LIVAS“LIdar climatology of Vertical Aerosol Structure for space-based lidar simulation studies” is an ESA project aiming to provide a global and extensive aerosol and cloud optical database, to be used for current and future space-borne lidar end-to-end simulations of realistic atmospheric scenarios, as well as retrieval algorithm testing activities. LIVAS provides a global 3-dimensional aerosol and cloud optical climatology and a collection of case studies focused on atmospheric episodes related to specific aerosol/cloud types (extended atmospheric scenes, i.e. Saharan dust events, smoke and volcanic eruption events, polar-stratospheric clouds etc). In order to cover the different spectral domains for HSRL and IPDA lidars, the compiled database addresses the three harmonic operating wavelengths of Nd-YAG lasers (355 nm, 532 nm and 1064 nm) as well as typical wavelengths of IPDA lidars in the SWIR spectral domain (1570 nm and 2050 nm).
LIVAS products are based on CALIPSO observations at 532 and 1064 nm and aerosol-type dependent spectral conversion factors derived from ground-based measurements or suitable optical models, in order to convert CALIPSO products to 355, 1570 and 2050 nm. Specifically, for the conversion of 532 nm CALIPSO backscatter and extinction products to 355 nm, the aerosol-type-dependent extinction- and backscatter-related conversion factors are extracted from multi-year, ground-based aerosol measurements of the EARLINET aerosol network, as well as models provided in the literature. The aerosol classification methodology followed within the ESA-CALIPSO study is adopted and applied on EARLINET data for an extended time range in order to calculate the required conversion factors for each CALIPSO aerosol type. Since EARLINET ground-based lidars are spectrally limited between 355 and 1064 nm, for the conversion of 532 nm CALIPSO backscatter and extinction products to 1570 and 2050 nm, the aerosol-type-dependent extinction- and backscatter-related conversion factors are calculated using appropriate scattering codes. Typical size distributions and refractive indexes are initialized for scattering calculations based on selected datasets (AERONET), as well as existing aerosol models (OPAC) and related literature. The scattering calculations are expanded in the UV for the evaluation of the conversion factors against EARLINET retrievals, in order to ensure the consistency of the conversions from the UV to IR. The obtained aerosol-type-dependent conversion factors for UV and IR are finally applied to the CALIPSO Level 2, Version 3.02 products for each aerosol type following the CALIPSO aerosol classification scheme, and the final aerosol climatology at 355, 532, 1064, 1570 and 2050 nm is derived. Depolarization spectral conversions have not been applied in LIVAS since multi-wavelength polarization measurements were not available during the project lifetime. This is a topic requiring further research which would benefit from the quality-assurance procedures for polarization measurements that have been recently implemented in EARLINET within the framework of the ACTRIS project.
Α global cloud climatology is also provided based on CALIPSO observations at 532 nm. With respect to clouds, the wavelength conversion is of minor importance due to, more or less, neutral scattering behavior along the range of LIVAS wavelengths. In addition, a climatology for the stratospheric features detected by CALIPSO is provided as well.
Finally, a set of selected scenes of extreme atmospheric phenomena (e.g. dust outbreaks, volcanic eruptions, wild fires, polar stratospheric clouds) is provided. The conversion factors for the selected scenes are delivered after thorough investigation of each case study, based on collocated ground-based data and the literature. Whenever this is not possible (as for the IR conversion) the climatological LIVAS conversion factors are used.
LIVAS climatology The final LIVAS climatology includes 4-year (01/01/2008 – 31/12/2011) time-averaged global gridded data for the following parameters: • Averaged aerosol extinction and backscatter at 355, 532, 1064, 1570 and 2050 nm • Averaged particle depolarization at 532 nm. • Averaged cloud extinction and backscatter at 532 nm. • Averaged cloud depolarization at 532 nm. • Averaged stratospheric extinction and backscatter at 355, 532, 1064, 1570 and 2050 nm • Seasonal variability of aerosol and clouds at 532 nm • Averaged extinction and backscatter at 532 nm per aerosol/cloud type • Statistics on aerosol and cloud types observed The horizontal spatial resolution of LIVAS climatology is 1X1 degree, covering all longitudes, and latitudes between 82°N and 82°S. The vertical resolution of CALIPSO Level 2 product is adopted for the final LIVAS products, namely 60 m in the tropospheric region between the surface and 20 km and 180 m in the stratospheric region between 20 and 30 km. The high vertical resolution of LIVAS product ensures the realistic simulations of the atmosphere variability in lidar end-to-end simulators.
LIVAS selected scenes In the frame of LIVAS, vertical distributions of extensive and intensive optical properties are delivered for specific atmospheric scenes. These scenes represent extreme phenomena for a number of aerosol and cloud types. LIVAS products for the selected scenes contain curtains for each aerosol and cloud type, for the following parameters: • Aerosol extinction and backscatter at 355, 532, 1064, 1570 and 2050 nm • Particle depolarization ratio at 532 nm • Cloud extinction and backscatter at 532 nm • Cloud depolarization ratio at 532 nm The original CALIPSO horizontal resolution of 5 km is maintained for the selected scenes. The vertical resolution follows CALIPSO’s Level 2 product, with 60 m resolution from -0.4 to 20 km and 120 m resolution from 20 to 30 km.