Evaluating a three-beam lidar wind-profiling method in urban areas Siân Lane, Janet Barlow, Humphrey Lean With thanks to Curtis Wood, Halo Photonics, John Lally, and Ewan O’Connor Need to add Met Office logo; think we should acknowledge Curtis and Ewan, John Lally and Halo guys for tech support *

Remote sensing allows measurement throughout the urban boundary layer. Most urban studies rely on rooftop or street-level measurements. A lack of information about, for example, wind speeds above cities makes comprehensive evaluation of numerical weather prediction (NWP) for urban areas impossible. Roof/street-level observations are only representative of the immediate surroundings. Adapted from Shepherd (2005)

Doppler Lidars provide information about BL structure, winds, and turbulence Backscatter (proxy for aerosol density) Lower aerosol density Higher aerosol density Height (km) Vertical wind speed Look in Ewan’s archive for the version with corrected backscatter; worth labelling that it’s the vertical component of velocity shown here, you will discuss horizontal components later? There needs to be a question/statement c. slide 2 or 3 saying exactly what we *don’t* know and what the point of this talk is…lack of information about winds means you can’t evaluate NWP, for example…to link in with later slides. Try to include words on the slide to reinforce the point you make verbally. Convective BL Time (UTC) www.actual .ac.uk *

Instrument sites London

The surrounding surface is very heterogeneous (parks, urban, river) Regent’s Park 1.6 km Westminster City Council BT Tower 1.6 km Note heterogeneous surface (parks, urban, river), point out where instruments are Would be worth having a zoomed out map of London in SE England, our sites in London as it is an international audience You could insert references in smaller font near bottom to highlight Barlow et al 2009 paper on WCC site, Wood et al 2010 BT Tower to say there is previous work describing and justifying these sites; some info on what instruments (show pic of lidar), what heights, what variables you are taking to satisfy tech queries – may be worth adding extra slide with photos of instruments at the sites River Thames Hyde Park See Barlow et al. 2009 and Wood et al. 2010 for previous work at these sites

Halo Photonics Streamline pulsed Doppler lidar Fully programmable scanner Gate length = 30 m 80 measurement gates Instrument location = 22 m above ground level Min. measurement height = 90 m above lidar (112 m above ground) Max. Doppler velocity = ±11 ms-1 Resolution = 0.023 ms-1 Measures velocity along the beam

Instruments at BT Tower (190 m) Gill instruments R3-50 ultrasonic anemometer Measures horizontal and vertical components of wind. Sampling frequency = 20 Hz Vaisala WXT 520 weather transmitter Measures horizontal wind velocity. Sampling frequency = 1 Hz Scaffolding tower

Doppler beam swinging method Three-beam wind-profiling method Derives wind speeds from one vertical and two tilted beams 2 s of data taken consecutively in each direction (40,000 pulses) Short scan time means flow will not change much over scan period. Interval between scans = 120 s See Pearson et al. (2009) for comparison with other methods in a rural setting. θ = 15° Watch out for a question like what error is introduced as the lidar beams are looking at separate patches of air? Maybe reference Pearson et al saying DBS agrees well with VAD… *

Doppler beam swinging method Three-beam wind-profiling method Derives wind speeds from one vertical and two tilted beams 2 s of data taken consecutively in each direction (40,000 pulses) Short scan time means flow will not change much over scan period. Interval between scans = 120 s See Pearson et al. (2009) for comparison with other methods in a rural setting. θ = 15° Watch out for a question like what error is introduced as the lidar beams are looking at separate patches of air? Maybe reference Pearson et al saying DBS agrees well with VAD… *

Comparison with sonic anemometer May – Nov 2011 -Comparison period = May 2011 – Nov 2011 **write this on the slide! If you have tech/numerical details, let people read them as well as hear them, LOTS of non-native English speakers… -Mid-point of comparison gate - Assumptions: 1) Horizontal distance between beams is not important (~ 50m @ BT Tower height). Flow more homogeneous at height. 2) Difference in height between gates due to slanted beams is not important (mid-point of tilted beams @180m) ?worth mentioning that data returned from bottom 3 gates is not quality By making this comparison we treat the lidar, which averages over a volume, as equivalent to an anemometer, which measures at a point. We must assume: 1) The horizontal distance between the beams (~50 m) is not important. 2) The difference in gate height due to using slanted beams (6.5 m) is not important.

