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

2019 ◽  
Author(s):  
Hendrik Andersen ◽  
Jan Cermak ◽  
Julia Fuchs ◽  
Peter Knippertz ◽  
Marco Gaetani ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Synoptic Scale ◽  
Namib Desert ◽  
Sea Level Pressure ◽  
Mean Sea Level ◽  
Air Masses ◽  
Low Clouds ◽  
Low Cloud

Abstract. 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 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.

scholarly journals Synoptic-scale controls of fog and low-cloud variability in the Namib Desert

2020 ◽  
Vol 20 (6) ◽  
pp. 3415-3438 ◽  
Author(s):  
Hendrik Andersen ◽  
Jan Cermak ◽  
Julia Fuchs ◽  
Peter Knippertz ◽  
Marco Gaetani ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Cloud Cover ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Synoptic Scale ◽  
Namib Desert ◽  
Data Set ◽  
Air Masses ◽  
Low Cloud ◽  
Low Cloud Cover

Abstract. 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 in order to identify synoptic-scale patterns associated with fog and low-cloud variability in the central Namib during two seasons with different spatial fog occurrence patterns. It is found that during both seasons, mean sea level pressure and geopotential height at 500 hPa differ markedly between fog/low-cloud and clear days, with patterns indicating the presence of synoptic-scale disturbances on fog and low-cloud days. 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 of the marine boundary layer, increasing low-cloud cover, especially over the ocean where the anomaly is strongest; (2) local wind systems are modulated, leading to an onshore anomaly of marine boundary-layer air masses. This is consistent with air mass back trajectories 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 are 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 information on large-scale dynamics. 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 regional fog in the Namib is predominantly of an advective nature and that fog and low-cloud cover is effectively maintained by increased cloud-top 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 and maintenance. The results are the basis for a new conceptual model of the synoptic-scale mechanisms that control fog and low-cloud variability in the Namib Desert and will guide future studies of coastal fog regimes.

scholarly journals NaFoLiCA: Synoptic-scale controls of fog and low cloud variability in the Namib Desert

2020 ◽  
Author(s):  
Hendrik Andersen ◽  
Jan Cermak ◽  
Julia Fuchs ◽  
Peter Knippertz ◽  
Marco Gaetani ◽  
...  
Keyword(s):  
Marine Boundary Layer ◽  
Reanalysis Data ◽  
Synoptic Scale ◽  
Data Set ◽  
Air Masses ◽  
Low Clouds ◽  
New Findings ◽  
Low Cloud ◽  
Different Seasons

<p>This contribution presents new findings on synoptic-scale mechanisms that control the day-to-day variability of fog and low clouds (FLCs) in the Namib region.<br>FLCs are a defining element of the Namib-region climate and a crucial source of water for many species and ecosystems. Still, little is known on the processes driving Namib-region FLCs, in large part due to the very sparse observational records. Specifically, there is an ongoing debate in the scientific literature concerning the relevance of different mechanisms responsible for fog formation in the region. In this contribution, a new long-term satellite-based data set of FLC occurrence is used in conjunction with reanalysis data and backtrajectories to systematically analyze dynamical and thermodynamical differences between days with and without FLCs in the central Namib during two different seasons. The main findings are:</p><ol><li>Central-Namib FLCs are nearly always associated with the advection of marine-boundary-layer air masses.</li> <li>The variability of the overall FLC coverage in the central Namib is largely driven by dynamics at the synoptic scale.</li> <li>Seasonally different synoptic-scale mechanisms determine the probability of the occurrence of FLCs in the central Namib.</li> </ol><p>The findings lead to a better understanding of Namib-region FLCs and help broaden the understanding of low clouds along the southwestern African coastline and the southeast Atlantic. </p>

scholarly journals On the transitions in marine boundary layer cloudiness

2010 ◽  
Vol 10 (5) ◽  
pp. 2377-2391 ◽  
Author(s):  
I. Sandu ◽  
B. Stevens ◽  
R. Pincus
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Data Sets ◽  
Lagrangian Analysis ◽  
Air Masses ◽  
Upper Tropospheric Humidity ◽  
The Mean ◽  
Subtropical Oceans ◽  
Cloud Evolution

