Weak-wind flows in plant canopies

Advanced Resolution Canopy FLow Observations (ARCFLO)

arcos-schematicInvestigators

Sponsored by

nsf-logo
National Science Foundation, CAREER Award 0955444


Objective(s)

The project will study how air moves in plant canopies such as crops and forests where winds are usually relatively weak, and how it affects the transport of heat, water, and momentum. The research component includes new field observations across a large spectrum of sites with contrasting characteristics of canopy architecure and terrain including the OSU Botany & Plant Pathology Field lab, the HJ Andrews Experimental forest, and two AmeriFlux Sites in Oregon. The findings from this unique set of experiments will be the key step toward the long-term goal to develop a novel improved framework to describe the flow and its transport under weak-wind conditions for a continuous variation of overstory density and stratification. Observations will be made with a unique combination of new and classic micrometeorological techniques including optical fiber measurement of temperature structure (DTS), acoustic remote sensing (SODAR), ultrasonic anemometers, thermo-/ hygrometers, and laser-illuminated flow visualizations as illustrated in the schematic experimental setup.

Background, broader impacts

The level of scientific understanding of weak-wind transport is very limited, and commonly used forecast tools and mathematical formulations don't apply. This information is also needed to correctly predict the spread of pollutants or contaminants in nocturnal atmospheric conditions, and to more precisely estimate carbon sequestration and evapotranspiration rates from tall vegetation. The broader impacts of the project are improved formulations of surface fluxes for regional and large-scale weather and climate models, as well as dispersion and diffusion models. A set of practical recommendations for the applied flux community will be formulated to reduce uncertainties in carbon and energy budgets, and to better predict the water availability in forests. In addition to a graduate student, an Oregon K-12 high school teacher will be working with the PI through a partnership with the Oregon Natural Resources Education Program at OSU. The PI will also work with the Willamette National Forest to improve management decisions for fire fighting.

Valley Circulation Experiment at HJ Andrews Forest started

VALCEX team at HJA Primet stationResearch: The VALey Circulation EXperiment (VALCEX) at the HJ Andrews Experimental Forest started on October 15 as a precampaign to next year's Advanced Resolution Canopy FLow Observations (ARCFLO) 2012 experiment in Watershed 1 funded by the NSF Career grant. The objective of the precampaign is to gain a better understanding of the airflow in the larger HJA valley, particularly its diurnal dynamics, vertical structure, and cold-air drainage and pooling. 

Jerilyn installing a sonic

The flows in the forested valley are extremely weak year-round and are strongly influenced by the topography. The bmM group installed two acoustic profilers (Sodars) and two sonic anemometers to measure height-dependent wind speed, wind direction, and turbulence from 7 m to approximately 400 m above the valley floor in high temporal resolution. Jerilyn Walley from The Evergreen State College in Olympia, WA will do the data analysis as her MS research project.(Picture left) The VALCEX team: Jerilyn Walley, Evan Hayduk, Chris Thomas, John Moreau (f.l.t.r). (Picture right) Jerilyn installing a sonic anemometer to measure wind speed and direction.

Accounting for canopy coupling improves NEE estimates in dense, tall forests

New framework to compute NEE in dense forestsNew publication: A new paper by Thomas et al. published in Agricultural and Forest Meteorology describes a novel approach of how to improve annual and seasonal estimates of net ecosystem exchange (NEE) of carbon dioxide by analyzing the degree of communication of air between the sub-canopy, canopy, and above-canopy air layers. Particularly dense forests suffer from persistent decoupling between these air layers because of the dense foliage acting as a mechanical barrier, but also because of the low wind speeds suppressing turbulent motions that mix the air. Using year-round observations of the flow, turbulence, and carbon and water fluxes at two levels collected at a Douglas-Fir site in Oregon's Coast Range over a period of 6 years we developed a framework and an associated set of equations to derive biologically meaningful NEE estimates for these dense forests, which are underrepresented in global analysis. The novelty of this framework is to indirectly account for advective losses of ecosystem respiration, which act to overpredict the net carbon uptake, when the layers are decoupled. The NEE estimates from this improved framework reduce the carbon sink strength of this high-leaf area index forest by approximately 50 % compared to estimates from traditional single-level observations and data processing.

