The SHEBA Flux-PAM data are available at two time resolutions, five-minute averages and one-hour averages.
T. Uttal |
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
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.
Name | Variable, units | Limits | 5-min | 1-hour |
---|---|---|---|---|
P | pressure, mb | 970, 1050 | x | x |
T.plat | PRT temperature, °C | x | ||
T.therm | thermistor temperature, °C | x | ||
T | air temperature, °C | -45, 5 | x | |
RH | relative humidity wrt water, %RH | x | ||
RHice | relative humidity wrt ice, %RH | 60, 115 | x | |
Rsw.in | incoming shortwave radiation, W/m⊃2 | -15, 900 | x | x |
Rsw.out | outgoing shortwave radiation, W/m⊃2 | -15, 900 | x | x |
Rpile.in,out | pyrgeometer thermopile, W/m⊃2 | x | ||
Tcase.in,out | pyrgeometer case temperature, °C | x | ||
Tdome.in,out | pyrgeometer dome temperature, °C | x | ||
Rlw.in | incoming longwave radiation, W/m⊃2 | 100, 350 | x | |
Rlw.out | outgoing longwave radiation, W/m⊃2 | 150, 350 | x | |
lev.rad.x,y | rad. array two-axis levels, degrees | x | ||
Rlwdiff.in.ARM, ETL | incoming longwave radiation
differences, NCAR - ARM(ETL), W/m⊃2 | x | ||
Tsoil, Tice** | ice temperature, °C | -25, 5 | x | x |
Gsoil, Gice** | ice heat flux, W/m⊃2 | -125, 100 | x | x |
u,v | horizontal velocities, m/s | 0, 17 | x | |
spd | wind speed, m/s | 0, 17 | x | |
dir | wind direction wrt true N, m/s | 0, 360 | x | |
w | vertical velocity, m/s | 0, 0.5 | x | |
tc | sonic virtual temperature, °C | x | ||
u'u' | turbulent variance, m²/s² | 0, 4* | x | x* |
u'v' | turbulent covariance, m²/s² | ± 0.5* | x | x* |
u'w' | turbulent covariance, m²/s² | ± 0.6* | x | x* |
u'tc' | turbulent covariance, °C-m/s | ± 0.3* | x | x* |
v'v' | turbulent variance, m²/s² | 0, 2.5* | x | x* |
v'w' | turbulent covariance, m²/s² | ± 0.3* | x | x* |
v'tc' | turbulent covariance, °C-m/s | ± 0.1* | x | x* |
w'w' | turbulent variance, m²/s² | 0, 0.8 | x | x |
w'tc' | turbulent covariance, °C-m/s | ± 0.08 | x | x |
tc'tc' | turbulent variance, °C² | 0, 0.1 | x | x |
uflux | momentum flux, Nt/m² | x | ||
H | heat flux, W/m² | x | ||
freq.sonic | sonic pulses/second per path | x | ||
u,v,wsamples | sonic pulses per sample, n | x | ||
samples.sonic | percentage of good data, % | 0, 100 | x | |
u,v,w,tcflag | software-detected data outliers, n | x | ||
Vazimuth | azimuth of sonic v axis wrt N, ° | 0, 360 | x | |
boom | azimuth of sonic boom wrt N, ° | 0, 360 | x | x |
heater | fraction of time heater is turned on | x | ||
latitude | latitude, decimal degrees | 74, 81 | x | |
longitude | longitude, decimal degrees | -169, -142 | x | |
lat,long.deg | latitude, longitude degrees | x | ||
lat,long.min | latitude, longitude minutes | x | ||
lat,long.sec | latitude, longitude seconds | x | ||
compass | compass reading, decimal degrees | x | ||
beacon | fraction of time beacon is turned on | x | ||
Tbox.eve | electronics internal temperature, °C | x | ||
vdc.batt | battery voltage, V | x | ||
vdcmax,min.batt | max, minimum battery voltage, V | x | ||
vdc.teg | system voltage, V | x | ||
vdcmax,min.teg | max, minimum system voltage, V | x | ||
i.charge | battery charging current, amps | x | ||
imax.charge | max charging current, amps | x | ||
i.load | system load current, amps | x | ||
Z.TRH.meas | TRH sensor height above snow, interpolated from direct measurements, meters | x*** | ||
Z.TRH.snowline | TRH sensor height above snow, interpolated using snow-line data, meters | x*** | ||
Z.sonic.meas | sonic anemometer height above snow, interpolated from direct measurements, meters | x*** | ||
Z.sonic.snowline | sonic anemometer height above snow, interpolated using snow-line data, meters | x*** |
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.
Most parameters have been sampled by the EVE data system at a rate of 1 Hz, while the sonic anemometer data have been ingested at a rate of 20.83 Hz for the R2 Solents and 10 Hz for the ATIs. In order to determine turbulent variances and fluxes from the high-rate sonic data, EVE calculated and archived a complete set of centered 5-minute covariances:
<u'u'> | |||
<u'v'> | <v'v'> | ||
<u'w'> | <v'w'> | <v'w'> | |
<u'tc'> | <v'tc'> | <w'tc'> | <tc'tc'> |
During data post-processing, these covariances were rotated into a coordinate system with its vertical (w) axis normal to the plane of the mean flow. However the (five-minute) data in the horizontal plane determined by that rotation remain in a coordinate system nominally aligned with the sonic measurement array. The data may be rotated about the vertical axis into a geographic coordinate system aligned with true North using the variable Vazimuth.
The hour-averaged sonic data were calculated with the following steps:
<u'w'>_{15} = M_{15}(<u'w'>) + M_{15}(<u><w>) - M_{15}(<u>)M_{15}(<w>)
where <u'w'>_{15} is the 15-minute covariance, <u> and <w> are 5-minute means, <u'w'> is a 5-minute covariance, and M_{15}(x) is the average of three 5-minute values of x.
The hour-averaged data also include the calculation of momentum and heat fluxes,
uflux = rho M_{60}(<u'w'>_{15})
H = rho C_{P} M_{60}(<w'tc'>_{15})
where M_{60}(x_{15}) is the average of four 15-minute values of x. Daily average values of air density rho and the specific heat of air C_{P} were used in these calculations, in order to assure that they were immune to the possiblity of errors in the individual 5-minute values of temperature, pressure and humidity. Note also that the heat flux is calculated using the sonic virtual temperature rather than true air temperature.
Note that the sonic data have not been edited for possible flow distortion when the mast is upwind of the sonic. The mast is upwind of the sonic when dir = boom ± 180°. This is particularly an issue for the ATI sonics, which were mounted at the same height as the sonic boom and thus below the top of the vertical mast. However it also may be an issue for the asymmetric R2A Solents. Although the Solents where mounted above the boom and above the top of the vertical mast, the three vertical struts supporting the R2A sonic array are clustered within a 120° sector which was usually oriented toward the mast.
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