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Everett C. Nickerson

Abstract

Previous investigations of boundary layer adjustment, such as those by Panofsky and Townsend, and more recently by Townsend, have been based upon similarity arguments. In this paper, however, the adjustment of a neutrally stratified boundary layer to a sudden change in surface roughness is treated as an initial value problem.

Changes in wind speed obtained from the numerical integration of a set of nonlinear boundary layer equations are significantly larger than previous theoretical predictions. Numerical results comparable to the similarity predictions can be obtained by neglecting the effects of vertical motion. Numerical techniques such as those developed herein should be of value in modeling nonuniform boundary layer flows that are beyond the scope of current similarity solutions.

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Everett C. Nickerson and Virginia E. Smiley

Abstract

A transcendental equation is presented for the Monin-Obukhov length L based upon (i) the Businger-Dyer surface layer formulations, (ii) parameterizations of the moisture flux, ground storage, and radiation terms in the surface energy budget; and (iii) the wind and temperature at 10 m above a surface characterized by a roughness length z o. The surface temperature, friction velocity and sensible heat flux are obtained from the computed value of L.

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Everett C. Nickerson and Michael A. Dias

Abstract

Aircraft observations west of the island of Hawaii in June 1980 during the Hawaii Mesoscale Energy and Climate (HAMEC) Project have provided the first in situ measurements of airflow within atmospheric vortices downwind of a tall island. A band of low-level westerly winds was observed to extend more than 150 km west of the island along the axis line separating cyclonic vortices to the north of that line from anticyclonic vortices to the south. The theoretical downstream propagation speed of those vortices is obtained from the solution to a quadratic equation, and while previous satellite studies of atmospheric vortices used the larger root (i.e., ∼80% of the ambient flow), the present data are consistent with the smaller root ∼20%). The turbulent Reynolds number for the flow is 140, and the corresponding vortex shedding time is 32 h.

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Evelyne Richard, Patrick Mascart, and Everett C. Nickerson

Abstract

Numerical simulations of the 11 January 1972 windstorm in Boulder, Colorado, were carded out using a hydrostatic model with a turbulent kinetic energy parameterization to investigate the role of fictional effects in the development of nonlinear mountain waves. Sensitivity tests to the roughness length specification and to the turbulent mixing and dissipation length formulations show that surface friction delays the onset of the strong surface winds and also prevents the downstream propagation of the zone of maximum windspeed. Shear production within convectively stable regions is the dominant mechanism for the production of the turbulent kinetic energy. Moreover, these results are consistent with the hypothesis that a hydrostatic amplification mechanism is capable of accounting for the development of strong downslope winds.

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Romualdo Romero, Sergio Alonso, Everett C. Nickerson, and Clemente Ramis

Abstract

The influence of vegetative cover on the development of mountain waves is analyzed using a two-dimensional meso-β model. The model includes a detailed representation of surface fluxes and friction that evolve in time as the incoming solar radiation interacts with the soil and vegetation. Simulations of a zonal flow over a north-south-oriented ridge covered by different types of vegetation are presented and examined. The intensity of downslope winds and turbulent kinetic energy structure appear to be especially sensitive to the presence and type of vegetation. Model-predicted rainfall is also examined, indicating an enhancement when mountainous areas are covered by conifer forest.

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Nadine Chaumerliac, Evelyne Richard, Robert Rosset, and Everett C. Nickerson

Abstract

Two widely used microphysical schemes are compared to evaluate their possible impact on wet deposition mechanisms. They are based upon different spectral distributions for raindrops (Marshall-Palmer and lognormal distributions) and use different formulations for the autoconversion and evaporation process, as well as for the fall velocity of raindrops. A comparative study of these two schemes is carried out for a two-dimensional mountain wave simulation in a mesoscale meteorological model. Differences in the spatial and temporal evolution of microphysical fields are investigated. The two schemes are compared for simple chemical scenarios: gas dissolution in cloud and rain, gas scavenging by raindrops, and wet deposition. Results contrast the differing behavior of the two schemes in describing processes such as the direct scavenging of gases by raindrops and the release of chemical species back into the atmosphere because of below-cloud evaporation of rain.

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Everett C. Nickerson, Evelyne Richard, Robert Rosset, and David R. Smith

Abstract

A three-dimensional meso-β model with parameterized microphysics is presented. The model is capable of simulating orographically forced clouds, rain, and airflow. Tests using a two-dimensional version confirm the ability of the model to replicate the linear and nonlinear mountain wave simulations of previous authors. The model is applied to the Rhine valley and surrounding mountainous areas, the Vosges in France and the Black Forest in Germany. Model-predicted rainfall over the mountainous areas is in good agreement with observations in both magnitude and location; however, an absence of model-predicted cloud cover over the Rhine valley suggests the need for an improved mesoscale initialization procedure.

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Evelyne Richard, Nadine Chaumerliac, Jean Francois Mahfouf, and Everett C. Nickerson

Abstract

Orographic precipitation enhancement associated with the feeder mechanism proposed by Bergeron has been simulated using a two dimensions model based upon primitive equations including detailed parameter microphysics. A case-by-case comparison is made between model results and each of 14 well-documented precipitation episodes in southern Wales. The model reproduces the observed strong dependence of the precipitation enhancement on the low-level wind speed, as well as the weak dependence on the upwind precipitation rate. Model results also demonstrate that a satisfactory treatment of orographically enhanced precipitation requires the linking of the dynamical, thermodynamical and microphysical processes.

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