148 research outputs found
A process-based evaluation of dust-emitting winds in the CMIP5 simulation of HadGEM2-ES
Despite the importance of dust aerosol in the Earth system, state-of-the-art models show a large variety for North African dust emission. This study presents a systematic evaluation of dust emitting-winds in 30 years of the historical model simulation with the UK Met Office Earth-system model HadGEM2-ES for the Coupled Model Intercomparison Project Phase 5. Isolating the effect of winds on dust emission and using an automated detection for nocturnal low-level jets (NLLJs) allow an in-depth evaluation of the model performance for dust emission from a meteorological perspective. The findings highlight that NLLJs are a key driver for dust emission in HadGEM2-ES in terms of occurrence frequency and strength. The annually and spatially averaged occurrence frequency of NLLJs is similar in HadGEM2-ES and ERA-Interim from the European Centre for Medium-Range Weather Forecasts. Compared to ERA-Interim, a stronger pressure ridge over northern Africa in winter and the southward displaced heat low in summer result in differences in location and strength of NLLJs. Particularly the larger geostrophic winds associated with the stronger ridge have a strengthening effect on NLLJs over parts of West Africa in winter. Stronger NLLJs in summer may rather result from an artificially increased mixing coefficient under stable stratification that is weaker in HadGEM2-ES. NLLJs in the BodĂŠlĂŠ Depression are affected by stronger synoptic-scale pressure gradients in HadGEM2-ES. Wintertime geostrophic winds can even be so strong that the associated vertical wind shear prevents the formation of NLLJs. These results call for further model improvements in the synoptic-scale dynamics and the physical parametrization of the nocturnal stable boundary layer to better represent dust-emitting processes in the atmospheric model. The new approach could be used for identifying systematic behavior in other models with respect to meteorological processes for dust emission. This would help to improve dust emission simulations and contribute to decreasing the currently large uncertainty in climate change projections with respect to dust aerosol
Idealized large-eddy simulations of nocturnal low-level jets over subtropical desert regions and implications for dust-generating winds
Model simulations of complex dust emissions over the Sahara during the West African monsoon onset
The existing limitations in ground-based observations in remote areas in West Africa determine the dependence on numerical models to represent the atmospheric mechanisms that contribute to dust outbreaks at different space-time scales. In this work, the ability of the Weather Research and Forecasting model coupled with the Chemistry (WRF-Chem) model using the GOCART dust scheme is evaluated. The period comprises the West African Monsoon onset phase (the 7th to 12th of June, 2006) coinciding with the AMMA Special Observing Period (SOP). Different features in the horizontal and vertical dynamical structure of the Saharan atmosphere are analyzed with a combination of satellite and ground-based observations and model experimentation at 10 and 30âkm model resolution. The main features of key Saharan dust processes during summer are identifiable, and WRF-CHEM replicates these adequately. Observations and model analyses have shown that cold pools (haboobs) contributed a substantial proportion of total dust during the study period. The comparative analysis between observations and WRF-Chem simulations demonstrates the model efficiency to simulate the spatial and 3D structure of dust transport over the Sahara and Sahel. There is, therefore, a strong basis for accurate forecasting of dust events associated with synoptic scale events when model dust emission parameterization is suitably calibrated
Idealized large-eddy simulations of nocturnal low-level jets over subtropical desert regions and implications for dust-generating winds
