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J. M. Wilczak

Abstract

Ensemble vertical cross sections are derived for the velocity and temperature fields associated with large-scale eddy (LSE) temperature ramp structures in the turbulent, convective atmospheric surface layer. The turbulent fluxes of heat and momentum, as well as the third-order moments that represent the fluxes of vertical and horizontal heat, stress and turbulent kinetic energy are interpreted in terms of the circulations associated with the LSE structures. Heat flux and stress correlation coefficients are related to the phase relationships between vertical velocity, longitudinal velocity, and temperature. In particular, w′ and T′ are found to be in phase essentially everywhere. In contrast, u′ and w′ have a height dependent phase lag which leads to specific regions within the LSE structure where the momentum flux is consistently counter-gradient.

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J. M. Wilczak and J. A. Businger

Abstract

Abstract not available.

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J. M. Wilczak and J. W. Glendening

Abstract

In northeastern Colorado a frequently observed feature of the surface wind field is a stationary, terrain-induced mesoscale gyre, which is often associated with the formation of severe weather. Because of the gyre's proximity to the Denver metropolitan area, local weather forecasters frequently refer to it as the “Denver Cyclone. ” The development of one such cyclone, which occurred on 1 August 1985, is documented with mesonet, radiosonde, wind-profiler, radiometer and tower data. Mixed-layer model simulations of this event closely agree with the observed gyre structure and indicate that the gyre is associated with a plume of warmer potential temperature air, which originates from a ridge of higher terrain to the south of Denver, and advects northward into the area of gyre formation. A mixed-layer vorticity budget demonstrates that the formation of the gyre results from the baroclinic and slope effects on the turbulent stress divergence profile.

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J. M. Wilczak and J. W. Glendening

Abstract

In northeastern Colorado a frequently observed feature of the surface wind field is a stationary, terrain-induced mesoscale gyre, which is often associated with the formation of severe weather. Because of the gyre/s proximity to the Denver metropolitan area, local weather forecasters frequently refer to it as the “Denver Cyclone”. The development of one such cyclone, which occurred on 1 August 1985, is documented with mesonet, radiosonde, wind-profiler, radiometer and tower data. Mixed-layer model simulations of this event closely agree with the observed gyre structure and indicate that the gyre is associated with a plume of warmer potential temperature air, which originates from a ridge of higher terrain to the south of Denver, and advects northward into the area of gyre formation. A mixed-layer vorticity budget demonstrates that the formation of the gyre results from the baroclinic and slope effects on the turbulent stress divergence profile.

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J. M. Wilczak and J. E. Tillman

Abstract

During April 1978, a field experiment was undertaken at the Boulder Atmospheric Observatory (BAO), near Boulder, Colorado, to investigate convective plumes in the atmospheric surface layer.

The plume translational velocities are determined for a wide range of stabilities, using an array of 16 temperature sensors, spanning a 100 m × 40 m area at a height of 4 m, and a three-dimensional sonic anemometer. The translational velocities are calculated from the phase information of the temperature cross spectrum, and by measuring the transit times of the plumes between sensors aligned in a direction parallel to the wind. Velocities obtained by the two methods are shown to be in rough agreement. Individual plume velocities are found to vary in proportion to the plume height.

The three-dimensional plume structure is investigated using both the horizontal array and the 300 m BAO tower. Under slightly unstable, high wind speed conditions, the majority of the plumes are distinctly elongated in the mean wind direction, with typical longitudinal and lateral dimensions of several hundreds and several tens of meters, respectively. As the instability increases the plume dimensions are found to decrease in the downwind direction, and to increase in the crosswind direction.

For very unstable, low wind speed conditions, the plumes are most often observed to be elongated in the lateral direction, although occasionally the isotherm patterns display a meandering behavior, with the trailing edge of one plume becoming the leading edge of the next.

The plume tilt is found to become more nearly vertical as −z/L increases, due to the surface shear layer becoming small in comparison with the height of the measurements. In addition, the plumes are found to have greatly varying vertical extents, and often several small plumes will merge together to form a larger plume at a higher level.

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James M. Wilczak and Alfred J. Bedard Jr.

