Vikram Sarabhai Space Centre

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 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.

We present a turbulence index independent framework for simultaneously deriving solar wind velocity and coronal electron density in the near-Sun region using the spectral broadening of spacecraft radio signals. The formulation accommodates arbitrary turbulence spectral indices (p), providing a direct analytical link between the observed Doppler spectra and underlying plasma parameters without assuming a fixed turbulence regime. This generalization extends conventional radio occultation techniques and enables consistent interpretation across multiple radio frequencies. We apply the method to X-band ($\sim$ 8.41 GHz) radio occultation measurements from JAXA's Akatsuki spacecraft during the 2016 and 2022 Venus-Earth superior conjunctions, spanning heliocentric distances of 1.4-10 $\mathrm{ R}_{\odot }$ and sampling both equatorial streamer regions and mid-latitude coronal holes. The retrieved electron densities exhibit systematic trends consistent with empirical coronal models and in situ observations. By coupling the measured spectral widths with a turbulence-based frequency-scaling relation, we obtain a compact expression that links spectral broadening, solar wind speed, and electron density, applicable for any turbulence index p. Fast-solar-wind intervals, characterized by nearly isotropic turbulence, yield speed estimates in close agreement with expectations, while the anisotropic nature of the slow solar wind introduces small but systematic deviations. Our results refine earlier work and demonstrate that explicit consideration of near-coronal turbulence anisotropy is essential for accurate solar-wind parameter retrievals.

Interactions of solar wind dynamic pressure (SWDP) discontinuities with Earth's magnetosphere cause geomagnetic Sudden Commencements (SCs). Typically, positive/negative SCs occur at low latitudes due to enhancements/reductions in SWDP. However, anomalous dawn-side SCs of opposite polarity were recently reported during the 10 May 2024, superstorm [Nilam et al., 2025, https://doi.org/10.1029/2025GL117032]. This study examines SC responses to positive and negative pressure discontinuities on the May 10 and October 10 storms under similar storm phases and local times. Both events consistently revealed anomalous dawn-side low latitude SCs opposite to those at other longitudes. We suggest the main impulse of the Disturbance Polar (DP) field extending equatorward as the most likely source. Under highly compressed background magnetosphere conditions, field-aligned currents associated with DP fields can shift to lower L-shells, producing such anomalous SCs at dawn-side low latitudes. These findings provide new insights into dawn-side magnetosphere-ionosphere coupling during intense storms.