Richard M. Eckman

Observations and Numerical Simulations of Winds within a Broad Forested Valley

Abstract The Tennessee River Valley in the eastern part of Tennessee is a broad valley with a moist climate and extensive forest cover. A series of 50–100-m-high parallel ridges forms corrugations along the floor of the valley. Tower measurements and numerical simulations are used in this paper to study the channeling of the winds in this valley over the diurnal cycle. At night, pressure-driven channeling caused by geostrophic imbalances is often present, but this channeling tends to be concentrated on the leeward side of the valley (relative to the winds aloft). The channeling is significantly weaker during the afternoon, so the winds on top of the corrugations are more closely aligned with those aloft. In the bottomlands between the corrugations, however, the daytime winds are more effectively channeled parallel to the valley axis. Although thermally driven winds are not dominant in the Tennessee Valley, a discernible pattern of upvalley daytime winds and downvalley nighttime winds is observed in the to...

Re-examination of empirically derived formulas for horizontal diffusion from surface sources

Field measurements indicate that the dispersion parameter σy is proportional to xp, where x is the downwind distance and the power p is generally less than unity but greater than 12. The conventional explanation for this less-than-linear growth is that the diffusion is undergoing a transition from the near-field to the far-field limits of Taylors equation. An alternate explanation is presented in this paper for surface releases. This explanation assumes that the less-than-linear growth of σy with x is due to the logarithmic wind profile. A simple roughness- and stability-dependent diffusion model based on this explanation is shown to compare well with field measurements of σy. In neutral conditions, the model indicates that σy should be proportional to x0.86 and z00.14, where z0 is the roughness length.

Simulation of Tracer Concentration Data in the Brush Creek Drainage Flow Using an Integrated Puff Model

Abstract During the 1984 ASCOT field study in Brush Creek Valley, two perfluorocarbon tracers were released into the nocturnal drainage flow at two different heights. The resulting surface concentrations were sampled at 90 sites, and vertical concentration profiles at 11 sites. These detailed tracer measurements provide a valuable dataset for developing and testing models of pollutant transport and dispersion in valleys. In this paper, we present the results of Gaussian puff model simulations of the tracer releases in Brush Creek Valley. The model was modified to account for the restricted lateral dispersion in the valley, and for the gross elevation differences between the release site and the receptors. The variable wind fields needed to transport the puffs were obtained by interpolation between wind profiles measured using tethered balloons at five along-valley sites. Direct turbulence measurements were used to estimate diffusion. Subsidence in the valley flow was included for elevated releases. Two te...

A Pressure-Sphere Anemometer for Measuring Turbulence and Fluxes in Hurricanes

Abstract Turbulence and air-surface exchange are important factors throughout the life cycle of a tropical cyclone. Conventional turbulence instruments are not designed to function in the extreme environment encountered in such storms. A new instrument called the Extreme Turbulence (ET) probe has been developed specifically for measuring turbulence on a fixed tower in hurricane conditions. Although the probe is designed for surface deployment, it is based on the same pressure-sphere technology used for aircraft gust probes. The ET probe is designed around a 43-cm-diameter sphere with 30 pressure ports distributed over its surface. A major obstacle during development was finding a method to prevent water from fouling the pressure ports. Two approaches were investigated: a passive approach using gravity drainage and an active approach using an air pump to flush water from the ports. The probes were tested in both dry and wet conditions by mounting them on a vehicle side by side with more conventional instru...

Mass and momentum balance in the Brush Creek drainage flow determined from single-profile data

Abstract Fluxes and flux-divergences of mass and momentum in Brush Creek Valley, computed from measurements taken by Tethersondes and Doppler sodars in the 1984 ASCOT experiment, are presented. Estimates of mass influx from open sidewalls in Brush Creek, derived from concurrent tower measurements, are also given. Mass and momentum fluxes calculated from single-profile data were within a factor of 1.5 of those obtained by integrating Doppler lidar data. Flux-divergences for budget calculations should be derived from a Doppler lidar or equivalent remote sensor data, because single-profile measurements were found to have sampling errors which are too large for reliable flux divergence estimates. The mass influx from the sidewalls was insufficient to account for the mass flux-divergence in the main valley. This imbalance in the drainage flow mass budget is speculated to be due to the inflow from the small box-canyon tributaries, rather than from subsidence of air above the main valley.

Influence of the sampling time on the kinematics of turbulent diffusion from a continuous source

A kinematic description is presented of how turbulent diffusion from a continuous source varies with the sampling time in stationary, homogeneous turbulence. Unlike most previous theories, the sampling is assumed to take place at fixed downstream distances from the source. It is shown that the sampling-time effects depend on two-particle velocity statistics. Thus, time-average diffusion at fixed downstream distances is more akin to relative diffusion than to absolute diffusion. Two simple diffusion models are developed from the kinematic equations. These models are in fairly good agreement with diffusion data obtained both in a wind tunnel and in the field. Moreover, these models have significant practical implications. For example, the models indicate that care must be taken when using band-pass spectral filtering as a paradigm for turbulent diffusion. Also, the models show that the mean flow speed U has an important influence on the sampling-time effects. To account for U properly, diffusion measurements with differing sampling times Λ should be compared using the product U Λ, and not just Λ.