Seasonal variations in Tropical Tropopause Layer Thickness (TTL) (m) for clear sky and cloudy conditions defined in terms of METEOSAT IR (Infra Red) Brightness temperatures. 

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... at 88 km during 0830 – 1730 IST. Figure 10 shows the short term variabilities of the zonal and meridional components of the diurnal tide observed in the MLT region by the MF radar at Tirunelveli for the period February – March 2006. The observations indicate the presence of 15 – 20 day modulation of diurnal tide at MLT heights, which is an important finding of the MF radar observations at Tirunelveli. One possible source for the observed variability of the diurnal tide in the MLT region could be the deep convection in the lower atmosphere. The earlier work of Gurubaran et al. (2005), based on simultaneous MF radar observations from Jakarta and Tirunelveli, clearly indicated a possible role of deep tropical convection in causing the diurnal tide variabilities on seasonsal time scales. To explore the possibility of short term variations in convective activity having relationship with the observed tidal variabilities, the variations of daily OLR are examined along with that of tidal amplitude. Figure 11 shows the contour plot of OLR observed over 70 0 – 210 0 E at 7.5 0 N for the period February – March 2006, along with the amplitude of the diurnal tide at MLT heights and the variation of OLR averaged over 120 0 – 160 0 E. The similarity between the variations in the OLR field and the amplitude of the diurnal tide suggests a possible link between the observed MLT tidal variabilities and the variabilities in the deep tropical convection through the non-migrating tides it generates (Gurubaran et al., 2008). The other important studies carried out under theme 3 include: MST radar observations of tropical tropopause variations; the tropopause dynamics and its implication to stratosphere–troposphere exchange of minor constituents; the solar wind-magnetosphere-ionosphere coupling effects observed in low latitude ionosphere. Under the Multi-institutional proposal on Coordinated measurements for understanding “Tropical Tropopause Dynamics”, extensive analysis of collocated measurements from MST Radar, Lidar, GP Sondes and of satellite data from KALPANA-1 and METEOSAT have been carried out on detection of Semi- Transparent Cirrus (STC) and on properties of cirrus clouds in the Tropical Tropopause Layer, characterization of Tropopause layer in terms of MST Radar observed major convective outflow, studying variations in TTL during METEOSAT observed clear sky and convective conditions for four seasons summer, monsoon, winter, and spring. Rajeev et al. (2008), based on IR (Infra Red) and WV (Water Vapour) Brightness temperatures from KALPANA-1 and cirrus COD (Cloud Optical Depth) obtained from collocated Lidar observations at Gadanki (13.5_N, 79.2_E ) for 12 nights in different seasons under contrasting cloud conditions, have shown that Satellite-derived STC amount is highly reliable when COD > 0.02 and is an underestimate when it decreases below 0.02, especially when they appear as broken clusters. However, in most of the cases, satellite could detect very thin STC with COD < 0.02 when they are widespread. Furthermore, Parameswaran et al. (2009) showed that very thin Sub-visible type cirrus clouds forming very close to TTL top remain mostly aligned with cold point tropopause, whereas, cloud-tops of thicker and structured TTL cirrus are mostly aligned with the top of TTL and their base show temporal variations in accordance with that of TTL-base. Here, TTL is defined (Tropical Tropopause Layer ) as the altitude region between the top of the tropospheric convective outflow (based on MST data) and the cold point tropopause. They also examined association of tropospheric turbulence with cirrus by using lidar-derived cloud scattering properties and MST radar derived turbulence parameters and reported that the turbulent kinetic energy dissipation rate ( ε ) shows a prominent increase in TTL and the vertical structure of the cloud responds favourably to that of ε . Mehta et al. (2008), using the MST radar observations at Gadanki, estimated the altitudes of major convective outflow in the troposphere ( represented as convective tropopause) for the period May 2006 to April 2007. They showed that altitudes of estimated convective outflow match well with Lapse Rate Minimum altitudes (the altitude of trough of Lapse Rate in the vertical closest to the cold point tropopause) with an RMS difference of 0.8 km as shown in Figure 12 . The altitude of major convective outflow from the radar gives a good measure of the base of the tropical tropopause layer (TTL). Narendra Reddy et al. (2009) analysed high vertical resolution GP Sonde measurements at 12 GMT (Vaisala RS-80, RS-92, Japan make Meisei RS-01GH) archived for location Gadanki (13.5 0N, 79.2 OE) along with METEOSAT IR brightness temperatures for the period from 19 April 2006 to 31 January 2009 for characterizing tropopause variations and classified TTL thickness