Assimilation of aircraft observations over the Indian monsoon region: Investigation of the effects of COVID‐19 on a reanalysis

Since March 2020, the COVID‐19 pandemic has significantly reduced the availability of global aircraft‐based observations (ABOs), which has been restored later in 2021. This study focuses on the impact of ABOs on a regional reanalysis. Indian Monsoon Data Assimilation and Analysis (IMDAA) is a regional reanalysis for a period from 1979 to 2020 (originally up to 2018) over India and surrounding regions produced at the National Centre for Medium Range Weather Forecasting (NCMRWF), India, in collaboration with the UK Met Office. A comparison of the impact of ABOs on other conventional and satellite observations assimilated in the NCMRWF global model and IMDAA during 2019 and 2020 revealed the importance of ABOs, particularly in IMDAA, since it did not assimilate the latest satellite data as the IMDAA system was frozen in October 2016. A data denial experiment that removes all the ABOs from the IMDAA assimilation system for a period from March to November 2019 is designed. The results from the IMDAA reanalysis run, which assimilates ABOs during the same period, are compared with the data denial experiment. Assimilation of ABOs strengthened the upper tropospheric circulation, the Tropical Easterly Jet (TEJ), during the Indian summer monsoon compared to the data denial experiment. Analysis of the features of two cyclones that developed over the North Indian Ocean during the study period revealed that ABO assimilation played a key role in simulating the track and intensity of these cyclones when they were in the ‘severe’ category. Since the sample is small, more cyclone cases need to be analysed to consolidate the result. Global coverage of aircraft observations received at the National Centre for Medium Range Weather Forecasting (NCMRWF) during the 1200 UTC assimilation cycle of 10 April 2019 (a) and (b) 2020. (c) Time series of number of radiosonde profiles over the Indian landmass received at NCMRWF at 0000 UTC of 2019 and 2020. Inside rectangles in (a) and (b) represent the Indian Monsoon Data Assimilation and Analysis (IMDAA) domain.