Ned Patton

Project Scientist
TIIMES - MMM
BEACHON

Contact Information:
PO Box 3000, Boulder, CO 80307-3000
Office: FL3 - 3056
Telephone: 303-497-8958
Email: patton@ucar.edu
Home Page

Project Summary:

Click on picture to view the entire figure.

CHATS Field Site
(photo credit: Ned Patton)

Ned's Photos from CHATS

CHATS:

I was scientifically in charge and responsible for organizing/coordinating the Canopy Horizontal Array Turbulence Study (CHATS). The key focus of this experiment was to establish the impact of vegetation and the clustering tendency of branches/leaves on within-canopy turbulence. We measured the flow field in a novel way which will allow for the testing of previously proposed models of within- and above-canopy turbulence. The intent is to guide efforts aimed at improving models of canopy-modified turbulent flows. Although there is little impact on the fluid dynamics between times when there are no leaves on the trees and times when there are leaves, canopies in full-leaf can have a dramatic impact on the source/sink distribution of scalars. Of particular interest is the ability for the canopy to modify the stability of the within-canopy layers compared to that aloft and the impact on turbulent motions and the transport of biogenic species. There are a number of sub-foci that emerged during the course of designing the experiment. Some of these include:

1. Linkages between fluctuating pressure fields and canopy structures,

2. Impact of canopy turbulence on the transport of biogenic species on the production of secondary organic aerosols,

3. Impacts of vertically distributed foliage and its dynamical and radiative impact on sub-canopy diffusion,

4. Linkages between larger-scale flows and canopy exchange of momentum, heat, water, CO2 and other more reactive compounds.

Click on picture to view the entire figure.

CHATS Field Helicopter Flyover.
(photo credit: Ned Patton)

CHATS took place in a walnut orchard just north of Dixon, CA. CHATS involved four separate funding sources (NSF OFAP, NSF/NCAR/EOL/TIIMES/BEACHON, the Army Research Office, and the Arizona State University), about twenty NCAR scientists/employees, and seven different university collaborators and their associated groups. Organizing and planning this research program took a substantial amount of time and effort, however I anticipate ground breaking science to come from the measurements taken during campaign.

Roughness Sublayer:

Myself, Roger Shaw (University of California, Davis) and John Finnigan (CSIRO, Australia) made great strides this year in characterizing the dominant organized eddy motion responsible for the exchange of momentum, heat and scalars in the roughness sublayer (RSL). We have proposed a model that qualitatively explains the unique features found in the RSL (compared to the inertial layer above) and the mechanisms responsible for generating the characteristic eddy structure. Due to vertically distributed momentum absorption by canopy drag, the mean velocity profile exhibits an inflection at the canopy top. This inflection in the velocity profile selects the scale of the most amplified two- and three-dimensional disturbances and relates them to the vorticity thickness at the canopy top. 'Stuart' vortices generated by the instability in the velocity profile can be deflected both upward and downward by turbulent motions. Downward deflected vortex loops (or hairpins) experience greater rotation and straining by the mean shear and consequently their vorticity is amplified more-so than upward deflections. These head-down hairpin vorticies are associated with sweep events (high momentum being brought downward) which have been shown to be responsible for between 60% and 80% of the exchange between the canopy layers and aloft. We presented a paper on this topic at the 17th AMS Boundary Layers and Turbulence meeting in San Diego in May, 2006, and are actively writing a manuscript on this topic which will be submitted to the Journal of Fluid Mechanics in the upcoming months. Ultimately we hope to develop a low-dimensional model incorporating these effects for use in models that can not resolve canopy turbulence.

Canopy Drag code:

Last year, I extended NCAR's curvilinear-LES code to be able to resolve the impact of canopy drag. This new tool allows for the simulation of turbulence interacting with both orography and vegetation. It is imperative to note that resolved vegetation is not the same as specifying a roughness length. The pressure drag associated with the canopy can make the hill appear steeper than the same hill with an equivalent specified roughness, which can force the flow to separate in the hill-lee for hills that would not otherwise induce flow separation. This year that code was extend such that the canopy can now act as a source/sink of both passive and active scalars. This extension is critical to assess the impact of atmospheric stability on canopy-resolving flows over hills. It is also an essential step in the quest to parameterize the impacts of canopy-covered hills on turbulent exchange of momentum, heat, moisture and reactive species in larger-scale simulations that can not resolve turbulence.

