Methods

    CEMRC operates two identical meteorological towers at sampling sites in the vicinity of the WIPP (Fig. 2). The Near Field site is located approximately 1 km northwest of the WIPP site at an elevation of 1088 m (latitude 32° 22'40.385"N; longitude 103° 47'55.425"W). The Cactus Flats site is located approximately 19-km southeast of the WIPP site at an elevation of 1041 m (latitude 32° 13'05.451"N; longitude 103° 41'42.583"W).

    Each station consists of a 10-meter tower equipped with sensors for temperature, relative humidity, barometric pressure, solar radiation, wind speed and direction, and vertical wind speed. Data are collected every second, with averaging times of ten minutes. In addition, the maximum wind speed and total precipitation occurring over the ten-minute averaging period are recorded.

    Temperature, relative humidity and all wind parameters are measured at a height of 10 m above the surface. Precipitation, barometric pressure and solar radiation are measured at heights of 0.4, 1 and 2 m, respectively. The barometric pressure sensors are adjusted for temperature, but are not referenced to mean sea level. The solar radiation sensors (pyranometers) measure the energy flux per unit area (W m^-2) of both direct and diffuse sky radiation.

    The data are stored in electronic dataloggers and downloaded twice weekly. Once downloaded, the data are screened for outliers and other anomalies and uploaded to a main database. Performance checks of the sensors are conducted quarterly, and sensors are re-calibrated at least annually.

    This report summarizes meteorological data collected over the 12 month period from December 1997 through November 1998. In addition, data collected at the sites from 1 January through 30 November 1997 (11 month period) are compared with data from the same time interval during 1998.

Results

    For the 1998 sampling period, data recovery exceeded 97% for all sensors at both the Cactus Flats and Near Field sites. Data recovery was slightly lower during the months of October and November due to a six day period (30 October - 6 November) when new meteorological equipment was installed at the sites. Other short-term (typically less than two hours) data losses occurred throughout the year due to sensor maintenance, repair, performance testing and malfunction.

    Averaged over the year, winds were from a southeasterly direction (E, ESE, SE and SSE quadrants, inclusive) 41% and 44% of the time at the Cactus Flats and Near Field sites, respectively (Fig. 3). However, there were some distinctive seasonal variations in wind direction (Figs. 4-5). Wind direction was highly variable during the winter and spring (December through May) when compared with the summer and fall (June through November). During summer and fall, wind from the southwestern quadrant occurred over 50% of the time, but dropped to less than 30% during the winter and spring. The inter-annual and intra-annual variability in wind direction are important parameters in modeling dispersion pathways for potential airborne releases from the WIPP.

    Wind velocities were very similar between sites. Wind velocities (10-minute means) were less than 5.4 m s^-1 over 77% of the time, with speeds frequently from 3.1 to 5.4 m s^-1. Calm periods (wind velocities < 0.1 m s^-1) occurred less than 1% of the time over the year. Wind velocities > 5.4 m s^-1 occurred less than 25% of the time, but were more frequent during the spring, and typically came from west and west-northwest. The highest wind velocities recorded at each site were 33.6 m s^-1 (75 mph) on 21 July, and 23.3 m s^-1 (52 mph) on 2 April, at the Near Field and Cactus Flats sites, respectively.

    Air temperatures at Near Field ranged from –10.5 to 43.6 ^oC and from –10.1 to 43.6 ^oC at Cactus Flats. The maximum temperatures were recorded on 27 June at both sites, and lowest temperatures were recorded on 12 December and 27 December at the Near Field and Cactus Flats sites, respectively. The annual mean temperatures were 18.7 ^oC and 18.2 ^oC at Near Field and Cactus Flats, respectively. At both locations, December was the coldest month (mean = 5.0 ^oC at Near Field; mean = 4.8 ^oC at Cactus Flats) and June was the hottest month (mean = 28.9 ^oC at Near Field; mean = 29.6 ^oC at Cactus Flats) (Fig. 6).

    The annual mean relative humidity at Near Field was 41.8% and ranged from 5.3 to 100%. Humidity at the Cactus Flats site was very similar to Near Field, averaging 42.0% and ranging from 5.5 to 101%. Mean relative humidities were lowest when temperatures peaked in late spring and early summer (Fig. 7). It should be noted that the accuracy of the relative humidity sensors declines at relative humidities below 12% and above 94%, and readings outside these ranges should be interpreted with caution.

    Barometric pressure did not exhibit an obvious seasonal trend at either site (Fig. 8). The annual mean was 890.9 mb at Cactus Flats and 896.0 mb at the Near Field site. The apparent 5.1 mb difference between the sites can be attributed to a 41 m difference in elevation, and this difference is not significant if corrected using standard barometric conversions that incorporate elevation (U.S. Department of Commerce Weather Bureau, 1963, Manual of Barometry, Vol. 1, Washington D.C.).

    Solar radiation flux (W m^-2) was integrated over daily intervals to calculate total energy received per unit area (MJ m^-2). As is typical, solar radiation received at the sites peaked in the summer and was lowest during the winter months (Fig. 9). This pattern is due to a combination of increasing solar radiation intensity, less cloud cover and additional hours of daylight during the summer months. Over the year, the daily total solar radiation ranged from 1 to 33 MJ m^-2 at Near Field and 1 to 36 MJ m^-2 at Cactus Flats.

    Over the year, a total of 12.88 cm of precipitation was measured on 37 days at Cactus Flats and 17.28 cm of precipitation was measured on 39 days at Near Field (Fig. 10). At both sites, the month of December had the highest number of days (9) on which precipitation was recorded. At Cactus Flats, December was also the month with the highest total precipitation (3.96 cm). In contrast, at the Near Field site, July was the month with the highest total amount of precipitation (5.53 cm).

    Overall, 1998 was a much drier year than 1997. In 1997, a total of 38.5 and 27.9 cm of precipitation was recorded at the Near Field and Cactus Flats sites, respectively, between 1 January and 30 November. Over the same time period in 1998 (December, 1997 excluded from the comparison), 12.3 cm of precipitation was recorded at Near Field and 8.9 cm recorded at the Cactus Flats site. In addition to being drier, mean annual temperatures were approximately two degrees higher in 1998 than 1997. Mean temperatures were higher in 1998 than in 1997 for every month except March and August. Given the differences in precipitation and temperature it is not surprising that total solar radiation was also higher in 1998, with Cactus Flats receiving 7984 MJ m^-2 in 1998 compared with 6294 MJ m^-2 in 1997 and the Near Field site receiving 7222 MJ m^-2 in 1998 compared to 6130 MJ m^-2 in 1997. In contrast, annual mean wind patterns (velocity and direction) were very similar between years, although some differences appeared in seasonal wind patterns.