TITLE: Ice Camp Surface Mesonet NCAR PAM-III (GIF) AUTHORS: Edgar L. Andreas CRREL 72 Lyme Rd Hanover NH 03755-1290 Tel: (603) 646-4436 eandreas@nwra.com Christopher W. Fairall NOAA/ESRL DSRC, Bldg 33 325 Broadway Boulder CO 80305-3337 Tel: (303) 497-3253 chris.fairall@noaa.gov Peter S. Guest NPS Monterey, CA 93943 Tel: (831) 656-2451 pguest@nps.navy.mil www.weather.nps.navy.mil/~psguest/sheba/ P. Ola G. Persson NOAA/ESRL DSRC, Bldg 33 325 Broadway Boulder CO 80305-3337 Tel: (303) 497-5078 ola.persson@noaa.gov 1.0 DATA SET OVERVIEW SHEBA (Surface Heat Budget of the Arctic) is an interdisciplinary program to study the interactions of clouds, atmospheric radiation, and the surface energy balance over the Arctic Ocean. The field phase involved the deployment of numerous measurement systems on board and in the vicinity of the Canadian ice breaker N.G.C.C. Des Groseilliers, which was frozen into the Arctic ice pack north of Alaska from October 1997 to October 1998. During this period, Ice Station SHEBA drifted from approximately 75 N, 144 W to 80 N, 166 W. The research teams at the ice camp included the SHEBA Atmospheric Surface Flux Group (ASFG), which was responsible for direct measurement of the fluxes contributing to the surface energy balance. The principal investigators in this group are Ed Andreas (U.S. Army Cold Regions Research and Engineering Laboratory), Chris Fairall (NOAA Environmental Technology Laboratory), Peter Guest (Naval Postgraduate School), and Ola Persson (NOAA Environmental Technology Laboratory and the Cooperative Institute for Research in Environmental Sciences). The Atmospheric Surface Flux Group instrumented a 20 m micrometeorological tower located about 1 km from the Des Groseilliers and also requested the use of four NCAR Flux-PAM stations to measure fluxes over a variety of different surface types. 2.0 INSTRUMENT DESCRIPTION Flux-PAM remote meteorological stations are one of the principal components of the NCAR Integrated Surface Flux Facility (ISFF). These stations measure the standard meteorological variables of wind, temperature, humidity, pressure and precipitation plus net radiation, soil heat flux and the eddy fluxes of momentum, water vapor and sensible heat. To provide maximum siting flexibility, Flux-PAM stations are powered by solar-charged batteries and the sensor are mounted on an internally-guyed mast with a tripod base. The Flux-PAM data-processing computer, known as EVE, ingests data from the sensors at rates commensurate with their individual response characteristics and calculates 5-min means, variances and covariances. These data statistics are stored locally on EVE and also transmitted to a base computer system for archival, analysis and display. Data transmission is accomplished in real time either through the GOES satellite and/or, for networks of limited spatial extent, by means of line-of-sight RF modems. For SHEBA, RF modems were used to transmit the data from each station to a base computer located on the Des Groseilliers. In order to meet the research requirements of the SHEBA Atmospheric Surface Flux Group, the Flux-PAM stations required several modifications. Propane-fueled thermoelectric generators were used to power the stations, supplemented during the spring and summer with solar panels and (at two stations) wind generators. For operation in the extreme cold, the thermoelectric generator and propane bottles were packaged with the 12V batteries and data computer in an insulated container. For station portability, each of these containers was mounted on a freighter sled that could be towed by snowmobile. In order to accurately determine true wind direction on the drifting ice pack, an electronic compass was mounted on the meteorological mast to measure station orientation. A strobe beacon and GPS receiver were added to help service personnel locate the sites, to archive station coordinates, and for accurate time-keeping. Several adaptations were also made to the standard complement of Flux-PAM sensors. Because of sensor maintenance requirements, no attempt was made to directly measure eddy fluxes of water vapor. The standard net radiometer was replaced by a 4-component measurement of incoming and outgoing, short and long-wave radiation utilizing aspirated Kipp and Zonen pyranometers and Eppley pyrgeometers. Since the standard platinum-resistance thermometer used by Flux-PAM has a lower limit of -40 C, it was supplemented with a thermistor calibrated over the range -55 C to 10 C. 3.0 DATA COLLECTION AND PROCESSING The SHEBA Flux-PAM data are available at two time resolutions, five-minute averages and one-hour averages. The basic data