Impacts of the Saharan air layer on the physical properties of the Atlantic tropical cyclone cloud systems: 2003–2019

Hao Luo,Yong Han

doi:10.5194/acp-21-15171-2021

Abstract

Abstract. It is generally known that the tropical cyclone (TC) cloud systems (TCCSs) in the North Atlantic region frequently occur during boreal summer, while the Saharan dust outbreaks occur concurrently. The Sahara air layer (SAL), an elevated layer containing Saharan dry air and mineral dust, has crucial impacts on the generation and evolution of TCs. However, the effects of SAL on the physical (macro and micro) characteristics of the Atlantic TCCSs have not been well constrained, and the interaction mechanisms between them still need further investigation. In this study, our primary interest is to distinguish the various effects of SAL on different intensities of TCs and further find out the probable causes of the varied feedback mechanisms. Therefore, we attempt to identify whether and how the effects of the SAL play a positive or negative role on the TCCSs and to draw a qualitative conclusion on how SAL affects the various intensities of the TCs. This paper focuses on the 70 TC samples from July to September in the years of 2003–2019 to investigate the physical effects of SAL on three intensities of TCs, i.e., the tropical depression (TD), tropical storm (TS), and hurricane (HU). The results show that SAL has a positive impact on the macro properties of HU but significantly suppresses the TD. It appears that the SAL attributes little to the variation of the ice cloud effective radius (CERi) for TS, whereas CERi changes significantly and differentially for TD and HU. When affected by SAL, the probability density function (PDF) curve of CERi generally shifts to the smaller value for TD, whereas the PDF curve becomes flatter for HU. Our analysis indicates that the various responses of TCCSs to SAL are determined by the combined effects of dry air masses, the dust aerosols as ice nuclei, and the thermodynamic and moisture conditions. Based on the observation data analysis, a concept scheme description has been concluded to deepen our recognition of the effects of SAL on the TCCSs.

Highlights

Tropical cyclones (TCs) at landfall are risky natural disasters that endanger human life and property (Zhou et al, 2018; Parks et al, 2021)
The North Atlantic is one of the areas with the most frequent TCs in the world, and these TC cloud systems (TCCSs) are often affected by the Saharan air layer (SAL), which is an elevated layer of Saharan dry air and mineral dust, especially during the boreal summer between July and September (Dunion and Velden, 2004)
We discuss the impacts of SAL on the ice clouds of the Atlantic TCCSs through the dust semi-direct and indirect effects, as well as the evaporation effect by the dry air masses in the SAL

Summary

Introduction

Tropical cyclones (TCs) at landfall are risky natural disasters that endanger human life and property (Zhou et al, 2018; Parks et al, 2021). Accurate TC forecasts and data analysis are essential for the protection of human beings (Gray, 1968; Sun and Zhao, 2020). In the past several decades, a series of advances in remote sensing observations, numerical simulations, and data assimilations have improved our recognition of TC cloud systems (TCCSs) microphysics, dynamics, and thermodynamics (Ooyama, 1969; Dunion and Velden, 2004; Holt et al, 2015). The North Atlantic is one of the areas with the most frequent TCs in the world, and these TCCSs are often affected by the Saharan air layer (SAL), which is an elevated layer of Saharan dry air and mineral dust, especially during the boreal summer between July and September (Dunion and Velden, 2004).

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Results

Conclusion