The full citation is:

Thomas, C.K., Martin, J.G., Law, B.E., Davis, K., 2013. Toward biologically meaningful net carbon exchange estimates for tall, dense canopies: multi-level eddy covariance observations and canopy coupling regimes in a mature Douglas-fir forest in Oregon. Agric. For. Meteorol. 173, 14–27. DOI:10.1016/j.agrformet.2013.01.001

Bright Air

BRIGHT AIR

Thermal Image of Watershed 1 at HJ Andrews showing the nocturnal temperature gradient

The Biomicrometeorology (BMM) group has been studying the nocturnal boundary layer dynamics of mountain valleys. One area of particular interest is the dynamics of cold air pools. Cold air pools, or inversions, are of interest for many reasons. One of the most well known is that these layers can trap contaminants/pollutants. In urban environments this can lead to health hazards for large populations. In rural environments, such as forested portions of mountainous regions, a better understanding of the dynamics of cold air pools can lead to better estimates of surface fluxes (e.g. carbon, momentum, heat, etc..) that in turn can lead to improved large scale models of weather and climate.

In this study a thermal IR camera is used to remotely sense the foliage temperature in a mountain valley. On clear windy nights one can assume that the foliage temperature is a proxy for the air temperature at a given location. Based on this assumption it is possible to map some of the dynamics of the atmosphere in the valley. Filtering and processing the raw imagery will be challenging. The camera will sense all the radiative flux in its field of view. However, not all of the flux sensed will have originated from the object of interest which, in this case, is the forest canopy. All flux not originating from the forest canopy must be filtered out. Once the image has been filtered it must be georeferenced and orthorectified before useful analyses can be performed.  After geoprocessing is complete, the BMM group will have a georeferenced time series of the temperature gradient for an entire forested mountain valley. 

For more thermal imagery and videos relating to this project, take a look at our Bright Air Channel on YouTube.

 

Investigators:

Christoph Thomas (PI)

Chris Johnson

Experimental energy and gas exchange ATS546

Snapshots from ATS546

Advanced undergraduate and graduate level class on field experimental methods to quantify the atmospheric carbon dioxide, water, methane, heat, momentum, and radiative exchange at the vegetation-land-ocean-air interface. Techniques include bulk and gradient approaches, and eddy covariance. The central activity consists of student teams designing and conducting a field experiment, analyzing and interpreting observations, and presenting results. The class is delivered as a mix of introductory lectures (2 weeks), design and calibration of instrument in the laboratory (2 weeks), field deployment and data collection (1 to 2 weeks), data analysis and interpretation in the computer lab (2 weeks), and presentation of group and individual projects (1 week).

Instructor: Christoph Thomas will teach this class in spring term 2013, 2015, etc

 

Course highlights:

Please contact us if you consider taking this class or if you have any questions about it content. 

How to build an effective, economic radiation shield for thermometers

Thomas & Smoot (2013):Radiation shield intercomparison using thermal Infrared camerasNew publication: We present the design and evaluate the performance of a double-walled electrically aspirated radiation shield for thermometers measuring air temperature and its gradients in the atmospheric surface layer. Commercially available shields are often cost-prohibitive and require significant power, which aggravates their use in remote sensor networks. We performed rigorous tests to quantify its solar radiation error and wake production, and to characterize the observer effect of the forced aspiration on vertical temperature gradients in the calm and stable boundary layer. Construction requirements were to design a unit that uses inexpensive off-the-shelf components, to assemble easily, to facilitate reconfiguration to accommodate various sensors, and to reduce power consumption with the goal of reducing costs and enabling use in sensor networks in remote locations. The cost savings compared to a commercially available triple walled shield of equal performance are on the order of several hundred dollars since the presented shield costs only about 50 USD.

Full citation:

Thomas, C.K., Smoot, A.R., 2013. An effective, economic, aspirated radiation shield for air temperature observations and its spatial gradients. J. Atmos. Ocean. Technol. 30, 526–537. DOI: 10.1175/JTECH-D-12-00044.1

New paper on scalar transport in forests

Coherent structure transport of respiration pulses, Zeeman et al (2012) New research paper: This paper links the turbulent transport in forest canopies via organized motion (coherent structures) to the carbon cycle. It explores the transport path of respiration pulses originating from the forest floor and the sub-canopy vegetation to better predict daytime ecosystem respiration, which is a highly sought-after quantity that is not readily available from eddy covariance measurements that only provide the net ecosystem exchange. Methods included state-of-the-art wavelet detection and analysis of coherent structures and a relaxed eddy accumulation techniques applied to turbuence time series collected at multiple levels at 3 forests in the US and Europe. The study's main finding is that the respiration pulses are mainly transported by coherent structures, which may help refine future conditional sampling approached to improve estimates of carbon cycle components in forest ecosystems.

Full citation:

Zeeman, M.J., Eugster, W. and Thomas, C.K., 2012. Concurrency of coherent structures and conditionally sampled daytime sub-canopy respiration. Boundary-Layer Meteorol.: in press. 10.1007/s10546-012-9745-2.

