34 research outputs found
Rainfall dynamics and change in East Africa
No full textEast Africa is vulnerable to changes in rainfall because of its reliance on agriculture, which supports livelihoods and drives economic development, combined with reduced adaptive capacity. Historical rainfall conditions have included an overall drying trend in the March to May season, called the long rains, as well as extreme flooding events. However, the mechanisms controlling rainfall variability in East Africa are not yet fully understood, and global climate models do not have adequate representation of present-day conditions. The models on average underestimate rainfall totals during the long rains and overestimate them during the short rains of October to November. Future projections from these models are also uncertain; the models disagree on the sign and the magnitude of changes, and the ensemble mean rainfall increase in the long rains is in opposition to recent observed drying. It is therefore difficult to plan for climate change adaptation, as neither a wetter nor a drier future can be confidently predicted. To address these issues, this thesis investigates the physical processes that affect East African rainfall on different spatial scales, and their representation in global climate models. The Indian Ocean Walker Circulation (IOWC) is an important large-scale feature which drives descending air over East Africa, and the strength of this descent is negatively correlated with rainfall totals in both rainy seasons (r = -0.57 - -0.85, p<0.05). It is also a source of model rainfall biases, as models with more (less) intense Walker Circulations produce less (more) rainfall over Kenya (r ~ 0.8, p<0.05). The Turkana Low-Level Jet (LLJ) is a meso-scale feature which affects regional rainfall through a variety of mechanisms. Its strength is negatively correlated with rainfall in Kenya, but positively correlated with rainfall in the continental interior (r = ± 0.6, p<0.1). Correlations between the LLJ strength and the large-scale zonal pressure gradient imply a link between the IOWC and the LLJ (r =± 0.5, p<0.1) The LLJ may be decreasing in strength on decadal timescales (-0.4 - -0.8 ms -1 decade-1, p<0.05), consistent with observed climate change, but reanalysis representations of its variability and annual cycle differ. It is not adequately simulated in models as a result of simplified topography representation, and thus further contributes to the uncertainty in projections; given that the IOWC is also constrained by topography, this is suggested as a key focus for future model development. Future projections of increasing rainfall over Kenya are associated with increasing ascent over East Africa (r ~-0.8, p<0.05) and enhanced easterly winds over the Indian Ocean (r~-0.5, p<0.05), reflecting a weakening of the IOWC. Using an assessment of the ability of models to represent the IOWC to constrain future projections, it is found that models with more realistic regional dynamics only feature significant increasing rainfall trends in April, November, and December. It is therefore concluded that the ensemble mean rainfall increase across the long rains may not be plausible, that the long rains may contract in future, and that rainfall increases in the short rains are more likely
Atmospheric controls on mineral dust emission from Southern African source areas and their representation in numerical models
No full textMineral aerosol (dust) is a critical component of the earth system, affecting climate processes and biogeochemical cycles; accurate simulation of dust in coupled models is therefore essential, so as to avoid uncertainties in the representation of current and future climate. Evaluation of dust in climate models shows biases in simulated emissions and concentrations, with models failing to capture the observed wind speed distribution in dust source areas. A number of atmospheric processes have been identified as critical to the uplift of dust from Saharan source areas, with their misrepresentation in models a likely source of the bias in the simulated surface wind speed distribution. Very little work has been conducted in southern Africa; this is despite the identification of major source areas using remotely sensed data. The research conducted as part of this thesis therefore aims to identify the atmospheric
processes driving dust emissions from southern Africa source areas, and evaluate their representation in numerical models.
High-resolution, in situ meteorological observations have been collected at the Etosha Pan, the most prominent dust source area in southern Africa. This data, used in conjunction with atmospheric reanalyses, has been used to characterise the meteorology of the source area across the two main dust seasons. Utilising a suite of satellite products, emissions from the pan (and southern Africa more generally) have been tracked across a range of temporal scales, which in turn has permitted an assessment of the atmospheric processes driving uplift. The in situ, reanalysis and satellite data also provide a constraint on model fields, and have been used to evaluate the representation of dust processes across southern Africa in the latest phase of coupled models.
Two mesoscale atmospheric processes have been shown to be especially important to dust emissions across southern Africa; namely, a nocturnal low level jet (NLLJ) during austral winter, and an integrated anabatic-sea breeze during austral summer. NLLJ’s are important emission mechanisms in North African source areas, however this is the first time the jet at Etosha has been linked to dust uplift. Thermally and topographically induced regional airflows have been observed along the west coast of southern Africa during austral summer, however this is the first time the sea breeze system has been shown to affect Etosha’s meteorology and hence dust emissions. The representation of these mechanisms in coupled models is mixed, and together with the treatment of land surface properties, helps to explain the inability of the models to capture the spatial distribution in uplift across southern Africa
Characteristics of Saharan dust emission mechanisms in boreal summer: a satellite and modelling approach
No full textMineral dust is one of the most abundant atmospheric aerosols and has a wide range of impacts on climate, meteorology, health and biogeochemistry. North African sources are responsible for about half of the total atmospheric burden, peaking as an emission source in the boreal summer months of June, July and August. In-situ data from the Fennec observation campaign of 2011 and 2012 indicates that cold pool outflows (CPOs) from convective downdrafts are the primary meteorological driver of these summertime emissions, followed by nocturnal low-level jet (LLJ) breakdown, both mesoscale phenomena which numerical models struggle to represent faithfully. Very few surface observations are available to characterise these emission mechanisms, however, and the extent to which Fennec observations are representative remains unclear. Satellite data from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) offers both the volume of data and resolution to observe emission mechanisms on the long term.
