Saurabh Das

• Ph.D.
• Professor (Assistant) at Indian Statistical Institute

Geomagnetic storms resulting from solar disturbances impact telecommunication and satellite systems. Satellites are positioned at Lagrange point L1 to monitor these disturbances and give warning 30 min to 1 h ahead. As propagation delay from L1 to Earth depends on various factors, estimating the delay using the assumption of ballistic propagation c...

Coronal mass ejections (CMEs) that cause geomagnetic disturbances on the Earth can be found in conjunction with flares, filament eruptions, or independently. Though flares and CMEs are understood as triggered by the common physical process of magnetic reconnection, the degree of association is challenging to predict. From the vector magnetic field...

Climatic changes and the recent surge in weather extremities are alarming concerns for society today. Tropical squall events with extremely heavy rainfall and lightning are among the very severe ones among such activities. Usually, squalls have a leading convective edge followed by a stratiform trail; however, the reverse is also observed in rare c...

Coronal Mass Ejections (CMEs) that cause geomagnetic disturbances at Earth can be found in conjunction with flares, filament eruptions, or independent. Though, flares and CMEs are understood as triggered by a common physical process magnetic reconnection, yet, the degree of association is challenging to predict. Using eight Machine Learning models,...

Lightning is one of the most severe weather events causing significant loss of human lives and resources. Increasing number of lightning fatalities due to recent climatic changes is emerging out to be a serious concern for India during last few years. Proper characterization and parameterization of the same, therefore, is extremely crucial. However...

Here, we explore the different characteristics of a possible coupling between tropospheric and ionospheric activities during the impact of tropical cyclones (TC) like Amphan and Nisarga in the Indian subcontinent. We have analyzed the effect of TCs Amphan and Nisarga on the low latitude ionosphere using the measurements from several IGS stations ar...

Experimental measurement of satellite signal at 20.2 GHz using GSAT-14 satellite beacon has been carried out over the hilly tropical location Umiam, Shillong during 2017-2019. Collocated rainfall measurements have been done using a laser precipitation monitor for the same duration. The data have been analyzed to understand the signature of orograph...

Accurate measurement of atmospheric water vapour is crucial in forecasting any weather extremity. The complex dynamics of the same, however, makes the process extremely challenging. The conventional techniques (e.g. Radiosonde) used to measure atmospheric water vapour is very costly. Moreover, such measurement can be attempted only a few times a da...

The effect of climate change on precipitation pattern is emerging out as a serious global challenge. However, the quantification and assessment of the climate change effect on precipitation are a challenging issue given the spatio-temporal inhomogeneity of rainfall. Previous studies mostly used only rain intensity information for the identification...

This article implements a Convolutional Neural Network (CNN)-based deep-learning model for solar-wind prediction. Images from the Atmospheric Imaging Assembly (AIA) at 193 Å wavelength are used for training. Solar-wind speed is taken from the Advanced Composition Explorer (ACE) located at the Lagrangian L1 point. The proposed CNN architecture is de...

This paper presents an investigation to explore different characteristics of a possible coupling between tropospheric and ionospheric activity using IIT Indore GPS station data, during the Tropical cyclone (TC) Nisarga, which hit the west coast the Indian subcontinent. Studies reveal that a TC can generate a wide spectrum of Gravity Waves(GWs) from...

This article implements a Convolutional Neural Network (CNN)-based deep learning model for solar-wind prediction. Images from the Atmospheric Imaging Assembly (AIA) at 193\.A wavelength are used for training. Solar-wind speed is taken from the Advanced Composition Explorer (ACE) located at the Lagrangian L1 point. The proposed CNN architecture is d...

Satellite signal at the Ka -band is measured at a hilly, high rain fall region, Umiam, Shillong (25.6768° N, 91.9270 °E) for three years using GSAT 14 satellite. Scintillation has been extracted using a Butterworth filter of tenth order and 0.025 Hz cutoff frequency. Experimental results are then compared with the existing ITU-R scintillation mod...

Validation of synthetic storm technique (SST) has been done for prediction of attenuation due to rain over a hilly, high-rainfall, tropical location, Shillong (25°N, 91°E), India. In this letter, considering one year of rain attenuation and rain rate data shows that SST predicted attenuation overestimates while comparing the available experimental...

Some second order rain attenuation statistics such as fade duration and fade slope are investigated on the basis of experimental measurements of received signals using the GSAT-14 satellite beacon signal at 20.2 GHz for three years (2014–2016) over the tropical location Ahmedabad (23.02 ⁰ E, 72.51⁰ N), India with an Elevation angle of 63⁰. Existing...

