Abstract
Abstract. The influence of wind and humidity on aerosol optical depth (AOD) over the Arabian sea is being investigated using MODIS (Moderate Resolution Imaging Spectroradiometer) Level 3 (Collection-5) and NCEP (National Centres for Environmental Prediction) reanalysis data for the second phase of the Arabian Sea Monsoon Experiment (ARMEX-II) over the South East Arabian Sea (SEAS) in the pre-monsoon period (14 March–10 April 2003). In order to qualify MODIS data for this study, MODIS aerosol parameters were first compared with ship borne Microtops measurements. This showed correlations 0.96–0.97 in the case of spectral AODs and a correlation 0.72 for the angstrom exponents. The daily AOD data from MODIS and winds from NCEP reveal that the ship observed episodic enhancement and decay of AOD at the TSL (Time Series Location) during 23 March–6 April 2003 was caused by the southward drift of an aerosol pocket driven by an intensification and reduction of surface pressure in the North Western Arabian Sea with a low altitude convergence prevailing over SEAS. The AOD increase coincided with a decrease in the Angstrom exponent and the fine mode fraction suggesting the pocket being dominated by coarse mode particles. A partial correlation analysis reveals that the lower altitude wind convergence is the most influential atmospheric variable in modulating AOD over the ARMEX-II domain during the TSL period. However, surface winds at a distant zone in the north/north west upwind direction also had a moderate influence, though with a lag of two days. But this effect was minor since the winds were not strong enough to produce marine aerosols matching with the high AODs over the ARMEX-II domain. These findings and the similarity between MODIS column mass concentration and the ship borne QCM (Quartz Crystal Microbalance) measured coarse mode mass concentration, suggest that the aerosol pocket was mostly composed of coarse mode mineral dust in the lower atmospheric altitudes transported from the Arabian deserts.
Highlights
Atmospheric aerosols play a significant role in the earth’s climate by perturbing the radiation energy balance of the earthatmosphere system (Charlson et al, 1992; Andreae, 1995; Hansen et al, 1997; Boucher et al, 1998; Haywood and Boucher, 2000; IPCC, 2001)
Following the method of partial correlation analysis (Spiegel and Stephens, 2000), the daily aerosol optical depth (AOD) distribution at 550 nm from MODIS Level 3 data were used along with NCEP reanalysis fields to study the modulation of AOD by ocean surface winds, convergence of aerosols by winds at different atmospheric altitudes and by hygroscopic growth of aerosols induced by relative humidity (RH)
A comparison of MODIS derived spectral AOD with the Microtops measurements over South East Arabian Sea shows good agreement with correlation coefficients 0.96–0.97 with mean deviations around 0.04 while the Angstrom exponents derived from their spectral AODs agree with a correlation 0.72
Summary
Atmospheric aerosols play a significant role in the earth’s climate by perturbing the radiation energy balance of the earthatmosphere system (Charlson et al, 1992; Andreae, 1995; Hansen et al, 1997; Boucher et al, 1998; Haywood and Boucher, 2000; IPCC, 2001). We first present a comparison of the spectral AOD and derived Angstrom exponent from MODIS with the spectral AOD measured using a sun photometer onboard the oceanographic research vessel (ORV) Sagar Kanya and the derived Angstrom exponent, during its cruise for the ARMEX-II (Arabian Sea Monsoon Experiment) over the South East Arabian Sea (SEAS) in the pre-monsoon season of the year 2003 This initial exercise was carried out with the aim of establishing a quantitative reliability of MODIS aerosol products that are subsequently used for our investigation. Following the method of partial correlation analysis (Spiegel and Stephens, 2000), the daily AOD distribution at 550 nm from MODIS Level 3 data were used along with NCEP reanalysis fields to study the modulation of AOD by ocean surface winds, convergence of aerosols by winds at different atmospheric altitudes and by hygroscopic growth of aerosols induced by relative humidity (RH)
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