VHF radar echoes in the vicinity of tropopause during the passage of tropical cyclone: First observations from the Gadanki MST radar

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi: /2007jd009014, 2008 VHF radar echoes in the vicinity of tropopause during the passage of tropical cyclone: First observations from the Gadanki MST radar S. S. Das, 1 A. K. Patra, 2 and D. Narayana Rao 2 Received 28 May 2007; revised 22 November 2007; accepted 21 January 2008; published 15 May [1] First observations on the characteristics of VHF radar echoes in the vicinity of tropopause (VOT) during depression or passage of tropical cyclone made using the Gadanki MST radar are presented. The most significant and new observation is that signal-to-noise ratio (SNR) in zenith and off-zenith beams are nearly equal in the VOT when a cyclone is located close to the radar site. During depression, however, a slight decrease in echo strength with zenith angle was found. Observations made during such disturbed conditions are in contrast to the usual aspect-sensitive characteristics of the radar echoes. Spectral widths during disturbed atmospheric conditions in general are found to be much larger than that of normal condition. Further, spectral widths observed in the vertical beam during cyclone are considerably larger than that during normal condition. These features, however, have not been noticed at other height regions during such atmospheric conditions. All these observations clearly indicate preferential enhancement in turbulence activity in the VOT. These observations constitute the first experimental evidence of such a zenith angle dependence of VHF radar echo characteristics in the VOT during depression and cyclone. These results are presented and discussed in the light of present understanding of turbulence activity in the VOT. Citation: Das, S. S., A. K. Patra, and D. Narayana Rao (2008), VHF radar echoes in the vicinity of tropopause during the passage of tropical cyclone: First observations from the Gadanki MST radar, J. Geophys. Res., 113,, doi: /2007jd Introduction [2] VHF radar echoes are known to be aspect sensitive in general [e.g., Gage and Green, 1978; Roettger and Liu, 1978]. Studies suggested that aspect sensitive echoes are manifested either due to thin stable layer providing sharp refractive index gradient [e.g., Gage and Green, 1978] or shear driven steep layer structures [e.g., Yamamoto et al., 2003; Ghosh et al., 2004]. VHF radar echoes from the vicinity of tropopause (VOT) are found to be highly aspect sensitive [Gage and Green, 1978; Roettger and Liu, 1978; Tsuda et al. 1986; Hocking et al., 1990]. It has been observed that signal strength decreases rapidly up to 10 zenith angle with an average rate of 1.2 db/degree and beyond 10 zenith angle, it is 0.6 db/degree [e.g., Tsuda et al., 1997]. These have been attributed to Fresnel reflection/ scattering from sharp gradients in the radio refractive index [Gage and Balsley, 1980]. These aspect sensitive echo characteristics, however, are observed during normal weather conditions (in the absence of any deep convection or cyclone). Jain et al. [2000] have shown that during convective activity over the radar site SNR observed in the VOT by vertical and 10 off-zenith beams are nearly equal. 1 Centre for Wind Energy Technology, Ministry of New and Renewable Energy, Government of India, Chennai, Tamilnadu, India. 2 National Atmospheric Research Laboratory, Department of Space, Government of India, Tirupati, India. Copyright 2008 by the American Geophysical Union /08/2007JD They have also shown that once the convection passed away from the radar site, VHF radar echoes regained its normal aspect sensitive behavior. Radar echo characteristics during depression/cyclonic activity, however, have not been investigated. [3] Cyclonic storms that occur in the Bay of Bengal while approaching the land often cover the region over Gadanki and it would be interesting to examine how the radar echo characteristics could vary in relation to tropical cyclone. In this context, studies related to the stratosphere-troposphere exchange (STE) processes could be quite relevant. The radar echo characteristics are expected to indicate the stability of the VOT in relation to cyclone and hence some indication of STE process. Although, alone with radar data, it may be difficult to study the STE processes, the radar observations could provide valuable information on dynamics and turbulence that play important role in the STE processes. [4] Experiments conducted using the Gadanki Mesosphere- Stratosphere-Troposphere (MST) radar during normal and cyclonic weather conditions show remarkably different echo characteristics in the VOT. The main intent of this paper is to present and discuss these new results in the light of present understanding of the dynamical processes involved in the VOT. 2. Experimental Method and Data Analysis [5] The MST radar located at Gadanki (13.5 N, 79.2 E) is a high power coherent pulsed Doppler radar operating at 53 MHz. A description of this radar system can be found 1of10

