An optimum initial manifold for improved skill and lead in long-range forecasting of monsoon variability
by K. C. Gouda, S. Joshi & Nagaraj Bhat
Using an initial manifold approach, an ensemble forecast methodology is shown to simultaneously increase lead and realizable skill in long-range forecasting of monsoon over continental India. Initial manifold approach distinguishes the initial states that have coherence from a collection of unrelated states. In this work, an optimized and validated variable resolution general circulation model is being adopted for long-range forecasting of monsoon using the multi-lead ensemble methodology. In terms of realizable skill (as against potential) at resolution (~60km) and lead (2–5 months) considered here, the present method performs very well. The skill of the improved methodology is significant, capturing 9 of the 12 extreme years of monsoon during 1980–2003 in seasonal (June–August) scale. Eight-member ensemble-average hindcasts carried out for realizable skill with lead of 2 (for June) to 5 (for August) months and an optimum ensemble is presented.
Source: https://doi.org/10.1007/s00704-021-03589-x and https://link.springer.com/article/10.1007/s00704-021-03589-x
A note on stress rotations due to the 2004 Mw 9.2 Sumatra-Andaman megathrust earthquake
by K Silpa and Anil Earnest
Here we study the evolution of stress and its rotations in the Sumatra-Andaman Subduction Zone (SASZ) after the 2004 Mw 9.2 megathrust earthquake, using moment tensor stress inversions. Models indicate returning of North Andaman stress regime to the inter-seismic oblique-compression, after a short stint of extension. Likewise, Little Andamans experience extension indicating a trench-ward guidance of extensional stresses. Similarly, Nicobar regime is presently normal-oblique, from early post-seismic reverse-oblique, and North Sumatra experiences pure-reverse regime. The influence of the Wharton Basin stress field and 2012 earthquakes on the SASZ fore-arc deformation is also probed. A near-complete co-seismic stress-relief is observed at Nicobar, followed by North Sumatra and Little Andaman. Co-seismic and post-seismic model comparisons indicate strong correlation with zones of co-seismic stress relief and regions of rapid post-seismic reloading. The North Andaman shows a northward compressive shear than the margin-normal component of subduction, as evident from prominent oblique stress regime with lower co-seismic stress relief and lack of post-seismic back rotations. At Andaman Spreading Ridge (ASR), transition from oblique to extensional stress indicates lateral shear to back-arc spreading enhancement.
A new rain-based index for the Indian summer monsoon rainfall
by Sulochana Gadgil, K Rajendran and D S Pai
Abstract: Most of the studies of the observed variability of the Indian summer monsoon rainfall (ISMR), its prediction and of its impact have involved analysis of an index for ISMR derived by Parthasarathy et al. (1995) or the all-India rainfall during the summer monsoon, available from the India Meteorological Department (IMD) website. Both these indices are based on the average rainfall over the meteorological subdivisions of India. Rajeevan et al. (2006) first derived a gridded rainfall data set for the Indian region which was at a resolution of 1° and subsequently, Pai et al. (2014) have derived a finer resolution (0.25°) rainfall data set for the same region. At present, these data sets are widely used by modelers to generate the ‘observed’ ISMR for assessment of the skill of their models. However, in different studies, different regions are used for averaging the grid data to obtain the ‘observed’ ISMR. For proper assessment and comparison of the skill of the simulations/predictions by different models/versions, it is important that the same region be used for averaging the rainfall to obtain the observed ISMR in each case. Here, we suggest what we consider as the appropriate regions for averaging the rainfall in terms of the 1° and 0.25° to derive/represent ISMR, on the basis of the present understanding of the monsoonal regions and the Indian summer monsoon. We show that the interannual variation of the ISMR thus derived (by averaging rainfall over the regions identified in this study) from gridded data sets is largely consistent with the indices derived as the area weighted sub-divisional rainfall data used in the indices used earlier.
Citation: Sulochana Gadgil, K Rajendran and D S Pai (2019): A new rain-based index for the Indian summer monsoon rainfall, MAUSAM, 70 (3), 485-500
A comprehensive study of atmospheric dynamics associated with cloudburst events in 2022 over Indian Himalayan Region
by Payoshni Samantray and Krushna Chandra Gouda
This comprehensive study aims to investigate the cloudburst events that occurred in the Indian Himalayan Region (IHR) during the year 2022, focusing on their characteristics, structure and dynamics. Cloudburst events have been observed in different states of the Himalayan region, with their unique geographical features, and are susceptible to extreme weather phenomena, which pose significant challenges to the local communities and infrastructure. A majority of cloudburst events occur within the folds of valleys of the Indian Himalayas, where elevations range from 325 to 4073 m. As the year 2022 witnessed frequent cloudburst events in the IHR (Himachal Pradesh recorded around 24 cloudburst events, Uttarakhand recorded 18 and Jammu and Kashmir recorded 24 events), an attempt is being made for the comprehensive analysis of the role of atmospheric dynamics to result in such extreme events in the valley region. The analysis clearly advocates that certain atmospheric phenomena, such as frontal boundaries like temperature, humidity, convective available potential energy, convective inhibition, or atmospheric disturbances, can act as triggers for cloud burst events. The spatial distribution of different thermodynamic parameters during the previous day, event day and the day after the cloudburst events are also analysed to quantify the role of atmospheric dynamics in the temporal distribution. This study has highlighted the importance of low-level jets and moisture transport in forming the convective systems that lead to cloudbursts over the Himalayan region due to wind patterns.
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