Direct Radiative Effect of Intense Dust Outbreaks in the Mediterranean

Abstract:

The broader Mediterranean basin is affected by intense desert dust outbreaks in spring. In the present study, we make use of satellite observations and modelling to investigate dust radiative impacts during three consecutive dust outbreaks occurred over the Mediterranean in the period 9/4-15/4/2008. The direct radiative effect (DRE) is estimated by using two simulations run with the NMMB/BSC-Dust model, where the interaction between dust aerosols and radiation is activated and deactivated, respectively. The simulation domain covers the North Africa, the Middle East and Europe at 0.25ºx0.25° and 40σ-layers. The first outbreak took place over the central and eastern Mediterranean on the 9^th reaching aerosol optical depths (AODs) close to 1. The second one, with AODs up to 2, lasted from 10^th to 14^th affecting mainly the central Mediterranean. The third one, with AODs up to 5, affected the Iberian Peninsula on the 15^th. DREs are computed for the outgoing radiation at the top of the atmosphere (TOA), the absorbed radiation into the atmosphere (ATMAB), for the downwelling (SURF) and the absorbed (NETSURF) radiation at surface, for the shortwave (SW), longwave (LW) and NET (SW+LW) radiation. According to our results, it is evident that DREs’ spatial patterns are driven by those of AOD. Negative (cooling) instantaneous DRE_TOA, DRE_SURF and DRE_NETSURF values up to -500W/m², -700W/m² and -600W/m², respectively, and positive (warming) instantaneous DRE_ATMAB up to 340W/m² are found for the SW spectrum, during daytime. Opposite but less pronounced effects are encountered for the LW radiation and during nightime. Due to these perturbations on the radiation field, the surface temperature is reduced locally by up to 8°C during daytime and increased by up to 4°C during nightime. It is found that the regional average NET DREs can be as large as -12W/m², -45W/m², -30W/m² and 27W/m² for TOA, SURF, NETSURF and ATMAB, respectively. Impacts on atmospheric stability and dust emissions are also investigated.