Abstract
Abstract. The sulphate aerosols mass and number concentration during the Indian Ocean Experiment (INDOEX) Intensive Field Phase-1999 (INDOEX-IFP) has been simulated using an interactive chemistry GCM. The model considers an interactive scheme for feedback from chemistry to meteorology with internally resolving microphysical properties of aerosols. In particular, the interactive scheme has the ability to predict both particle mass and number concentration for the Aitken and accumulation modes as prognostic variables. On the basis of size distribution retrieved from the observations made along the cruise route during IFP-1999, the model successfully simulates the order of magnitude of aerosol number concentration. The results show the southward migration of minimum concentrations, which follows ITCZ (Inter Tropical Convergence Zone) migration. Sulphate surface concentration during INDOEX-IFP at Kaashidhoo (73.46° E, 4.96° N) gives an agreement within a factor of 2 to 3. The measured aerosol optical depth (AOD) from all aerosol species at KCO was 0.37 ± 0.11 while the model simulated sulphate AOD ranged from 0.05 to 0.11. As sulphate constitutes 29% of the observed AOD, the model predicted values of sulphate AOD are hence fairly close to the measured values. The model thus has capability to predict the vertically integrated column sulphate burden. Furthermore, the model results indicate that Indian contribution to the estimated sulphate burden over India is more than 60% with values upto 40% over the Arabian Sea.
Highlights
The aerosol effects on climate can be large and complex due to the fact that aerosols chemical composition, abundance and size distribution are highly variable, both spatially and temporally
The simulated sulphate concentrations at the surface are compared with measured concentrations during INDOEXIFP (February–March 1999) at Kaashidhoo Climate Observatory (KCO), Maldives in the Indian Ocean
KCO was established as a part of Indian Ocean Experiment (INDOEX) in the island of Kaashidhoo (4.965◦ N, 73.466◦ E) about 500 km southwest of the southern tip of India, where the aerosol chemical, radiative, and microphysical properties were continuously measured since February 1998
Summary
The aerosol effects on climate can be large and complex due to the fact that aerosols chemical composition, abundance and size distribution are highly variable, both spatially and temporally. In order to further understand sulphate aerosolclimate interactions, the present study is carried out with the Laboratoire de Meteorologie Dynamique interactive chemistry global model (Verma et al, 2007) to produce the simulation of INDOEX-IFP. Ramanathan et al (2001) have presented the INDOEX results for the oceanic regions adjacent to the Indian subcontinent, which reveal the presence of a dense anthropogenic aerosol layer over China, South and Southeast Asia. The main motivation of this study is to reproduce the observed variations in the sulphate aerosol mass and number concentration during the Indian Ocean Experiment from January to March 1999 using LMD interactive chemisry GCM. The sulphate aerosol/chemistry is interactively included in the model The model considers both sulphur mass and number concentrations as a truly interactive part and not merely a passive component in atmospheric chemistry (Verma et al, 2007). The experiments are designed to assess impacts of sulphate aerosols on INDOEX region
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