Papers by Rajib Chattopadhyay
Journal of Geophysical Research: Atmospheres, 2013
The paper describes a probabilistic prediction scheme of the intraseasonal oscillation of Indian ... more The paper describes a probabilistic prediction scheme of the intraseasonal oscillation of Indian summer monsoon (ISM) in the extended range (ER, ~3–4weeks) using a self‐organizing map (SOM)‐based technique. SOM is used to derive a set of patterns through empirical model reduction. An ensemble method of forecast is then developed for these reduced modes based on the principle of analogue prediction. A total of 900 ensembles is created based on the variations of one of the parameters, like length of the observation sample, number of patterns, number of lags, and number of input variables, keeping the others constant. Deterministic correlation skill at fourth pentad lead (15–20 days) from the current model is 0.47 (for development period, 1951–1999) and 0.43 (for hindcast period, 2000–2011) over the monsoon zone of India. This method effectively takes care of the stochastic uncertainties associated with a deterministic prediction scheme and provides better guidance to the user communit...
Theoretical and Applied Climatology, 2021
The economy and livelihood of the State of West Bengal in India are mainly dependent on agricultu... more The economy and livelihood of the State of West Bengal in India are mainly dependent on agriculture. Rice is one of the major crops of this state, and it contributes a significant proportion to the rice production of India. The present study deals with the prediction of Kharif rice over Gangetic West Bengal using forecast products from IITM-IMD extended range prediction (ERP) system. It is a multi-model ensemble prediction system that comprises 16 different ensemble members obtained from the Climate Forecast System (CFSv2) and the stand-alone atmospheric component of CFSv2 (i.e., GFSv2). The performance of ERP rainfall forecast has been assessed over a relatively larger domain covering Indian landmass before its utilization in crop yield prediction. Satisfactory skills, e.g., higher correlation and lower normalized root mean squared error (nRMSE), have been found in ERP rainfall forecast during the first 3 weeks for most of the initial conditions (ICs). Next, bias-corrected ERP weekly forecast data of incoming solar radiation, rainfall, and maximum and minimum temperatures have been incorporated into a process-based crop model (CERES-rice) available in the Decision Support System for Agro-technology Transfer (DSSAT). ERP-driven crop model has performed better to reproduce inter-annual variability of observed rice yield compared to yield simulated using crop model driven by climatology alone. Also, the ERP-driven model has been able to capture the below- and above-normal yield categories relatively better than the climatology-driven model. Hence, the incorporation of ERP in crop models may provide value-added prediction, which will be helpful for the stakeholders and decision-makers.
The seamless forecast approach of subseasonal to seasonal scale variability has been succeeding i... more The seamless forecast approach of subseasonal to seasonal scale variability has been succeeding in the forecast of multiple meteorological scales in a uniform framework. In this paradigm, it is hypothesized that reduction in initial error in dynamical forecast would help to reduce forecast error in extended lead-time up to 2-3 weeks. This is tested in a version of operational extended range forecasts based on Climate Forecast System version 2 (CFSv2) developed at Indian Institute of Tropical Meteorology (IITM), Pune. Forecast skills are assessed to understand the role of initial errors on the prediction skill for MJO. A set of lowest and highest initial day error (LIDE & HIDE) cases are defined and the error-growth for these categories are analysed for the strong MJO events during May to September (MJJAS). The MJO forecast initial errors are categorized and defined using the well-known multivariate MJO index introduced by Wheeler & Hendon (2004). The probability distribution of biva...
MAUSAM, 2021
Extended range forecast of sub seasonal variability beyond weather scale is a critical component ... more Extended range forecast of sub seasonal variability beyond weather scale is a critical component in climate forecast applications over Indian region. The sub-seasonal to seasonal (s2s) project, undertaken by WCRP, started in 2013 to improve the forecast beyond weather scale which is a challenging gap area in research and operational forecast domain. The primary objective of this s2s project is to provide the sub-seasonal to seasonal forecast in various lead times. Extended range forecast project at Indian Institute of Tropical Meteorology (IITM) is envisioned more than a decade ago in this way, keeping in view of the demands from the stakeholders to create a robust research background required for improved delivery of operational forecast in this scale. Extended range forecast implies an outlook with a lead-time of 2-3weeks. Several efforts have been undertaken to improve the extended range forecast in the past decade or so at IITM. The current study summarizes the development of extended range prediction system for operational implementation at IMD. The research and development that lead to the development of extended range prediction are summarized at first. It describes the efforts that were undertaken to establish the statistical properties of monsoon intraseasonal oscillations, establish the predictability horizon, represent the oscillations in state-of-art climate models and finally develop the latest state-of-art forecast technique. This technique is now operationally implemented at India Meteorological Department.