In this thesis, two sets of automated algorithms are applied to 14 years of summertime SEVIRI data, the first of which categorises automatically identified and tracked dust by emission mechanism and the second of which tracks and characterises dust-laden cold pool outflows. CPOs account for 82% of the total observed dust and 88% at the point of emission in the central and western Sahara during boreal summer, which is the highest estimate yet of their contribution. Whereas CPO dust is widespread and the majority of dust source regions are primarily CPO-driven, LLJs dominate a small number of hotspots such as the Tidihelt Depression. CPOs are far-reaching, with 22.5% travelling over 300 km. They also follow a clear diurnal cycle which favours emissions in the late afternoon and evening. Unlike dust emission, CPO frequency peaks in August in southern Algeria.
In the final component of the thesis, satellite observations are used to support a Met Office Unified Model experiment diagnosing the role of orography in driving dust-emitting LLJs over central Algeria. Fluvial drainage from mountains is thought to contribute erodible sediment to western Saharan dust sources, but their effect upon emission mechanisms there is untested. Removing the Hoggar mountains reduces LLJ emission frequency by approximately 30% as an elevated heating anomaly helps sustain the strong pressure gradient driving low-level winds across the region in summer.
This thesis offers a meteorological perspective on satellite dust source maps, showing the contribution of erosivity to known summertime emission hotspots on climatic timescales. The fact that most central and western Saharan dust sources are predominantly activated by CPOs poses a challenge for numerical modelling of the dust cycle given the inadequate representation of downdrafts in models with convective parameterization
Data for amt-2023-67
No full textThe uploaded files have been used in Sayedain et al. (2023)
Sayedain SA, O’Neill NT, King J, Hayes PL, Bellamy D, Washington R, Engelstaedter S, Vicente-Luis A, Bachelder J, Bernhard M. Analysis of Lhù’ààn Mân’(Kluane Lake) dust plumes using passive and active ground-based remote sensing supported by physical surface measurements. Atmospheric Measurement Techniques, Sept. 2023
An analysis of the role of the atmosphere in modulating desert dust variability : controls on emission and atmospheric transport
No full textEThOS - Electronic Theses Online ServiceGBUnited Kingdo
Atmospheric controls on the annual cycle of North African dust
No full text[1] Dust emitted from desert regions and transported in the atmosphere has been recognized for its potential to alter the Earth’s climate and environments. Satellite data show that the largest source regions of dust (i.e., hot spots) are located in dry, nonvegetated areas of West Africa and central Chad. Dust emissions from these sources follow a distinct seasonal cycle. Whereas our understanding of processes controlling the dust cycle of the Chad dust source has been improved through recent studies, our understanding of the West African sources is limited because of the remoteness of the sources and lack of surface observations. Using a satellite-derived dust index and reanalysis atmospheric fields, we show that the annual dust cycle at the West African dust hot spots is not related to changes in mean surface wind strength but is linked to small-scale high-wind events. We find that the annual dust cycle correlates well with changes in near-surface convergence associated with the annual north-south movement of the Inter-Tropical Convergence Zone (ITCZ). Dust emissions in West Africa are highest in June coinciding with the crossing of the convergence zone on its northward bound over the dust hot spots. The increase in convergence leads to enhanced surface gustiness suggesting that dry convection associated with an increase in the occurrence of small-scale high-wind events and vertical velocity are the main processes controlling the annual dust cycle at the West African dust sources
Aerosol concentration and meteorological data at Etosha Pan, Namibia (July 2015 - June 2016)
No full textAerosol concentration and meteorological data collected at five meteorological stations situated around Etosha Pan in Namibia during the year July 2015 to June 2016. Data are logged every 10 minutes. LiDAR data measured at Okaukuejo on 8/9/10 July 2015. Data support the publication Wiggs et al. 2022, Quantifying mechanisms of aeolian dust emission: field measurements at Etosha Pan, Namibia. Journal of Geophysical Research: Earth Surface.
Met and Dust Data.xlsx = Excel file detailing 10 minute measurements of aerosol concentration and meteorological data (2015-2016).