2 Table 1. Radar Parameters Used for the Experiments Using the Gadanki MST Radar Parameter Value Inter pulse period (ms) 1000 Pulse width (ms) 16 Pulse code complementary code with 1 ms baud Range resolution (m) 150 No. of beams E (20,18,16,14,12,8,6,4,2), Z, W (2,4,6,8,10,12,14,16,18,20) N (20,18,16,14,12,8,6,4,2), Z, S(2,4,6,8,10,12,14,16,18,20) a No. of FFT points 256 No. of coherent integrations 64 Incoherent integrations 1 a E, W, N, S and Z represent East, West, North, South and Zenith beam, respectively and number in bracket indicates the oblique angle in degree. elsewhere [Rao et al., 1995]. The radar beam can be steered with 1 step between ±20 off-zenith in the two orthogonal planes (i.e., east-west (EW) and north-south (NS) planes) of the antenna array. [6] In order to study the characteristics of echoes in the VOT in normal (i.e., clear-air) and disturbed (i.e., depression/cyclone) weather conditions, the radar was operated in special experimental mode employing multibeam configuration. For this purpose, the radar beam was swung from 20 off-zenith of east to 20 off-zenith of west and 20 offzenith of north to 20 off-zenith of south with an interval of 2. Pulse returns were sampled at 1 ms, providing range resolution of 150 m. This scheme of beam scanning operation in the two planes could be completed in 12 min. Details of the experimental parameters used for the present study are given in Table 1. If we assume that the basic features of the atmosphere do not change much in 6 min (time required to complete one set of beam scanning in one plane), then one snap shot in principle should provide spatial variation of echo intensity. We, however, know that neither the atmospheric targets responsible for radio wave scattering are passive tracers nor do they drift with uniform velocity. Thus a quick scan of radar experiment would provide an approximate picture of the spatial distribution of echoing regions. It may be mentioned that using fast scanning capability of the MU radar with varied zenith and azimuth angles, Worthington [2004] has studied the spatial variability in echo power. Here we attempt to mainly characterize the radar echoes as a function of zenith angle. For this purpose, we have carried out experiments in different atmospheric conditions to study the radar echo characteristics. Details on the days of experiment and atmospheric conditions are given in Table 2. [7] The observational data were gathered in the form of Doppler power spectrum and stored for off-line processing. In the offline, we computed the three lower order moments and finally obtained signal-to-noise ratio (SNR), mean Doppler velocity and spectral width (two times the square root of variance). Later, we generated maps of SNR as a function of zenith angle, referred to as fan-sector SNR map. As we will see, these maps provide a comprehensive picture of the echo characteristics as a function of zenith angle in different atmospheric conditions. 3. Observational Results and Discussion [8] In an attempt to understand the radar echoes in the VOT, we use observations made in both normal and disturbed atmospheric weather conditions. By normal, we mean that either the atmosphere is free from cloud or have some cloud, but not have the type that signify any largescale unstable system. From this viewpoint, deep depression or cyclone is ascribed to disturbed condition. We should mention that we have not considered localized convection in the present study. In the following, we present observations made during normal and cyclonic/depression conditions Observations During Normal Atmospheric Condition [9] Figures 1a and 1b show the observations made during normal weather condition. Figure 1a shows observations Table 2. Dates and Times of Observations Used for the Present Study and Background Atmospheric Conditions Seen in the Satellite Image Date and Time of Radar Condition/Case Observations Normal Case-I 28 Apr :00 IST Normal Case-II 14 May :00 IST Cyclone case-i approaching 12 Dec 2003 overhead dissipated 18:50 IST 14 Dec :30 IST 16 Dec :00 IST Cyclone case-ii 14 May :20 IST Cyclone case-iii 12 Jun :15 IST Cyclone case-iv 14 