Vikram Sarabhai Space Centre

We investigate the thermal structure of the Venusian middle atmosphere between 45 and 80 km using radio occultation (RO) measurements from Venus Express and Akatsuki spanning 2006-2024. Retrieved temperature profiles are compared with climatological predictions from the Venus-GRAM and the Venus Climate Database (VCD). Systematic deviations exceeding 10 K are observed between the RO temperatures and model climatologies, particularly at high latitudes in both hemispheres. The long-term dataset further reveals a possible decadal-scale temporal variability in temperatures across the low-to-mid latitude regions. This variability becomes less coherent at higher altitudes. A combination of post-stratification and bootstrap analysis on the temperature anomalies indicates that the observed temporal variability at low-to-mid and polar latitudes is not readily explained by the latitudinal sampling bias quantified using the adopted analysis, while the trends at mid-to-high latitudes are affected by sparse and uneven RO sounding. Sensitivity tests using modified cloud albedo inputs in VCD simulations show that adjusting cloud radiative forcing partially reconciles the discrepancies, especially at the lower altitudes, below 60 km. However, above the cloud top, at the lower latitudes, the divergence from the observations increases significantly, thereby failing to provide a general reconciliation between VCD and RO. These results highlight the need for updated empirical climatologies incorporating recent RO measurements and for improved physical parameterizations in Venus general circulation models to better capture the global variability in the planet's middle atmosphere.

This study presents a comparative analysis of aerosol black carbon (BC) characteristics during 2015–2019 at two tropical locations in Southern India: Thumba (a coastal semi-urban site) and Ponmudi (a rural highland station) in the Western Ghats. These sites, separated by 1 km in elevation and 35 km in distance, experience similar synoptic-scale air masses but different mesoscale meteorological conditions. The analysis revealed substantial heterogeneity with a higher annual mean BC at Thumba (2375±1492 ng m−3) than at Ponmudi (1143±794 ng m−3). Notwithstanding the observed similar annual pattern (though with varying BC amplitudes) at two sites, contrasting diurnal BC patterns highlighted the role of short-scale driver processes. Diurnally, BC displayed a ‘two-peaks and a midday-trough’ pattern at Thumba, driven by local atmospheric boundary layer dynamics and sea-land breeze interactions. Ponmudi exhibited a distinct ‘afternoon BC peak followed by a plateau’, which can be attributed to mesoscale transport dynamics, including convective uplift of aerosols and transport by sea-breeze from the coastal region towards the hills, and their further elevation by upslope winds to the hilltop. The WRF-Chem model simulations supported the above mechanism, highlighting the complex interactions of topography, local meteorology, transport pathways, and mesoscale processes in controlling BC variability in tropical environments.

Orography and surface winds modulate the organization and characteristics of precipitation over mountain areas. Four different surface directional wind (SDF) flow/patterns, namely, upslope-seaside, along-seaside, upslope-landside, and along-landside are considered as the trigger mechanism for convective organizations (COs) over Western Ghats (WGs). Four types of COs are defined here, namely, deep convective-cores (DCCs), wide convective-cores (WCCs), deep-wide convective-cores (DWCs) and broad stratiform-region (BSRs) using space-based precipitation radars. Statistical analysis revealed the strong dependency of precipitation characteristics of COs on SDFs, and the variabilities among different SDFs are higher for deeper COs compared to wider COs. Higher rain rate (RR) during the upslope-seaside and along-seaside SDFs is the result of efficient collision-coalescence processes at the western slope of WGs as wind starts rising along the slope. It precipitates locally to produce higher RR but with less mass weighted mean hydrometeors diameter (Dm), because of drier atmosphere at lower altitudes. Along-landside and upslope-landside SDFs have higher echo top heights (ETHs) and Dm over topographic regimes of WGs, with less RR and surface rainfall, whereas less ETH and Dm are observed over Arabian Sea. RR is higher at the western flank of WGs and starts decreasing as it approaches towards the eastern flank of WGs for all types of SDFs. The role of the orography is most important at higher pressure levels (1000−850 hPa, e.g., lowest altitudes), where a sudden increase in relative humidity (RH) is observed parallel to the western slope of WGs, which could lead to rapid condensation and fall of precipitation. Upslope-landside and along-landside SDFs show a sudden dip in relative humidity (RH) (at 1000−850 hPa) and reflect the less evaporation because of higher-sized hydrometeors. Present study reveals that in future we must have to consider the importance of surface wind while modeling the convective organization over WGs. Topography and surface winds modulate the precipitation characteristics over mountain areas. Impact of the surface wind flow and topography along with the background atmospheric conditions on precipitation are crucial for the correct estimate of surface precipitation in numerical weather prediction models. Present study reveals a strong dependency of the interaction of surface wind with the mountain on the precipitation characteristics and their microphysical behaviour over the Western Ghats. Moisture availability, orography and updraft play a vital role in deciding the precipitation characteristics including their horizontal extension, vertical growth, hydrometeors size and their fall intensity. Higher moisture availability can increase the hydrometeors size, only if updraft speed provides them enough time to grow, and if they do, hydrometeors fall in the early stage of their life, and restrict their vertical growth. While approaching the orographic features an abrupt change is observed in the precipitation related parameters, and associated with the orographic features of the mountains. For example, a higher updraft can carry the higher sized hydrometers at higher altitude even in the moist conditions, and precipitate at the lee side on the mountain.

