we did before the HSRL. Ed Eloranta University of Wisconsin ...

LLidar Basics: why do need an HSRL… and some example of what we did before the HSRL. Ed Eloranta University of Wisconsin-Madison http://lidar.ssec.wisc.edu

Laser

Telescope

Range Finding Lidars

Uses: Lunar ranging Mapping – land use Polar ice caps Vegetation mapping Ceilometers Satellite tracking

Laser pulse length ~ nanoseconds

Requirements: Short pulse length Accurate timing Narrow beam width Sky noise suppression Accurate pointing

range = cT / 2

Range = c T /2 Where: c = speed of light T = photon time of flight

Laser

Telescope

Aerosol backscatter profiling lidar

Lidar Equation

The Geometry Correction, η(r)

Telescope objective At long ranges the telescope focal point is close to the field stop and all of the collected photons pass through.

At close range the telescope focal point Is farther behind the field stop and only those photons hitting near the center of the objective pass through the field stop.

Field stop

Geometry effect on profiles with 100 micoradian and 2 milliradian fov’s

2 mr fov

100 micro radian

Altitude = 3 km

100 m

Photon interactions with the atmosphere

Photon absorbed

No interaction—photon transmitted

Photon scattered—no wavelength change (elastic scattering)

Photon scattered at new wavelength (inelastic scattering)

Examples: Raman scattering Fluorescence Doppler

Lidar Equation

Volume absorption cross section, βa m-1

Consider absorption by gases and particles in a unit volume cube of the atmosphere

Replace the gases and particles by an absorbing patch that intercepts the same fraction of the photons as the actual atmospheric volume

Volume absorption cross section = βa = area of patch/volume of cube ; units = 1/length

Volume scattering cross section, βs m-1

Consider scattering by molecules and particles in a unit volume cube of the atmosphere

Place an reflecting patch that deflects the same fraction of the photons as are scattered in all directions by the actual atmospheric volume

Volume scattering cross section = βs = area of mirror/volume of cube ; units = 1/length

Volume cross sections

βa = absorption cross section βs = scattering cross section βe = βa + βs = extinction cross section βRayleigh = molecular scattering cross section βRaman = Raman cross section

Lidar Equation

The phase function describes the angular distribution of scattered photons

Incoming light beam Scattered photons

θ and ϕ are scattering angles in spherical coordinates F0 = Incident irradiance (W/m2) I0 v= Scattered radiant intensity (W/sr) dv = incremental scattering volume

For most lidar systems the scattering is always at 180o , but it may also vary with range. Therefore the phase function becomes

Raman Lidar

Raman water vapor lidar

Differential Absorption Lidar (DIAL)

Traditional aerosol lidar can not distinguish between changes in target reflectivity and attenuation between the lidar and the target

1975

29-April-1976 10:49 AM 1 pps ruby laser

1983

University of Wisconsin Volume Imaging Lidar(1987-1998) Transmitter: Wavelength = 1064 nm Ave power = 40 W Rep rate = 100 Hz Recevier: Telescope dia = 0.5 m Optical bandpass = 1 nm Quantum efficiency = 35 % Range resolution = 15 m Max scan rate = 20o s-1

Four-dimensional lidar Imaging of the atomsphere Ed Eloranta University of Wisconsin http://lidar.ssec.wisc.edu

Lidar (top) compared to two versions of LES (bottom two panels) Lake-ICE January 13, 1998

Single RHI plane PPI plane 2D Correlations as function of offshore distance, alt= 5m alt=5m 1.8 km 2.7 km 3.6 km 4.5 km

| | | | 0 5km 0 5km

Comparisons of structure lifetime between lidar data and LES via correlation decay

Waves downwind of Lake Michigan shore shown in a range-height display