Jun :20 IST Cyclone case-v 13 Oct :40 IST Remarks Clear-air sky over the radar site. Some patchy clouds were observed. Well-developed cyclone center was at 1000 km away and cyclonic band structures were observed over the radar site. Well-developed cyclone was 200 km away from the radar site. Cyclone dissipated and the sky overhead was close to normal condition. Low-pressure developed and dense clouds were observed over the radar site. Low-pressure developed over wide area ( sq.km) and the band structures were observed over the radar site. Low-pressure developed and well-defined band structures were observed overhead the radar site. Low-pressure developed over the radar site and but dense patchy clouds were present over the radar site. 2of10

3 Figure 1. Fan-sector map of signal-to-noise ratio in east-west plane (top left panel), north-south plane (top right panel), averaged of both planes (bottom left panel), and satellite image of cloud activity (bottom right panel) corresponding to normal weather condition observed on (a) 28 April 2005 at 11:00 IST and (b) 14 May 2005 at 17:00 IST. The circle in bottom right panel represents the radar site. Arrow represents tropopause height. made at 11:00 IST (Indian Standard time) (IST = GMT+5:30 h) on 28 April The top two panels represent fan-sector SNR maps obtained from a single scan cycle in EW and NS planes, respectively. The bottom left panel shows SNR averaged over 4 scan cycles (24 min observations) made in both planes. Arrow in each panel represents the tropopause height obtained from radiosonde observations made from Chennai. The bottom right panel depicts the satellite picture of cloud activity corresponding to radar observations. The circle marked inside the satellite picture represents the location of the radar site (Gadanki). [10] Similar features observed at 17:00 IST on 14 May 2005 are shown in a similar way in Figure 1b. On this day, however, some cloud patches were present overhead, as can be seen from satellite picture. Despite the fact that some clouds were present over the radar site, the fan-sector SNR maps clearly show aspect-sensitive characteristics of the echoes occurring in the height range of km. SNR is found to decrease as a function of zenith angle and the rate of decrease is estimated to be 1 db/degree between 0 to 10 zenith angle and 0.5 db/degree between 10 to 20 zenith angle. [11] These observations are quite similar to those reported earlier using the Gadanki MST radar corresponding to the normal weather condition [Jain et al., 1997; Ghosh et al., 2004]. They are also quite similar to those reported from other radar observations [e.g., Tsuda et al., 1986, 1997; Hooper and Thomas, 1995; Palmer et al., 1998]. Such an angular dependence of echo power has been attributed to anisotropic behavior of the atmospheric scatterers. Close to zenith and in the VOT, scattering is believed to be dominated by sharp vertical gradient in refractive index with a horizontal correlation length comparable to first Fresnel dimension as compared to anisotropic scattering generated by turbulence. At angles >10, they are mainly due to refractive index variation caused by turbulence. At altitude below the VOT, some aspect sensitivity is often observed and is attributed to anisotropic turbulence [Doviak and Zrnic, 1984; Hocking and Roettger, 2001]. We, however, do not discuss this aspect here and confine our presentation and discussion to the VOT Observations During Depression/Cyclonic Condition [12] A low-pressure area was formed in the west of the Bay of Bengal during late evening hours on 12 December 2003, which gradually transformed into deep-depression and finally turned into a tropical cyclone during the midnight hours of 14 December Then it moved slowly toward northwest and finally dissipated in the early morning hours on 16 December These observations are presented in Figures 2a 2c. Figure 2a shows the observations made during 18:50 19:06 IST on 12 December 2003 in a similar way as that of Figure 1. It may be noticed that the cyclone was well developed at this time but the cyclone center was located at a radial distance of 1000 km from the radar site. Note that some of the band structures, however, are close to the radar site. As can be seen in the top left panel, the radar echoes from altitude of about 16 km observed by east beam are somewhat stronger than those observed in the other beam directions. The average picture of SNR variation as a function of zenith angle (bottom left panel), however, shows no remarkable difference when compared with that of normal condition. [13] Later, when the cyclone center moved close to the radar location, radar echo power with zenith angle was seen to have significant change. Figure 2b shows the observations made at 23:30 IST on 14 December 2003 when the 3of10

