The spatial distribution of mineral dust and its shortwave radiative forcing over North Africa: modeling sensitivities to dust emissions and aerosol size treatments

Abstract

Abstract.A fully coupled meteorology-chemistry-aerosol model (WRF-Chem) is applied to simulate mineral dust and its shortwave (SW) radiative forcing over North Africa. Two dust emission schemes (GOCART and DUSTRAN) and two aerosol models (MADE/SORGAM and MOSAIC) are adopted in simulations to investigate the modeling sensitivities to dust emissions and aerosol size treatments. The modeled size distribution and spatial variability of mineral dust and its radiative properties are evaluated using measurements (ground-based, aircraft, and satellites) during the AMMA SOP0 campaign from 6 January to 3 February of 2006 (the SOP0 period) over North Africa. Two dust emission schemes generally simulate similar spatial distributions and temporal evolutions of dust emissions. Simulations using the GO-CART scheme with different initial (emitted) dust size distributions require ∼40% difference in total emitted dust mass to produce similar SW radiative forcing of dust over the Sahel region. The modal approach of MADE/SORGAM retains 25% more fine dust particles (radius<1.25 µm) but 8% less coarse dust particles (radius>1.25 µm) than the sectional approach of MOSAIC in simulations using the same sizeresolved dust emissions. Consequently, MADE/SORGAM simulates 11% higher AOD, up to 13% lower SW dust heating rate, and 15% larger (more negative) SW dust radiative forcing at the surface than MOSAIC over the Sahel region. In the daytime of the SOP0 period, the model simulations show that the mineral dust heats the lower atmosphere with an average rate of 0.8 ± 0.5 K day −1 over the Niamey vicinity and 0.5 ± 0.2 K day −1 over North Africa and reduces the downwelling SW radiation at the surface by up to 58 W m −2 withCorrespondence to: C. Zhao (chun.zhao@pnl.gov) an average of 22 W m −2 over North Africa. This highlights the importance of including dust radiative impact in understanding the regional climate of North Africa. When compared to the available measurements, the WRF-Chem simulations can generally capture the measured features of mineral dust and its radiative properties over North Africa, suggesting that the model is suitable for more extensive simulations of dust impact on regional climate over North Africa.

Radiative impact of mineral dust on monsoon precipitation variability over West Africa

“…The WRF-Chem model was used by Zhao et al (2010) N) are used to minimize the influence from the lateral boundary conditions. We use the same model configuration as Zhao et al (2010), except that the rapid radiative transfer model (RRTMG) (Mlawer et al, 1997;Iacono et al, 2000) for both SW and LW radiation is updated to include aerosol direct radiative effect.…”

“…Zhao et al (2010) investigated the sensitivity of the WRF-Chem simulated dust radiative forcings to dust emission and aerosol schemes and evaluated the model results using data from the African Monsoon Multidisciplinary Analysis (AMMA) campaign, which is a major international campaign in 2006 to improve understanding of climate, monsoon, and hydrological cycle of West Africa and the impacts from aerosols (Redelsperger et al, 2006). Following a similar modeling approach as Zhao et al (2010), the objective of this study is to further understand the various radiative forcings of Saharan dust and how they jointly affect the surface energy budget, atmospheric diabatic heating, and hydrological cycle at a regional scale. Building on our recent study (Zhao et al, 2010), this work (1) examines both SW and LW radiative forcings of Saharan dust during the wet monsoon season over West Africa, and (2) investigates the potential dust effects on precipitation during the WAM season.…”

“…Following a similar modeling approach as Zhao et al (2010), the objective of this study is to further understand the various radiative forcings of Saharan dust and how they jointly affect the surface energy budget, atmospheric diabatic heating, and hydrological cycle at a regional scale. Building on our recent study (Zhao et al, 2010), this work (1) examines both SW and LW radiative forcings of Saharan dust during the wet monsoon season over West Africa, and (2) investigates the potential dust effects on precipitation during the WAM season. The paper is organized as follows.…”

“…This study uses a regional model (Weather Research and Forecasting (WRF) model (Skamarock et al, 2008)) with relatively high horizontal resolution of 36×36 km 2 and online dust calculation (Zhao et al, 2010) to understand the response of WAM precipitation to Saharan dust. The WRF model has been used in many regional air quality and climate studies (e.g., Fast et al, 2006 andLeung et al, 2006;Wang and Liu, 2009;Qian et al, 2009;Zhang et al, 2009) including some over Africa (e.g., Vigaud et al, 2009;Zhao et al, 2010). In this study, we use a version of WRF (WRF-Chem) that simulates trace gases and particulates simultaneously with the meteorological fields (Grell et al, 2005).…”

“…In this study, we use a version of WRF (WRF-Chem) that simulates trace gases and particulates simultaneously with the meteorological fields (Grell et al, 2005). Zhao et al (2010) investigated the sensitivity of the WRF-Chem simulated dust radiative forcings to dust emission and aerosol schemes and evaluated the model results using data from the African Monsoon Multidisciplinary Analysis (AMMA) campaign, which is a major international campaign in 2006 to improve understanding of climate, monsoon, and hydrological cycle of West Africa and the impacts from aerosols (Redelsperger et al, 2006). Following a similar modeling approach as Zhao et al (2010), the objective of this study is to further understand the various radiative forcings of Saharan dust and how they jointly affect the surface energy budget, atmospheric diabatic heating, and hydrological cycle at a regional scale.…”