The critical role of light-absorbing aerosol black carbon (BC) in modifying the Earth's atmosphere and climate system warrants detailed characterization of its microphysical properties. The present study examines the BC microphysical properties (size distributions and mixing state) and their impact on the light-absorption characteristics over a semi-urban tropical coastal location in Southern Peninsular India. The measurements of refractory BC (rBC) properties, carried out using the single particle soot photometer during 2018-2021, covering four distinct air mass conditions (Marine, Continental, Mixed-1, and Mixed-2), were used for this purpose. These were supported by measurements of non-refractory submicron particulate matter (NR-PM1) mass loadings and the core-shell Mie theory model for BC-containing particles. The results suggested that the BC particles exhibited varying sizes (mass median diameters from 0.181 +/- 0.079 mu m to 0.202 +/- 0.064 mu m) and relative coating thicknesses (RCT) (1.3-1.6) under distinct air mass conditions. These characteristics reflected varying source/sink strengths, aging processes of BC, and potential condensable coating material. The aerosol system during the Marine air mass period has lower BC (similar to 0.67 +/- 0.57 mu g m(-3)) and NR-PM1 (12.06 +/- 10.81 mu g m(-3)) mass concentrations, and the lowest RCT on BC (similar to 1.34 +/- 0.14). However, the other periods with continental influence depicted significant coatings on BC (mean RCT >1.5). The coatings on BC particles exhibited daytime enhancement, driven by photochemically produced condensable material, a contrasting diurnal pattern to that of other BC properties. Interestingly, the RCT on BC increased and/or remained invariant with increasing relative humidity (RH) until RH <= 82 %, and subsequently showed a decline. This is possibly due to deliquescence and subsequent detachment of the coating material (likely, inorganics) from the BC cores. Despite this, the BC particles retained coatings of similar to 30-50 % of their core sizes even under high RH (>85 %), indicating the potential role of secondary organics as coatings. The changes in the BC mixing state resulted in a significant alteration to its light-absorption properties. The mean light-absorption enhancement of BC (compared to uncoated BC) ranged from 1.36 +/- 0.14 for the Marine air mass periods to 1.58 +/- 0.15 for the Continental air mass periods, whereas the overall mass absorption cross-sections of BC varied between 7.91 +/- 0.91 to 9.03 +/- 0.84 m(2)/g at 550 nm. The key implication of this study is that changes to the BC mixing state, caused by multiple underlying processes unique to tropical atmospheric conditions, can lead to a significant enhancement in its light-absorption characteristics, which can lead to a notable increase in the positive radiative forcing of BC.

The mesosphere-lower thermosphere (MLT) region is a highly dynamic region of the atmosphere that is strongly influenced by a broad spectrum of atmospheric waves. Among these, the quasi-two-day wave (QTDW) plays a prominent role in modulating MLT dynamics. In this study, we utilize more than 11 years (November 2013-March 2025) of Sri Venkateswara University (SVU) meteor radar observations made at a low-latitude site (13.63 degrees N, 79.4 degrees E), SVU, Tirupati, India, to investigate the characteristics and impacts of the QTDW, as well as its contribution to momentum and heat transport in the MLT region, which remains poorly understood. The horizontal momentum flux ( zonal heat flux (), and meridional heat flux were analyzed using horizontal winds and the ambipolar diffusion coefficient (derived from meteor decay time). The ambipolar diffusion coefficient, which is directly related to the background temperature, was used for the heat flux estimation. The results reveal distinct seasonal and height-dependent variations. In general, the horizontal momentum flux and zonal heat flux exhibited opposite trends relative to the background zonal mean wind during the solstices above 90 km -positive flux during the winter solstice and negative flux during the summer solstice. The meridional heat flux remained positive throughout, with enhanced values during the solstices and in October. This study is the first to quantify QTDW-induced horizontal momentum and heat fluxes using meteor radar observations over a low-latitude MLT region, providing new insights into wave-mean flow interactions.