Assessing the quality of Synthetic Aperture Radar …...Assessing the quality of Synthetic Aperture Radar (SAR) wind retrieval in coastal zones using multiple Lidars Tobias Ahsbahs

Assessing the quality of Synthetic Aperture Radar (SAR) wind retrieval in coastal zones using multiple Lidars

Tobias Ahsbahs

Merete Badger, Ioanna Karagali, Xiaoli Larsen

DTU Wind Energy, Technical University of Denmark

Add Presentation Title in Footer via ”Insert”; ”Header & Footer”

What is the coastal zone?

• Coastal zone here defined as the region over water where the boundary layer is not in equilibrium.

• “Results suggest that the distance from the coastline over which wind speed vertical profiles are not at equilibrium with the sea surface (which defines the coastal zone) extends to 20 km and possibly 70 km from the coast”. 1

• Typically, up to 50km distance to shore is define as the coastal zone for wind energy applications.

2

1 Barthelmie et.al., Offshore Coastal Wind Speed Gradients: issues for the design and development of large offshore windfarms, Wind Engineering, 2009



SAR wind retrieval

3

Radar backscatter High resolution o cean wind

speed at10 m



From radar backscatter to wind speed

• Radar backscatter is related to the

”roughness” of the ocean surface

• Geophysical Model Function (GMF)

• CMOD5.n for this study

Input:

– Angle between wind

direction and radar beam

– Radar backscatter

signal

Output:

– Wind speed at 10m

– RMSE typically 1.3m/s

4

Hasager, C.B. et al. (2015): Offshore wind climatology based on synergetic use of Envisat ASAR, ASCAT and QuikSCAT. Remote Sensing of Environment, 156, 247–263. 10.1016/j.rse.2014.09.030.



Motivation for coastal winds from SAR

Why do we want SAR wind in the coastal zone?

• High resolution can resolve horizontal wind speed gradients

• Large data archive available

What is the problem?

• Coastal effects like currents, breaking waves, limited fetch etc.

• Performance of the GMFs in the coastal zone unknown

Approach:

• Use ground based remote sensing techniques to measure the wind in the coastal zone.

• Compare to available Sentinel-1 data

5



The RUNE project

Reducing Uncertainty of Near-shore wind resource Estimates using onshore lidars

6

• Data from December

2015 to March 2016

• West Coast of Denmark

• Remote sensing instruments for wind speed measurements



Radial velocity sampling • Wind vector consists of 3

components (u, v, w)

• Radial velocity is a projection of the true wind speed along the laser’s line of sight

• One LiDAR can only measure a portion of the wind vector!

Thanks to Elliot Simon for this slide

7



The RUNE project

For more information: Peña et.al., Report on the coastal experiment and first inter-comparisons between measurement systems, 2016

A near shore wind measurement campaign

8

Multiple Lidars for:

• Wind profiles

• Sector scans (SC)

• Dual Doppler (DD)

Additional data

• WRF model runs

• SAR images

• Høvsøre tall met mast



The RUNE project

For more information: Peña et.al., Report on the coastal experiment and first inter-comparisons between measurement systems, 2016

A near shore wind measurement campaign

Multiple Lidars for:

• Wind profiles

• Sector scans (SC)

• Dual Doppler (DD)

Additional data

• WRF model runs

• SAR images

• Høvsøre tall met mast

9



SAR and lidar wind measurements from the RUNE project

10

SAR image of Danish West Coast

Zoom into the RUNE area

Scanning lidar 10min mean speed

Image: Sentinel-1, Processing with SAROPS from APL/NOAA



Comparing Lidar and SAR. HOW?

11

• Colocate in time and space

• Logarithmic wind profile for vertical displacement

• Assuming homogenuity along the coast in the SAR wind



Comparisons of 10m winds

12

Assuming homogenuity along the coast

Scanning lidar 10min mean speed

SAR and scanning lidar comaprison at 10m



Scanning lidars and SAR wind – over the transect

13

Left: Differences U(10m) DD-SAR wind. Right: Differences U(10m) SC-SAR wind

• 10m wind from the lowest lidar level

• Cases: 10 for DD, 11 for SC

• RMSE: 1.4 m/s DD, 1.8 m/s SC

• Bias: 0.6 m/s DD, 0.2 for SC



Average from 1500m to 5000m

14

• Box average of the SAR wind

• Mean over 10m wind from the lowest lidar

• All wind directions

• All stabilities

• RMSE comparable to literature



Average from 1500m to 5000m – easterly wind

15



• Wind from the land

• All stratifications

• Large RMSE

• Internal boundary layer unaccouted



Average from 1500m to 5000m – westerly wind

16



• Wind from the sea

• All stratifications

• RMSE comparable to literature

• Still considerable deviations



Non-logarithmic cases

17

• Use 10m wind from all lidar levels

• 10m wind inconsistent

• Non-logarithmic profile



Non-logarithmic cases

18

• Use 10m wind from all lidar levels

• 10m wind inconsistent

• Non-logarithmic profile



Average from 1500m to 5000m – westerly wind excluding 2 cases

19




• Excluding inconsistent 10m wind

• Better RMSE

• Even fewer cases



Average from 1500m to 5000m – westerly wind and neutral stratification

20




• Neutral (|L|>500m) at 40m (onshore mast)

• Good RMSE

• Very few cases



Conclusions

• SAR wind retrievals in the coastal zone can work.

• Using Lidar measurements above 10m for comparisons is challenging.

• Cases with westerly winds and neutral stratification agree very well.

Outlook:

• More data from other locations for better statistics.

• More detailed investigation of each case in a case study.

• Use SAR for investigation of coastal gradients.

Acknowledgement:

ESA for the use of Sentinel-1 data

Thanks to ForskEL for funding the RUNE experiment and DTU’s technical staff for the execution

21

Documents

Assessing the quality of Synthetic Aperture Radar …...Assessing the quality of Synthetic Aperture Radar (SAR) wind retrieval in coastal zones using multiple Lidars Tobias Ahsbahs