Probing the Fog Life Cycles in the Namib Desert

Spirig, Robert; Vogt, Roland; Larsen, Jarl Are; Feigenwinter, Christian; Wicki, Andreas; Franceschi, Joel; Parlow, Eberhard; Adler, Bianca; Kalthoff, Norbert; Čermák, Jan; Andersen, Hendrik; Fuchs, Julia; Bott, Andreas; Hacker, Maike; Wagner, Niklas; Maggs‐Kölling, Gillian; Wassenaar, Theo; Seely, Mary

doi:10.1175/bams-d-18-0142.1

Cited by 29 publications

(32 citation statements)

References 25 publications

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“…The findings clearly demonstrate the strong dependence of central-Namib FLC occurrence on the advection of moist marine boundary-layer air masses, contrasting the notion of predominant radiation fog (Kaseke et al, 2017), but 10 in agreement with many other studies (e.g. Seely and Henschel, 1998;Formenti et al, 2018;Andersen et al, 2019;Spirig et al, 2019). These results are supported by a principal component analysis of near-surface winds that show a clear connection of FLC cover to synoptic-scale dynamics.…”

supporting

confidence: 91%

“…In order to obtain insights about the physical properties of the air masses, the temperature, potential temperature, specific humidity, and relative humidity are tracked along the trajectories. The location is chosen, as it is the main site of both historic and present-day scientific activity in the region (Lancaster et al, 1984;Seely and Henschel, 10 1998; Kaseke et al, 2017;Spirig et al, 2019).…”

Section: Trajectory Analysismentioning

confidence: 99%

“…These differences are caused by the subsiding dry continental easterly air masses that dominate on clear days, whereas on FLC days, a slight onshore flow of the more humid marine boundary layer air is observed in the central Namib. Over land, these marine air masses flow against the dominant continental easterly winds (Lindesay and Tyson, 1990), producing a northerly wind flow at ≈ 15 • N (not shown) that has been found to be associated with 20 fog occurrence in the central Namib (Seely and Henschel, 1998;Spirig et al, 2019). Above the moist marine boundary layer, the free troposphere is relatively dry on FLC days during both seasons, a feature which is not visible in the columnar TCWV composites during AMJ as it is masked by the moist anomaly in the marine boundary layer (Fig.…”

Section: Seasonal Differences In Synoptic-scale Mechanismsmentioning

confidence: 99%

“…While it seems settled that, at least at the scales considered in this study, FLC occurrence is mostly driven by advective processes, the quantitative contributions of humidity and temperature changes and radiative cooling for low-cloud formation in 5 the Namib during the advection of marine boundary-layer air masses are still unclear. A heat budget analysis as in e.g., Adler et al (2019) or Babić et al (2019), based on ground-based measurements conducted during the field campaign of the Namib Life Cycle Analysis (NaFoLiCA) project (Spirig et al, 2019), is necessary to better understand the origin, development and life cycle of FLCs within the advected marine boundary-layer air masses. Future work should also focus on understanding the local and possibly synoptic-scale drivers of the vertical structure of Namib-region FLCs on diurnal to seasonal scales, and 10 the day-to-day variability of (marine) boundary-layer humidity.…”

mentioning

confidence: 99%

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Synoptic-scale controls of fog and low clouds in the Namib Desert

Andersen¹,

Čermák²,

Fuchs³

et al. 2019

Preprint

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<p><strong>Abstract.</strong> Fog is a defining characteristic of the climate of the Namib Desert and its water and nutrient input are important for local ecosystems. In part due to sparse observation data, the local mechanisms that lead to fog occurrence in the Namib are not yet fully understood, and to date, potential synoptic-scale controls have not been investigated. In this study, a recently established 14-year data set of satellite observations of fog and low clouds in the central Namib is analyzed in conjunction with reanalysis data to identify typical synoptic-scale conditions associated with fog and low-cloud occurrence in the central Namib during two seasons that characterize seasonal fog variability. It is found that during both seasons, mean sea level pressure and geopotential height at 500&#8201;hPa differ significantly between fog/low-cloud and clear days, with patterns indicating seasonally different synoptic-scale disturbances on fog and low-cloud days: cut-off lows during September, October, and November, and breaking Rossby waves during April, May, and June. These regularly occurring disturbances increase the probability of fog and low-cloud occurrence in the central Namib in two main ways: 1) an anomalously dry free troposphere in the coastal region of the Namib leads to stronger longwave cooling, especially over the ocean, facilitating low-cloud formation, and 2) local wind systems are modulated, leading to an onshore anomaly of marine boundary-layer air masses. This is consistent with air mass backtrajectories and a principal component analysis of spatial wind patterns that point to advected marine boundary- layer air masses on fog and low-cloud days, whereas subsiding continental air masses dominate on clear days. Large-scale free-tropospheric moisture transport into southern Africa seems to be a key factor modulating the onshore advection of marine boundary-layer air masses during April, May, and June, as the associated increase in greenhouse gas warming and thus surface heating is observed to contribute to a continental heat low anomaly. A statistical model is trained to discriminate between fog/low-cloud and clear days based on large-scale mean sea level pressure fields. The model accurately predicts fog and low-cloud days, illustrating the importance of large-scale pressure modulation and advective processes. It can be concluded that Namib-region fog is predominantly of advective nature, but also facilitated by increased radiative cooling. Seasonally different manifestations of synoptic-scale disturbances act to modify its day-to-day variability and the balance of mechanisms leading to its formation. The results are the basis for a new conceptual model on the synoptic-scale mechanisms that control fog and low clouds in the Namib Desert, and will guide future studies of coastal fog regimes.</p>

