NCAR Integrated Surface Flux Facility at CME04

Pine tower
Pine tower for CME (view from Aspen tower)

Introduction

This document describes the operation and measurements of the Integrated Surface Flux Facility (ISFF) during the Carbon in the Mountains Experiment, Niwot Ridge, Colorado in Spring and Summer, 2004.

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CME/ACME Links

Schedule

10 June 2004
Data collection starts at Willow
28 June 2004
Data collection starts at Pine
23-30 July 2004
Collected data from Washington State University SF6 sensor (hydra sampling)
29 July 2004
Data collection starts at Aspen
9 Sep 2004
Hydra inlets reconfigured -- 0.5, 2, and 4m inlets along transect
10 Sep 2004
Hydra sampling changed during morning/evening transitions
17 Sep 2004
Hydra inlets reconfigured again -- out to 260m
4 Oct 2004
Data collection ends at Aspen and Willow
5 Oct 2004
Data collection ends at Pine

Measurement Sites

Table of Sites

Station# Name Latitude
(deg N) 		Longitude
(deg W) 		Elevation
(m)
1 Willow 40.035888178° 105.552138222° 3089
2 Pine 40.035931579° 105.547123249° 3051
3 Aspen 40.034959920° 105.542495110° 3012

Instrumentation

Layout of all sensors on the 3 towers in PNG and PostScript formats.

The instrumentation consists of:

CO2 sensing is different at each tower.

Data Collection Notes

This was the second use of the new PC104-based data systems. In general, they performed well. Issues are:

Bigger issues involved data network:

Sensor Notes

Sonic anemometer

Height.Site Boom Angle (deg rel N) Actual Height (m)
1m.w 179.7 1*
2m.w 179.9 2*
6m.w 178.3 6*
10m.w 178.1 10*
17m.w 178.9 17*
1m.p 158.4 1.39
2m.p 159.3 2.22
6m.p 161.0 5.82
10m.p 160.1 9.49
17m.p 162.2 16.77
30m.p 163.5 29.69
1m.a 182.9 1.18
2m.a 182.7 2.26
6m.a 172.1 6.43
10m.a 182.0 10.28
17m.a 184.7 17.62
30m.a 186.5 30.54
* = Not measured during operations, but should have been close due to the tower/clamps that were used at willow.

The tops of timbers used for the base of the willow and aspen towers were used to define "0" height. For the pine tower, a point on the ground directly below the 1m sonic was used (since the terrain sloped significantly.)

Sensor issues are:

None of these problems should affect the data that were archived, so no action has been taken.

Other issues requiring some action were:

Naturally, there are various rain-related problems. Many are dealt with by our despiking routine, but there clearly are cases (19 Aug) when the statistics are affected. These have not yet been edited from the data set.

Krypton hygrometer

Issues are:

Fast-response CO2 sensors (HiLo's + open path)

As mentioned above, their were two types of co2 flux sensors used during CME04. Willow and Aspen had "HiLo" systems that used a large pump to bring air from inlets near the CSAT sonic anemometers down to a closed path analyzer at the tower base. A valve cycled sampling each inlet for 5 minutes and occasionally calibration gases. Thus, 5-minute average flux values are available from each level every 10 minutes.

Pine had an open-path co2 sensor at 2m which ran continously. (No co2 flux measurement was made at 30m at Pine, since this should have been nearly the same as at the CUFF tower.)

These data have been organized in the NetCDF files as one set of co2 measurements (co2, co2'co2', w'co2', etc.) made at 2m height dimensioned by the 3 tower sites. The upper-level co2 measurements are reported as a one-site at 17m (Willow) and one site at 30m (Aspen).

Issues are:

See the following discussion for the treatment of these issues.

Willow HiLo

Willow's HiLo was reasonably straightforward. It only had 4 configurations: The dessicant/scrubber used for the reference cell was never changed, however it was always used cycling through the reference cell and should not have been depleted.

