Some reports of snowfall from fog during the UK winter of 2008/09

Snowfall during anticyclonic, non-frontal, and foggy conditions is surprising. Because it is often not forecast, it can present a hazard to transport and modify the surface albedo. In this report, we present some observations of snowfall during conditions of freezing fog in the UK during the winter of 2008/09

Applying an analytical framework to production process improvement

Thesis (M.B.A.)--Massachusetts Institute of Technology, Sloan School of Management; and, (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering; in conjunction with the Leaders for Manufacturing Program at MIT, 2007.Includes bibliographical references (p. 74).As the medium voltage switchgear industry moves from air insulated to gas insulated technology, Siemens Frankfurt factory is introducing a new gas insulated product line that will improve their relative market position. It is their intent to design a product and supporting production system that will enable substantial cost and lead time reduction over existing Siemens gas insulated switchgear products. This thesis outlines a framework for analyzing the existing production process from 'customer order to customer delivery', identifying areas of opportunity, valuing projects aimed at achieving these opportunities, and prioritizing highest value projects for implementation. To provide a rigorous analytical approach to project selection, it was important to rethink existing ways of valuing inventory holding costs, material handling costs, and lead time. By uncovering hidden costs and benefits for each, projects that otherwise seemed unattractive become important to achieving overall factory objectives. Conversely, other projects that had been historically pushed by factory leadership were shown to generate little overall return on investment.(cont.) By using the approach outlined in this thesis, improved alignment was achieved across departments on several high value projects. This alignment positioned the factory to move forward with plans for successful implementation. It is the authors' hope that Frankfurt not only finalizes implementation of high value projects identified during this analysis, but also use the framework provided for future analysis and continued improvement.by Curtis J. Underwood & Jacob R. Wood.S.M.M.B.A

Scorch marks from the sky

Abstract An example of a meteorological data-source in widespread use that has probably only been partially exploited is the Campbell-Stokes sunshine recorder: invented in the late 19 th century to provide a measurement of the duration of bright sunlight by making a burn mark on a treated 'burn card'. Each burn provides a continuous record of the state of the sky during daylight hours, by recording the absence or the presence (and indeed burning power) of the Sun's rays. Beyond just the length of the burn, which was routinely converted to the daily total sunshine duration as the primary quantity sought, the detailed properties of the burn would also have been influenced by the sky through which the solar radiation passed. Therefore the burn marks contain, to some extent, additional embedded information on the state of the sky. Analogue sunshine recorders are now increasingly being replaced by modern electronic sunshine sensors which measure the sunshine duration in a different way. Because the electronic measurement is solely that of sunshine duration, there is likely to be a temptation to regard the transcription of the sunshine duration from the burn cards as sufficient data with which to form a combined long data series; and as a presumed safe basis on which to destroy the original burn cards. Our purpose here is to emphasize that the original burn cards probably contain additional sky state information-beyond that of just sunshine duration-for which, in common with other original geophysical archive sources, they may be irreplaceable. There are good reasons to seek new information on past cloud properties quantitatively, not least because of the importance of clouds in Earth's radiation balance (Solomon et al., 2007). The burn card of the Campbell-Stokes sunshine recorder The Campbell-Stokes sunshine recorder (CSSR) records sunlight without a mechanical tracking system, through use of a spherical glass lens On cloud-free days, as the relative positions of the Sun and Earth change, a continuous burn mark is scorched across the length of the card. There is an onset threshold of solar radiation to initiate the burn, which varies slightly and is affected by the state of the card (such as its moisture content; Painter, 1981) and the quality and physical state of the lens (Curtis, 1898). At the onset o

A wind-tunnel study of flow distortion at a meteorological sensor on top of the BT Tower, London, UK

High quality wind measurements in cities are needed for numerous applications including wind engineering. Such data-sets are rare and measurement platforms may not be optimal for meteorological observations. Two years' wind data were collected on the BT Tower, London, UK, showing an upward deflection on average for all wind directions. Wind tunnel simulations were performed to investigate flow distortion around two scale models of the Tower. Using a 1:160 scale model it was shown that the Tower causes a small deflection (ca. 0.5°) compared to the lattice on top on which the instruments were placed (ca. 0–4°). These deflections may have been underestimated due to wind tunnel blockage. Using a 1:40 model, the observed flow pattern was consistent with streamwise vortex pairs shed from the upstream lattice edge. Correction factors were derived for different wind directions and reduced deflection in the full-scale data-set by <3°. Instrumental tilt caused a sinusoidal variation in deflection of ca. 2°. The residual deflection (ca. 3°) was attributed to the Tower itself. Correction of the wind-speeds was small (average 1%) therefore it was deduced that flow distortion does not significantly affect the measured wind-speeds and the wind climate statistics are reliable

