Characteristics of the transmitter and receiver systems of the Durban Rayleigh Light Detection and Ranging.http://sajs.co.za/index.php/SAJS/article/downloadSuppFile/612/30421
Comparisons between LIDAR, SABER and HALOE Climatological temperature The contour maps of monthly mean temperature distributions obtained from the Durban LIDAR, SABER and HALOE data plotted in a grid of month versus altitude are depicted in Figure 2. In all three contour plots, high temperatures at a height region between ~40 km and 55 km can be clearly seen and are as a result of the absorption of ultraviolet radiation from the sun by ozone. The LIDAR, SABER and HALOE temperatures do not seem to change significantly from month to month below an altitude of about 40 km. This pattern is similar to the findings of Chandra et al.27 for middle atmospheric temperature over Mt Abu, India (24.5°N, 72.7°E). However, it must be mentioned that, in their study, there were no measurements during the months of July and August because of the monsoon season. The thermal structure observed by the LIDAR over Durban at the stratopause (at ~42 km – 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. The maximum temperature is found to be about 270 K. The minimum temperature (about 260 K in the stratopause) is observed during the period from August to September. The SABER stratopause temperature during the extended summer season (from October to February) had a maximum temperature of 260 K. During the winter period the stratopause temperature was found to be ~250 K. Data from the HALOE satellite for the stratopause during summer, autumn and spring showed a maximum temperature of 260 K. In essence, the stratosphere structure, as seen by the three instruments (LIDAR, SABER and HALOE), depicted a familiar feature (an annual oscillation) in the mid-latitude regions. Previous studies have also shown the presence of an annual oscillation in the mid-latitudes and high-latitudes.6,15,28 Batista et al.29 also reported a domination of annual oscillations in the stratosphere in their study using Rayleigh LIDAR measurements obtained from 1993 to 2006 at São josé dos Campos, Brazil (23.2°S, 45°W). In contrast to the annual oscillation in the stratosphere of the mid-latitudes and high-latitudes, the sub-tropical latitude observations show a strong semi-annual oscillation.9 The Durban station is situated at ~30°S, and the middle atmosphere at this latitudinal position can be influenced by the sub-tropical and mid-latitude meridional exchanges (including the surf zone). The observation of maximum temperatures during the summer and minimum temperatures during the winter stratosphere is in phase with the solar flux. There were fluctuations observed in the LIDAR temperature climatology as a result of dynamic events, which were smoothed in the satellite (SABER and HALOE) observations. The LIDAR observations were about 10 K warmer in height regions between 40 km and 55 km, compared to the satellite data for the majority of the months. This temperature difference may have been because of dynamic activities which could not be detected by the satellites. In fact, the ground-based LIDAR experiment allows a better vertical resolution (0.3 km) than the satellites. Indeed, the LIDAR is more sensitive than the satellites to small-scale dynamic disturbances, such as gravity waves and atmospheric tides. The SABER and HALOE data exhibited almost the same thermal structure for the middle atmosphere over Durban. Both SABER and HALOE indicated a cold mesospheric winter, above ~63 km. Similar results were also reported by Batista et al.29 on their study of monthly climatology and trends in the 35 km – 65 km altitude over São josé dos Campos, Brazil. For a better comparison between the LIDAR and satellite observations, it is necessary to calculate the temperature differences between the measurements obtained from each instrument. The contour plots in Figure 3 and Figure 4 illustrate the temperature differences between the LIDAR and SABER and LIDAR and HALOE measurements, respectively, plotted as a function of month versus height. These differences were obtained by subtracting the satellite (SABER and HALOE) climatology from the LIDAR climatology data, (TLIDAR-TSABER) and (TLIDAR-THALOE). The differences between the LIDAR and SABER measurements and between the LIDAR and HALOE measurements are similar. Generally, both contour plots are dominated by higher LIDAR temperature values throughout the middle atmosphere, except at heights above 50 km during the equinoxes. The differences between the LIDAR and SABER measurements (Figure 3) were higher (~20 K) in the height range of 40 km – 50 km during the months from March to July. On the other hand, the differences between the LIDAR and HALOE measurements (Figure 4) were greatest (~10 K) in the 35 km – 45 km height range during the months from February to July. Another peak of ~10 K can be seen during August–September. The same feature is also observed in Figure 3, which shows