arXiv subjects

Rajib Chattopadhyay

Publications and source records attributed to Rajib Chattopadhyay.

12 recordsLinked to original sources

arXiv2025

Emerging trend in the east-west Dipole Pattern in Indian Summer Monsoon Rainfall and the associated impact on Regional Dynamics

Traditionally, during the monsoon season, more rainfall is received along the Western Ghats, the Northern Gangetic plains, the central belt, and northeast India. However, recently, there has been a shift in this canonical monsoon rainfall pattern on the monthly to seasonal scale. In this study, we quantify an east-west asymmetric trend in monthly to seasonal rainfall due to the increased rainfall over the northwestern part of the country. An Empirical Orthogonal Function (EOF) analysis has been performed to understand the spatial and temporal variation of the monsoon. EOF mode 3 shows such a distinct east-west dipole pattern, highlighting the existence of a modal feature representing the recent trend in the rainfall distribution. The physical nature of this mode is also established. The regression pattern of the rainfall anomalies to the Webster-Yang Index (Webster and Yang, 1992) exhibits a similar east-west pattern that further confirms the physical existence of this east-west rainfall modal dipole pattern. Since rainfall across the northwest is directly linked to the Arabian Sea and rainfall over the eastern region to the Bay of Bengal, the characteristics of these two regions are studied separately. Over the Arabian Sea, there is a significant negative trend in the Sea Level Pressure (SLP) anomalies and an increase in the specific humidity, causing greater moisture convergence. In contrast, over the Bay of Bengal, the SLP shows an increasing trend. The SST warming over the Arabian Sea is higher than that of the Bay of Bengal. Further, while investigating the zonal wind(u) at 850hPa, it shows an increasing trend along the northern branch that is more directed towards the northwestern part of the country. These factors together create dynamically favorable conditions for enhanced convection and thus receive more rainfall across the northwest compared to the northeast India.

physics.ao-ph↗

arXiv2025

Dynamics of Heatwave Intensification over the Indian Region

In a warming world, heatwaves over India have become intense and are causing severe health impacts. Studies have identified the presence of amplified Rossby waves and their association with the intensification of heatwaves. Earlier studies have identified two dominant modes of temperature variability in India and their possible role in the development of dry (mode 1) and moist (mode 2) heatwaves. These modes are associated with midlatitude Rossby waves intruding over the Indian region. However the role of regional forcing and the teleconnection behind the intensification of the heatwaves over India is missing. The present study has analyzed the dynamical mechanisms for the regional intensification of the circulation features associated with the dominant moist heatwave mode (mode 2). Considering the predominant barotropic nature of the observed circulation features of the mode, a simple barotropic vorticity equation model forced with extratropical and regional vorticity sources is used to understand the intensification of the heat waves. It was found that a wave response initiated by a cyclonic vorticity over the Bay of Bengal superimposes with the mid-latitude anticyclonic vorticity generated Rossby waves intruding over India. This superimposition results in the amplification and persistence of the anticyclonic vorticity phase over the Northwest Indian region, leading to the intensification of circulation. It was also found that the barotropically forced intensified circulation leads to the intensification of the heat stress. Under a climate change scenario, different circulation regimes, characterized by zonal stationary wave number and jet speed, which can favor the intensification are also identified.

physics.ao-ph↗

arXiv2025

Towards Location-Specific Precipitation Projections Using Deep Neural Networks

Accurate precipitation estimates at individual locations are crucial for weather forecasting and spatial analysis. This study presents a paradigm shift by leveraging Deep Neural Networks (DNNs) to surpass traditional methods like Kriging for station-specific precipitation approximation. We propose two innovative NN architectures: one utilizing precipitation, elevation, and location, and another incorporating additional meteorological parameters like humidity, temperature, and wind speed. Trained on a vast dataset (1980-2019), these models outperform Kriging across various evaluation metrics (correlation coefficient, root mean square error, bias, and skill score) on a five-year validation set. This compelling evidence demonstrates the transformative power of deep learning for spatial prediction, offering a robust and precise alternative for station-specific precipitation estimation.

physics.ao-ph↗

arXiv2023

FlashBench: A lightning nowcasting framework based on the hybrid deep learning and physics-based dynamical models

Lightning strikes are a well-known danger, and are a leading cause of accidental fatality worldwide. Unfortunately, lightning hazards seldom make headlines in international media coverage because of their infrequency and the low number of casualties each incidence. According to readings from the TRMM LIS lightning sensor, thunderstorms are more common in the tropics while being extremely rare in the polar regions. To improve the precision of lightning forecasts, we develop a technique similar to LightNet's, with one key modification. We didn't just base our model off the results of preliminary numerical simulations; we also factored in the observed fields' time-dependent development. The effectiveness of the lightning forecast rose dramatically once this adjustment was made. The model was tested in a case study during a thunderstorm. Using lightning parameterization in the WRF model simulation, we compared the simulated fields. As the first of its type, this research has the potential to set the bar for how regional lightning predictions are conducted in the future because of its data-driven approach. In addition, we have built a cloud-based lightning forecast system based on Google Earth Engine. With this setup, lightning forecasts over West India may be made in real time, giving critically important information for the area.

