Subnivean Instrument Description

The instrument array at the subnivean is designed to measure and compute the surface energy and mass balances at a "point". Radiative fluxes are measured directly, turbulent energy fluxes are computed using an aerodynamic profile method, and mass fluxes (snowmelt) is measured using a system of draining lysimeters. The instrument array is described in detail below.

Shortwave Radiation

Incident and Reflected Shortwave

The principal incident shortwave and reflected shortwave radiation measurements are made with a Kipp & Zonen CM14 "Albedometer". It is a single body instrument that houses both an upward- and downward-looking pyranometer. The instrument complies with the specifications for 'secondary standards' as published in the Guide to Meteorological Instruments and Methods of Observation, Fifth Edition of the World Meteorological Organization (WMO). The CM14 has a response time of <15 seconds, a spectral range of 305-2800 nm (50% points, or 335-2200 nm (95% points). It has a Schott K5 optical glass (2mm) dome.

A secondary incident global shortwave measurement is made in conjunction with the measurement of direct- and diffuse-beam solar radiation. Two LiCor LI-200SA pyranometers are used for this purpose; one is unshaded (global incident shortwave), while the second is shaded by a shadowband (diffuse incident shortwave). Direct beam radiation is computed by subtracting the diffuse from the global. The shadowband is similar in design to the model supplied by Eppley, and the same correction coefficients are used as in the Eppley model. The Li-Cors have a non-uniform spectral response: their response is very low at 400 nm and increases nearly linearly to a maximum at about 950 nm, and then decreases nearly linearly to a cutoff near 1200 nm. Thus changes in the spectral composition of the incident light cause errors in the Li-Cor output. This is an important consideration given that the spectral quality of diffuse and global sunlight is ordinarily different, but the calculated error terms appear to be relatively small.

Longwave Radiation

Atmospheric Incident and Surface Emitted Longwave Radiation

Incident and emitted longwave measurements are made with a Kipp & Zonen CG2 "Net Pyrgeometer". It is a single body instrument that houses two separate CG1 pyrgeometer sensors. It is sensitive to radiation in the range from 5 to roughly 25um. Since pyrgeometer measurements are dependent on the temperature of the sensor itself, this variable is monitored using a dedicated YSI 44031 thermistor at the surface of each thermopile. The CG2 has a response time of 5 seconds (63%) and 25 seconds (99%). The factory estimated accuracy of measurement is +/- 10%.

Net Radiation

Although the four components of net radiation are measured individually as discussed above, an independent net radiation measurement is made using a REBS Q*7 Net Radiometer. This instrument has a spectral response of 0.25 to 60 um, and so measures a somewhat larger portion of the EM spectrum than the instruments described above. The instrument is ventilated to prevent snow accumulation and frost development.

Air Temperature and Relative Humidity

Air temperatures and relative humidities in the vertical profile used for flux computations are measured using Vaisala Model HMP35C Temperature and Relative humidity Probes. These probes are housed in 1 Gill 12-plate radiation shield, but are not ventilated. (Calm conditions are rare at Niwot Ridge, and the lack of mechanical ventilation does not appear to be much of a problem.) For purposes of computing turbulent fluxes using the aerodynamic profile method, our concern is with the relative accuracy of the temperature and relative humidity measurements between levels. Therefore all the Vaisala instruments are placed on the same level and calibrated to a single instrument on a regular basis.

Air Temperature

The Vaisala HMP35C contains a Fenwal Electronics UUT51J1 thermistor. The overall probe accuracy is a function of three error sources; in the "worst case" all errors add to an accuracy of +/- 0.4 C over a range of -33 C to +48 C. The error is typically less than this.

Relative Humidity

The Vaisala HMP35C contains a Vaisala capacitive relative humidity sensor. The measurement range is 0-100%. The accuracy is specified at +/- 1% against factory references, +/- 2% against field references in the range of 0-90%, and +/- 3% against field references in the range of 90-100%. It has a 15s response time (90%).

Wind Speed and Direction

Wind speed and direction are measured using R.M. Young 05103 Wind Monitors. These are propellor-type anemometers which measure wind speed and direction with the single instrument. Four of these instruments are mounted in a vertical profile for turbulent flux computations.

Wind Speed

The R.M. Youngs measure wind speed in the range of 0-60 m/s with a gust survival of up to 100 m/s. Their threshold sensitivity is 2.0 m/s.

Wind Direction

The R.M. Youngs measure a full 360 degrees of wind direction using a balanced vane. Their threshold sensitivity is 1.0 m/s at 10 degrees displacement, and 1.5 m/s at 5 degrees displacement. Due to datalogger constraints, only the 2.0 m and 6.0 m wind directions are currently recording.

Soil Heat FLux

The heat flux of the soil is measured directly using a REBS HFT-1 Soil HEat Flux Plate installed at 5 cm depth. The REBS palte has a thermal conductivity of 1.00 W/m/K, a diameter of 38.6 mm, and a thickness of 3.9 mm.

Snow Depth

Snow depth is monitored constantly beneath the instrument tower using a Campbell Scientific UDG01 Ultrasonic Depth Gauge. This sensor measures the distance from the sensor to the surface; it is currently mounted at 6.0 above the ground, so during peak snowpack conditions it is 2-3 m above the snow surface. The primary components of this sensor are the Polaroid Ultrasonic transducer and the Polaroid 6500 Sonar Ranging Module. The instrument bounces an ultrasonic wave off the surface and listens for the return echo. The time from transmit to return of the echo is the basis for determining the distance to the surface. The sensor has a measurement range of 0.1-6 m, and an accuracy of +/- 1 cm or 0.4% of Distance to Target, whichever is greatest. The measurement resolution is 0.5 mm. The IFOV of the sensor is approximately 20 degrees, so at low snow depths (large distances) echoes are sometimes recorded from the tower itself, resulting in a multiple signal.

Lysimeters

Snowmelt water draining from the base of the snowpack is measured using two series of lysimeters. The first series consists of two 1mx1m lysimeters that drain into the subnivean laboratory, where the flow is measured using Campbell Scientific TE525 Tipping Bucket Rain Gages, which is a smaller adaptation of the standard Weather Bureau tipping bucket gage.

The second lysimeter series consists of 16 0.2 m^2 lysimeters arranged equally spaced in a ring approximately 10 m in diameter. These lysimeters, designed to capture the spatial variability of snow meltwater, drain into the subnivean laboratory where the flow is measured using 16 Davis Instruments Rain Collector II tipping bucket gages.

Barometric Pressure

Barometric pressure is measured by capacitance with an AIR-DB-2BX, with a measurement range of 600 to 1060 mb and an accuracy of +/- 0.5 mb.

Data Recording

The principal data recording is performed using a Campbell Scientific CR21x Datalogger. Additional measurements are made using a Campbell Scientific CR10 Data Logger. Data is currently stored on a Campbell SM192 storage module, which is interchanged each Tuesday.

If any problems arise please e-mail

Todd Ackerman (todda@culter.colorado.edu)