The meteorological data were primarily recorded as hourly mean values. A summary of the weather data collected each year at DELTA is provided by a table (TABLE: variables measured & periods of sensor deployment). A description of the data available for each variable is provided below.
As shown in the table, measurement of all variables except precipitation and vapor pressure was done every 60 seconds; vapor pressure was calculated from measurements at the same frequency. Every hour on the hour the preceding 60 values were averaged, yielding hourly mean values for all variables except precipitation and wind.
Precipitation amount measurements were made at least twice daily, coinciding with the PCSP Weather Observations at 0600 h (or 0700 h) and 1800 h (or 1900 h), Eastern Standard Time. Observations were also made irregularly to assess the precipitation rate during events. Observations were generally also made late in the evening to allow determination of daily precipitation totals.
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)
Wind speed and direction was measured every 60 seconds, and processed on the hour every hour, and at the end of each day, using a wind vector algorithm. The algorithm computed mean wind speed, wind vector magnitude, wind vector direction, and the standard deviation of direction. The mean wind speed and wind vector magnitude values were converted from mph to m/s during the data adjustment period.
Wind direction at both sites 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 at Delta ranged from 2.5° to 8.8° , and adjustments were not made.
All radiation measurements through the 1992 season were made based on solar noon, or local apparent time (LAT). This was done to allow comparison with AES hourly irradiance and net all-wave values at weather station Alert, which are made using LAT. Computations indicated that solar noon during the field season occurred between 8 and 18 minutes after noon EST.
In the UV-B dataset, a three to four hour data gap exists approximately every three days, due to sensor disconnection during battery charging. The YES UVB-1 sensor has a high current draw to an internal temperature control system, which maintains optical components at a fixed temperature of 45° C. Periodic resistance measurements were made of an internal thermistor to insure that the specified temperature was maintained.
Net all-wave irradiance (Q*) was measured directly during the 1991 field season, and determined as the sum of net solar and net thermal irradiance in 1992: