The Big Thompson Storm:Weather Patterns 3

© Fernando Caracena, 2016

3-D Atmospheric Structure of the West US

One way of looking at the volume of the atmosphere over a large area is through a vertical stack of analyses on constant pressure surfaces. The pressure surface analyses presented below are on what are called mandatory levels by meteorologists , which are: surface, 1000, 925, 850, 700, 500, 400, 300, 250, 200, 150, 100, 70, 50, and 10 mb. Here the analysis extends only from 850 to 100 mb because 850 mb is slightly below the surface at a few locations of the West, and 100 mb is at or near the top of the troposphere. Although the 850 mb surface is just below the ground at the Wyoming , Denver CO, and Albuquerque NM sounding sites. The surface pressure is extrapolated by the Weather Service below the ground to fill the mandatory levels below ground. In the case of the Denver sounding, the terrain elevation is at 1611 m, MSL, whereas the 850 mb pressure level is at 1520 m, MSL in Fig. 1 for an extrapolation of 91 meters. The temperature and dew point temperature at these sites are presented as missing data, and are therefore not included in the analyses of these fields.

An important feature at 850 mb is the elevation in the level of the 850 mb pressure surface located north of a polar front that extended over NE Colorado across the Nebraska Kansas border into Illinois.

Fig. 1 A contour of the 850 mb pressure surface in tens of meters MSL from radiosonde measurements for 31 July 1976, 6:00 AM, MDT (1200 GMT). Areas where the dew point temperature is closer to the ambient temperature are shaded in green.

A tight temperature gradient and trough (dip in the pressure surface) on the 850 mb pressure surface are characteristics of a polar front situated over NE Colorado (Fig. 1). These features and surface weather observations (not shown) indicate that a cold front was present across NE Colorado extending into central Kansas and eastward to Missouri. Low-level air within 6 °C of saturation was present behind the front in a band over the northern states including eastern Montana, northern Nebraska,  North and South Dakota, and eastward into the Great Lakes region..

Fig. 2 As in Fig.1 caption, but the analysis is for the 700 mb pressure surface.

The 700 mb pressure surface (Fig.2) is somewhat above 10,000 ft (10 kft), MSL. At Denver, the height of this surface is indicated as 3170 m, which is 10.4 kft. This level is just about equal to the tops of the first range of mountains that rise above the High Plains in Colorado.  For example, Mount Thorodin, a favorite local climbing spot, which is located at  9.88452°N , 105.42875°W SW of Boulder CO is 10540 ft (3213 m), MSL.

At 700 mb on 31 July 1976, the morning before the storm, all of Colorado was covered by the plume of air within 6 °C of saturation, which had emerged from the tropics over western Mexico.  Note that at this time, the 700 mb cold front was located farther north as indicted by the tight thermal gradient that extended south eastward from SE Montana, across SW South Dakota, and into Iowa. A trough in the 700 mb height field south of Colorado is a reflection of a short wave trough propagating northward.

Another short wave trough is evident at this pressure level over the Great Lakes, which was responsible for bringing down the polar front that pushed into the Denver area.

Fig. 3 01 August 1976 0000 GMT 500 mb analysis.

The 500 mb analysis (Fig. 3) at this time, is included here for convenience. It is discussed in a previous posting in this series, "The Big Thompson Storm:Weather Patterns 2". Note that the 500 mb surface passes over the Denver Area in the rodiosonde observation at 5920 m, or 19.42 kft, MSL. This surface clears the tops of the highest peaks in Colorado by about 5 Kft.

Fig. 4 As in Fig. 1 caption but the analysis is on the 300 mb pressure surface and the green shaded areas are within 10°C of saturation.

The short wave signal is more evident in the 300 mb pressure surface analysis(Fig. 4) than at the lower levels. The height of this surface over Denver at the time is indicated in the figure to be 9670. m, which is 31,730 ft, MSL. This is about the height where weak scattered shower clouds (cumulonimbus clouds) top out. Note that that morning the ridge axis at 300 mb was just about to pass over the Denver area, and that the thermal pattern was weak. Also, the area within 10°C of saturation probably marks an area where some cirrus, cirrocumulus, and alto cumulus clouds are likely to be found (for photographs of cloud types, see here.).

Note that at 300 mb the winds over Denver would be expected to rotate counter clockwise form weak south westerly to weak south easterly as the ridge line moved north. Also note that at this level the two short wave troughs discussed above are clearly evident.

Fig. 5 As in Fig. 1, but the analysis is on a 250 mb pressure surface.

The northward propagating trough to the south of Colorado is still visible on the 250 mb pressure surface analysis that morning (Fig. 5). The height of this surface above Denver (1090 m) is about 35.8 kft. The thermal pattern at this level is weak, registering in the range of -43 to -44°C over Colorado. Note that at this height, the radiosonde humidity sensors are no longer capable of determining the frost point temperature, hence those numbers are reported as missing (999).

Fig. 6 As in Fig. 1 caption, but the analysis is on the 100 mb pressure surface.

The height of the 100 mb pressure surface was 16.61 km (54.63 kft) over Denver that morning. This level is close to the level of the tropopause over the tropics, and above it, over the poles. Note that the temperature, over what was a thermal ridge at levels 300 mb and below, are here depressed, so that a thermal trough aloft overlays the lower thermal ridge. The coldest temperatures (less than -70) here are found over Texas, Oklahoma, and across the Gulf States along the Gulf of Mexico. The maximum height of the 100 mb pressure surface is situated in the figure over north central Kansas. Also present at 100 mb is a reflection of the short wave trough passing over the Great lakes, the warmest air at this level overlaying the cool air brought into the area behind the polar front that trailed the short wave trough to the NE.

The discussion continues in the next post.












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