North Eastern Space Applications Centre

Aerosols are the suspended solid or liquid particles in the atmosphere and are one of the major drivers of climate change. While natural aerosols are present everywhere, anthropogenic aerosols have variable concentration in atmosphere. NER of India, particularly the Brahmaputra valley is one of the highest aerosol concentrated area in India due to its geographical location. NESAC works towards scientific understanding of the optical, physical, dynamical and radiative processes responsible for the climatic impact of aerosols over NER of India. The activities in this area are done primarily under the national project, Aerosol Radiative Forcing over India (ARFI), coordinated by Space Physics Laboratory, Thiruvananthapuram.

Data from several surface based instruments (Multi wavelength radiometer, Sunphotometer, Aethalometer, Nephelometer, Electric low pressure impactor, Cloud Condensation Nuclei (CCN) counter, Microaeth, etc) and satellite derived aerosol products are used to characterize the aerosol. Continuous data from three fixed observatories (at NESAC, Umiam, Meghalaya; Tawang, Arunachal Pradesh; and Lachung, Sikkim) are used for the purpose. Additionally, land campaigns are also conducted to collect short term data over different locations over NER of India. Four such campaigns have been conducted along the East-West and North-South corridor of the NE region and also along the Northern Arunachal Pradesh.

The effect of aerosol has become a matter of concern, because it is directly affecting our life from the point of view of climate or indirectly in terms of environment, health etc. So, aerosol studies have become a hot topic within the Atmospheric Science research community. However, such studies are vastly dependent on in-situ measurements and a systematic data collection will directly help the academic centers to conduct aerosol-based climate change as well as air pollution studies over NER. The fruitful research work will eventually help the common people.

The estimation of aerosol radiative forcing (RF) comes with large uncertainties and aerosols dominate the uncertainty in the total anthropogenic RF. This uncertainty arises primarily due to unavailability of adequate information on spatial and temporal distribution of aerosol across the globe. This uncertainty can be greatly reduced through accurate measurements of optical, physical, and chemical properties of aerosols through continuous measurements from ground-based networks, ships, aircrafts, balloons, and satellites, collecting data through field experiments supported by numerical modelling. Unfortunately, the north eastern part of India has not been explored much either in the form of fixed station measurements or during any of the large-scale campaigns over India except partly during the CAIPEEX program. Therefore, to examine the spatial variability of the aerosol characteristics, several land campaigns were conducted by NESAC which will help the Academic Institutions for doing aerosol based studies over NER; will help to improve the microphysical schemes in numerical weather prediction models.

Director, NESAC may be contacted to obtain further information on the topics.

The atmospheric boundary layer (ABL) is the lowest part of the atmosphere and its behaviour is directly influenced by its contact with a planetary surface. The ABL plays an important role in modulating local weather and climate. The study of spatial and temporal characteristics of ABL parameters is very important to understand the mixing processes in the ABL, transport of mass, momentum and energy between the surface layer and free troposphere as well as air quality prediction and improvement of the atmospheric model. Mountainous terrain like the NER of India, with its pronounced steep slopes and complex topographic features, produces an intrinsic spatial inhomogeneity in the boundary layer response to surface forcing, making the process more complex.

Several studies have been taken up at NESAC to understand the surface-air interaction and exchange processes, energy and radiation balance of the Earth-atmosphere system over complex mountainous region. Main objective of the activities are to improve the understanding and parameterization of the ABL processes in numerical models, diurnal evolution of ABL and its role in pollutant dispersal, atmospheric turbulence, surface roughness, hydrological processes, etc. The activities are carried out under the national project on Network of Observatories for Boundary Layer Experiments (NOBLE) of ISRO Geosphere Biosphere program. Data from several instruments like, Automatic Weather Station (AWS), 31 meter Meteorological Tower equipped with 3D Sonic anemometer (CSAT3), Pisharoty sonde (launched using hydrogen gas filled balloons), Tethered balloon, etc are used to investigate the ABL evolution over the complex terrain. Three land campaigns have been conducted to study the diurnal variation of katabatic and anabatic wind flow along the Brahmaputra Valley and its impact on local to regional weather.

The ABL studies conducted at NESAC will directly help the research community to develop improved parameterization schemes for numerical models that will eventually help in improving the weather forecasting.

Although ABL studies have many benefits, their direct impact on numerical weather forecasting (NWP) is significant. NWP models are unable to explicitly resolve the small scale or complex processes such as effect of ABL, turbulence, cloud microphysics etc. Therefore, the processes are simplified by the models using certain parameterizaion schemes. However, the performance of the schemes is region specific and it is essential to understand the sensitivity of schemes over a particular region for its optimal use. In order to customize such schemes for a specific region, in-situ measurements are very important. The study carried out by NESAC will definitely help the research community in this direction.

