Notice

3.1. Scope

This section describes the rationale for station siting, and summarizes the station locations, the characteristics of the stations themselves, data-assurance procedures, the soil-moisture measurement strategy, and special considerations that need to be addressed to keep the stations running.

3.2 Surface Flux Stations and Sampling Strategy

3.2.1. Site Selection Rationale

The aggregate method of average surface flux estimation was adopted by CASES 97. This method determined the site selection strategy outlined below. In this technique, the domain-averaged surface flux is equal to the weighted average of the individual surface-station flux values, weighted by the fraction of land type that station represents. `Land type' is defined in terms of wetness, vegetation cover, elevation, slope, and soil characteristics. Thus surface flux stations were sited to:

• Emulate the general conditions found in Butler County Kansas, which comprises most of the CASES 97 domain (Table 3.1).
• Represent different slopes and altitude (ridge top, flood plain, etc.).

In addition, we sought to:

• Provide a reasonable geographic distribution.

Stations with similar characteristics will be compared to test the representativeness of the stations for various land types.

Table 3.1 summarizes the distribution of stations by land type and the match to Butler county conditions, which we have assumed represents the conditions of the experimental domain.

Table 3.1: Land Cover Characteristics Summary.
South and North wind numbers differ because of two stations located at the border of two crop types (McClure and Chinn).

Due to the limited number of surface flux stations available to CASES 97, the following surface types were chosen as most important:

• Grassland/Pastureland: Grassland is often mowed in July. Thus, during CASES 97 grassland should remain undisturbed. Most of the pastureland will be grazed during CASES-97. In the spring, many farmers burn off their pastureland to increase the ratio of new, tender shoots to dead grass from the previous year. Grassland and pastureland will provide most of the increase in overall domain greenness expected to be observed during CASES 97.
• Winter Wheat: Winter wheat will be well into its vegetation stage and will be about 45 cm high during CASES 97. Because winter wheat will remain green over the course of CASES 97, winter wheat stations will provide a background level of greenness..
• Crops other than Winter Wheat: For CASES-97, these sites are located on tilled soil about to be planted or in the early stages of crop growth. Greenness and evaporation should change significantly at these sites.

Table 3.2 summarizes the general albedo, evapotranspiration (vegetation controlled), and evaporation (controlled by bare soil type) characteristics of these three surface types.

Table 3.2: Characteristics of Main Surface Types

Table 3.3 gives the location and characteristics of each station.

Table 3.3. Final sites with latitudes and longitudes and heights

3.2.2 Surface Flux Station Measurements

Each surface flux station will provide the following basic measurements (details provided in Tables 3.4-3.7)

• Average fluxes of water-vapor, sensible heat, momentum flux.
• Mean, variance, and skewness of wind components, temperature, and water-vapor.
• Surface Radiation balance.
• Soil Moisture and Temperature at least one level.
• Rainfall amount.

3.2.2.1. NCAR/NSF Surface Stations

NCAR/NSF has 8 stations in the field, 6 basic Portable Automated Mesonet (PAM) flux stations that provide five-minute averaged quantities of the above quantities, and 2 Atmosphere-Surface Turbulence Exchange Research (ASTER) sites, which provide five-minute fluxes. ASTER sites also provide higher-resolution time-series data of the wind components, thermodynamic quantities and other special measurements. The five-minute fluxes and means can be used to reconstruct fluxes for longer periods (divisible by five) from the five-minute means and fluxes by (a) computing `low-frequency' fluxes from the means (after subtracting a `grand mean' from each variable for the averaging period chosen) and (b) adding the result to the average of the five-minute `high-frequency' fluxes for the period chosen.

Figure 3.1 is a schematic of the station layout, which is designed to maximize the quality of observations in fair weather. The wind roses for April and May appear in Figure 3.2.

Figure 3.1 Schematic of instrument placement on NCAR/NSF surface stations.

ASTER (Table 3.4)

The main ASTER `supersite,' just east of Rock, Kansas, is on a south-sloping winter wheat field, and is close to the ASTER Base Trailer, which doubles as the Surface Operations Center (Section 2.1.3). The satellite ASTER station is located on grass, about 4.5 miles SE of the ASTER Base (Fig. 1.3, Table 3.3). The two stations transmit data through line-of-sight radio communication.

In addition to 5-in fluxes, the two ASTER stations will provide wind and thermodynamic data at rates of 1 and 10 Hz for more detailed analysis. In addition, special instrumentation will be divided between these two sites, including:

• Soil moisture profile (both sites, Lundquist and LeMone).
• Photo-active radiation (at 7; PAR, NCAR).
• Ozone fluxes (at 7; PI: Tony Delany, UCAR).
• All-sky camera (at 7; PI: Dr. Charles Long, CIRES/CU).
• Laser ceiliometer (at 7; Long).
• Veggie meter (at 7, later at 8)

Table 3.4. ASTER Instrumentation in CASES

PAM III: Table 3.5

The 6 PAM stations are in a variety of locations (Table 3.2, Stations 1-6 in Fig. 1.3). Each station is powered by solar collectors. The stations are equipped to provide the basic flux and mean measurements at 5-in intervals. These data can be converted to averages over longer periods by adding the fluxes associated with the five-minute means. Soil moisture is sampled at 5 cm (Section 3.6).

