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Assessing the performance of the WRF model in simulating squall line processes over the South African highveld
(MDPI, 2025-09-06) Mbokodo , Innocent L.; Burger , Roelof P.; Fridlind, Ann; Ndarana, Thando; Maisha , Robert; Chikoore, Hector; Bopape, Mary-Jane M.
Squall lines are some of the most common types of mesoscale cloud systems in tropical and subtropical regions. Thunderstorms associated with these systems are among the major causes of weather-related disasters and socio-economic losses in many regions across the world. This study investigates the capability of the Weather Research and Forecasting (WRF) model in simulating squall line features over the South African Highveld region. Two squall line cases were selected based on the availability of South African Weather Service (SAWS) weather radar data: 21 October 2017 (early austral summer) and 31 January–1 February 2018 (late austral summer). The European Centre for Medium-Range Weather Forecasts ERA5 datasets were used as observational proxies to analyze squall line features and compare them with WRF simulations. Mid-tropospheric perturbations were observed along westerly waves in both cases. These perturbations were coupled with surface troughs over central interior together with the high-pressure systems to the south and southeast of the country creating strong pressure gradients over the plateau, which also transports relative humidity onshore and extending to the Highveld region. The 2018 case also had a zonal structured ridging High, which was responsible for driving moisture from the southwest Indian Ocean towards the eastern parts of South Africa. Both ERA5 and WRF captured onshore near surface (800 hPa) winds and high-moisture contents over the eastern parts of the Highveld. A well-defined dryline was observed and well simulated for the 2017 event, while both ERA5 and WRF did not show any dryline for the 2018 case that was triggered by orography. While WRF successfully reproduced the synoptic-scale processes of these extreme weather events, the simulated rainfall over the area of interest exhibited a broader spatial distribution, with large-scale precipitation overestimated and convective rainfall underestimated. Our study shows that models are able to capture these systems but with some shortcomings, highlighting the need for further improvement in forecasts.
Investigating the Hi distribution and kinematics of ESO444-G084 and [KKS2000]23 : new insights from the MHONGOOSE survey
(EDP Sciences, 2025-07) Namumba, B.; Ianjamasimanana, R.; Koribalski, B.S.; Bosma, A.; Athanassoula, E.; Carignan, C.; Józsa, G.I.G.; Kamphuis, P.; Deane, Roger; Sikhosana, S.P.; Verdes-Montenegro, L.; Sorgho, A.; Ndaliso, X.; Amram, P.; Brinks, E.; Chemin, L.; Combes, F.; De Blok, W.J.G.; Deg, N.; English, J.; Healy, J.; Kurapati, S.; Marasco, A.; Mc Gaugh, S.S.; Oman, K.A.; Spekkens, K.; Veronese, S.; Wong, O.I.
We present the HI distribution, kinematics, mass modeling, and disk stability of the dwarf irregular galaxies ESO444-G084 and [KKS2000]23 using high-resolution, high-sensitivity MHONGOOSE survey data from MeerKAT. ESO444-G084 shows centrally concentrated HI emission, while [KKS2000]23 exhibits irregular high-density clumps. Total HI fluxes measured down to 10^19 and 10^18 cm^-2 are nearly identical, indicating that the increased HI diameter at lower column densities results mainly from the larger beam, with no significant extra emission detected. We derive total HI masses of (1.1 +/- 0.1) x 10^8 and (6.1 +/- 0.3) x 10^8 solar masses for ESO444-G084 and [KKS2000]23, respectively. Using PyFAT and TiRiFiC, we extract 3D rotation curves that reveal disk-like kinematics in both galaxies. ESO444-G084 shows a warp beyond ~1.8 kpc and a fast-rising curve consistent with a centrally concentrated dark matter distribution, while [KKS2000]23's more gradual rise suggests a more extended halo. Mass modeling with an isothermal halo and stellar mass-to-light ratios of 0.20 for ESO444-G084 and 0.18 for [KKS2000]23 yields consistent results. We analyze disk stability using spatially resolved Toomre Q and gas-to-critical surface density ratios, linking these with H-alpha and FUV-based star formation. ESO444-G084 supports localized star formation despite global stability, while [KKS2000]23 appears gravitationally unstable yet lacks H-alpha, suggesting that turbulence, gas depletion, or past feedback suppresses star formation. No inflows or outflows are detected, indicating internal processes regulate star formation. This study highlights the interplay between HI morphology, kinematics, dark matter distribution, and disk stability, showing how internal processes shape dwarf galaxy evolution.
