An analysis of aeolian dust in climate models Evan, Amato T.; Flamant, Cyrille; Fiedler, Stephanie ...
Geophysical research letters,
28 August 2014, Letnik:
41, Številka:
16
Journal Article
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Aeolian dust is a key aspect of the climate system. Dust can modify the Earth's energy budget, provide long‐range transport of nutrients, and influence land surface processes via erosion. ...Consequently, effective modeling of the climate system, particularly at regional scales, requires a reasonably accurate representation of dust emission, transport, and deposition. Here we evaluate African dust in 23 state‐of‐the‐art global climate models used in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. We find that all models fail to reproduce basic aspects of dust emission and transport over the second half of the twentieth century. The models systematically underestimate dust emission, transport, and optical depth, and year‐to‐year changes in these properties bear little resemblance to observations. These findings cast doubt on the ability of these models to simulate the regional climate and the response of African dust to future climate change.
Key Points
CMIP5 models underestimate African dust emission and transportThe dust size distribution is biased toward small particles in CMIP5 modelsCMIP5 models do not represent coupled processes that involve African dust
Understanding the West African monsoon (WAM) dynamics in the mid-Holocene (MH) is a crucial issue in climate modeling, because numerical models typically fail to reproduce the extensive precipitation ...suggested by proxy evidence. This discrepancy may be largely due to the assumption of both unrealistic land surface cover and atmospheric aerosol concentration. In this study, the MH environment is simulated in numerical experiments by imposing extensive vegetation over the Sahara and the consequent reduction in airborne dust concentration. A dramatic increase in precipitation is simulated across the whole of West Africa, up to the Mediterranean coast. This precipitation response is in better agreement with proxy data, in comparison with the case in which only changes in orbital forcing are considered. Results show a substantial modification of the monsoonal circulation, characterized by an intensification of large-scale deep convection through the entire Sahara, and a weakening and northward shift (∼6.5°) of the African easterly jet. The greening of the Sahara also leads to a substantial reduction in the African easterly wave activity and associated precipitation. The reorganization of the regional atmospheric circulation is driven by the vegetation effect on radiative forcing and associated heat fluxes, with the reduction in dust concentration to enhance this response. The results for the WAM in the MH present important implications for understanding future climate scenarios in the region and in teleconnected areas, in the context of projected wetter conditions in West Africa.
Simulations are performed for the period 2000–2015 by two different regional climate models, ALADIN and RegCM, to quantify the direct and semi-direct radiative effects of biomass-burning aerosols ...(BBAs) in the southeast Atlantic (SEA) region. Different simulations have been performed using strongly absorbing BBAs in accordance with recent in situ observations over the SEA. For the July–August–September (JAS) season, the single scattering albedo (SSA) and total aerosol optical depth (AOD) simulated by the ALADIN and RegCM models are consistent with the MACv2 climatology and MERRA-2 and CAMS-RA reanalyses near the biomass-burning emission sources. However, the above-cloud AOD is slightly underestimated compared to satellite (MODIS and POLDER) data during the transport over the SEA. The direct radiative effect exerted at the continental and oceanic surfaces by BBAs is significant in both models and the radiative effects at the top of the atmosphere indicate a remarkable regional contrast over SEA (in all-sky conditions), with a cooling (warming) north (south) of 10 ∘S, which is in agreement with the recent MACv2 climatology. In addition, the two models indicate that BBAs are responsible for an important shortwave radiative heating of ∼0.5–1 K per day over SEA during JAS with maxima between 2 and 4 km a.m.s.l. (above mean sea level). At these altitudes, BBAs increase air temperature by ∼0.2–0.5 K, with the highest values being co-located with low stratocumulus clouds. Vertical changes in air temperature limit the subsidence of air mass over SEA, creating a cyclonic anomaly. The opposite effect is simulated over the continent due to the increase in lower troposphere stability. The BBA semi-direct effect on the lower troposphere circulation is found to be consistent between the two models. Changes in the cloud fraction are moderate in response to the presence of smoke, and the models differ over the Gulf of Guinea. Finally, the results indicate an important sensitivity of the direct and semi-direct effects to the absorbing properties of BBAs. Over the stratocumulus (Sc) region, DRE varies from +0.94 W m−2 (scattering BBAs) to +3.93 W m−2 (most absorbing BBAs).