Since the Li6251 appears to measure CO2 density, we convert the calibration cylinder mixing ratios to density using the measured Li6251 sample cell pressure and temperature. A plot of all the data are shown here. There appears to be an offset shift in the Li6251 at the time of the HiCal change, though I can't remember any action that should have induced this offset. The lines shown are eyeball fits to the two offsets, adjusted to make the following plot fit well. We check this fit by plotting the difference in CO2 mixing ratio between HiCal and LoCal. This plot shows pretty good agreement with the horizontal lines representing the cylinder concentration differences. Note that this plot is generated using one gain for the analyzer for the entire experiment and the same offset is used for both HiCal and LoCal. Thus, the analyzer was relatively stable. The differences between the Scott Marin cylinders are better behaved than between the Airgas and Scott Marin cylinders. Also, a drift is seen in the last 1-2 weeks which could indicate the AirGas also becoming empty. Since the gain appears to be constant in these calibrations, we will apply it to the June/July data when calibration gases were not available.

Lag times have been measured in the lab and determined by cross-correlations using field data. The inlet tubing was 19.5 m long with 3/8-inch ID. The flow rate through each tube was 39 lpm, producing a delay of 2.1s. For the lower ambient pressure at Niwot Ridge, the regenerative blower that was used is specified to have a higher flow rate, thus the delay would be 1.8s.

The in-field cross-correlations between sonic temperature and co2 ranged from 2.5 to 6s. However, many of these estimates had low correlation coefficients. Lag times computed from high-correlation cases (near local noon) were: 

    MST         correlation     lag(2m/17m) 
 8 Jul 1300      -0.8/-          2.9/-
15 Jul 1000      -0.9/-0.9       3.0/2.7
18 Jul 1200        - /-0.8        - /2.6
 8 Aug 1100        ? /-0.7       3.1/2.9
 9 Sep 1200      -0.8/-0.6       3.0/2.8
 3 Oct 1300      -0.6/-0.5       2.9/2.8
Thus, as expected if the pump were running similarly, the lag times appear to be approximately constant at 3.0s for the 2m inlet and 2.8s for the 17m inlet. The reason for the 0.2s difference between these inlet lags is not obvious. The 2m tubing was mostly coiled, whereas the 17m was straight, so we assume that the coiled tubing had somewhat more drag.

Also, a lag of 2.8 or 3.0s is signficantly larger than the calculated value of 1.8s. The calculation didn't take into account lags in the analyzer itself, though 1.0s seems rather large. We'll try to measure this component of the lag.

Lenshow and Raupach also allow us to estimate the filtering effect of this length of tubing. For the flow rate of (39 lpm)(700mb/840mb), U = 7.6 m/s and for temperature of 10 C, Re = 5100, which is turbulent. Thus, they predict a half-power frequency of 2.8 Hz. This will cause some of the flux to be lost, but we will not attempt a correction here.

Thus, the data will be processed simply using a constant analyzer gain and a constant delay time for each inlet. We will convert the measured CO2 density to CO2 mixing ratio using measured analyzer cell pressure and temperature.

Aspen HiLo

This system had two major problems -- the Li7000 calibration was quite bad until recalibrated in the field on 23 Aug and the main blower was too weak until changed 22 Sep. A chronology of major events is:

The data from 30-31 Aug when the reference gas ran out have been removed.

The Li7000 reports CO2 mixing ratio, but like the 6251 is fundamentally a density-measuring device. Just to be safe, I've worked up the calibrations as density. It was necessary to break the data into 4 periods to obtain reasonable calibrations:

These fits are shown in a scatterplot and the difference in cylinder amounts in another plot. This difference plot still shows a drift from 10-13 Sep, which I assume are bad HiCal values prior to the cylinder completely running out. (Values are missing completely from this plot from 13-22 Sep, when the Pcell values indicated that HiCal was empty.)

After these calibrations, a plot of the CO2 densities 30m-2m generally are as expected. However, from 6-22 Sep, there is a step change in differences apparent from the daytime data. My guess is that this shows the failure of the regenerative blower which we finally fixed by replacing it. Alternatively, this plot could indicate a problem with the system after the new pump was installed. Indeed, the 2m data after the pump change are somewhat strange, with daytime lags being impossible to determine (see next paragraph), however this also could be due simply to changing photosynthesis.