Nocturnal low-level jets (LLJs) are maxima in the wind profile, which often form above the stable nocturnal boundary layer. Over the Sahara, the world's largest source of mineral dust, this phenomenon is of particular importance to the emission and transport of desert aerosol. We present the first ever detailed large-eddy simulations of dust-generating LLJs. Using sensitivity studies with the UK Met Office large-eddy model (LEM), two key controls of the nocturnal LLJ are investigated: surface roughness and the Coriolis force. Functional relationships derived from the LEM results help to identify optimal latitude-roughness configurations for a maximum LLJ enhancement. Ideal conditions are found in regions between 20 and 27°N with roughness lengths >0.0001 m providing long oscillation periods and large jet amplitudes. Typical LLJ enhancements reach up to 3.5 m s-1 for geostrophic winds of 10 m s-1. The findings are largely consistent with results from a theoretical LLJ model applied for comparison. The results demonstrate the importance of latitude and roughness in creating regional patterns of LLJ influence. Combining the functional relationships with high-resolution roughness data over northern Africa gives good agreement with the location of morning dust uplift in satellite observations. It is shown that shear-induced mixing plays an important role for the LLJ evolution and surface gustiness. With decreasing latitude the LLJ oscillation period is longer and, thus, shear-induced mixing is weaker, allowing a more stable nocturnal stratification to develop. This causes a later and more abrupt LLJ breakdown in the morning with stronger gusts, which can compensate for the slower LLJ evolution that leads to a weaker jet maximum. The findings presented here can serve as the first step towards a parametrization to improve the representation of the effects of nocturnal LLJs on dust emission in coarser-resolution models.European Research Counci
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Idealized large-eddy simulations of nocturnal low-level jets over subtropical desert regions and implications for dust-generating winds
Nocturnal low-level jets (LLJs) are maxima in the wind profile, which often form above
the stable nocturnal boundary layer. Over the Sahara, the worldâs largest source of mineral
dust, this phenomenon is of particular importance to the emission and transport of
desert aerosol.We present the first ever detailed large-eddy simulations of dust-generating
LLJs. Using sensitivity studies with the UK Met Office large-eddy model (LEM), two
key controls of the nocturnal LLJ are investigated: surface roughness and the Coriolis
force. Functional relationships derived from the LEM results help to identify optimal
latitudeâroughness configurations for a maximum LLJ enhancement. Ideal conditions are
found in regions between 20 and 27âŚN with roughness lengths >0.0001m providing long
oscillation periods and large jet amplitudes. Typical LLJ enhancements reach up to 3.5msâ1
for geostrophic winds of 10msâ1. The findings are largely consistent with results from
a theoretical LLJ model applied for comparison. The results demonstrate the importance
of latitude and roughness in creating regional patterns of LLJ influence. Combining the
functional relationships with high-resolution roughness data over northern Africa gives
good agreement with the location of morning dust uplift in satellite observations. It is
shown that shear-induced mixing plays an important role for the LLJ evolution and surface
gustiness. With decreasing latitude the LLJ oscillation period is longer and, thus, shearinduced
mixing is weaker, allowing a more stable nocturnal stratification to develop. This
causes a later and more abrupt LLJ breakdown in the morning with stronger gusts, which
can compensate for the slower LLJ evolution that leads to a weaker jet maximum. The
findings presented here can serve as the first step towards a parametrization to improve the
representationof the effectsofnocturnal LLJsondust emission in coarser-resolution models
Harmattan, Saharan heat low, and West African monsoon circulation: modulations on the Saharan dust outflow towards the North Atlantic
The outflow of dust from the northern African continent towards the North Atlantic is stimulated by the atmospheric circulation over North Africa, which modulates the spatio-temporal distribution of dust source activation and consequently the entrainment of mineral dust into the boundary layer, as well as the transport of dust out of the source regions. The atmospheric circulation over the North African dust source regions, predominantly the Sahara and the Sahel, is characterized by three major circulation regimes: (1) the harmattan (trade winds), (2) the Saharan heat low (SHL), and (3) the West African monsoon circulation. The strength of the individual regimes controls the Saharan dust outflow by affecting the spatio-temporal distribution of dust emission, transport pathways, and deposition fluxes.
