Tutorials - Emigrant Pass Observatory

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1. How exactly do we use the Emigrant Pass Observatory and boreholes to study climate?

Diffusion of a sinusoidal annual surface temperature wave into the subsurface. Note the phase lag and attenuation of the surface temperature.
Boreholes, long used to investigate heat flowing out of the Earth through measurements of temperature with depth, are also an important source of information of changing temperatures at the surface of the earth. Changing surface temperatures diffuse into the subsurface, as described by the one-dimensional heat diffusion equation, , where T is temperature, z is depth, t is time, and α is thermal diffusivity. Because it is a diffusive process, there is both an attenuation of the temperature amplitude and a phase lag (figure - right) in the ground temperatures. In the figure to the right we show a simplified annual sinusoidal surface temperature wave (figure - right, blue line z = 0 m) with a mean temperature of ≈10°C and an amplitude of 20°C. At a depth of 1 m (figure - right, red line), the amplitude of the temperature wave is 73% of the surface amplitude, and the peaks and troughs occur 18 days later than at the surface. At a depth of 5 m (figure - right, green line) the amplitude is further attenuated to 41% of the surface amplitude, and the peak occurs 91 days after the surface. Also noteworthy in this simplified model is that the surface temperature is below freezing for 121 days, whereas at 1 meter the temperature is below freezing for only 94 days. The ground at 5 m depth never freezes, with a minimum temperature of 5.9°C. It is this attenuation property that allows one to avoid freezing water pipes by burying the pipes to a particular depth.

One way to investigate this phenomenon is through repeat logging of boreholes. By measuring the temperature at a given borehole over time, changes in air temperature can be seen in the ground temperature changes. Even more helpful is to monitor the air and ground temperatures at the same location, as well as other meteorological variables such as changing solar irradiance and precipitation. Snow, for example, insulates the ground from the very cold conditions in the air, resulting in the ground being warmer than would be expected. Further, ground conditions like excessive vegetation or shade, in association with incoming solar irradiance, can result in cooler ground temperatures. A site such as EPO where changing conditions can be continually monitored allows scientists to connect ground and air temperatures, particularly in an effort to investigate past climate changes.

2. An example from our studies - Repeat Logging

Basic aspects of using borehole temperatures to understand surface temperature change.

The process of diffusion of surface temperature into the subsurface can further be used to examine changing surface air temperature (SAT). This is due to changes in SAT (and hence surface ground temperatures) being seen as transient departures from the background, steady-state thermal regime measured in boreholes. The figure to the right illustrates a hypothetical, fluctuating SAT plotted with the mean as zero (a) with ground temperatures measured at three distinct times (1956, 1976, and 1998) as indicated by triangles. Measurements of a borehole temperature-depth profile at these three times are shown with respect to the linear, background thermal gradient (b). As the SAT changes (in this case it warms, cools, and warms again) the transient anomaly responds accordingly. Because the transient component is often small, it is convenient to remove the background thermal gradient and present the transient anomaly as reduced temperature (c) with an expanded temperature scale.
It is most informative to examine temperature changes between logs by differencing them relative to the initial log (d). Differencing allows one to eliminate changes not related to climate, particularly if the air temperature can be modeled to fit the changes seen in the differenced temperature. For more information, see our publications and presentations pages and look for repeat logging presentations and papers.

3. Analyzing EPO data

Meteorological variables and ground temperatures at EPO for the year 2007.

The figure to the left shows meteorological variables and ground temperature measured at EPO during 2007. The annual wave of SAT (a) is seen to be highly fluctuating, and generally cooler than the ground temperatures. Ground temperatures can be seen to follow the SAT, with similar patterns of temperature change throughout the year, especially for the shallow measurements. The attenuation and the phase lag of the ground temperatures are more apparent in the deeper measurements. The amplitude of the temperature fluctuation at 1 m is greatly subdued not only annually, but high frequency variation is not seen at shorter time scales. There is also a notable lag in the time of peak SAT and peak ground temperature at 1 m.

Other available data include precipitation in the form of both rain (b) and snow (c), as well as solar insolation (d). Precipitation at EPO is very low, with small rain events throughout the year (b). The majority of the precipitation comes in the form of snow, with some years having little snow (e.g., 2006 – 2007) and others having considerably more (e.g., 2007 – 2008; c). Solar radiation at the site varies throughout the year (d) and is the primary driver of temperatures recorded at EPO (Putnam and Chapman, 1996; Bartlett et al., 2006).

4. Analyzing EPO data II

Air and ground temperatures at EPO over the course of one week in October of 2009. The phase lag and attenuation of the air temperature is clearly seen in the subsurface temperatures.

A closer examination of the SAT and the ground temperatures are shown in the figure to the right. Over the course of one week, the SAT has a daily variation of greater than 10°C and is highly variable throughout the day. The shallowest ground temperature at 2.5 cm depth follows the general trend seen in the SAT, but with an obvious time lag. The ground temperature at 2.5 cm also is much warmer at its peak than the SAT. This observation can be directly related to the heating the granite surface at EPO receives from the incoming solar radiation (Putnam and Chapman, 1996). Also notable is the attenuation of the ground temperatures with depth when compared with the SAT (figure - right).