• Volume 116, Issue 5

      October 2007,   pages  369-463

    • Current status of multimodel superensemble and operational NWP forecast of the Indian summer monsoon

      Akhilesh Kumar Mishra T N Krishnamurti

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      In the last thirty years great strides have been made by large-scale operational numerical weather prediction models towards improving skills for the medium range time-scale of 7 days.This paper illustrates the use of these current forecasts towards the construction of a consensus multimodel forecast product called the superensemble.This procedure utilizes 120 of the recent-past forecasts from these models to arrive at the training phase statistics. These statistics are described by roughly 107 weights.Use of these weights provides the possibility for real-time medium range forecasts with the superensemble.We show the recent status of this procedure towards real-time forecasts for the Asian summer monsoon.The member models of our suite include ECMWF,NCEP/EMC, JMA,NOGAPS (US Navy),BMRC,RPN (Canada)and an FSU global spectral forecast model. We show in this paper the skill scores for day 1 through day 6 of forecasts from standard variables such as winds,temperature,500 hPa geopotential height,sea level pressure and precipitation.In all cases we noted that the superensemble carries a higher skill compared to each of the member models and their ensemble mean.The skill matrices we use include the RMS errors,the anomaly correlations and equitable threat scores.For many of these forecasts the improvements of skill for the superensemble over the best model was found to be quite substantial.This real-time product is being provided to many interested research groups.The FSU multimodel superensemble,in real- time,stands out for providing the least errors among all of the operational large scale models.

    • Link between convection and meridional gradient of sea surface temperature in the Bay of Bengal

      D Shankar S R Shetye P V Joseph

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      We use daily satellite estimates of sea surface temperature (SST)and rainfall during 1998 –2005 to show that onset of convection over the central Bay of Bengal (88-92°E, 14-18°N)during the core summer monsoon (mid-May to September)is linked to the meridional gradient of SST in the bay.The SST gradient was computed between two boxes in the northern (88-92°E, 18-22°N) and southern (82-88°E, 4-8°N) bay; the latter is the area of the cold tongue in the bay linked to the Summer Monsoon Current.Convection over central bay followed the SST difference between the northern and southern bay (𝛥 𝑇) exceeding 0.75°C in 28 cases.There was no instance of 𝛥 𝑇 exceeding this threshold without a burst in convection.There were,however,five instances of convection occurring without this SST gradient.Long rainfall events (events lasting more than a week)were associated with an SST event (𝛥 𝑇 ≥ 0.75°C);rainfall events tended to be short when not associated with an SST event.The SST gradient was important for the onset of convection, but not for its persistence:convection often persisted for several days even after the SST gradient weakened.The lag between 𝛥 𝑇 exceeding 0.75°C and the onset of convection was 0-18 days,but the lag histogram peaked at one week.In 75% of the 28 cases,convection occurred within a week of 𝛥 𝑇 exceeding the threshold of 0.75°C. The northern bay SST, T_N contributed more to 𝛥 𝑇 but it was a weaker criterion for convection than the SST gradient.A sensitivity analysis showed that the corresponding threshold for T_N was 29°C. We hypothesise that the excess heating (∼1° C above the threshold for deep convection)required in the northern bay to trigger convection is because this excess in SST is what is required to establish the critical SST gradient.

    • GPS satellite and receiver instrumental biases estimation using least squares method for accurate ionosphere modelling

      G Sasibhushana Rao

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      The positional accuracy of the Global Positioning System (GPS)is limited due to several error sources.The major error is ionosphere.By augmenting the GPS,the Category I (CAT I)Precision Approach (PA)requirements can be achieved.The Space-Based Augmentation System (SBAS)in India is known as GPS Aided Geo Augmented Navigation (GAGAN).One of the prominent errors in GAGAN that limits the positional accuracy is instrumental biases.Calibration of these biases is particularly important in achieving the CAT I PA landings.In this paper,a new algorithm is proposed to estimate the instrumental biases by modelling the TEC using 4th order polynomial.The algorithm uses values corresponding to a single station for one month period and the results confirm the validity of the algorithm.The experimental results indicate that the estimation precision of the satellite-plus-receiver instrumental bias is of the order of ± 0.17 nsec.The observed mean bias error is of the order − 3.638 nsec and − 4.71 nsec for satellite 1 and 31 respectively.It is found that results are consistent over the period.

