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Problems with the Mean-Sea-Level Pressure Field over the Western United States
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
2004 (English)In: Monthly Weather Review, ISSN 0027-0644, E-ISSN 1520-0493, Vol. 132, no 8, 1952-1965 p.Article in journal (Refereed) Published
Abstract [en]

Reduction of station pressure to mean sea level (MSL) is a difficult procedure. In general, the temperature structure of the fictitious air column between station height and MSL is not known and has to be estimated somehow. Normally, station pressure is reduced to MSL only for stations with relatively low elevations above sea level (ASL). At higher stations, station pressure is usually converted to the height of the closest standard pressure surface.

In the United States, however, station pressure is reduced to MSL for stations as high as 2000 m ASL. In order to reduce the amplitude of the annual MSL pressure variation at stations situated above 305 m ASL (hereinafter referred to as “plateau stations”), a so-called plateau correction is applied at these stations. The correction increases reduced MSL pressure when the actual temperature at the station is greater than the yearly mean temperature at the same station, and vice versa. The correction can therefore change both magnitude and direction of MSL pressure gradients. This is illustrated by means of the average monthly MSL pressure differences between the two cities of Yuma (southwestern Arizona) and Las Vegas (Nevada). Reduced MSL pressure values from plateau stations are used operationally in producing MSL pressure charts.

Similar methods for the reduction of station pressure to MSL are used in the postprocessing procedure of numerical atmospheric models, in order to obtain pressure or geopotential fields below the lowest level of the numerical model. Temperatures, on the other hand, are normally extrapolated from the lowest levels of the numerical model by means of a standard-atmosphere temperature lapse rate. For this reason, fields below the model's orography can be out of hydrostatic balance. This was found to be the case for the elevated regions of the western United States, where the lowest level of a global atmospheric model is usually at a height of around 1500 m ASL.

Nine days of measurements from a part of the lower Colorado River valley are used to evaluate such fields over the southwestern United States during summer. Mesoscale model simulations were carried out using fields from the NCEP–NCAR reanalysis system as basic-state conditions. Model-predicted winds were then compared to measured winds in that part of the lower Colorado River valley, situated approximately 100 km to the south-southeast of Las Vegas. The results showed that, in the lowest 1000 m ASL or so, model-predicted winds within the valley agreed far better with observed winds, when input geopotential fields were hydrostatically recalculated below 850 hPa before using them as basic-state conditions in the mesoscale model.

Ten years of geopotential fields were hydrostatically recalculated below 850 hPa. The hydrostatically recalculated 1000-hPa geopotential fields for summer show an average position of the thermal low that is about 450 km to the north and somewhat to the east, compared to the position in the original NCEP–NCAR 1000-hPa summer geopotential fields. In addition, the thermal low is about 40 gpm (≈5 hPa) deeper in the recalculated 1000-hPa geopotential fields. During winter, however, differences between hydrostatically recalculated 1000-hPa geopotential fields and original NCEP–NCAR 1000-hPa geopotential fields were very small.

Place, publisher, year, edition, pages
2004. Vol. 132, no 8, 1952-1965 p.
National Category
Natural Sciences
URN: urn:nbn:se:uu:diva-90528DOI: 10.1175/1520-0493(2004)132<1952:PWTMSL>2.0.CO;2OAI: oai:DiVA.org:uu-90528DiVA: diva2:162911
Available from: 2003-05-16 Created: 2003-05-16 Last updated: 2014-01-22Bibliographically approved
In thesis
1. Mesoscale Simulations of Atmospheric Flow in Complex Terrain
Open this publication in new window or tab >>Mesoscale Simulations of Atmospheric Flow in Complex Terrain
2003 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The MIUU mesoscale model was further developed, in order to include information on large-scale atmospheric fields from global or regional atmospheric climate- and weather-prediction models. For this purpose, a new lateral boundary condition was developed and implemented into the model. The new lateral boundary condition is a combination of two existing conditions, namely the flow relaxation scheme and the tendency modification scheme.

Tests indicated that an optimum lateral boundary configuration would be obtained with moderate to strong flow relaxation at higher levels, small flow relaxation at lower levels (within the atmospheric boundary layer), upstream advection at the outermost 4 grid points, and 6% horizontal grid stretching starting at a substantial distance from the lateral boundaries. The flow relaxation coefficients should be specified carefully, in order to minimize the reflection of all kinds of waves at the lateral boundaries.

The summer thermal low in the mean-sea-level pressure field over North America is traditionally analyzed over the northern end of the Gulf of California. The position of this low is influenced by the application of the so-called plateau correction in obtaining mean-sea-level pressure values from highly elevated stations in North America. A model study indicated that the low should be located approximately 450 km to the north and somewhat to the east of the above location.

A statistical comparison of model results from two mesoscale models against upper-air and surface measurements from several sites was carried out. Statistical methods, however, give only an insufficient picture of overall model performance. A comparison between predicted and measured tracer concentrations could be used to better evaluate the overall performance of different models.

Sound propagation in the atmosphere was predicted in a mountain valley using a mesoscale atmospheric model together with a sound propagation model. This suggests that forecasts of sound propagation should be possible in future.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2003. 42 p.
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 847
Meteorology, Mesoscale modeling, Flow in complex terrain, Atmospheric dispersion, Model comparison, Boundary conditions, Numerical modeling, Atmospheric pressure, Gravity waves, Meteorologi
National Category
Meteorology and Atmospheric Sciences
Research subject
urn:nbn:se:uu:diva-3461 (URN)91-554-5650-2 (ISBN)
Public defence
2003-06-06, SGU's lecture room, Sveriges Geologiska Undersökning (SGU), Uppsala, 13:00
Available from: 2003-05-16 Created: 2003-05-16Bibliographically approved

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