for four seasons March-April-May, June-July-August, September-October-November and December-Jan-February as shown in Figure 13 for sky conditions varying from clear sky (BRT >280 K-Red) to deepest (BRT < 210 K- Blue), very deep ( 210 K< BRT < 235 K-Green), deep ( 235 K< BRT < 260 K-Yellow), middle level ( 260 K< BRT < 270 K-Black) cloud systems. The range (270 K< BRT < 280-Majenta ) may correspond to either cirrus or low level clouds which need further investigation to classify. It is striking to note that TTL thickness during clear sky conditions can be lowest or as low as that during convective conditions irrespective of the season except in winter season (DJF). Furthermore, Kusuma Rao and Narendra Reddy (2009) showed that TTL thickness shrinks to ~1 Km during “Disturbed conditions” over Bay of Bengal (11.90 N, 88.60 E) due to lowering of CPT Altitude and as well due to increase in the Lapse Rate Minimum Altitude associated with deepest cloud systems based on ship based GP Sonde measurements (76 profiles in 12 Days of disturbed and non- disturbed conditions) during JASMINE- an international programme. A comprehensive study has been made of the climatology of the equatorial and low latitude ionosphere-thermosphere system using long series of data from Indian ionosonde network and the multi-frequency radio beacon observations of total electron content (TEC) and ionospheric scintillations. The data from SROSS – C2 has been used to study the characteristics of equatorial midnight temperature maximum (MTM) and also to develop the electron density and electron temperature models for the topside ionosphere over the Indian zone. The electron density data from SROSS – C2 was used in conjunction with IRI to demonstrate the limitations of the median models to represent the spatial gradients in the equatorial and sub-tropical latitudes in the Indian sector. Using ionosonde data and the IRI model, complete electron density profiles were derived to study the temporal variability of the centroid of the density distribution in the equatorial anomaly region. The study has shown that uniform ionospheric pierce point altitude can not be assumed for GPS applications in the anomaly region of the Indian sector ( Niranjan et al., 2007). Radio beacon studies have been made extensively by many institutions across the country for well over three decades, building up a large database for TEC, and scintillations at VHF and L band for the equatorial and low latitude Indian sector (Rama Rao ,2007). The TEC and scintillation data have assumed great significance in view of their importance to the performance assessment of the satellite based communication and navigation systems (e.g.,Das Gupta et al., 2004; Rama Rao et al., 2006). The TEC and scintillation models have been developed by individual groups, making use of limited data sets. However, comprehensive climatological models have yet to be developed, based on the data generated over the whole country for well over three decades. These models should find application in optimal design of space based communication and navigation systems, apart from being of value for better understanding of the long term variabilities of the equatorial and low latitude ionosphere. The development of application oriented ionospheric models for short and long term predictions of F region parameters over Indian zone has been a significant accomplishment of the studies carried out under space climatology. Two types of models have been developed, one based on second degree (SD) empirical relationships of F F2 and M(3000)F2 to the monthly mean sunspot number and the 0 other based on multi-regression analysis (MRA) involving expressions relating F F2 and M(3000)F2 to S and Ap indicies, representing the solar and 0 10.7 geomagnetic activities, respectively (Dabas et al., 2008). The best fit coefficients for the SD model were obtained, making use of more than two solar cycles of ionosonde data from 14 stations, including a few from neighboring countries. The MRA model was developed for Delhi using more than half a solar cycle of digital ionosonde data. The best fit MRA model coefficients for each month at hourly intervals were obtained separately for magnetically quiet (Ap < 25) and disturbed (Ap > 25) conditions. Figure 14 shows a comparison of the SD model contour plot of f F2 for March 1958, representing high solar activity, with the corresponding 0 observed and IRI model plots. The comparison clearly shows that the SD model is a significantly better representation of the ionosphere over 0 – 45 0 N in the Indian sector compared to the IRI model. Figure 15 presents storm time variations of the F region parameters for the MRA model along with the corresponding observed and IRI model plots for the magnetic storm of October 28 – November 1, 2003. The MRA model has done fairly well in reproducing the storm time behavior of F F2 and hmF2, not only the 0 gross variations but short term fluctuations as well. The comparison clearly brings out the intrinsic limitations of the IRI in reproducing not only the short term ...