National Academies Press: OpenBook

The Earth's Electrical Environment (1986)


Suggested Citation:"STRONG CONVECTION." National Research Council. 1986. The Earth's Electrical Environment. Washington, DC: The National Academies Press. doi: 10.17226/898.
Page 85

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS 85 STRONG CONVECTION Development of a severe convective system depends on a stratification of temperature, moisture, and horizontal wind that produces extreme buoyancy forces tending to accelerate the motion of both ascending and descending air parcels. A steepening of the temperature-lapse rate is not alone sufficient, because there are many situations in which the energy of overturning can be dissipated in minor events about as fast as it becomes available through solar heating or other processes. Regimes of frequent events of weak to moderate intensity characterize air masses that are moist and weakly unstable through great depths. Usually a combination of the following conditions is associated with extreme convection in middle latitudes: (a) The airmass is convectively unstable. That is, there is a considerable lapse rate of temperature, and moisture is abundant only at low altitudes. (b) A disturbance in the larger-scale flow (e.g., a short-wave trough in the westerly current aloft, often having a marked low-pressure system at the Earth's surface) provides generalized lifting, which causes the convective instability to become realized. (c) The moist lower layer is separated from the dry zone by a stable layer or even a temperature inversion, which inhibits early overturning within the airmass and premature loss of potential energy. (d) There usually is differential temperature advection (warm air advection at low altitudes and, occasionally, cold air advection aloft). (e) A marked increase of the wind with height has a dual enhancing effect. First, warm air from convective towers is carried rapidly downstream by the strong winds aloft, preventing its local accumulation aloft and permitting longer duration of local convection; second, variation of the wind with altitude can cause the updraft column to slope with height and contribute to an organization of the flow that enables the intrinsic coolness of the air at middle levels to be manifested in overall storm energy. In brief, the precipitation formed and carried to great heights in strong updrafts can descend into intrinsically cold middle-level air, hastening that air's descent. In this case, where precipitation does not descend in the updraft in which it was formed, the storm may acquire a quasi-steady character. The conditions described above are often associated with weather-map features like those shown in Figure 7.4. Figure 7.5 shows a schematic vertical cross section through a quasi-steady severe storm that might form under the conditions depicted in Figure 7.4 . The more detailed plan view of such a storm (Figure 7.6) illustrates two downdrafts. The forward-flank downdraft in the rain area is largely an effect of the weight of condensation products; the rear-flank downdraft is thought to owe its existence in part to a barrier effect on the ambient winds produced by ascent of air from low altitudes in a shearing environment. Another cause of the rear-flank downdraft is evaporative cooling of air, dry and intrinsically cold at heights of 3 to 4 km, by precipitation descending into it from greater altitudes. Finally, Brandes (1984) proposed a mechanism by which this rear-flank downdraft would be stimulated by formation of a tornado or other vortex at low altitudes. Figure 7.4 Idealized sketch of a middle-latitude weather situation especially favorable for development of severe thunderstorms. Thin lines denote sea-level isobars around a low-pressure center with cold and warm fronts. Broad arrows represent low-level jetstream (LJ), polar jet (PJ), and subtropical jet (SJ). The LJ advects moisture-rich air from subtropical regions to provide the basic fuel for convection. Severe storms (hatched area) are most likely to start near I and gradually shift toward the east while building southward. Severe thunderstorms also occur with many variations on this basic theme. From Barnes and Newton (1985). Figures 7.5 and 7.6 show asymmetries that are critical features of persistent severe-traveling-storm complexes; Figure 7.3, in contrast, presents more symmetrical features, especially in its first and third frames. South of the severe- storm center in Figure 7.6, where a mesocyclone and possibly a tornado are located, a line of convective clouds (the flanking line) marks the intersection of air descended from middle levels with warm air rushing to

Next: HAIL »
The Earth's Electrical Environment Get This Book
Buy Paperback | $75.00
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

This latest addition to the Studies in Geophysics series explores in scientific detail the phenomenon of lightning, cloud, and thunderstorm electricity, and global and regional electrical processes. Consisting of 16 papers by outstanding experts in a number of fields, this volume compiles and reviews many recent advances in such research areas as meteorology, chemistry, electrical engineering, and physics and projects how new knowledge could be applied to benefit mankind.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook,'s online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!