Estimate of surface direct radiative forcing of desert dust from atmospheric modulation of the aerosol optical depth

Abstract

Measurements carried out on the island of Lampedusa, in the central Mediterranean, on 7 September 2005, show the occurrence of a quasi-periodic oscillation of aerosol optical depth, column water vapour, and surface irradiance in different spectral bands. The oscillation has a period of about 13 min and is attributed to the propagation of a gravity wave able to modify the vertical structure of the planetary boundary layer, as also confirmed by satellite images. The wave occurred during a Saharan dust event. The oscillation amplitude is about 0.1 for the aerosol optical depth, and about 0.4 cm for the column water vapour. The modulation of the downward surface irradiances is in opposition of phase with respect to aerosol optical depth and water vapour column variations. The perturbation of the downward irradiance produced by the aerosols is determined by comparing the measured irradiances with estimated irradiances at a fixed value of the aerosol optical depth, and by correcting for the effect of the water vapour in the shortwave spectral range. The direct radiative forcing efficiency, i.e., the radiative perturbation of the net surface irradiance produced by a unit of optical depth aerosol layer, is determined at different solar zenith angles as the slope of the irradiance perturbation versus the aerosol optical depth. The estimated direct surface forcing efficiency at about 60° solar zenith angle is −(181 ± 17) W m⁻² in the shortwave, and −(83 ± 7) W m⁻² in the photosynthetic spectral range. The estimated daily average forcing efficiencies are of about −79 and −46 W m⁻² for the shortwave and photosynthetic spectral range, respectively

Solar radiation measurements compared to simulations at the BSRN Izaña station. Mineral dust radiative forcing and efficiency study

“…For example, Li et al [] found diurnal Δ DF eff‐slope of −65±3 Wm −2 per unit of AOD at 550 nm for mineral dust events in the tropical Atlantic region. These values are comparable to the Δ DF eff‐slope values reported by other studies [ Balkanski et al , ; Di Sarra et al , ; Valenzuela et al , ; Di Sarra et al , ]. When comparing to other mineral dust sources, we observe that, on average, Saharan mineral dust seems to be less efficient at surface [e.g., Bush and Valero , , ; Yoon et al , ], due in part to the different mineralogical compositions [e.g., Arimoto et al , ; Shen et al , ; Formenti et al , ].…”

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

“…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.…”

“…The authors (Barragan et al, 2016) estimated that the direct radiative forcing in the infrared band ranges from +3.5 to +8.0 W/m 2 and 1.7 to 6.9 W/m 2 at the surface and the TOA, correspondingly. Another measurement (di Sarra et al, 2013), performed in the central Mediterranean, found that the direct radiative forcing in the solar band at the surface is approximately À181 W/m 2 at the 60°solar zenith angle. For dusty days during the springtime in Beijing, China, the dust direct radiative forcing ranges from À65.9 to À115.7 W/m 2 and À166.6 to À236.0 W/m 2 at the TOA and the surface, correspondingly, for a period from 2001 to 2014 (Yu et al, 2016).…”

Direct radiative effects during intense Mediterranean desert dust outbreaks

“…Di Sarra et al, 2008;. Thus, a number of studies focused on Mediterranean dust outbreaks' impacts on the SW (Meloni et al, 2004;Gómez-Amoet al, 2011;Antón et al, 2012;Di Sarra et al, 2013;Obregón et al, 2015), LW (Antón et al, 2014;Sicard et al, 2014a) and NET Romano et al, 2016) radiation. However, the obtained results were representative at a local scale and, considering the high spatial variability of desert dust outbreaks, the optimum solution of assessing in a comprehensive way their impacts on weather and climate is provided by atmospheric-dust models.…”