Research - Aerosols and Climate - Dust

Longwave Dust Aerosol forcing from MISR/MODIS/CERES satellite data 

Desert dust is considered to be one of the major sources of tropospheric aerosol loading, and play an important role in climate forcing studies. Widely prevalent in the tropics, dust aerosols are effective in reflecting solar energy back to space thereby "cooling" the earths surface. Besides their radiative impact in the shortwave portion of the electro-magnetic spectrum, dust aerosols also have an important radiative effect in the longwave (LW). Having mean particle sizes on the order of several micrometers, dust aerosols can effectively reduce the earths LW emission by re-emitting at a colder temperature when compared to the surface and thereby "warming" the earth. However, numerical simulations of dust LW forcing remain a challenging task because of the high spatio-temporal variation of dust properties. Using observations from the Multi-angle Imaging Spectroradiometer (MISR), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Clouds and the Earth's Radiant Energy System (CERES) instruments onboard the Terra satellite; we present a new technique for studying longwave (LW) radiative forcing of dust aerosols over the Saharan desert for cloud-free conditions. The satellite data is used to detect aerosols, obtain aerosol properties and compute the radiative energy budget at the top of atmosphere. We also discuss the effects of water vapor and surface albedo on our results.

Our results show that the monthly-mean LW forcing for September 2000 is 7 Wm-2 and the LW forcing efficiency (LWeff) [Longwave forcing per unit aerosol optical thickness at 0.55 mm] is 15 Wm-2. Using radiative transfer calculations, we show that simultaneous measurements of the vertical distribution of aerosols, surface temperature and water vapor are critical to the understanding of dust aerosol forcing, and must come from other sources. Using well calibrated, spatially and temporally collocated data sets, we have combined the strengths of three sensors from the same satellite to quantify the LW forcing, and show that dust aerosols have a "warming" effect over the Saharan desert that will counteract the shortwave cooling effect of other aerosols. - See Figure 1.

For further information check out the following publications.
Zhang, J., and S.A. Christopher, Longwave Radiative Forcing of Dust Aerosols over the Saharan Desert estimated from MODIS, MISR, and CERES observations from Terra, Geophysical Research Letters, 30(23), doi:10.1029/2003GL018479, 2003, (pdf file).

Satellite Remote Sensing and Numerical Modelling of Dust Aerosols

Aerosols both natural and anthropogenic are major contributors to the radiation balance of the earth-atmosphere system. They reflect and absorb incoming solar radiation called the direct effect and they also affect cloud properties, cloud lifetime and precipitation patterns called the indirect effect. Understanding the spatial distribution of aerosols, their properties and their role in interacting with solar and thermal radiation is crucial before realistic predictions on global climate can be made. Significant progress has been made in understanding the role of aerosols in climate largely in part due to high quality satellite, ground-based and suborbital data coupled with advances in numerical modelling of aerosols.

Although detailed classifications exist, broad categorizations denote dust and marine aerosols as natural and Black Carbon, Organic Carbon, Sulfate as anthropogenic aerosols. However there are considerable uncertainties exist in determining the anthropogenic influences on dust aerosol production. The term Direct Radiative Effect (DRE) is used for the study of natural aerosols and Direct Climate Forcing (DCF) usually denotes the study of anthropogenic aerosols. Dust aerosols are generated primarily over the desert rregions and world wide dust emissions can range from 1000-3000 Tg per year constituting a large fraction of the total aerosol emissions per year. Dust aerosols typically have large particle sizes and near source regions such as deserts the particle radius can be >5m and these aerosols can be transported thousands of kilometres affecting the radiation balance of the earth-atmosphere system significantly.
The goal of our research is to use multiple satellite data sets to monitor dust aerosols, separate dust from other anthropogenic and natural components and to study the climatic impact of dust. We are also focused on examining not only the solar radiative impact but the thermal impact of dust aerosols which could be significant due to their large particle sizes.

For further details see:

Christopher, S.A. and T. Jones, Satellite-based Assessment of Cloud-free Net Radiative Effect of Dust Aerosols over the Atlantic Ocean, Geophysical Research Letters, (pdf file)- revised September 11, 2006 - 2006GL027783R
Wang, J., U. Nair, and S.A. Christopher (2004), GOES-8 Aerosol Optical Thickness Assimilation in a Mesoscale Model: Online Integration of Aerosol Radiative Effects,    J. Geophysical Research-Atmospheres,  Vol. 109, No. D23, D23203 04 (pdf file).
Christopher, S.A., J. Wang, Q. Ji and S-C. Tsay, Estimation of Shortwave Dust Aerosol Radiative forcing during PRIDE, J. Geophys.  Res., 108(D19), 8956, doi:10.1029/2002JD002787, 2003, (pdf file).
Zhang, J., and S.A. Christopher, Longwave Radiative Forcing of Dust Aerosols over the Saharan Desert estimated from MODIS, MISR, and CERES observations from Terra, Geophysical Research Letters, 30(23), doi:10.1029/2003GL018479, 2003 (pdf file)