physics.ao-ph↗

arXiv2023

Intraseasonal Oscillation of Land Surface Moisture and its role in the maintenance of land ITCZ during the active phases of the Indian Summer Monsoon

What is the role of soil moisture in maintaining the land ITCZ during the active phase of the monsoon? This question has been addressed in this study by using ERA5 reanalysis datasets, and then we evaluate the question in the CFS model-free run. Like rainfall, soil moisture also show intraseasonal oscillation. Furthermore, the sub-seasonal and seasonal features of soil moisture are different from each other. During the summer monsoon season, the maximum soil moisture is found over western coastal regions, central parts of India, and the northeastern Indian subcontinent. However, during active phases of the monsoon, the maximum positive soil moisture anomaly was found in North West parts of India. soil moisture also play a pre-conditioning role during active phases of the monsoon over the monsoon core zone of India. When it is further divided into two boxes, the north monsoon core zone, and the south monsoon core zone, it is found that the preconditioning depends on that region's soil type and climate classification. Also, we calculate the moist static energy (MSE) budget during the monsoon phases to show how soil moisture feedback affects the boundary layer MSE and rainfall. A similar analysis is applied to the model run, but it cannot show the realistic preconditioning role of soil moisture and its feedback on the rainfall as in observations. We conclude that to get proper feedback between soil moisture and precipitation during the active phase of the monsoon in the model, the pre-conditioning of soil moisture should be realistic.

physics.ao-ph↗

arXiv2023

On the Relative Role of East and West Pacific Sea Surface Temperature (SST) Gradients in the Prediction Skill of Central Pacific NINO3.4 SST

Dominant modes of SST in the west and east Pacific show strong but regionally different gradients caused by waves, internal dynamics, and anthropogenic warming, which drives air-sea interaction in the Pacific. The study discusses the relative contribution of SST gradients over the western and eastern Pacific to the prediction skill of SST in the central Pacific, where El-Nino, La-Nina, or El-Nino Modoki events project significantly. For this, the analysis develops a convolutional neural network (CNN) based prediction model to predict the Nino3.4 SST. CNN-based prediction models use a spatial filter at the initial stage, which is highly efficient in capturing the edges or gradients and hence are useful to understand the role of SST spatial gradients in the prediction skill. The study reports three CNN-based model experiments. The first one is a CTRL experiment that uses the whole equatorial Pacific domain SST pattern. The second and third models use the equatorial eastern and western Pacific domain SST only. Another novel feature of this study is that we have generated a large number of ensemble members (5000) through random initialization of CNN filters. It is found that random initialization affects the forecast skill, and the probability density function of the correlation skill of the 5000 models at each lead time shows a gaussian distribution. The model experiments suggest that the west Pacific SST model provides better Nino3.4 skills as compared to the east Pacific skill. The CNN-based model forecast based on the SST pattern, thus, shows the impact of the SST spatial pattern on the ENSO forecast.

physics.ao-ph↗

arXiv2022

On the modern deep learning approaches for precipitation downscaling

Deep Learning (DL) based downscaling has become a popular tool in earth sciences recently. Increasingly, different DL approaches are being adopted to downscale coarser precipitation data and generate more accurate and reliable estimates at local (~few km or even smaller) scales. Despite several studies adopting dynamical or statistical downscaling of precipitation, the accuracy is limited by the availability of ground truth. A key challenge to gauge the accuracy of such methods is to compare the downscaled data to point-scale observations which are often unavailable at such small scales. In this work, we carry out the DL-based downscaling to estimate the local precipitation data from the India Meteorological Department (IMD), which was created by approximating the value from station location to a grid point. To test the efficacy of different DL approaches, we apply four different methods of downscaling and evaluate their performance. The considered approaches are (i) Deep Statistical Downscaling (DeepSD), augmented Convolutional Long Short Term Memory (ConvLSTM), fully convolutional network (U-NET), and Super-Resolution Generative Adversarial Network (SR-GAN). A custom VGG network, used in the SR-GAN, is developed in this work using precipitation data. The results indicate that SR-GAN is the best method for precipitation data downscaling. The downscaled data is validated with precipitation values at IMD station. This DL method offers a promising alternative to statistical downscaling.

physics.ao-ph↗

arXiv2021

Machine learning for Earth System Science (ESS): A survey, status and future directions for South Asia

This survey focuses on the current problems in Earth systems science where machine learning algorithms can be applied. It provides an overview of previous work, ongoing work at the Ministry of Earth Sciences, Gov. of India, and future applications of ML algorithms to some significant earth science problems. We provide a comparison of previous work with this survey, a mind map of multidimensional areas related to machine learning and a Gartner's hype cycle for machine learning in Earth system science (ESS). We mainly focus on the critical components in Earth Sciences, including atmospheric, Ocean, Seismology, and biosphere, and cover AI/ML applications to statistical downscaling and forecasting problems.