New pressure sensor network in forest deployed

Pressure sensor and sonic anemometerNew field experiment: A new kind of environmental sensor that can measure the barometric pressure and its fluctuation at a very fast rate and with very high precision was deployed today. The sensors were deployed in a network consisting of 3 identical instruments mounted in a vertical profile on a 140 ft tall tower in a dense Douglas-fir forest in the Oregon Coast Range. It is part of the Advanced Resolution Canopy FLow Observation (ARCFLO) 2013 experiment funded by the National Science Foundation as the central experimental activity of Chris Thomas' CAREER award. The series of ARCFLO experiments aims at identifying the spatial and temporal dynamics of weak-wind flows in vegetated canopies to help build an integrative theoretical framework for airflow and transport that is valid for a continuous variation of surface conditions and stratification.

Pressure sensor test on the OSU campus

As inlet for the barometric pressure signal serves a so-called pressure port (white instrument on the left in the upper picture), which consists of 4 parallel discs around a cylinder to align the airflow horizontally and to minimize artifacts from the fluctuating airflow. The pressure sensors are interrogated 20 times per second (at 20 Hz) and have a resolution of 1/100 of one Pascal (N m-2). Each pressure port is paired with a sonic anemometer (instrument on the right in the picture) to combine the pressure signal with that of the velocity of the airflow.

The particular goal of these novel observations is to better understand the pressure transport and its role in creating turbulence kinetic energy (TKE) in forests that are characterized by weak winds. The transport and mixing in weak-wind environments such as forests and urban areas is poorly understood, and can, e.g., aggravate the computation of ecologically meaningful estimates of carbon and water exchange between the forest and the atmosphere.

Precise measurements of barometric pressure fluctuations are very challenging and the methodology is still under development, very few studies have been reported in the literature. Prior to field deployment, the sensors were tested on the roof of Weniger Hall on the OSU campus (picture to the right).

Publications on DTS over streams and coherent structures in forests

New research paper publishedPublications: Two new research papers were recently published in collaboration with the Department of Biological and Ecological Engineering at OSU and the Department of Micrometeorology at the University of Bayreuth, Germany. The first paper was published in Water Resources Research and demonstrates the utility of Distributed Temperature Sesnsing to estimating shade over streams. The second paper analyzes horizontal and vertical transport of energy and matter by coherent structures in a tall spruce canopy. The full citations are:

 

Petrides, A.C., Huff, J., Arik, A., Van de Giesen, N., Kennedy, A.M., Thomas, C.K. and Selker, J.S., 2011. Shade Estimation Over Streams Using Distributed Temperature Sensing. Water Resourc. Res., 47: W07601.

 

Serafimovich, A., Thomas, C.K. and Foken, T., 2011. Vertical and horizontal transport of energy and matter by coherent motions in a tall spruce canopy. Boundary-Layer Meteorol.: DOI 10.1007/s10546-011-9619-z.

Thermal-IR imagery may help map cold-air drainage in complex terrain

Thermal-Infrared image of mouth of Lookout Creek valley (HJA) Research:As part of the Advanced Resolution Canopy Flow Observation (ARCFLO) 2012 study at the HJ Andrews LTER site funded by the National Science Foundation, a tandem camera system was installed on Blue River Ridge that looks down on the lower Lookout Creek Valley. The system consists of a thermal infrared (IR) and a visible & near infrared camera which take an image every 5 min. The scientific idea is to use the thermal signal of the foliage as a proxy for air temperature to study and map the dynamics of the nocturnal, weak-wind boundary layer in this mountain valley. Valley-scale cold air drainage and cold pools are important features in complex terrain. The tandem camera system also complements extensive efforts to characterize the valley-scale air circulation using acoustic ground-based remote sensing (VALCEX). The core of the experimental efforts during ARCFLO 2012 takes place in Watershed 1, where a dense network of wind, temperature, carbon dioxide, and water vapor sensors will be installed to investigate how wind, heat, carbon dioxide and water vapor communicate across the landscape in this steep mountaineous terrain. This experiment is a central reearch activity of the NSF Career award in Physical and Dynamical Meteorology.

A sequence of images taken by the tandem camera system on 27 and 28 June 2012 is shown as an exmaple:

(First panel): Cold-air drainage and pooling dominate temperatures in the bottom of the valley early at night, while higher elevations and indiviudal tall trees show up as warm.

(Second panel): Early in the morning before sunrise, temperature differences are smaller, while the general trend of lower temperatures in the bottom of the valley compared to the sloped is conserved. A secondary gradient of decreasing temperatures deeper into the Lookout Creek valley (into the foreground of the image) is noted.