Climate variability associated with the West African monsoon (WAM) has important environmental and socio-economic impacts in the region. However, state-of-the-art climate models still struggle in ...producing reliable climate predictions. An important cause of this low predictive skill is the sensitivity of climate models to different forcings. In this study, the mechanisms linking the WAM dynamics to the CO
2
forcing are investigated, by comparing the effect of the CO
2
direct radiative effect with its indirect effect mediated by the global sea surface warming. The July-to-September WAM variability is studied in climate simulations extracted from the Coupled Model Intercomparison Project Phase 5 archive, driven by prescribed sea surface temperature (SST). The individual roles of global SST warming and CO
2
atmospheric concentration increase are investigated through idealized experiments simulating a 4 K warmer SST and a quadrupled CO
2
concentration, respectively. Results show opposite and competing responses in the WAM dynamics and precipitation. A dry response (−0.6 mm/day) to the SST warming is simulated in the Sahel, with dryer conditions over western Sahel (−0.8 mm/day). Conversely, the CO
2
increase produces wet conditions (+0.5 mm/day) in the Sahel, with the strongest response over central-eastern Sahel (+0.7 mm/day). The associated responses in the atmospheric dynamics are also analysed, showing that the SST warming affects the Sahelian precipitation through modifications in the global tropical atmospheric dynamics, reducing the importance of the regional drivers, while the CO
2
increase reinforces the coupling between precipitation and regional dynamics. A general agreement in model responses demonstrates the robustness of the identified mechanisms linking the WAM dynamics to the CO
2
direct and indirect forcing, and indicates that these primary mechanisms are captured by climate models. Results also suggest that the spread in future projections may be caused by unbalanced model responses to the CO
2
direct and indirect forcing.
Trade-wind cumuli constitute the cloud type with the highest frequency of occurrence on Earth, and it has been shown that their sensitivity to changing environmental conditions will critically ...influence the magnitude and pace of future global warming. Research over the last decade has pointed out the importance of the interplay between clouds, convection and circulation in controling this sensitivity. Numerical models represent this interplay in diverse ways, which translates into different responses of trade-cumuli to climate perturbations. Climate models predict that the area covered by shallow cumuli at cloud base is very sensitive to changes in environmental conditions, while process models suggest the opposite. To understand and resolve this contradiction, we propose to organize a field campaign aimed at quantifying the physical properties of trade-cumuli (e.g., cloud fraction and water content) as a function of the large-scale environment. Beyond a better understanding of clouds-circulation coupling processes, the campaign will provide a reference data set that may be used as a benchmark for advancing the modelling and the satellite remote sensing of clouds and circulation. It will also be an opportunity for complementary investigations such as evaluating model convective parameterizations or studying the role of ocean mesoscale eddies in air–sea interactions and convective organization.
We present a new characterisation of the three‐dimensional (3D) distribution of dust over the Sahara during summer, exemplified for June 2011. Our approach, called AEROIASI, is based on the ...innovative retrieval of vertical profiles of the dust extinction coefficient from daily cloud‐free hyperspectral Infrared Atmospheric Sounder Interferometer (IASI) satellite observations. AEROIASI observations clearly agree with other widely used measurements (from lidar and radiometers). The 3D characterisation is focused on the dust maximum in June 2011, located in the central Sahara (17–23°N, 1–7°E) and linked to the major atmospheric dynamical drivers associated with the West African Monsoon (WAM) system. AEROIASI shows the near‐surface dust load to be dominated by five major emission events occurring every 3–4 days. These all occur when the study region is under the influence of northward bursts of the WAM and convection‐related cold pools, likely associated with orographic forcing by the Aïr Mountains. During the earliest (June 10) and the dustiest (June 17) cases, northward advection of moisture over the hotspot is favoured by the superposition of cyclonic circulations related to an extratropical disturbance northwest of the Sahara and to the Saharan heat low over Mauritania, respectively. Convection over the hotspot also triggers wave‐like disturbances that travel westwards. The three dustiest events are characterised by elongated dust fronts moving northwards, with a leading edge spanning 200–300 km horizontally and extending from the surface up to 2 km of altitude. Further south, the dust layer progressively elevates to 3.5 km along the slanted isentropes at the interface of the monsoon and the harmattan, increasingly losing contact with the ground. When northerlies blow over the study region, elevated dust layers at 3–5 km are observed, which are transported southwards within the Saharan air layer and westwards along the northern edge of the African easterly jet (after June 13).
We present a new characterisation of the three‐dimensional (3D) distribution of dust over the Sahara during summer, using innovative observations of dust vertical profiles derived from hyperspectral satellite measurements of the Infrared Atmospheric Sounder Interferometer. The 3D characterisation is focused on the dust maximum in June, located in the central Sahara and linked to atmospheric dynamical drivers associated with the West African monsoon system. We show the first observational characterisation of the 3D structure of dust plumes associated with convection‐related cold pools.
The present study examines the impact of the environmental moisture structure
in the lower troposphere (below 2 km above sea level, a.s.l.) on the
precipitation development, observed in southern ...France during Intensive
Observation Period (IOP) 13 of the first Special Observation Period of the
Hydrological cycle in the Mediterranean Experiment (HyMeX SOP-1), through a
series of sensitivity experiments using the non-hydrostatic mesoscale
atmospheric numerical model (Meso-NH). The control simulation (CNTL) and all
the other 12 sensitivity experiments examined in this study succeed in
reproducing a heavy precipitation event (HPE) in the coastal mountainous
region of Var in south-eastern France as observed. The sensitivity
experiments are designed to investigate the response of the HPE to the
variability of the water vapour content upstream in the moist marine
atmospheric boundary layer (MABL) and the drier air above. The comparisons
between CNTL and the 12 sensitivity experiments show how the life cycle of
precipitation associated with the HPE, but also the upstream flow (over the
sea), is modified, even for moisture content changes of only 1 g kg−1
below 2 km a.s.l. Within the low-level wind convergence between southerlies
and south-westerlies, a small increase of moisture content in the MABL
prolongs moderate precipitation (≥5 mm in 15 min) and enlarges the
area of weak precipitation (≥1 mm in 15 min). The moistening in the
1–2 km a.s.l. layer, just above the MABL, prolongs the duration of
moderate precipitation, for a similar total precipitation amount as in CNTL.
