The aerosol-induced impacts on South Asian monsoon estimated from Fengyun-2 observations of the cloud effective particle radius

Aerosols, the tiny particles suspended in the atmosphere, can exert an important impact on radiative forcing and cloud-precipitation system of the earth. Recent studies have shown that a marked increase of absorbing aerosols in South Asia has altered the characteristic of Asian monsoon. General circulation model simulations by Ramanathan et al.(2005) suggested that the aerosol-induced solar dimming leads to a large surface cooling and a decrease in evaporation from India and North Indian Ocean, which resulting in the increase of tropospheric stability over South Asia, weakening of monsoon circulation and decrease in summer rainfall.

On the other hand, Lau et al. (2006) and Lau and Kim (2006) proposed the elevated heat pump hypothesis. They suggested that absorbing aerosols such as desert dust and anthropogenic black carbon enhance the heating in the middle/upper troposphere over slopes of Tibetan Plateau, which lead to the strengthening of the meridional tropospheric temperature gradient, thus resulting in an enhancement of drawing warm moist air from below and an increase of summer monsoon rainfall over northern India.

Gauman et al. (2009) conducted observations of tropospheric temperature during 29 years from 1979 to 2007 using satellite-borne microwave sounder unit. They revealed widespread warming over the Himarayan-Gangetic region and strengthening of the land-sea thermal gradient which can influence the monsoon rainfall variability.

Although these studies investigated various direct effects affecting on Asian monsoon, it is not easy to explain the aerosol effects on Asian monsoon water cycle in total, because it depends on many factors. Especially the indirect effect of aerosols and the possible interaction with the monsoon dynamics remains unknown. Therefore, it is important to obtain observational data of cloud particle properties such as an effective radius over South Asia toward the deeper understanding of interaction between aerosol and Asian monsoon circulation.

In this study, the cloud effective particle radius is estimated using geostationary satellite data in order to evaluate aerosol indirect effects over South Asia and its impact on Asian monsoon. Data from two geostationary meteorological satellites, MTSAT-2 and Fengyun-2E are used in this study. MTSAT-2 is operational in a geostationary orbit at 145°E, and Fengyun-2E at 105°E. Their sensor specifications are very similar and both of them equip with four infrared sensors and one visible sensor including 3.7 µm band channel. The use of ‘geostationary satellite' has a merit that it provides high temporal resolution, typically one or half hour, rather than the polar orbital satellites such as Terra/MODIS whose observation interval is approximately once or twice a day. Therefore, by using geostationary satellite, it is expected to be able to capture the progress of clouds varying from hour to hour.

Our retrieval method basically follows Kaufman and Nakajima (1993) in that the cloud effective particle radius is estimated from cloud reflectance at 3.75 µm. In order to keep the quality of data, we first estimate the cloud effective particle radius from MTSAT-2 data, which is calibrated according to GSICS information. Cloud pixels appropriate for analysis are chosen from satellite pictures (Choi et al., 2009). Then, the cloud reflectance at 3.7µm is calculated according to Oku et al. (2010). The reflectance at 3.7µm is then converted to the effective radius according to Kaufman and Nakajima (1993). The validity of cloud effective radius obtained by this procedure has been confirmed by comparing the result with those obtained by the Terra/MODIS cloud products.

The method is applied to Fengyun-2E data in the same way but with additional processes. Since Fengyun-2E data often contains non-negligible geometric error, geometric calibration is performed by comparing the GLCC land use data and Fengyun-2E visible albedo data. For the radiance calibration a calibration curves between MYSAT-2 and FY-2E channels are made by comparing the data in common scope in simultaneous observation time. The FY-2E data are then calibrated by MTSAT-2 data which are already calibrated by GSICS information.

In FY-2E-derived effective particle radius data, general characteristic that the cloud effective particle radius is smaller in continental clouds than in maritime clouds is confirmed. The diurnal variation of cloud effective particle radius is also suggested. We are continuing further retrieval towards the investigation of seasonal variation of the cloud effective particle radius. In the presentation, we will introduce the up-to-date retrieval results and give discussion on the temporal evolution of the cloud effective particle radius around South Asian region. Then, we will propose observational insight in association with aerosol indirect effects and its possible impacts on Asian monsoon.

References

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