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

Nihanth W. CherukuruaRonald CalhounaManuela LehnerbSebastian HochbDavid Whitemanb

a Arizona State University, Environmental Remote sensing group, Tempe , Arizona

b University of Utah, Mountain Meteorology Group, Salt Lake City, Utah

Instrument Configuration for Dual-Doppler Lidar Coplanar scans:

METCRAX IIObjectiveOverview of the techniqueDomain dimensions and scan speedWeighting functionEquivalence of LS method with the average method (LS= Least squares)Error analysisField experiment resultsSummary and future workOutline#2Resolve 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 ?ObjectiveX () Y()VL.O.S(Elevation angle)VrA single Lidar measures the radial velocity.We can combine the radial velocities from different look directions to reconstruct the actual wind velocity. V = (u2 + v2 )Vr = u cos + w sinL.O.S = Line of sight3Why Dual Doppler ?ObjectiveVL.O.S from Lidar 1Vr1A single Lidar measures the radial velocity.We can combine the radial velocities from different look directions to reconstruct the actual wind velocity. V = (u2 + v2 )Vr = u cos + w sinL.O.S = Line of sightL.O.S from Lidar 2Vr2Resolve 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

(Newsom et al. 2008Hill et al. 2010 Stawiarski et al. 2013)

4Overview of the TechniqueLidar 1Lidar 2Set 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

5Lidar 1Lidar 2ROverview of the Technique

3. At each grid intersection point define a region.4. Assign the range gates to the corresponding grid intersection point.

6

Lidar 1Lidar 2Overview of the TechniqueRepeat for all points 7Grid spacing must be greater than range gate length i.e, R (p/2)Total scan time should be :

Domain Dimensions and scan speed

Lidar 1Lidar 2

(PRF=Pulse Repeatition Frequency)8Weighting 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)

9R'ABCDEFGHpR

R= R +(p/2)

Weighting Function - The weighting function relates to the portion of the range gate that falls within the circle of influence (R)

Equivalence of LS method with the average method1. From the LS method2. Upon expanding each term of the matrix:10

3. Replacing the elevation angles with one single value corresponding to the grid centre:Where,4. Rearranging the terms in matrix form5. Above system of equations is valid when the 2x2 matrix on LHS has an inverse11Equivalence of LS method with the average method

12Error Analysis (From Lidar Simulator)- Temporal error- Spatial error- Retrieval errorError due to the the cells with skewed LOS from Lidars. i.e., angle between the LOS close to 0 degrees or 180 degrees.timeErrors are dependent on both averaging and range gate size

(Lhermitte and Miller 1970 )

13Red : Temporal errorsBlue : Spatial errors: errors in u: errors in wTemporal errors increase with pulse averaging.Temporal errors increase with range gate lengthSpatial errors are independent of averaging usedSpatial errors increase with range gate size

(a)(b)Error Analysis (From Lidar Simulator)14Lidar-1 parametersPRF15kHzAveraging10,000Range gate24 mElevation angle at the beginning of the scan150 Elevation angle at the end of the scan10 Time taken for the one scan30 sScan rate4.5 deg/sSector140 Lidar-2 parametersAccumulation time0.75 sRange gate25 mElevation angle at the beginning of the scan-10 Elevation angle at the end of the scan20 Time taken for the one scan30sScan rate1.0 deg /sSector30 Domain specificationsNumber of cells in X direction36Number of cells in Y direction19Cell dimensions26.8 m x 26.8m

Lidars on site

* WALL-E_03.jpg. Digital image. Wikia. N.p., 27 Mar. 2008. Web. 8 Aug. 2014. *15

Field Experiment results16Summary and Future WorkCoplanar 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.

AcknowledgementsResearch work supported by National Science Foundation (NSF) grants AGS-1160730 and 1160737.- We would like to thank Dr. William O.J Brown from NCAR.Questions