015 ). Other regime classifications have been suggested

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Carsten Abraham and Adam H. Monahan

et al. 2017b ; Abraham and Monahan 2019a , b , hereafter AM19a and AM19b ). This study uses data from several tower sites in different meteorological settings to investigate the long-term occupation and transition statistics of SBL regimes. Furthermore, the relationship between external drivers and regime occupation and regime transitions are assessed. While this two-regime classification is the simplest among those that have been proposed for the SBL, we have demonstrated that it provides

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Carsten Abraham and Adam H. Monahan

regimes and transitions between them are poorly represented in weather and climate models, due both to coarse resolution (vertical and horizontal) and to an imperfect understanding of all the diverse physical processes governing the SBL (e.g., Holtslag et al. 2013 ; Mahrt 2014 ). In this study the climatological evolution and structure of meteorological state variables during times of transitions between these two SBL regime states are investigated across different tower sites, and compared to state

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James S. Risbey, Terence J. O’Kane, Didier P. Monselesan, Christian Franzke, and Illia Horenko

DPM are supported by the Ocean and Climate Dynamics program of CSIRO. TJO is supported by an Australian Research Council Future Fellowship. CF is supported by the Deutsche Forschungsgemeinschaft through the CliSAP Cluster of Excellence. We are grateful for the thorough and constructive reviews of the manuscript. REFERENCES Barnston , A. G. , and R. E. Livezey , 1987 : Classification, seasonality, and persistence of low-frequency atmospheric circulation patterns . Mon. Wea. Rev. , 115

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Goodwin Gibbins and Joanna D. Haigh

1. Introduction a. Motivation The climate is, fundamentally, an entropy-producing system. The movement of energy from warmer regions, where it is supplied to the climate, to cooler regions, where it leaves, is an inevitable consequence of the second law of thermodynamics and drives the motion and activity of the climate. The energy transfers are mediated by a myriad of irreversible processes: for example, wind, rain, and radiation. Each process produces entropy, which must be exported from the

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E. Tyrlis and B. J. Hoskins

: Rossby wave breaking in the Southern Hemisphere wintertime upper troposphere. Mon. Wea. Rev. , 131 , 2623 – 2634 . Robertson , A. W. , and M. Ghil , 1999 : Large-scale weather regimes and local climate over the western United States. J. Climate , 12 , 1796 – 1813 . Thorncroft , C. D. , B. J. Hoskins , and M. E. McIntyre , 1993 : Two paradigms of baroclinic-wave life-cycle behaviour. Quart. J. Roy. Meteor. Soc. , 119 , 17 – 55 . Tyrlis , E. , 2005 : Aspects of Northern

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Toshiki Iwasaki, Takamichi Shoji, Yuki Kanno, Masahiro Sawada, Masashi Ujiie, and Koutarou Takaya

and Ho 2005 ; Park et al. 2011 ). In isobaric coordinates, however, the time-mean wind hardly indicates the cold airmass flux, because it includes warm winds. Therefore, previous studies treated the cold air outbreaks as anomalous events deviated from the normal climate. Direct estimates, however, have not been made of the stationary component of polar cold airmass flux yet. Isentropic coordinates are convenient to trace the airmass trajectory. Harada (1962) studied the rapid advancement of the

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Brian E. J. Rose and John Marshall

– 352 . Marshall , J. C. , 1981 : On the parameterization of geostrophic eddies in the ocean. J. Phys. Oceanogr. , 11 , 257 – 271 . Marshall , J. C. , and F. Molteni , 1993 : Toward a dynamical understanding of planetary-scale flow regimes. J. Atmos. Sci. , 50 , 1792 – 1818 . Marshall , J. C. , D. Ferreira , J-M. Campin , and D. Enderton , 2007 : Mean climate and variability of the atmosphere and ocean on an aquaplanet. J. Atmos. Sci. , 64 , 4270 – 4286 . Matteucci

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A. W. Robertson, M. Ghil, and M. Latif

. Livezey, 1987: Classification, seasonality, and persistence of low-frequency atmospheric circulation patterns. Mon. Wea. Rev., 115, 1083–1126. Bengtsson, L., K. Arpe, E. Roeckner, and U. Schulzweida, 1996: Climate predictability experiments with a general circulation model. Climate Dyn., 12, 261–278. Blackmon, M. L., and N.-C. Lau, 1980: Regional characteristics of the Northern Hemisphere wintertime circulation: A comparison of the simulation of a GFDL general circulation model with

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Carsten Abraham and Adam H. Monahan

1. Introduction Observations of the nocturnal stable boundary layer (SBL) show abrupt changes of physical properties, motivating a classification into physically distinct regimes. The simplest classification considers two regimes: one very stable with weak and intermittent turbulence, and another weakly stable with sustained turbulent activity (e.g., Sun et al. 2012 ; Vignon et al. 2017b ). Transitions between these regimes remain one of the least understood phenomena in the planetary

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