4 Figure 2. Same as Figure 1 but for observations made on (a) 12 December 2003 at 18:50 IST when a cyclone was approaching the radar site, (b) 14 December 2003 at 23:30 IST when a cyclone was overhead, and (c) 16 December 2003 at 08:00 IST when the cyclone is dissipated. cyclone center is close to the radar site. In this case, it is estimated that the center of the cyclone was 200 km away from the radar site. During this condition, well-developed band-like structures were observed just over the radar site. The fan-sector maps clearly show strong echoes at all zenith angles at altitudes close to 16 km unlike that observed in normal condition. [14] Observations made at a later time, when the cyclone is dissipated, such a feature disappeared and the fan-sector maps showed usual aspect sensitive echoes in the VOT. Observations made at 8 IST on 16 December 2003 are presented in Figure 2c to depict the post-cyclone scenario. As can be seen from Figure 2c, cyclonic cloud is still present, but in the N NE direction from the radar site. This possibly implies that overhead presence of cyclonic cloud and/or associated dynamics is essential for the manifestation of the unusual features of radar echoes that we are observing during cyclone. [15] In order to confirm that the radar echoes in the VOT during near-overhead cyclone condition are very different from normal aspect sensitive echoes, experiments were repeated on 4 more cyclonic events. Observations made at 12:20 IST on 14 May 2004, 10:15 IST on 12 June 2004, 16:20 IST on 14 June 2004 and 11:40 IST on 13 October 2005 are presented in Figures 3a 3d, respectively. The fansector maps observed on all the four events show features 4of10

5 Figure 3. Same as Figure 1 but for observations made on (a) 14 May 2004 at 12:20 IST, (b) 12 June 2004 at 10:15 IST, (c) 14 June 2004 at 16:20 IST, and (d) 13 October 2005 at 11:40 IST. For all these four cases, the depression was overhead the radar site. quite similar to that of 14 December 2003 (shown in Figure 2b). In these case also, strong echoes were observed in all zenith angles, which are very different from usual aspect sensitive echoes in this height region. Note that the height of occurrence of such features varied slightly from one event to another, but in all the cases unusual features were observed in the VOT. Also some aspect sensitive echoes could be noticed below the height of the unusual echoing region, but this feature is not consistent in all the cases. [16] In order to examine the height of the echoes with reference to tropopause, we have obtained temperature profiles with height resolution of m from routine radiosonde measurements made at Chennai (13.04 N, E). On one of the days (14 May 2004), simultaneous observation of temperature profile was made at Gadanki. Temperature profile was obtained using GPS sonde, providing height resolution of 10 m. It may be mentioned that for observations made from Chennai, we have chosen those temperature measurements where the balloon reached the tropopause and are also close to the time of radar observations presented in this study. It may be noticed that except for 14 May 2004, temperature observations were not made simultaneously. Temperature observations made on normal days have also been considered to compare them with those of disturbed days. Temperature profiles corresponding to normal and disturbed days are presented in Figures 4a and 4b, respectively. As is evident, temperature profiles corresponding to disturbed days show significant structures 5of10

6 Figure 4. Height profiles of temperature obtained from radio/gps sonde flights during (a) normal and (b) disturbed conditions on different days. as compared to normal days. On 14 May 2004, both temperature profiles (made from Gadanki at 12:30 IST and from Chennai at 17:30 IST) show remarkable structures as a function of height. Because of high-resolution capability of GPS sonde measurements as compared to radiosonde, fine structures in temperature present on 14 May 2004 could be revealed. Interestingly, on this day, the cold point tropopause and the height of the unusual radar echoes coincided extremely well. On the other days also the height of the echoing layer was close to the cold point tropopause. Notably, on 12 and 14 June 2004, the cold point tropopause was observed at relatively higher altitude than on other days and radar echoes were also observed at higher