Is the Asian Brown Haze affecting cloud properties over the Indian Ocean? Do these empirical findings alter regional and global climate assessments?

We have analyzed over 4 years of MODIS data from NASA's Terra satellite on aerosol fine fraction optical depth and effective size distributions of warm and cirrus clouds over South East Asia and the Indian Ocean [1]. Our analysis demonstrates that the effective radius of cloud droplets decreases over the seas adjacent to the Indian sub-continent during periods of enhanced anthropogenic pollution from South Asia. This is expected since increases in aerosol number cause the nucleation of more cloud droplets, thus making their size smaller. The aerosol indirect effect results in brighter and longer-lived clouds that partially mask the warming effect of greenhouse gases. In contrast, we observe that the effective radius of ice crystals in cirrus clouds increases over this region during the pollution period. These observational results are summarized in Figure 1 (below) [1]. We suggest that this surprising increase of ice crystal size is caused by heterogeneous ice crystal formation on mixed black carbon/organic/sulfate particles that, and we support it with both in situ data collected during INDOEX and global model simulations. Our observations provide the first evidence of an aerosol effect on cirrus clouds over vast regions of the Arabian Sea and the Bay of Bengal during the Indian dry season pollution months. In spite of the opposing effects of aerosols on the size of ice crystals and clouds water droplets, the indirect aerosol effects on ice and water clouds can both lead to a cooling and thus provide a larger masking effect on the greenhouse warming than estimated thus far.

Our global model simulations indicate that aerosol mediated increase in cirrus cloud size amounts to a forcing of 10 W m-2 over vast regions of the Arabian Sea and Bay of Bengal,and should be included in regional and global climate assessments. We are including these new finding in our new assessments of the impact of the Asian Brown Haze on ocean surface temperatures, the monsoon, and regional water cycle using coupled climate models at NCAR, NASA-GISS and in Europe. In particular our simulations of the ocean show that the aerosol haze can cool the Indian Ocean surface by several degrees, in qualitative agreement with recent measurements of warming trends as a function of depth [2].

Figure 1: The average effective radius of water droplets during the months of June to September 2003 (panel a) and November 2003 to March 2004 (panel b). The corresponding scale in microns is on the top of the figure. A general decrease of water droplets effective radius is observed during the winter pollution period. Also shown is the average effective radius of the ice crystal size distribution during the months of June to September 2003 (panel c) and November 2003 to March 2004 (panel d). The corresponding scale in microns is on the bottom of the figure. The size of ice crystals over the seas adjacent to Indian sub-continent is generally larger during the winter than in the summer.

Performing reliable in-situ measurements of absorbing (black carbon) aerosols in collaborations with DOE, NSF, and NOAA sponsored field campaigns.

We are acquiring the first commercial of the photo-acoustic instrument developed by Pat Arnott (UNV) and manufactured by Droplet Measurement Technologies (DMT) that is much more sensitive than the current versions. The instrument directly measures both absorption and scattering yielding accurate values for the single-scatter albedo of the aerosols. The instrument is being enhanced to have 3 wavelength capabilities (761nm, 550nm, 405 nm) to gain spectral information on the optical properties of aerosols. We are working closely with UNV and DMI on the instrument, algorithms, and calibration. We participated in the MASE campaign, where we deployed an older version of the PA instrument on the CIRPAS twin otter (John Seinfeld, PI) that was equipped with numerous microphysical and chemical instruments including a TOF-MS. We illustrate the ability of our PA in identifying pollution plumes encountered in the vicinity of marine stratus cloud decks and measuring absorption and scattering directly. This allows us to measure the single scatter albedo (SSA) directly as illustrated in Figure 2 (below). We are working closely with Prof. Seinfeld's group at Caltech on the data analysis from the MASE campaign, and learn more about correlations between black carbon, carbonaceous aerosols, and size distributions in and near marine stratus clouds. In particular, we are working hard to separate black carbon in cloud drops from that of interstitial air, which may be possible due to the fortuitous configuration of our PA and a PSAP that sampled cloud drops.

We are also scheduled to participate in the Mexico City campaign next year, and are exploring a deployment in the Maldives in collaboration with Prof. Ramanathan.

Figure 2: Absorption (top) and scattering cross-sections (bottom) (blue in inverse megaMeters) for a 17 July 2005 flight of the CIRPAS Twin otter during the MASE campaign. The pressure is in red. Two pollution events are highlighted and the single scatter albedo derived for these in the middle panel.

References:

[1] Chylek P., M. K. Dubey, U. Lohmann, V. Ramanathan, Y. J. Kaufman, G. Lesins, G. Altmann, and S. Olsen, Indirect Effect of Anthropogenic Aerosol on Cirrus Clouds: Satellite Based Observations, LA-UR-05-4861, under review by Nature, August 2005.

[2] Barnett, T. P., D. W. Pierce, K. M. AchutaRao, P. J. Gleckler, B. D. Santer, J. M. Gregory, W. M. Washington, Penetration of human-induced warming into the world's oceans, Science; 8 July 2005; 309, 5732, pp. 284-287.