MAUSAM, 2021
The seasonal genesis parameters for tropical cyclogenesis developed by W. M. Gray, is widely used... more The seasonal genesis parameters for tropical cyclogenesis developed by W. M. Gray, is widely used for the climatological and seasonal monitoring of cyclogenesis over the tropical oceans. Over the North Indian Ocean (NIO), cyclogenesis and evolution is monitored and predicted in the short, medium and extended ranges by India Meteorological Department with the implementation of different deterministic and probabilistic forecasting techniques. This paper provides a review of an in-house developed tropical cyclone prediction system involving an improved storm evolution index and an objective tracking algorithm for detecting cyclogenesis, evolution and storm tracks from postprocessed Multi-model ensemble (MME) outputs from the Climate Forecast System-based Grand Ensemble Prediction System (CGEPS) implemented for operational extended range prediction. In the first part, the reliability of cyclogenesis prediction when more than one storm systems develop simultaneously is discussed using a case study. Prominent cyclogenesis indices and constituent parameters are used to analyse the atmospheric and oceanic features which affected the evolution two consecutive storms over NIO by using ERA-Interim daily averaged datasets. The performance of indices from MME outputs is also analysed. Further the reliability of objective track prediction system is discussed by using ERA-5 and ERA-Interim datasets. Finally, the performance of the CGEPS-MME in predicting the recent tropical cyclones, Amphan and Nisarga are discussed in detail. Real-time implementation of this prediction system has proven to be critical in providing early guidance on the formation MAUSAM, 72, 1 (January 2021) of storms, enabling the cyclone warning community to be on alert thereby providing enough lead time for better planning and mitigation strategies.
The intra-seasonal fluctuations of Indian summer monsoon rainfall (ISMR) are mainly controlled by... more The intra-seasonal fluctuations of Indian summer monsoon rainfall (ISMR) are mainly controlled by northward propagating Monsoon Intra-seasonal Oscillation (MISO) and eastward propagating Madden Julian Oscillation (MJO). In the current study, we examine the relationship between the intra-seasonal fluctuations (active and break spells) of ISMR with the phase propagation and amplitude of MISO and MJO. We notice that active spells generally occur during MISO phase 2–5 (MJO phase 3–6), and break spells mainly occur during MISO phase 6–8 (MJO phase 6–8 and 1). The association of active/break spells with MISO phases is more prominent than with MJO phases. We show the phase composite of unfiltered and regression based reconstructed rainfall for eight MISO and MJO phases, and the same is consistent with the earlier findings. We notice that the reconstructed field shows a systematic and well-organised northward propagation compared to the unfiltered field. Phase composite also indicates that ...
Frontiers in Climate, 2021
In an endeavor to design better forecasting tools for real-time prediction, the present work high... more In an endeavor to design better forecasting tools for real-time prediction, the present work highlights the strength of the multi-model multi-physics ensemble over its operational predecessor version. The exiting operational extended range prediction system (ERPv1) combines the coupled, and its bias-corrected sea-surface temperature forced atmospheric model running at two resolutions with perturbed initial condition ensemble. This system had accomplished important goals on the sub-seasonal scale skillful forecast; however, the skill of the system is limited only up to 2 weeks. The next version of this ERP system is seamless in resolution and based on a multi-physics multi-model ensemble (MPMME). Similar to the earlier version, this system includes coupled climate forecast system version 2 (CFSv2) and atmospheric global forecast system forced with real-time bias-corrected sea-surface temperature from CFSv2. In the newer version, model integrations are performed six times in a month f...
