Level 2 Meteorology for all LTER Sites

Level 2 Meteorology entails hourly, or at least synoptic (four times daily at 0000, 0600, 1200, and 1800 hr GMT), reporting throughout a day. This reporting is necessarily automated. A day is defined as a 24-hour period from local midnight to midnight as measured by standard time in the time zone.

An established LTER site is expected to maintain at least one Level 2 meteorological station for intersite comparison and standardization purposes.

In addition to basic climatic parameters obtained at a Level 1 station, a Level 2 station obtains the more detailed meteorological data appropriate for a research site. Table 6 lists the variables to be recorded at Level 2. All variables except radiation should be recorded at least at synoptic times but preferably hourly.

Another important distinction of Level 2 meteorological systems is the capability for continuous, unattended operation, as required by the periodic (hourly or synoptic) measurements. The relatively low cost of so-called electronic data loggers makes automatic recording an especially attractive method for handling the additional recording requirements of a Level 2 station. Most LTER sites have already acheived this level of observation.

Instrumentation

Level 2 instrumentation makes periodic measurements of maximum, minimum and (separately) mean air temperature, precipitation (as water-equivalent), wind (speed and direction), relative humidty, and global solar radiation, as summarized in Table 6. The air temperature and precipitation sensors at a Level 2 station are often are more sensitive and need to be exposed in a different manner than those at a Level 1 station. Some "packaged" meteorological stations come with masts and equipment enclosures that obviate the need for the standard NWS-type shelters and other equipment described in sections 2 and 3 above.

Table 6. LEVEL 2 Meteorological Station

The nonrecording precipitation gage should be located no nearer the instrument shelter than twice the height of the instrument shelter. At exposed windswept sites, a windshield may be required for the precipitation gage. The gage must be elevated above maximum snow depth, and, if possible, operation should continue during freezing weather.

The recording precipitation gage may be either a weighing or tipping type gage. Gages should record to at least 0.5 mm (0.02inch) unless a NWS recommended Fisher Porter gage or gage from an elctronic data logger system, is used. Both standard and recording precipitation gages will be maintained at the same site. Recording gages will be impractical for some LTER sites in winter unless exposure and servicing can be provided in deep snow and the gage heated. It is a common acceptable practice to use a mixture or any combination of antifreeze, alchohol and oil, in the storage container of the raingage in order to use the instrument in the winter time. The water equivalent depth of snow will be recorded (USDC, 1989).

Both precipitation gages should not be closer to trees, buildings or the instrument shelter than twice the height of the obstruction. Standards for precipitation measurement given in for lower levels also apply.

The hygrothermograph will not be needed if an electronic relative humidity sensor is available. But note comments elsewhere concerning the calibraation of such sensors.

The anemometer should be located away from obstructions which would interfere with wind flow over the instrument. The anemometer will be mounted with the cups at 10 meters (Note this is a change from the first edition of the standards). Maintenance on the bearings and spindle will be performed twice yearly as recommended in the Observer Handbook No. 2 (USDC, 1989) or the instrument manufacturers manual. Wind travel may be accumulated by an internal counter or at a separate recorder. The optional wind direction variable will require a recording system. Level 2 stations require increased reliance upon recording instruments. The individual LTER site may elect to install a data logging system rather than separate recorders.

The global incoming radiation sensor must be fully exposed to the sky in all directions (not shaded by vegetation, buildings, or topography). An exception may be made if all of an LTER site is similarly shaded by topographic obstacles. A fully exposed sensor is preferable because the data have wider application and the effect of shading can be subtracted from full sky data. The sensor should be inspected daily, the glass kept clean, and the sensor and recorder recalibrated every 18 months.

Measurements

Level 2 measurements are to be made hourly, or at least synoptically, and reported daily. Synoptic observation times are well defined by the WMO as being 0000, 0600, 1200, annd 1800 GMT. Hourly times should always be referenced to local standard time, not daylight-savings time or other special times which may be in effect at a site.

Table 7 LEVEL 2 Meteorological Measurements

Preprocessing

Substantial preprocessing, including quality control, at the site is required for Level 2 data.

Assuming hourly data are summaries of short time interval observations, air temperature and relative hmidity data should include the instantaneous maximum and minimum and 60 minute average for each variable where available. Precipitation and solar radiation should be hourly totals. These hourly measurements are a minimum and additional parameters may need to be recorded at some sites.

