Characteristics of the atmospheric boundary layer during transient conditions of the Indian summer monsoon.

Knowledge of atmospheric boundary layer height (ABL) during transient monsoon conditions is essential to understand the role of the monsoon on the exchange between ABL and the free troposphere (FT). The role of the Indian summer monsoon on the day-to-day variations of the ABL height over the five selected stations distributed across the northern (New Delhi), central (Nagpur), western (Mumbai), eastern (Kolkata), and southern peninsular (Gadanki) India has been investigated. In this study, radiosonde datasets over 2004–2019 are utilized to obtain the ABL height during the active and break phases of the monsoon. The India Meteorological Department (IMD) gridded rainfall dataset identifies the active and break phases based on the central India rainfall. We have observed the contrasting wind range of spatial variation in the meteorological features during the active and break monsoon conditions over these stations. Within the ABL, the temperature becomes lower and relative humidity (RH) higher during the active than the break phase over different stations across India. The inland stations show pronounced active and break differences compared to the coastal stations. All the stations have a larger active and break difference in surface meteorological parameters during the daytime compared to nighttime. The particulate matter (PM) 2.5 concentration is higher during the break phase when compared to the active phase. The active and break difference in the ABL height strongly depends on the time and space scales besides the meteorological factors. On a day-to-day scale, the relationship between the ABL and active and break phases is not straightforward, as their relationship is nonlinear. The relationship between the ABL height and active and break monsoon phases is stronger for the central inland stations than the coastal stations. Over southern peninsular India, ABL height is more related to wet and dry conditions based on the local rainfall than the central India monsoon rainfall. The ABL height becomes shallower (0.6–0.8 km) over central India and east and west coastal regions in north India, while deeper ABL (~ 1.0–1.8 km) over northwestern India and peninsular inland regions during the active phase and vice versa during the break phase. [ABSTRACT FROM AUTHOR]