1. Aerosol radiative forcing and implication for circulation
in the MENA region
M. Zampieri, R. Abida, H. Bangalath, H.V. Nguyen and G.L. Stenchikov
King Abdullah University of Science and Technology
Abstract
Aerosols play an important role in the climate of the arid MENA region (Middle East and Northern Africa) which is characterized by frequent dust storms and large aerosol generation. We present preliminary results of our work at King Abdullah University of Science and Technology (Thuwal, Saudi Arabia):
1. the inter-comparison of several Aerosol Optical Depth datasets for the MENA region, obtained from the Multi-angle Imaging SpectroRadiometer (MISR) satellite observations and the dataset that is used by the radiation package of the High Resolution Atmospheric Model (HIRAM), developed at GFDL, which we run at KAUST
2. the contribution of aerosol to the radiative forcing at the surface and at the top of the atmosphere (TOA) from the Modern Era Retrospective-analysis for Research and Applications (MERRA) reanalysis and from the HIRAM simulation;
3. the performed sensitivity tests to the dust content in the region by doubling and halving the distribution values, and we analyze the results in terms of surface temperature response at the regional scale, and the response on the circulation and the hydrological cycle at a larger scale.
Data
The MISR instrument on the Terra satellite acquires data with multiple angular views of the atmosphere and surface (Diner et al., 1998). These views are used to retrieve a number of physical aerosol parameters as well as aerosol microphysical properties (size and shape). MISR performs retrievals of the fraction of AOD due to “fine” (particle radii <0.35 μm), “medium” (particle radii between 0.35 and 0.7 μm) and “large” (particle radii >0.7 μm) particles as well as the fraction of AOD due to “spherical” and “nonspherical” particles from measurements in the four MISR spectral bands. Here, we have analyzed the latest version (Version 31) of the Level 3 monthly total aerosol optical depth product of MISR from January 2001 to December 2008.
The Modern Era Retrospective-Analysis for Research and Applications (MERRA, Rienecker et al. 2011) covers the period from 1797 to present at a resolution of 1/2 by 2/3 degrees. A particularity of this reanalysis is to include the effect of aerosols, which are especially important for the MENA region, given the constant presence of dust.
HIRAM: TOA SW-LW Aerosol Radiative Climatology over
HIRAM: Surface SW-LW Aerosol Radiative Climatology over
MERRA: TOA SW-LW Aerosol Radiative Climatology over
MERRA: Surface SW-LW Aerosol Radiative Climatology over
Aerosol Optical Depth from GFDL Longterm Climatology and MISR ( )
By doubling the dust amount surface temperature drops by up to 1 degree in the Sahel region. The local response to the direct effect of aerosol overlaps to an interesting dynamical response that, in winter, amplifies the cooling over the Arabian Peninsula. In summer, the cooling of the Sahel produce a northward shift of the precipitation associated to the inter-tropical convergence zone (ITCZ). Cloudiness in the MENA region increases, probably as a result of the cooler atmosphere and of the northward shift of ITCZ.   T
HIRAM DJF 2m temp sensitivity to doubling dust
HIRAM DJF precip sensitivity to doubling dust (mm/day)
HIRAM DJF 2m temp sensitivity to halving dust
HIRAM DJF precip sensitivity to halving dust (mm/day)
  The model simulation is performed from 1950 for 10 years to compute the aerosol radiative forcing and for 5 years for the sensitivity studies to dust content using the GFDL HIRAM model (Zhao et al., 2009). Relative to GFDL AM2 (GAMDT 2004), HIRAM employs a modified shallow convection parameterization that is extended to describe the deep convection. HIRAM employs a cubed-sphere implementation (~0.25 deg resolution) of a finite-volume dynamical core and is coupled to LM3, a new land model with eco-system dynamics and hydrology. Prescribed SST and sea ice distribution are adopted as the bottom boundary conditions over the Sea. Prescribed time varying greenhouse gases and stratospheric/troposperic aerosol distribution datasets are used to reproduce the observed radiative forcing in the model. Tropospheric aerosols are obtained from a preliminary simulation using the chemistry-transport model MOZART model, which is jointly developed by NCAR, Max-Planck-Institute for Meteorology, and NOAA/GFDL. The effects of volcanoes are included in the HIRAM. Direct injection of sulfur into the stratosphere from volcanic eruptions is not included, nor is carbonyl-sulfide chemistry, a major source of background stratospheric aerosol. To compensate, in the stratosphere, a time series of volcanic optical properties is specified as in Stenchikov et al. (2006).
  Discussion
The AOD comparison between the aerosol dataset used in HIRAM and the MISR product reveals some consistency, especially in the pattern that characterizes the summer season. In winter, HIRAM distribution shows a maximum in the Central Sahel, between 10N and 20N, which is actually shifted south in the satellite observations. In general, HIRAM AOD is underestimated respect to the MISR retrieval by approximately 1/3 in the peak values.
The comparison between MERRA and HIRAM aerosol radiative forcing is consistent with the fact that the aerosol distribution in the model is underestimated. In fact, MERRA radiative forcing seems to be more consistent with the distribution retrieved by MISR. At the surface, aerosols are responsible for more than 15 W/m2 warming in the shortwave and 60 W/m2 in the longwave over Upper Egypt and Sudan in summer. In winter, radiative forcing is lower, and the two contributions almost balance each other. The distribution produced by MERRA is much wider and also covers North-Western Africa. The peak values are approximately double the corresponding amount computed by HIRAM. Similar conclusion can be drawn for the analysis of the comparison between the results at the top of the atmosphere (TOA).
HIRAM JJA 2m temp sensitivity to doubling dust
HIRAM JJA precip sensitivity to doubling dust (mm/day)
HIRAM JJA 2m temp sensitivity to halving dust
HIRAM JJA precip sensitivity to halving dust (mm/day)
The temperature response of the halving dust experiment is the opposite of the doubling dust one in the tropical region, as it would be expected from a quasi-linear interpretation of the direct effect of dust on the surface energy budget. In this case, we also observed a southward displacement of the precipitation associated to the summer ITCZ.
Future work will also include the use of observations from SEVIRI, MODIS and MISR to construct aerosol optical characteristics and distribution for model calculations.
References
Diner, D.J. et al., 1998, Multi-angle Imaging SpectroRadiometer (MISR) instrument description and experiment overview, IEEE Transactions on Geoscience and Remote Sensing, 36,
GFDL GAMDT, 2004, The new GFDL global atmosphere and land model AM2-LM2: Evaluation with prescribed SST simulations, J. Climate, 17,
Rienecker, Michele M. et al., 2011, MERRA: NASA’s Modern-Era Retrospective Analysis for Research and Applications. J. Climate, 24, 3624–3648.
Stenchikov, G. et al., 2006, Arctic Oscillation response to volcanic eruptions in the IPCC AR4 climate models. J. Geophys. Res., 111, D07107.
Zhao M., I. M. Held, S.-J. Lin, G. A. Vecchi, 2009, Simulations of Global Hurricane Climatology, Interannual Variability, and Response to Global Warming Using a 50-km Resolution GCM. J. Climate, 22, 6653–6678.
King Abdullah University of Science and Technology, 2011