Preface to the INCOMPASS Special Collection

The South Asian summer monsoon provides 80% of annual precipitation to over a billion people. Among other users of water resources, the monsoon is particularly important for agriculture and power generation, itself in support of food security and employment. Accurate prediction of the amount and timing of monsoon rainfall is vital to society across many time-scales, from daily weather forecasting up to multi-decadal changes in the mean state. While much research continues into monsoon modelling and prediction, large model biases prevail. A key opportunity lies in better measurement and understanding of processes operating at small space and time scales, particularly interactions between the surface, boundary layer and cloud dynamics which profoundly influence monsoon convection. These processes are often parametrized in models. It is against this background that the Earth System Science Organization (ESSO) of the Ministry of Earth Sciences (MoES), Govt. India, in collaboration with the Natural Environment Research Council (NERC), United Kingdom, funded the Drivers of variability in the South Asian monsoon programme. For NERC, this was part of a larger NERC action on “Drivers of variability in atmospheric circulation”, while for ESSO-MoES, this activity fell under its Monsoon Mission to improve prediction of the monsoon on short-range to interannual time-scales. This funding avenue provided the unique opportunity for the Indo-UK scientific community to collaborate on a joint observational mission to India (described in Turner et al., 2020), including the first deployment of a foreign atmospheric research aircraft to the country. The INCOMPASS aircraft mission, from bases in Lucknow and Bengaluru, took place between May and July 2016 using the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft, run jointly by NERC and the Met Office. This is a four-engine jet aircraft, capable of carrying a large suite of instruments and up to 19 scientists on missions lasting up to 5 hours. Use of the aircraft was shared with the sister project SWAAMI (South West Asian Aerosol–Monsoon Interactions: see, for example, Brooks et al., 2019). As described in the lead article of this Special Collection (Turner et al., 2020), the INCOMPASS field campaign is designed to better understand how an air parcel is modified as it travels towards India, crossing coastlines, mountains and a variety of land surface types and soil moisture patterns. Work motivated by INCOMPASS and published in this Special Collection has already suggested a new paradigm for the monsoon onset (Parker et al., 2016), as a tug-of-war between advancing tropical flow and a retreating dry intrusion emanating northwest of India. The erosion of this dry intrusion has been shown in modelling work to be aided by the detrainment of moisture from shallow convection (Menon et al., 2018), while the role of the extratropics has been further highlighted (Volonté et al., 2020), showing that monsoon progression is a jerky process. While other monsoon processes such as ocean– atmosphere interaction have been heavily studied in the past, the role of the land surface is often overlooked. In particular, how the land surface feeds back on the progression of the monsoon through its seasonal cycle, or