Seminars on precipitation modeling

Prof. Dr. Heini WERNLI

Institute of Atmospheric and Climate Science, ETH Zürich

9:00 – 10:00

“On the linkage between cloud physics and extratropical dynamics”

There are several ways how clouds influence the weather. The obvious ways are that clouds produce precipitation and reduce solar surface radiation. A more subtle influence of clouds on weather comes from the latent heat release in clouds when water vapour condenses and cloud droplets freeze. This energy input to the atmospheric circulation can have important consequences: latent heating, in particular in the so-called warm conveyor belt airstream, contributes to the intensification of extratropical cyclones, and it can modify Rossby waves, the jet stream and the formation of blocks in the upper troposphere. This presentation will explain the fundamentals how these cloud-dynamics interactions occur, and it will present results from two currently ongoing projects at ETH Zurich. These projects quantify the effect of below-cloud microphysical processes (e.g., snow sublimation) on cyclones and the jet stream, and elucidate the role of embedded convection in warm conveyor belts on these weather systems.

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Dr Olivier CAUMONT

CNRM (Météo-France/CNRS)

10:00 – 11:00

« Convective-scale data assimilation for Numerical Weather Prediction: Progress and challenges»

Since a decade or so, operational weather services have started to use regional Numerical Weather Prediction (NWP) systems at resolutions of the order of a few kilometres. The main objective of these systems is to provide useful forecasts of high-impact, small-scale weather phenomena, such as deep convection. Although such so-called 'convective-scale' models proved their ability to simulate realistic thunderstorms, the accurate prediction of deep convection remains an open challenge. One of the reasons why convective-scale models still struggle to predict deep convection accurately lies in the difficulty of making the most of observations to supply them with accurate initial conditions - a process known as 'data assimilation'. This presentation will first briefly review the basics of NWP and data assimilation. Then, progress made so far, open questions, and promising ways forward will be presented.

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Dr. Ulrich Blahak

DWD, Germany

11:00 – 12:00

« Radar data assimilation in the framework of SINFONY »

At DWD a new internal project has been set up recently to develop its future seamless ensemble prediction system for storm-scale forecasting from observation time up to +6 h / +12 h forecasts.

The focus is on severe summertime convective events with their associated hazards (heavy precipitation, hail, wind gusts, etc.).

Up to now, for the first 1-2 h this relies mostly on observation-bsaed nowcasting products, whereas convection-allowing ensemble NWP (COSMO-DE-EPS) is only able to reach/outperform the quality of nowcasting at later times. New NWP forecasts are started only every 3 h and after a rather long cut-off time to wait for incoming observational data.

Moreover, nowcasting and ensemble NWP are treated as two separate and independent methods, and there are few common products available for the forecasters.

The goal of the new project is to narrow down these gaps, on the one hand by enhancements to both nowcasting and NWP separately and on the other hand by mutual information exchange and combination, to further enhance the quality of both. High-resolution observational data (radar, satellite, lightning, GPS-derived moisture, etc.) will be exploited.

For this presentation, the focus will be on the radar data assimilation aspect of the project, which includes direct assimilation of 3D volume scans of radial wind and reflectivity of the German C-Band weather radar network into the km-scale convection-allowing COSMO-DE model. The data are directly assimilated using DWD's LETKF system and the conventional radar forward operator EMVORADO. Results from several weeks of intense convection in May/June 2016 over Germanyare shown.