Assessment of BT Tower site Previous work has shown that sensors at this site may be affected by turbulent wakes. See Barlow et al. (2011) for a detailed description

Wind direction (degrees from north) TI=σU/U U = wind speed Gill R3-50 Turbulence intensity Wind direction (degrees from north) Vaisala WXT 520 Barlow et al 2010 is a paper quantifying flow distortion around sensors on top of BT – this may be worth a mention, or in fending off queries Worth changing the title: it sounds like a rubbish site!! And include a photo of the scaffolding to indicate why there is potential wakes up there The text on the bottom is not clear – I don’t understand your point. Conclude with some statement as to how much data has been excluded due to this effect These data need to be filtered out Turbulence intensity *

Assessment of lidar and WCC site Lidar is not affected by turbulent wakes. Terrain and topography do not affect lidar measurements at this height – tested by comparison with data from same instrument at different site. The instrument was found to be off-horizontal by ~2°. This was corrected using a rotation matrix before further analysis * = KCL o = WCC Ulidar-UWXT Wind direction (degrees from north)

Wind speed (60 minute average) Weighted best fit 60 minute average used to include sufficient data from lidar. Some of RMSE can be explained by standard error (average SE = 0.4 ms-1). Some difference likely due to large separation between instruments. RMSE larger at higher wind speeds – smaller turbulence scales? Lidar wind speed (ms-1) No. of data points -Most of RMSE can be accounted for by standard error -Some differences due to separation -Differences seem to increase w. increasing wind speed -> smaller scales of turbulence? Investigate integral length scale using sonic data? **units on graphs please! Emphasise that the line is best fit, NOT 1:1; bigger font for regression line ** Stuart has sent through a paper which estimates error due to beam separation we should try even a back of envelope calculation on this Y=0.98x+0.56 RMSE=1.4 ms-1 Gill R3-50 wind speed (ms-1)

At Usonic<10 ms-1, average (Usonic-Ulidar) is consistently ~5 ms-1 1 ms-1 bins Error bars show standard deviation Much less data at wind speeds > 10 ms-1 Flow may be more consistent over large distances at high wind speeds. Usonic-Ulidar (ms-1) Usonic (ms-1)

Wind direction (60 minute average) 1:1 line 1:1 line Lidar (degrees from north) No. of data points -~20 degree offset between lidar & sonic -~20 degree offset between lidar & WXT in other direction Which is right? (inclined to trust the lidar) Highest density shows SW is most common direction (right for UK) Mention flow distortion around WXT ** mark in that these ARE 1:1 lines Ok, this needs a bit of clarification: which instrument were you using for flow distortion plot earlier? Can you show both sonic AND WXT? The 20deg offset in the sonic is a bit embarassing – we should really know better… Gill R3-50 (degrees from north) Vaisala WXT 520 (degrees from north) ~20° offset between lidar and sonic. Highest density data around SSW-SW – typical for the UK. ~20° offset between lidar and WXT in other direction – Alignment issue at BT? Flow distortion more noticeable.

Application: NWP evaluation 1230 UTC 2330 UTC Height (m) Differences too small to be significant Definitely different 60 min. mean wind speed (ms-1) 60 min. mean wind speed (ms-1)

Conclusions 60 minute mean wind speeds measured using the three-beam DBS method of wind-profiling are known to within 1.4 ms-1. A systematic difference between the lidar and the sonic anemometer is seen at Usonic<10 ms-1 The three-beam DBS method tested here is suitable for wind profiling-in urban areas, and for NWP evaluation. Put your email on the bottom of slide, maybe make room for logos as well, just as a reminder sian.lane@metoffice.gov.uk www.actual.ac.uk *

References Barlow, J. F., A. Dobre, R. J. Smalley, S. J. Arnold, A. S. Tomlin, and S. E. Belcher, 2009: Referencing of street-level ows measured during the DAPPLE 2004 campaign. Atmospheric Environment, 43 (34), 5536-5544. Barlow, J. F., J. Harrison, A. G. Robins, and C. R. Wood, 2011: A wind-tunnel study of flow distortion at a meteorological sensor on top of the BT Tower , London , UK. Journal of Wind Engineering and Industrial Aerodynamics, 99 (9), 899-907. Pearson, G., F. Davies, and C. Collier, 2009: An Analysis of the Performance of the UFAM Pulsed Doppler Lidar for Observing the Boundary Layer. Journal of Atmospheric and Oceanic Technology, 26 (2), 240-250. Shepherd, J. M., 2005: A Review of Current Investigations of Urban-Induced Rainfall and Recommendations for the Future. Earth Interactions, 9 (12), 1-27. Wood, C. R., A. Lacser, J. F. Barlow, A. Padhra, S. E. Belcher, E. Nemitz, C. Helfter, D. Famulari, and C.S.B. Grimmond, 2010: Turbulent Flow at 190 m Height Above London During 2006-2008: A Climatology and the Applicability of Similarity Theory. Boundary-Layer Meteorology, 137 (1), 77-96.