The studies on Atmospheric chemistry are focused on characterization of major trace gases, understanding the chemical process of generation and dissipation of these gases, trace gas transport modelling, understanding the interaction between the atmospheric parameters and the trace gases, etc. Atmospheric science lab at NESAC is equipped with trace gas analyzers for measuring surface level concentration of SO₂, CO, NOx , O₃, CH₄ and non methane hydrocarbons. Along with the in-situ measurement, satellite data are also taken into account to study the long term pattern of trace gases and their impact on environment. Forecasting of trace gas concentration using numerical models like WRF-CHEM and REGCM are also taken up.

The study will help create a standard database that could be used for assessing regional climate change. The database will also help the research community engaged in atmospheric pollution studies, pollution forecasting, etc.

North Eastern Region of India is vulnerable to various kinds of weather-induced natural hazards such as Flood, Landslides, Severe storms, Lightning, Cyclone. The NER of India suffers huge loss of lives and revenue due to the devastation brought by these extreme events. Early warnings of these events are vital for this region. Numerical Weather prediction (NWP) model is a powerful tool that is being used globally for early warning of weather events. The Weather Research and Forecasting (WRF) model is used at NESAC to conduct research for improvement of short and medium range weather forecast over NER of India. The model is run using a high performance computing (HPC) facility available at NESAC. The model is configured with three nested domains with resolutions varying from 27 km to 3 km. Background error statistics for this region is generated using NMC method.

The potential of NWP models depend on the initial conditions employed for integrating the model. Observational data must be modified in a dynamically consistent fashion in order to obtain a suitable set for model initialization. Such processes are called Data Assimilation (DA). Constant efforts are being made to upgrade the quality of initial condition using various DA techniques such as 3DVAR, 4DVAR, Hybrid ETKF-3DVAR. Observed data from AWS stations, Radiosonde, GFS-Prepbufr and Satellites are used for data assimilation. As in situ measurements are not well distributed spatially, satellite plays an important role for this region. Radiances, wind data from satellites such as INSAT-3D, SCATSAT-1, AMSU, HIRS, MHS are assimilated in the model. Several experiments are done with Hybrid: Hybrid-50, where 50% weight is given to each static and ensemble based background error covariance (BEC) and Hybrid-75 where 75% weight is given to ensemble based BEC.

Model validation is done by comparing daily accumulated WRF rainfall with respect to AWS rainfall data and GPM IMERG (GPM-MS) rainfall product available at 0.1 x 0.1 degree resolution and temporal resolution of 30 min. The validation strategy entails the estimation of the performance metrics such as Bias, RMSD, ETS, Statistically Significant (p<0.05) Pearson’s Correlation Coefficient (CC). Other meteorological parameters are compared with NCEP reanalysis, Era interim data, ASCAT Wind. WRF generated Rainfall data has been used since 2010 by NESAC hydrology team for Flood Early Warning System (FLEWS) project to operationally forecast flood over different flood prone river basins in NER.

The NWP activity at NESAC helps the following community/organizations:

• ASDMA and other State Disaster Management Authorities of NER
• Flood and River Erosion Management Agency of Assam (FREEMA)
• North East Electric Power Corporation (NEEPCO)
• Meghalaya Basin Development Authority
• Indian Council of Medical Research (ICMR)
• State Departments: Agriculture, Forestry & Environment, Water Resource,
• Academic Institutes: ICAR, AAU, NEHU, Dibrugarh University, Guwahati University, IITM

NESAC has been working with the Assam State Disaster Management Authority (ASDMA) to develop flood forecasting system (FLEWS) for the Brahmaputra and Barak valley in Assam. Atmospheric Science Team of NESAC has developed adequate vertical expertise in development of meteorological support tools that helps the Hydrology team of NESAC to generate and disseminate timely and customized information to move from flood disaster response to improved flood disaster preparedness and to effectively support flood control and management in the flood-prone areas of the State. The project is now going to be implanted all over NER.

Apart from the rainfall forecast, other important meteorological parameters such Wind at different atmospheric levels, Relative Humidity and Temperature at surface level are published on daily basis on NESAC website which will help the general public to get an idea of the anticipated weather condition over NER.

Severe weather events like severe storms, lightning, intense rainfall, etc are very common over the NER of India. In India, each year lightning alone kills more than 2,500 people and it ranks first in terms of killing among all the natural calamities (Report of National Crime Record Bureau, India). The NER of India is also no different. With the constantly increasing use of electronic devices in the industrial and domestic sphere, the vulnerability to lightning damages is growing in leaps and bounds. Lightning is often associated with gusting winds, intense rainfall, hails, etc.

