7.3 Gravity Wave Potential Energy
7.3.2 Results: Measured Gravity Wave Potential Energy
As mentioned earlier, the SABER instrument obtains temperature profiles from 52◦S to 83◦N during its north-looking mode for 60 days before switching to an analogous south- looking mode and then repeats that sequence for the subsequent months (Remsberg et al., 2003). In this way, the polar cap data is available only in 60-day segments, with no in- formation for the 60 days preceding. Due to this reason, only data from 1 September to 19 September was available. This implies that only data from the first 3-4 days of the occurrence of the major SSW could be obtained over SANAE (72◦S, 3◦W), because the stratospheric warming started around 15 September and persisted for about 10 days.
Figure (7.7) shows the height profiles of mean potential energy with standard error of the mean, for August and September 2002 (warming year) and 2003 (normal year), respecti- vely.
-50 0 50 100 150 200 250 300 30
40 50 60 70 80 90
100 August (2002)
Height (km)
P.E (J/kg) 30-50 0 50 100 150 200 250 300
40 50 60 70 80 90
100 September (2002)
Height (km)
P.E (J/kg)
(a) (b)
-50 0 50 100 150 200 250 300
30 40 50 60 70 80 90
100 August (2003)
Height (km)
P.E (J/kg)
-50 0 50 100 150 200 250 300
30 40 50 60 70 80 90
100 September (2003)
Height (km)
P.E (J/kg)
(c) (d)
Figure 7.7: Height profiles of monthly mean potential energy per unit mass (black line) and the asso- ciated standard error of the mean (grey shading) for the gravity wave perturbations over SANAE.
The variation of the potential energy with height is clearly observed during August and September and in 2002 and 2003 (see Figure (7.7)). The potential energy is observed to be gradually increasing with height up to values of∼150 J.kg−1 at heights above∼65 km.
Even though the September 2002 potential energy mean profile contains only the first 19 days of the month, the enhancement in the potential energy can be clearly observed.
The September 2002 potential energy height variation unlike the other panels show three distinct peaks with values of ∼100 J.kg−1, ∼200 J.kg−1, and ∼300 J.kg−1 in the height region of 45-50 km, 65-73 km, and 85-93 km.
CHAPTER 7. INFLUENCE OF GRAVITY WAVES DURING THE SSW 123
0 200 400
30 40 50 60 70 80 90
100 September (2002)
Height (km)
P.E (J/Kg) 30 0 200 400
40 50 60 70 80 90
100 September (2003)
Height (km)
P.E (J/Kg)
(a) (b)
Figure 7.8: Height profiles of monthly mean potential energy per unit mass (black line) and the asso- ciated standard error of the mean (grey shading) for gravity wave perturbations over 52◦S.
In order study the gravity wave associated potential energy for the whole month of Sep- tember, those profiles which are closest to -52◦ latitude were used. Figure (7.8) shows the height profiles of mean potential energy with standard error of a mean, for September 2002 and 2003, respectively. Again when comparing the two years there is a clear enhan- cement in September 2002 potential energy profile through the middle atmosphere. This enhancement in the potential energy is evidence of the effect of the major SSW. The MLT peaks observed in Figure (7.7b) are also evident in the profile of September 2002 (Figure (7.8a)). However, in this profile, the peak at the height around 70 km seems to be the strongest (∼350 J.kg−1).
The values of potential energy observed at a height region below 40 km in the September 2002 profile (Figure (7.7b) and Figure (7.8a)) are consistent with observations by Ratnam et al. (2004b). Ratnam et al. (2004b) studied the dynamics of the potential energy only in the stratosphere during the 2002 major SSW using the CHAMP/GPS temperature profiles, and compared it to the 2003 observations. In their study, they showed that during the 2002 major SSW the potential energy in the stratosphere had higher values outside and in the edges of the polar vortex, while the values were lower inside the polar vortex. The above mentioned observation could explain the reason for observed higher values of potential energy at latitudinal region closest to -52◦ (Figure (7.8a)) when compare to the potential energy at region closest to -72◦ latitude (Figure (7.7b)), observed in this study. For more details on the latitudinal and longitudinal variation of potential energy and the causative mechanisms, the reader may refer to Tsuda et al. (2000) and Ratnam et al. (2004a).
Figure (7.8a) indicates clearly that gravity wave amplitudes were increasing with height up to their braking point in the MLT. The observations indicate that during the year 2002 winter, the amplification of planetary waves in the middle atmosphere led to the reversal of the mean wind. This disturbance of the mean wind in the stratosphere allowed more
gravity waves of tropospheric origin to propagate to the MLT. Gravity waves propagation in the middle atmosphere depends largely on the background wind. As they propagate upward in the middle atmosphere they undergo a selection process due to critical level filtering (Taylor et al., 1993) owing to the effect of the background wind. Assuming that the background wind varies slowly on the spatial and temporal resolution of the gravity waves, a local dispersion relation holds, and the vertical wavenumberm can be given by
m2≈ N2
c−ukˆh2 (7.12)
and the vertical group velocityCgz is given by Cgz ≈ N kh
m2 (7.13)
where N is the Brunt V¨ais¨al¨a frequency, c = ω/k is the horizontal phase speed of the wave with radial frequencyω,kh is the horizontal wavenumber, and u is the background wind. The above two equations show that as a wave travels with the background wind,m increases, the vertical wavelength is shortened, and the vertical group velocity decreases.
The shortening of the wavelength and the reduction of the phase speed implies that the viscosity will have a larger effect, and thus the attenuation of the wave (e.g. Stockwell and Lowe (2001)).
245 250 255 260 265 270
0 500 1000 1500
2000 Potential Energy [September 2002]
Day #
P.E (J/kg)
95 km 90 km 85 km 80 km 70 km
245 250 255 260 265 270
500 1000 1500
2000 Potential Energy [September 2003]
Day #
P.E (J/kg)
(a)
(b)
Figure 7.9: Potential energy for the days from 1 to 30 September 2002 (a) and 2003 (b), respectively.
Figure (7.9) shows the daily variation of the potential energy over the altitude region
CHAPTER 7. INFLUENCE OF GRAVITY WAVES DURING THE SSW 125
of 70 km, 80 km, 85 km, 90 km, and 95 km, for the days of September 2002 (warming year) and 2003 (normal year), respectively. These profiles were obtained from the SABER temperature profiles which are closest to -52◦ latitude. When comparing the two figures (Figure (7.9a and b)), it can be clearly observed that there was an enhancement in the year 2002 potential energy in the MLT region during the period of the occurrence of the major SSW. During this period (day 260-273), the potential energy at the height region between 70 km and 90 km is observed to have a maximum peak which reach a value of
∼1500 J.kg−1 on day number 267 (24 September 2002) at the height region of 85 km and 70 km. At the height region between 90 km and 100 km, the potential energy is observed to be weaker, presumably due to the gravity waves damping their momentum in this region, leading to the MLT cooling which is evident in Figure (6.14).