Investigation of local-scale extreme convective wind events and their interaction with building structures in urban areas

• Applicants: Professor Dr. Michael Kunz; Professor Dr.-Ing. Bodo Ruck
• Subject Area: Fluid Mechanics ; Atmospheric Science
• Term: from 2013 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 245745015

Severe convective gusts may reach wind speeds that exceed by far those related to synoptic-scale winter storms. Associated with this is a considerable damage potential for buildings, critical infrastructure, or crops. Due to the local-scale nature of deep moist convection, essential information concerning probability, spatial extent and maximum speeds of the convective gusts can neither be obtained directly from numerical model data nor estimated from data of available observation stations with the necessary spatial representativeness. Moreover, point measurements at single stations cannot reproduce the additional amplification of gusts caused by the interaction with built-up structures. Therefore, severe convective gusts are underrepresented in the wind loading codes and standards for buildings and structures, which are usually based on time series obtained from meteorological stations. This leads to a considerable lack of knowledge over several scales, from the generation of the gusts within cumulus clouds over their characteristics and climatology to the flow propagation over built-up structures including the impacts.The overall objective of the proposed project is to reduce this knowledge gap by temporally and spatially high-resolving observations over several scales and to estimate the impact they exert on urban structures. In the meteorological part of the project, data from different observation systems and networks (measuring masts, Lidar, Radar) are combined and statistically analyzed with the aim to investigate the relation between vertical downdrafts and horizontal gusts depending on the prevailing meteorological conditions. Maximum gust wind speeds related to different convective systems are estimated by applying appropriate empirical parameterization schemes to the observations. Based on these methods and the obtained results the climatology including the probability of severe convective gusts shall be derived from high-resolution reanalysis data over a long-term period. Convective gusts shall be investigated in detail in the fluid mechanical part of the proposal, which aims at improving the basic knowledge about the interaction of downdrafts and gusts with typical urban built-up structures by realistic experiments within an atmospheric boundary layer wind tunnel. For this, a wind tunnel test section shall be constructed with a gust generator, which generates gusts of pre-defined duration, extent, and tilt. Embedded in a horizontal flow field, the gusts shall be released from a moving source in order to simulate realistic wind loading on defined building configurations. Typically, the gust hits the ground with an oblique trajectory, which can be conceived of as the superposition of gust velocity, velocity of convective system and base flow velocity. The gust dynamics and interaction shall be investigated with high temporal and spatial resolution using a time-resolved PIV system and highly sensitive pressure tapping technique.

• DFG Programme: Research Grants