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Sun J.,U.S. National Center for Atmospheric Research | Lenschow D.H.,U.S. National Center for Atmospheric Research | Mahrt L.,Oregon State University | Nappo C.,CJN Research Meteorology
Journal of the Atmospheric Sciences | Year: 2013

Relationships among the horizontal pressure gradient, the Coriolis force, and the vertical momentum transport by turbulent fluxes are investigated using data collected from the 1999 Cooperative Atmosphere-Surface Exchange Study (CASES-99). Wind toward higher pressure (WTHP) adjacent to the ground occurred about 50% of the time. For wind speed at 5 m above the ground stronger than 5 m s-1, WTHP occurred about 20% of the time. Focusing on these moderate to strong wind cases only, relationships among horizontal pressure gradients, Coriolis force, and vertical turbulent transport in the momentum balance are investigated. The magnitude of the downward turbulent momentum flux consistently increases with height under moderate to strong winds, which results in the vertical convergence of the momentum flux and thus provides a momentum source and allows WTHP. In the along-wind direction, the horizontal pressure gradient is observed to be well correlated with the quadratic wind speed, which is demonstrated to be an approximate balance between the horizontal pressure gradient and the vertical convergence of the turbulent momentum flux. That is, antitriptic balance occurs in the along-wind direction when the wind is toward higher pressure. In the crosswind direction, the pressure gradient varies approximately linearly with wind speed and opposes the Coriolis force, suggesting the importance of the Coriolis force and approximate geotriptic balance of the airflow. A simple one-dimensional planetary boundary layer eddy diffusivity model demonstrates the possibility of wind directed toward higher pressure for a baroclinic boundary layer and the contribution of the vertical turbulent momentum flux to this phenomenon. © 2013 American Meteorological Society.

Nappo C.,CJN Research Meteorology | Sun J.,U.S. National Center for Atmospheric Research | Mahrt L.,NorthWest Research Associates, Inc. | Belusic D.,Monash University
Bulletin of the American Meteorological Society | Year: 2014

Internal gravity waves not only effectively influence global atmospheric circulations in the middle and upper atmosphere, but also generate turbulence in the stably stratified atmospheric boundary layer, which leads to the transport of momentum, heat, aerosols, and biogenic and anthropogenic gases. One of the most challenging problems facing atmospheric modelers is the parameterization of turbulence and gravity waves in the stable planetary boundary layer (PBL). Currently numerical models perform much worse for the stably stratified nocturnal PBL than the daytime convective PBL. Discontinuities in density and wind speed can generate unstable Kelvin-Helmholtz waves that break down into turbulence. Waves in the PBL that persist for several cycles with near-constant frequency and amplitude are often observed to appear intermittently throughout a night. The majority of these waves propagate horizontally within ducts or waveguides extending from the ground surface to an upper reflecting level.

Arnqvist J.,Uppsala University | Bergrstrom H.,Uppsala University | Nappo C.,CJN Research Meteorology
Agricultural and Forest Meteorology | Year: 2016

In this paper, we document the existence of wave-like motions above a forest canopy using data taken from a 138. m high tower placed within a forest. Characteristics of the waves are examined in relation to their possible effects on wind energy. It is shown that when the wave signal is relatively clean, the phase lag between horizontal and vertical velocity is close to 90°, which limits the contribution of the waves to the downward momentum flux. Numerical solutions of the linear wave equations agree with measurements in terms of wave period and the vertical shape of the wave amplitude. Linear analysis show that shear instability is the cause of unstable wave growth, and that the fastest growing unstable wave number typically has a period of 10-100. s. In addition to the shear instability, the linear analysis predicts that under certain conditions instabilities of the Holmboe kind can develop over forests. © 2015 Elsevier B.V.

Aylor D.E.,U.S. Department of Soil and Water | Schmale D.G.,Virginia Polytechnic Institute and State University | Shields E.J.,Cornell University | Newcomb M.,Virginia Polytechnic Institute and State University | Nappo C.J.,CJN Research Meteorology
Agricultural and Forest Meteorology | Year: 2011

A means for determining the aerial concentration, C (sporangiam-3), of plant pathogenic spores at various distances from a source of inoculum is needed to quantify the potential spread of a plant disease. Values of C for Phytophthora infestans sporangia released from an area source of diseased plants in a potato canopy was quantified in three ways: (1) by using Rotorods to sample the air just above the source, (2) by using unmanned aerial vehicles to sample the air at altitudes up to 90m above the source and at downwind distances up to 500m from the source, and (3) by using a Lagrangian stochastic simulation of sporangia flight trajectories to tie these two measurements together. Experiments were conducted using three potato crops over two years. Model predictions of time-average, crosswind-integrated concentrations were highly correlated (r=0.9) with values of C measured using the unmanned aerial vehicles. The model describes the release and dispersal of sporangia from a potato canopy to a downwind distance of 500m. Thus, it may have utility as a part of an area-wide decision support system by helping to predict risk of disease spread between neighboring or distant potato fields. © 2010 Elsevier B.V.

Nappo C.J.,CJN Research Meteorology | Hiscox A.L.,Louisiana State University | Miller D.R.,University of Connecticut
Boundary-Layer Meteorology | Year: 2010

This note presents initial results of an analysis of the stationarity of turbulence kinetic energy and the persistence of winds within the stable boundary layer (SBL). Measurements were made at 1.5 and 11 m above ground level from 0100 to 0600 local time on five nights during the JORNADA field experiment. The average stationarity ranged from about 160 to about 570 s. Wind persistence ranged from about ± 40° (3-min average) to about ± 36° (30-min average) on a weakly stable night, and from about ± 40° (3-min average) to about ± 27° (30-min average) on an strongly stable night. It is shown that, at 1.5 m, which we take to be within the surface layer, the average duration of stationarity of turbulent kinetic energy tends to correlate with the kurtosis of the heat flux; however, at 11 m, which we take to be outside of the surface layer, this correlation is poorly approximated. © 2010 Springer Science+Business Media B.V.

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