Search Results

You are looking at 1 - 10 of 79 items for

  • Author or Editor: Roland Stull x
  • Refine by Access: All Content x
Clear All Modify Search
Roland Stull

Abstract

Three of the atmospheric datasets that were originally used to verity statistical dispersion theory are reevaluated. These datasets are described as well by transilient turbulence theory as by statistical theory over the range of time periods of interest for practical dispersion problems. The limitations of transilient turbulence theory, particularly its inability to duplicate the linear growth of plume-width standard deviation at infinitesimally short times, are also investigated.

Full access
Roland Stull

Abstract

An equation is presented for wet-bulb temperature as a function of air temperature and relative humidity at standard sea level pressure. It was found as an empirical fit using gene-expression programming. This equation is valid for relative humidities between 5% and 99% and for air temperatures between −20° and 50°C, except for situations having both low humidity and cold temperature. Over the valid range, errors in wet-bulb temperature range from −1° to +0.65°C, with mean absolute error of less than 0.3°C.

Full access
Roland Stull
Full access
Xingxiu Deng
and
Roland Stull

Abstract

A technique is developed to anisotropically spread surface observations in steep valleys. The goal is to create an improved objective analysis for the lowest, terrain-following numerical weather prediction (NWP) model level in mountainous terrain.

The method is a mother–daughter (MD) approach, where the amount of information transferred from one grid point (the mother) to all neighboring grid points (the daughters) depends on elevation differences. The daughters become mothers and further share information with their neighboring grid points. This iterative method allows information to follow valleys around ridges, while reducing spread over the ridge top. The method is further refined to account for land–sea anisotropy.

This approach is tested in the objective analyses of surface potential temperatures over the steep mountainous and coastal terrain of southwestern British Columbia, Canada. Analysis results are compared with other existing schemes using the Advanced Regional Prediction System Data Assimilation System (ADAS). It is found that the MD approach outperforms the other schemes over mountainous and coastal terrain.

Full access
Roland B. Stull

Abstract

Not all cumulus clouds can vent mixed-layer air into the free atmosphere. Therefore, three subtypes of fair-weather cumulus clouds are identified based on the nature of their interaction with the mixed layer: forced, active and passive clouds. Forced clouds, the visible tracers within the tops of some mixed-layer thermals, are totally embedded within the mixed layer. Active clouds reach above their level of free convection and are responsible for inhibiting mixed-layer growth and for venting pollutants from the mixed layer. Passive clouds are the decaying remnants of formerly active clouds, and are disconnected from the mixed layer.

Full access
Roland B. Stull

Abstract

The stable-layer thickness h and near-surface potential temperature strength Δθs, of the nocturnal boundary layer (NBL) are shown to have a “background” square-root of time dependence. Superimposed upon this background are other time variations caused by changes in bulk turbulence parameter B and average surface heat flux H : h = 5(−HtB)&frac12 and &minusΔθs = (−HtB −1)&frac12). As an intentionally different approach to the NBL problem B is modeled in terms of forcings external to the NBL rather than in terms of internal variables such as friction velocity or Obukhov length. Nocturnal boundary layer observations from the Wangara and Koorin field experiments in Australia are used to guide some dimensional arguments to yield B − (ŪG UG −1)(|fUG|Zs )3/2/(−QHg), where U G is the geostrophic wind vector, f the Coriolis parameter, g the acceleration due to gravity, Zs is a site and wind-direction-dependent empirical parameter and the overbear indicates time-average since transition (near sunset). Apparently, Zs is a measure of the influence of terrain features such as roughness and slope on NBL development. The resulting model is shown to be adaptable to frost-warning and air-quality applications.

Full access
Roland B. Stull

Abstract

In Part I, external forcings such as pressure gradient, terrain roughness and imposed cooling were used to forecast the thickness and strength of an exponentially-shaped (ES) nocturnal boundary layer (NBL) temperature profile. In Part II, it is suggested that the evolution of the ES temperature profile can be explained by simple models for background radiative, surface-induced radiative, and turbulence contributions to the total cooling. One partitioning model sets the ratio of turbulent to surface-induced radiative components to be a constant (∼3.35). The exponentially-shaped heat-flux profile implied by that ratio agrees favorably with the Minnesota field experiment profile of Caughey et al. Differences between an ES and a mixed-layer (ML) model for the NBL am presented using potential energy (PE) arguments, where a thinner ML yields the same PE change as a thicker ES. Differences are also apparent using eddy diffusivity (K) theory, where the bulging K-profile for a ML is dissimilar to the linear K-profile found for an ES. The implications of using velocity scales from Part I with the PE calculations done here are that over 90% of the turbulence kinetic energy is dissipated by viscosity, as opposed to smaller percentages suggested by others.

Full access
Edi Santoso
and
Roland Stull

Abstract

No abstract available.

Full access
Edi Santoso
and
Roland Stull

Abstract

In the middle of the convective atmospheric boundary layer is often a deep layer of vertically uniform wind speed (M UL), wind direction, and potential temperature (θ UL). A radix layer is identified as the whole region below this uniform layer, which includes the classic surface layer as a shallower subdomain. An empirical wind speed (M) equation with an apparently universal shape exponent (A) is shown to cause observations from the 1973 Minnesota field experiment to collapse into a single similarity profile, with a correlation coefficient of roughly 0.99. This relationship is M/M UL = F(z/z R ), where F is the profile function, z is height above ground, and z R is depth of the radix layer. The profile function is F = (z/z R ) A exp[A(1 − z/z R )] in the radix layer (z/z R ⩽ 1), and F = 1 in the uniform layer (z R < z < 0.7z i ). The radix-layer equations might be of value for calculation of wind power generation, wind loading on buildings and bridges, and air pollutant transport.

The same similarity function F with a different radix-layer depth and shape exponent is shown to describe the potential temperature (θ) profile: (θθ UL)/(θ 0θ UL) = 1 − F(z/z R ), where θ 0 is the potential temperature of the air near the surface. These profile equations are applicable from 1 m above ground level to the midmixed layer and include the little-studied region above the surface layer but below the uniform layer. It is recommended that similarity profiles be formulated as mean wind or potential temperature versus height, rather than as shears or gradients versus height because shear expressions disguise errors that are revealed when the shear is integrated to get the speed profile.

Full access
Roland B. Stull

Abstract

A first-order turbulence theory is developed that describes eddy-like mixing. Named transilient theory after a Latin word meaning “leap across” this approach models the turbulent mixing between arrays of points separated in space. It differs from eddy-diffusivity theory in that it is not restricted to turbulent transfer between adjacent points. By explicitly including “large eddy” effects it can handle mixing across zero-gradient and counter-gradient situations such as found in convective mixed layers. Applications might include pollutant dispersion, boundary layer modeling and cloud entrainment studies.

Full access