Dispersion of massless particles in complex boundary layers :

In collaboration with Jeff Weil (CIRES, University of Colorado, Boulder) and Peter Sullivan (NCAR), we are actively researching dispersion of massless particles in complex boundary layers. We have made substantial progress on the first component of this research where we are studying the impact of atmospheric stability on dispersion. A key finding from this research is that under stably-stratified flow conditions, the mean wind profile remains ``unmixed" which has distinct ramifications for dispersion of elevated particle releases. In particular, cross-wind integrated concentration profiles exhibit a more rapid downward dispersion than upward which is caused by stronger turbulence below the source. Stratification therefore also impacts the height to which the plume grows. For the case we've been focusing on, the mean plume height for surface releases are found to reach the middle of the 200m-deep boundary layer at a distance of 64km downstream. That said, however, the growth rate is faster than predicted by surface-layer similarity theory. We are now actively working on the second component of this research investigating the impact of vegetation on dispersion.

Drainage flows on carbon dioxide exchange:

In collaboration with researchers at the University of Texas, San Antonio (Shaolin Mao, Zhi-Gang Feng and Stathis Michaelides) we are looking at the impact of drainage flows on carbon dioxide exchange. A manuscript has been submitted to Boundary-Layer Meteorology discussing the impact of an imposed wall-jet (similar to what might form in the case of drainage flows) on carbon dioxide exchange. The manuscript focuses on the impact of such a drainage flow on point measurements taken above the canopy, which has importance for interpretation of ongoing long-term CO2 measurement campaigns in complex terrain (such as AmeriFlux, FluxNet and NCAR's CME).

Vertical transport & mixing:

In collaboration with Jordi Vilà (Wageningen University, The Netherlands), Mary Barth (NCAR), and Si-Wan Kim (NOAA Aeronomy Lab, Boulder) we are continuing our research on the ability for shallow cumulus clouds to enhance vertical transport and mixing and to scatter solar radiation. Last year, we reported on the impact of clouds on a simple set of chemical reactions. We have now extended that work to incorporate the full MOZART chemical mechanism. Our earlier research showed that the dilution of the PBL due to the deepening of the PBL by clouds can decrease reactant mixing ratios by 10-50 percent compared to cases with no clouds. Additionally, we found that clouds transport chemical species to higher elevations and can be left in the residual layer following the afternoon collapse of the PBL. We found that a mass-flux parameterization accurately predicts within-cloud species transport. We also found that scattering of radiation by cloud drops substantially affect local mixing ratios, but have little effect on time- and spatially-averaged mixing ratios. Analysis of the results with the more-complete chemistry mechanism are on-going.

PBL Turbulence:

In collaboration with Jordi Vilà at Wageningen University, The Netherlands, we are investigating the impact of land-surface heterogeneity and the strength of the capping inversion on the rate of entrainment by PBL turbulence.

Impact of vegetation on vertical scalar transport:

With colleagues at the Pennsylvania State University (Weiguo Wang and Ken Davis) we investigated the impact of vegetation on vertical scalar transport. We used measurements taken on a 400m tall tower to evaluate the top-down/bottom-up gradient and variance functions that I previously proposed from large-eddy simulation results. The observations show less canopy-induced modification than the previously presented numerical results. There are a number uncertainties in the data, but we attribute most of the differences to the canopy's architecture and source/sink distribution. A manuscript on this topic appeared recently in Boundary-Layer Meteorology.

Flow heterogeneity:

In collaboration researchers at Yale University (Xuhui Lee and his postdoc Jianping Huang), we are working on a project to establish a mechanistic understanding of the interplay among flow heterogeneity in the atmospheric boundary layer (ABL), land surface heterogeneity, and the vegetation-air fluxes of energy, water and CO2. We are investigating mechanisms by which mesoscale motions in the ABL influence vegetation-air exchange. It is hypothesized that two important ABL processes - entrainment and flow heterogeneity - cause biases in the observation and model estimates of vegetation-air exchange and that the degree of bias is different for active (heat and water) and passive (CO2) scalars. We recently submitted a manuscript to Boundary-Layer Meteorology on this topic and are working on two others.