(Third panel): The sunlight heats up the foliage of indivdiual tall trees towering above the main canopy and the north-facing slope of the Blue River Ridge. A very cold, cloud-topped inversion above the Blue Lake reservoir can be seen near the top of the image.

(Fourth panel): Surface heating has lead to higher temperature in the bottom of the valley.

Sequence of images taken by the tandem camera system installed on the Rd130 tower at the HJA LTER site, (c) bmm group

Welcome Javier to the group!

Javier OrozcoNew member: Javier, welcome to the bmM group! Javier is an undergraduate student in Mechanical Engineering, who will be working over the next two summers as member of our group through the 'Increasing Diversity in Earth Science' (IDES) program. IDES is a COAS program, which aims at increasing the number of students from underrepresented groups in Earth Sciences, and is sponsored through the Opportunity for Enhancing Diversity in Geosciences (OEDG) section at the National Science Foundation. Javier will be working with our new SUbcanopy Sonic Anemometer Network (SUSAN), which was also funded by the National Science Foundation to work on weak-wind canopy flows.   

Welcome Kyle to the group!

Kyle Jones

New member: Kyle joined the bmM group this summer to participate and support in the Advanced Resolution Canopy Flow Observations (ARCFLO) 2013 experiment. He will work on building, testing, and deploying the new high-resolution, fast-response pressure transducer array funded through a supplement to the NSF Career award. Kyle is an OSU student in the Enviornmental Sciences Undergraduate Program and enters his senior year this fall. He will also conduct an internship and collect research credits this coming Fall. Welcome Kyle, good to have you!   

Pre-campaign to compare effectiveness of radiation shields

NSF Precampaign for comparison of radiation shieldsA field comparison between different radiation shields for thermo-hygrometers has started at the BPP research site near Corvallis as a pre-campaign to ARCFLO-2011. The purpose of this study is to find out whether relatively inexpensive 'home-made' radiation shields constructed from PVC pipe are as effective in minizming the radiative heating error of air temperature measurements caused by sunlight as more expensive commercially available models. Data from 3 different models of thermo-/hygrometers housed in 6 different types of radiation shields including aspirated, naturally ventilated, commercially available, and homemade painted and unpainted shields will be collected over the next weeks. Our goal is to find a less obstructive design to minimize the flow distortion caused by the radiation shield that would affect readings of a closely collocated sonic anemometer and cost savings.

Christoph Thomas receives NSF Career award

NSFResearch and teaching award: Christoph Thomas received a prestigious CAREER award from National Science Foundation to work on "A new direction into atmospheric near-surface transport for weak-wind conditions in plant canopies". The project will study how air moves in plant canopies such as crops and forests where winds are usually relatively weak, and how it affects the transport of heat, water and momentum. The level of scientific understanding of weak-wind transport is very limited, and commonly used forecast tools and mathematical formulations don't apply. This information is also needed to correctly predict the spread of pollutants or contaminants in nocturnal atmospheric conditions, and to more precisely estimate carbon sequestration and evapotranspiration rates from tall vegetation. 

The CAREER (Faculty Early Career Development) program is NSF's most prestigious award in support of junior faculty who exemplify the role of teachers-scholars through outstanding research, excellent education and the integration of both components in the context of their institutions. The project will start this September and continue for 5 years.

The research component of the project includes new field observations across contrasting sites including the OSU Botany & Plant Pathology Field lab, the HJ Andrews Experimental forest, and two AmeriFlux Sites in Oregon. The results from this unique data set will be the key step toward the long-term goal to develop a novel improved framework to describe the flow and its transport under weak-wind conditions for a continuous variation of overstory density and stratification.  Observations will be made with a unique combination of new and standard micrometeorological techniques including optical fiber measurement of temperature structure (DTS), acoustic remote sensing (SODAR), ultrasonic anemometers, and laser-illuminated flow visualizations.  This research will be integrated with a strong teaching component of educational activities in the classroom, field site visits, and development of a new graduate-level field course for students from several disciplines including atmospheric sciences, forestry, engineering, and agriculture.

The broader impacts of the project are improved formulations of surface fluxes for regional and large-scale weather and climate models, as well as dispersion and diffusion models. A set of practical recommendations for the applied flux community will be formulated to reduce uncertainties in carbon and energy budgets, and to better predict the water availability in forests. In addition to a graduate student, an Oregon K-12 high school teacher will be working with Dr. Thomas on this project by partnering with the Oregon Natural Resources Education Program at OSU. He will also work with the Willamette National Forest to improve management decisions for fire fighting. 

See also the OSU press release from July 2010.