Modelling for Science, for a better future - some recent outcomes
Urban extreme rainfall events: categorical skill of WRF model simulations for localized and non-localized events
by G. N. Mohapatra, V. Rakesh and K. V. Ramesh
An objective method is used for determining the rainfall threshold for identifying extreme rainfall events (EREs) over the urban city, Bangalore, using observed rainfall data for a period of 35 years (1971–2005). Using this threshold, 52 EREs were identified during the period 2010–2014 using high-resolution rain-gauge observations. From these EREs, 15 localized and non-localized events were chosen based on spatial distribution to examine the forecast skill of the Weather Research and Forecasting (WRF) model. Apart from the conventional verification methods, a number of skill scores and indices were defined for a comprehensive evaluation of rainfall model skill. In general, the forecast underpredicted the magnitude of localized and non-localized EREs for the majority of cases; however, the model overpredicted light rainfall (≤10 mm day−1). The model showed a success rate of 59% in simulating light rainfall for localized EREs while 12% of events were missed and 29% were wrongly predicted. The success rate was significantly reduced at higher rainfall categories for localized and non-localized EREs, where the forecast missed the majority of rainfall events. The Reliability Index (RI) computed clearly showed that model skill is relatively higher for non-localized EREs compared to localized EREs. The average forecast reliability for non-localized and localized EREs were 74 and 51%, respectively. For localized EREs, model skill is relatively higher in rainfall location prediction (61%) compared to area (44%) and intensity (46%) prediction; while in the case of non-localized EREs, model skill is similar for location, intensity and area prediction. It is found that coupling an urban canopy model with WRF reduces the model errors particularly for lower rainfall thresholds.
Source: http://onlinelibrary.wiley.com/doi/10.1002/qj.3087/full
Subsurface profiling along Banni Plains and bounding faults, Kachchh, Western India using microtremors method
by Dhananjay A. Sant, Imtiyaz A. Parvez, Govindan Rangarajan, Satish J. Patel, Madhuri N. Bhatt and T.A. Sanoop Salam
The present article is a maiden attempt to map shallow subsurface rheological interfaces laterally across the Banni Plains and to decode geometry of the antecedent faults associated with the Kachchh Mainland Fault using the microtremor method. We conducted microtremor data acquisition for thirty-one sites along N-S transect from Loriya in Mainland Kachchh to Bhirandiara towards Patcham Island. Results from H/V spectral ratio technique show presence of two distinct rheological interfaces characterised by the resonant frequency (fr) ranges 0.23–0.27 Hz and 0.8252–1.5931 Hz respectively. The above frequency ranges are correlated with the depths of the Mesozoic-Basement (M-B) interface and the Quaternary-Tertiary (Q-T) interface. Using either the velocity (Vs) of seismic waves at the M-B and Q-T interfaces (calculated as 1830 m/s and 411 m/s respectively) or the standard non-linear regression relationship derived for the Banni Plains (h = 110.18fr−1.97) we estimate the depth range for M-B interface to be 1442–1965 m and for Q-T interface to be 44–160 m. The subsurface profile across the Banni Plains educe cluster of four faults that develop an array of imbricate faults at the forefront of the Kachchh Mainland Fault within the Banni Footwall Syncline. The geometry of the faults suggests a ‘positive flower structure’ indicating step-overs and strain restraining bends displaying push-ups resulting from localized shortening between converging bends of Kachchh Mainland Fault and the South Wagad Fault. The Banni Footwall Syncline preserves evidence of two episodes of deformations. The initial deformation event led to subsidence within the Kachchh Mainland Fault Zone bringing Mesozoic sequence juxtaposed to the basement rocks, whereas the later event is dominated by an uplift developing a positive flower structure in the Kachchh Mainland Fault Zone. Finally, the present study provides a mechanism to investigate faults and fault geometries correlating surface structural grains with subsurface structures.
Source: http://www.sciencedirect.com/science/article/pii/S1367912017303085
Subsurface profiling of granite pluton using microtremor method: southern Aravalli, Gujarat, India
by Aditya U. Joshi, Dhananjay A. Sant, Imtiyaz A. Parvez, Govindan Rangarajan, Manoj A. Limaye, Soumyajit Mukherjee, Mitesh J. Charola, Meghnath N. Bhatt and Sagar P. Mistry
We report, using the microtremor method, a subsurface granitic pluton underneath the Narukot Dome and in its western extension along a WNW profile, in proximity of eastern fringe of Cambay Rift, India. The dome and its extension is a part of the Champaner Group of rocks belonging to the Mesoproterozoic Aravalli Supergroup. The present finding elucidates development of an asymmetric double plunge along Narukot Dome. Microtremor measurements at 32 sites were carried out along the axial trace (N95°) of the dome. Fourier amplitude spectral studies were applied to obtain the ratio between the horizontal and vertical components of persisting Rayleigh waves as local ambient noise. Fundamental resonant frequencies with amplitude ≥1-sigma for each site are considered to distinguish rheological boundary. Two distinct rheological boundaries are identified based on frequency ranges determined in the terrain: (1) 0.2219–10.364 Hz recorded at 31 stations identified as the Champaner metasediment and granite boundary, and (2) 10.902–27.1119 Hz recorded at 22 stations identified as the phyllite and quartzite boundary. The proposed equation describing frequency–depth relationship between granite and overlaying regolith matches with those already published in the literature. The morphology of granite pluton highlights the rootless character of Champaner Group showing sharp discordance with granitic pluton. The findings of manifestation of pluton at a shallower depth imply a steep easterly plunge within the Champaner metasediments, whereas signature of pluton at a deeper level implies a gentle westerly plunge. The present method enables to assess how granite emplacement influences the surface structure.
Source: https://link.springer.com/article/10.1007/s00531-017-1482-9
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