The drier MABL and 1–2 km a.s.l. layer shorten the lifetime of
precipitation and reduce the total precipitation amount with respect to CNTL.
We also found that the moisture in the MABL has a stronger impact on
producing enhanced precipitation (both in terms of amount and intensity) than
the moisture just above (1–2 km a.s.l.). Also, it is worth noting that
adding moisture in the MABL does not necessarily lead to enhanced
precipitation amount. In moistening the MABL, the duration of moderate
precipitation increases with increasing moisture as does the area covered by
weak precipitation, while the area covered by the intense precipitation (≥30 mm) decreases. Despite a simplified moisture-profile modification
approach, this study suggests that moisture structure in the lower
troposphere is key for accurate prediction at short-term range of the timing
and location of precipitation in the coastal mountainous region in southern
France.
Dynamical processes leading to dust emission over Iran and surrounding countries in the summer as well as the subsequent transport of dust toward northwest Iran are analyzed on the basis of two case ...studies using a suite of ground‐based and spaceborne remote sensing platforms together with modeling tools. Ground‐based lidar measurements acquired in Zanjan provide new insight into the vertical distribution of dust linked to transport over northwest Iran and highlight the importance of low‐level transport of dust from both Iraq and Iran for air quality issues in Tehran. During the 3–5 August 2007 case, dust emission regions are located in Syria/Iraq and close to Qom, Iran, in a large intermittent salt lake in the western part of the Dasht‐e Kavir desert. The visibility in Tehran associated with this event decreases significantly (reaching 7 km) on 5 August 2007 only. During the 11–13 September 2008 case, the dust transported to northwest Iran originates from Syria/Iraq only. The visibility in Tehran during this case is low throughout the period, sometimes less than 5 km due to the transport of dust at low levels. In both cases, emissions in Syria and Iraq occur in response to strong Shamal winds. However, transport of dust toward Iran takes place at different levels: above 700 hPa in August and below 700 hPa in September. This is found to be related to the presence of strong northeasterly winds over the Zagros Mountains as well as in its lee (south of the range) in the August case only. In August also, dust emissions in the Qom region results from strong winds blowing over the Dasht‐e Kavir desert.
We describe the daily evolution of the three-dimensional (3D) structure of a major dust outbreak initiated by an extratropical cyclone over East Asia in early March 2008, using new aerosol retrievals ...derived from satellite observations of IASI (Infrared Atmospheric Sounding Interferometer). A novel auto-adaptive Tikhonov-Phillips-type approach called AEROIASI is used to retrieve vertical profiles of dust extinction coefficient at 10 microns for most cloud-free IASI pixels, both over land and ocean. The dust vertical distribution derived from AEROIASI is shown to agree remarkably well with along-track transects of Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) spaceborne lidar vertical profiles (mean biases less than 110 meters, correlation of 0.95, and precision of 260 meters for mean altitudes of the dust layers). AEROIASI allows the daily characterization of the 3D transport pathways across East Asia of two dust plumes originating from the Gobi and North Chinese deserts. From AEROIASI retrievals, we provide evidence that (i) both dust plumes are transported over the Beijing region and the Yellow Sea as elevated layers above a shallow boundary layer, (ii) as they progress eastward, the dust layers are lifted up by the ascending motions near the core of the extratropical cyclone, and (iii) when being transported over the warm waters of the Japan Sea, turbulent mixing in the deep marine boundary layer leads to high dust concentrations down to the surface. AEROIASI observations and model simulations also show that the progression of the dust plumes across East Asia is tightly related to the advancing cold front of the extratropical cyclone.
Changes in the emission, transport and deposition of aeolian dust have profound effects on regional climate, so that characterizing the lifecycle of dust in observations and improving the ...representation of dust in global climate models is necessary. A fundamental aspect of characterizing the dust cycle is quantifying surface dust fluxes, yet no spatially explicit estimates of this flux exist for the World’s major source regions. Here we present a novel technique for creating a map of the annual mean emitted dust flux for North Africa based on retrievals of dust storm frequency from the Meteosat Second Generation Spinning Enhanced Visible and InfraRed Imager (SEVIRI) and the relationship between dust storm frequency and emitted mass flux derived from the output of five models that simulate dust. Our results suggest that 64 (±16)% of all dust emitted from North Africa is from the Bodélé depression, and that 13 (±3)% of the North African dust flux is from a depression lying in the lee of the Aïr and Hoggar Mountains, making this area the second most important region of emission within North Africa.
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