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Toward a Systematic Evaluation of Warm Conveyor Belts in Numerical Weather Prediction and Climate Models. Part I: Predictor Selection and Logistic Regression Model

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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Tobias Selz, Lucas Fischer, and George C. Craig

interactions of dynamics and moisture in the atmosphere are not fully understood, the observations of power-law scaling allow for a reality check of NWP models and GCMs concerning water vapor variability ( Kahn et al. 2011 ). While this is not a traditional validation measure, a model’s ability to reproduce the observed scale dependence behavior provides a statistical measure of the accuracy of its moist dynamics and physics ( Skamarock 2004 ). In other words, if a model does not reproduce the real

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Christian Barthlott and Corinna Hoose

and simulating clouds in different environments are needed. The COSMO model uses a rotated latitude–longitude grid with terrain-following hybrid height coordinates. Deep convection is resolved explicitly, while shallow convection is parameterized using a modified Tiedtke mass-flux scheme with moisture-convergence closure ( Tiedtke 1989 ). Shallow convection is nonprecipitating and limited to a cloud depth of 250 hPa. Only temperature and moisture are directly affected by shallow convection (for

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Tobias Selz

basically determines the intrinsic limit of weather prediction up to planetary scales. Already Lorenz (1969) suggested this with his experiment C. It has been further confirmed by the sensitivity of the magnitude of a large-scale error after a certain lead time on moisture ( Zhang et al. 2007 ) or on the convection scheme Selz and Craig (2015a) . Indeed, error-growth experiments with global numerical models suffer from the low grid resolution and the need of a parameterization scheme for deep

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