Lecture 24. Wind Lidar (6) Direct Motion Detection Lidar

Transcription

1 Lectue 24. Wind Lida (6) Diect Motion Detection Lida Diect Motion Detection Wind Lida Lida tacking of aeosol motions Lase time-of-flight velocimety Lase Dopple velocimety Compaison of wind lida techniques Summay

2 Diect Motion Detection fo Wind Use the definition of velocity, i.e., velocity is the deivative of displacement vecto Wind taces ae needed to tack the motion, i.e., the position changes with time Aeosols, clouds, o smokestack plumes, i.e., any inhomogeneities in the atmosphee povide excellent taces. Common appoaches fo detecting motion emotely Cosswind detemination by patten coelation (1) Tacking aeosols, clouds, plumes, tails by images (2) Tacking Aeosol/cloud motion by lidas Lase Time-of-Flight Velocimety (LTV) Lase Dopple Velocimety (LDV) v = d dt

3 Coss-Coelation of Cloud Patten The inhomogeneities, such as aeosol paticles, cloud doplets, smokestack plumes, show pattens easily ecognized with naked eyes. If the positions of these pattens ae tacked at consecutive time, then the wind that causes the pattens to shift can be deived. One way of doing so is to take images of such patten at two points in time, t 1 and t 2. And if the geometic paametes such as distance, angle of obsevation, and imaging scale ae known, then the two-dimensional patten H(x, y) of the object can be detemined fom the images. Then it is sufficient to find those two values ( x, y) by which the second image must be shifted to give maximum similaity with the fist one. This is to maximize the coss-coelation coefficient between the two images: Q( x, y) = H(x,y,t 1 )H(x x,y y,t 2 )dxdy = maximum The two-component velocity vecto in the plane pependicula to the line of sight is then given by the simple elation: u ho = 1 t 2 t 1 ( x, y)

4 Coss-Coelation of Cloud Patten Some exciting applications of this appoach include (1) tacking a plume sent by a ocket (chemical elease) to deive wind vecto and how it vaies with time; (2) tacking long lifetime meteo tails to deive wind vecto Leonid Meteo Showe at SOR -- [Dummond et al., JGR, 106 (A10), , 2001]

5 Lida Tacking of Aeosol Motions Using lida to tack aeosol/cloud pattens is a much efficient way and can measue wind duing both day and night. Lida signals backscatteed fom the planetay bounday laye ae dominated by scatteing fom aeosol paticles. The fluctuations in aeosol content ae easily detected with lida. By obseving the dift of these spatial inhomogeneities, lida can be used to detemine wind velocities emotely. Tempoal and/o spatial coelation techniques using lida pofiles of aeosol backscatte intensity wee developed by Eloanta et al. in 1970s at the Univesity of Wisconsin-Madison. In the example on the next page, the lida is elevated by a small angle and is apidly scanned between thee closely spaced azimuth angles. The hoizontal wind component pependicula to the lida beam is obtained by measuing the time inteval needed fo aeosol inhomogeneities to dift fom one azimuth angle to the next. The longitudinal component of the wind is detemined fom the adial displacement that occus duing this coss-path dift time. Today scanning HSRL has made the wind measuement via tacking aeosol/cloud to a high degee of sophistication (Eloanta goup).

6 Lida Tacking of Aeosol Motions [Soga et al., JAM, 1980]

7 Lase Time-of-Flight Velocimety (LTV) This dual-beam technique measues the speed of a coss wind by detemining a paticle s time of flight acoss two appoximately paallel beams with a small spatial sepaation, as illustated in the plot. The output of a cw lase is focused into two paallel beams of equal intensity, with a beam-to-beam sepaation D. A single aeosol paticle taveling acoss both focused spots scattes two light pulses (flashes) by the time of flight T, which depends on its speed and the pedetemined sepaation distance D. The pependicula component of wind speed is then given by V = D /T Field demonstate went up to 100 m ange unde natual aeosol conditions. CSU [Batlett and She, Opt. Lett., 1, 175, 1977]

8 Lase Dopple Velocimety (LDV) Two lase beams split fom the same lase beam coss with each othe and fom intefeence patten, acting as a peiodic field of egions with high and low intensity. Paticles tansvesely coss the field and scatte light (stong and weak) peiodically with a fequency that is popotional to thei speed. u = f 2sin( /2) u is the speed pependicula to intefeence patten, is the lase wavelength, f is the fequency of paticle scatteing light, is the angle between two lase beams.

9 One Way to Undestand LDV The intefeence between two lase beams foms a lattice with the inteval given by d = 2sin( /2) Paticles pass though the lattice with a speed of u, so the fequency of paticles scatteing stong light is given by f = 1 T = 1 d / u = u d f u = df = 2sin( /2)

10 Anothe Way to Undestand LDV When a paticle scattes light in the intesection, both lase beams ae scatteed, suffeing Dopple shift due to the motion of the paticle. Due to the slightly diffeent angle of the beams, the Dopple shifted lase fequencies ae slightly diffeent, given by 1 v f 1 = f p e 1 /c 1 v L f 2 = f p e 2 /c L 1 v p e p /c 1 v p e p /c The light eceived at the photo-detecto is a supeposition of the two scatteed light beams - a supeposition of the amplitudes, not intensities. E 1 = A 1 sin(2 f 1 t) E 2 = A 2 sin(2 f 2 t) e 1, e 2, e x, e y, e p ae unit vectos

11 Continued fo LDV The supeposed amplitude is given by So the intensity at the photo-detecto is DC components E = E 1 + E 2 Filteed out by bandwidth I E E = A 2 1 sin 2 (2 f 1 t) + A 2 2 sin 2 (2 f 2 t) + A 1 A 2 cos[2 ( f 1 + f 2 )t] + A 1 A 2 cos[2 ( f 1 f 2 )t] The beat fequency shown at the photo-electic signal is detemined by v f D = f 1 f 2 = f p ( e 2 e 1 ) 2sin( /2) L = f L v p 2sin( /2) e x = v x c c So the tansvese velocity is v x = f D 2sin( /2) e 2 e 1 = e x 2sin( /2) Note: the measued velocity component is the tansvese component, not the adial component. This is diffeent fom the moden Dopple wind lida.