Intense geomagnetic storms can have a strong impact on the signals (termed ionospheric scintillations) emitted by any global navigation satellite system (GNSS). The paper reports the first studies of scintillations at the Indore region on the NavIC signals due to the impact of the intense geomagnetic storm event reported on September 8, 2017, at 01...

Intense geomagnetic storms can have a strong impact on the signals (termed as ionospheric scintillations) emitted by any global navigation satellite system (GNSS). The paper reports the first ever scintillations at Indore region on the NavIC signals due to impact of the intense geomagnetic storm event reported on September 8, 2017 at 01:51 and 13:0...

Detection of coronal holes (CHs) in the solar disk is one of the important tasks in the domain of astronomy. In this work, a technique based on image segmentation has been used for the detection of the CHs in the solar disk image. Here in the work, the task of image segmentation has been carried out using fast fuzzy c-means algorithm (FFCM). In the...

The characteristic features of inter-seasonal variability of precipitation microphysics for the year 2019 over Mumbai and Chennai-the two urban-coastal cities of Indian sub-continent have been studied in this paper. The importance of choosing the regions lies in the fact that these two cities are well-known for the occurrences of severe rainfall ev...

The study of solar coronal holes (CHs) is important in the understanding of solar physics and the prediction of space weather events, which have significant impact on space-based instruments, communication and navigation systems. With the availability of the multi-wavelength Atmospheric Imaging Assembly (AIA) instrument on board Solar Dynamics Obse...

Lightning is one of the most important extreme weather conditions and is associated with heavy convective rain and thunderstorms. However, the present understanding of such systems as well their effect on precipitation characteristics are insufficient for proper modelling. In this paper, the rain microphysics is studied at Kolkata using an optical...

Dynamic characteristics of Ka band rain fade (or rain attenuation) over tropical region are relatively less studied, though they are essential for implementation of suitable fade mitigation techniques. In this paper, dynamic characteristics of rain fade at 20 GHz and 30 GHz from GSAT-14 satellite are reported with experimental measurements at Ahmed...

Noise leads to the ambiguity in regions of interest detection by corrupting the pixel information and is a vital problem in image processing domain. A novel hybrid technique based on fuzzy filtering and fuzzy thresholding is proposed here to extract the object regions accurately in presence of Gaussian noises. The proposed method is automated, does...

Propagation impairment due to rain is one of the most critical considerations for satellite communication (Satcom) links above 10 GHz. The existing studies at tropical and equatorial regions are mostly focused on rain attenuation characteristics derived from rainfall characteristics, which may not be adequate to represent actual rain attenuation ch...

This paper emphasizes on NavIC’s performance in ionospheric studies over the Indian subcontinent region. The study is performed using data of one year (2017–18) at IIT Indore, a location near the northern crest of Equatorial Ionization Anomaly (EIA). It has been observed that even without the individual error corrections, the results are within plu...

The presence of coronal holes in solar disk plays an important role in influencing the space weather and generation of the solar wind. As such there lies a requirement in proper study and prediction of coronal holes occurs in the solar disk. This, in turn, arises the necessity of detection of coronal holes present in the solar disk. In this work, a...

This paper emphasizes on NavIC's performance in ionospheric studies over the Indian subcontinent region. The study is performed using data of one year (2017-18) at IIT Indore, a location near the northern crest of Equatorial Ionization Anomaly (EIA). It has been observed that even without the individual error corrections, the results are within $\p...

Navigation satellites provide a good opportunity to monitor both ionospheric and tropospheric conditions by using propagation delays in the L-band signal. However, the availability of S-band frequency in the Indian Regional Navigation Satellite System, Navigation with Indian Constellation (NavIC), provides a unique opportunity to utilize signal str...

Site diversity is one of the few fade mitigation techniques that are actively explored to overcome the serious fading condition encountered in high-frequency satellite communication, particularly over tropical regions. However, the study of site diversity performance is limited over tropical regions due to unavailability of simultaneous fade measur...

Characterization of precipitation is important for proper interpretation of rain information from remotely sensed data. Rain attenuation and radar reflectivity (Z) depend directly on the drop size distribution (DSD). The relation between radar reflectivity/rain attenuation and rain rate (R) varies widely depending upon the origin, topography, and d...

Quantitative precipitation estimations (QPE) from remote sensing observations requires understanding of the rain characteristics. Rain drop size distribution (DSD) is the fundamental parameter for characterizing rain and depends strongly on the climate, origin and precipitation type. Past studies are mostly concentrated on the DSD variations on loc...

Automatic nowcasting of convective initiation and thunderstorms has potential applications in several sectors including aviation planning and disaster management. In this paper, random forest based machine learning algorithm is tested for nowcasting of convective rain with a ground based radiometer. Brightness temperatures measured at 14 frequencie...

