LIDAR Ben Kravitz November 5 2009 What is LIDAR Stands for LIght Detection And Ranging Micropulse LASERs Measurements of usually backscatter from clouds and aerosols in the transmission path Uses frequencies in the near UV visible and near IR One of the most common wavelengths is 532 nm LIDAR systems ground based this lecture MPLNET colocated with AERONET sites other unaffiliated LIDARs space based CALIPSO next half of the lecture Kinds of LIDAR backscatter LIDAR differential absorption LIDAR DIAL Doppler LIDAR fluorescence LIDAR Raman LIDAR Backscatter LIDAR By far the most common functions almost exactly like RADAR but at a different wavelength Differential 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 measure Doppler LIDAR Works on calculation of phase shift Very similar to wind profilers Fluorescence LIDAR Try to induce fluorescence in the species you re measuring Raman LIDAR Works on the principle of Raman scatter LIDAR Equation z CE0 Er t 2 r 180 z a 180 z exp 2 cr z ca z dz z 0 Er received power what the LIDAR actually measures E0 transmitted power what the LIDAR transmits C LIDAR constant unique to each LIDAR r 180 z Rayleigh molecular backscatter a 180 z aerosol backscatter cr Rayleigh molecular extinction z tc 2 ca aerosol extinction what we want LIDAR Equation If there is a layer of clear air above the aerosol layer Rayleigh scattering dominates and we can ignore aerosol backscatter z CE0 Er t 2 r 180 z exp 2 AOD 2 cr z dz z 0 Then we can solve for Aerosol Optical Depth AOD if we assume power decays at a certain rate with height according to Rayleigh scatter theory Determining C z CE0 Er t 2 r 180 z exp 2 AOD 2 cr z dz z 0 Perform the same aerosol optical depth measurement using a sun photometer next lecture and backsolve 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 Ratio z CE0 Er t 2 r 180 z a 180 z exp 2 cr z ca z dz z 0 Assume a relationship ratio between km 1 sr 1 and c km 1 c 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 dependence Extinction to Backscatter Ratio Example values 70 sr for tropospheric aerosols pollution 18 sr for water clouds near but not equal to 0 for clear air 40 sr for stratospheric aerosols Aerosol values can range between 15 and over 120 sr Deliquescence Certain aerosols are very good at picking up water molecules This changes their optical characteristics Wavelength Dependence Aerosol backscatter can be highly dependent upon the wavelength used This needs to be taken into account Typical retrieval Ground based LIDAR network MPLNET http mplnet gsfc nasa gov
View Full Document
Unlocking...