altitude (17 km). The cold point tropopause and the height of unusual radar echoes corresponding to the days considered are summarized in Table 3. As is evident from the table, the height difference of radar echoes and cold point tropopause is within ±500 m, which is of the order of the height resolution of the radiosonde measurements ( m). Hence the above observations clearly indicate that the unusual radar echoes observed during the cyclonic conditions are in the VOT. In the normal atmospheric condition, although strong echo was observed in the VOT, they were confined only to the near-vertical beams, clearly indicating the aspect sensitive nature of the echoes. [17] Cloud brightness temperatures obtained from satellite based measurements showed significantly low values (200 K) corresponding to the cyclonic events, suggesting the presence of high altitude cloud, as compared to the values observed in normal condition (270 K). Cloud brightness temperature of 200 K will correspond to cloud height of 15 km. While this provides qualitative information on the presence of high altitude cloud during cyclone as compared to the normal atmospheric condition, the correct altitude of the cloud top, however, is not known to compare it with the height of unusual radar echoes and tropopause. [18] To quantify zenith angle dependence of SNR during normal and cyclonic conditions and also to obtain a general picture, we have considered the heights of the strong echoing layer observed near the VOT with the vertical beam. Accordingly, SNR values corresponding to the nearest height observed in other beams have been chosen. Also we have used observations made in both planes (i.e., EW and NS) to derive height-averaged values. A careful scrutiny of the data suggests that features are slightly different during depression and cyclonic conditions and accordingly, we have considered them separately to study the features in detail. The events have been defined as depression or cyclone by India Meteorological Department, which follows the guidelines of World Meteorological Organization. For the normal condition we have used 8 cycles of measurements made on 28 April 2005 and 14 May For overhead cyclone condition, only 4 scan cycles of data gathered on 14 December 2003 have been used, since on other days, the conditions were categorized as depression. For depression, 14 cycles of observations made on 12 June Table 3. Heights of Radar Echoes and Cold Point Tropopause Observed on Different Days Condition/Case Date and Time of Radiosonde Observations Height of Radar Echoes Height of CPT Normal case-i 28 Apr :30 IST 17 km km Normal case-ii 14 May :30 IST 17 km km Cyclone case-i approaching overhead dissipated 12 Dec :30 IST 16 km 16.5 km 14 Dec :30 IST 16 km km 16 Dec :30 IST 15.9 km km Cyclone case-ii 14 May :30 IST 17.4 km km Cyclone case-iii 12 Jun :30 IST 17.8 km km Cyclone case-iv 14 Jun :30 IST 18 km km Cyclone case-v 13 Oct :30 IST 16 km km 6of10

7 Figure 5. Mean and standard deviations of signal-to-noise ratio of the echoes from the VOT as a function of zenith angle during (a) normal, (b) cyclone, and (c) depression observed on different days. 2004, 14 June 2004, and 13 October 2005 have been used. Mean and standard deviation of SNR as a function of zenith angle during normal, cyclone overhead, and depression conditions are presented in Figures 5a 5c, respectively. [19] The observations show that SNR variations as a function of zenith angle during normal and depression/ cyclonic conditions are very different. In normal condition, SNR is found to be maximum near-zenith and falls monotonically up to 7 off-zenith angle. After 7, SNR is found to be nearly equal or decreases very slowly with zenith angle. SNRs in off-zenith beams >7 are found to be about 14 db lower than that of near-zenith beam (SNR in the zenith beam is 13 db). It is estimated that the linear decrement is about 2dB/degree. In contrast, during cyclone, SNRs in all the beams are found to be nearly equal and are equal to that observed in zenith beam during normal condition. During depression, however, SNR is found to decrease slowly with zenith angle. It may be noted that SNRs in 20 off-zenith beam during depression and normal condition are nearly equal, suggesting that the scattering processes are leading to anisotropy. The linear decrement in SNR as a function of zenith angle during depression is calculated to be 0.3 db/degree. [20] To gain further insight, we examine spectral characteristics as a function of zenith angle corresponding to normal and cyclonic conditions. Figure 6 shows Doppler spectrum as a function