cs.LG↗

arXiv2021

Deep learning for improved global precipitation in numerical weather prediction systems

The formation of precipitation in state-of-the-art weather and climate models is an important process. The understanding of its relationship with other variables can lead to endless benefits, particularly for the world's monsoon regions dependent on rainfall as a support for livelihood. Various factors play a crucial role in the formation of rainfall, and those physical processes are leading to significant biases in the operational weather forecasts. We use the UNET architecture of a deep convolutional neural network with residual learning as a proof of concept to learn global data-driven models of precipitation. The models are trained on reanalysis datasets projected on the cubed-sphere projection to minimize errors due to spherical distortion. The results are compared with the operational dynamical model used by the India Meteorological Department. The theoretical deep learning-based model shows doubling of the grid point, as well as area averaged skill measured in Pearson correlation coefficients relative to operational system. This study is a proof-of-concept showing that residual learning-based UNET can unravel physical relationships to target precipitation, and those physical constraints can be used in the dynamical operational models towards improved precipitation forecasts. Our results pave the way for the development of online, hybrid models in the future.

physics.ao-ph↗

arXiv2021

Deep Learning Based Forecasting of Indian Summer Monsoon Rainfall

Accurate short range weather forecasting has significant implications for various sectors. Machine learning based approaches, e.g., deep learning, have gained popularity in this domain where the existing numerical weather prediction (NWP) models still have modest skill after a few days. Here we use a ConvLSTM network to develop a deep learning model for precipitation forecasting. The crux of the idea is to develop a forecasting model which involves convolution based feature selection and uses long term memory in the meteorological fields in conjunction with gradient based learning algorithm. Prior to using the input data, we explore various techniques to overcome dataset difficulties. We follow a strategic approach to deal with missing values and discuss the models fidelity to capture realistic precipitation. The model resolution used is (25 km). A comparison between 5 years of predicted data and corresponding observational records for 2 days lead time forecast show correlation coefficients of 0.67 and 0.42 for lead day 1 and 2 respectively. The patterns indicate higher correlation over the Western Ghats and Monsoon trough region (0.8 and 0.6 for lead day 1 and 2 respectively). Further, the model performance is evaluated based on skill scores, Mean Square Error, correlation coefficient and ROC curves. This study demonstrates that the adopted deep learning approach based only on a single precipitation variable, has a reasonable skill in the short range. Incorporating multivariable based deep learning has the potential to match or even better the short range precipitation forecasts based on the state of the art NWP models.

physics.ao-ph↗

arXiv2021

On the role of Initial Error Growth in the Skill of Extended Range Prediction of Madden-Julian Oscillation (MJO)

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 bivariate RMSE and error growth evolution (first order difference of index error for each successive lead days) with respect to extended range lead-time are used as metrics in this analysis. The result showed that initial error is not showing any influence in the skill of model after a lead time of 7-10 days and the error growth remains the same for both set of errors. A rapid error growth evolution of same order is seen for both the classified cases. Further the physical attribution of these errors is studied and found that the errors originate from the events with initial phase in Western Pacific and Indian Ocean. The spatial distribution of OLR and the zonal winds also confirms the same. The study emphasises the importance of better representation of MJO phases especially over Indian ocean in the model to improve the MJO prediction rather than focusing primarily on the initial condition

physics.ao-ph↗

arXiv2020

Deep-learning based down-scaling of summer monsoon rainfall data over Indian region

Downscaling is necessary to generate high-resolution observation data to validate the climate model forecast or monitor rainfall at the micro-regional level operationally. Dynamical and statistical downscaling models are often used to get information at high-resolution gridded data over larger domains. As rainfall variability is dependent on the complex Spatio-temporal process leading to non-linear or chaotic Spatio-temporal variations, no single downscaling method can be considered efficient enough. In data with complex topographies, quasi-periodicities, and non-linearities, deep Learning (DL) based methods provide an efficient solution in downscaling rainfall data for regional climate forecasting and real-time rainfall observation data at high spatial resolutions. In this work, we employed three deep learning-based algorithms derived from the super-resolution convolutional neural network (SRCNN) methods, to precipitation data, in particular, IMD and TRMM data to produce 4x-times high-resolution downscaled rainfall data during the summer monsoon season. Among the three algorithms, namely SRCNN, stacked SRCNN, and DeepSD, employed here, the best spatial distribution of rainfall amplitude and minimum root-mean-square error is produced by DeepSD based downscaling. Hence, the use of the DeepSD algorithm is advocated for future use. We found that spatial discontinuity in amplitude and intensity rainfall patterns is the main obstacle in the downscaling of precipitation. Furthermore, we applied these methods for model data postprocessing, in particular, ERA5 data. Downscaled ERA5 rainfall data show a much better distribution of spatial covariance and temporal variance when compared with observation.

physics.ao-ph↗