6,291 research outputs found
Dynamics of Earth's Hadley Circulation
No full textThis thesis advances our understanding of the mechanisms controlling the Hadley circulation, and its interaction with eddies on planetary scales in particular. On Earth, and more generally in a rapidly rotating and differentially heated planet, planetary scale eddies in the extratropics interact with the mean flow in the tropics, contributing to the driving of the Hadley circulation. A hierarchy of numerical models is used to simulate and understand the relative importance of eddies in the driving of the Hadley circulation. In a global warming experiment, the Hadley circulation is found to strengthen in colder climates and weaken in warmer climates, with a maximum strength in a climate close to present-day Earth’s. This nonmonotonicity is shown to be consistent with variations in the eddy activity in the midlatitudes. The cells are also found to widen over the entire range of this climate change. A criterion quantifying the importance of baroclinic waves in setting the depth of the troposphere, which is modified to account for the effect of convective adjustment on planetary Rossby waves activity, is used to explain the shifts in the terminus of the Hadley circulation for a wide range of climate scenarios. Additionally, by comparing simulations with and without ocean heat transport, it is shown that accounting for low-latitude ocean heat transport and its coupling to wind stress is essential to obtain Hadley circulations in a dynamical regime resembling Earth’s. These changes in the strength and extent are found to be captured in a simple one-dimensional model that relies on standard assumptions about the thermodynamic properties of the atmosphere in the low-latitude regions and with a simple representation of eddy fluxes. Further work with this model, which may be amenable to analytical progress, could provide a quantitative understanding for the sensitivity of the Hadley circulation in comprehensive GCM simulations of 21st century global warming scenarios.</p
tourr: An R Package for Exploring Multivariate Data with Projections
No full textThis paper describes an R package which produces tours of multivariate data. The package includes functions for creating different types of tours, including grand, guided, and little tours, which project multivariate data (p-D) down to 1, 2, 3, or, more generally, d (⤠p) dimensions. The projected data can be rendered as densities or histograms, scatterplots, anaglyphs, glyphs, scatterplot matrices, parallel coordinate plots, time series or images, and viewed using an R graphics device, passed to GGobi, or saved to disk. A tour path can be stored for visualisation or replay. With this package it is possible to quickly experiment with different, and new, approaches to tours of data. This paper contains animations that can be viewed using the Adobe Acrobat PDF viewer.
Postcard Written by Harold M. Hadley to the Bryant College Service Club Dated January 3, 1943
No full text[Transcription begins]
Harold M. Hadley, Y3c Navy Department Washington, D. C. Arlington Annex
Bryant Service Club Cr. of Hope St., and Young Orchard Ave., Providence, R. I.
1 – 3 – 43
Dear Friends,
Your cigarettes came through in good condition, and on time.
It sure is swell to hear from the old school. The Alumni Bulletin came just a few days before the Christmas package. I couldn’t help but think of all the good times I had in the happiest two years of my life.
Many things to all the gang that has taken our place.
Sincerely,
Harold M. Hadley [Transcription ends
Gray, T.-R.-G. and Parkinson, D. (Editors). — The Ecology of Soil Bacteria. An International Symposium. Liverpool. University of Liverpool Press, 1968
No full textHadley Malcolm. Gray, T.-R.-G. and Parkinson, D. (Editors). — The Ecology of Soil Bacteria. An International Symposium. Liverpool. University of Liverpool Press, 1968. In: La Terre et La Vie, Revue d'Histoire naturelle, tome 23, n°3, 1969. pp. 366-367
Nearly Optimal Bounds for Sample-Based Testing and Learning of k-Monotone Functions
No full textWe study monotonicity testing of functions f : {0,1}^d → {0,1} using sample-based algorithms, which are only allowed to observe the value of f on points drawn independently from the uniform distribution. A classic result by Bshouty-Tamon (J. ACM 1996) proved that monotone functions can be learned with exp(Õ(min{(1/ε)√d,d})) samples and it is not hard to show that this bound extends to testing. Prior to our work the only lower bound for this problem was Ω(√{exp(d)/ε}) in the small ε parameter regime, when ε = O(d^{-3/2}), due to Goldreich-Goldwasser-Lehman-Ron-Samorodnitsky (Combinatorica 2000). Thus, the sample complexity of monotonicity testing was wide open for ε ≫ d^{-3/2}. We resolve this question, obtaining a nearly tight lower bound of exp(Ω(min{(1/ε)√d,d})) for all ε at most a sufficiently small constant. In fact, we prove a much more general result, showing that the sample complexity of k-monotonicity testing and learning for functions f : {0,1}^d → [r] is exp(Ω(min{(rk/ε)√d,d})). For testing with one-sided error we show that the sample complexity is exp(Ω(d)).
Beyond the hypercube, we prove nearly tight bounds (up to polylog factors of d,k,r,1/ε in the exponent) of exp(Θ̃(min{(rk/ε)√d,d})) on the sample complexity of testing and learning measurable k-monotone functions f : ℝ^d → [r] under product distributions. Our upper bound improves upon the previous bound of exp(Õ(min{(k/ε²)√d,d})) by Harms-Yoshida (ICALP 2022) for Boolean functions (r = 2)
Observed poleward expansion of the Hadley circulation since 1979
No full textUsing three meteorological reanalyses and three outgoing long-wave radiation (OLR) datasets, we show that the Hadley circulation has a significant expansion of about 2 to 4.5 degrees of latitude since 1979. The three reanalyses all indicate that the poleward expansion of the Hadley circulation in each hemisphere occurs during its summer and fall seasons. Results from the OLR datasets do not have such seasonality. The expansion of the Hadley circulation implies a poleward expansion of the band of subtropical subsidence, leading to enhanced mid-latitude tropospheric warming and poleward shifts of the subtropical dry zone. This would contribute to an increased frequency of midlatitude droughts in both hemispheres
GCM simulations of the Indian Ocean dipole influence on East African rainfall: present and future.
