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Aerosol distribution over the Arabian Sea and the Indian subcontinent during the northern hemispheric summer is dominated by mineral dust transport from the West Asian desert regions. The radiative impact of these dust plumes is expected to have a prominent role in regulating the Asian Summer Monsoon circulation. While satellite observations have provided information in the spatial distribution of aerosols over the oceanic regions during the season, their utility over the land is rather limited. This study examines the transport of mineral dust over the West Asian desert, the Indian subcontinent and the surrounding oceanic regions during the summer monsoon season with the help of a regional scale model, WRF-Chem. Geographical locations of prominent dust sources, altitude ranges of mineral dust transport and their inter-annual variations are examined in detail. Multi-year model simulations were carried out during 2007 to 2012 with a model integration from 15 May to 31 August of each year. Six-year seasonal mean (June to August) vertically integrated dust amount from 1000 to 300 hPa level shows prominent dust loading over the eastern parts of Arabian desert and the northwestern part of India which are identified as two major sources of dust production. Large latitudinal gradient in dust amount is observed over the Arabian Sea with the largest dust concentration over the northwestern part and is primarily caused by the prevailing northwesterly wind at 925 hPa level from the Arabian desert. The model simulations clearly show that most of the dust distributed over the Indo-Gangetic plane originates from the Rajasthan desert located in the northwestern part of India, whereas dust observed over the central and south peninsular India and over the Arabian Sea are mainly transported from the Arabian desert. Abnormal dust loading is observed over the north Arabian Sea during June 2008. This has been produced as a result of the low pressure system (associated with the onset of summer monsoon) which entered in to the Arabian land mass, resulting strong wind and large dust storms over the region; the prevailing northwesterly wind transported these dust in to the Arabian Sea.

Six-hourly soundings (GPS sonde) were carried out at the central equatorial Indian Ocean (80o–83oE) during 25th September–10th October 2011 under the CINDY2011 (Cooperative Indian Ocean Experiment on Intra-seasonal variability in Year 2011) field campaign. One-degree interval soundings were also taken along a meridional section at 83oE from 5oN to 5oS during 12–20 October 2011 to supplement the time series data. Relative humidity (RH) and meridional wind component exhibit downward propagation of air mass in bands of high and low RH associated with northerly and southerly winds, respectively. Low (20–100 day) and high (2–10 day) frequency band pass filtered OLR data (NOAA-interpolated OLR) revealed the presence of Madden and Julian Oscillation (MJO) with 20- to 40-day periodicity, and weak Mixed Rossby Gravity (MRG) waves with 4- to 5-day periodicity. Eastward (westward) propagating MJO (MRG wave) with wave numbers 3–4 (4–5), amplitudes of anomaly 1.1–1.2 Wm−2 (1.8 Wm−2) were observed. The asymmetric bifurcation of warm surface water by the subsurface cold water off Sumatra generate asymmetric convective regimes in the vicinity of the equator probably triggered convection with periodicity similar to MRG waves. The intermittent surface convection associated is believed to be responsible for the ascending moisture to the middle troposphere prior to the initiation of MJO. The moisture pumped to the middle troposphere makes the layer convectively more unstable leading to the state of deep convection, a situation conducive for the MJO initiation processes.

The Advanced Research Weather Research and Forecasting (AR-WRF) model is used to study the influence of Western Ghats situated along the west cost of peninsular India in the mean characteristics of the Asian summer monsoon (ASM) through numerical simulations. A control simulation (CTRL) is carried out using 11-year (2000–2010) mean initial and lateral boundary conditions from the ERA-Interim reanalysis to simulate the mean atmospheric features of the ASM. The Modern-Era retrospective analysis for research and applications (MERRA) data along with the Tropical Rainfall Measuring Mission (TRMM, 3B42 daily rainfall) data are used to validate the CTRL simulation. The simulated dynamical features and precipitation characteristics during the ASM period agree well with the MERRA reanalysis and TRMM observations. In order to examine the role of Western Ghats on the mean characteristics of the ASM, a sensitivity simulation (NoWG) is carried out with orography reduced to surface over a domain bound between 5°–28°N and 72°–90°E, keeping all other conditions unchanged. This sensitivity analysis showed an enhancement in the low level monsoon flow over the Indian Ocean and peninsular India in the absence of Western Ghats. The prominent up-draft over the west coast of peninsular India observed in the CTRL simulation also decrease in the absence of Western Ghats. The simulated rainfall show a considerable decrease over the west coast and an enhancement over the east coast of peninsular India in the absence of Western Ghats. These simulations clearly depict the importance of Western Ghats in the circulation dynamics and rainfall features during the ASM period.

This research article aims at characterization of the sea-breeze circulation over Thumba (8.5°N, 76.9°E, India) in the winter season from December 2008 to February 2009, when this mesoscale circulation over the study domain was prominent. The characteristics of sea-breeze circulation cell comprising sea-breeze and compensatory return flow are investigated for clear-sky and cloudy days. The study indicated delayed onset of sea-breeze on the cloudy days as compared to the clear-sky days and the vertical thickness of sea-breeze circulation cell was found to be larger for the clear-sky days. Vertical thickness of the return flow for both clear-sky and cloudy days was larger than that of the sea-breeze flow. Simultaneous observations of upper-air meteorological parameters obtained through balloon-borne GPS sonde ascents carried out from Thumba and its adjoining coastal ocean on 29 January 2009 as part of the Winter phase of Integrated Campaign for Aerosols, gases and Radiation Budget (W-ICARB) are utilized for a case study towards investigation of diurnal evolution of the sea-breeze circulation cell. Results obtained from this study indicated systematic evolution of sea-breeze circulation over Thumba, however, it was not very clear over ocean which is attributed to cloudy conditions that prevailed on the day of measurement.