Table 3.5. Instrumentation at PAM III Stations

References:

 Militzer, J.M., M.S. Michaelis, S.R. Semmer, K.S. Norris, T.W. Horst, S.P. Oncley, A.C. Delany, and F.V. Brock, 1995: Development of the prototype PAM III/Flux PA surface meteorological stations. 9th AMS Symposium on Meteorological Observations and Instrumentation, Charlotte, NC.

 Horst, T.W., and S. P. Oncley, 1995: Flux-PAM measurements of scalar fluxes using cospectral similarity. 9th AMS Symposium on Meteorological Observations and Instrumentation. Charlotte, NC.

 3.2.2.2 The Qualls site is located in the NW part of the array, over grass.

Table 3.6. Qualls Instrumentation

 a. Time and heights

REBS=Radiation and Energy Balance Systems, Inc.

EI=Everest Interscience, Inc.

* Below ground surface. All other heights are above ground.

b. Errors and resolutions

c. Derived parameters

Sensible heat flux (eddy correlation and Bowen ratio)

Latent heat flux (Bowen ratio)

%Soil Water Content (Mass water *100/mass dry soil)

Friction Velocity (flux profile method)

3.2.2.3 ABLE Smileyberg Site is located on grass east of Smileyberg, Kansas (Table 3.3). The instrumentation will be powered from a bank of solar panels and batteries. The data will not be available in real time, and will have to be quality assured before release to general users.

Table 3.8. Instrumentation at ABLE Smileyberg site 

a. Meteorological data

 b. Flux data (heat, moisture, momentum, CO2)

 3.2.2.4 ARM CART Towanda site is located on alfalfa in the NW part of the site. Richard Cuenca and Shaun Kelly will supplement the Towanda site with time-domain refractometry soil-moisture data, infiltration data, soil hydrologic properties, and precipitation at a number of locations in the Towanda subbasin. In the optimum (south) fetch, coverage is alfalfa-brome. However, as of the beginning of CASES-97, the alfalfa at the site had been burned out. Location (Table 3.3) 37.842N, 97.020W, 395 m.

Table 3.9. Towanda Instrumentation (ARM CART Extended Facility #6)

a. Fluxes

b. Meteorological observations

*SWATS = Soil water and temperature sensors. They measure soil water potential, from which estimates of soil volumetric water content are made.

3.3. Data Assurance Procedures for Surface Stations

3.3.1 Intercomparison

• If portable instrument standards are available, they can be compared to tower measurements when the stations look for systematic offsets. This comparison can be done when visiting for soil-core samples.
• Aircraft will fly over some surface flux stations and sounding sites for comparison of measurements.
• Downwelling longwave radiation can be compared to that calculated from thermodynamic data from aircraft or radiosondes.

3.3.2. Ensuring site uniformity

Grass within fenced sites in pasture areas should be clipped to the same height as grass outside the fence,

to ensure measurements are representative of the surroundings.

3.3.2 Documentation of location

• Surface photographs will be taken in the four cardinal directions at all of the sites at 7 to 10 day intervals, and where possible, aerial photographs will be taken of the sites.
• Sometime during the experiment, slope and aspect and solar noon should be determined at each surface flux site and at the Argonne sites.
• Height and type of vegetation will be documented during the experiment with measurements and photographs.

3.4. Soil Moisture Measurement Strategy

Except for the ARM flux station at Towanda (Table 3.9) and the ASTER sites (Table 3.4), soil moisture measurements are limited to one level. A profile is would be more desirable, since soil moisture near the surface is affected by weather events, while soil moisture at deeper levels has a longer memory, integrating the effects of weather events.

The following strategy was adopted to minimize the single-level problem:

• Observe soil moisture at the 5-cm level. This level is responsive to surface evaporation on week time scales, appropriate to the one-month duration of the experiment. With this strategy, we can observe episodes of gradual drying out of the soil when a heavy rain is followed by several dry days (dry-down events) if they occur.
• Place the soil moisture sensor (for PAM and ASTER, Tables 3.4, 3.5) horizontally to increase the footprint of the measurement.
• For PAM and ASTER, volumetric estimates of soil moisture from the buried soil-moisture systems will be supplemented by measurements of soil-moisture content in soil core samples (gravimetric method). In this technique, soil core samples are taken and weighted before and after drying. These samples will be taken at least twice during operations, on a rotating basis.
• ASTER 7 and ASTER 8 soil moisture will be sampled every three days using the TRIME system, starting with first IOP. At least 3 samples will be at 5 cm down to 67 cm at ASTER 7, and 88 cm at ASTER 8.

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Last Modified: 20 Aug 1997