26 research outputs found
Jack Shepperd /
No full textRunning title: The life and adventures of Jack Shepperd.Published in 30 numbers.Paper covers identify publisher as Foster and Hextall; printer as Elliot.Obediah Throttle is the pseud. of an author writing a parody of William Harrison Ainsworth's Jack Sheppard.Caption title: The life and surprising adventures of Jack Shepperd."Printed and published by James Caton, at the Office, 268 Strand--Price and one penny" and "Printed and published by James Caton, at the Office, 11, Catharine-Street, Strand, in weekly numbers-price one penny" on inside margins of parts.Wolff, R.L. 19th cent. fiction,Mode of access: Internet
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
A 14 year climatology of Saharan dust emission mechanisms inferred from automatically tracked plumes
No full text The central and western Sahara (CWS) is the largest source of mineral aerosols during boreal summer, but observed ground‐based data are extremely scarce and typically distant from key source regions. Knowledge of dust emission mechanisms has therefore been mostly limited to short‐term observations from a point or model approximations. To address this deficiency, dust plumes from the CWS are classified according to emission mechanism for June, July and August of 2004‐2017 using an automated inference method which accurately tracks the timing, convective association and geometry of plumes observed with the Spinning Enhanced Visible and Infrared Imager (SEVIRI) aboard Meteosat Second Generation satellites. From these characteristics, plumes are classified as either low‐level jet or cold pool outflow events. The extensive data set is used to generate the largest available climatology of dust emission sources and Saharan emission mechanisms. Automated inference compares well with ground‐based measurements from the Fennec Campaign (76% accuracy) as well as with an entirely manual approach (88% accuracy). Cold pool activity accounts for 82% of total observed dust and 88% at the point of emission. Dust from cold pools evolves seasonally from hotspots around the Mali‐Niger‐Algeria border triple point towards the central Sahara to the northwest, while dust from low‐level jets is organised along the axis of the northeasterly Harmattan, and dominates emission within the Tidihelt Depression of central Algeria. The widespread importance of cold pool outflows in this research supports the findings of the Fennec Campaign, but low‐level jets remain highly significant in certain isolated hotspots. </p
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
An Update to the Central England Temperature Series—HadCET v2.1
No full textThe Central England Temperature (CET) series is one of the longest instrumental climate records in the world. The CET record from 1659 represents a roughly triangular area of England extending from the Lancashire plain in the north, to London in the south‐east and south‐west of the Midlands of England. HadCET is a composite series produced by the Met Office Hadley Centre, using data from a succession of observing sites for which the data have been adjusted to remove inhomogeneities to be consistent with the original long running series and be updated in near real time. This paper documents a technical update to the HadCET which is referred to as HadCET version 2 (v2), and at time of publication v2.1.0.0 is the latest available version
Wind‐Driven Coastal Polynya Variability Drives Decadal Ice‐Shelf Melt Variability in the Amundsen Sea
No full textAbstract The ice shelves in the Amundsen Sea are being melted rapidly by warm Circumpolar Deep Water (CDW), causing sea‐level rise. Ice‐shelf melt variability is controlled by the speed of a shelf‐break undercurrent which transports CDW onto the continental shelf. We study decadal variability of the undercurrent and ice‐shelf melting using new regional ice‐ocean model perturbation experiments. The perturbation experiments suggest that the undercurrent decadal variability is controlled by variable coastal sea‐ice freshwater fluxes, these driven by winds mechanically opening and closing coastal polynyas. With the perturbation experiments we also quantify a positive feedback mechanism between the undercurrent and ice‐shelf melting which is responsible for 25% of their decadal variability
Reanalysis representation of low-level winds in the Antarctic near-coastal region
No full textLow-level easterly winds encircling Antarctica help drive coastal currents which modify transport of circumpolar deep water to ice shelves, and the formation and distribution of sea ice. Reanalysis datasets are especially important at high southern latitudes where observations are few. Here, we investigate the representation of the mean state and short-term variability of coastal easterlies in three recent reanalyses, ERA5, MERRA-2 and JRA-55. Reanalysed winds are compared with summertime marine near-surface wind observations from the Advanced Scatterometer (ASCAT) and surface and upper air measurements from coastal stations. Reanalysis coastal easterlies correlate highly with ASCAT (r= 0.91, 0.89 and 0.85 for ERA5, MERRA-2 and JRA-55, respectively) but notable wind speed biases are found close to the coastal margins, especially near complex orography and at high wind speeds. To characterise short-term variability, 12-hourly reanalysis and coastal station winds are composited using self-organising maps (SOMs), which cluster timesteps under similar synoptic and mesoscale influences. Reanalysis performance is sensitive to the flow configuration at stations near steep coastal slopes, where they fail to capture the magnitude of near-surface wind speed variability when synoptic forcing is weak and conditions favour katabatic forcing. ERA5 exhibits the best overall performance, has more realistic orography, and a more realistic jet structure and temperature profile. These results demonstrate the regime behaviour of Antarctica's coastal winds and indicate important features of the coastal winds which are not well characterised by reanalysis datasets
An extensive investigation of the ability of the ICOLMDZ model to simulate a katabatic wind event in Antarctica