    • Strike slip faulting inferred from offsetting of drainages: Lower Narmada basin, western India

      Rachna Raj

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      The detailed analysis of landforms,drainages and geology of the area between the rivers Amaravati and Karjan was carried out in order to understand the tectonic history of the lower Narmada basin. Movement along the various faults in the area was studied on the basis of the drainage offsetting. Horizontal offsetting of stream channels was found quite demonstrable along NNW –SSE trending transverse faults.Tectonic landforms including systematic de flection of stream channels and ridges, alignment of fault scarp and saddles and displacement in the basement rocks and alluvial deposits show that the area is undergoing active deformation driven by the NSF system.

    • Polyphase deformation and garnet growth in politic schists of Sausar Group in Ramtek area, Maharashtra, India: A study of porphyroblast–matrix relationship

      A Chattopadhyay N Ghosh

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      Polyphase deformation and metamorphism of pelitic schists of Chorbaoli Formation of Sausar Group in and around Ramtek area,Nagpur district,Maharashtra,India has led to the development of garnet and staurolite porphyroblasts in a predominantly quartz –mica matrix.Microstructural study of oriented thin sections of these rocks shows that garnet and staurolite have different growth histories and these porphyroblasts share a complex relationship with the matrix.Garnet shows at least two phases of growth –first intertectonic between D1 and D2 (pre-D2 phase)and then syn-tectonic to post-tectonic with respect to D2 deformation.Growth of later phase of garnet on the earlier (pre-D2 garnet grains has led to the discordance of quartz inclusion trails between core and rim portion of the same garnet grain.Staurolite develops only syn-D2 and shows close association with garnet of the later phase.The peak metamorphic temperature thus coincided with D2 deformation,which developed the dominant crenulation schistosity ($S_2$ ,regionally persistent in the terrain.The metamorphic grade reached up to middle amphibolite facies in the study area, which is higher than the adjoining southern parts of Sausar Fold Belt.

    • Advective heat transfer and fabric development in a shallow crustal intrusive granite – the case of Proterozoic Vellaturu granite, south India

      Dilip Saha Sukanya Chakraborti

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      Syntectonic plutons emplaced in shallow crust often contain intermediate-to low-temperature deformation microstructures but lack a high-temperature, subsolidus deformation fabric,although the relict magmatic fabric is preserved. The Proterozoic Vellaturu granite emplaced at the eastern margin of the northern Nallamalai fold belt,south India during the late phase of regional deformation has a common occurrence of intermediate-to low-temperature deformation fabric, superimposed over magmatic fabric with an internally complex pattern. But high-T subsolidus deformation microstructure and fabric are absent in this pluton.The main crystal plastic deformation and fluid enhanced reaction softening was concentrated along the margin of the granite body. Resulting granite mylonites show Y-maximum c axis fabric in completely recrystallized quartz ribbons,dynamic recrystallization of perthites,and myrmekite indicative of fabric development under intermediate temperature (∼500-400° C). The weakly-deformed interior shows myrmekite,feldspar microfracturing and limited bulging recrystallization of quartz.The abundance of prism subgrain boundaries is indicative of continuing deformation through low-temperature(∼300° C).The relative rates of cooling in fluenced by advective heat transfer and deformation of the pluton seem to control the overall subsolidus fabric development.The rapid advective heat transfer from the interior in the early stages of subsolidus cooling was followed by slow cooling through intermediate temperature window as a well-developed phyllosilicate rich mylonitic skin around the granite body slowed down conductive heat loss.Low-T crystal plastic deformation of quartz was effected at a late stage of cooling and deformation of the shallow crustal granite body emplaced within the greenschist facies Nallamalai rocks.

    • Rainfall intensity characteristics at coastal and high altitude stations in Kerala

      V Sasi Kumar S Sampath P V S S K Vinayak R Harikumar

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      Rainfall intensities measured at a few stations in Kerala during 2001 –2005 using a disdrometer were found to be in reasonable agreement with the total rainfall measured using a manual rain gauge. The temporal distributions of rainfall intensity at different places and during different months show that rainfall is of low intensity (> 10 mm/hr),65%to 90%of the time.This could be an indication of the relative prevalence of stratiform and cumuliform clouds.Rainfall was of intensity > 5 mm/hr for more than 95%of the time in Kochi in July 2002,which was a month seriously deficient in rainfall,indicating that the deficiency was probably due to the relative absence of cumuliform clouds.Cumulative distribution graphs are also plotted and fitted with the Weibull distribution.The fit parameters do not appear to have any consistent pattern. The higher intensities also contributed signi ficantly to total rainfall most of the time,except in Munnar (a hill station). In this analysis also,the rainfall in Kochi in July 2002 was found to have less presence of high intensities. This supports the hypothesis that the rainfall de ficiency was probably caused by the absence of conditions that favoured the formation of cumuliform clouds.

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