12 Actual Measuement Result of LDV

13 Compaison of Wind Techniques Use wind-dependent effects o use definition of wind Diect Motion Detection Technique: (using the definition of velocity ) (1) Tacking aeosol/cloud motions (2) Lase Time-of-Flight Velocimety (3) Lase Dopple Velocimety Dopple (Shift) Wind Technique: = k v Geostophic wind detection: v = d (t) dt (1) Coheent (Heteodyne) Detection Dopple Wind Lida (2) Diect Detection Dopple Wind Lida = 2k v Tempeatue Pessue Gadients Geostophic Wind o

14 Moe on Diect Detection Dopple Lida Diect Detection Dopple Wind Lida - cuently we can think of - includes 1. Finge Imaging with Faby-Peot Etalon 2. Scanning Faby-Peot Intefeomete 3. Edge filte based on etalons 4. Edge filte based on iodine absoption lines 5. Edge filte based on atomic absoption lines (Ba, Na, K, )

15 Altitude (km) Wind Techniques vs Altitude PMC Aeosols Clouds MSIS90 Tempeatue Statopause Mesopause Topopause Tempeatue (K) Themosphee Aiglow & Meteoic Layes OH, O, Na, Fe, K, Ca Ozone PSC Mesosphee Statosphee Toposphee km: esonance fluoescence (Na, K, Fe) Dopple technique (DDL) FPI: Faby-Peot Intefeomete Diect detection Dopple lida (DDL) techniques using molecula scatteing and/o aeosol scatteing In toposphee: Coheent Detection Dopple tech, Diect Detection Dopple tech, Diect motion Detection tech (tacking aeosols, LDV, LTV)

16 Compaison of Wind Techniques Technique Dopple Wind Technique (Diect Detection o Coheent Detection): wind dependence of Dopple fequency shift (1 time Dopple shift fo single absoption o emission pocess) (2 times Dopple shift fo Mie and Rayleigh scatteing) Diect Motion Detection Technique: deivative of displacement (the definition of velocity) (diect application of velocity definition o cosscoelation coefficient) Lidas Resonance Fluoescence Dopple Lida: Dopple fequency shift and boadening of esonance fluoescence absoption cosssection (scan and atio techniques) Rayleigh/Mie Diect Detection Dopple Lida : Dopple fequency shift of molecula and/o aeosol scatteing using edge filtes (absoption lines o etalons) o finge imaging o scanning FPI Coheent Detection Dopple Lida: Dopple fequency shift of aeosol scatteing using heteodyne detection tech High-Spectal-Resolution Lida: tacking aeosol / cloud motion though time (Scanning) Aeosol Lida: tacking aeosol motion though time Lase Time-of-Flight Velocimete: measuing time-of-flight of aeosol acoss two focused and paallel lase beams Lase Dopple Velocimete: measuing the fequency of aeosol scatteing acoss the intefeence finges of two cossed lase beams Applications Mesosphee and Lowe Themosphee tempeatue and wind ( km) Lowe mesosphee, statosphee and toposphee wind (up to km) Toposphee wind, especially in bounday layes (up to 15 km), whee aeosols ae abundant Toposphee wind, whee aeosols and clouds ae abundant Toposphee wind, whee aeosols and clouds ae abundant Within the fist km ange, laboatoy, machine shop, etc. Within the bounday layes, wind tunnel, poduction facility, machine shop, laboatoy, etc

17 Summay Mainly two methods to measue tue wind velocity: use the definition of velocity (diect motion detection) o use the Dopple effect (Dopple wind techniques). Using the definition of velocity (deivative of displacement), the diect motion detection of aeosols, clouds, o smoke plumes, by images and lidas can obtain wind with high esolution mostly in lowe atmosphee o in industial shop, lab o wind tunnel. Using the Dopple effect, the Dopple wind lida can extend the wind measuements up to the lowe themosphee, using the esonance fluoescence, molecula, and aeosol scatteing. Two main types of Dopple wind lidas ae the coheent detection Dopple lidas (CDL) and diect detection Dopple lidas (DDL).

18 Summay Coheent detection Dopple lida utilizes the aeosol scatteing in the lowe atmosphee to mixtue the etun signal with local oscillato. By heteodyne detection, CDL can achieve vey high accuacy (0 bias) and high pecision (< 10 cm/s), although it only woks in the lowe atmosphee with abundant aeosols. This is vey impotant fo weathe foecast, pollution study and defense applications. Diect detection Dopple lida uses atomic absoption lines, the edge filtes, o finge-imaging techniques to disciminate o analyze the fequency o spectum of the etun lida signals (Dopple shifted and/o boadened). Potentially, DDL can measue both wind and tempeatue if sufficient spectal infomation is povided o inquied. Fo atmospheic science study, especially fo waves coupling fom lowe to uppe atmosphee, I think that DDLs have vey high potentials fo the futue, especially the combination of esonance DDL in MLT egion with non-esonance DDL in the toposphee, statosphee and lowe mesosphee, we may be able to pofile the wind and tempeatue fom gound all the way up to 120 km. This will be a beakthough fo atmospheic science community.