Abstract
Abstract. The study is focused on Intensive Observation Period (IOP) 14 of the Hydrological Cycle in the Mediterranean Experiment first Special Observing Period (HyMeX SOP 1) that took place from 17 to 19 October 2012 and was dedicated to the study of orographic rain in the Cévennes–Vivarais (CV) target area. During this IOP a dense dust plume originating from northern Africa (the Maghreb and Sahara) was observed to be transported over the Balearic Islands towards the south of France. The plume was characterized by an aerosol optical depth between 0.2 and 0.8 at 550 nm, highly variable in time and space over the western Mediterranean Basin. The impact of this dust plume, the biggest event observed during the 2-month-long HyMeX SOP 1, on the precipitation over the CV area has been analyzed using high-resolution simulations from the convection permitting mesoscale model Meso-NH (mesoscale non-hydrostatic model) validated against measurements obtained from numerous instruments deployed specifically during SOP 1 (ground-based/airborne water vapor and aerosol lidars, airborne microphysics probes) as well as space-borne aerosol products. The 4-day simulation reproduced realistically the temporal and spatial variability (including the vertical distribution) of the dust. The dust radiative impact led to an average 0.6 K heating at the altitude of the dust layer in the CV area (and up to +3 K locally) and an average 100 J kg−1 increase of most unstable convective available potential energy (and up to +900 J kg−1 locally) with respect to a simulation without prescribed dust aerosols. The rainfall amounts and location were only marginally affected by the dust radiative effect, even after 4 days of simulation. The transient nature of this radiative effect in dynamical environments such as those found in the vicinity of heavy precipitation events in the Mediterranean is not sufficient to impact 24 h of accumulated rainfall in the dust simulation.
Highlights
most unstable convective available potential energy (MuCAPE) can be viewed as an integrator of all stabilization/destabilization effects in the atmosphere and is easier to interpret than fields of DUST/NODUST temperature differences whose interpretation can differ between daytime and nighttime and in the presence of key features of the water cycle, i.e., clouds and water vapor. We look at these processes in a more systematic way over the Cévennes– Vivarais (CV) area during Intensive Observation Period (IOP) 14
The direct radiative impact of a moderately intense dust plume (AOD between 0.2 and 0.8 at 550 nm) from northern Africa on precipitation over the southern fringes of the Massif Central was investigated during IOP 14 of the HyMeX SOP 1
The simulation with prognostic dust was first shown to simulate the most intense dust plume observed during the HyMeX SOP 1, in terms of spatiotemporal variability and, and more importantly, in terms of vertical distribution and dust aerosol concentration
Summary
Terrigenous dust aerosols are mobilized in arid and semi-arid environments, they affect precipitation thousands of kilometers away from the source regions through complex semi-direct radiative effects (e.g., Lau et al, 2006; Huang et al, 2006b, 2009, 2014; Solmon et al, 2008; Chaboureau et al, 2011; Zhao et al, 2012) and indirect radiative effects (e.g., Rosenfeld et al, 2001; van den Heever et al, 2006; Huang et al, 2006a, 2014; Creamean et al, 2013). The study is focused on Intensive Observation Period (IOP) 14 of the HyMeX SOP 1 which took place from 17 to 19 October 2012 and was dedicated to studying orographic rain in the Cévennes–Vivarais (CV) target area During this IOP a dense mineral dust plume originating from northern Africa (the Maghreb and Sahara) was observed to be transported towards the southern French coastline and over the Balearic Islands and to lead to AODs (aerosol optical depths) between 0.2 and 0.8 at 550 nm over the western Mediterranean Basin and to be highly variable in time and space. This case study was selected owing to the fact that the dust outbreak observed during IOP 14 was the biggest event observed in the course of the HyMeX SOP 1 It was chosen because only orographic rain was forecasted to occur on the southern slopes of the Massif Central, i.e., precipitation forced by orography and not embedded in large-scale clusters such as those initiated along a cold front.
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