Scientific Reports, 2020
Among the other diseases, malaria and diarrhoea have a large disease burden in India, especially ... more Among the other diseases, malaria and diarrhoea have a large disease burden in India, especially among children. Changes in rainfall and temperature patterns likely play a major role in the increased incidence of these diseases across geographical locations. This study proposes a method for probabilistic forecasting of the disease incidences in extended range time scale (2–3 weeks in advance) over India based on an unsupervised pattern recognition technique that uses meteorological parameters as inputs and which can be applied to any geographical location over India. To verify the robustness of this newly developed early warning system, detailed analysis has been made in the incidence of malaria and diarrhoea over two districts of the State of Maharashtra. It is found that the increased probabilities of high (less) rainfall, high (low) minimum temperature and low (moderate) maximum temperature are more (less) conducive for both diseases over these locations, but have different thres...
Pure and Applied Geophysics, 2020
Eastward propagating Madden-Julian Oscillation (MJO) is a dominant mode of the intraseasonal vari... more Eastward propagating Madden-Julian Oscillation (MJO) is a dominant mode of the intraseasonal variability and hence a potential source of intraseasonal predictability. Therefore, advancing MJO prediction using state-of-the-art dynamical model is of utmost importance for improving intraseasonal prediction. The prediction skill and predictability of MJO are assessed using 44 members ensemble hindcast (16 years data; 2001-2016) of CFSv2 based extended range prediction (ERP) system of IITM as well as 10 member ensemble hindcast (16 years data; 2001-2016) of ECMWF S2S dataset. The MJO is diagnosed using a newly developed Extended Empirical Orthogonal Function (EEOF) analysis. Near equatorial (15°S-15°N) model anomaly fields are projected onto the leading pair of observed eigen modes. The leading pair of observed eigen modes are obtained based on the EEOF analysis of the combined field of zonal wind at 200 hPa (U200), zonal wind at 850 hPa (U850) and velocity potential at 200 hPa (chi200). Model forecasted principal components (PCs) are quantitatively compared with observed PCs using bivariate correlation coefficient and root mean square error (RMSE). We find that MJO could be predicted up to around 22 days (around 31 days) for IITM ERP system (ECMWF S2S dataset) as measured by anomaly correlation coefficient remains larger than 0.5 and RMSE remains lower than 1.4. This prediction skill is quite low compared to potential predictability, which is estimated as more than 40 days both for IITM-ERP and ECMWF system. MJO prediction skill varies with initial MJO phase, particularly at the longer lead. This variation is more significant for the ECMWF system. Model (both for IITM-ERP and ECMWF) predicted amplitude drops at a faster rate and phase propagation speed for almost all initial phase is slower and amplitude is weaker compared to the observation. It could be concluded that even the state-of-the-art models [IITM-ERP (basically NCEP CFSv2) and ECMWF] are also not free from systematic errors/biases. Hence, there is an enormous space for improving MJO prediction skill by reducing these errors/biases in the dynamical model and error in the initial condition.
Journal of Hydrology X, 2019
To study the atmospheric moisture transport processes from the source to a receiver region, isoto... more To study the atmospheric moisture transport processes from the source to a receiver region, isotopic analysis of rainwater from Port Blair (as source region), the Andaman Islands, Bay of Bengal has been carried out. In addition to the island site, rainwater collected at three sites on the Indian mainland (as receiver region) namely Nagpur, Kolkata, and Tezpur has also been isotopically analyzed. We present a daily variation of long-term (2012-2015) record of rain isotope from Port Blair to understand the moisture dynamics on a sub-seasonal timescale. It was observed that the Port Blair rainwater oxygen isotope ratios (d 18 O) maintain a temporally dependent correlation with the average rainfall over the Core Monsoon Zone (CMZ) of India, though the seasonal rainfall over these two regions appears to be mutually independent. Rain isotopes are controlled more by the moisture dynamics rather than the individual rainfall events. Consequently, the correlation between rainfall over the CMZ and d 18 O of Port Blair rain provides conclusive evidence of the transport of the Bay of Bengal (BoB) moisture to the central and the northeast Indian regions during the summer monsoon season. The spatial dependency of Port Blair rain-isotopes with the CMZ rain shows interannual variability and also indicates different pathways of the BoB moisture to the Indian mainland within a given season. The pathways of the source moisture to the receiver are identified by the trajectory analysis with Port Blair being the source area. We observed analogous variation in d 18 O of island rain and that of the mainland rain, and d 18 O at these two regions is modulated by the monsoon intra-seasonal oscillation (MISO). The correlation between the two isotopic records appears to arise from the propagation of the rain/cloud band from the Bay to the Indian landmass and the associated moisture transport linked to the MISO.
