C5 - Forecast uncertainty for peak surface gusts associated with European cold-season cyclones

Principal investigators: Prof. Dr. Peter Knippertz, Dr. Ulrich Corsmeier

Other researcher: Lea Eisenstein (PhD)

Former researcher: Dr. Florian Pantillon (Postdoc)

Predictions of the track and intensity of severe cyclones have substantially improved over recent decades but accurate predictions of the location, timing, and intensity of peak surface gusts remain challenging. Such information is of utmost importance for storm damage and warnings to the population.

This project aims at a systematic quantification of forecast uncertainty for extreme wind gusts over Germany associated with cyclonic storms during the winter half year, taking into account the involved dynamical processes from the synoptic scale to boundary layer turbulent mixing. This research is two-fold in the sense that both model deficiencies and the chaotic behavior of the system to be forecast limit predictability. The project will combine new observational data from Doppler lidars and research aircraft with recently available limited area ensemble prediction systems (LEPSs) and high-resolution models for detailed process studies in an innovative and systematic way.

The general approach is to break down sources of uncertainty according to the different scales involved using appropriate modeling and observational datasets and methods:

• On the synoptic scale the focus will be on the track and intensity of the cyclone associated with the extreme gusts over Germany. Forecast uncertainty will be quantified statistically for the top 2% of gust events using global ensemble prediction systems and objective tracking methods. Standard gust parameterizations will be applied to these datasets. [A]
• For the investigation of mesoscale processes, suitable case studies will be selected from the sample identified under [A]. Physical processes of main interest in this context are "sting jets", descending wind speed maxima in the middle troposphere in the area of bent-back warm fronts, and moist convection, for example as occurring along cold fronts, and their associated cold pools. Observational datasets to be used in this context include gust measurements from the network of the German Weather Service (DWD), space-borne scatterometer data over the North and Baltic Seas, radiosondes, and wind profilers. Forecast uncertainty will be assessed on the basis of LEPSs, taking into account uncertainty resulting from the driving synoptic scale analyzed under [A]. Targeted high-resolution simulations using the COSMO model will be conducted for detailed process studies, e.g., of sting jet formation.
• Factors influencing the prediction of gusts, which are not resolved by current weather prediction models, include turbulent mixing of momentum into and across the boundary layer, particularly in areas of large shear at mid-levels as for example underneath sting jets. These aspects will be addressed with observations from ground-based and aircraft-borne wind lidar operated by the Karlsruhe Institute of Technology as well as wind towers and wind profilers, which will be compared to Large Eddy simulations. Selected cases will include existing data and new observations taken as part of the project.

The final outcome from this project will be an integral assessment of the relative contributions from different scales and processes to forecast uncertainty of strong wind gusts over Germany during the winter half year as well as of the quality of widely used gust parameterizations. This will enable the provision of concrete guidance to operational forecasters and model developers.