of zenith angle observed in the VOT during normal atmospheric condition. Left and right panels present spectral features in the EW and NS planes, respectively. These spectra were observed at 11:00 11:12 IST on 28 April As shown, the spectra are narrow and are nearly equal at all zenith angles. Also note that signal strength decreases as a function of zenith angle and becomes remarkably low beyond 12 zenith angle, which are consistent with the fan sector maps presented for normal cases. These figures also demonstrate that the mean Doppler shift increases with zenith angle depending upon the background wind as expected. [21] Figure 7 shows an example corresponding to cyclone in a similar way as that of Figure 6. In this case also, the mean Doppler increases as a function of zenith angle as expected due to finite background wind. Spectral widths in this case, however, are much larger than the normal condition. Spectral width in cyclonic condition is found to be more than double in general and as high as 4 times of that of normal condition. Also we find that the largest spectral width is found to be around 10 zenith angle. Figure 6. Doppler spectra as a function of zenith angle in (a) east-west and (b) north-south directions corresponding to normal weather condition observed on 28 April 2005 at 11:00 IST. 7of10

8 Figure 7. Same as Figure 6 but for observations made on 14 December 2003 during 23:30 23:42 IST corresponding to cyclone. [22] To show the variability of spectral feature as a function of zenith angle in different phases of cyclone, we present a series of such maps in Figure 8 corresponding to the cyclonic event of 13 October Note that in this case, we start presenting the observations when the cyclone was active and located close to the radar site and gradually to the diminishing state of the cyclone. Note the time of observation in each panel. It is very clear from these figures that when the cyclone is close to the radar site, spectral widths are large and as the cyclonic activity decays, spectral widths gradually decrease and tend to show normal behavior. [23] From the observations presented above, we find that during cyclonic condition, SNRs of radar echoes from the VOT observed in the off-zenith beams are nearly equal to that of zenith beam. During, depression, however, a small decrement in SNR with zenith angle has been observed. Thus observations in both conditions are different from normal aspect sensitive behavior of the echoes observed during normal conditions. Large spectral width with enhanced echo power as compared to normal condition was also observed, which indicates enhanced level of turbulence. While the enhanced spectral width in the off-zenith beam could be partly due to shear broadening effect, the enhanced spectral width observed in all beam have to be related to enhanced turbulence. Using the MU radar observations, Hermawan et al. [1998] have shown that aspect sensitive echoing regions are closely associated with the stability of the atmosphere. Using coordinated observations made by the MU radar and radiosonde during the passage of a typhoon, Sato et al. [1991] have observed the presence of isotropic scattering. In the same region after the passage of the typhoon, the scattering property changed to anisotropic, which was found to be associated with temperature inversion. The isotropic scattering was attributed to convectively unstable region or low-stability. Near isotropic scattering observed in the VOT by Jain et al. [2000] during tropical convection was also attributed to convective instability. Hence isotropic characteristics of the echoes with enhanced spectral width in the VOT observed during cyclone can be attributed to the prevailing unstable conditions in the VOT. Using Gadanki radar observations, Singh et al. [1999] have shown enhanced turbulent activity at altitudes close to tropopause. They have explained their observations in terms of KHI manifestation. Using the Equatorial Atmosphere Radar (EAR) located at Kototabang, Indonesia, Yamamoto et al. [2003] have also shown enhanced turbulent activity in the VOT arising from KHI. Using the same radar (i.e., EAR) observations, Fujiwara et al. [2003] have reported enhanced turbulence at the tropopause and shown that they are convectively generated in the breaking phase of an equatorial Kelvin wave. Strong isotropic radar echo enhancement has also been observed in the VOT by the Middle and Upper atmosphere radar situated at midlatitude location Shigaraki, Japan and has been attributed to KHI [Luce et al., 2002]. These observations, however, were conducted in normal