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Horizontal convective rolls (HCR) are sub-mesoscale motions that influence the transport of heat, momentum, and pollutants within the boundary layer. In this work, a high-resolution (0.666 km) Weather Research and Forecasting (WRF) model is employed to simulate the structure of the HCRs over Kalpakkam along the southeast coast of India. The sensitivity of HCR simulation to model land surface physics is studied with two land surface models (LSM), (i) Noah and (ii) Noah multi-parameterization (NMP), for three selected days (15 April 2013, 07 May 2015, 28 March 2018) during summer synoptic conditions. On all three selected days, the boundary layer rolls formed over a period of about 2–3 h in the morning under moderate winds (4–5.0 m/s), moderate vertical wind shear (2.4–3.5 m/s) in the lower atmosphere, and slightly unstable conditions [gradient Richardson number (RiG) −4.5 to −5.0] in both simulations and observations, indicating that thermal instability is the chief mechanism in their development. Simulated mean surface meteorological parameters by NMP were found to be in better agreement with observations than Noah. Results suggest that the LSMs mainly affected the simulated turbulent roll structure in terms of updraft cores and their horizontal and vertical extent by variation in simulated surface energy fluxes, boundary layer structure, wind shear, and stability. The structure of simulated HCRs is better represented by NMP due to the improvements in the flux distribution and surface properties. Simulations using the FLEXPART dispersion model for a hypothetical case of tracer release indicated an uneven spatial concentration pattern due to upward and downward motions in the region of HCRs. The stronger winds and stronger flow convergence in Noah and higher heat flux and more unstable conditions in NMP led to differences in the simulated tracer concentrations in the two cases. [ABSTRACT FROM AUTHOR]

A spectrally integrated clear-sky and three-layer cloudy-sky models were developed to determine atmospheric transmittances and instantaneous surface insolation. Half-hourly observations at 8 km spatial resolution in optical and thermal infrared bands from an Indian geostationary satellite (Kalpana-1) Very High Resolution Radiometer (VHRR) sensor were used to provide inputs to these models in addition to global 8 day aerosol optical depth and columnar ozone. Sensitivity analysis of the clear-sky model showed a higher influence of aerosol on global insolation, diffuse insolation, and its fraction as compared with water vapour and ozone. The root mean square error (RMSE) of insolation estimates of the daily integral was found to be 2.05 MJ m−2(∼11.2% of measured mean) with a high correlation coefficient (r = 0.93) when compared within situmeasurements during 1 August 2008 to 31 March 2010 over six locations in India. The errors were found to reduce to 7.5% over 3 to 5 day averages. The comparison of annual estimates and equivalent reanalysis fields showed a mean difference of the order of ±1.7 MJ m−2over the majority of the Indian landmass. [ABSTRACT FROM PUBLISHER]

Abstract: Simultaneous measurements of aerosol absorption and scattering coefficients for the PM2.5 aerosols particles were done at Delhi during April 2008–March 2009 to estimate the aerosol single scattering albedo (SSA) and the Angstrom absorption exponents at the surface. The annual average SSA at 0.55 μm was found to be 0.70 ± 0.07 with only slight variations during the four seasons, summer (0.63 ± 0.06), monsoon (0.69 ± 0.07), winter (0.74 ± 0.03) and spring (0.72 ± 0.04). However, large variations in average absorption and scattering coefficients were seen during these four seasons. The average absorption coefficients during summer, monsoon, winter and spring were found to be 62.47 ± 21.27, 50.95 ± 43.61, 189.65 ± 85.94 and 90.65 ± 33.06 Mm−1 respectively. The corresponding scattering coefficients were 110.46 ± 36.15, 95.34 ± 49.46, 565.59 ± 274.59 and 236.56 ± 96.25 Mm−1. The Angstrom absorption exponent (ασ(abs)) remained close to unity throughout the year averaging at 1.02 ± 0.08, 1.02 ± 0.10, 1.04 ± 0.11, and 1.03 ± 0.05 during summer, monsoon, winter and spring seasons respectively, strongly indicating that the absorption at Delhi aerosol is mainly due to the abundance of black carbon of fossil fuel origin. [ABSTRACT FROM AUTHOR]