4.3 Climatological Comparison
4.3.2 Overall Monthly Temperature Variations
The contour maps of monthly mean temperature distributions obtained from the Durban LIDAR, SABER (3041 day and night profiles), and HALOE (81 day and night profiles) data plotted in a grid of month versus altitude are shown in Figure (4.4). In all three contour plots high temperatures at a height region between ∼40 km and 55 km can be clearly seen due to the absorption of the ultraviolet radiation from the sun by ozone. The LIDAR, SABER, and HALOE temperatures do not show a significant change with month below about 40 km altitude in the monthly mean distributions. This observation is similar to the findings by Chandra et al. (2005) on their middle atmospheric temperature study over Mt. Abu (24.5◦N, 72.7◦E), India. However, it should be mentioned that in their study, no measurements were made during July and August due to the monsoon season.
The thermal structure observed by the LIDAR over Durban at the stratopause (∼42-50 km) shows two distinct maxima, with one during the period from February to July and the other during the period from September to December. This maximum temperature is about 270 K. The minimum temperature (about 260 K) is observed during August to September in the stratopause.
CHAPTER 4. LIDAR OBSERVATIONS 49
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Figure 4.4: The contour plots of temperature variation with altitude and month for the LIDAR (top panel), SABER (middle panel), and HALOE (lower panel) observations.
The SABER stratopause temperature during the spring/summer season (from October to February) has a maximum temperature of 260 K. During the winter period the stratopause temperature is found to be∼250 K. For HALOE the summer and equinoxes stratopause is observed with a maximum of 260 K. Thus, the stratosphere structure as seen by the three instruments (LIDAR, SABER, and HALOE) depicts a familiar feature (annual oscillation)
in the mid-latitude regions. Previous studies have also shown a strong presence of annual oscillation in the mid-latitudes and high latitudes (e.g. Chanin (1991); Hauchecorne et al.
(1991); Gobbi et al. (1995)). Batista et al. (2009) reported a domination of annual oscilla- tion in the stratosphere in their study using Rayleigh LIDAR measurements obtained from 1993 to 2006 at S˜ao jos´e dos Campos, Brazil (23.2◦S, 45◦W). In contrast to the annual oscillation in stratosphere of the midlatitudes and high-latitudes, the low-latitude obser- vations show a strong semi-annual oscillation (e.g. Sivakumar et al. (2003)). The Durban station is situated at ∼29◦S, and the middle atmosphere at this latitudinal position can be influenced by the subtropical and mid-latitude dynamical patterns. The observation of maximum temperatures during the summer and minimum temperatures during the winter stratosphere is in phase with the solar flux.
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Figure 4.5: Contour plot of temperature differences between the LIDAR and SABER (a), LIDAR and HALOE (b), and LIDAR and CIRA-86 model (c).
Fluctuations observed in the LIDAR temperature climatology are due to dynamical events which are smoothened in the satellite (SABER and HALOE) observations. The LIDAR observations are about 10 K warmer in the height regions between 40 and 55 km compa- red to the satellites observations for most of the months. This can be associated with the
CHAPTER 4. LIDAR OBSERVATIONS 51
dynamical activities which could not be detected by the satellites. In fact, the ground- based LIDAR experiment allows a better vertical resolution (150 m) than the satellite observations. The calibration of the instrument may also play a role, as, for example, LIDAR instrument assumes the ideal gas law. Furthermore, LIDAR measurements are more sensitive to small-scale dynamical disturbances, such as gravity waves and atmos- pheric tides than satellite measurements. The SABER and HALOE show almost the same thermal structure of the middle atmosphere over Durban. Both instruments (SABER and HALOE) show cold mesospheric winter above∼63 km. Similar results were also reported by Batista et al. (2009) in their study of monthly climatology and trend in the 35-65 km altitude over S˜ao jos´e dos Campos, Brazil.
For a better comparison between LIDAR observations and satellite observations it is ne- cessary to calculate temperature differences between the instruments. The contour plots in Figure (4.5) show the temperature differences between the LIDAR (a) and SABER (b) and HALOE (c) and CIRA-86 model, plotted as a function of month versus altitude. These differences were obtained by subtracting the satellite (SABER and HALOE) and model (CIRA-86) climatology from the LIDAR climatology. The differences between LIDAR and SABER and LIDAR and HALOE and CIRA-86 are observed to be similar. Generally, all contour plots are dominated by higher LIDAR temperature values throughout the middle atmosphere, except at a height above 50 km during the equinoxes. The differences bet- ween LIDAR and SABER show evidence of higher values (20 K) located in the height range ∼40-50 km during March to July. The differences between LIDAR and HALOE show the highest values (10 K) in the height range∼35-45 km during February to July.
Another peak of 10 K is noticed during September to August. The differences between LIDAR and CIRA-86 model show the highest values (10 K) in the height range∼35-45 km during April to September. The same feature is observed in all three figures. The double lobe structure of minimum temperature in February to April and September to November over 50 km could possibly reflect the presence of semi-annual oscillation. Another factor which could lead to the formation of the double lobe structure could simply be a general reflection of the larger discrepancies between the systems during the equinoxes. This is confirmation of a semi-annual oscillation which was observed in this study. The smaller differences in SABER, HALOE, and CIRA-86 during November and December may be associated with lack of LIDAR observations during these months.
Studies such as Leblanc et al. (1995) made a comprehensive comparison of the LIDAR and HALOE satellite observations and found differences to be as large as 15 K between the two instruments in the mesospheric inversion region. Hervig et al. (1996) compared the LIDAR temperature measured at different stations in the Northern Hemisphere to that of HALOE and rockets and found that the measurements typically have random differences less than 5 K for the latitude below ∼60 km. The differences shown in Figure (4.5b) exhibit the highest values (10 K) in the height range∼35-45 km during February to July.
Another peak of 10 K is noticed during September-August.