atmospheric conditions (i.e., not having any depression/cyclone around). The observations presented here thus differ from those of others in the sense that they all are related to disturbed atmospheric conditions. For one of the events (14 May 2004) on which temperature observation was available, we have evaluated the atmospheric condition in terms of Richardson number (Ri). Figures 9a 9c show height profiles of potential temperature, wind speed and Richardson number (Ri), respectively. We find that Ri is less than 0.25 at 17.3 km, an unstable condition for the likelihood of shear instability (i.e., KHI). Interestingly, enhanced SNR and turbulence have also been observed on other days during both depression/cyclonic conditions. Simultaneous temperature measurements, however, are not available to derive Ri so that the strong echoing regions can be characterized in terms of shear driven instability. One important point to be noted is that in the observations of both Singh et al. [1999] and Yamamoto et al. [2003], the unstable region was above the cold point tropopause and was associated with large wind shear in zonal wind. Although temperature measurements were not simultaneous on all the cases, the present observations also indicate that the strong echoing regions were mostly located above the cold point tropopause, a feature quite similar to those of 8of10

9 Figure 8. Same as Figure 7 but for observations made on 13 October 2005 during 11:40 12:40 IST. normal time KHI manifestation. Hence it is quite likely that shear and temperature structures associated with tropical cyclones might have been providing suitable condition for the KHI to be operative. This however, needs to be ascertained with simultaneous measurements of temperature and radar observations employing various modes of radar observations to characterize the radar echoes in the fan sector map and KH billow structures. The breaking of Kelvin waves [Fujiwara et al., 2003] to generate turbulence through convective instability also could be equally important. Further investigations are essential to evaluate these 9of10

10 Figure 9. Height profiles of (a) potential temperature, (b) wind speed, (c) Richardson number derived from GPS sonde and radar measurements made on 14 May two possibilities in understanding the observations related to cyclone presented here. 4. Summary and Concluding Remarks [24] We have shown that VHF radar echo characteristics in the VOT during depression/cyclonic conditions deviate from it normal time aspect sensitive characteristics. From the observations, it is evident that equally strong echoes were observed in all zenith angles in the VOT during cyclonic condition. In case of depression, however, a slight decrease in echo strength with zenith angle was found. Further, it is shown that the spectral widths observed in all the zenith angles during disturbed conditions are more (sometimes 4 times) than that observed during normal atmospheric conditions. We have also shown that SNR and spectral width return to their normal values once the cyclone dissipates. These unusual characteristics of radar echoes are mostly observed above the cold point tropopause as evident from the temperature profiles. It is found from one case study that such enhancement in turbulence activity is associated with large wind shear characterized by Ri < On the basis of this fact we surmise that KHI operating above the cold point tropopause might be involved in manifesting the enhanced turbulence observed in our data. We, however, do not know whether all the features presented here are simply related to KHI. The observations on the echo characteristics in the VOT during cyclone reported here are new and need to be studied further to understand their generation mechanism. [25] Acknowledgments. The Gadanki MST radar belongs to National Atmospheric Research Laboratory (NARL), an autonomous organization under Department of Space, Government of India. We would like to thank NARL technical staff for their support in conducting these experiments. 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Fukao (2003), Kelvin-Helmholtz instability around the tropical tropopause observed with Equatorial Atmosphere Radar, Geophys. Res. Lett., 30(9), 1476, doi: /2002gl S. S. Das, Centre for Wind Energy Technology, Ministry of New and Renewable Energy, Government of India, Velachery-Tambaran High Road, Pallikaranai, Chennai, Tamilnadu , India. (dassiddhu@rediffmail. com) A. K. Patra and D. Narayana Rao, National Atmospheric Research Laboratory, Department of Space, Government of India, P.B. 123, Tirupati , India. 10 of 10