|
|
CAMEX-4 LIDAR Atmospheric Sensing Experiment (LASE) Table of Contents
NASA's Lidar Atmospheric Sensing Experiment (LASE) system was operated during the the Fourth Convective and Moisture Experiment (CAMEX-4) to characterize water vapor fields in the vicinity of hurricanes. While remote sensing of the hurricane environment was the primary objective of CAMEX-4, there were also separate flights to study thunderstorm structure, precipitation systems, and atmospheric water vapor profiles. This portion of CAMEX-4 was known as KAMP, Keys Area Microphysics Project. The objective of the KAMP flights was to improve quantitative precipitation estimates from passive and active microwave instruments. LASE was operated during both CAMEX-4 and KAMP. LASE was operated from the NASA DC-8 aircraft in the nadir and zenith modes simultaneously. Six DC-8 flights were made in the vicinity of hurricanes and four KAMP flights were made into areas of heavy rain convection. In addition, there were two instrument check flights and one calibration flight over the Andros Island site during the CAMEX-4 period between August 18 and September 24, 2001. LASE collected profile data using a combination of three water vapor absorption cross-sections over a period of more than 70 hours. Data may be obtained from the GHRC (see contact information below), or directly from the CAMEX-4 web page. The LASE homepage is found at http://asd-www.larc.nasa.gov/lidar/cmx4/camex4.html from which additional information may be found. Principles of OperationLASE is an airborne DIAL (Differential Absorption Lidar) system used to measure water vapor, aerosols, and clouds throughout the troposphere. This system uses a double-pulsed Ti:sapphire laser, which is pumped by a frequency-doubled flashlamp-pumped Nd:YAG laser, to transmit light in the 815-nm absorption band of water vapor. The Ti:sapphire laser wavelength is controlled by injection seeding with a diode laser that is frequency locked to a water vapor line using an absorption cell. LASE operates by locking to a strong water vapor line and electronically tuning to any spectral position on the absorption line to choose the suitable absorption cross-section for optimum measurements over a range of water vapor concentrations in the atmosphere. During CAMEX-4, LASE operated from the NASA DC-8 using strong and weak water vapor lines in both the nadir and zenith modes, thereby simultaneously acquiring data below and above the aircraft. The strongly absorbing, temperature insensitive water vapor line at 817.2231 nm (12236.5603 cm-1) with a line strength of 4.060E-23 cm, linewidth of 0.0839 cm-1, and lower energy state of 224.838 cm-1 was used during CAMEX-4. Line strength accuracy is estimated to be 2% and linewidths have agreed with other measurement to within 2 0iving an overall accuracy of absorption cross-section of less than 3% (Poinsardin and Browell, 1997). Effective absorption cross-section profiles were calculated at the on-line and off-line wavelengths and the side-line positions, and corrections for Doppler broadening, pressure shift, water vapor line width, spectral purity, molecular density, and aerosol scattering ratio were used in water vapor mixing ratio retrievals (Ismail et al., 1989). Absolute water vapor distributions will be derived from the LASE measurements across the troposphere from 0-14 km over a mixing ratio range of about 20 g/kg to 0.01 g/kg. The initial archive contains only the nadir water vapor distributions. The LASE nadir water vapor profiles have a vertical resolution of 330 m. For all flights, the LASE nadir water vapor profiles have a temporal averaging period of 3 minutes, which corresponds to a horizontal distance of about 42 km. Other temporal averaging periods can be produced upon request. Previous water vapor comparisons have shown the LASE water vapor mixing ratio measurements have an accuracy of better than 60r 0.01 g/kg, whichever is larger, across the troposphere (Browell et al., 1997). In addition to measuring water vapor mixing ratio profiles, LASE simultaneously measures aerosol backscattering profiles at the off-line wavelength near 815 nm. Profiles of the aerosol scattering ratio, defined as the ratio of aerosol scattering to molecular scattering, are determined by normalizing the scattering in the region containing enhanced aerosol scattering to the expected scattering by the "clean" (molecular only) atmosphere at that altitude. For these CAMEX-4 measurements, the LASE nadir and zenith aerosol scattering ratio profiles have a vertical resolution of 60 m for all nadir measurements and for zenith measurements in flights 8, 10, 11, and 12 (zenith measurements in all other flights have a resolution of 2 km due to extensive clouds). Both nadir and zenith aerosol scattering ratio profiles have a horizontal resolution of 9 seconds (~2.1 km). Data Naming Conventions and Data FormatThe data files archived at the GHRC are tarred into daily files that have the format:
where c4dlase is the dataset identifier, 2001.246 is the year.day-of-year, 010409 is the mission number When 'untarred', the daily file will extract two data files and numerous image files. Their naming convention is as follows:
where 'bc' and 'wc' are file type identifiers used in the read software, 'cm4' identifies this as a CAMEX-4 file, yyyy is the four digit year, mm is the two digit month and dd is the two digit day. In the image file, nnnn represents the different types of image files (e.g. asr, h2o, etc). These are described in greater detail in LASE_CAMEX-4_data_documentation.pdf which is the document prepared by the data producer. Format specification for Data Exchange by Steve E. Gaines and R. Stephen Hipskind may be found either online at http://cloud1.arc.nasa.gov/solve/archiv/archive.tutorial.html which has been converted to PDF format here. LASE uses format number 2310. There is a program written to read the Gaines/Hipskind format 2310 for LASE/CAMEX-4 ascii archive files. It is echoed below, or may be found in a file named c4dlase_rd_cm4_2310.pro. Contact Information To order these data or for further information, please contact:
|