Modification of Temperature Lapse Rates and Cloud Properties during a Spatiotemporally Extended Dust Aerosol Episode (16–18 June 2016) over the Mediterranean Basin Based on Satellite and Reanalysis Data

Maria Gavrouzou,Nikolaos Hatzianastassiou,Nikolaos Mihalopoulos,Christos J Lolis,Marios-Bruno Korras-Carraca

doi:10.3390/rs14030679

Abstract

A spatiotemporally extended dust aerosol episode that occurred over the Mediterranean Basin (MB) from 16 to 18 June 2016 is investigated using observational satellite and reanalysis data, focusing on the effects of high dust loads on cloud formation and temperature fields, including the creation of temperature inversions. The atmospheric conditions before and during the 3-day dust aerosol episode case (DAEC) are also analyzed. The dust episode, which is identified using a contemporary satellite algorithm, consists of long-range transport of African dust to the western and central MB. The day to day, before and during the DAEC, atmospheric circulation, dust-cloud interactions, and dust effect on temperature are examined using a variety of Moderate Resolution Imaging Spectroradiometer (MODIS) Level-3 Collection 6.1 satellite and Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis data. According to the obtained results, the dust export from N. Africa, which occurs under the prevalence of a trough over the western MB, and a ridge over the central MB, extends from southwest to northeast along two axes, one in the western and another in the central Mediterranean, covering remote areas up to the coasts of southern Europe, including the Balearic and Tyrrhenian Seas, the Italian peninsula, the Ionian and Adriatic Seas, and the Balkan peninsula. The analysis provides evidence of the formation of mixed-phase clouds, with high cloud-top heights (CTH higher than 10 km) and low cloud-top temperatures (CTT as low as 230 K), which spatiotemporally coincide with the high dust loadings that provide the necessary CCN and IN. Dust aerosols are transported either in the boundary layer (within the first 1–2 km) of areas close to the North African dust source areas or in the free troposphere over the Mediterranean Sea and the Italian and Balkan peninsulas (between 2 and 8 km). Distinct and extended layers of remarkable temperature inversions (up to 20 K/km) are created below the exported dust layers in the boundary layer of Mediterranean Sea areas, while weak/reduced lapse rates are formed over continental areas of MB undergoing the dust transport. Such modifications of temperature fields are important for the dynamics of the atmosphere of MB.

Highlights

A detailed analysis of an evolving episode of dust transport (DAEC) over the Mediterranean Basin and of the associated atmospheric circulation, cloud formation, and three-dimensional temperature structure is made by using observational Moderate Resolution Imaging Spectroradiometer (MODIS) satellite and MERRA-2 reanalysis data
Emphasis is given to the assessment of the effect of dust aerosol episode case (DAEC) on the formation and the properties of clouds, as well as on the
38◦ N–7.5◦ E and 40◦ N–12.5◦ E, i.e., over the western Tyrrhenian Sea off the coasts of Sardinia, high dust optical depth (DOD) values occur below 3800 m, in the whole boundary layer down to the Earth’s surface. These high DOD values can be attributed to the proximity of this region to the African source region of the dust export and the absence of sedimentation of the heavy Dust aerosols (DA)

Summary

Introduction

Aerosols participate in chemical reactions, absorbing and scattering the shortwave and terrestrial radiation and acting as cloud condensation nuclei (CCN) and ice nuclei (IC) Through their interaction with radiation, aerosols affect the Earth’s radiation budget, modifying the atmospheric temperature structure and driving to temperature inversions [2]. The aerosol cloud interactions (ACI) vary temporally and spatially, strongly depending on cloud types, regional aerosol background, atmospheric circulation, regional geomorphological characteristics, as well as other atmospheric parameters such as relative humidity (RH) and wind vertical velocities [7] As it concerns warm clouds, additional aerosols increase the number and reduce the size of droplets, resulting in increased cloud reflectivity and a longer lifetime (“Twomey effect” [8]). Aerosols affect clouds through their semi-direct effect, which accounts for local warming within the troposphere caused by absorbing aerosols, such as black carbon, leading to changes in atmospheric stability and cloud cover [9,10,11]

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