Data Access
ASTER data are stored in two forms:
ASTER Overview
The National Center for Atmospheric Research (NCAR) Atmosphere-
Surface Turbulent Exchange Research (ASTER) facility was used as
the basis of the FLAT experiment. ASTER's propeller-vane anemometers were used
to measure $d\overline{U}/dz$ needed in the TKE shear production term, ASTER's
sonic anemometers were used to measure u* and
d(u^2+v^2+w^2)/dz needed
in the TKE turbulent transport term and were combined with
ASTER's platinum temperature sensors and Krypton hygrometer to
measure w'Tv' needed in the buoyant production term.
Fast response pressure sensors, provided by NOAA/WPL, were placed
near the sonic anemometers to measure w'p' needed in the
pressure transport term. Finally, a hot-wire anemometer, provided by
the University of California at Irvine, made high-frequency velocity
measurements used to calculate the dissipation rate of
TKE.
The FLAT experiment was carried out from September through November, 1990. This time of year was chosen to maximize the chances for near-neutral stability conditions (overcast, strong winds) to occur, yet to obtain data in convective conditions early in the experiment.
Just a few photos have been digitized. Oncley has more images on slides:
Location
The FLAT experiment required that the site be horizontally homogeneous
(hence the name FLAT). A site was used near Carpenter, Wyoming (see
Road Map) which had a uniform slope for almost 8~km to the west
of about 1/4~degree (see Topographic Map). The area was
agricultural, though some of the land was in the Crop Reserve Program
(CRP) which grows into wild grassland. Although the fields
were broken into many strips of various
sizes (see Crop Map), the surface roughness was nearly the same for
all of the non-CRP land during the fall when the experiment occured.
There was an albedo difference in these surfaces however, which may have
created internal boundary layers.
The masts were oriented to have good fetches from SW through N (see
Mast Layout) since west was thought to be the predominate direction for
strong winds.
The actual coordinates for the site were: 41.0668 deg N, 104.2966 deg W.
Sensors
Although this was ASTER's first deployment, several extensions from the
"generic" layout were made. Since the measurement of vertical gradients
were important, two of the masts were extended to allow measurements at
15~m. The mast spacing was either 10~m or 15~m to prevent the guy wires
from these taller masts from interfering. As seen in the
Channel Configurations, the propeller mast had 6 levels from 1 to 15~m,
the psychrometer mast had 5 levels from 1 to 10~m (plus a sixth level
placed on the ati mast), and the ati sonic anemometer mast had three levels
at 4, 7, and 13~m for measuring turbulent quantities. The uw sonic anemometer mast was
kept uncluttered (by the pressure ports) to estimate flow distortion. The
hot wire was placed near the 7~m uw sonic to enable {\it in situ}
calibration. The Krypton hygrometer was placed near the 4~m uw sonic
since it was not felt that there would be a strong variation in humidity
flux with height. A "sawhorse" supported infrared and visible upward
and downward-looking radiometers plus at least one net radiometer. Soil
temperature and heat flux measurements were made and soil moisture was
checked sporadically (see Soil Moisture summary) using a gravimetric method.
Two types of slow-response
barometers were used to measure the low-frequency pressure variations which
were cut off by the fast-response pressure sensors.
ASTER also hosted several other "piggy-back" investigations.
In addition, nine PAM stations were deployed to document the mesoscale flow. See the separate PAM page.
Data Processing Notes
This section is not complete.
Daily Weather Plots
The following plots summarize conditions during each day of the project.
Each plot covers one Julian day (0000-2359 GMT) and is labeled with time
in GMT at the bottom and local time (MST) at the top.
The top panel has the mean temperature and humidity measured by the
psychrometer along with the pressure from the PAM barometer at 7 m. Sharp
drops in humidity (for example, at 1500 on day 291) are caused by the
wet-bulb thawing out after having been frozen and reading saturated values.
The next panel is wind speed and wind direction, usually measured from
the propeller-vane anemometer at 10 m. The dotted line represents direction
of winds coming perpendicular to the mast array. Acceptable fetches
should be for winds 45 degrees south through 75 degrees
north of this direction (260 - 020 degrees). The next lower panel shows
the net radiation, and sensible, latent, and soil heat flux contributions
to the energy balance. The bottom panel shows the Monin-Obukhov stability
parameter z/L evaluatated at 10 m from measurements from the 7 m UW mast
along with u* from these measurements. A line for neutral
conditions (z/L = 0) is shown for reference.
Since these fluxes and derived parameters are based on smoothed, 5-minute
average statistics, they should not be used quantitatively and are only
shown for guidance in selecting periods to analyze further.
Other plots
Precipitation during FLAT (from the nearest PAM station) is shown on
the bottom panel of "Summary of conditions". Only one rain event
on day 303 is seen. Smaller events on days 281, 282, 308, and 309 are
probably snow melting in the rain gauge. The upper panel shows the albedo
of the planted soil where the ASTER masts were located. These values after
day 281 and 308 are high and then decrease, corresponding to melting
snow cover on the surface. (Values greater than one probably are due to
snow masking the upward-looking radiometer.) The nominal albedo is seen
to be about 0.22. Lower values are seen which may be due to darker soil
when wet. Albedos measured from the PAM station, located on a adjacent stubble
field, during the last two weeks of the experiment are also shown. These
values are lower than expected since the stubble appeared lighter than the
soil. Further work will be necessary to verify the PAM radiometer calibration.