D. Narayana Rao

Layered clouds in the Indian monsoon region

Contrary to the prevalent belief that tropical region is characterized by convective clouds rather than by layer clouds, we have suggested that deep convective clouds occur on meso-scale, but layer clouds occur on larger synoptic-scale with a relatively small region of deep convective clouds. Sustenance of deep convective clouds is inhibited by the presence of inertio-gravity waves, which have alternating layers of upward and downward motion in the vertical. We have also shown that inertio-gravity waves generate regions of relatively strong horizontal velocity, vertically separated by layers of relatively weak horizontal velocity. Layers of strong horizontal velocity are created by inertio-gravity wave system through convergence of vertical flux of horizontal momentum. We have also suggested that horizontal convergence/divergence of moisture flux is generated by inertio-gravity waves, giving rise to vertically alternating layers of high/low humidity, and visible or sub-visible clouds. Layers of high humidity become layers of strong radar reflectivity at frequency of 53 MHz at which MST Radar at Gadanki, near Tirupati, India, operates. These observations, more than 2,50,000 in number, for vertical grid points, spread over all the months of the year, have helped us, among other observations, to arrive at these conclusions. Further, the analysis suggests that the main source of strong MST radar reflectivity is not mechanical turbulence as is commonly believed.

Lidar observations of sporadic Na layers over Gadanki (13.5° N, 79.2° E)

We studied the characteristics of sporadic sodium layers (SSLs) observed with the sodium (Na) resonance scat- tering lidar at Gadanki (13.5 N, 79.2 E). The SSLs were observed on a total of 63 occasions during 464 h of Na lidar observations from January 2005 to February 2006. The ob- servations showed that one SSL event occurred, on average, every 7 h. The most prominent sporadic layer, which formed on 12 February 2005, exhibited a peak density of 60 722 Na atoms/cm 3 around 92 km and it was nearly twice the peak density reported from elsewhere using ground-based obser- vations. In general, the SSLs exhibited the following char- acteristics: (1) they developed at heights between 88 and 98 km with an average height around 94 km; (2) maximum den- sity occurred during the early morning hours between 02:00 and 05:00 IST; (3) the ratio of the maximum peak Na den- sity to the average density was normally around 3 to 5 and it exceeded even 10 in some cases; (4) the events lasted from a few minutes to several hours. The formation period of the SSLs was longer compared to the decay period of the SSLs. Most of the SSL events showed downward motions.

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.