Theoretical and Applied Climatology, 2018
Indian summer monsoon rainfall is characterized by a considerable variability on the spatial and ... more Indian summer monsoon rainfall is characterized by a considerable variability on the spatial and temporal scale, though the standard deviation of rainfall on the temporal scale is much smaller (ca. 10% of annual long-term mean) compared with that of the spatial scale. It is known that the seasonal rainfall amplitude over India as well as the statistical properties of the subseasonal variability is strongly linked to the Pacific teleconnection on interannual timescales, such as El Niño-Southern Oscillation (ENSO) and La Niña, and the teleconnection is primarily arising from central and eastern Pacific Ocean. But, the west Pacific controls on the southwest monsoonal flow, and in turn, the modulation of rainfall variability in the subseasonal scale by this low-frequency seasonal to interannual background over India is poorly understood. Here, we examine the moisture pathways by which the spatial pattern of rainfall variability in the subseasonal scale is envisaged to be arising due to the interplay between the Bay of Bengal (BoB) summer monsoon circulation and the South China Sea (SCS) moisture dynamics under different background conditions provided by ENSO or the subseasonal movement of the Inter Tropical Convergence Zone (ITCZ). We observe that the south BoB branch of the summer monsoon could occasionally extend up to 120°E in the SCS and reverse back to Indian mainland. We propose a mechanism by which the south BoB branch interacts with the SCS (viz. western Pacific) atmospheric system, thereby facilitating a pathway of the west Pacific moisture intrusion into the Indian subcontinent. The study concludes that the changes in surface temperature/pressure spatial pattern between India and SCS region play an important role in the distribution of the rainfall. In this study, for the first time, the life cycle of south Bay monsoon flow and its interaction with the SCS are discussed, likely to warrant special attention for modelers trying to understand the spatial distribution of rainfall over India.
Journal of Geophysical Research, 2011
Prediction of interannual variability (IAV) of Indian summer monsoon (ISM) rainfall is limited by... more Prediction of interannual variability (IAV) of Indian summer monsoon (ISM) rainfall is limited by "internal" dynamics, and the monsoon intraseasonal oscillations (MISOs) seems to be at the heart of producing internal IAV of the ISM. If one could find an identifiable way through which these MISOs are modulated by slowly varying "external" forcing, such as El Niño-Southern Oscillation (ENSO), the uncertainty in the prediction of IAV could be reduced, leading to improvement of seasonal prediction. Such efforts, so far, have been inconclusive. In this study, the modulation of MISOs by ENSO is assessed by using a nonlinear pattern recognition technique known as the Self-Organizing Map (SOM). The SOM technique is efficient in handling the nonlinearity/event-to-event variability of the MISOs and capable of identifying various shades of MISO from large-scale dynamical/thermodynamical indices, without providing information on rainfall. It is shown that particular MISO phases are preferred during ENSO years, that is, the canonical break phase is preferred more in the El Niño years and the typical active phase is preferred during La Niña years. Interestingly, if the SOM clustering is done by removing the ENSO effect on seasonal mean, the preference for the break node remains relatively unchanged; whereas, the preference reduces/vanishes for the active node. The results indicate that the El Niño-break relationship is almost independent of the ENSO-monsoon relationship on seasonal scale whereas the La Niña-active association seems to be interwoven with the seasonal relationship.
