R. R. Burton

THE JET2000 PROJECT Aircraft Observations of the African Easterly Jet and African Easterly Waves

Abstract Scientific background and motivation for the JET2000 aircraft observing campaign that took place in West Africa during the last week of August 2000 are presented. The Met Research Flight C130 aircraft made two flights along the African easterly jet (AEJ) between Sal, Cape Verde, and Niamey, Niger, and two “box” flights that twice crossed the AEJ at longitudes near Niamey. Dropsondes were released at approximately 0.5°–10° intervals. The two box flights also included low-level flights that sampled north–south variations in boundary layer properties in the baroclinic zone beneath the AEJ. Preliminary results and analysis of the JET2000 period including some of the aircraft data are presented. The JET2000 campaign occurred during a relatively dry period in the Niamey region and, perhaps consistent with this, was also associated with less coherent easterly wave activity compared to other periods in the season. Meridional cross sections of the AEJ on 28 and 29 August (after the passage of a mesoscale ...

Evaluation of high frequency radar wave measurement

Abstract The spatial coverage, temporal availability and spectral and parameter accuracy of wave measurements using radars operating at the upper end of the high frequency (HF) radio band are discussed. The two radars used are the Ocean Surface Current Radar (OSCR) developed in the UK and the Wellen Radar (WERA) developed in Germany. The measurements show that useful accuracy is obtainable with very good potential for operational coastal monitoring. Direction biases over most of the frequency range are less than 15°. The relative bias in significant waveheight estimation in high sea-states is within ±5%. Although there is some deterioration in spectral performance in high sea-states, this is primarily confined to amplitudes at the higher ocean wave frequencies where a modelling approach could be adopted. Gaps in spatial and temporal coverage and some reduction in accuracy in mean direction estimation are due to surface current variability or antenna sidelobes.

Modelling of diffraction in pillar vertical cavity surface-emitting lasers with embedded Bragg layers

An analytical expression for the reflectivity between the active cavity of a circular symmetric pillar vertical cavity surface-emitting laser (VCSEL) and Bragg mirror, which takes account of diffraction at the interface, is derived. The method (mode-matching in Hankel space) is also used to determine the reflectivity with some of the Bragg layers embedded in the cavity region. The dependence of reflectivity upon various parameters, including number of Bragg pairs, cavity radius and mode number, is discussed. The validity of using the plane wave reflectivity value (which ignores diffraction effects) is seen to depend significantly upon these parameters. As an indicator of the importance of taking these effects into account for pillar diameters of ∼6 μm and less, the variation of threshold gain with cavity radius is given for a commonly used type of VCSEL structure.

Ground-level climate at a peatland wind farm in Scotland is affected by wind turbine operation

The global drive to produce low-carbon energy has resulted in an unprecedented deployment of onshore wind turbines, representing a significant land use change for wind energy generation with uncertain consequences for local climatic conditions and the regulation of ecosystem processes. Here, we present high-resolution data from a wind farm collected during operational and idle periods that shows the wind farm affected several measures of ground-level climate. Specifically, we discovered that operational wind turbines raised air temperature by 0.18 °C and absolute humidity (AH) by 0.03 g m−3 during the night, and increased the variability in air, surface and soil temperature throughout the diurnal cycle. Further, the microclimatic influence of turbines on air temperature and AH decreased logarithmically with distance from the nearest turbine. These effects on ground-level microclimate, including soil temperature, have uncertain implications for biogeochemical processes and ecosystem carbon cycling, including soil carbon stocks. Consequently, understanding needs to be improved to determine the overall carbon balance of wind energy.

The Two-Dimensional Response of a Tropical Jet to Propagating Lines of Convection

This paper uses simple one- and two-dimensional models to investigate the influence of a propagating line of convective forcing on a tropical jet, representative of the African easterly jet. The results are used to infer changes in the environment of the forcing region, which would in reality tend to influence the evolution of the storm through convective mechanisms, which are not resolved here. From linear analytical solutions with a rigid lid it is found that the influence of the propagation of the forcing region is to intensify the response on the upstream side of the forcing: this result is confirmed in two-dimensional nonlinear simulations. When linear wave modes are computed in a basic state that includes the jet structure, small sensitivities to the basic-state jet are found. The two-dimensional nonlinear model has been used further to compute the change in the environmental structure as a result of the forcing. Principally, it is found that the modes of response to the forcing may be associated with characteristic changes in the basic-state shear, convective available potential energy (CAPE), and convective inhibition (CIN), which would be expected to have significant influence on the convective system itself.