Wind

The preprocessing of the wind data is complex. Observers using elctronic data processing eqipment should follow the suggestion provided in Appendix 4. Total wind travel, observed for a 24 hour period, will be converted to mean daily wind speed in meters per second. Where calm wind conditions are the rule, listing of minimum wind speed may be omitted. At sites using data loggers the proceedure for obtaining daily mean wind values is outlined in Appendix 4. At sites experiencing diurnal wind shifts, the report may list day and night means in addition to a single 24 hour mean. Wind direction may be recorded as an instantaneous observation once an hour (or as the most common direction in a five minute interval at times when the direction is highly variable) or summarized as the mean direction for each hour. As a minimum, direction will be listed for eight points plus the calm condition. Where measured, wind direction will be reported as the number of hourly observations in each of 8 directions, plus calm, for each 24 hour period, for example:

Where data loggers are used vector mean wind direction may be computed and reported following the guidlines suggested in Appendix 4. Whichever method is chosen should be noted in the metadata for the variable.

Precipitation

Because the standard precipitation gage is considered the more accurate, the recording gage values are adjusted to equal the standard gage total. Daily precipitation will be reported in millimeters for the LTER site reports. Hourly precipitation totals will be tabulated and available at each site but not included in intersite reports.

Relative Humdity

The electronic sensor or hygrothermograph will provide a continuous record of temperature and relative humidity. The hygrothermograph record should be adjusted to read within 1 deg C of the maxmin thermometers. The accuracy of the hygrograph response to relative humidity will be verified using a portable psychrometer which draws a constant air stream over wet and dry bulb thermometers. The relative humidity reading of hygrograph and psychrometer will be compared at high and low relative humidities. The use and adjustment of the hygrothermograph are discussed in Field Manual for Research in Agricultural Hydrology Chapter 3 (Brakensiek, Osborn and Rawls, 1979) which serves as the guideline document for the hygrothermograph.

Global Solar Radiation

Global solar radiation may be integrated by the data logger. Total global incoming solar radiation will be reported in MJ/sq. m/day. More on this topic is provided in Appendix 1.

Additional Considerations

Because of their complexity, sensors in Level 2 systems typically require periodic calibration. Suggested methods of calibration are provided below.

No specific Level 2 meteorological equipment is recommended by the committee since each site should be free to select its own system. However, the selected system should be able to, at a minimum:

1) make the indicated measurements at hourly (or at least synoptic) intervals,

2) record the measurements for periodic collection, either by direct media transfer or by telemetry

3) translate and relay the recorded measurements on command in suitable form to an to an external computer.

The following are some criteria that LTER sites may use for selecting automated meteorological systems:

1) Components of a system must be physically and electrically compatible. Response speed and signal level of sensor and recorder must match to avoid degrading raw data. Recorder and translator must match to avoid losing data. Unless the user is prepared to assume system design responsibility, components should be bought from a single supplier who guarantees system compatibility.

2) The recorded data should be accessible in the field for checks and calibration as well as being easily translatable to a record which can be read and processed by computer for summary reporting, and further analysis.

3) The system should be able to operate during expected environmental conditions. Estimate climatic extremes at your site and specify that the equipment will operate within these limits. Components that seem particularly susceptible to cold (less than minus 10 deg.C) and moisture (RH greater than 90%) conditions include hard copy paper printers and cassette recorders. Modems are more reliable if a phone line can reach a station. Cellular phones and telemetering systems have also proved reliable in extreme conditions.

4) Be sure that the manufacturer can provide fast service and backup for your equipment and/or have two or more compatible systems to interchange components. If components are interchanged make sure intercalibration factors are available. Budget for repairs and recalibrations.

Although the LTER program does not endorse any particular manufacturer of electronic data sensing and recording instrumentation, it is noted that a large number of LTER sites use equipment made by Campbell Scientific inc, of Logan Utah; accordingly, Apppendix 2 includes a typical configuration from this manufacturer. Other manufacturers may be able to provide similar, acceptable systems

All original data records (tapes, charts, etc) should be kept, and the earliest listing of raw electronic data should become part of the permanent record for the site.