Sound Propagation using LES:

ollaboration with researchers at the U.S. Army Cold Regions Research and Engineering Laboratory (D. Keith Wilson, Edgar Andreas, John Weatherly), we recently published a manuscript in the Journal of the Acoustical Society of America - Express Letters, discussing predictive skill for outdoor sound propagation which was assessed using high-resolution atmospheric fields from large-eddy simulations. Propagation calculations through the full LES fields were compared to calculations through subsets of the LES fields that have been processed in typical ways, such as mean vertical profiles and instantaneous vertical profiles synchronized to the sound propagation. It was found that mean sound pressure levels can be predicted with low errors from the mean profiles, except in refractive shadow regions. Prediction of sound pressure levels for short-duration events is much less accurate, with errors of 8-10 dB for near-ground propagation being typical.

Community Service:

• K-12 Activiy: Hosted thirty third grade students for an afternoon at CHATS. Described the experiment, why we were there, and answered numerous questions., Elementary School from Davis, CA, Dixon, CA, USA
• Member - Boundary Layers and Turbulence, American Meteorological Society (AMS)

Presentations:

• Turbulent flow over isolated ridges; influence of vegetation, , Davis USA, November 2006

TIIMES External Collaborators:

Gene Allwine, Washington State University
Roni Avissar, Duke University
Walter Bach, Army Research Office
Ronald Calhoun, Arizonal State University
Omduth Coceal, University of Reading
Ken Davis, The Pennsylvania State University
Steven Edburg, Washington State University
Ian Faloona, University of California
John Finnigan, Commonwealth Scientific and Industrial Research Organisation (CSIRO)
Sara Friedman, Massachusetts Institute of Technology
Ian Harman, Commonwealth Scientific and Industrial Research Organisation (CSIRO)
Jian-Ping Huang, Yale University
John Kain, National Severe Storms Laboratory (NSSL) - NOAA
Jan Kleissl, University of California
Brian Lamb, Washington State University
Xuhui Lee, Yale University
Stathis Michaelides, University of Texas
Kyaw Tha Paw U, University of California
Roger Shaw, University of California, Davis
Jordi Vila, Wageningen University
Weiguo Wang, Pacific Northwest National Laboratory
Jeffrey Weil, University of Colorado
D. Keith Wilson, Cold Regions Research and Engineering Laboratory

Publications:

Huang, J., X. Lee, E. G. Patton, 2007: A modeling study of flux imbalance and the influence of entrainment in the convective boundary layer. Bound.-Layer Meteor.. (Submitted)

Wilson, D. K., E. L. Andreas, C. L. Pettit, E. G. Patton, P. P. Sullivan, J. W. Weatherly, 2007: Characterization of uncertainty in outdoor sound propagation predictions. J. Acoust. Soc. Am. Ex. Lett., 121(5), EL177-EL183, doi: 10.1121/1.2716159.

Wang, W., K. J. Davis, C. Yi, E. G. Patton, M. P. Butler, D. M. Ricciuto, P. Bakwin, 2007: A note on the top-down and bottom-up gradient functions over a forested site. Bound.-Layer Meteor., 124, 305-314, doi: 10.1007/s10546-007-9162-0.

Mayor, S. D., S. M. Spuler, B. M. Morley, S. C. Himmelsbach, R. A. Rilling, T. M. Weckwerth, E. G. Patton, D. H. Lenschow, 2007: Elastic backscatter lidar and in situ observations of sea-breeze fronts in Dixon, California. 7th Conf. on Coastal Atmospheric and Oceanic Prediction and Processes, San Diego, CA, US, AMS.

Wilson, D. K., E. L. Andreas, C. L. Pettit, E. Patton, P. P. Sullivan, J. W. Weatherly, 2006: On the predictability of sound propagation from vertical profile observations. 12th Intl. Symp. on Long Range Sound Propagation, New Orleans, LA, US.