Direct radiative effects during intense Mediterranean desert dust outbreaks

The direct radiative effect (DRE) of 20 intense and widespread dust outbreaks that affected the broader Mediterranean basin during the period March 2000 – February 2013, has been calculated with the regional NMMB-MONARCH model. The DREs have been calculated based on short-term simulations (84 hours) for a domain covering the Sahara and most part of the European continent. At midday, desert dust outbreaks induce locally a NET (shortwave plus longwave) strong atmospheric warming (DREATM values up to 285 Wm −2 ), a strong surface cooling (DRENETSURF values down to −337 Wm −2 ) whereas they strongly reduce the downward radiation at the ground (DRESURF values down to −589 Wm −2 ). During nighttime, reverse effects of smaller magnitude are found. At the top of the atmosphere (TOA), positive (planetary warming) DREs up to 85 Wm −2 are found over highly reflective surfaces while negative (planetary cooling) DREs down to −184 Wm −2 are computed over dark surfaces at noon. Desert dust outbreaks significantly affect the regional radiation budget, with regional clear-sky NET DRE values ranging from −13.9 to 2.6 Wm −2 , from −43.6 to 4 Wm −2 , from −26.3 to 3.9 Wm −2 and from −3.7 to 28 Wm −2 for TOA, SURF, NETSURF and ATM, respectively. Although the shortwave (SW) DREs are larger than the longwave (LW) ones, the latter are comparable or even larger at TOA, particularly over the Sahara at midday. As a response to the strong surface cooling during daytime, dust outbreaks cause a reduction of the regional sensible and latent heat fluxes by up to 45 Wm −2 and 4 Wm −2 , respectively, averaged over land areas of the simulation domain. Dust outbreaks reduce the temperature at 2 meters by up to 4 K during day, whereas a reverse tendency of similar magnitude is found during night. Depending on the vertical distribution of dust loads and time, mineral particles heat (cool) the atmosphere by up to 0.9 K (0.8 K) during daytime (nighttime) within atmospheric dust layers. Beneath and above the dust clouds, mineral particles cool (warm) the atmosphere by up to 1.3 K (1.2 K) at noon (night). When dust radiative effects are taken into account in numerical simulations, the total emitted dust and dust AOD, computed on a regional mean basis, are decreased (negative feedback) by 19.5 % and 6.9 %. The consideration of dust radiative effects in numerical simulations improves the model predictive skills. More specifically, it reduces the model positive and negative biases for the downward surface SW and LW radiation, respectively, with respect to Baseline Surface Radiation Network (BSRN) measurements. In addition, they also reduce the model near-surface (at 2 meters) nocturnal cold biases by up to 0.5 K (regional averages), as well as the model warm biases at 950 and 700 hPa, where the dust concentration is maximized, by up to 0.4 K.