Ben KravitzNovember 5, 2009LIDARWhat is LIDAR?Stands for LIght Detection And RangingMicropulse LASERsMeasurements of (usually) backscatter from clouds and aerosols in the transmission pathUses frequencies in the near UV, visible, and near IROne of the most common wavelengths is 532 nm•ground-based (this lecture): MPLNET (colocated with AERONET sites), other unaffiliated LIDARs•space-based: CALIPSO (next half of the lecture)LIDAR systemsKinds of LIDAR•backscatter LIDAR•differential absorption LIDAR (DIAL)•Doppler LIDAR•fluorescence LIDAR•Raman LIDARBackscatter LIDAR•By far the most common•functions almost exactly like RADAR but at a different wavelengthDifferential Absorption LIDAR (DIAL)•Calculation of molecular species in the atmosphere•Transmit pulses at two different frequencies determined by the absorption line of the species you want to measureDoppler LIDAR•Works on calculation of phase shift•Very similar to wind profilersFluorescence LIDAR•Try to induce fluorescence in the species you’re measuringRaman LIDAR•Works on the principle of Raman scatterLIDAR EquationEr(t)=CE0z2�βr(180,z)+βa(180,z)�exp�−2�z0�cr(z�)+ca(z�)�dz��Er = received power (what the LIDAR actually measures)E0 = transmitted power (what the LIDAR transmits)C = LIDAR constant (unique to each LIDAR)βr(180,z) = Rayleigh (molecular) backscatterβa(180,z) = aerosol backscattercr = Rayleigh (molecular) extinctionca = aerosol extinction (what we want)z = tc/2LIDAR EquationIf there is a layer of clear air above the aerosol layer, Rayleigh scattering dominates, and we can ignore aerosol backscatter:Er(t)=CE0z2βr(180,z)exp�−2 · AOD − 2�z0cr(z�) dz��Then we can solve for Aerosol Optical Depth (AOD) if we assume power decays at a certain rate with height according to Rayleigh scatter theoryDetermining CPerform the same aerosol optical depth measurement using a sun photometer (next lecture) and backsolveEr(t)=CE0z2βr(180,z)exp�−2 · AOD − 2�z0cr(z�) dz��Sometimes finding a layer of clear air above the aerosol layer is tricky or impossible. We need a method that will work in all circumstances.Extinction-to-Backscatter RatioEr(t)=CE0z2�βr(180,z)+βa(180,z)�exp�−2�z0�cr(z�)+ca(z�)�dz��Assume a relationship (ratio) betweenβ (km-1 sr-1) and c (km-1)c/β (sr)Sources of Error•Overlap function - loss in signal strength at close range (less than 4 km) due to poor focusing by the detector (optics)•Afterpulse function - cross-talk between the laser pulse and the detector, includes internal noise•Multiple scattering•Incorrect estimate of extinction-to-backscatter ratio•Deliquescence•Wavelength dependenceExtinction-to-Backscatter RatioExample values~70 sr for tropospheric aerosols/pollution18 sr for water cloudsnear (but not equal to) 0 for clear air~40 sr (?) for stratospheric aerosolsAerosol values can range between 15 and over 120 srDeliquescenceCertain aerosols are very good at picking up water molecules. This changes their optical characteristics.Wavelength DependenceAerosol backscatter can be highly dependent upon the wavelength used. This needs to be taken into account.“Typical”retrievalGround-based LIDAR networkMPLNET
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