This study aims at investigating the atmospheric circulation pattern over North Africa with regard to its role favouring dust emission and dust export towards the tropical North Atlantic. The focus of the study is on summer 2013 (June to August), during which the SALTRACE (Saharan Aerosol Long-range TRansport and Aerosol-Cloud interaction Experiment) field campaign also took place. It involves satellite observations by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) flying on board the geostationary Meteosat Second Generation (MSG) satellite, which are analysed and used to infer a data set of active dust sources. The spatio-temporal distribution of dust source activation frequencies (DSAFs) allows for linking the diurnal cycle of dust source activations to dominant meteorological controls on dust emission. In summer, Saharan dust source activations clearly differ from dust source activations over the Sahel regarding the time of day when dust emission begins. The Sahara is dominated by morning dust source activations predominantly driven by the breakdown of the nocturnal low-level jet. In contrast, dust source activations in the Sahel are predominantly activated during the second half of the day, when downdrafts associated with deep moist convection are the major atmospheric driver. Complementary to the satellite-based analysis on dust source activations and implications from their diurnal cycle, simulations on atmosphere and dust life cycle were performed using the mesoscale atmosphereâdust model system COSMO-MUSCAT (COSMO: COnsortium for Small-scale MOdelling; MUSCAT: MUltiScale Chemistry Aerosol Transport Model). Fields from this simulation were analysed regarding the variability of the harmattan, the Saharan heat low, and the monsoon circulation as well as their impact on the variability of the Saharan dust outflow towards the North Atlantic. This study illustrates the complexity of the interaction among the three major circulation regimes and their modulation of the North African dust outflow. Enhanced westward dust fluxes frequently appear following a phase characterized by a deep SHL. Ultimately, findings from this study contribute to the quantification of the interannual variability of the atmospheric dust burden
A model-based analysis of meteorological processes important for North African dust emission
Dust aerosol is abundant and important in the Earth system due to its influence on the radiation balance, the hydrological cycle, human health and ecosystems. Reducing the currently large uncertainties in dust effects requires realistic simulations of the lifecycle of dust aerosol which depends on the time, location and amount of dust emission. Emission estimates from models show a large variety, the reduction of which requires a systematic evaluation of dust-emitting winds. Different processes are known, but their relative importance was previously poorly quantified. This work investigates dust-emitting winds in North Africa based on single meteorological processes which helps guiding future
model development. Based on 32 years of ERA-Interim data and a dust model, the emission amounts associated with nocturnal low-level jets (NLLJs), atmospheric depressions and mobile, long-lived cyclones are estimated climatologically for the first time. The results highlight NLLJs as an important driver for dust emission, particularly in the BodĂŠlĂŠ Depression during winter.
Associated maxima in mid-morning emission underline the importance of temporally high-resolved winds for dust modelling. ERA-Interim systematically underestimates NLLJ core wind speeds, likely due to artificially increased mixing in stable boundary layers. Derived emission
frequencies over the BodĂŠlĂŠ Depression agree well with observations, but differ elsewhere.Atmospheric depressions, often in the form of heat lows and lee depressions, occur frequently and coincide with the majority of dust emission. Few depressions develop into mobile and long-lived cyclones which coincide with particularly intense events. The largest emission fractions associated with cyclones are found in northeast Africa during spring, primarily at day with a small emission reduction by soil moisture. Smaller West African areas show similar fractions, likely associated with nearsurface signatures of African Easterly Waves. Comparing results derived from ERA-Interim against the Earth system model of the UK Met Office shows considerable disagreement in NLLJ core wind speeds and dust emissions. In depth analysis underlines the urgency for model development that improves the synoptic-scale
conditions and the stable boundary layer. Such model improvements hold the potential to advance the scientific understanding of dust aerosol in the Earth syste
A parameterization of convective dust storms for models with mass-flux convective schemes
Cold pool outflows, generated by downdrafts from moist convection, can generate strong winds and therefore uplift of mineral dust. These so-called âhaboobâ convective dust storms occur over all major dust source areas worldwide and contribute substantially to emissions in northern Africa, the worldâs largest source. Most large-scale models lack convective dust storms, because they do not resolve moist convection, relying instead on convection schemes. We suggest a parameterization of convective dust storms to account for their contribution in such large-scale models. The parameterization is based on a simple conceptual model, in which the downdraft mass flux from the convection scheme spreads out radially in a cylindrical cold pool. The parameterization is tested with a set of Unified Model runs for June and July 2006 over West Africa. It is calibrated with a convection-permitting run, and applied to a convection-parameterized run. The parameterization successfully produces the extensive area of dust-generating winds from cold pool outflows over the southern Sahara. However, this area extends farther to the east and dust generating winds occur earlier in the day than in the convection-permitting run. These biases are due to biases in the convection scheme. It is found that the location and timing of dust-generating winds are weakly sensitive to the parameters of the conceptual model. The results demonstrate that a simple parameterization has the potential to correct a major and long-standing limitation in global dust models
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