Julia H. Keller

The TIGGE Project and Its Achievements

AbstractThe International Grand Global Ensemble (TIGGE) was a major component of The Observing System Research and Predictability Experiment (THORPEX) research program, whose aim is to accelerate improvements in forecasting high-impact weather. By providing ensemble prediction data from leading operational forecast centers, TIGGE has enhanced collaboration between the research and operational meteorological communities and enabled research studies on a wide range of topics.The paper covers the objective evaluation of the TIGGE data. For a range of forecast parameters, it is shown to be beneficial to combine ensembles from several data providers in a multimodel grand ensemble. Alternative methods to correct systematic errors, including the use of reforecast data, are also discussed.TIGGE data have been used for a range of research studies on predictability and dynamical processes. Tropical cyclones are the most destructive weather systems in the world and are a focus of multimodel ensemble research. Their ...

Utility of Hovmöller diagrams to diagnose Rossby wave trains

The study investigates and compares various methods that aim to diagnose Rossby wave trains with the help of Hovm¨oller diagrams. Three groups of methods are distinguished: The first group contains trough-and-ridge Hovm¨oller diagrams of the meridional wind; they provide full phase information, but differ in the method for latitudinal averaging or weighting. The second group aims to identify Rossby wave trains as a whole, discounting individual troughs and ridges. The third group contains diagnostics which focus on physical mechanisms during the different phases of a Rossby wave train life cycle; they include the analysis of eddy kinetic energy and methods for quantifying Rossby wave breaking. The different methods are analysed and systematically compared with each other in the framework of a two-month period in fall 2008. Each method more or less serves its designed purpose, but they all have their own strengths and weaknesses. Notable differences between the individual methods render an objective identification of a Rossby wave train somewhat elusive. Nevertheless, the combination of several techniques provides a rather comprehensive picture of the Rossby wave train life cycle, being broadly consistent with the expected behaviour from previous theoretical analysis.

An Eddy kinetic energy view of physical and dynamical processes in distinct forecast scenarios for the extratropical transition of two tropical cyclones

AbstractThe extratropical transition (ET) of Hurricane Hanna (2008) and Typhoon Choi-Wan (2009) caused a variety of forecast scenarios in the European Centre for Medium-Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS). The dominant development scenarios are extracted for two ensemble forecasts initialized prior to the ET of those tropical storms, using an EOF and fuzzy clustering analysis. The role of the transitioning tropical cyclone and its impact on the midlatitude flow in the distinct forecast scenarios is examined by conducting an analysis of the eddy kinetic energy budget in the framework of downstream baroclinic development. This budget highlights sources and sinks of eddy kinetic energy emanating from the transitioning tropical cyclone or adjacent upstream midlatitude flow features. By comparing the budget for several forecast scenarios for the ET of each of the two tropical cyclones, the role of the transitioning storms on the development in downstream regions is investigated. Di...

HErZ: The German Hans-Ertel Centre for Weather Research

AbstractIn 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 fur 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 ...

A quantitative assessment of the sensitivity of the downstream midlatitude flow response to extratropical transition of tropical cyclones

During extratropical transition (ET) of tropical cyclones (TC) reduced predictability for the midlatitude flow is often observed. In this study we assess the sensitivity of the midlatitude flow response to ET. To this end, a simple novel metric, the “equivalent forecast hour difference,” is introduced and applied to experimental simulations for eight recent ET events, in which the TC has been removed or relocated. Early during ET, the midlatitude flow response is sensitive to ridgebuilding directly downstream of the transitioning TC. In cases with strong ridgebuilding and Rossby wave triggering, the sensitivity remains high. The midlatitude flow response is even more sensitive to error in the initial position of the TC. This study newly quantifies the high degree of sensitivity of the downstream midlatitude flow response to ET. It corroborates the need for correct tropical cyclogenesis and TC track forecasts for improving midlatitude numerical weather prediction during ET.

Earth System Science Frontiers: An Early Career Perspective

AbstractThe exigencies of the global community toward Earth system science will increase in the future as the human population, economies, and the human footprint on the planet continue to grow. This growth, combined with intensifying urbanization, will inevitably exert increasing pressure on all ecosystem services. A unified interdisciplinary approach to Earth system science is required that can address this challenge, integrate technical demands and long-term visions, and reconcile user demands with scientific feasibility. Together with the research arms of the World Meteorological Organization, the Young Earth System Scientists community has gathered early-career scientists from around the world to initiate a discussion about frontiers of Earth system science. To provide optimal information for society, Earth system science has to provide a comprehensive understanding of the physical processes that drive the Earth system and anthropogenic influences. This understanding will be reflected in seamless pre...

Forecast Variability of the Blocking System over Russia in Summer 2010 and Its Impact on Surface Conditions

AbstractThe Euro–Russian atmospheric blocking pattern in the summer of 2010 was related to high-impact weather, including a mega–heat wave in Russia. A set of scenarios for the synoptic evolution during the onset, mature stage, and decay of the block are extracted from the THORPEX Interactive Grand Global Ensemble multimodel ensemble forecast. These scenarios represent the key features of the forecast variability of the block and of the resulting surface impacts. Two heat indices and a fire index are computed to highlight the forecast variability in societal impacts. The study is a proof of concept, showing how information about surface impacts can be derived from available operational ensemble forecasts in an effective manner, and pointing to possible difficulties in this approach. Comparing the forecast for the heat wave’s impact on large spatial domains, and on a near-gridpoint scale, identifies challenges forecasters may face when predicting the development of a heat wave.Although the block’s onset wa...

Enhanced Tropospheric Wave Forcing of Two Anticyclones in the Prephase of the January 2009 Major Stratospheric Sudden Warming Event

AbstractTropospheric forcing of planetary wavenumber 2 is examined in the prephase of the major stratospheric sudden warming event in January 2009 (MSSW 2009). Because of a huge increase in Eliassen–Palm fluxes induced mainly by wavenumber 2, easterly angular momentum is transported into the Arctic stratosphere, deposited, and then decelerates the polar night jet. In agreement with earlier studies, the results reveal that the strongest eddy heat fluxes, associated with wavenumber 2, occur at 100 hPa during the prephase of MSSW 2009 in ERA-Interim. In addition, moderate conditions of the cold phase of ENSO (La Nina) contribute to the eddy heat flux anomaly. It is shown that enhanced tropospheric wave forcing over Alaska and Scandinavia is caused by tropical processes in two ways. First, in a climatological sense, La Nina contributes to an enhanced anticyclonic flow over both regions. Second, the Madden–Julian oscillation (MJO) has an indirect influence on the Alaskan ridge by enhancing eddy activity over t...

Amplification of the Downstream Wave Train during Extratropical Transition: Sensitivity Studies

AbstractA tropical cyclone (TC) undergoing extratropical transition (ET) may support the amplification of a Rossby wave train in the downstream midlatitudes. Within the context of downstream baroclinic development, the TC acts as an additional source of eddy kinetic energy (). Previous studies concluded that the impact depends, in particular, on the phasing between the TC and the midlatitude flow and the continuation of the generation during ET. These studies did not quantify the impact of ET on the within a downstream Rossby wave train.The present study uses ensemble sensitivity analysis to examine the sensitivity of downstream Rossby wave train amplification to the budget of the transitioning TC and of the upstream midlatitude features for Typhoon Choi-Wan (2009) and Hurricane Hanna (2008) in ECMWF ensemble forecasts. The amplification of the downstream wave train is measured using the amplitude of its associated maxima. The sensitivity of the maximum’s intensity at a particular forecast time to the bud...