archived during field operations are five-minute averages calculated at each Flux-PAM site by the EVE data systems. The five-minute data were stored on removable media on the local station data systems and also transmitted by RF modem to the base computer on the Des Grosseiliers. These two overlapping data sets have been merged during post-project processing to fill in non-coincident data gaps present in both sets. Various corrections have been applied to the five-minute data during post-project processing. Most of these have been discussed in the sections of the report that correspond to individual sensors. They include * interpolating between pre- and post-project radiometer calibrations, * correction to the compass reading for offsets generated when the beacon was on, * use of a 1-hour median filter to remove known spikes in the compass and hygrothermometer data in October 1997, * subtracting offsets from the radiometer level data, * calibration correction to the Tsoil/ice sensor at Station 2 after April 11, * correction to Solent virtual temperature data prior to June 21, * where possible, the wind measurements from the sonic anemometer have been rotated into a coordinate system with its vertical axis normal to a plane defined by the local mean wind field. Missing or obviously erroneous compass data have not been replaced in the 5-minute data record (with the exception noted above), but obviously erroneous data have been flagged as missing, including compass data collected during operation of the sonic heaters. For most of the project, the orientation of the stations, and thus the compass data, varied rather slowly. Thus it was thought to be generally acceptable to fill in the missing data by interpolation (and extraplotion) during the calculation of derived variables such as Vazimuth, dir, or boom. Hourly averages of most data parameters, centered on the half-hour, have been calculated during post-project processing. The hour-average parameters generally, but not always, correspond to the five-minute parameters. For example, the pyrgeometer thermopile and temperature measurements have been combined to calculate the long-wave radiation flux, the PRT and thermistor data have been combined into a single value for the air temperature, and the PRT and RH (with respect to water) data have been combined to calculate the relative humidity with respect to ice. Prior to calculating hour averages, two editing steps were applied to the data to minimize the contribution of occasional erroneous spikes. First absolute range limits, listed in Table 10.1, were imposed on the data. Observations exceeding these limits were flagged as missing. Second, a running 3-point (15 minute) median filter was applied to the five-minute time series, which replaced the central data point with the median of the data within the 15-minute window. If the data within the 15-minute window are monotonically increasing or decreasing (or constant), they are unaffected by the median filter. However, valid maxima or minima within the window are suppressed in the same manner as erroneous spikes, reducing the variance of the five-minute time series. Reduction of the five-minute variance is not expected to have a significant effect on the hour average. 4.0 DATA REMARKS Some information on the NCAR Flux-PAM stations that was taken from the Roughness Lengths Over Snow paper: "At the other four SHEBA sites, we deployed portable automated mesonet (PAM) stations-these are the flux-PAM stations from NCAR's instrument pool (Militzer et al. 1995). These PAM stations featured either ATI or Gill sonic anemometer/thermometers mounted at heights between 2 and 4 m above the surface. We named our first four PAM sites Atlanta, Baltimore, Cleveland, and Florida after the teams playing in the Major League Baseball Championship Series in the fall of 1997 while we were building the SHEBA ice camp. The Atlanta, Baltimore, and Florida sites lasted for the entire experiment. A pressure ridge engulfed the Cleveland PAM station in late January 1998, and the station was offline for several months for refurbishing. We redeployed it at a site called Seattle in spring 1998 and, later in the summer, moved it to Maui." 5.0 REFERENCES SHEBA FLUX-PAM Project Report - http://www.eol.ucar.edu/rtf/projects/sheba/ Sheba Atmospheric Surface Flux Group Home Page - http://www.weather.nps.navy.mil/~psguest/sheba/index.html Parameterizing the Turbulent Surface Fluxes Over Summer Sea Ice paper - http://www.esrl.noaa.gov/psd/people/ola.persson/polar_studies/conference/Andreas_8th_Polar.pdf Roughness Lengths Over Snow paper - http://ams.confex.com/ams/pdfpapers/68601.pdf