1.5. Meteorological Measurements

T. Mefford (Editor) and B. Halter

1.5.1. Meteorology Operations

Introduction

The climatology of surface meteorological observations at the four CMDL observatories is based on hourly average measurements of the resultant wind direction and speed, barometric pressure, ambient and dewpoint temperatures, and the precipitation amount. The meteorological sensors in use were selected for their high accuracy as well as their ability to withstand the extreme conditions of the polar regions. Data is recorded as 1-min averages so that the variability within the hourly averages can be determined. To the extent that is possible, World Meteorological Organization (WMO) siting standards [WMO, 1969] are followed. Thermometers are also positioned at the top of the sampling towers at BRW, MLO, and SPO to measure the temperature gradient to determine the stability of the surface boundary layer.

A detailed description of the PC-based data acquisition system may be found in Peterson and Rosson [1994]. Table 1.5 describes the instrument deployment as of December 31, 1997.

TABLE 1.5. CMDL Meteorological Sensor Deployment December 31, 1997

Heights are in meters above surface, except for the pressure transducer and mercurial barometer, which is with respect to MSL.

†Propeller Anemometer, model no. 05103, R. M. Young Company, Traverse City, Michigan.

‡Pressure Transducer, model no. 270, Setra Systems, Acton Massachusetts.

§Platinum Resistance Probe, Logan 4150 Series, Logan Enterprises, Liberty, Ohio.

Thermometer, positioned at the top of the local sampling tower to facilitate an estimation of boundary layer stability.

**Hygrothermometer, Technical Services Laboratory model no. 1088-400, Fort Walton Beach, Florida.

At BRW the complement of sensors measuring meteorological variables remained unchanged. Corrections to the configuration of the digital data cables and the communications board in the computer were made to bring the system into conformance with standards for the stable RS-485 data communications between sensor interface modules and the computer.

At MLO, maintenance of the array of meteorological sensors included the replacement of some weather beaten components such as AC power cords and the R.M. Young anemometers at the 10.2 and 38.2-m levels in May 1997. In mid-August 1997 a lightning strike, which interrupted many of the projects at the observatory, damaged the meteorological data acquisition computer as well as some of the electrical components on the tower. The computer was replaced and spare sensor parts were sent to MLO. The system was back up and running about a month after the lightning strike. The data cable from the tower to the data acquisition computer was replaced in October 1997.

At SMO a new data cable connecting the tower sensor to the computer in the main observatory building was pulled through an underground conduit and put into operation in May 1997. At the same time, some deficiencies in the interconnections among sensor interface modules and the computer were corrected. Calibration checks on wind and temperature sensors were performed, while periodic comparison of the pressure sensor with the observatory’s mercurial barometer continued. Corrosion by sea salt aerosol, which continually hits the outdoor components, continued to cause occasional instrument malfunction. Most susceptible are those components associated with sensors which must have a continuous stream of outside air drawn past them in order to make valid measurements, such as the Technical Services Laboratory (TSL) dewpoint hygrometer with its exposed chilled mirror and printed circuit board. Replacement of this sensor is being considered.

At SPO the meteorological data acquisition computer and barometric sensor were transferred from the old Clean Air Facility to the new Atmospheric Research Observatory in January 1997. An Antarctic Support Associates (ASA) surveyor determined a change in elevation of +2.35 m for the pressure sensor. The other meteorological sensors had been remounted on the meteorological tower that was moved in December 1995 [Peterson and Rosson, 1994]. Table 1.6 summarizes the changes to the sensor heights after the tower move. The tower is located approximately 91.4-m grid north-northwest from the new building. Data cables and the AC power line running to the relocated tower from the old Clean Air Facility were removed, and new cables were run from the new building to the tower along a line of wooded stanchions. A separation of 1 to 2 feet was maintained between the AC power and data lines. The mercurial barometer, used as a check on the long-term stability of the pressure sensor, was returned to the manufacturer for reconditioning and calibration. The barometer in the ASA meteorology office was used to make these periodic checks until November 1997 when the CMDL barometer was received and installed in the new building. Calibration checks of temperature and wind sensors, including interface electronics, were performed. Heights of the tower sensors were maintained by raising them to compensate for drifted snow accumulation around the tower.

TABLE 1.6. SPO Sensor Instrument Heights Before and After Tower Move

Heights are in meters above snow surface, except for barometric pressure, which is with respect to MSL.

Refer to Table 1.5 for sensor information.

Data Management

The meteorological data acquisition system gathers data from sensors that operate continuously at each of the four CMDL observatories. Data are transferred to Boulder on a daily basis via the Internet. Preliminary hourly averages of wind direction and speed, barometric pressure, ambient and dewpoint temperature, and precipitation amounts are sent to the station personnel. Each month, a climatic summary is prepared from edited data and distributed within CMDL and to each of the observatories.

A comparison of the number of data points recorded against that expected for the year was used to monitor the system’s performance. Table 1.7 shows the performance of each system during 1996 and 1997. On average the meteorological data acquisition system for the four observatories operated 98.16% and 95.99% of the time for 1996 and 1997 respectively. Due to the remoteness of the observatories, power outages are common and are the main reason for data loss. Hardware failure, system restarts, and system maintenance are the other reasons for data loss. At BRW, during the winter periods, rime, snow, and ice occasionally would build up on the sensors and have to be removed by the station personnel. At MLO high winds can cause electrostatic buildup on the 38-m anemometer. The solution was to temporarily disconnect the AC power that would reset the sensor modules. The biggest cause of data loss at MLO was due to the lightning strike in August 1997 when the system was down for about a month until it was replaced. At SMO, the biggest cause of data loss was due to the buildup of corrosion on the sensor connector pins and moisture getting into the RS-485 communications line. This produced noise in the communications line that the data acquisition system was not able to handle.

TABLE 1.7. CMDL Meteorological Operations Summary

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