Abstract. Satellite observations and meteorological reanalysis are used to examine the transition from unbroken sheets of stratocumulus to fields of scattered cumulus, and the processes controlling them, in four subtropical oceans. A Lagrangian analysis suggests that both the transition, defined as the temporal evolution in cloudiness, and the processes driving the transition, are quite similar among the subtropical oceans. The increase in sea surface temperature and the associated decrease in lower tropospheric stability appear to play a far more important role in cloud evolution than other factors including changes in large scale divergence and upper tropospheric humidity. During the summer months, the transitions in marine boundary layer cloudiness appear so systematically that their characteristics obtained by documenting the flow of thousands of individual air masses are well reproduced by the mean (or climatological) fields of the different data sets. This highlights interesting opportunities for future observational and modeling studies of these transitions.

scholarly journals Large-scale and synoptic meteorology in the south-east Pacific during the observations campaign VOCALS-REx in austral Spring 2008

2011 ◽  
Vol 11 (10) ◽  
pp. 4977-5009 ◽  
Author(s):  
T. Toniazzo ◽  
S. J. Abel ◽  
R. Wood ◽  
C. R. Mechoso ◽  
G. Allen ◽  
...  
Keyword(s):  
Large Scale ◽  
Marine Boundary Layer ◽  
Synoptic Scale ◽  
The South ◽  
Austral Spring ◽  
Scale Parameters ◽  
Cloud Properties ◽  
Low Cloud ◽  
Crude Approximation ◽  
Spatio Temporal

Abstract. We present a descriptive overview of the meteorology in the south eastern subtropical Pacific (SEP) during the VOCALS-REx intensive observations campaign which was carried out between October and November 2008. Mainly based on data from operational analyses, forecasts, reanalysis, and satellite observations, we focus on spatio-temporal scales from synoptic to planetary. A climatological context is given within which the specific conditions observed during the campaign are placed, with particular reference to the relationships between the large-scale and the regional circulations. The mean circulations associated with the diurnal breeze systems are also discussed. We then provide a summary of the day-to-day synoptic-scale circulation, air-parcel trajectories, and cloud cover in the SEP during VOCALS-REx. Three meteorologically distinct periods of time are identified and the large-scale causes for their different character are discussed. The first period was characterised by significant variability associated with synoptic-scale systems interesting the SEP; while the two subsequent phases were affected by planetary-scale disturbances with a slower evolution. The changes between initial and later periods can be partly explained from the regular march of the annual cycle, but contributions from subseasonal variability and its teleconnections were important. Across the whole of the two months under consideration we find a significant correlation between the depth of the inversion-capped marine boundary layer (MBL) and the amount of low cloud in the area of study. We discuss this correlation and argue that at least as a crude approximation a typical scaling may be applied relating MBL and cloud properties with the large-scale parameters of SSTs and tropospheric temperatures. These results are consistent with previously found empirical relationships involving lower-tropospheric stability.

scholarly journals Sunlight-absorbing aerosol amplifies the seasonal cycle in low cloud fraction over the southeast Atlantic

2021 ◽  
Author(s):  
Jianhao Zhang ◽  
Paquita Zuidema
Keyword(s):  
Boundary Layer ◽  
Seasonal Cycle ◽  
Large Scale ◽  
Cloud Fraction ◽  
Small Scale ◽  
Aerosol Cloud ◽  
Ascension Island ◽  
Tropospheric Aerosol ◽  
Low Cloud ◽  
Aerosol Cloud Interactions

Abstract. Many studies examining shortwave-absorbing aerosol-cloud interactions over the southeast Atlantic apply a seasonal averaging. This disregards a meteorology that raises the mean altitude of the smoke layer from July to October. This study details the month-by-month changes in cloud properties and the large-scale environment as a function of the biomass-burning aerosol loading at Ascension Island from July to October, based on measurements from Ascension Island (8º S, 14.5º W), satellite retrievals and reanalysis. In July and August, variability in the smoke loading predominantly occurs in the boundary layer. During both months, the low-cloud fraction is less and is increasingly cumuliform when more smoke is present, with the exception of a late morning boundary layer deepening that encourages a short-lived cloud development. The meteorology varies little, suggesting aerosol-cloud interactions consistent with a boundary-layer semi-direct effect can explain the cloudiness changes. September marks a transition month during which mid-latitude disturbances can intrude into the Atlantic subtropics, constraining the land-based anticyclonic circulation transporting free-tropospheric aerosol to closer to the coast. Stronger boundary layer winds help deepen, dry, and cool the boundary layer near the main stratocumulus deck compared to that on days with high smoke loadings, with stratocumulus reducing everywhere but at the northern deck edge. Longwave cooling rates generated by a sharp water vapor gradient at the aerosol layer top facilitates small-scale vertical mixing, and could help to maintain a better-mixed September free troposphere. The October meteorology is more singularly dependent on the strength of the free-tropospheric winds advecting aerosol offshore. Free-tropospheric aerosol is less, and moisture variability more, compared to September. Low-level clouds increase and are more stratiform, when the smoke loadings are higher. The increased free-tropospheric moisture can help sustain the clouds through reducing evaporative drying during cloud-top entrainment. Enhanced subsidence above the coastal upwelling region increasing cloud droplet number concentrations may further prolong cloud lifetime through microphysical interactions. Reduced subsidence underneath stronger free-tropospheric winds at Ascension supports slightly higher cloud tops during smokier conditions. Overall the monthly changes in the large-scale aerosol and moisture vertical structure act to amplify the seasonal cycle in low-cloud amount and morphology, raising a climate importance as cloudiness changes dominate changes in the top-of-atmosphere radiation budget.

