The Big Thompson Storm:Weather Patterns I

© Fernando Caracena, 2016

Large Scale Patterns

In 1976 there was a geostationary satellite that took photographs of clouds over the earth from space. Large bed fax charts regularly printed out various weather charts transmitted from Washington DC. Attendants cut theses charts from the machine and hung them on appropriate hooks on the wall . Satellite images came in on a smaller, photographic quality fax machine. In several ways, that period was the infancy of the age of modern meteorology.

The modern technology that is used no locally was already in place, but in a more centralized way. There were big computers in DC that assembled weather observations; analyzed them; used these analyses to initialize numerical prognostic models; and produce the graphics output. The images were then faxed out to the subscribers. At the weather offices and research sites, further analysis was done on paper.

I was excited to participate in the emerging computer revolution based on internet access and local processing, the tools of which  we were then developing on workstations, if you were rich, or personal computers (PC)s, if you had a small budget. Some groups in our laboratory had refrigerator-sized minicomputers, such as the PDP-11. Nowadays, an individual has affordable tools at his finger tips that give him the power to do the kind of analyses that it took a team to do in 1976.

I produced a series of analyses of the large scale of weather conditions over the US West (Fig. 1), which were produced in minutes on my PC.

Fig.1 An analysis of the wind streamlines, temperature contours and areas (shaded green) where the dewpoint temperature depression is less than 6 °C  on the 500 mb surface. Also depicted are the observed winds in meteorological notation and the height of the surface in tens of meters.


For the layperson, weather charts can be somewhat complex to look at, perhaps too complex to be meaningful. In Fig. 1, the observations taken through radiosondes and launched almost simultaneously at various, discrete sites are plotted as numbers and wind barbs at their respective locations. These observations, made at 1200 GMT(or Z) on Friday 30 July 1976, pertain to the 500 mb pressure surface, which is 50,000 Pa. (1 Pascal=1 Newton /square meter and 1mb = 100 Pa.) The bold, black numbers plotted directly above the radiosonde balloon launching location (marked in the figure with a + ) indicate the height of the 500 mb surface above mean seal level (MSL) in tens of meters. For example, 588 means 5880 meters, which is 5.88 km, MSL. The temperature and dew point temperature are also plotted, but as smaller numbers.

At that time, at the APCL, Mesoscale Research group, many of our conclusions were made base on analyses that we made using raw observations, using numerical weather forecast and analysis products only as a guide. An analysis, such as in Fig. 1, cost some time to produce. The analyst had to copy the data onto a background map, then pencil in his analysis. If the analysis were destined to be included as a publication figure, then this product was sent to a draftsman who would render the analysis as an India-inked figure of publication quality.

 Height Field of the 500 mb Pressure Surface

Fig. 2 As in Fig. 1 caption, but here the analysis is of the 500 mb height field in tens of meters, MSL.


One can look at atmospheric pressure two ways: (1) the pressure at a fixed elevation above sea level, or (2) the height above mean sea level (MSL) of a given pressure surface. The second choice is the standard in meteorological analyses. An analysis of the MSL contours of the 500mb, pressure surface (Fig. 2) shows a pattern similar to that of the wind field analysis by streamlines (Fig. 1).

Both analyses of the wind field and the height field of the 500 mb level show a wave coming up from the south over the Southwestern United States. This is called in meteorological parlance, a short wave trough. At this level, this wave is travelling in a plume of very warm moist air emerging from the tropics. This northward flow of moist air is produced by a shift in the large scale flow over the Southwest US during the warm season, which is part of the North American Monsoon.

(Continued here)




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