NESAC has developed a methodology for nowcasting of severe thunderstorms by integrating data from weather stations, Doppler weather radars, satellites, and numerical weather prediction models. A thunderstorm potential map is prepared for entire NER of India, indicating the areas where thunderstorm development is expected within next three hours from the time of issue of the forecast. The forecast is made available through NESAC website and also are directly communicated to the concerned stakeholders in each state of NER of India.

NESAC has also initiated research to provide location based lightning warning services. The data from lightning detectors from different network over NER of India are used and assimilated is numerical weather prediction models to provide location specific lightning as well as severe weather warning with an actionable lead time.

In NESAC, TS Nowcasting is provided in two forms, first by preparing a thunderstorm potential map based on numerical weather prediction model outputs, and then by preparation of thunderstorm bulletins based on real time weather watch using data from Doppler weather radar, Satellites, and automatic weather stations. The services are provided from 15th March to 15th June every year, as frequency of TS is highest during this period.

The severe weather nowcasting services from NESAC are consumed by the following community/organizations:

• ASDMA and other State Disaster Management Authorities of NER
• Flood and River Erosion Management Agency of Assam (FREEMA)
• North East Electric Power Corporation (NEEPCO)
• District Disaster Management Authorities from NE region of India

The TS bulletin and potential map was issued thrice daily with TS bulletin having validity of four hour and TS potential map having validity of three hours. The TS potential map uses WRF model based forecast of atmospheric instability indices (like convective available potential energy, convective inhibition energy, K index, Lifted Index, etc) and relative humidity. The TS potential map predicts the potential location where thunderstorm occurrence is likely during a given time. TS warnings in both TS bulletin and potential map is categorized in five category (Nil, Low, Moderate, High, and Severe) based on the likely intensity of the thunderstorm. So, the users can get the idea about the potential thunderstorm occurrence zone based on the different categories. Similarly the lightning forecasting services are provided for every hour during the peak lightning period, viz. Mid March to mid June every year. The forecast is disseminated through NESAC website as well as through direct communication.

NESAC with support from ISRO, has been putting maximum effort to improve the surface based meteorological observations over the NER of India. Towards this a network of Automatic Weather Stations (AWS), one S-band Polarimetric Doppler Weather Radar (DWR), an UHF Wind profiler has been installed in NER of India. Additionally, NESAC has facilitated installation of lightning detectors by National Remote Sensing Centre (NRSC) and Indian Institute of Tropical Meteorology (IITM).

The first polarimetric S-band DWR in India has been installed at Sohra (Cherrapunjee), Meghalaya. The radar provides continuous data on rainfall and wind over a circular area having radius of 300 km with the radar at the centre. The data from the radar is made available in real time through India Meteorological Department and MOSDAC websites. The data from the DWR are used primarily for the nowcasting of severe thunderstorms and operational forecasting by IMD and Indian Air Force (IAF).

Numerical Weather Prediction models are used globally for early warning of catastrophic weather events such as Flood, Thunderstorm, Lightning, Tropical Cyclone etc. Though we have made remarkable improvements in the forecast of weather systems during the last decade, the prediction of severe convective systems such as thunderstorms, extreme localized rainfall, and lightning are yet very challenging. One potential reason for frequent failure of localized convection forecast is the lack of high resolution observation over a region that would represent the small-scale features of a convective system. NESAC with support from ISRO, has been putting maximum effort to improve the surface based meteorological observations over the NER of India.

The data collected by NESAC using different instruments helps the following community/organizations:

• Flood and River Erosion Management Agency of Assam (FREEMA)
• North East Electric Power Corporation (NEEPCO)
• Meghalaya Basin Development Authority
• Indian Air Force (Eastern Air Command)
• Indian Council of Medical Research (ICMR)
• India Meteorological Department
• State Departments: Agriculture, Forestry & Environment, Water Resource,
• Academic Institutes: ICAR, AAU, NEHU, Dibrugarh University, Guwahati University, IITM

The performance of any NWP model is based on the initial conditions required for running a model. However, these initial conditions are based on globally available observed data, and these data do not include local atmospheric information that is important for simulating severe weather systems. The accuracy of the model forecast can only be increased by incorporating these local observations from different sources such as AWS station, Radiosonde, DWR, Satellite etc., in the NWP model using data assimilation techniques. Also, the observed data are very crucial for model validation. The observations from Doppler Weather Radar (DWR) play a significant role for nowcasting of severe weather system.

Most of the services provided by this group are shared through NESAC website. However, Director, NESAC may be contacted to obtain further information on the topics.