1. 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 *

2. 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)

3. 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)
.ac.uk *

4. Instrument sites
London

5. 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 and Wood et al for previous work at these sites

6. 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 = ms-1
Measures velocity along the beam

7. 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

8. 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… *

9. 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… *

10. 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 (~ 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
?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.

11. 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

12. 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 *

13. 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)

14. 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)

15. 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)

16. 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.

17. 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)

18. 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 on the bottom of slide, maybe make room for logos as well, just as a reminder *

19. 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 campaign. Atmospheric Environment, 43 (34),
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),
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),
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 : A Climatology and the Applicability of Similarity Theory. Boundary-Layer Meteorology, 137 (1),