Long-term assessment of basic meteorological field variability is an important factor that influences the Indian summer monsoon and consequently affects the socio-economic aspects of India. In this study, the spatial and temporal variation of meteorological parameters during summer monsoon season using NCEP/NCAR reanalysis datasets for the period of 70 years (1948–2017) has been analyzed in climatology, early-late phase and multidecadal epochs over India and its regions. Statistical techniques such as the standardized anomaly index of surface temperature, rainfall and zonal and meridional wind (at 850 and 200 hPa) and temporal analysis of Mann–Kendall trend test over six selected regions, viz., NorthIndia (NI), Central India (CI), Southern India (SI), Arabian Sea (AS), Bay of Bengal (BoB) and Equatorial Indian Ocean (EIO) reveal higher variability during summer monsoon season from 1948 to 2017. The significant spatial changes in the value of standard deviation and coefficient of variation confirm the early-late phase and multidecadal modulation of the seasonal variability of selected climatic parameters. The results indicate that the escalation in the surface temperature multidecadal variability and trend has dominating characteristics over NI, CI and SI regions at an alarming range (0.5–1.0°C). The major hotspots of increasing early-late phase and multidecadal variability and average precipitation have been found over BoB, EIO and SI (${\sim}$1–3.5 mm/day). The decreasing changes in the mean rainfall pattern and associated variability is strongly linked with increasing surface warming and significant reduction in the strength of surface zonal wind over BoB, IO, SI and CI region which cause the weakening of important atmospheric circulations such as the role of Somali jet and strong low-level jet (LLJ) during Indian summer monsoon season. Also, the meridional wind at the surface and upper level has shown significant enhancement over AS and EIO. The recent decadal anomaly (2008–2017) is really a matter of concern as precipitation and wind circulation anomaly at 850 and 200 hPa have shown decreasing trends over all the regions. In recent years, the variation in meteorological parameters and distribution are asymmetrical during summer monsoon season in changing climate.

In-situ vicarious calibration method for INSAT-3D imager visible (550–750 nm) has been carried out for characterizing the radiometric uniformity of the site. The field campaigns were conducted over two sites Thar Desert, Jaisalmer and Bhuj, Gujarat, during 2013–2017. The measurements of ground reflectance using a Spectroradiometer (spectral ranges 350–2500 nm), aerosol optical depth (AOD) using a sunphotometer and total column ozone using an Ozonometer, respectively, and other meteorological variables were obtained. Based on these measurements, simulations of radiance reaching the top of the atmosphere (TOA) was carried out using radiative transfer model 6S (second simulation of satellite signal in solar spectrum). Calibration coefficients of INSAT-3D were estimated by linear regression analysis based on INSAT-3D measured and 6S simulated radiance for both the sites. For the Jaisalmer site, the results suggested a very high relative error between INSAT-3D measured and 6S derived radiance with high spectral variability in ground reflectance and less statistical correlation. Whereas, for the Bhuj site, close agreement was observed between INSAT-3D measured and 6S derived radiance with uniform ground reflectance for all the days of the campaign. The mean difference in the relative errors is ${\le}$3% over the Bhuj site. In addition, to check its suitability, the temporal and spectral stability of the Bhuj site was also analyzed using moderate-resolution imaging spectroradiometer (MODIS) and advanced very high-resolution radiometer (AVHRR) data. The analysis suggested that the Bhuj site has been preferred for post-launch vicarious calibration of the optical sensor of INSAT-3D/R satellites as well as the upcoming geostationary INSAT satellite series.