ARL and Forest Meteorology -- the Past, the Future, and the Interim.

ARL and Forest Meteorology – the Past, the Future, and the Interim.

Bruce B. Hicks NOAA/ARL/ATDD, Oak Ridge, TN 37830

The arrival of the atomic age caused a rethinking of the atmospheric sciences. Whereas the focus had previously been on how changes in the atmosphere affected people and society, there was suddenly a new awareness of a need to consider how the activities of mankind could affect the atmosphere. The atmospheric resource on which mankind depended was seen to be at risk. The Air Resources Laboratory grew out of this perception, with an initial focus on radioactivity but quickly extending to all forms of pollution that affect the air we breathe.

While early work concentrated on understanding where and how material injected into the atmosphere was dispersed, concerns about the residence time of injected pollutants immediately led to studies of the rate at which they are removed from the air — mainly by scavenging by precipitation and by turbulent (and gravitational) exchange when it is not raining. The ability of the surface to capture pollutants coming into contact with its soils, vegetation, buildings, and so on, became a major issue, with awareness that the relevant research needed to accommodate several extreme cases. In particular, early studies focused on the air-surface exchange properties of the oceans, arid surfaces (like those in which nuclear facilities and experiments were concentrated), and forested areas like those where rates of atmospheric deposition were anticipated to be higher.

Walker Branch Watershed tower studies

Forest meteorology emerged as a major focus of research in ARL in the 1970s, with the identification of the Walker Branch watershed (at Oak Ridge, TN) as a site for studies of a forest environment typical of most of the eastern USA. The forest was classified as mixed deciduous, with a dominance of tulip poplar and oak. A tall tower was erected, extending through the canopy up to a height of about 55 m, instrumented with standard meteorological sensors enabling regular intensive investigation. The leader of this activity was Dr. Boyd Hutchson, with team members including Detlef Matt, Bob McMillen and Dennis Baldocchi. The products of his team’s research included descriptions of leaf area distributions and the variables that became characteristic meteorological indices – roughness lengths and displacement heights. However, it was the exploration of leaf angle distributions and consequences of the penetration of solar radiation through the foliage that became a signature achievement of this work.

In the 1980s, concerns about “acid precipitation” generated a need to consider the deposition of airborne trace gases and particles from the air to foliage. This resulted in an expanded and more pragmatic forest meteorology program. To the dismay of forest meteorology purists, the focus of Walker Branch research shifted from the physical and biological aspects of the forest-atmosphere interaction to the role of forests as sinks for harmful atmospheric pollutants. ARL served as a forest meteorology “host” for many collaborative studies, with leading researchers from the national laboratories and universities. The ARL forest meteorology capabilities were a component of many acid deposition programs elsewhere, e.g. in Germany, Brazil, and Canada. This “dry deposition” attention was in direct line with the original focus of ARL – on the processes that controlled the atmospheric inventory of contaminants injected as a result of human activities. Once again, national attention was on the danger confronting the atmospheric resource on which society and the environment depended.

At the time of the transition from academic investigations of the forest environment to its role as a cleanser of the atmosphere, ARL produced a major collection of research contributions that soon became a standard reference source for forest meteorologists of all kinds — “The Forest-Atmosphere Interaction” (B. A. Hutchison and B. B. Hicks, eds.). This volume brought together the latest thinking of many scientists into a single volume that served (and continues to serve) as an entry point into explorations of the forested environments that typify much of the global land mass. It also underscored the use of micrometeorological eddy covariance methods in studies of air pollution mitigation by forests, leading to work that rapidly grew as direct measurements of carbon dioxide uptake by forests became possible using methods that were initiated by ARL at its Walker Branch location.

The era of single-tower forest measurement in ARL came to a hiatus in the 1990s, when interest was diverted to forested areas rather than to a specific forested location. Airborne eddy covariance methods permitted examination of fluxes over terrain not readily addressable using multiple towers, The methods developed by ARL were employed in many studies conducted as multi-organizational efforts through the subsequent decades, making use of numerous airborne systems, both piloted and, more recently, unmanned vehicles.

The CHESS location and observations.

In the early 2000s, the Walker Branch site was threatened by plans for an extensive nuclear research facility, to be located a short distance upwind. In reaction to this, and with awareness of the rapidly evolving need to improve the depiction of natural forest environments in increasingly sophisticated numerical models, a new location was located some 10 km distant. A 60 m tower was set up, instrumented with sonic anemometers and multiple temperature sensors at 5 m intervals. After several years of measurement as the forest grew around the tower, the facility is now being upgraded by the installation of carbon dioxide and water vapor sensors.

In the meantime, the dataset already accumulated has proved exceedingly revealing. Examples of recent analyses of CHESS data will be given, focusing on (a) the variability of the classical micrometeorological “constants” z0 and d, and (b) on the behavior of sweeps and ejections detected using the tower temperature data.

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