Stratosphere -Troposphere Exchange-Cyclone (STE-C)
(Under CAWSES-India Phase II Program)
The upper troposphere and lower stratosphere (UTLS) is a complex region of the Earth’s atmosphere and the exchange process takes place across the tropopause which is the interface between troposphere and stratosphere. Over the globe, a complex mixing process takes place over the tropics top, which remove water vapour from mixture of air as it enters the stratosphere. Thus, stratosphere-troposphere exchange (STE) and vice-versa processes have been a topic of research for past several decades. In this aspect various ground based, in-situ, satellite measurements, and modelling have been carried out to improve our understanding of STE processes and its associated dynamics. In spite of various experiments and campaign carried out in past, to divulge the important aspects of STE, especially during disturbed weather conditions (viz. cyclone and convection), there are still many problems unresolved and the area is still a focus of increasing scientific interest.
The development of cyclone/depression can overshoot the tropospheric humid air into the stratosphere and if the tropopause temperature is cold enough (<191 K), it will freeze-dry. This will also perturb the tropopause characteristics. Similarly, if tropopause is weaken, the dry zone rich stratospheric air can also penetrate to middle troposphere through tropopause, viz. stratospheric intrusion. Many attempts have been made to quantify such intrusion of stratospheric dry ozone rich air into troposphere and explained in terms of cold-front, low pressure system and tropopause fold, cut-off low, etc. Stratospheric air is easily distinguishable from the tropospheric air because of its high ozone concentration and low water vapour. Many researchers have demonstrated that transition of ozone over tropics is associated with cyclone and convection. Earlier studies have also showed that maximum tropospheric ozone is found in the vicinity of cyclone. Apart from the disturbed weather condition, earlier studies have also shown that the equatorial Kelvin waves enhanced the downward transport of stratospheric air in to the troposphere. However, our present understanding of stratospheric intrusion into the troposphere especially associated with equatorial waves and tropical cyclone as more mass sinking around the convective towers than rising in them is not adequate.
The previous studies on STE during clear-sky and disturbed weather conditions over Indian region are based mainly on the vertical velocity measurement with MST radar, satellite and in situ GPS sonde observations. One of the important constituents of the STE study is ozone, a major greenhouse gas and a major component of the oxidation cycle for the atmosphere. Earlier the continuous measurements of ozone profiling were lacking. A project proposal entitled, ‘Stratosphere Troposphere Exchange-Cyclone (STE-C)’ under CAWSES India Phase-II programme to understand the exchange processes, instabilities, wave dynamics and tropopause characteristics at the upper troposphere and lower stratosphere (UTLS) region under cyclonic and clear-air conditions has been approved by Chairman, ISRO.
This is a collaborative experimental project between Space Physics Laboratory (SPL), Trivandrum and National Atmospheric Research Laboratory (NARL), Gadanki. Simultaneous observations of ozonesonde and radiosonde were made from Trivandrum (8.5°N, 77°E) and Gadanki (13.5°N, 79.2°E) during clear-sky as well as for the entire episode of the tropical cyclones Nilam (30 October 2012 to 7 November 2012) and Phailin (11-15 October 2013). Apart from the balloon born experiments, MST (Mesosphere-Stratosphere-Troposphere) radar was also operated to measure the high resolution vertical velocity and turbulence intensity. Satellite measurements such as SAPHIR (Sondeur Atmosphérique du Profil d’Humidité Intertropicale par Radiométrie) on board the Megha-Tropiques satellite, Aura-MLS (Micro-Limb Sounder) and COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) were also used in the present campaign/study.
Contact Person : Dr. Siddarth Shankar Das (siddarth_das[at]vssc[dot]gov[dot]in)