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supporting

confidence: 91%

Section: Trajectory Analysismentioning

confidence: 99%

Section: Seasonal Differences In Synoptic-scale Mechanismsmentioning

confidence: 99%

mentioning

confidence: 99%

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Synoptic-scale controls of fog and low clouds in the Namib Desert

Andersen¹,

Čermák²,

Fuchs³

et al. 2019

Preprint

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“…During nighttime moderate winds blow from E. The final two days show a change in wind regime in which the wind rotates back to NW directions after sunset. This situation is typically connected to inland advection of stratus/fog (Spirig et al 2019).…”

Section: Meteorological Conditionsmentioning

confidence: 99%

The Namib Turbulence Experiment: Investigating Surface–Atmosphere Heat Transfer in Three Dimensions

Hilland

Bernhofer

Bohmann

et al. 2022

Bulletin of the American Meteorological Society

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The Namib Turbulence EXperiment (NamTEX) was a multi-national micrometeorological campaign conducted in the Central Namib Desert to investigate three-dimensional surface layer turbulence and the spatio-temporal patterns of heat transfer between the sub-surface, surface, and atmosphere. The Namib provides an ideal location for fundamental research that revisits some key assumptions in micrometeorology that are implicitly included in the parameterizations describing energy exchange in weather forecasting and climate models: Homogenous flat surfaces, no vegetation, little moisture, and cloud-free skies create a strong and consistent diurnal forcing, resulting in a wide range of atmospheric stabilities. A novel combination of instruments was used to simultaneously measure variables and processes relevant to heat transfer: A three km fibre-optic distributed temperature sensor (DTS) was suspended in a pseudo-three-dimensional array within a 300 m x 300 m domain to provide vertical cross-sections of air temperature fluctuations. Aerial and ground-based thermal imagers recorded high resolution surface temperature fluctuations within the domain and revealed the spatial thermal imprint of atmospheric structures responsible for heat exchange. High-resolution soil temperature and moisture profiles together with heat flux plates provided information on near-surface soil dynamics. Turbulent heat exchange was measured with a vertical array of five eddy-covariance point measurements on a 21-m mast, as well as by co-located small- and large-aperture scintillometers. This contribution first details the scientific goals and experimental set-up of the NamTEX campaign. Then using a typical day, we demonstrate i) the coupling of surface layer, surface, and soil temperatures using high-frequency temperature measurements, ii) differences in spatial and temporal standard deviations of the horizontal temperature field using spatially distributed measurements, and iii) horizontal anisotropy of the turbulent temperature field.

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Nematode communities indicate diverse soil functioning across a fog gradient in the Namib Desert gravel plains

Treonis

Marais

Maggs‐Kölling

2022

Ecology and Evolution

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Soil nematodes are fundamentally aquatic animals, requiring water to move, feed, and reproduce. Nonetheless, they are ubiquitous in desert soils because they can enter an anhydrobiotic state that allows them to persist when water is biologically unavailable. In the hyper‐arid Namib Desert of Namibia, rain is rare, but fog routinely moves inland from the coast and supports plant and animal life. Very little is understood about how this fog may affect soil organisms. We investigated the role of fog moisture in the ecology of free‐living, soil nematodes across an 87‐km fog gradient in the gravel plains of the Namib Desert. We found that nematodes emerged from anhydrobiosis and became active during a fog event, suggesting that they can utilize fog moisture to survive. Nematode abundance did not differ significantly across the fog gradient and was similar under shrubs and in interplant spaces. Interplant soils harbor biological soil crusts that may sustain nematode communities. As fog declined along the gradient, nematode diversity increased in interplant soils. In areas where fog is rare, sporadic rainfall events can stimulate the germination and growth of desert ephemerals that may have a lasting effect on nematode diversity. In a 30‐day incubation experiment, nematode abundance increased when soils were amended with water and organic matter. However, these responses were not evident in field samples, which show no correlations among nematode abundance, location in the fog gradient, and soil organic matter content. Soil nematodes are found throughout the Namib Desert gravel plains under a variety of conditions. Although shown to be moisture‐ and organic matter‐limited and able to use moisture from the fog for activity, variation in fog frequency and soil organic matter across this unique ecosystem may be biologically irrelevant to soil nematodes in situ.

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Probing the Fog Life Cycles in the Namib Desert

Cited by 29 publications

References 25 publications

Synoptic-scale controls of fog and low clouds in the Namib Desert

Synoptic-scale controls of fog and low clouds in the Namib Desert

The Namib Turbulence Experiment: Investigating Surface–Atmosphere Heat Transfer in Three Dimensions

Nematode communities indicate diverse soil functioning across a fog gradient in the Namib Desert gravel plains

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