Climate Dynamics, 2013
Atmospheric dynamical mechanisms have been prevalently used to explain the characteristics of the... more Atmospheric dynamical mechanisms have been prevalently used to explain the characteristics of the summer monsoon intraseasonal oscillation (MISO), which dictates the wet and dry spells of the monsoon rainfall. Recent studies show that ocean-atmosphere coupling has a vital role in simulating the observed amplitude and relationship between precipitation and sea surface temperature (SST) at the intraseasonal scale. However it is not clear whether this role is simply 'passive' response to the atmospheric forcing alone, or 'active' in modulating the northward propagation of MISO, and also whether the extent to which it modulates is considerably noteworthy. Using coupled NCEP-Climate Forecast System (CFSv2) model and its atmospheric component the Global Forecast System (GFS), we investigate the relative role of the atmospheric dynamics and the ocean-atmosphere coupling in the initiation, maintenance, and northward propagation of MISO. Three numerical simulations are performed including (1) CFSv2 coupled with high frequency interactive SST, the GFS forced with both (2) observed monthly SST (interpolated to daily) and (3) daily SST obtained from the CFSv2 simulations. Both CFSv2 and GFS simulate MISO of slightly higher period (*60 days) than observations (*45 days) and have reasonable seasonal rainfall over India. While MISO simulated by CFSv2 has realistic northward propagation, both the GFS model experiments show standing mode of MISO over India with no northward propagation of convection from the equator. The improvement in northward propagation in CFSv2, therefore, may not be due to improvement of the model physics in the atmospheric component alone. Our analysis indicates that even with the presence of conducive vertical wind shear, the absence of meridional humidity gradient and moistening of the atmosphere column north of convection hinders the northward movement of convection in GFS. This moistening mechanism works only in the presence of an 'active' ocean. In CFSv2, the lead-lag relationship between the atmospheric fluxes, SST and convection are maintained, while such lead-lag is unrealistic in the uncoupled simulations. This leads to the conclusion that high frequent and interactive oceanatmosphere coupling is a necessary and crucial condition for reproducing the realistic northward propagation of MISO in this particular model. Keywords Summer monsoon intraseasonal oscillation Á Northward propagation Á Atmospheric dynamics Á Ocean-atmosphere coupling Á Climate Forecast System This paper is a contribution to the Topical Collection on Climate Forecast System Version 2 (CFSv2). CFSv2 is a coupled global climate model and was implemented by National Centers for Environmental Prediction (NCEP) in seasonal forecasting operations in March 2011.
Atmospheric Science Letters, 2013
Impact of bias correction of sea surface temperature (SST) forecast on extended range (ER, ∼3–4 w... more Impact of bias correction of sea surface temperature (SST) forecast on extended range (ER, ∼3–4 weeks) prediction skill is studied using the bias‐corrected forecasted SST from Climate Forecast System version 2 (CFSv2) as the boundary condition for running the Global Forecast System version 2 (GFSv2) model. Potential predictability limit is comparable (∼16 days) for both bias‐corrected GFSv2 (GFSv2bc) and CFSv2. Prediction skills of active and break spells and of low‐frequency monsoon intraseasonal oscillations is higher for GFSv2bc at all lead pentads. Although initially same, predictability error after 14 days grows slightly faster for GFSv2bc compared to CFSv2. Bias correction in SST has minimal impact in short‐to‐medium range, while substantial influence is felt in ER between 12–18 days.
International Journal of Climatology, 2020
The water vapour variation in the upper troposphere and lower stratosphere (UTLS) is of high sign... more The water vapour variation in the upper troposphere and lower stratosphere (UTLS) is of high significance due to its impact on global warming. In this article, we present an association of occurrence frequency of double tropopauses (DTs) with convective clouds and transport of water vapour in the UTLS over subtropical South Asia using multiple multi‐decadal datasets (e.g., radiosonde temperature profiles (1977–2017), Atmospheric Infrared Sounder (2003–2017), ERA‐Interim reanalysis (1979–2017) and Microwave Limb Sounder (2004–2016). The diagnostic analysis of temperature, water vapour and potential vorticity indicates that convective clouds occurring during DTs enhance water in the altitude layer near the DTs. DTs are frequent (~5–55%) over the subtropical South Asia (25°–30°N) and associated with an enhancement of water vapour mixing ratios by ~5–40% (0.2–7.5 ppmv) above the lower tropopause. The radiosonde observations show a positive trend (~0.27 ± 0.12 to 0.4 ± 0.2%/year) in the ...