No full textKatabatic winds are a key feature of the climate of Antarctica, but despite decades of extensive studies, substantial biases remain in their representation in atmospheric models. However, it is often difficult to identify the origin of those biases amongst the model resolution, physical content, and large-scale forcings aspects. This study conducts an extensive investigation of the ability of the ICOLMDZ atmospheric model to simulate Antarctic katabatic winds by disentangling uncertainties associated with parameter calibration, from those associated with horizontal resolution as well as structural deficiencies in the model with a particular attention given to turbulent diffusion. We carefully select a katabatic-driven wind event in clear-sky conditions in Adélie Land, and perform perturbed parameter experiments at three different horizontal resolutions (10, 20 and 40 km). ICOLMDZ is able to reproduce wind observations, but the parametric uncertainty remains large and structural differences not associated with parameter calibration nor horizontal resolution are found for turbulence and near-surface temperature. A parametric analysis reveals that the most critical parameter controlling the magnitude of near-surface winds is roughness length, whereas near-surface temperatures are mainly controlled by snow near-infrared albedo. Sensitivity to horizontal resolution reveals that the 40 km configuration compares least favourably with the observations, and that the 10 and 20 km ensembles cannot be distinguished due to a too wide parametric uncertainty. We then discuss three aspects of katabatic winds modeling that we deem critical but underappreciated: the parameterization of roughness length over snow, oscillations in katabatic flows, and the representation of subgrid-scale orographic drag. This study underlines in particular the need for a more physical parameterization of roughness length to correctly represent near-surface wind along the slopes of Antarctica.</p
Sensitivity of near-surface marine winds and wind stress in coastal Antarctica to regional atmospheric model configuration
No full textNear-surface marine winds in coastal Antarctica have global importance, as they affect ocean circulation and sea-ice variability. We test the sensitivities of simulated near-surface winds and wind stress in coastal Antarctica to uncertain aspects of regional atmospheric model configuration. The UK Met Office Unified Model (MetUM) is run in a limited-area setup over 10 months, evenly split between austral summer and winter. Tests include varying horizontal grid spacing, stable boundary-layer representation, surface exchange of momentum, and subgrid orographic drag. We focus especially on winds over the Cape Darnley polynya, which is important for Antarctic Bottom Water formation. Output from the MetUM correlates well with low-level winds from sonde, station, and satellite observations, especially in the summer months. However, the shape of the vertical profile of wind speed depends strongly on model configuration. Enabling the subgrid orography scheme and enhancing the scale of subgrid mountains leads to major reductions in near-surface wind speed over the steep coastal slopes and over the Cape Darnley polynya, which we attribute to an upslope shift in the extent of katabatic flow. Near-surface winds and wind stress over near-shore ocean regions are also highly sensitive to the configuration of sea-ice roughness, which disproportionately affects strong winds, again leading to especially large impacts over Cape Darnley. Stable boundary-layer representation has a moderate impact on boundary-layer winds but a relatively small effect at the surface. Varying horizontal grid spacing between 4, 12, and 40 km has a relatively minor impact on winds, even though the steepness of the coastal terrain is greatly affected. The results underscore the sensitivity of Antarctic marine coastal winds to some highly model-dependent aspects of atmospheric model physics, with implications for simulation of atmosphere–ocean coupling, sea-ice dynamics, and Antarctic Bottom Water production
Dynamics of extreme wind events in the marine and terrestrial sectors of coastal Antarctica
No full textAntarctic coastal surface winds affect ice-sheet stability, sea ice, and local ecosystems. The strongest coastal winds are especially important due to the nonlinear relationship between wind speed and wind stress. We investigate the dynamics of extreme coastal winds using a simplified momentum budget calculated across the period 2010–2020 from the ERA5 reanalysis. The pressure-gradient forcing term in the budget is decomposed into a large-scale component and one associated with the temperature deficit layer. The role of budget terms across the coastal sector is compared for weak and strong winds. We then calculate composites of the top 100 easterly wind events across six east Antarctic coastal sectors, identifying terms responsible for the evolution of coastal extremes. A simple balance of terms exists offshore, dominated by large-scale forcing, contrasting with the complex balance in the onshore sector where katabatic forcing is large. Large-scale forcing explains 57% of offshore coastal wind-speed variance overall, improving to 81% when budget terms associated with the temperature deficit layer and horizontal advection are included, with significant regional variation. The residual term plays an increasingly active role as wind speed increases. Extremes in all coastal sectors are associated with a synoptic-scale transient dipole of pressure anomalies driving warm-air advection. Although katabatic forcing is a very large term in magnitude, it is found to play a passive role, declining as wind speeds increase during extreme conditions. In some regions, an anomalous southerly component develops during extremes, which we attribute to an ageostrophic barrier wind. This research underscores the major role for large-scale forcing in Antarctica's coastal winds, but also reveals a significant regional locally driven component. The results have implications for improving numerical model simulations of coastal easterlies and for studying their impacts on ocean circulation, sea ice, and ice-shelf basal melt
