GCIP/EOP NOAA/ATDD Fort Peck, Montana Long Term Flux Site 1.0 General Information This long-term flux monitoring site was installed in November of 1999, adjacent to the NOAA SURFRAD facility, which is within the LSA-NW region. The tower (48 18.473' N, 105 6.032' W) was to the west of the SURFRAD instrumentation. Natural grassland surrounds the 3 meter tower in all sectors providing a minimum fetch of 200 meters over relatively flat terrain. Data on the soil physical properties and characterization of the vegetation are currently being gathered. The Leaf area index is estimated to be around 2.5. 2.0 Methodology Traditionally, the use of the eddy correlation method (Businger, 1987; Baldocchi et al., 1988) has been constrained to mainly short term intensive field campaigns. Improvements in instrument design, stability, and power requirements over the past decade now allow for nearly continuous measurements of sensible and latent energy fluxes using the eddy covariance technique. Using this technique, the average vertical turbulent eddy fluxes of sensible and latent heat (and other scalars) are determined as ____ w'X' = $(w-{w})(X-{X}) --------------- n where w is the vertical velocity component of the wind vector, and X is the scalar of interest (e.g. water vapor concentration). Here, the {bracketed} quantities denote an average or "mean" that is subtracted from the instantaneous values to obtain the fluctuating component. The $ represents the summation from i = 1 to n. Average vertical turbulent ____ fluxes (w'X') are computed in real time using a digital recursive filter (200 s time constant) for the determination of a running "mean" from which the instantaneous values are subtracted. An averaging period of 30 minutes (denoted by the overbar) is used and is considered large enough for statistical confidence in the covariance quantity but is short enough to resolve the structure of the diurnal cycle. Wind vector measurements made at experimental sites that are not perfectly flat can result in non-zero vertical wind velocities measured from the "vertical " coordinate system of the measurement platform. At the end of an average period, vertical turbulent fluxes perpendicular to the mean horizontal wind (which generally follows the contour of the land surface) are obtained by mathematically rotating the coordinate system of the measurement frame of reference (sonic anemometer) to obtain a zero _ _ mean vertical and transverse velocity (w=v=0). Details of this procedure described by Wesely (1969) are outlined by Businger (1986) and Baldocchi et al., (1988). The three components of the wind vector are determined with a sonic anemometer ( R3Gill Instruments, Hampshire, England). The stable long-term operational characteristics of this instrument and its ability to continue measurements during cold weather and light rain events (Yellard et al., 1994), as well as its low power consumption, were important considerations in the selection of this anemometer. The symmetric head design of the R2 with its slender support structure produces little flow distortion (Grelle and Lindroth, 1994) and is well suited for measurements in the the relatively flat and open locations of the Little Washita Watershed and Champaign, Illinois sites Fast response water vapor and CO2 concentration measurement are made with an open-path, fast response infrared gas analyzer (Auble and Meyers, 1992). This sensor was used extensively for flux measurements during recent ARM (Doran et al., 1992) and BOREAS (Baldocchi et al., 1997) experiments. In a recent evaluation of open and closed path sensors for water vapor and CO2 concentrations, Leuning and Judd (1996) found that for the measurement of CO2, this sensor displayed minimal cross sensitivity to water vapor (see Leuning and Moncrieff, 1990). 3.0 Data Format Standard meteorological data collected at each site. The following are the variables, in order as they appear in the file. jday Julian Day time LST, half hour ending w_speed propeller anemometer (2 m) w_dir wind direction (2 m) Ta air temperature (C), at 2 m RH relative humidity at 2 m Pres surface pressure in mb Rg incoming global radiation (W/m2) Par_in incoming visible radiation (0.4-0.7 um) in uE/m2/s Par_out outgoing or reflected visible light Rnet net radiation (W/m2) GHF soil or ground heat flux (W/m2) rain total rain for half hour (inches) wet wetness sensor (in voltage with higher values indicating wetness) IRT surface or skin temp (C) 2_cm soil temp at 2 cm (C) 4_cm soil tmep at 4 cm 8_cm soil temp at 8 cm 16_cm soil temp at 16 cm 32_cm soil temp at 32 cm 64_cm soil temp at 64 cm u_bar average wind vector speed (m/s) u'w' kinematic shear stress (m2/s2) u'2 streamwise velocity variance (m2/s2) v'2 crosswind velocity variance (m2/s2) w'2 vertical velocity variance (m2/s2) H sensible heat flux (W/m2) LE latent energy flux (W/m2) CO2 CO2 flux (mg/m2/s) LW_in Incoming longwave radiation (W/m2, from SURFRAD) sm_7.5 soil volumetric water content at 7 cm zone sm_15 soil volumetric water content at 15 cm zone sm_30 soil volumetric water content at 30 cm zone sm_60 soil volumetric water content at 60 cm zone The eddy covariance sensors are located at 3 m AGL. The standard meteorological sensors (Table 1) are sampled every 2 s with a datalogger and multiplexor (CR23x, Campbell Scientific, Inc.) and averages are computed every 30 minutes, coincident with the eddy covariance data 4.0 Data Acquisition A laptop computer is configured in a mulitasking mode to simultaneously perform three operations. For the first and foremost task, measurements of the three componenets of the wind vector along with the speed of sound (from which the virtual temperature can be derived) are digitally sent from the sonic anemometer (which includes the digitized H2O and CO2 signals from the IRGA) to the laptop computer, which is housed in a small environmental enclosure. In the second task, the computer retrieves the standard meteorological data from the CR23X datalogger every 30 minutes and appends the data to an existing file. After midnight, the covariance data and standard meteorological data are copied to separate files with a name, year and calendar day header. The computer is equipped with a modem and cellular phone in order to retrieve the data and conduct occasional system checks. On average, data are retrieved from the laptop computers about once every two days.