Estimating extinction coefficient and aerosol concentration profiles in the atmospheric surface boundary layer with commercial lidar ceilometers

Transcription

1 Estimating extinction coefficient and aerosol concentration profiles in the atmospheric surface boundary layer with commercial lidar ceilometers Christoph Münkel Senior Scientist Vaisala GmbH, Hamburg, Germany

2 Contents Ceilometer optical design types Single lens ceilometers Vaisala CL31 and CL51 Capturing boundary layer dynamics Mixing layer height assessment Extinction profiles Comparing ceilometer attenuated backscatter with particle concentration Data example from a clear summer day Discussion of possible use of ceilometers for the determination of beam attenuation in tower plants Page 2 / / Presentation at DNICast workshop / Vaisala

3 Biaxial optics design Vaisala LD-40, Lufft CHM15k NIMBUS LD-40 Overlap < 50 % below 200 m Overlap 0 % below 60 m evaluation depends on double and stray single scattering Page 3 / / Presentation at DNICast workshop / Vaisala

4 Split lens optics design Campbell Scientific CS135 Overlap < 5 % below 50 m Overlap < 20 % below 100 m Overlap < 60 % below 200 m Page 4 / / Presentation at DNICast workshop / Vaisala

5 Single lens ceilometer Vaisala CL31 Overlap < 20 % below 20 m Overlap < 60 % below 40 m Overlap 100 % above 80 m Simple and reliable instrument design. Sufficient overlap already 20 m above the system. More than 5000 units in operation. Page 5 / / Presentation at DNICast workshop / Vaisala

6 Single lens ceilometer Vaisala CL51 Lev-Yatir, Israel Wien Hohe Warte, Austria Station Nord, Greenland operated by KIT/IMK-IFU operated by ZAMG operated by DTU Wind Energy, Roskilde Unchanged optical setup compared to CL31. Larger lens and modified electronics increase SNR significantly. Qualified instrument for boundary layer investigation. Designed for harsh environments. Page 6 / / Presentation at DNICast workshop / Vaisala

7 A typical CL51 backscatter profile density plot Cloud Melting layer Residual layer Nocturnal layer Precipitation Virga Page 7 / / Presentation at DNICast workshop / Vaisala

8 Boundary layer dynamics Graswang 870 m Page 8 / / Presentation at DNICast workshop / Vaisala

9 Boundary layer dynamics Rottenbuch 765 m Page 9 / / Presentation at DNICast workshop / Vaisala

10 Boundary layer dynamics Fendt 610 m Page 10 / / Presentation at DNICast workshop / Vaisala

11 Boundary layer dynamics North Sea CL51 and Vaisala PWD52 on FINO3 platform Page 11 / / Presentation at DNICast workshop / Vaisala

12 Simplified lidar equation c 2 P( x, ) P0 A O( x) t ( x, ) ( x, ) 2 2x instrument specific attenuated backscatter Measured power from distance x Only the attenuated backscatter from distance x can be derived from the measurement, not the backscatter coefficient. Relationship between transmittance and extinction coefficient. Page 12 / / Presentation at DNICast workshop / Vaisala

13 Mixing layer height - summer Gradient method: Attenuated backscatter gradient minimum hints on aerosol layer top. Page 13 / / Presentation at DNICast workshop / Vaisala

14 Mixing layer height - winter Page 14 / / Presentation at DNICast workshop / Vaisala

15 Extinction profiles c 2 P( x, ) P0 A O( x) t ( x, ) ( x, ) 2 2x instrument specific attenuated backscatter Ceilometers report the attenuated backscatter part of the simplified lidar equation. According to ISO (ground-based remote sensing of visual range by lidar), profiles of the extinction coefficient α can be derived from this if α m -1 (corresponding to MOR 2000 m), a linear and range-independent relation of α and β is assumed. Vaisala ceilometers CL31 and CL51 report extinction profiles on request. Page 15 / / Presentation at DNICast workshop / Vaisala

16 A haze event in Beijing attenuated backscatter Cleaning the window increases the reported attenuated backscatter value Page 16 / / Presentation at DNICast workshop / Vaisala

17 A haze event in Beijing extinction profiles increase information content Attenuated backscatter too low for application of Klett-Fernald algorithm Formation of a dense haze layer only visible in extinction profiles Cleaning the window does not affect extinction Page 17 / / Presentation at DNICast workshop / Vaisala

18 Extinction coefficient verification with forward scatter visibility meter PWD52 Page 18 / / Presentation at DNICast workshop / Vaisala

19 Concentration PM10, PM2.5 in g m -3 Comparing ceilometer attenuated backscatter with PM10 and PM2.5 concentration From March 1, 2002 to April 23, 2003, there had been a campaign in Hannover, Germany, comparing attenuated backscatter average between 0 m and 30 m with in-situ PM10 and PM2.5 concentration values situated 20 m above the ceilometer PM10 from in situ sensor PM10 from ceilometer PM2.5 from in situ sensor Day in April 2002 Page 19 / / Presentation at DNICast workshop / Vaisala

20 Attenuated backscatter vs. PM10 concentration, all dry weather periods Results published in Münkel et al: Retrieval of mixing height and dust concentration with lidar ceilometer (Boundary-Layer Meteorol (2007) 124: ) Page 20 / / Presentation at DNICast workshop / Vaisala

21 PM2.5 concentration vs. CL51 attenuated backscatter from 100 m Tang et al.: Impact of emission controls on air quality in Beijing during APEC 2014: lidar ceilometer observations Atmos. Chem. Phys., 15, , Page 21 / / Presentation at DNICast workshop / Vaisala

22 Attenuated backscatter on a clear summer day (20 C 26 C, MOR > m) Increased attenuated backscatter below 200 m Mixed layer up to 1300 m Page 22 / / Presentation at DNICast workshop / Vaisala

23 4 min * 40 m average Page 23 / / Presentation at DNICast workshop / Vaisala

24 Is it possible to derive extinction profiles from these summer day attenuated backscatter profiles? Technically, the Klett-Fernald algorithm has sufficient data points to run on these data, but the assumptions listed in ISO are not fulfilled evaluation of near range attenuated backscatter values requires a good knowledge of the overlap function Page 24 / / Presentation at DNICast workshop / Vaisala

25 Some suggestions how to better apply ceilometers for the determination of beam attenuation in tower plants 1. Use ground visibility determined by a forward scatter visibility meter to improve initial guess for the Klett-Fernald algorithm 2. Tilt the ceilometer an elevation angle of 30 instead of 90 doubles the data points available 3. Develop a more robust form of the Klett-Fernald algorithm tailored to this application 4. Change the firmware of the ceilometer to concentrate measurements to the near range and thus improve SNR Page 25 / / Presentation at DNICast workshop / Vaisala

26 Some suggestions how to better apply ceilometers for the determination of beam attenuation in tower plants 5. Estimate the lidar ratio L = (0, )/ (0, ) from the ground visibility and the attenuated backscatter the ceilometer measures at ground level and use the assumptions ²(x, ) is reasonably close to 1 to assume (x, ) ²(x, ) (x, ) scatterer properties remain unchanged through the sample volume and thus lidar ratio is constant through the sample volume to calculate (x, ) = L* (x, ) ²(x, ) 6. Estimate a factor linking (x, ceilometer ) to (x, tower ) because the ceilometer is using one infrared wavelength while the tower plant uses a wide spectrum range Page 26 / / Presentation at DNICast workshop / Vaisala

27 Thank you very much for your attention Page 27 / / Presentation at DNICast workshop / Vaisala