Evaluation of Newly Developed Similarity Functions under Stable Condition with a Mesoscale Model over the Indian Region

The Monin-Obukhov similarity theory is widely used to parameterize turbulent fluxes in weather prediction and climate models. The theory utilizes empirical correction functions, commonly known as similarity functions to incorporate the effects of atmospheric stability. Recently, Sharan and Srivastava (2019) have pointed out the inconsistency of commonly used similarity functions in weather and climate models and subsequently proposed new similarity functions for stable surface layer (Srivastava et al., 2020) which are valid for Indian region. In the present study, these similarity functions are implemented in the MM5 surface layer module of the Weather Research and Forecasting model version 4.2 (WRFV4.2), and their impact on near-surface variables over Indian region is analysed. Simulations were carried out with three nested domains at a very fine spatial resolution centred around a flux tower installed at Ranchi (India). We utilized modified surface layer scheme along with different planetary boundary layer (PBL) schemes available in the WRFV4.2 model and carried out simulations for the entire year of 2009 to analyze the impact of modified surface layer formulations on the seasonal evolution of surface variables and turbulent fluxes. The model simulations have been evaluated against multiple global and regional reanalysis data sets which are available on different temporal scales from daily to hourly and at a fine spatial resolution. Furthermore, we have also evaluated the diurnal pattern of turbulent fluxes with respect to the observations from the flux tower installed at Ranchi (India). Initial results suggest that during day-time convective conditions, most of the surface variables and fluxes simulated from revised scheme have good agreement with the default scheme. With the updated similarity functions the modified surface layer scheme produced slightly smaller values of heat flux and 2m temperature and higher values of 10-m wind speed in the night-time. Further, recommendations have been made for improvement in the surface layer parameterization over Indian region. References: Srivastava, P., & Sharan, M. (2019). Analysis of dual nature of heat flux predicted by Monin-Obukhov similarity theory: An impact of empirical forms of stability correction functions. Journal of Geophysical Research: Atmospheres, 124, 3627 3646. Srivastava, P, Sharan, M, Kumar, M, Dhuria, AK. On stability correction functions over the Indian region under stable conditions. Meteorol Appl. 2020.