Automatic nowcasting of convective initiation and thunderstorms has potential applications in several sectors including aviation planning and disaster management. In this paper, random forest based machine learning algorithm is tested for nowcasting of convective rain with a ground based radiometer. Brightness temperatures measured at 14 frequencie...

This paper investigates the variability of raindrop size distribution (DSD) and rain integral parameters at Ahmedabad, a tropical location, in relation to the radar estimation of rainfall. Rain DSDs for the years 2006-2007 at Ahmedabad (23°04’ N, 72°38’ E) have been measured using a disdrometer. Variability of DSD is evaluated for different seasons...

In the present study, the effectiveness of nowcasting convective activities using a microwave radiometer has been examined for Kolkata (22.65° N, 88.45° E), a tropical location. It has been found that the standard deviation of brightness temperature (BT) at 22 GHz and instability indices like Lifting Index (LI), K Index (KI) and Humidity Index (HI)...

A priori estimation of delays with error variance of atmospheric radio wave propagation is important for precise position estimation and integrity of the system like Global Positioning System (GPS). Tropospheric effects are less severe than the ionospheric counterpart, but behave in a complex manner, making it difficult to isolate them. In this pre...

Precipitable water vapour (PWV) is an important input for numerical weather prediction model, meteorology and highprecision navigational applications. Conventional methods for the determination of PWV using radiosonde are not sufficient owing to poor temporal resolution, whereas radiometer-derived PWV is reliable only in fair weather conditions. Gl...

The propagation of satellite signals at frequencies above 10 GHz has been studied to investigate the characteristic features of tropical rain. The propagation effects, namely, attenuation, scintillation and depolarization caused by varying structures of rain have been investigated to indicate their interrelationship. The varying relationship betwee...

Radar remote sensing of precipitation is important for disaster management and early warning systems. Interpretation of radar reflectivity to derive rain information is, however, not straight forward due to presence of various drop size distributions and rain structure. Information of vertical rain structure is important for accurate quantitative p...

The characteristics of convective and stratiform precipitation over the high-altitude tropical station, Mahabaleshwar (17.92 °N, 73.85°E; 1438 m above sea level) has been studied in the present paper by using multi-frequency radars and disdrometer measurements. The station, receiving an average annual rainfall of 5000 mm represents one of the impor...

Nowcasting of heavy rain events using microwave radiometer has been carried out at Kolkata (22.65oN, 88.45oE), a tropical location. Microwave radiometer can produce the temperature and humidity profiles of the atmosphere with fairly good accuracy. Definite changes are observed in temperature and humidity profiles before and at the onset of heavy ra...

Convective storms popularly known as Nor’westers are very common in eastern and north-eastern parts of India during the pre-monsoon period (March-May). These convective activities cause profuse damage to life and property. Thus Nowcasting of such activities is necessary. Generally, spatial and temporal interpolation of satellite and radar images is...

Convective activities during Indian summer monsoon (June-September) have been studied at Kolkata (22.57oN, 88.37oE). A multi technique observation has been performed using a multi frequency Radiometer, a Micro Rain Radar (MRR), a Disdrometer, and an Electric Field Monitor (EFM) to find some unique signatures of convective events. It has been seen t...

Convective storms generated from Nor’westers are common in the eastern and north-eastern parts of India during the pre-monsoon period (March-May). This type of activities causes immense damage to agriculture, fishing, aviation and life. Conventionally, spatial and temporal interpolation techniques are used to nowcast convection using satellite, rad...

Rain attenuation is very severe at Ka and higher frequencies especially in tropical regions. The conventional fade mitigation techniques are not be able to mitigate this severe fade and hence, suitable diversity technique is required for this purpose. In this paper, rain characteristics and slant path rain attenuation at 30 GHz using synthetic stor...

Nowcasting of thunderstorm has important implication on minimizing the loss of properties and life. Conventional approaches for such purpose using radiometer measured brightness temperature of only water vapor line usually tend to overestimate the occurrence probability. In this paper, we demonstrate that use of brightness values of oxygen absorpti...

Abstract—Nowcasting of thunderstorm has important implication on minimizing the loss of properties and life. Conventional approaches for such purpose using radiometer measured brightness temperature of only water vapor line usually tend to overestimate the occurrence probability. In this paper, we demonstrate that use of brightness values of oxygen...

Nowcasting of thunderstorm has important use in minimizing the loss of property and life. Conventional approaches for such purpose using brightness temperature (from microwave radiometer) of only water vapor line dissatisfactory predict rainfall. In this paper, we show that implementation of brightness values of oxygen and water vapor absorption li...

Radio signals transmitted from satellites are refracted, delayed and depolarirized by the ionosphere and the neutral troposphere while they propagate through the atmosphere producing adverse effects in earth-space communication systems and navigational systems. Water vapour, which is a minor constituent of troposphere, is responsible for such propa...

