(Clark, 1972). The vertical speed of the front of one
disturbance in a very idealized stratosphere was found to be about 7.6 km/day by Clark (op. cit), and Matsuno (1971) quotes
- 1
a value of " less than 5 k m day '' . Thus one would expect a delay of several days between the effects of the front of such a distu rbance on the 10 mb level (30 km) and the mesosphere
(70 km).
Suppose instead that a planetary wave of given latitudinal and longitudinal wavenumber and given period is present in the upper stratosphere and mesosphere. Then correlations between stratospheric and mesospheric pararoeters above a given point on the earth will depend on the difference in the phase of the wave between stratospheric and mesospheric heights. The
vertical wavelength depends on the horizontal dimensions and the period of the wave (Lindzen, 19 67) and probably also on the background zonal wind. From the article by Lindzen (op. cit) it appears that planetary waves with vertical wavelengths of less than 30 km are possible. This could lead to differences of several days between the occurrence of a given phase at
stratospheric and at mesospheric heights. 6.2.3 Stratospheric Warmings
During the winter of 195 1-52 Scherhag (19 62) discovered a sudden increase in stratospheric temperature over Berlin, and since then other sudden warming events have been observed
(Scherhag, 1 9 6 9). At 6 0°N they may show up as temperature increases of about 5o0c in less than five days (Webb 19 6 6 , page 171). At times these temperature changes are accompanied
74 by changes in the zonal stratospheric circulation from east ward to westward (Murgatroyd, 19 70) . If the circulation does not revert to the normal winter eastward flow the event is called a final warming. Final warmings are also observed in the southern hemisphere (Phillpot, 1969) . Apparent mid-winter warmings at 30 rnb in the southern hemisphere stratosphere may actually be due to the passage of migratory warm ridges
associated with an n = 2 planetary scale wave (Phi llpot, op. ci t) .
Rocket measurements of temperature in the northern hemi sphere have shown that the disturbance associated with a stratospheric warming extends at least into the upper mesosphere, but above a given location on the earth the temperature change has different signs at different heights (Labitzke 1972a, 1972b) (see figure 6.3) . Also there may be warmings in the upper stratosphere and mesosphere which have little effect below 25 km. It should be noted that a given warming does not show up over the entire mid-latitude strato sphere at the same time ; there may be several days between the onset of a warming over Europe and its appearance above North America (Scherhag, 1969) .
It is possible that the warmings are due to interactions between planetary waves and the zonal flow (Matsuno, 1971) . However, it is important to realize that a large temperature increase at a given height at the time of a large stratospheric warming is not simply the temperature perturbation of a
7 5
6. 2. 4 Internal Gravity Waves
Internal gravity waves in the atmosphere have periods between about five minutes and several hours (Hines 1972) . If they are not dissipated or reflected their amplitudes increase exponentially with height (planetary waves also behave in this fashion) . Their vertical scale size is from 5 to 50 km , and the corresponding horizontal scale size 10 to 300 km. It is thought that the main sources of the waves are in the tropo sphere, such as mountain lee waves, weather systems and jet streams. Typical vertical trace speeds associated with
individual waves are 1-20 m/sec, and horizontal trace speeds are typically 20 to 100 m/sec. The patterns of irregular wind fields due to the interference of superposed waves move with similar speeds. Not all of the waves from the troposphere will reach the mesosphere since waves of some frequencies are reflected due to the background temperature and wind. At
' critical levels ' where the frequency of the wave relative to the background wind is Doppler shifted to zero the waves are absorbed giving rise to heating and a change in the mean flow
(Hines and Reddy, 1967) . Gravity waves may also give rise to turbulence, by energy deposition at critical levels and also due to superadiabatic temperature gradients and wind shears
associated with the waves. Turbulence gives rise to mixing of ionizable constituents (it contributes to the eddy diffusion coefficient discussed in chapter 9) .
Because planetary waves give rise to changes in the back ground flow and temperature as seen by gravity waves, fluctuat ions in the stratosphere due to planetary waves might give rise to variations in the transmission of gravity waves into the mesosphere (Hines 1972) . Stratospheric warmings might also affect the gravity wave flux.
80 7 0 (/) 60 0:: w 1-- w 50 ;;; :.: 4 0 30 2 0 F'i r, 6 l) • •
4•
W E ST G E I R IN I S H ( 57°N , 0 7°W) L...
'-80 -70 -6Q -50 -40 -30 -20 -10 0 - EARLY WINTER I - EARLY WIN TERa
(Betore Mojo, wo,mn9) 2 - PEAK OF WARMING J ,._ . _,, ADVANCED STAGE 4 00 0 00 TERMI NATION 0 +10 +20 +30 +40 80 70 60 50 40 30 20
Typical mean temperature profiles for West Geirinish (Labi tzke , 1972b) ,E .; 10 MB 20 - TEMPERATURE / I (BERL I N ) o- -o f I f I I I I I I I I IONOSPHERIC A9S0RPTION- (LIN0AU) •--• \ g 10
JI
0I I
if
- 1 0 - 1:! - 8 - 4 0 \ \nnr• frum epoch of 1 0 111U warming Flg. 1
400
300
340
Superposed epoch comparison of daily ionospheric
absorption index and stratospheric temperature . (Shapley & Beynon, 1965)
6.3 MESOSPHERIC OBSERVATIONS AT THE TIME OF STRATOSPHERIC WARMINGS
76
The term " stratospheric warming" tends to be applied rather loosely in ionospheric physics. In some mid-winter warmings there i s breakdown of the stratospheric polar vortex
(these are called maj or warmings) and temperature variations may be observed well down in the lower stratosphere, below 20 km (Labitzke 19 7 2b) . As has been discussed there are some warmings when there is little discernable temperature change at 20 km (50 mb) but sti ll a large warming in the upper
stratosphere, or there may be temperature increases at 30 mb due to planetary wave passage. For these reasons there are problems in using, say, the 30 mb temperature above a given station as an indication that a warming has occurred as it i s not clear to which of the above classes the " warming" may belong, and a warming which occurred only in the upper stratosphere might not be detected.
There are several cases on record of anomalies in the
lower ionosphere at the time of stratospheric warmings .
Belrose (1967) found that during the maj or warming of
February 19 5 2 there was an increase in the vertical incidence