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A. P. Van Ulden and A. A. M. Holtslag

Abstract

This paper gives the outline of a “meteorological preprocessor” for air pollution modeling. It is shown how significantly more information can be extracted from routinely available measurements than the traditional Pasquil stability classes and power law wind speed profiles. Also it is shown how additional special measurements—if available—can be accommodated. The methods are primarily intended for application in generally level, but not necessarily homogeneous terrain. The improved characterization of the state of the planetary boundary layer allows a more modern and probably more accurate description of diffusion. The paper is an extended version of an introductory paper presented during the “Workshop on Updating Applied Diffusion Models” Clearwater, Florida, January 1984.

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A. A. M. Holtslag and A. P. Van Ulden

Abstract

In this paper a simple empirical scheme is presented, which gives hourly estimates of the surface fluxes of heat and momentum from routine weather data during daytime. The scheme is designed for grass surfaces, but it contains parameters which take account of the surface properties in general. The required input weather data are no more than a single wind speed, air temperature at screen height and total cloud cover. The output of the scheme is in terms of the Monin-Obukhov similarity parameters; it is obtained by using estimates for the mean values of the surface radiation and energy budget. For the climate of the Netherlands good agreement is found between a full year of observations and estimates made with the scheme. For all data it appears that root-mean-square errors are σ = 90 W m−2 for the incoming solar radiation, σ = 63 W m−2 for the net radiation, σ = 34 W m−2 for the sensible heat flux, σ = 0.01 m s−1 for the friction velocity and σ = 0.67 × 10−3 for the similarity ratio between the surface roughness length and the Obukhov stability parameter. A discussion is given an the surface parameters and coefficients of the scheme.

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J. Reiff, D. Blaauboer, H. A. R. De Bruin, A. P. Van Ulden, and G. Cats

Abstract

A model has been constructed for the purpose of forecasting at specific places the potential temperature, the specific humidity and the depth of the boundary layer. The model, an Air Mass Transformation model (AMT model), consists of a trajectory model and a one-dimensional boundary layer model. The horizontal and vertical advection of air masses is described by trajectories. The trajectories are computed from the analyzed geostrophic wind fields of a large-scale grid-point model. The lowest trajectory advects the air in the boundary layer. The air-mass transformations in the boundary layer are described by a simple boundary layer model. During unstable conditions, a slab model is used; during stable conditions, linear potential temperature profiles and humidity profiles are assumed. Along the lowest trajectory, the surface fluxes of heat and water vapor are computed from a simple parameterization scheme. The initial conditions for the state of the boundary layer are obtained from an analysis scheme. In this scheme, the routinely collected radiosonde data in the source area are used. So far the model has been tested for the period July–September 1981. During this period, twice-daily 12-hour model runs have been made. DeBilt (52°N, 5°E) was chosen as the test site. The following results have been obtained:

  1. The model temperature “forecasts” show a 0.85 correlation coefficient with the observed values at noon and 0.80 at midnight.
  2. The model has a strong potential to forecast the occurrence of boundary layer clouds.
  3. A vertical resolution of about 50 mb in the analysis scheme is necessary to obtain these results.
  4. Specific humidity forecasts based on pure (12-hour) advection show correlation coefficients of 0.81 and 0.77 with the observed humidity at noon and midnight respectively.
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P. Bechtold, S. K. Krueger, W. S. Lewellen, E. van Meijgaard, C.-H. Moeng, D. A. Randall, A. van Ulden, and S. Wang

Several one-dimensional (ID) cloud/turbulence ensemble modeling results of an idealized nighttime marine stratocumulus case are compared to large eddy simulation (LES). This type of model intercomparison was one of the objects of the first Global Energy and Water Cycle Experiment Cloud System Study boundary layer modeling workshop held at the National Center for Atmospheric Research on 16–18 August 1994.

Presented are results obtained with different 1D models, ranging from bulk models (including only one or two vertical layers) to various types (first order to third order) of multilayer turbulence closure models. The ID results fall within the scatter of the LES results. It is shown that ID models can reasonably represent the main features (cloud water content, cloud fraction, and some turbulence statistics) of a well-mixed stratocumulus-topped boundary layer.

Also addressed is the question of what model complexity is necessary and can be afforded for a reasonable representation of stratocumulus clouds in mesoscale or global-scale operational models. Bulk models seem to be more appropriate for climate studies, whereas a multilayer turbulence scheme is best suited in mesoscale models having at least 100- to 200-m vertical resolution inside the boundary layer.

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