Currently, global positioning system (GPS) satellites transmit the Klobuchar coefficients to estimate ionospheric delay for the single-frequency users. These coefficients are broadcast to the users on the basis of seasonal ionospheric variations and average solar flux. In low-latitude Indian region, prediction of the delay using these coefficient i...

Increased stratospheric water vapor is one of the signi�cant causes of global warming as increased stratospheric water vapor acts to cool the stratosphere but it warms the underlying troposphere. The sun can influence the clouds by mediating through Galactic cosmic rays (GCR) which controls the nucleation of water droplets in the atmosphere. The ro...

Rain estimation from both space based and ground based radar rely on the proper relationship on radar reflectivity and rain rate and usually assumed to be of the form Z=AR b . Vertical rain rate and radar reflectivity data from MRR observations over Kolkata (22 º 3 E, 88 º 2 N) for monsoon and post monsoon seasons of the year 2009 were studied to u...

Hydrometeors pose a serious threat to satellite communication operating above 10 GHz. The designing line of sight link usually is based on the ITU-R models, which are often inadequate for tropical region. ITU-R model uses a yearly averaged constant rain height for the attenuation calculation, which may not be valid for tropics. This paper reports t...

A long-term observation over three solar cycles indicates a perceptible influence of solar activity on rainfall and associated parameters in the Indian region. This paper attempts to reveal the solar control on the cloud liquid water content (LWC) and integrated water vapor (IWV) along with Indian Summer Monsoon (ISM) rainfall during the period of...

Rain attenuation is an important aspect of signal propagation above 10 GHz frequency. The attenuation time series generation from point rain rate measurement is crucial due to unavailability of actual signal measurements. In this paper, a simple and realistic approach has been demonstrated for better estimation of rain attenuation using Ku-band sig...

The exponential growth of wireless technologies in last few decades coupled with the recent congestion in the L, C and X bands has triggered the use of higher frequency bands like Ku and Ka. However, earth-space satellite communications at frequencies above 10 GHz are highly susceptible to the absorption and scattering caused by the rain drops. In...

Satellite Based Augmentation Systems (SBAS) for navigational services rely heavily on the accu-rate estimation of the Total Electron Content (TEC) along the slant ray path. The Wide Area Augmentation Sys-tem (WAAS) estimates and transmits the delay corrections by assuming a 5x5 degree ionospheric grid (in lati-tude and longitude) at 350 k m altitud...

The microstructure of rain has been studied with observations using a vertical looking Micro Rain Radar (MRR) at Ahmedabad (23.06°N, 72.62°E), a tropical location in the Indian region. The rain height, derived from the bright band signature of melting layer of radar reflectivity profile, is found to be variable between the heights 4600 m and 5200 m...

1] Intersite separation (ISS) between two terminals is one of the important parameters required for implementation of site diversity (SD), as a fade mitigation technique (FMT), in moderate to heavy rain conditions usually encountered in tropical climates. This paper presents the findings of an experiment during the monsoon periods of years 2007 and...

The optimum microwave link design requires the knowledge of rain rate distribution as well as of the duration statistics of the rain events. The rain rates measurements at Ahmedabad, a tropical location in India, are analysed and models for various event parameters are developed using three years of continuous measurements. The average number of ca...

Rain drop size distributions (DSD) are measured with disdrometers at five different climatic locations in the Indian tropical region. The distribution of drop size is assumed to be lognormal to model the rain attenuation in the frequency range of 10–100 GHz. The rain attenuation is estimated assuming single scattering of spherical rain drops. Diffe...

Generation of rain attenuation time series is important for understanding the rain fade characteristics in the absence of actual fade measurements. In this paper, performance of synthetic storm technique (SST) is assessed for time series generation over Ahmedabad, a tropical location in India. Radiometric measurements at 20.2 GHz are used for atten...

The effect of rain attenuation in high frequency signals need proper understanding of the variation of drop size distribution (DSD) separately in convective and stratiform rain due to different characteristics. This paper presents the analysis of variation of vertical rain rate profile for these two types from micro rain radar observations as well...

In the U.S. Wide Area Augmentation System, European Geostationary Navigation Overlay Service, and Indian Global Positioning System Aided Geo Augmented Navigation, a near real-time grid-based single-shell model is proposed to correct the ionospheric delay at the user aircrafts. The single-shell model is based on the assumption that the whole ionosph...

Future Satellite Based Navigation Systems based on GPS, like US Wide Area Augmentation Sysytem (WAAS), are of global interest to the scientific community. Precise estimation of ionospheric delay is most crucial for successful implementation of the systems. Due to the complex ionospheric structure and large variation of Total Electron Content (TEC)...