No full textSix coupled GCMs are assessed in terms of their ability to simulate observed characteristics of East African rainfall, the Indian Ocean dipole and their temporal correlation. Model results are then used to analyze the future behaviour of rainfall and the DMI. All models simulate reasonably well the spatial distribution and variability of annual and seasonal rainfall over the 1961–1990 period. Model simulation of observed DMI characteristics is less consistent with observations, however, five models reproduce similar correlations to those observed between the DMI and East African short rains (SON). In the future, there are no clear inter-model patterns of rainfall or DMI behaviour. In this sample of models four (two) out of six simulate modest increases (decreases) in annual rainfall by the 2080s. For SON, three of the six models indicate a trend towards increasingly positive phase of the DMI, two indicate a decrease and one shows no substantial change
Changes in the strength and width of the Hadley Circulation since 1871
No full textRecent studies demonstrate that the Hadley Circulation has intensified and expanded for the past three decades, which has important implications for subtropical societies and may lead to profound changes in global climate. However, the robustness of this intensification and expansion that should be considered when interpreting long-term changes of the Hadley Circulation is still a matter of debate. It also remains largely unknown how the Hadley Circulation has evolved over longer periods. Here, we present long-term variability of the Hadley Circulation using the 20th Century Reanalysis. It shows a slight strengthening and widening of the Hadley Circulation since the late 1970s, which is not inconsistent with recent assessments. However, over centennial timescales (1871–2008), the Hadley Circulation shows a tendency towards a more intense and narrower state. More importantly, the width of the Hadley Circulation might have not yet completed a life-cycle since 1871. The strength and width of the Hadley Circulation during the late 19th to early 20th century show strong natural variability, exceeding variability that coincides with global warming in recent decades. These findings raise the question of whether the recent change in the Hadley Circulation is primarily attributed to greenhouse warming or to a long-period oscillation of the Hadley Circulation – substantially longer than that observed in previous studies
Tracking sperm whales using passive acoustics and particle filters
No full textPassive acoustics provides a powerful tool for marine mammal research and mitigation of the risk posed by high energy anthropogenic acoustic activities through monitoring animal positions. Animal vocalisations can be detected and utilised in poor visibility conditions and while animals are dived. Marine mammal research is often conducted on restricted financial budgets by non-government organisations and academic institutions from boats or ships towing hydrophone arrays often comprising only two elements. The arrival time-delay of the acoustic wavefront from the vocalising animals across the array aperture is computed, often using freely available software, and typically regarded as the bearing of the animal to the array. This methodology is limited as it provides no ranging information and, until a boat manoeuvre is performed, whether the animal is to the left or right of the array remains ambiguous. Methods of determining range that have been suggested either negate the fact the animal is moving, rely on robust detection of acoustic reflections, rely on accurate equipment calibration and knowledge of the animal’s orientation or require modification of hydrophone equipment. There is a clear need to develop an improved method of estimating animal position as relative bearing, range and elevation to a hydrophone array or boat based on time-delay measurements. To avoid the costs of upgrading hydrophone arrays, and potentially the size of the vessels required to tow them, a software solution is desirable. This thesis proposes that the source location be modelled as a probability density function and that the source location is estimated as the mean. This is developed into a practical method using particle filters to track sperm whales. Sperm whales are the ideal subject species for this kind of development because the high sound pressure levels of their impulsive vocalisations (up to 236 dB re 1 ?Pa) makes them relatively simple to detect. Simulation tracking results demonstrate particle filters are capable of tracking a manoeuvring target using time-delay measurements. Tracking results for real data are presented and compared to the pseudotrack reconstructed from a tag equipped with accelerometers, magnetometers, a depth sensor and an acoustic recorder placed on the subject animal. For the majority of datasets the animal is tracked to a position relatively close to the surface sighting position. Sperm whales are typically encountered in groups, therefore a viable tracking solution needs to be capable of tracking multiple animals. A multiple hypothesis tracking method is proposed and tested for associating received vocalisations with animals, whereby vocalisations are correctly associated for periods exceeding 15 minute
Single-electron switching on chips: The nanoscale circuit has become reality
No full textVery little distinguishes the first transistor presented by Bell Telephone Labs researchers Bardeen, Shockley, and Brattain in 1947 from the Single Electron Transistor (set) developed by tu delft physicist Pieter Heij. The set,which switches using single electrons, can only be observed with the aid of an electron microscope. Over the past four years, Heij has worked at the Quantum Transport group of professor Dr. Ir. Hans Mooij and Dr. Peter Hadley to develop techniques for joining sets into larger units that can be used for logical operations. This involved the extremely accurate positioning of successive layers of atoms, some of which were only just thick enough to enable electrons to tunnel through. The current models still require extremely low temperatures to work
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