Abstract

The construction, calibration, and application of a microbarometer that is capable of accurately measuring turbulence pressure fluctuations is described. The microbarometer consists of a quad-disk pressure probe and a highly sensitive high-pass pressure sensor. The accuracy of the instrument is tested by evaluating the budget of streamwise horizontal heat flux in the atmospheric surface layer. In this budget, shear and stratification production balance a pressure covariance term and a small turbulent transport term. The measured pressure covariance term is found to close the heat flux budget to within approximately 15%.

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J. M. Wilczak and Joost A. Businger

Abstract

The energetics of the dry convective boundary layer is studied by partitioning the turbulent heat flux into thermally indirect (w′θ′<0) and thermally direct (w′θ′>0) components as a function of z/Zi. It is found that except for the inversion transition region, the thermally direct and indirect fluxes each have linear profiles. The integrated profiles indicate that a fraction A of the buoyant production of thermal kinetic energy is available to do the work of entrainment, where A is the boundary layer entrainment coefficient. A simple mixing analysis shows that this would require the integrated production of energy due to surface heating to be independent of the entrainment process. Sub-partitioning the thermally indirect flux into two components (w′<, θ′>0 and w′>0, θ′<0) reveals that an upward flux of cold air is the dominant thermally indirect term throughout the bulk of the mixed layer. Further, in the inversion transition layer the measured negative mean entrainment beat flux (w′θ′)i is principally due to a net upward flux of locally colder air, and not to a net downward flux of locally warmer air. These results are interpreted in terms of a highly idealized dome-wisp model of the entrainment mechanism.

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J. M. Wilczak and T. W. Christian

Abstract

Observations taken during the Convection Initiation and Downburst Experiment (CINDE) are used to describe the formation and structure of an orographically induced mesoscale vortex that frequently occurs in northeastern Colorado. This vortex, known locally as the Denver Cyclone due to its proximity to the Denver metropolitan area, is frequently associated with severe weather. We present a case study of the Denver Cyclone of 25 June 1987, that formed during the late morning hours and remained nearly stationary for over 24 hours.

Interesting features of the case study vortex are: low-level convergence into the center of the cyclone during nighttime hours but divergence at the center when daytime heating becomes significant; a very shallow initial vertical extent at night, growing to nearly 1500 m during the daytime hours; a cold pool of air on the west side of the vortex, with highest surface potential temperatures present in a warm plume on the east side; a perturbation low pressure of ∼150 Pa in the region of warmest potential temperatures; a sloping zone of low-level convergence, in the region of lower pressure, that triggers intense convective activity, and an upwind tilt of the center axis of the vortex.

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J. M. Wilczak and Joost A. Businger

Abstract

A method is developed for retrieving turbulent pressure fluctuations from tower measurements of velocity and temperature, through use of the equations of motion. This method is applied to a series of large-scale eddies which are defined by their characteristic temperature ramp structure. The variance of pressure is found to follow local free-convection.

Large-scale eddy (LSE) pressure fields are used to estimate the pressure transport and pressure-gradient interaction terms in the convective surface-layer budgets of heat flux, stress and turbulent kinetic energy. The LSE pressure terms are found to balance the budgets to within 20–30% of the size of the largest budget terms.

Ensemble fields are formed by averaging individual LSE pressure transport and pressure-gradient interaction fields. The basic characteristics of these ensemble pressure covariance fields are easily related to the cross-products of the ensemble fields of p′, ∂p′/∂x, w′, and so on. This offers a simple way of visualizing the source of the budget pressure covariances in terms of the average LSE structure.

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J-W. Bao, S. A. Michelson, and J. M. Wilczak

Abstract

A few roughness length schemes for surface sensible and latent heat fluxes have been developed to fit observations under low and moderate wind (<20 m s−1) conditions. It is not clear to what degree these schemes can be extrapolated to cases of high wind conditions (>25 m s−1). In this study, numerical experiments are carried out to reveal the sensitivity of a simulated hurricane to the roughness length schemes for heat fluxes. It is shown that great disparity exists in the response of the model-simulated hurricane to the schemes. This suggests that further research involving both theory and observations is required in order to reduce the uncertainties in numerical simulation of air–sea fluxes under high wind conditions.

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