K 300 K

104m whence internal gravity waves in the atmosphere have a typical period of 2n/N ~ 9min.5 Internal gravity waves are ubiquitous in the atmosphere and are continually excited by, for example, horizontal winds blowing over hills and mountains, and convective plumes buffeting a stable layer above, among other things. On occasion, when the air is nearly saturated, they are made visible by the presence of regular bands of clouds in the crest of each wave (note, not all visible bands of clouds are produced in this way). These clouds often have a characteristic ''lens'' shape, hence the name lenticular. (See Figs. 4.13 and 4.14.) One can (very roughly) estimate an expected horizontal wavelength for the waves as follows. If each parcel of air oscillates at frequency N but is carried along by the wind at speed u such that a stationary pattern results, as in Fig. 4.15, then the expected wavelength is 2nu/N, around 5-10 km for the N estimated above and a wind of 10-20ms-1. Regular cloud features of this type are especially dramatic in the vicinity of mountain ranges (such as the Sierra Nevada and the Continental Divide of North America), as illustrated in Fig. 4.13 and shown in the photograph in Fig. 4.14. Another rather spectacular example of internal gravity waves in the atmosphere is shown in Fig. 4.15 and has an uncanny resemblance to the wake left by a ship (superimposed on the figure).

We shall see in Chapter 9 that the interior of the ocean is also stably stratified; internal gravity waves are a ubiquitous feature of the ocean too. Features indicative of such waves have also been observed in the Martian and Jovian atmospheres.

4.4.2. Temperature inversions

In abnormal situations in the troposphere in which T increases with height, the atmosphere is very stable; the restoring force

5The frequency given by Eq. 4-22 is an upper limit. We have considered the case in which the parcels of air oscillate exactly vertically. For parcel oscillations at an angle a to the vertical, the frequency is N cos a.

FIGURE 4.13. A schematic diagram illustrating the formation of mountain waves (also known as lee waves). The presence of the mountain disturbs the air flow and produces a train of downstream waves (cf. the analogous situation of water in a river flowing over a large submerged rock, producing a downstream surface wave train). Directly over the mountain, a distinct cloud type known as lenticular (''lens-like'') cloud is frequently produced. Downstream and aloft, cloud bands may mark the parts of the wave train in which air has been uplifted (and thus cooled to saturation).

FIGURE 4.13. A schematic diagram illustrating the formation of mountain waves (also known as lee waves). The presence of the mountain disturbs the air flow and produces a train of downstream waves (cf. the analogous situation of water in a river flowing over a large submerged rock, producing a downstream surface wave train). Directly over the mountain, a distinct cloud type known as lenticular (''lens-like'') cloud is frequently produced. Downstream and aloft, cloud bands may mark the parts of the wave train in which air has been uplifted (and thus cooled to saturation).

FIGURE 4.14. Looking downwind at a series of lenticular wave clouds in the lee of the Continental Divide of North America. Photo courtesy of Dale Durran, University of Washington.

on a lifted air parcel is large, and the atmosphere is thus particularly resistive to vertical motion. Such "inversions" can be produced in several ways. Low-level inversions (at altitudes of a few hundred meters) are commonly produced during calm winter nights from radiative cooling of the surface (see Fig. 4.16a).

Note that the inversion may be self-reinforcing; under conditions of slight wind, turbulence carries heat from aloft and limits the cooling of the surface. If an inversion forms, the resistance to vertical motion suppresses the turbulence and allows the surface layer to cool, thus strengthening the inversion. (Fog may then form in the cold surface layer below the inversion.) Apart from its thermal impact, the inversion may trap surface air and thus allow pollutants to build up in the surface layer.

A second type of low-level inversion, common in many subtropical regions of the Earth, is known as the trade-wind, or just trade, inversion. As we shall see in Chapter 5, air in the subtropics is, on average, descending and thus warms adiabatically, according to Eq. 4-14, as it does so. As shown in Fig. 4.16b, this can produce a persistent inversion (at altitudes between 400m and 2 km, depending on location). Many

FIGURE 4.15. Atmospheric gravity waves formed in the lee of Jan Mayen island (only 50km long, situated 375 miles north-northeast of Iceland), observed in February 2000. The wind is blowing from the WSW. A volcano—called Beerenberg—forms the north end of the island and rises to a height of over 2 km. Note the similarity between the atmospheric wake and that formed on a water surface by a ship, superimposed on the figure.

FIGURE 4.15. Atmospheric gravity waves formed in the lee of Jan Mayen island (only 50km long, situated 375 miles north-northeast of Iceland), observed in February 2000. The wind is blowing from the WSW. A volcano—called Beerenberg—forms the north end of the island and rises to a height of over 2 km. Note the similarity between the atmospheric wake and that formed on a water surface by a ship, superimposed on the figure.

FIGURE 4.16. (a) Low-level inversions are commonly produced during calm winter nights from radiative cooling of the surface. (b) A trade inversion created by descent and adiabatic warming typical of subtropical regions.

subtropical areas have pollution problems that are exacerbated by the trade inversion (see Fig. 4.17). In some cases, the vertical trapping of air by the inversion can be compounded by horizontal trapping by mountains. Los Angeles and Mexico City (and many other cities) suffer from this situation.

Solar Power Sensation V2

Solar Power Sensation V2

This is a product all about solar power. Within this product you will get 24 videos, 5 guides, reviews and much more. This product is great for affiliate marketers who is trying to market products all about alternative energy.

Get My Free Ebook


Post a comment