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S. A. Tjemkes
and
J. Reiff

Abstract

Based on model calculations with a combined radiation-turbulence model, a simple relation to estimate the minimum surface temperature of a saturated vegetated clay soil from the 2 m air temperature is proposed. The parameterization scheme is tested against observations taken at the meteorological observational site near the village of Cabauw in the Netherlands. This comparison shows that the parameterization scheme is able to reproduce the minimum surface temperature under these circumstances with approximately 2.5°K.

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A. P. M. Baede
,
W. J. A. Kuipers
, and
J. Reiff

Abstract

A crucial step in the calculation of spectra from objective analyses is the transformation of the analysis to the latitude-longitude grid by some interpolation method. By analysing a synthetic two-dimensional function of known spectral content, it is shown that an interpolation method based on double Fourier expansion yields better results than the conventional linear interpolation. In particular, the minus-third- power profile of the high-wavenumber part of the spectrum is very well reproduced.

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J. Reiff
,
Gregory S. Forbes
,
F. Th M. Spieksma
, and
J. J. Reynders

Abstract

An African dust event reaching northwestern Europe has been documented and examined more thoroughly than ever before, using satellite imagery, upper-air soundings, surface observations, X-ray analyses of the dust composition, low-level dust concentration measurements, and objectively calculated air trajectories. This multidisciplinary study has lead to a consistent explanation of a number of initially puzzling observations. The dust case has been used to verify trajectory calculations based upon wind analyses produced at the European Centre for Medium Range Weather Forecasts (ECMWF). The calculations show excellent agreement with the observations: The error in the horizontal displacements turned out to be at most 200 km, while the traveling distance of the dust was at least 3000 km. The upper limit of the error in the vertical displacement is 50 mb, while the total upward displacement of the dust between the boundary layer in northwestern Africa and the dust layer above northwestern Europe 3 days later was about 300 mb. In appendix C, the trajectory model that is used in this case is given and compared to other approaches.

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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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