Emigrant Pass Observatory - Insights on air and ground temperature tracking

Conductive heat transfer in the shallow subsurface coupled with the low thermal diffusivity of Earth materials allows surface ground temperature (SGT) histories to be reconstructed from subsurface temperature-depth measurements. If ground and air temperatures faithfully track each other, then these SGT histories provide a complement to surface air temperature (SAT) records by extending the history of surface temperature changes. Recent studies however have questioned the tracking of air and ground temperatures. To understand this tracking the Thermal Processes Research Group at the University of Utah built a climate and ground temperature observatory in northwestern Utah in 1994. The Emigrant Pass Observatory (EPO) consists of a standard meteorological station placed adjacent to a geothermal borehole (GC-1) drilled in 1978. We record meteorological and shallow ground variables at the site simultaneously. The combined dataset provides the opportunity to observe the ground-air interaction in near real time. EPO offers the observational evidence necessary to test theoretical studies of ground-air interactions. Eight years of measurements are analyzed to understand the coupling between common meteorological measurements and subsurface temperatures. The temperature difference between the ground surface and the standard air temperature measurement at 2 m, Δ T_g-a, averages 2.55° C (+/-0.01) from 1993-2001. The temperature difference varies from -5° C to +10° C when averaged over a diurnal cycle, and from 2.50° C to 2.60° C when averaged over an annual cycle. Incident solar radiation is the primary variable in determining the temperature difference between the ground surface and air temperature (Δ T_g-a=1.48° C per 100 Wm^-2), but with significant uncertainty, +/-8° C, due to complex heat transfer from latent heat, precipitation, snow and wind. Models of the snow data from EPO demonstrate that seasonal snow cover can either warm or cool the ground relative to the annual mean and that the snow effect is an order of magnitude smaller than the radiation effect. Divergence between the observed temperatures at 1 m in the subsurface and the air temperature modeled as a transient boundary layer forcing is less than 0.04° C per annum. The EPO observations verify that ground temperatures track air temperatures at frequencies important for climate change studies exceptionally well.