Probing the formation of megaparsec-scale giant radio galaxies. I. Dynamical insights from MHD simulations
(EDP Sciences, 2025-01) Giri, Gourab; Bagchi, Joydeep; Thorat, Kshitij; Deane, Roger; Delhaize, Jacinta; Saikia, D.J.; gourab.giri@up.ac.za
CONTEXT : Giant radio galaxies (GRGs), a minority among the extended-jetted population, form in a wide range of jet and environmental configurations, complicating the identification of the growth factors that facilitate their attainment of megaparsec scales.
AIMS : This study aims to numerically investigate the hypothesized formation mechanisms of GRGs extending ≳ Mpc to assess their general applicability. METHODS : We employ triaxial ambient medium settings to generate varying levels of jet frustration and simulate jets with low and high power from different locations in the environment, formulating five representations.
RESULTS : The emergence of distinct giant phases in all five simulated scenarios suggests that GRGs may be more common than previously believed, a prediction to be verified with contemporary radio telescopes. We find that different combinations of jet morphology, power, and the evolutionary age of the formed structure hold the potential to elucidate different formation scenarios. The simulated lobes are over-pressured, prompting further investigation into pressure profiles when jet activity ceases, potentially distinguishing between relic and active GRGs. We observed a potential phase transition in giant radio galaxies, marked by differences in lobe expansion speed and pressure variations compared to their smaller evolutionary phases. This suggests the need for further investigation across a broader parameter space to determine if GRGs fundamentally differ from smaller RGs. Axial ratio analysis reveals self-similar expansion in rapidly propagating jets, with notable deviations when the jet forms wider lobes. Overall, this study emphasizes that multiple growth factors at work can better elucidate the current-day population of GRGs, including scenarios e.g., growth of GRGs in dense environments, GRGs of several megaparsecs, GRG development in low-powered jets, and the formation of X-shaped GRGs.
MeerKAT 1.3 GHz observations of the wide-angle tail radio galaxy J1712-2435
(IOP Publishing, 2025-08) Cotton, W.D.; Giri, Gourab; Agnihotri, P.J.; Saikia, D.J.; Thorat, Kshitij; Camilo, F.
We present full polarization MeerKAT images of the wide-angle tail, giant radio galaxy J17122435 at 1.3 GHz with 7.\asec5 resolution and an RMS sensitivity of 8 Jy beam. Due to the angular proximity to the Galactic Center (l=359.6, b=+8.5) the immediate environment is not well understood but there are massive clusters nearby. Emission can be traced over an extent of 34.\amin6 which at the redshift of 0.024330 corresponds to a projected length of 1.02 Mpc. The inner jets are quite straight but then bend and completely decollimate into extended plumes nearly orthogonal to the initial jet directions at a projected distance of approximately 100 kpc. The nearly unity brightness ratio of the inner jets suggest that they are orientated within a few degrees of the plane of the sky. The 1400 MHz power is 3.9 W Hz, somewhat below the FRI/FRII divide. The total power emitted is estimated to be 5.6 erg sec over the range 10 MHz to 100 GHz. The source dynamics are modeled with magneto-hydrodynamics simulations; the result is a rough reproduction of the source's radio morphology / appearance. This study further highlights the merit of alternative scenarios, calling for future observational and numerical efforts.
Low-velocity precessing jets can explain observed morphologies in the Twin Radio Galaxy TRGJ104454+354055
(IOP Publishing, 2025-07) Mondal, Santanu; Giri, Gourab; Joshi, Ravi; Wiita, Paul J.; Krishna, Gopal; Ho, Luis C.
Our understanding of large-scale radio jets in merger systems has been drastically improved in the era of VLA, VLBA/EVN, uGMRT, and MeerKAT. Twin Radio Galaxies (TRGs) are the rare interacting galaxy pairs where both supermassive black holes host kiloparsec-scale bipolar radio jets. Only recently was a third TRG discovered and it shows significantly different jet morphologies than the previous two. Due to both the extreme paucity and complexity of such systems, the launching of their jets as well as their mutual interaction during the propagation through the ambient medium are not well understood. We have performed 3D hydrodynamic simulations to study the bipolar jets in the third TRG, J104454+354055. Our study indicates that the precession of mutually tilted bipolar jets originating from the two galactic nuclei separated by tens of kiloparsecs and propagating at low velocities can explain the observed morphologies. The simulated jet precession timescales are short compared to the overall dynamical timescale of the jets and could originate from Lense-Thirring effects in the accretion disks. This approach to understanding the TRG jet dynamics could also be applied to other TRG systems with similar helical morphologies that may be discovered in the upcoming era of the SKA and its pathfinder surveys.