Consistency of dimensional distributions and refractive indices of desert dust measured over Lampedusa with IASI radiances

Giuliano Liuzzi,Paola Formenti,Claudia Di Biagio,Guido Masiello,Carmine Serio,Daniela Meloni

doi:10.5194/amt-10-599-2017

Abstract

Abstract. In the context of the ChArMEx campaign, we present here some results concerning the quantitative comparison between simulated and observed radiances in the presence of atmospheric desert dust, between June and July 2013 in the southern Mediterranean Basin, in the air mass above the island of Lampedusa. In particular, comparisons have been performed between radiances as observed by the Infrared Atmospheric Sounder Interferometer (IASI) and those simulated using the σ-IASI-as radiative transfer model, which takes into account aerosol extinction effect through a set of fast parameterizations. Simulations have been carried out using different sets of input complex refractive indices, which take into account the parent soils of the aerosols. Their accuracy also relies on the quality of the characterization of desert dust microphysical properties, achieved through direct measurements in the ChArMEx experiment. On the one hand, the fact that the model can ingest such a variable input proves its feasibility. On the other hand, this work goes through a direct validation of different refractive index sets for desert dust in the thermal infrared, and pursues an assessment of the sensitivity of IASI data with respect to the dimensional distribution of desert dust particles. Results show a good consistency between calculations and observations, especially in the spectral interval 800–1000 cm−1; further, the comparison between calculations and observations suggests that further efforts are needed to better characterize desert dust optical properties in the shortwave (above 2000 cm−1). Whatever the case, we show that it is necessary to properly tune the refractive indices according to the geographical origin of the observed aerosol.

Highlights

The discussion about the possibility to detect and characterize atmospheric aerosols in the thermal infrared spectral range is relatively recent (Hollweg et al, 2006; Clarisse et al, 2013; Cuesta et al, 2015; Sellitto and Legras, 2016) and very active, since infrared satellite observations enable an extensive spatial–temporal characterization of aerosol optical and microphysical properties, and quantitative description of their transport processes (e.g. Peyridieu et al, 2013; Sellitto et al, 2016; Prata and Prata, 2012)
A more detailed focus on these regions is given in the same figures, which is important since the aerosol extinction effect is heavily manifested therein, with differences in brightness temperature units that can be as large as 5 K for the case of 22 June
It is proper to point out that both the surface temperature and the water vapour columnar amount provided by ECMWF and used by the model have been slightly tuned in order to better match Infrared Atmospheric Sounder Interferometer (IASI) observations, which is necessary to correct some well-known biases typical of ECMWF reanalyses, and because we do not perform any retrieval of the true state vector

Summary

Introduction

The discussion about the possibility to detect and characterize atmospheric aerosols in the thermal infrared spectral range is relatively recent (Hollweg et al, 2006; Clarisse et al, 2013; Cuesta et al, 2015; Sellitto and Legras, 2016) and very active, since infrared satellite observations enable an extensive spatial–temporal characterization of aerosol optical and microphysical properties, and quantitative description of their transport processes (e.g. Peyridieu et al, 2013; Sellitto et al, 2016; Prata and Prata, 2012). Proficient exploitation of satellite infrared data requires a good knowledge of the spectral properties of the most common atmospheric aerosols in this spectral range In this context, there can be several sources of uncertainty, such as poor characterization of complex refractive indices, the particle size distribution, the aerosol’s vertical profile on the spatial scale of interest (Vandenbussche et al, 2013), and the degree of robustness of radiative transfer, which should be able to consistently reproduce the radiative impact of aerosols. The two aspects have the same importance, since both the quality and the amount of infrared data produced by satellite-based sensors have steeply increased in the last decade In this respect, before going through the exploration of aerosol properties, we describe the radiative transfer model we use in this work, called σ -IASI-as, fo-

Objectives

Results

Conclusion