The generation and propagation of planetary Rossby waves

Imperial Users onl

Non-hydrostatic effects on mountain wave breaking in directional shear flows

Mountain waves excited by narrow 3D orography are investigated using idealized numerical simulations of atmospheric flows with directional wind shear. The stability of these waves is compared with the stability of hydrostatic mountain waves. The focus is on understanding how wave breaking is modified via gravity wave-critical level interaction, when non-hydrostatic (dispersive) effects arise. The influence of nonhydrostatic effects on wave breaking appears to be a function of the intensity of the background shear, increasing the stability of the flow (inhibiting wave breaking) for weak wind shear, but decreasing it instead (enhancing wave breaking) for stronger wind shear

No Horns Allowed

Picture Book

Tracking down the origin of NWP model uncertainty : coarse-graining studies

Ponencia presentada en: Workshop on representing model uncertainty and error in numerical weather and climate prediction celebrado del 20 al 24 de junio de 2011 en Reading, Inglaterra.Current implementations of the perturbed parametrization tendency method for representing uncertainty rely on ad hoc assumptions about its magnitude and its spatial and temporal correlation scales. Ideally one would use observational data to ascertain the statistical character of parametrization tendency errors and use the resulting probability distribution functions to devise and calibrate the perturbed tendency approach. The reality is that observations rarely have the coverage, representativity and accuracy to form a useful comparison with model data. A less satisfactory alternative is to use high resolution modelling to provide a ‘truth’ simulation and then compare this with an equivalent but lower resolution simulation. Tendency fields from both simulations are coarse-grained to a resolution compatible with the assumed horizontal correlation scale in the perturbed tendency method and the bias-corrected differences between them are used to quantify statistical uncertainty. Early results using the ECMWF IFS forecasts appear to show that the variance of the coarse-grained tendency differences is proportional to the tendency in the lower-resolution forecast. However the current perturbed parametrization tendency scheme at ECMWF assumes that the standard deviation of the perturbations is proportional to the tendency itself. Probability distribution functions of the high-resolution model tendency, sub-sampled by narrow ranges of the low-resolution model tendency, seem to be consistent with an underlying Poisson process

Social categories guide young children’s preferences for novel objects

To whom do children look when deciding on their own preferences? To address this question, 3-year-old children were asked to choose between objects or activities that were endorsed by unfamiliar people who differed in gender, race (White, Black), or age (child, adult). In Experiment 1, children demonstrated robust preferences for objects and activities endorsed by children of their own gender, but less consistent preferences for objects and activities endorsed by children of their own race. In Experiment 2, children selected objects and activities favored by people of their own gender and age. In neither study did most children acknowledge the influence of these social categories. These findings suggest that gender and age categories are encoded spontaneously and influence children's preferences and choices. For young children, gender and age may be more powerful guides to preferences than race.Psycholog

The Conceptualization of Sisterhood Within the Collegiate Sorority: An Exploration

This article expands the work of Cohen, McCreary and Schutts (2017) by devising, testing and validating a scale that measures five distinct schema of sisterhood. The scale development process resulted in a 24-item measure made up of five correlated dimensions: shared social experiences, belonging, support and encouragement, accountability, and common purpose. The five-factor model was stable across multiple samples. The construct validity of the sisterhood scale, including convergent and discriminant validity was also demonstrated

Social Information Guides Infants' Selection of Foods

Two experiments investigated the influence of socially conveyed emotions and speech on infants' choices among food. After watching films in which two unfamiliar actresses each spoke while eating a different kind of food, 12-month-old infants were allowed to choose between the two foods. In Experiment 1, infants selected a food endorsed by a speaker of their native language who displayed positive affect over a food endorsed by a foreign-language speaker who displayed negative affect. In Experiment 2, both actresses displayed positive affect, but they spoke in different languages, and infants again selected the food associated with the speaker of their native language. The findings contrast with previous research in which infants and toddlers have shown little selectivity when presented with foods that differ in their intrinsic properties such as color, texture, and familiarity. Although infants may lack capacities for evaluating foods on their own, they do look to other people for guidance in food selection.Psycholog

A linear model of gravity wave drag for hydrostatic sheared flow over elliptical mountains

