Vikram Sarabhai Space Centre

About: Vikram Sarabhai Space Centre is a facility organization based out in Thiruvananthapuram, India. It is known for research contribution in the topics: Aerosol & Ultimate tensile strength. The organization has 2092 authors who have published 3058 publications receiving 47975 citations. The organization is also known as: VSSC.

Do the stability indices indicate the formation of deep convection

Abstract: The present study investigates the relation between the stability indices and different types of precipitating clouds during the active and the suppressed periods of deep convection of the Madden–Julian oscillation. This is achieved by utilizing three-hourly radiosonde (RS92) data and merged cloud radar data over the Gan Island (0.69°S, 73.15°E) from October 2011 to January, 2012. The active and the suppressed periods are defined based on the rainfall. Three periods of active (15–27 October, 15–28 November and 15–27 December) and suppressed periods (01–14 November, 0–14 December and 01–14 January) are identified. During the above periods, the stability indices are calculated to distinguish the background meteorological conditions. The analysis shows that during both the active and the suppressed periods, the magnitude of the stability indices are not much different. During both the periods, the indices attain their respective threshold corresponding to the occurrence of deep convection. However, the third suppressed period shows a dry condition compared to the other two suppressed periods. The relation between the stability indices and the precipitating cloud categories (shallow, congestus and deep) indicate that even though the threshold in the stability indices were attained, deep convective clouds were not observed during the suppressed periods. The active period correlates well with the stability indices. Therefore, the stability indices do not clearly and directly determine the state of the atmosphere during deep convection. The result shows stability indices need to be substantially improved in the context of deep convection prediction.

Photochemistry of atomic oxygen green and red-doublet emissions in comets at larger heliocentric distances

Abstract: Context. In comets, the atomic oxygen green (5577 A) to red-doublet (6300, 6364 A) emission intensity ratio (G/R ratio) of 0.1 has been used to confirm H2 O as the parent species producing forbidden oxygen emission lines. The larger (>0.1) value of G/R ratio observed in a few comets is ascribed to the presence of higher CO2 and CO relative abundances in the cometary coma.Aims. We aim to study the effect of CO2 and CO relative abundances on the observed G/R ratio in comets observed at large (>2 au) heliocentric distances by accounting for important production and loss processes of O(1 S) and O(1 D) atoms in the cometary coma.Methods. Recently we have developed a coupled chemistry-emission model to study photochemistry of O(1 S) and O(1 D) atoms and the production of green and red-doublet emissions in comets Hyakutake and Hale-Bopp. In the present work we applied the model to six comets where green and red-doublet emissions are observed when they are beyond 2 au from the Sun.Results. The collisional quenching of O(1 S) and O(1 D) can alter the G/R ratio more significantly than that due to change in the relative abundances of CO2 and CO. In a water-dominated cometary coma and with significant (>10%) CO2 relative abundance, photodissociation of H2 O mainly governs the red-doublet emission, whereas CO2 controls the green line emission. If a comet has equal composition of CO2 and H2 O, then ~50% of red-doublet emission intensity is controlled by the photodissociation of CO2 . The role of CO photodissociation is insignificant in producing both green and red-doublet emission lines and consequently in determining the G/R ratio. Involvement of multiple production sources in the O(1 S) formation may be the reason for the observed higher green line width than that of red lines. The G/R ratio values and green and red-doublet line widths calculated by the model are consistent with the observation.Conclusions. Our model calculations suggest that in low gas production rate comets the G/R ratio greater than 0.1 can be used to constrain the upper limit of CO2 relative abundance provided the slit-projected area on the coma is larger than the collisional zone. If a comet has equal abundances of CO2 and H2 O, then the red-doublet emission is significantly (~50%) controlled by CO2 photodissociation and thus the G/R ratio is not suitable for estimating CO2 relative abundance.