scholarly journals Contrasting characteristics of open- and closed- cellular stratocumulus cloud in the Eastern North Atlantic

2021 ◽  
Author(s):  
Michael P. Jensen ◽  
Virendra P. Ghate ◽  
Dié Wang ◽  
Diana K. Apoznanski ◽  
Mary J. Bartholomew ◽  
...  
Keyword(s):  
Boundary Layer ◽  
North Atlantic ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Open Cell ◽  
Cold Air ◽  
Closed Cell ◽  
Boundary Layer Cloud ◽  
Eastern North Atlantic ◽  
Layer Cloud

Abstract. Extensive regions of marine boundary layer cloud impact the radiative balance through their significant shortwave albedo while having little impact on outgoing longwave radiation. Despite this importance, these cloud systems remain poorly represented in large-scale models due to difficulty in representing the processes that drive their lifecycle and coverage. In particular, the mesoscale organization, and cellular structure of marine boundary clouds has important implications for the subsequent cloud feedbacks. In this study, we use long-term (2013–2018) observations from the Atmospheric Radiation Measurement (ARM) Facility's Eastern North Atlantic (ENA) site on Graciosa Island, Azores, Portugal to identify cloud cases with open- or closed-cellular organization. More than 500 hours of each organization type are identified. The ARM observations are combined with reanalysis and satellite products to quantify the cloud, precipitation, aerosol, thermodynamic and large-scale synoptic characteristics associated with these cloud types. Our analysis shows that both cloud organization populations occur during similar sea surface temperature conditions, but the open-cell cases are distinguished by stronger cold-air advection and large-scale subsidence compared to the closed-cell cases, consistent with their formation during cold-air outbreaks. We also find that the open-cell cases were associated with deeper boundary layers, stronger low-level winds, and higher-rain rates compared to their closed-cell counterparts. Finally, raindrops with diameters larger than one millimeter were routinely recorded at the surface during both populations, with a higher number of large drops during the open-cellular cases. The similarities and differences noted herein provide important insights into the environmental and cloud characteristics during varying marine boundary layer cloud mesoscale organization and will be useful for the evaluation of model simulations for ENA marine clouds.

scholarly journals On the relationship between acetone and carbon monoxide in air masses of different origin

2003 ◽  
Vol 3 (1) ◽  
pp. 1017-1049
Author(s):  
M. de Reus ◽  
H. Fischer ◽  
F. Arnold ◽  
J. de Gouw ◽  
R. Holzinger ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Carbon Monoxide ◽  
Field Experiments ◽  
Marine Boundary Layer ◽  
Biogenic Emissions ◽  
Model Calculations ◽  
Single Observation ◽  
Tropical Regions ◽  
Air Masses ◽  
Clean Air

Abstract. Carbon monoxide and acetone measurements are presented for five aircraft measurement campaigns at mid-latitudes, polar and tropical regions in the northern hemisphere. Throughout all campaigns, free tropospheric air masses, which were influenced by anthropogenic emissions, showed a similar linear relation between CO and acetone, with a slope of 21–25 pptv acetone/ppbv CO. Measurements in the anthropogenically influenced marine boundary layer revealed a slope of 13–16 pptv acetone/ppbv CO. The different slopes observed in the marine boundary layer and the free troposphere indicate that acetone is emitted by the ocean in relatively clean air masses and taken up by the ocean in polluted air masses. In the lowermost stratosphere, a good correlation between CO and acetone was observed as well, however, with a much smaller slope (~5 pptv acetone/ppbv CO) compared to the troposphere. This is caused by the longer photochemical lifetime of CO compared to acetone in the lower stratosphere, due to the increasing photolytic loss of acetone and the decreasing OH concentration with altitude. No significant correlation between CO and acetone was observed over the tropical rain forest due to the large direct and indirect biogenic emissions of acetone. The common slopes of the linear acetone-CO relation in various layers of the atmosphere, during five field experiments, makes them useful for model calculations. Often a single observation of the CO-acetone correlation, determined from stratospheric measurements, has been used in box model applications. This study shows that different slopes have to be considered for marine boundary layer, free tropospheric and stratospheric air masses, and that the CO-acetone relation cannot be used for air masses which are strongly influenced by biogenic emissions.