An analytical model of orographic gravity wave drag due to sheared flow past elliptical mountains is developed. The model extends the domain of applicability of the well-known Phillips model to wind profiles that vary relatively slowly in the vertical, so that they may be treated using a WKB approximation. The model illustrates how linear processes associated with wind profile shear and curvature affect the drag force exerted by the airflow on mountains, and how it is crucial to extend the WKB approximation to second order in the small perturbation parameter for these effects to be taken into account. For the simplest wind profiles, the normalized drag depends only on the Richardson number, Ri, of the flow at the surface and on the aspect ratio, γ, of the mountain. For a linear wind profile, the drag decreases as Ri decreases, and this variation is faster when the wind is across the mountain than when it is along the mountain. For a wind that rotates with height maintaining its magnitude, the drag generally increases as Ri decreases, by an amount depending on γ and on the incidence angle. The results from WKB theory are compared with exact linear results and also with results from a non-hydrostatic nonlinear numerical model, showing in general encouraging agreement, down to values of Ri of order one

Monolithic dual-wavelength InP/AlGaInP quantum dot lasers

This thesis describes the development of a monolithic dual-wavelength laser based on an InP/AlGaInP quantum dot (QD) laser structure. Each wavelength is sourced from the same active region and can be operated simultaneously or independently, with light being emitted from a common aperture. The inhomogeneity of the QD material provides a wide distribution of energies, resulting in a broad and relatively flat-topped gain spectrum, which is ideal for sourcing multiple wavelengths. Measurements of optical absorption, gain and laser threshold current densities were used to characterise the optical properties of InP/AlGaInP QDs and ascertain a suitable structure from which to fabricate the dual-wavelength source. A growth temperature of 710 °C resulted in the lowest threshold current densities, and the incorporation of tensile strain into the upper confining layers was found to reduce the temperature dependence. Optical gain measurements were used to assess how state-filling and temperature govern the gain-peak wavelength. For a fixed gain at low injection the wavelength dependence follows that of the band gap (≈ 0.17 nm/K), but at higher levels of injection it becomes relatively temperature-insensitive. A minima in wavelength sensitivity corresponded to a net gain of ≈ 28 cm-1. Edge-emitting lasers with a wavelength temperature dependence as low as 0.03 nm/K were demonstrated for temperatures up to 107 °C (380 K). An Ar-Cl2 based inductively-coupled plasma (ICP) etch process, suitable for fabricating sub-micron features, was developed to create the necessary device architecture. Using the effects of state-filling and spectrally preferential feedback, coupled-cavity ridgewaveguide lasers with unequal length sections were used to generate two wavelengths, with separations up to 61.5 ± 0.2 nm. Time resolved spectra were used to demonstrate dual-mode operation, where both wavelengths were observed to emit simultaneously. This is a promising result as it suggests that this device could potentially be used as a compact terahertz sourc

Large-scale length and time scales for use with stochastic convective parameterization

Many numerical models for weather prediction and climate studies are run at resolutions that are too coarse to resolve convection explicitly, but too fine to justify the local equilibrium assumed by conventional convective parameterizations. The Plant-Craig (PC) stochastic convective parameterization scheme, developed in this paper, solves this problem by removing the assumption that a given grid-scale situation must always produce the same sub-grid-scale convective response. Instead, for each timestep and gridpoint, one of the many possible convective responses consistent with the large-scale situation is randomly selected. The scheme requires as input the large-scale state as opposed to the instantaneous grid-scale state, but must nonetheless be able to account for genuine variations in the largescale situation. Here we investigate the behaviour of the PC scheme in three-dimensional simulations of radiative-convective equilibrium, demonstrating in particular that the necessary space-time averaging required to produce a good representation of the input large-scale state is not in conflict with the requirement to capture large-scale variations. The resulting equilibrium profiles agree well with those obtained from established deterministic schemes, and with corresponding cloud-resolving model simulations. Unlike the conventional schemes the statistics for mass flux and rainfall variability from the PC scheme also agree well with relevant theory and vary appropriately with spatial scale. The scheme is further shown to adapt automatically to changes in grid length and in forcing strength