P. Vishnu Prasanth

The first lidar observations of the nighttime sodium layer at low latitudes Gadanki (13.5 ◦ N, 79.2 ◦ E), India

We report on the first lidar observations of the nighttime mesospheric sodium layer from Gadanki (13.5°N, 79.2°E) site in India. The lidar measurements of upper atmospheric sodium made on 6 nights between the 10 and 16 January 2005 are presented in this paper. The Gadanki lidar uses a Nd:YAG pumped dye laser, tuned to the sodium D2 line (589.0 nm), as a transmitter. Using the system, sodium number density profiles have been obtained with a vertical resolution of 300 m, a time sampling of 120 s. During the initial six nights of observation, the peak sodium concentration is found at a height of 95 km, and the top side scale height is usually about 2 km. On three occasions, a secondary peak was observed at heights between 87 and 92 km. Measurements at Gadanki site indicate that the mean sodium abundances appear to decrease after sunset and increase before sunrise. The average nocturnal columnar abundances were in the range 2–8.9 × 109 cm2. The nightly mean centroid heights range between 92.9 and 95.2 km and the rms widths vary between 4.3 and 4.9 km. On some nights, wave like structures in the sodium layer were observed with wavelength of about 3 km and downward phase velocities of about 1 km/hr. Four sporadic layers were observed during the initial 54 h of observation. The formation and decay of an intense sporadic sodium layer was observed on the night of 11 January 2005. The layer was found to develop between 93 and 90 km altitude and appear between 0230 and 0430 LT.

Cirrus observations using ground-based LIDAR and Terra MODIS instrument

A climatological study of cirrus occurrence has been carried out using the ground-based lidar observations over Gadanki (13.5°N, 79.2° E) during 1998-2004. The Moderate Resolution Imaging Spectroradiometer (MODIS) measurements on the Terra spacecraft are also used for remote sensing of high clouds cirrus from space during 2001-2004. The interannual study using LIDAR and MODIS shows an enhancement in the cirrus occurrence during 2001 and fewer amount during 2002. The interseasonal variation of cirrus occurrence frequencies shows much of the occurrences during the monsoon season. Further lidar observations shows that the cirrus cloud tops typically extended to near the 16.53 km, the average tropical tropopause height. The distribution of maximum cloud base height frequency is confined to 10-12km. Frequency of occurrence of cloud physical thickness with respect to cloud base height (Zb) gives a higher occurrence between 11- 15 km and typically the thickness of 2-4 km. At the cloud base height Zb>15km, which is in the vicinity of tropopause, the cirrus is found to have lesser thickness. A significant observation from this statistical study over this latitude shows appearance of cirrus at two different altitudes because of different formation mechanisms. We will also discuss the formation mechanisms for the occurrence of tropical cirrus at this latitude.