In general, the lag correlations were as expected: 

    MST         correlation     lag(2m/30m) 
 8 Aug 1100      -0.7/-0.9       5.6/4.7
25 Aug 1300	 -0.7/-0.8	 5.3/4.7
 9 Sep 1200      -0.2/-0.6        - /4.6
20 Sep 1100	 -0.4/-0.7	 5.1/4.4
23 Sep 2300	 -0.9/-0.5	 3.0/3.1
26 Sep 1200	 -0.2/-0.9	  - /3.1
29 Sep 0600	 -0.8/-0.8	 3.3/3.1
 3 Oct 1300      -0.3/-0.9        - /3.1
Note that the correlation coefficients were quite small for the 2m data during the day starting in September, making the time lag impossible to find. However, the nighttime data still had large correlation coefficients with a stable lag. Also note that the lags have a step change at the time of pump replacement, as expected. Finally, note that, like at Willow, the 2m lag is larger than the 30m lag. The data have been processed using constant lags of 5.6 and 4.7s for 2m and 30m, respectively, prior to 23 Sep and 3.3 and 3.1s afterwards.

Pine Open-path

No particular problems were noted with this sensor, other than expected bad values during rain. Removing data when the krypton hygrometer reads a low voltage (below 0.01V) cleans up the vast majority of these cases. The remaining outliers were mostly removed by applying a simple range check (440 < co2 < 638).

Even after this filtering of the data, the computed fluxes were different from those computed using the HiLos. This difference may be real, since the 2m level at Pine would be expected to be heavily influenced by the soil directly below. However, this also may be due in part to the sensor placement (see photo), where the Li7500 was 23cm in back (North) and 53cm below the center of CSAT3 array. According to "How Close is Close Enough?" (Kristensen et al.), the vertical displacement should have only a small effect on the fluxes, however their result would not necessarily apply within a canopy. The horizontal displacement will have a time lag, but this would be difficult to account for in data processing, especially with the light and variable (temporally and spatially) winds within the canopy. For now, the data have been processed with no lag.

To determine the approximate magnitude of the effect of displaced sensors on fluxes in this environment, we can compare h2o fluxes from the Li7500 and krypton hygrometer (which was only 20 cm below the sonic array). A scatterplot shows that many of these Li7500 fluxes are about 90% of the krypton fluxes, which is in general agreement with Kristensen et al., however there is a lot of scatter -- including many cases with near zero krypton h2o flux, but significant Li7500 h2o flux. If the fluxes were well behaved (i.e. only attentuated by the displacement), the flux could be calculated using a Modified Bowen Ratio method, e.g. w'co2' = w'tc' (sigma_co2/sigma_tc).

Hydra

These data have been processed by Sean Burns and are now available here. Some issues were:

T/RH

These were mounted at nearly the same heights as the sonic anemometers at pine and aspen. At willow, they were approximately 0.5m lower than the sonics. On 28 June, the 1m TRH at willow was moved up from 0.5m to 0.95m.

Laboratory post-calibration results for these sensors are described in a temperature postcal report and a humidity postcal report. Temperatures all agreed to within 0.05 C and humidities to 1% (except for one sensor that was 2% off.) Nevertheless, RH data reported during saturated conditions ranged from 97-105% and T.6m.a appeared to have a bias of +0.2 C on 21 Sep. No adjustments have been made to the data set (yet).

Noise was seen on the data from several sensors. Alternate data samples differed by 0.1 C typically. Lab testing was unable to duplicate this problem and field testing was unable to identify either the source or the reason for susceptibilitiy to this noise. (A change of power supply appeared to reduce, but didn't eliminate, the noise.) For 5-minute average statistics, the noise would be averaged out if it were symmetric, but this is not known. Thus, values for these sensors [LIST] could be off by as much as [LIST].

Other issues are:

Barometer

NO PROBLEMS. (There may be some initial data at each tower with pressure port disconnected.) (There are some "aspen" data on ~20 June which are bogus.)

Radiometers

Rnet/PAR. Issues are:

Tsfc

Issues are:

Tsoil sensors

Laboratory post-calibration results for these sensors are described in a Tsoil postcal report.

Issues are:

Detailed Information about the Tsoil Implementation.

Photographs

Also see Sean Burns' photo/event calendar and those on the MMM CME04 Page.

Field Logbook

A computer-readable field logbook of comments by NCAR and other personnel is available in read-only html form.

Data Tables and Plots

Click on the above link to view tables and plots.

The tables display mean, deviation, number of missing values, and number of clipped values for each variable, from 0:00-6:00, 6-12:00, 12-18:00 and 18-24:00 each day.

The 5 minute statistics for most variables are shown on time-series plots.

Data Download

The NCAR data are available for download in the following forms: We also have a Table of Variables available in these files.

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© NCAR/Earth Observing Laboratory
This page was prepared by Gordon Maclean and Steve Oncley, NCAR Research Technology Facility