scholarly journals Contrasting characteristics of open- and closed-cellular stratocumulus cloud in the eastern North Atlantic

2021 ◽  
Vol 21 (19) ◽  
pp. 14557-14571
Author(s):  
Michael P. Jensen ◽  
Virendra P. Ghate ◽  
Dié Wang ◽  
Diana K. Apoznanski ◽  
Mary J. Bartholomew ◽  
...  
Keyword(s):  
Boundary Layer ◽  
North Atlantic ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Open Cell ◽  
Cold Air ◽  
Closed Cell ◽  
Boundary Layer Cloud ◽  
Eastern North Atlantic ◽  
Layer Cloud

Abstract. Extensive regions of marine boundary layer cloud impact the radiative balance through their significant shortwave albedo while having little impact on outgoing longwave radiation. Despite this importance, these cloud systems remain poorly represented in large-scale models due to difficulty in representing the processes that drive their life cycle and coverage. In particular, the mesoscale organization and cellular structure of marine boundary clouds have important implications for the subsequent cloud feedbacks. In this study, we use long-term (2013–2018) observations from the Atmospheric Radiation Measurement (ARM) Facility's Eastern North Atlantic (ENA) site on Graciosa Island, Azores, Portugal, to identify cloud cases with open- or closed-cellular organization. More than 500 h of each organization type are identified. The ARM observations are combined with reanalysis and satellite products to quantify the cloud, precipitation, aerosol, thermodynamic, and large-scale synoptic characteristics associated with these cloud types. Our analysis shows that both cloud organization populations occur during similar sea surface temperature conditions, but the open-cell cases are distinguished by stronger cold-air advection and large-scale subsidence compared to the closed-cell cases, consistent with their formation during cold-air outbreaks. We also find that the open-cell cases were associated with deeper boundary layers, stronger low-level winds, and higher rain rates compared to their closed-cell counterparts. Finally, raindrops with diameters larger than 1 mm were routinely recorded at the surface during both populations, with a higher number of large drops during the open-cellular cases. The similarities and differences noted herein provide important insights into the environmental and cloud characteristics during varying marine boundary layer cloud mesoscale organization and will be useful for the evaluation of model simulations for ENA marine clouds.

Spatial Variability of Liquid Water Path in Marine Low Cloud: The Importance of Mesoscale Cellular Convection

2006 ◽  
Vol 19 (9) ◽  
pp. 1748-1764 ◽  
Author(s):  
Robert Wood ◽  
Dennis L. Hartmann
Keyword(s):  
Spatial Variability ◽  
Liquid Water ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Liquid Water Path ◽  
Water Path ◽  
Geographical Regions ◽  
Low Cloud ◽  
Moderate Resolution Imaging Spectroradiometer ◽  
Subtropical Oceans

Abstract Liquid water path (LWP) mesoscale spatial variability in marine low cloud over the eastern subtropical oceans is examined using two months of daytime retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Terra satellite. Approximately 20 000 scenes of size 256 km × 256 km are used in the analysis. It is found that cloud fraction is strongly linked with the LWP variability in the cloudy fraction of the scene. It is shown here that in most cases LWP spatial variance is dominated by horizontal scales of 10–50 km, and increases as the variance-containing scale increases, indicating the importance of organized mesoscale cellular convection (MCC). A neural network technique is used to classify MODIS scenes by the spatial variability type (no MCC, closed MCC, open MCC, cellular but disorganized). It is shown how the different types tend to occupy distinct geographical regions and different physical regimes within the subtropics, although the results suggest considerable overlap of the large-scale meteorological conditions associated with each scene type. It is demonstrated that both the frequency of occurrence, and the variance-containing horizontal scale of the MCC increases as the marine boundary layer (MBL) depth increases. However, for the deepest MBLs, the MCC tends to be replaced by clouds containing cells but lacking organization. In regions where MCC is prevalent, a lack of sensitivity of the MCC type (open or closed) to the large-scale meteorology was found, suggesting a mechanism internal to the MBL may be important in determining MCC type. The results indicate that knowledge of the physics of MCC will be required to completely understand and predict low cloud coverage and variability in the subtropics.

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