Preprints
https://doi.org/10.5194/egusphere-2024-1627
https://doi.org/10.5194/egusphere-2024-1627
23 Jul 2024
 | 23 Jul 2024

A satellite-based analysis of semi-direct effects of biomass burning aerosols on fog and low cloud dissipation in the Namib Desert

Alexandre Mass, Hendrik Andersen, Jan Cermak, Paola Formenti, Eva Pauli, and Julian Quinting

Abstract. In the Namib Desert fog is the only regular water input and thus a crucial water source for its fauna and flora. Each year between June and October, in some synoptic settings, absorbing biomass burning aerosols (BBA) are overlying the stratocumulus clouds in the adjacent Southeast Atlantic, and sometimes are reaching the coastal fog and low clouds (FLCs) in Namibia. In this study, a novel 15-year data set of geostationary satellite observations of FLC dissipation time in the Namib Desert is used together with reanalysis data in order to better understand possible semi-direct effects of BBA on the dissipation of FLCs in the Namib. This is done by investigating both FLC dissipation time and synoptics depending on BBA loading. It is found that FLC dissipation time is significantly later on high BBA loading days. BBA are transported to the Namib along moist free-tropospheric air by a large-scale anticyclonic recirculation pattern. At the surface, the associated longwave heating strengthens a continental heat low, which modifies the circulation and boundary layer moisture along the coastline, complicating the attribution of BBA effects. During high BBA days, the vertical profiles of the temporal development of air temperatures highlight contrasting day and nighttime processes modifying the local inversion. These processes are thought to be driven by greenhouse warming by the moisture in the BBA plumes and BBA absorption (only during daytime). A statistical learning framework is used to quantify meteorological and BBA influences on FLC dissipation time. The statistical model is able to reproduce the observed differences in FLC dissipation time between high and low BBA days and attributes these differences mainly to differences in circulation, boundary layer moisture and near-surface air temperature along the coastline. However, the model is underfitting and is not able to reproduce the majority of the FLC dissipation variability. While the model does not suggest that BBA patterns are important for FLC dissipation, the findings show how the moist BBA plumes modify local thermodynamics to which FLC dissipation is shown to be sensitive. The findings highlight the difficulties of disentangling meteorological and aerosol effects on cloud development using observations.

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Alexandre Mass, Hendrik Andersen, Jan Cermak, Paola Formenti, Eva Pauli, and Julian Quinting

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-1627', Anonymous Referee #2, 13 Aug 2024
    • AC1: 'Reply on RC1', Alexandre Mass, 15 Oct 2024
  • RC2: 'Comment on egusphere-2024-1627', Anonymous Referee #1, 20 Aug 2024
    • AC2: 'Reply on RC2', Alexandre Mass, 15 Oct 2024

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2024-1627', Anonymous Referee #2, 13 Aug 2024
    • AC1: 'Reply on RC1', Alexandre Mass, 15 Oct 2024
  • RC2: 'Comment on egusphere-2024-1627', Anonymous Referee #1, 20 Aug 2024
    • AC2: 'Reply on RC2', Alexandre Mass, 15 Oct 2024
Alexandre Mass, Hendrik Andersen, Jan Cermak, Paola Formenti, Eva Pauli, and Julian Quinting
Alexandre Mass, Hendrik Andersen, Jan Cermak, Paola Formenti, Eva Pauli, and Julian Quinting

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Short summary
This study investigates the interaction between smoke aerosols and fog and low clouds (FLCs) in the Namib desert between June and October. Here, a satellite-based dataset of FLCs, reanalysis data and machine learning are used to systematically analyze FLCs persistence under different aerosol loadings. Aerosol plumes are shown to modify local thermodynamics which increases FLC persistence. But fully disentangling aerosol effects from meteorological ones remains a challenge.