In this section, manipulations and adjustments to the measured data are summarized for each variable.
The 1992 air temperature values have been adjusted slightly, based on a four day post-field season intercomparison of the sensors used at Delta and Echo. Using an average difference in the comparison period values of 0.16° C, Delta values were adjusted up 0.08° C, and Echo values down 0.08° C.
At the end of each day the maximum and minimum values were recorded, from the 1440 values measured. The times at which the daily maximum and minimum temperatures occurred were also recorded. Daily mean temperatures were calculated as the average of all 1440 values, rather than by averaging the daily extremes. The daily maximum and minimum temperatures are based on daily extreme hourly mean values for 4 and 2 days, respectively, in 1990, due to a micrologger program change. The daily temperature range is also based on extreme hourly mean values for 2 days at Delta in 1991; in 1992 there were 5 maximum and 8 minimum daily temperatures determined this way. The maximum and minimum temperatures for these days are therefore conservative, and the times of maximum/minimum occurrence are for the extreme hourly means.
The 1992 RH values have been adjusted slightly, based on a four-day post-field season intercomparison of the sensors used at Delta and Echo. Based on an average difference in the values of 1.2 percent, Delta values were adjusted down 0.6 percent, and Echo values up 0.6 percent.
The daily maximum one-minute wind speed and the time of occurrence were recorded during each field season at Delta. These values were checked against the maximum hourly mean wind speeds each day, due to the occasional loss of daily data when program changes were made. In 1991 the maximum hourly mean wind was used as the daily maximum for one relatively calm day (DY 159). Four days in 1992 use the maximum hourly mean value.
Wind direction was referenced to true north. An attempt was made each year to orient the sensor using its shadow at solar noon (local apparent time), with varying degrees of success. During the 1991 and 1992 seasons, alignment was checked by recording the shadow orientation at a specific date and time. After the field season, solar azimuth was computed based on latitude, date and the observation time. The difference between the predicted and recorded shadow orientation ranged from 2.5° to 8.8° , and adjustments were not made.
No adjustments made.
No adjustments made.
No adjustments made.
Computation of L-down followed the method of Wardle and McArthur (1992) using the pyrgeometer signal, adjusting for ambient temperature, and correcting for short-wave interference:
where Vthp and Vg are the pyrgeometer and pyranometer signals, respectively, in mV; R and Rg are the responsivities, in mV W^-1 m^-2, of the pyrgeometer and pyranometer; E is the Stefan-Boltzmann Constant (5.67*10^-8 W m^-2 K^-4); and T is the absolute temperature of the pyrgeometer determined from the case thermistor (° K; see below). Although this equation was developed for pyrgeometers mounted inside NARC ventilated housings (the pyrgeometers at Taconite Inlet were not ventilated, due to logistical considerations), the method is preferable over the battery compensation circuit for unventilated instruments as well (McArthur, personal communication). The responsivity used for each pyrgeometer was the average of that determined in calibration before and after the field season. Comparison computations of L-down using the two different calibration values resulted in differences of only 1.5 percent (n = 60 hourly values), so averaging only altered the results slightly.
Field measurements of pyrgeometer case thermistor resistance were also made, using a DC half bridge (YSI model 44031 precision thermistor). These resistances were used to determine the pyrgeometer case temperature, allowing a correction to be entered into the L-down computation accounting for changes in ambient temperature. The equation used is from Wardle and McArthur (1992):
where T is the required absolute temperature (° K); To is 273.15° + 25° ; LGr is Loge(r/10000); and r is the measured resistance, in ohms. At Delta early in the season, a programming error resulted in loss of resistance measurements for a period of 70 hours. To compensate, a regression equation was determined for predicting case temperature from air temperature during similar weather (r^2 = 0.85), and used to estimate missing case temperature values.
Computation of thermal exitance from the surface (L-up) followed a modification of the Stefan-Boltzmann Law, for less than full radiators:
where the emissivity coefficient (G) is taken as 0.99 for snow (Dozier and Warren, 1982), and 0.95 for tundra (cf. Ohmura, 1980); the Stefan-Boltzmann Constant (E) is 5.67*10^-8 W m^-2 K^-4, and T is the adjusted surface temperature in ° K. The appropriate emissivity coefficient was determined by the albedo (a) at each measurement time: when albedo > 0.5 the surface was classified as snow covered.
The net radiometer at Delta was initially connected with the polarity reversed, for the period 1800 h on DY 150 through 1500 h on DY 155; all data during this period were therefore multiplied by -1. No other adjustments were made.
Precipitation amounts during snowfall events were determined by gage catch and from measurement of new snow accumulation. While new fallen snow generally melted from bare tundra surfaces, it was possible to assess accumulation on snow or ice covered areas (e.g. Lake C2). A precipitation amount was then determined using a measured or assumed snow density. These values were compared with those from the gage, and generally the larger value was used.
Precipitation type was also recorded, as either rain, snow, or mixed forms. The reliability of these observations depended upon the frequency with which precipitation type changed, and the degree to which the precipitation was a distinct type. Numerous events during the three field seasons involved multiple types of precipitation, and rapid changeovers between them. (See also Weather Observations section)