16 Aug 2021
16 Aug 2021
Status: a revised version of this preprint is currently under review for the journal ACP.

Atmospheric stratification over Namibia and the southeast Atlantic Ocean

Danitza Klopper1,2, Stuart J. Piketh1, Roelof Burger1, Simon Dirkse3, and Paola Formenti4 Danitza Klopper et al.
  • 1North-West University, School for Geo- and Spatial Sciences, Potchefstroom, South Africa
  • 2University of Limpopo, Department of Geography and Environmental Studies, Polokwane, South Africa
  • 3Namibia Meteorology Service, Windhoek, Namibia
  • 4Université de Paris and Univ Paris Est Creteil, CNRS, LISA, F-75013 Paris, France

Abstract. We currently have a limited understanding of the spatial and temporal variability in vertically stratified atmospheric layers over Namibia and the southeast Atlantic (SEA) Ocean. Stratified layers are relevant to the transport and dilution of local and long-range transported atmospheric constituents. This study used eleven years of global positioning system radio occultation (GPS-RO) signal refractivity data (2007–2017) over Namibia and the adjacent ocean surfaces, and three years of radiosonde data from Walvis Bay, Namibia, to study the character and variability in stratified layers. From the GPS-RO data and up to a height of 10 km, we studied the spatial and temporal variability in the point of minimum gradient in refractivity, and the temperature inversion height, depth and strength. We also present the temporal variability of temperature inversions and the boundary layer height (BLH) from radiosondes. The BLH was estimated by the parcel method, the top of a surface-based inversion, the top of a stable layer identified by the bulk Richardson number (RN), and the point of minimum gradient in the refractivity (for comparison with GPS-RO data). A comparison between co-located GPS-RO to radiosonde temperature profiles found good agreement between the two, and an average underestimation of GPS-RO to radiosonde temperatures of −0.45 ± 1.25 °C, with smaller differences further from the surface and with decreasing atmospheric moisture content. The minimum gradient (MG) of refractivity, calculated from these two datasets were generally in good agreement (230 ± 180 m), with an exeption of a few cases when differences exceeded 1000 m. The surface of MG across the region of interest was largely affected by macroscale circulation and changes in atmospheric moisture and cloud, and was not consistent with BLH(RN). We found correlations in the character of low-level inversions with macroscale circulation, radiation interactions with the surface, cloud cover over the ocean and the seasonal maximum in biomass burning over southern Africa. Radiative cooling on diurnal scales also affected elevated inversions between 2.5 and 10 km, with more co-occurring inversions observed at night and in the morning. Elevated inversions formed most frequently over the subcontinent and under subsidence by high-pressure systems in the colder months. Despite this macroscale influence peaking in the winter, the springtime inversions, like those at low levels, were strongest.

Danitza Klopper et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-668', Anonymous Referee #1, 11 Sep 2021
  • RC2: 'Comment on acp-2021-668', Anonymous Referee #2, 14 Sep 2021

Danitza Klopper et al.

Danitza Klopper et al.


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Short summary
The western coast of southern Africa is a key region of the Earth, with persistent clouds and particles also transported from distant forest fires. The atmosphere is stratified as a result of the different temperatures of the cold Atlantic ocean and the warm semi-arid land, and that affects how the particles will be distributed whilst in the atmosphere and how long they will persist. We used long term satellite and in situ observations to describe, for the first time, those main features.