Martina Frid – m.a.h.frid@pgr.reading.ac.uk
Beth Saunders – bethany.saunders@pgr.reading.ac.uk
Introduction
With a large (and growing) proportion of the global population living in cities, research undertaken in urban areas is important; especially in hazardous situations (heatwaves, flooding, etc), which become more severe and frequent due to climate change.
This post gives an overview of recent work done for The urbisphere; a Synergy Project funded by the European Research Council (urbisphere 2021), aiming to forecast feedbacks between weather, climate and cities.
Berlin Field Campaign
The project has included a year-long field campaign (Autumn 2021 – Autumn 2022) undertaken in Berlin (Fig. 1). A smart Urban Observation System was used to take measurements across the city. Sensors used include ceilometers, Doppler wind LIDARs, radiometers, thermal cameras, and large aperture scintillometers (LAS). These measurements were taken to provide new information about the impact of Berlin (and other cities) on the urban boundary layer. The unique observation network was able to provide dense, multi-scale measurements, which will be used to evaluate and inform weather and climate models.
Large Aperture Scintillometry in Berlin
The Berlin field campaign has included 6 LAS paths (Fig. 1). LAS paths consist of a transmitter and receiver mounted in the free atmosphere (Fig. 2), 0.5 – 5 km apart (e.g. Ward et al. 2014).
A beam of near-infrared radiation (wavelength of ~ 850 nm) is passed from the transmitter to receiver, where the beam intensity is measured. Changes in the refractive index of air are used to derive turbulent sensible heat flux. As the received intensity is the result of fluctuations all along the beam, derived quantities are spatially-integrated, and are therefore at a larger-scale compared to other flux measurement techniques (e.g. eddy-covariance).
Our Visit to Berlin
During the first week of August, we travelled to Berlin for three days of fieldwork, to prepare for an intense observation period (IOP). This trip included us installing sensors, and testing they worked as expected. We visited three observation sites: GROP (123 m above sea level, Fig. 2), OSWE (63 m, Fig. 2) and NEUK (60 m).
One of the main purposes of this visit was to align two of the LAS paths (including the one in Fig. 2). Initially, work is undertaken at the transmitter site (Fig. 3, top) to point the instrument in the approximate direction of the receiver using a sight (Fig. 3, right hand side photographs).
At the receiver site (Fig. 3, bottom), the instrument’s measurement of signal strength can be displayed on a monitor in real time. Using this output as a guide, small adjustments to the receiver’s alignment are made by loosening or tightening two bolts on the mount; one which adjusts the receiver’s pitch, and one with adjusts the yaw. This was carried out until we reached a peak reading in signal strength, indicating the path was aligned.
Our contribution to the IOP
Back in Reading, daily weather forecasts were carried out for the IOP, to determine when ground-based observations could be made. As the field campaign coincided with the central European heat wave, some of the highest temperatures were recorded during the IOP, and there was a need to forecast thunderstorm and the possibility of lightning strikes.
Ideal conditions for observations were clear skies and a consistent wind direction with height. A variety of different wind directions during the IOP was also preferable, to capture different transects of Berlin. For the selected days, group members in Berlin deployed multiple weather balloons simultaneously across multiple sites within the city and the outskirts. This was also timed with satellite overpasses. Observations of the mixing layer height (urban and suburban) were taken using a ceilometer mounted in a van, which drove along different transects of Berlin.
As the field campaign is wrapping up in Berlin, several instruments are now being moved to the new focus city: Paris. We are looking forward to this new period of interesting observations! Thank you and goodbye from us at the top of the GROP observation site!
References
urbisphere, 2021: Project Context and Objectives. http://urbisphere.eu/ (accessed 27/09/22)
Ward, H. C., J. G. Evans, and C. S. B. Grimmond, 2014: Multi-Scale Sensible Heat Fluxes in the Suburban Environment from Large-Aperture Scintillometry and Eddy Covariance. Boundary-Layer Meteorol., 152, 65–89.
The Social Metwork 