Improved spatial monitoring of air temperature in forested complex terrain: an energy-balance based calibration method

Fiber-optic distributed temperature sensing (DTS) systems have great potential for spatial monitoring in atmospheric boundary layer, hydrologic, and forestry applications. These novel systems have an advantage over arrays of conventional individual temperature sensors in that thousands of quasi-concurrent temperature measurements may be made at 1 meter increments along the entire length of a fiber several thousands of meters long by a single instrument, thus increasing measurement precision and spatial coverage substantially. However, like any other temperature sensor, the fiber temperature is influenced by energy exchange with its environment, particularly by radiant energy (short and longwave) modulated by wind speed.

The objective of this study is to develop a physical model of the energy balance of the DTS fiber, which can then be applied to compute the true air temperature for measurements in open and forested environments. Such observations will encourage the visualization of spatial temperature fields and facilitate the computation of advective transport of sensible heat on micro to mesoscales.

To better understand the physics controlling a fiber temperature reported by the DTS, laboratory trials of alternating black and white fiber-optic cables were installed on vertical wooden jigs inside a recirculating wind tunnel. A constant irradiance from six 600W halogen lamps was directed on a two meter section of fiber to permit controlled observations of the resulting temperature difference between the black and white fibers and a temperature reference as wind speed was varied. The net short and longwave radiation balance of each fiber was measured with an Eppley pyranometer and Kipp and Zonen pyrgeometer. Additionally, air temperature at a high accuracy was recorded from a platinum resistance thermometer in an aspirated radiation shield, and sonic anemometers were positioned to record wind speed and turbulence. Relationships between the temperature excess of each fiber, net radiation, and wind speed were developed and will be used to derive correction terms necessary to determine air temperature.

Preliminary results indicate 1) the temperature differences between a fiber and the temperature reference and between the fibers themselves increases with increasing radiation and decreases with increasing wind speed as expected, and 2) the temperature difference between an aspirated, radiation-shielded thermometer and a fiber is likely to be several degrees in typical forest conditions despite the small diameter of the fiber (0.9 mm).

Our ultimate goal is to use atmospheric DTS measurements of 3D temperature fields in a small steep-walled forested watershed to gain a better understanding and rigorous description of the processes of advective transport of sensible heat including gravity-driven cold air drainage in the canopy. Such knowledge will assist in the interpretation of observed trace gas and energy fluxes, as well as biological responses.