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Joël Arnault, Thomas Rummler, Florian Baur, Sebastian Lerch, Sven Wagner, Benjamin Fersch, Zhenyu Zhang, Noah Kerandi, Christian Keil, and Harald Kunstmann

1. Introduction Numerical atmospheric models generally consider terrestrial hydrological processes as only being vertical, in order to estimate the surface heat fluxes for constraining the atmospheric lower boundary condition. This is, for example, the case for the Weather Research and Forecasting (WRF) Model ( Skamarock and Klemp 2008 ) coupled with the Noah land surface model (LSM; Chen and Dudhia 2001 ). In this approach, the lateral redistribution of soil moisture according to the

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Peter Vogel, Peter Knippertz, Andreas H. Fink, Andreas Schlueter, and Tilmann Gneiting

organization and coupling to larger-scale circulations. The most important example of such a coupling on weather time scales are equatorial waves, classically referring to planetary-scale solutions of the shallow water equations for the tropics ( Matsuno 1966 ; Wheeler and Kiladis 1999 ). The coupling relies on a wave-induced modification of environmental conditions for convection such as convergence, stability, moisture availability, and shear ( Schlueter et al. 2019a , b ). Although a relatively high

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Roderick van der Linden, Andreas H. Fink, Joaquim G. Pinto, and Tan Phan-Van

, northeastern Vietnam is also frequently affected by the rainbands that are associated with this front ( Xu et al. 2009 ). The South China Sea usually is the main moisture source, but sometimes moisture is also transported from the Bay of Bengal to the mei-yu front ( American Meteorological Society 2016 ). Other synoptic systems affecting the study region with copious rains are tropical cyclones (TCs) that originate in the northwest Pacific and enter the South China Sea. Although the TC season peaks in

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Thomas Engel, Andreas H. Fink, Peter Knippertz, Gregor Pante, and Jan Bliefernicht

lesser extent, from evaporation at the continental surface. A convergent humidity flux [with the specific humidity q (kg kg −1 ) and horizontal wind vector (m s −1 )] then causes an accumulation of moisture in the atmospheric column as TPW (kg m −2 ): where p sfc (hPa) is the surface pressure and g = 9.81 m s −2 the gravitational acceleration. The convective available potential energy (CAPE; J kg −1 ; Emanuel 1994 ) is a common measure of convective instability: Here, α pcl and α env

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Andreas Schlueter, Andreas H. Fink, and Peter Knippertz

. 2009 ). Moisture anomalies starting at low levels, rise to midlevels prior to the convective peak. A stratiform moist outflow is left behind after the passage of the deep convection. In addition to adiabatic heating associated with the vertical circulation, diabatic effects create heating that slows down the wave. The reader is referred to Kiladis et al. (2009) for a more detailed review of the theory, observational evidence, and properties of CCEWs. Two additional disturbances dominate rainfall

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Julian F. Quinting and Christian M. Grams

trajectories has significantly advanced our understanding of WCBs and their effect on the large-scale flow (e.g., Eckhardt et al. 2004 ; Grams et al. 2011 ; Madonna et al. 2014b ; Martínez-Alvarado et al. 2016 ). The inflow of WCBs is located in a cyclone’s warm sector ahead of the cold front (label 1 in Fig. 1 ). At this stage, air parcels still reside predominantly in the planetary boundary layer. WCB inflow is typically characterized by strong moisture flux convergence and a band of high water

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Kirstin Kober and George C. Craig

relevant to the initiation of convection. In general, the initiation of convection requires certain atmospheric conditions. In synoptic situations without large-scale forcing, convection can develop if instability, measurable by the convective available potential energy (CAPE), as well as local triggers to overcome a possible inversion above the boundary layer, measurable by the convective inhibition (CIN), are available together with moisture in specific heights ( Done et al. 2012 ). The distribution

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Andreas Schäfler, George Craig, Heini Wernli, Philippe Arbogast, James D. Doyle, Ron McTaggart-Cowan, John Methven, Gwendal Rivière, Felix Ament, Maxi Boettcher, Martina Bramberger, Quitterie Cazenave, Richard Cotton, Susanne Crewell, Julien Delanoë, Andreas Dörnbrack, André Ehrlich, Florian Ewald, Andreas Fix, Christian M. Grams, Suzanne L. Gray, Hans Grob, Silke Groß, Martin Hagen, Ben Harvey, Lutz Hirsch, Marek Jacob, Tobias Kölling, Heike Konow, Christian Lemmerz, Oliver Lux, Linus Magnusson, Bernhard Mayer, Mario Mech, Richard Moore, Jacques Pelon, Julian Quinting, Stephan Rahm, Markus Rapp, Marc Rautenhaus, Oliver Reitebuch, Carolyn A. Reynolds, Harald Sodemann, Thomas Spengler, Geraint Vaughan, Manfred Wendisch, Martin Wirth, Benjamin Witschas, Kevin Wolf, and Tobias Zinner

forecasts of an extratropical cyclone with severe impacts in western Europe to be very sensitive to the initial low-level moisture, which influenced the moisture supply in a WCB. At upper levels, global NWP models fail to maintain a sufficiently sharp tropopause, showing a decrease in sharpness with forecast lead time ( Gray et al. 2014 ). This influence on the waveguide can have major implications for the representation of the downstream propagation and amplification of Rossby waves in NWP ( Harvey et

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Michael Maier-Gerber, Michael Riemer, Andreas H. Fink, Peter Knippertz, Enrico Di Muzio, and Ron McTaggart-Cowan

moisture in the midtroposphere. They also speculate that the interactions of precursor features and processes required for TT further reduce the overall predictability as those constitute additional probability factors that multiplicatively extend the joint probability for non-TT pathways. Despite this probabilistic way of viewing TT predictability, it still remains unclear which factors of uncertainty cause predictability issues (i.e., strikingly rapid changes in forecast uncertainty with lead time

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Stephan Rasp, Tobias Selz, and George C. Craig

the cold front ( Harrold 1973 ). WCBs transport large quantities of heat and moisture poleward and upward and are associated with bands of clouds and precipitation in extratropical cyclones ( Browning 1990 ). Climatological analyses of WCBs have shown a preferred occurrence in the winter and over the oceans ( Madonna et al. 2014 ; Eckhardt et al. 2004 ). Furthermore, they are of particular importance for extratropical precipitation extremes ( Pfahl et al. 2014 ), especially when they interact

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