Estimation of mineral dust direct radiative forcing at the European Aerosol Research Lidar NETwork site of Lecce, Italy, during the ChArMEx/ADRIMED summer 2013 campaign: Impact of radiative transfer model spectral resolutions

Abstract

A field campaign took place in the western and central Mediterranean basin on June–July 2013 in the framework of the ChArMEx (Chemistry‐Aerosol Mediterranean Experiment, http://charmex.lsce.ipsl.fr/)/ADRIMED (Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region, http://adrimed.sedoo.fr/) project to characterize the aerosol direct radiative forcing (DRF) over the Mediterranean. This work focuses on the aerosol DRF estimations at Lecce (40.33°N; 18.11°E; 30 m above sea level) during the Saharan dust outbreak that affected southern Italy from 20 to 24 June 2013. The Global Atmospheric Model (GAME) and the Two‐Stream (TS) model were used to calculate the instantaneous aerosol DRF in the short‐wave (SW) and long‐wave (LW) spectral ranges, at the surface and at the top of the atmosphere (TOA). The main differences between the two models were due to the different numerical methods to solve the radiative transfer (RT) equations and to the more detailed spectral resolution of GAME compared to that of TS. 167 and 115 subbands were used by GAME in the 0.3–4 and 4–37 µm spectral ranges, respectively. Conversely, the TS model used 8 and 11 subbands in the same spectral ranges, respectively. We found on 22 June that the SW‐DRFs from the two models were in good agreement, both at the TOA and at the surface. The instantaneous SW‐DRFs at the surface and at the TOA varied from −50 to −34 W m−2 and from −6 to +8 W m−2, respectively, while the surface and TOA LW‐DRFs ranged between +3.5 and +8.0 W m−2 and between +1.7 and +6.9 W m−2, respectively. In particular, both models provided positive TOA SW‐DRFs at solar zenith angles smaller than 25° because of the mixing of the desert dust with anthropogenic pollution during its transport to the study site. In contrast, the TS model overestimated the GAME LW‐DRF up to about 5 and 7.5 times at the surface and at the TOA, respectively, when the dust particle contribution was largest. The low spectral resolution of the real (n) and imaginary (k) refractive index values was mainly responsible for the LW‐DRF overestimates of the TS model. However, we found that the “optimization” of the n and k values at 8.75 and 11.5 µm was sufficient in this study to obtain a satisfactory agreement between the LW‐DRFs from the two models, both at the TOA and at the surface. The impact of the spectral dependence of the water vapor absorption coefficients on the estimation of the flux without aerosol has also been addressed. Paper results did not reveal any significant impact due to the different numerical methods used by the two models to solve the RT equations.

The Dust Direct Radiative Impact and Its Sensitivity to the Land Surface State and Key Minerals in the WRF‐Chem‐DuMo Model: A Case Study of Dust Storms in Central Asia

“…The radiative forcing at the TOA ranges from 8.2 to 8.8 W/m 2 and from 7.3 to 7.8 W/m 2 on 7 May 2007, using the Maxwell-Garnett approximation and volume averaging method, respectively, in the longwave spectral range in contrast to a wider range from À6.0 to À12.0 W/m 2 and from À1.9 to À9.7 W/m 2 in the shortwave spectral range. Despite the low SSA computed in the model, our estimation of the forcing on 7 May at the TOA is still more negative in comparison to that (À6.0 to 8.0 W/m 2 ) over the western and central Mediterranean basin on 22 June (Barragan et al, 2016). As mentioned in the forcing 2-D distribution and can be seen in Table 3, at the TOA the sign of the direct radiative forcing changed to positive when using mineral fractions obtained based on the soil mineralogy map to calculate dust optical properties.…”

“…Many efforts have been directed to more accurately measuring meteorological variables (e.g., water vapor profile and air temperature) and relevant parameters of dust (e.g., the complex refractive index, the particle shape, and the size distribution) by employing various in-site measurements, numerical models (Barragan et al, 2016;Osipov et al, 2015), and remote sensing observations at both regional and global scales. At the global scale, the net direct radiative forcing estimation at the top of the atmosphere (TOA) due to anthropogenic dust contributions varies from À0.3 to +0.1 W/m 2 , and it differs among available publications (Hansen et al, 1998;Jacobson, 2001;Mahowald et al, 2010;Miller and Tegen, 1998;Sokolik & Toon, 1996;Yoshioka et al, 2007;Yue et al, 2010).…”

“…At the global scale, the net direct radiative forcing estimation at the top of the atmosphere (TOA) due to anthropogenic dust contributions varies from À0.3 to +0.1 W/m 2 , and it differs among available publications (Hansen et al, 1998;Jacobson, 2001;Mahowald et al, 2010;Miller and Tegen, 1998;Sokolik & Toon, 1996;Yoshioka et al, 2007;Yue et al, 2010). For a region, studies have been performed mainly at a much shorter time scale focusing on more accurately measuring meteorology variables and optical parameters (Barragan et al, 2016;di Sarra et al, 2013). These case studies mainly work on the instantaneous direct radiative forcing, which is a basis for estimating the time-averaged direct radiative forcing.…”

Dust Properties and Radiative Impacts at a Suburban Site during 2004–2017 in the North China Plain

“…These values were generally smooth, although weakest cooling effects of RFEBOA tended to be exhibited by dust from the northeast, which was partly because the largest dust SSA was associated with dust from this direction. In view of their seasonal statistics (Figure 9c) [11,56]. Relatively low cooling effect of RF in The excellent performance of the solar radiations calculated by AERONET inversion algorithm encouraged us to explore the dust aerosol radiative forcing as follows.…”

Calibration of Raman Lidar Water Vapor Mixing Ratio Measurements Using Zenithal Measurements of Diffuse Sunlight and a Radiative Transfer Model

“…Gaseous absorption (H 2 O, CO 2 , O 2 , and O 3 ) is treated with the correlated k distribution, using a line by line code [27] and multiple scattering effects are treated using the discrete ordinates method [28]. GAME allows accurate treatment of scattering and absorption by aerosols and molecules [29]. Calibrations were performed in cloudfree conditions for accurate radiance simulations.…”