MSG3-SEVI-MSG15-0100-NA-*-NA_dust_250.tif - SEVIRI false colour composite data related to LiDAR data 8-10 July 2015
Processed_Wind_Profile_81_YYYYMMDD_L2.nc - LiDAR horizontal wind speed 8-10 July 2015
Stare_81_YYYYMMDD_L2.nc - LiDAR aerosol backscatter 8-10 July 201
Satellite‐Derived Characteristics of Saharan Cold Pool Outflows During Boreal Summer
No full textCold pool outflows (CPOs) are thought to be the most significant meteorological mechanism of mineral dust emission from the world's largest source in the central and western Sahara in boreal summer. An absence of CPOs from numerical models and reanalyses used to simulate Saharan dust emission leads to considerable error in modeling of dust fluxes from the Sahara. As such, the role of CPOs in the observed variability of dust through the monsoon season remains unclear. To remedy these issues, an improved observational benchmark is needed. In this research, an automated approach to identify and track CPOs in dust imagery from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) is derived. The approach is found to flag 74.2% of events identified manually (26/35). 1,559 events are tracked for June, July and August of 2004–2017. CPOs follow a clear diurnal cycle, peaking at 1700–1900 Universal Time Coordinated. Propagation speeds decay exponentially through their lifetime, but on average speeds are 1.5 ms−1 higher at night. About 22.5% of the observed events exceed a total traveled distance of 300 km, with an overwhelming preference for northwestwards propagation. Common across the southern central and western Sahara, CPO activity shifts north through summer in line with observed dust emission. The exception to this is the development of an intense hotspot of CPO activity in southern Algeria in August, which does not parallel any known late season outbreaks of dust. The results underline the importance of the southernmost Saharan dust sources, activated by frequent CPO occurrence in early summer
Resolving the Turkana Jet—Impact of Model Resolution in Simulating Channel Flow and Inversions
No full textThe Turkana Jet plays a pivotal role in the meteorology of East Africa across timescales, and owes its existence to both large‐scale dynamics and the representation of intricate local‐scale processes. However, much of our understanding of the jet relies on reanalysis, and these along with climate models that produce important projections do not represent these local‐scale processes. We systematically investigate the impact of changing model horizontal and vertical resolution in simulating the Turkana Jet, and associated local and large‐scale processes. We perform simulations to coincide with the Radiosonde Investigation For the Turkana Jet (RIFTJet) campaign, enabling direct model‐sonde comparisons in unprecedented detail. We find that increasing horizontal model resolution significantly increases the strength of the jet throughout the channel by up to 30%, while vertical resolution changes have little impact. Horizontal resolutions finer than 2.2 km produce a nocturnal jet ∼2 m/s stronger than observed but perform better during the day. The elevated inversion, which is strongly tied to the strength of the jet, is much better represented in resolutions as high as 1.1 km, whereas the global model at resolution O(∼10 km) is unable to produce any nocturnal elevated inversion. Predictions of jet strength are improved at higher resolution, indicating an important role of local process given that models inherit the same large‐scale state. Despite further improvements at resolutions finer than 4.4 km, we recommend that 4.4 km is the minimum horizontal resolution required to capture realistic interactions between these processes. Underestimation of the Turkana Jet could cause considerable errors in moisture advection into Africa
Representation of the Indian Ocean Walker Circulation in climate models and links to Kenyan rainfall
No full textReliable climate change projections over East Africa are vital because of regional vulnerability to precipitation changes. However, global climate models from Coupled Model Intercomparison Project Phase 5 (CMIP5) display significant biases in their representation of key East African rainfall seasons, which call into question the reliability of projected climate change. We investigate the links between models' representation of rainfall over Kenya during the long and short rains and the proximate Walker circulation. There is a strong correlation in the short rains between model biases in Kenyan rainfall and in the mid‐to‐upper tropospheric vertical velocity associated with this circulation. The overturning Indian Ocean Walker cell at the equator is absent in 5/25 models during the short rains – these models exhibit wet biases. In the long rains, dry biased models overestimate the strength of the descending limb of the circulation over East Africa. Omega biases over the Congo Basin are linked to broader Walker circulation biases. During the long rains, models overestimate equatorial descent more generally across the Western Hemisphere Tropics (0°E–200°E). A significant correlation is obtained across the model ensemble between model rainfall over Kenya and Western Hemisphere equatorial ascent during November. Atmosphere‐only models display some improvements over coupled models, but biases of a similar magnitude remain. We therefore propose Indian Ocean Walker circulation errors as a key source of bias in CMIP5 East African rainfall. The results add to recent work on CMIP5 biases in this region, demonstrating that the Indian Ocean Walker circulation should be a focus for future model improvement and a consideration when assessing the reliability of climate projections over East Africa. Further work is needed on the causes of Walker circulation biases (in particular the role of SST), and on understanding the impact of Walker circulation biases on modelled tropical rainfall elsewhere in the world