Vertical velocity observed by Doppler lidar during cops ? A case study with a convective rain event

Convective and Orographically-Induced Precipitation Study (COPS), conducted in the Black Forest region in Southern Germany and Eastern France during the summer of 2007. From the 13 June to the 16 August 2007, the National Centre for Atmospheric Science (NCAS), Facility for Ground-based Atmospheric Measurement (FGAM) 1.5 lm scanning Doppler lidar was deployed at Super Site R, Achern, in the Rhine Valley, in order to contribute to the extensive COPS observation campaign. The FGAM Doppler lidar system provides measurements of radial wind and aerosol backscatter in the layer 100–1500 m. Profiles of horizontal wind velocity are presented, these being derived from performing azimuth scans. Profiles of vertical velocity, its variance and skewness derived from the vertical scans are also presented and discussed in the paper. Knowledge of vertical velocity skewness is important for the understanding of the structure and origin of turbulent convection in the atmospheric boundary layer (ABL). The skewness of vertical velocity can provide a measure of the asymmetry in the distribution of vertical velocity perturbations within the ABL and can be estimated using the Doppler lidar. In addition, we investigate the behaviour of the boundary layer using data from the FGAM Doppler lidar and Automatic Weather Station (AWS), the University of Basilicata Raman lidar (BASIL) and the DLR’s Poldirad C-band radar. A case study event on the 6 th August 2007 is selected and investigations of possible causes of layers with positive and negative skewness are presented, along with comparisons with output from the National Center for Atmospheric Research (NCAR) Weather Research and Forecasting (WRF) model to assess the accuracy of the model output, including location and timing of rainfall onset.

Doppler lidar observations of sensible heat flux and intercomparisons with a ground-based energy balance station and WRF model output

This is an open access article - Copyright @ 2009 E. Schweizerbartsche Verlagsbuchhandlung

Globally Significant CO2 Emissions From Katla, a Subglacial Volcano in Iceland

Volcanoes are a key natural source of CO2, but global estimates of volcanic CO2 flux are predominantly based on measurements from a fraction of worlds actively degassing volcanoes. We combine high‐precision airborne measurements from 2016 and 2017 with atmospheric dispersion modeling to quantify CO2 emissions from Katla, a major subglacial volcanic caldera in Iceland that last erupted 100 years ago but has been undergoing significant unrest in recent decades. Katlas sustained CO2 flux, 12–24 kt/d, is up to an order of magnitude greater than previous estimates of total CO2 release from Icelands natural sources. Katla is one of the largest volcanic sources of CO2 on the planet, contributing up to 4% of global emissions from nonerupting volcanoes. Further measurements on subglacial volcanoes worldwide are urgently required to establish if Katla is exceptional, or if there is a significant previously unrecognized contribution to global CO2 emissions from natural sources. We combine high‐precision airborne measurements from 2016 and 2017 with atmospheric dispersion modelling to quantify CO2 emissions from Katla, a major subglacial volcanic caldera in Iceland that last erupted 100 years ago but has been undergoing significant unrest in recent decades. Katlas sustained CO2 flux, 12‐24 kt/d, is up to an order of magnitude greater than previous estimates of total CO2 release from Icelands natural sources. Katla is one of the largest volcanic sources of CO2 on the planet, contributing up to 4% of global emissions from non‐erupting volcanoes. Further measurements on subglacial volcanoes world‐wide are urgently required to establish if Katla is exceptional, or if there is a significant previously unrecognized contribution to global CO2 emissions from natural sources.

Bragg Mirror Reflectance in a VCSEL Scanning Optical Microscope

It has been proposed that near-field scanning optical microscopy might be performed using a vertical cavity surface emitting laser (VCSEL) both as source and detector. The required high-resolution probing field could be obtained by covering the emitting Bragg stack with a high reflectivity metal film containing a small sub-micron diameter aperture, on axis. The change in complex reflectance from the Bragg stack for an object placed in the near field of the aperture might be expected to modulate the wavelength, or power output, from the VCSEL. This would provide the necessary contrast as the object is scanned past the aperture.