Atmospheric Chemistry and Physics, 2018
The highly vibrant Asian summer monsoon (ASM) anticyclone plays an important role in efficient tr... more The highly vibrant Asian summer monsoon (ASM) anticyclone plays an important role in efficient transport of Asian tropospheric air masses to the extratropical upper troposphere and lower stratosphere (UTLS). In this paper, we demonstrate long-range transport of Asian trace gases via eddy-shedding events using MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) satellite observations, ERA-Interim reanalysis data and the ECHAM5-HAMMOZ global chemistry-climate model. Model simulations and observations consistently show that Asian boundary layer trace gases are lifted to UTLS altitudes in the monsoon anticyclone and are further transported horizontally eastward and westward by eddies detached from the anticyclone. We present an event of eddy shedding during 1-8 July 2003 and discuss a 1995-2016 climatology of eddy-shedding events. Our analysis indicates that eddies detached from the anticyclone contribute to the transport of Asian trace gases away from the Asian region to the western Pacific (20-30 • N, 120-150 • E) and western Africa (20-30 • N, 0-30 • E). Over the last two decades, the estimated frequency of occurrence of eddy-shedding events is ∼ 68 % towards western Africa and ∼ 25 % towards the western Pacific. Model sensitivity experiments considering a 10 % reduction in Asian emissions of non-methane volatile organic compounds (NMVOCs) and nitrogen oxides (NO x) were performed with ECHAM5-HAMMOZ to understand the impact of Asian emissions on the UTLS. The model simulations show that transport of Asian emissions due to eddy shedding significantly affects the chemical composition of the upper troposphere (∼ 100-400 hPa) and lower stratosphere (∼ 100-80 hPa) over western Africa and the western Pacific. The 10 % reduction of NMVOCs and NO x Asian emissions leads to decreases in peroxyacetyl nitrate (PAN) (2 %-10 % near 200-80 hPa), ozone (1 %-4.5 % near ∼ 150 hPa) and ozone heating rates (0.001-0.004 K day −1 near 300-150 hPa) in the upper troposphere over western Africa and the western Pacific. forts to implement several mitigation strategies (Ohara et al., 2007). In situ observations, satellite measurements, trajectory analysis and model simulations show long-range transport of Asian trace gases to remote locations (e.g. North America, Europe) (Liang et al., 2004). The transported trace gases change the radiative balance, dynamics and chemical composition at the respective locations (Vogel et al., 2016). Satellite observations show increasing trends in sev-Published by Copernicus Publications on behalf of the European Geosciences Union.
MAUSAM, 2021
The evolution of operational extended range forecast (ERF) system of IMD starting from use of emp... more The evolution of operational extended range forecast (ERF) system of IMD starting from use of empirical models, dynamic models and the Multi-model Ensemble (MME) from 2009 to 2016 till the operational implementation of fully coupled model in 2016 is discussed. The coupled model implemented in IMD is the Climate Forecast System version 2 (CFSv2). The performance of ERF for southwest monsoon, northeast monsoon, cyclogenesis over the Bay of Bengal (BoB) and maximum and minimum temperature during the period from 2009 to 2018 have been discussed along with its prospects of its application in different sectors like, agriculture, hydrology, health, power etc. are also analysed. The performance of extended range forecasts for the southwest monsoon seasons clearly captured the intraseasonal variability of monsoon including delay/early onset of monsoon, active/break spells of monsoon and also withdrawal of monsoon in the real time in providing guidance for various applications. The MME based ERF also provides encouraging results to provide useful guidance upto 2/3 weeks about northeast monsoon and cyclogenesis potential during October to December (OND) over the north Indian Ocean, heat wave, cold wave during summer and winter with statistically significant correlation coefficient (CC) upto two weeks. For applications in agriculture sector meteorological subdivision level forecasts are prepared for two weeks for the purpose of agro advisory. In addition to the regular ERF products for application in agriculture and hydrology, additional products are being prepared like, Standarised Precipitation Index (SPI), land-surface hydrology products like soil moisture and runoff change, transmission windows products for vector borne diseases etc for applications in agriculture, hydrology and health sectors.
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Papers by Rajib Chattopadhyay