Temperature-depth profiles, measured in boreholes, contain
a temporal record of past changes in the Earth's surface
ground temperature and provide valuable constraints on
climatic variations over the last few centuries. But the
linkage between ground temperature and meteorological
variables including air temperature are imperfectly known.
To understand that linkage better, and to document in detail
how the surface ground temperature changes propagate into
the subsurface where they are later measured in temperature-
depth logs in boreholes, we have designed and installed a
geothermal climate-change observatory. Installed in arid
northwest Utah, our Emigrant Pass Observatory (EPO)
consists of an array of thermistor strings in the subsurface,
and a meteorological station at the borehole collar. Results
from our first complete annual cycle, November 1, 1993,
though October 31,1994 are presented. Ground and air
temperatures generally track each other but with important
time-varying offsets. The mean surface ground temperatures
for the period are 11.3 on the granite outcrop and 9.5
for the partially shaded regolith site; mean air
temperature at a 2 m mast height above the ground is 8.8
/deg. The ground-air temperature differences are variable on
time scales from days to seasons, largely governed by level
of absorbed solar radiation. Marginal precipitation (8.6 mm)
and ephemeral snow cover did not significantly disturb the
ground-air temperature difference during the year monitored.
Instrumental measurement of air temperature has non-
random sampling biases that present problems for observing
long term changes. The calculation of average annual air
temperature at EPO decreases by 0.34 K as the sampling rate
of air temperature is decreased from 60 s to every 12 hours.
The attenuation and phase lag of thermal waves with depth
confirm that heat conduction theory adequately describes the
transient temperature field at this site, and yield in situ
estimates of thermal diffusivity, a quantity needed to
reconstruct surface ground temperature histories. Thermal
diffusivity for the granite and regolith is 0.88 x
and 0.45 x
respectively. Energy flux calculations for the Emigrant Pass
Observatory site suggest that a geothermal climate-change
observatory has the capability of detecting a century scale
energy perturbation that is one part in a million of the
instantaneous flux.
AGU Index Terms: 1645 Solid Earth; 1600 GLOBAL CHANGE New category; 1694 Instruments and techniques; 1620 Climate dynamics
Keywords/Free Terms: Climate, geothermal, temperature, ground temperature, solar radiation, meteorologic, utah, air temperature
JGR-Solid Earth 96JB01903