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Kathrin Wapler and Bernhard Mayer

Abstract

Cloud-resolving models—in particular, large-eddy simulation (LES) models—are important tools to improve the understanding of cloud–radiation interactions. A method is presented for accurate, yet fast, three-dimensional calculation of surface shortwave irradiance within an LES model using the tilted independent column approximation with smoothing of the diffuse irradiance. The algorithm calculates a tilted optical thickness for each surface pixel that is then used as input to a one-dimensional radiative transfer code. In a sensitivity analysis, it is shown that this calculation can even be replaced by a simple precalculated lookup table that tabulates surface irradiance as a function of only solar zenith angle and cloud optical thickness. Because the vertical variability of the cloud is of little relevance for the surface irradiance, this approximation introduces little extra uncertainty. In a final step, surface irradiance is smoothed to account for horizontal photon transport between individual columns. The algorithm has been optimized for parallelization, which enhances its applicability in LES models. In this implementation, the total computational time of the LES model increased by only 3% relative to the reference run without radiation. Comparisons between the fast approximation and detailed three-dimensional radiative transfer calculations showed very good agreement for different cloud conditions and several solar zenith and azimuth angles, with a root-mean-square difference of 6%.

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Thomas Hengstebeck, Kathrin Wapler, Dirk Heizenreder, and Paul Joe

Abstract

The radar network of the German Weather Service [Deutscher Wetterdienst (DWD)] provides 3D Doppler data in high spatial and temporal resolution, supporting the identification and tracking of dynamic small-scale weather phenomena. The software framework Polarimetric Radar Algorithms (POLARA) has been developed at DWD to better exploit the capabilities of the existing remote sensing data. The data processing and quality assurance implemented in POLARA include a dual-PRF dealiasing algorithm with error correction. Azimuthal shear information is derived and processed in the mesocyclone detection algorithm (MCD). Low- and midlevel azimuthal shear and track products are available as composite (multiradar) products. Azimuthal shear may be considered as a proxy for rotation. The MCD results and azimuthal shear products are part of the severe weather detection algorithms of DWD and are provided to the forecaster on the NinJo meteorological workstation system. The forecaster analyzes potentially severe cells by combining near-storm environment data with the MCD product as well as with the instantaneous azimuthal shear products (mid- and low level) and their tracks. These products and tracks are used to diagnose threat potential by means of azimuthal shear intensity and track longevity. Feedback from forecasters has shown the utility of the algorithms to analyze and diagnose severe convective cells in Germany and in adjacent Europe. In this paper, the abovementioned algorithms and products are presented in detail and case studies illustrating usability and performance are shown.

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Kathrin Wapler, Todd P. Lane, Peter T. May, Christian Jakob, Michael J. Manton, and Steven T. Siems

Abstract

Nested cloud-system-resolving model simulations of tropical convective clouds observed during the recent Tropical Warm Pool-International Cloud Experiment (TWP-ICE) are conducted using the Weather Research and Forecasting (WRF) model. The WRF model is configured with a highest-resolving domain that uses 1.3-km grid spacing and is centered over Darwin, Australia. The performance of the model in simulating two different convective regimes observed during TWP-ICE is considered. The first regime is characteristic of the active monsoon, which features widespread cloud cover that is similar to maritime convection. The second regime is a monsoon break, which contains intense localized systems that are representative of diurnally forced continental convection. Many aspects of the model performance are considered, including their sensitivity to physical parameterizations and initialization time, and the spatial statistics of rainfall accumulations and the rain-rate distribution. While the simulations highlight many challenges and difficulties in correctly modeling the convection in the two regimes, they show that provided the mesoscale environment is adequately reproduced by the model, the statistics of the simulated rainfall agrees reasonably well with the observations.

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Clemens Simmer, Gerhard Adrian, Sarah Jones, Volkmar Wirth, Martin Göber, Cathy Hohenegger, Tijana Janjic´, Jan Keller, Christian Ohlwein, Axel Seifert, Silke Trömel, Thorsten Ulbrich, Kathrin Wapler, Martin Weissmann, Julia Keller, Matthieu Masbou, Stefanie Meilinger, Nicole Riß, Annika Schomburg, Arnd Vormann, and Christa Weingärtner

Abstract

In 2011, the German Federal Ministry of Transport, Building and Urban Development laid the foundation of the Hans-Ertel Centre for Weather Research [Hans-Ertel-Zentrum für Wetterforschung (HErZ)] in order to better connect fundamental meteorological research and teaching at German universities and atmospheric research centers with the needs of the German national weather service Deutscher Wetterdienst (DWD). The concept for HErZ was developed by DWD and its scientific advisory board with input from the entire German meteorological community. It foresees core research funding of about €2,000,000 yr−1 over a 12-yr period, during which time permanent research groups must be established and DWD subjects strengthened in the university curriculum. Five priority research areas were identified: atmospheric dynamics and predictability, data assimilation, model development, climate monitoring and diagnostics, and the optimal use of information from weather forecasting and climate monitoring for the benefit of society. Following an open call, five groups were selected for funding for the first 4-yr phase by an international review panel. A dual project leadership with one leader employed by the academic institute and the other by DWD ensures that research and teaching in HErZ is attuned to DWD needs and priorities, fosters a close collaboration with DWD, and facilitates the transfer of fundamental research into operations. In this article, we describe the rationale behind HErZ and the road to its establishment, present some scientific highlights from the initial five research groups, and discuss the merits and future development of this new concept to better link academic research with the needs and challenges of a national weather service.

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