the difference between the LIDAR and SABER data. The double lobe structure of minimum temperature in February–April and September–November over 50 km reflects the presence of a semi-annual oscillation. The smaller differences in both SABER and HALOE data during November and December may be associated with a lack of LIDAR observations during these months. Studies such as Leblanc et al.’s30 included comprehensive comparisons of the LIDAR and HALOE satellite observations and found differences between the two instruments to be as large as 15 K in the mesospheric inversion region. Hervig et al.23 compared LIDAR-measured temperature profiles to those of HALOE-measured and rocket-measured profiles and found that the measurements typically have random differences less than 5 K for heights below ~60 km. The differences shown in Figure 4 indicate that the greatest difference (10 K) was in the 35 km – 45 km height range during the months from February to July. We have also compared the Durban LIDAR temperature profile with the closest overpass temperature data measured by the SABER satellite. Figure 5 illustrates the SABER temperature profile and the Durban LIDAR temperature profile for the night of 27–28 November 2002. The LIDAR profile shows fluctuations in temperature as a result of vertical propagating planetary and gravity waves. However, the figure indicates a good agreement between the LIDAR and the SABER instruments. At a height of between 46 km and 53 km, the SABER temperature profile seems to be about 5 K – 7 K warmer than the LIDAR profile. At a height region between 34 km and 45 km, the SABER-measured temperatures seem to be about 4 K warmer than the LIDAR-derived temperatures. The observed differences in the middle atmosphere over Durban are understandably because of the two different techniques used by the two instruments and the actual observation time,9 and also because some middle atmospheric dynamics can be pictured by the LIDAR, but not by the satellite. Comparison of seasonal temperature profiles In order to examine the seasonal characteristics of temperature over Durban, and also to compare the seasonal means measured by the LIDAR against those measured by the satellites, the temperature profiles derived from the LIDAR, HALOE and SABER over Durban were grouped per season; the summer, autumn, winter and spring temperature profiles were derived from daily profiles averaged for the December–February, March–May, June–August and September–November periods, respectively. Figure 6 illustrates the seasonal averaged LIDAR temperature profiles with standard deviations. Superimposed on Figure 6 is the seasonal averaged closest overpasses temperature profiles measured by the HALOE and SABER satellites. Generally, the temperature profiles obtained from the LIDAR were found to be systematically higher than those obtained from the two satellites in the stratospheric height region between 33 km and 55 km. The stratopause is depicted at the same height in the LIDAR, SABER and HALOE profiles in summer and winter. However, during the autumn and spring seasons the LIDAR stratopause height was found to be ~5 km lower than those shown by the two satellite profiles. The HALOE temperature profiles were found to be systematically colder than the SABER temperature profiles for the height region below 40 km in autumn, winter and spring. Above 50 km, the satellite measurements had good agreement with the LIDAR measurements, apart from visible fluctuations within the LIDAR profiles, which can be associated with unusual spectacular events in the middle atmosphere that cannot be detected by satellites (e.g. upwards propagating gravity or planetary wave braking, stratospheric warmings and tidal waves).31 The differences in stratospheric temperature measurements between the LIDAR and the two satellites were found to be very small during summer and spring compared to those during winter and autumn. The observed differences are consistent with the findings of Sivakumar et al.32 in their study comparing Rayleigh LIDAR measurements with satellite (HALOE, SABER, GPS-CHAMP and COSMIC) measurements of the stratosphere and mesosphere temperature over a southern sub-tropical site, Reunion Island (20.0°S, 55.5°E). The temperature differences between the LIDAR and SABER seem to be larger than the differences between the LIDAR and HALOE. Temperature differences between the LIDAR and SABER at the height region between 40 km and 50 km were found to be 5 K – 12 K in autumn and 5 K – 10 K in winter. However, the temperature differences between the LIDAR and HALOE were found to be 3 K – 6 K at the height region between 35 km and 50 km, in both autumn and winter. These differences between the LIDAR and the two satellites may be as a result of the difference in the techniques used to retrieve the temperature, the observational time difference or the poor height resolution of SABER (~2 km) and HALOE measurements (3.7 km) in comparison with the resolution of LIDAR measurements (0.3 km). There are some events that appear to have been detected by the LIDAR but were smoothed out in the observations from the satellites. In the southern hemisphere autumn and winter, eastward propagating waves with periods between 7 and 23 days (usually with zonal wavenumber 2) and quasi-stationary planetary waves co-exist in the stratosphere. These planetary waves can lead to the occurrence of different processes in the stratosphere (e.g. wave–wave interactions, wave–mean flow interactions, mean flow reversals and stratospheric warmings). Thus, it is possible that the LIDAR measurements detected these processes, which led to the increase in the temperature difference during the autumn and winter seasons. Similar results were also reported by Randel et al.33 for the northern hemisphere where they compared data sampled at four LIDAR sites to Met Office stratospheric analyses. Moreover, the HALOE and SABER satellite data are for day and night-time observations, whilst the LIDAR measurements were made during the night only. Hervig et al.23 compared the sunrise and sunset observations of HALOE temperature measurements and found sunrise values to be 1 K – 5 K lower than the sunset values. They also compared the HALOE-measured temperature profiles (sunrise and sunset) to those of LIDAR-measured and rocketsonde-measured profiles and noticed that the measurements typically had differences less than 5 K for the height region below 60 km. The differences between the LIDAR and HALOE measurements reported in this study are in agreement with results from Hervig et al.23 Earlier results on an intercomparison study by Randel et al.33 based on different middle atmospheric temperature measurements also indicated that the LIDAR-measured temperature profiles differed by ±5 K. Recently, a study by Sivakumar et al.32 comparing Rayleigh LIDAR and satellite temperature measurements over a southern sub-tropical site (Reunion Island) reported a temperature difference of 5 K – 10 K in the stratosphere. Summary and conclusion Based on 5 years (1999–2004) of Rayleigh LIDAR measurements over Durban, a sub-tropical latitude station in South Africa in the southern hemisphere, the climatology of the temperature of the atmosphere between 30 km and 70 km has been studied and compared with the 4-year (2000–2004) and 5-year (1999–2004) observations from SABER and HALOE satellite instruments, respectively, for the first time. The LIDAR temperature measurements of the stratopause (at ~42 km – 50 km) showed two distinct maxima of 270 K – one during the period from February to July and the other during the period from September to December. All instruments indicated an annual oscillation in the stratosphere. These findings are consistent with previous observations made by Chanin and Hauchecorne6, Hauchecorne et al.15, Gobbi et al.28 and Batista et al.29 The satellites, SABER and HALOE, exhibited the same thermal structure of the middle atmosphere over Durban. Comparisons of seasonal mean temperatures obtained from the Durban LIDAR with HALOE and SABER seasonal mean temperatures were in good agreement. However, during autumn and winter the LIDAR measurements were 5 K – 12 K higher in the height region between 40 km and 55 km. The stationary planetary waves that usually propagate into the stratosphere during winter may be the main cause of a higher temperature difference between the LIDAR and satellite observations in winter and autumn. The difference in methods of data retrieval between the ground-based and space-based instruments, as well as the location and time of the measurements, might contribute to the overall differences between the LIDAR and satellite measurements. In all seasonal profiles, the LIDAR temperature measurements fluctuated above 50 km, indicating a wave-like structure which may be as a result of gravity and tidal waves. The effects of gravity waves and tidal waves over Durban will be examined in detail in a future study. This work was supported by the National Research Foundation of South Africa, the South African Antarctica Programme (SANAP), the French Embassy and CNRS. We also wish to acknowledge Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite, the Upper Atmosphere Research Satellite (UARS) Project (code 916) and the Distributed Active Archive Center (code 902) at the Goddard Space Flight Center, Greenbelt, Maryland, for providing HALOE satellite data. We would also like to acknowledge with thanks Guy Bain and Pi Pacific for operating the Durban LIDAR for many years. Competing interests We declare that we have no financial or personal relationships which may have inappropriately influenced us in writing this article. Authors’ contributions N. Mbatha performed the data analysis and produced the results of the paper. V. Sivakumar, H. Bencherif, S. Malinga and S.R. Pillay assisted in analysing the results and provided input during the construction of the manuscript. A. Moorgawa and M.M. 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