Examining the characteristics of planetary boundary layer height and its relationship with atmospheric parameters over Indian sub-continent

Abstract

Planetary boundary layer plays a critical role in the monsoon circulation studies at regional scales through exchange of land surface fluxes (heat and moisture) with atmosphere. The objective of the present study is to identify the spatial variability of long-term seasonal trends of planetary boundary layer height (PBLH) over Indian subcontinent and its association with atmospheri parameters. Monthly atmospheric boundary layer height data obtained from European centre for medium range weather forecast ERA-5 reanalysis has been used for the study during 1980–2018. Statistically robust non-parametric Mann Kendall test is used to investigate the significance of trend at 95% confidence interval. From the results it has been observed that decreasing trend for PBLH is noticed during pre-monsoon and monsoon seasons over Indian region. PBLH over central and north-west India is deeper (1600–1800 m) during pre-monsoon season whereas PBLH is shallow during other seasons. Seasonal variation of PBLH shows a strong negative association with the relative humidity (r more than −0.8) whereas it shows strong positive association with surface temperature (r > 0.7) and surface zonal winds (r > 0.5) over Indian subcontinent. Evaporative fraction which acts as a dominant controlling factor for the variation in PBLH is positively associated (r > 0.7) with the PBLH during monsoon season. Decreasing trend of PBLH during monsoon season may be attributed to the decrease in latent heat flux and increase in relative humidity that results in entrap of moisture at lower levels and may enhance the formation of extreme events. Inter annual variability of PBLH over Indian subcontinent demonstrates in-phase relationship with the deficit and excess monsoon years. Further to this, PBLH is found to be strongly negatively correlated with PM2.5 (r more than −0.5) during pre-monsoon and monsoon seasons suggesting an opposite phase relationship between the two parameters.