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Instrument Configuration for Dual-Doppler Lidar Coplanar scans: METCRAX II

Nihanth W. Cherukuru a Ronald Calhoun a Manuela Lehner b Sebastian Hoch b David Whiteman b. Instrument Configuration for Dual-Doppler Lidar Coplanar scans: METCRAX II. a Arizona State University, Environmental Remote sensing group, Tempe , Arizona

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Instrument Configuration for Dual-Doppler Lidar Coplanar scans: METCRAX II

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  1. Nihanth W. Cherukurua Ronald Calhouna Manuela Lehnerb Sebastian Hochb David Whitemanb Instrument Configuration for Dual-Doppler Lidar Coplanar scans: METCRAX II a Arizona State University, Environmental Remote sensing group, Tempe , Arizona b University of Utah, Mountain Meteorology Group, Salt Lake City, Utah

  2. Objective • Overview of the technique • Domain dimensions and scan speed • Weighting function • Equivalence of LS method with the average method (LS= Least squares) • Error analysis • Field experiment results • Summary and future work

  3. Objective • Resolve the flow in a 2D cross plane and study the spatial and temporal evolution of downslope windstorm type flows. • Need to determine the best possible scanning strategy and lidar parameters to perform a coplanar dual Doppler Lidar analysis for METCRAX II Why Dual Doppler ? Y(ĵ) L.O.S • A single Lidar measures the radial velocity. • We can combine the radial velocities from different look directions to reconstruct the actual wind velocity. V Vr X (î) θ(Elevation angle) V = √(u2 + v2 ) Vr= u cosθ + w sinθ L.O.S = Line of sight

  4. Objective • Resolve the flow in a 2D cross plane and study the spatial and temporal evolution of downslope windstorm type flows. • Need to determine the best possible scanning strategy and lidar parameters to perform a coplanar dual Doppler Lidar analysis for METCRAX II L.O.S from Lidar 2 (Newsom et al. 2008 Hill et al. 2010 Stawiarski et al. 2013) Why Dual Doppler ? L.O.S from Lidar 1 • A single Lidar measures the radial velocity. • We can combine the radial velocities from different look directions to reconstruct the actual wind velocity. V Vr2 Vr1 V = √(u2 + v2 ) Vr= u cosθ + w sinθ L.O.S = Line of sight

  5. Overview of the Technique Set the lidars such that the angle between their LOS not close to 0 or 180 degrees. Overlay a Cartesian grid over which the velocity components are retrieved Lidar 2 Lidar 1

  6. Overview of the Technique 3. At each grid intersection point define a region. 4. Assign the range gates to the corresponding grid intersection point. R Lidar 2 Lidar 1

  7. Overview of the Technique Repeat for all points … Lidar 2 Lidar 1

  8. Domain Dimensions and scan speed • Grid spacing must be greater than range gate length i.e, R ≥ (Δp/√2) • Total scan time should be : (PRF=Pulse Repeatition Frequency) Lidar 2 Lidar 1

  9. Weighting Function • The radial velocity measured at a range gate can be considered to be an average over a finite distance along the LOS. (Frehlich et al. 1998)

  10. R' R H G D C B F A ∆p E Weighting Function - The weighting function relates to the portion of the range gate that falls within the circle of influence (R) R’=R +(∆p/2)

  11. Equivalence of LS method with the average method 1. From the LS method 2. Upon expanding each term of the matrix:

  12. Equivalence of LS method with the average method 3. Replacing the elevation angles with one single value corresponding to the grid centre: Where, 4. Rearranging the terms in matrix form 5. Above system of equations is valid when the 2x2 matrix on LHS has an inverse

  13. Error Analysis (From Lidar Simulator) - Temporal error time - Spatial error - Retrieval error Error due to the the cells with skewed LOS from Lidars. i.e., angle between the LOS close to 0 degrees or 180 degrees. (Lhermitteand Miller 1970 ) Errors are dependent on both averaging and range gate size

  14. Error Analysis (From Lidar Simulator) • Temporal errors increase with pulse averaging. • Temporal errors increase with range gate length • Spatial errors are independent of averaging used • Spatial errors increase with range gate size Red : Temporal errors Blue : Spatial errors : errors in u : errors in w (a) (b)

  15. Lidars on site * * WALL-E_03.jpg. Digital image. Wikia. N.p., 27 Mar. 2008. Web. 8 Aug. 2014. <http://hero.wikia.com/wiki/File:WALL-E_03.jpg>

  16. Field Experiment results

  17. Summary and Future Work • Coplanar Dual Doppler Technique can be used to study complex flows. • Care must be taken in choosing the lidar parameters to get the best temporal and spatial resolution of the phenomenon. • Analysis can be simplified with certain assumptions and for small range gate lengths, the results are very close. • Investigate the significance of weighting function for alternate approaches and larger range gate lengths. Acknowledgements • Research work supported by National Science Foundation (NSF) grants AGS-1160730 and 1160737. • - We would like to thank Dr. William O.J Brown from NCAR.

  18. Questions

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