SHEBA Surface-Layer Scintillometer Data Compiled 9/10/1999. Provided by Ed Andreas and the SHEBA Atmospheric Surface Flux Group (Chris Fairall, Peter Guest, and Ola Persson) These data were collected with a Scintec (Tubingen, Germany) Surface-Layer Scintillometer System. The horizontal propagation path was 350 meters for the 1997 data and 300 meters for the 1998 data. The wavelength of the laser was 0.685 micrometers. The propagation paths were in the vicinity of the Atmospheric Surface Flux Group's main meteorological tower. The main variables reported are hourly averages of the refractive index structure parameter Cn2 (C, sub-n, squared) and the inner scale of turbulence el-0 (el, sub-zero). The following references describe why and how these are useful. Andreas, E.L., 1989: Two-wavelength method of measuring path-averaged turbulent surface heat fluxes. Journal of Atmospheric and Oceanic Technology, 6, 280-292. Andreas, E.L., Editor, 1990: Selected Papers on Turbulence in a Refractive Medium. SPIE Milestone Series, vol. 25, Society of Photo-Optical Instrumentation Engineers, Bellingham, WA, 693 pp. Andreas, E.L., 1990: Three-wavelength method of measuring path-averaged turbulent heat fluxes. Journal of Atmospheric and Oceanic Technology, 7, 801- 814. Andreas, E.L., 1992: Uncertainty in a path-averaged measurement of the friction velocity u*. Journal of Applied Meteorology, 31, 1312-1321. Hill, R.J., 1992: Review of optical scintillation methods of measuring the refractive-index spectrum, inner scale and surface fluxes. Waves in Random Media, 2, 179-201. Hill, R.J., 1997: Algorithms for obtaining atmospheric surface-layer fluxes from scintillation measurements. Journal of Atmospheric and Oceanic Technology, 14, 456-467. The scintillometer system makes a measurement every 6 seconds and reports minute averages of the Cn2 and el-0 values. One 6-second period during each minute is used for calibration--the laser is automatically turned off and the system measures the background. The system also does real-time quality control and therefore evaluates whether each 6-second block of data is good. If all 9 available data periods from each minute are good, the system assigns that period a quality number of 100%. If, for example, only 5 of the 9 available periods yielded reasonable Cn2 and el-0 values, it assigns a quality number of 55%. From these 1-minute quality numbers, we have assigned an overall quality number to each set of hourly values. If all 6-second samples during the hour produced Cn2 and el-0 values that satisfied the system's internal quality checks, the data have a quality number of 100%. If none of the 6-second samples were good, the data have a quality number of 0%. Cn2 and el-0 values associated with quality numbers of 25% or less are probably too unreliable to use. Several phenomena produce data that the system identifies as unreasonable and, thus, does not report. Fog, snow, and rain all produce scintillometer signals that do not have the characteristics of turbulence; the system flags these with quality numbers of 0. If the scintillometer signal changes intensity because of movements in the ice that affect its pointing, the system will also flag the data as bad. For the later 1998 data, the scintillometer was beginning to fail; these periods of low laser intensity and clouded receiver windows also produced low-quality data. The data file, SHEBASCT.PRN, is ASCII with the data in nine columns, as follows: Column 1: Year Column 2: Julian day of that year Column 3: Hour (in GMT). The data are hourly averages. This is the start of the averaging period. That is, the reported data were collected between Hour and Hour + 1. Column 4: Decimal Julian day, with 1 January 1997 as day 1. Column 5: Nominal path height of the laser. For the 1997 data, the path length was 350 meters; for the 1998 data, the path length was 300 meters. Column 6: Number of 1-minute samples collected during the hour and used to calculate the Cn2 and el-0 averages. Column 7: The quality number for that hour's data in percent. Column 8: The hourly averaged refractive index structure parameter (Cn2; C, sub-n, squared) in meters^(-2/3). Column 9: The hourly averaged inner scale of turbulence (el-0; el, sub-zero) in millimeters.