Observations of urban boundary layer structure during a strong urban heat island event

It has long been known that the urban surface energy balance is different to that of a rural surface, and that heating of the urban surface after sunset gives rise to the Urban Heat Island (UHI). Less well known is how flow and turbulence structure above the urban surface are changed during different phases of the urban boundary layer (UBL). This paper presents new observations above both an urban and rural surface and investigates how much UBL structure deviates from classical behaviour. A 5-day, low wind, cloudless, high pressure period over London, UK, was chosen for analysis, during which there was a strong UHI. Boundary layer evolution for both sites was determined by the diurnal cycle in sensible heat flux, with an extended decay period of approximately 4 h for the convective UBL. This is referred to as the “Urban Convective Island” as the surrounding rural area was already stable at this time. Mixing height magnitude depended on the combination of regional temperature profiles and surface temperature. Given the daytime UHI intensity of 1.5∘C, combined with multiple inversions in the temperature profile, urban and rural mixing heights underwent opposite trends over the period, resulting in a factor of three height difference by the fifth day. Nocturnal jets undergoing inertial oscillations were observed aloft in the urban wind profile as soon as the rural boundary layer became stable: clear jet maxima over the urban surface only emerged once the UBL had become stable. This was due to mixing during the Urban Convective Island reducing shear. Analysis of turbulent moments (variance, skewness and kurtosis) showed “upside-down” boundary layer characteristics on some mornings during initial rapid growth of the convective UBL. During the “Urban Convective Island” phase, turbulence structure still resembled a classical convective boundary layer but with some influence from shear aloft, depending on jet strength. These results demonstrate that appropriate choice of Doppler lidar scan patterns can give detailed profiles of UBL flow. Insights drawn from the observations have implications for accuracy of boundary conditions when simulating urban flow and dispersion, as the UBL is clearly the result of processes driven not only by local surface conditions but also regional atmospheric structure

Tracer concentration profiles measured in central London as part of the REPARTEE campaign

There have been relatively few tracer experiments carried out that have looked at vertical plume spread in urban areas. In this paper we present results from two tracer (cyclic perfluorocarbon) experiments carried out in 2006 and 2007 in central London centred on the BT Tower as part of the REPARTEE (Regent’s Park and Tower Environmental Experiment) campaign. The height of the tower gives a unique opportunity to study vertical dispersion profiles and transport times in central London. Vertical gradients are contrasted with the relevant Pasquill stability classes. Estimation of lateral advection and vertical mixing times are made and compared with previous measurements. Data are then compared with a simple operational dispersion model and contrasted with data taken in central London as part of the DAPPLE campaign. This correlates dosage with non-dimensionalised distance from source. Such analyses illustrate the feasibility of the use of these empirical correlations over these prescribed distances in central London

Large-eddy simulation of approaching-flow stratification on dispersion over arrays of buildings

The study investigates thermal stratification effects of approach flows on dispersion in urban environments. This is in some ways analogous to a well developed non-neutral flow (e.g. through a large urban area) approaching a neighborhoodscale urban region, where the effect of the local heat transfer was assumed less important. A generic urban-type geometry, i.e. a group of staggered cubes, wastaken as the first test case. The DAPPLE site, which was about a one-km2 region near the intersection of Marylebone Road and Gloucester Place in central London, was taken as the second test case. Only weakly unstable conditions (i.e.bulk Richardson number Rb &gt;= ?0.2) of approach flows were considered, with adiabatic boundary conditions at the ground and building surfaces. A number of numerical experiments including with various Rb were performed. The modelled mean concentration for Rb = ?0.1 gave the best agreement with the field data at all DAPPLE stations. This suggests that stratification effects on dispersion inweakly unstable conditions (e.g. in London) are not negligible

Boundary layer dynamics over London, UK, as observed using Doppler lidar during REPARTEE-II

Urban boundary layers (UBLs) can be highly complex due to the heterogeneous roughness and heating of the surface, particularly at night. Due to a general lack of observations, it is not clear whether canonical models of boundary layer mixing are appropriate in modelling air quality in urban areas. This paper reports Doppler lidar observations of turbulence profiles in the centre of London, UK, as part of the second REPARTEE campaign in autumn 2007. Lidar-measured standard deviation of vertical velocity averaged over 30 min intervals generally compared well with in situ sonic anemometer measurements at 190 m on the BT telecommunications Tower. During calm, nocturnal periods, the lidar underestimated turbulent mixing due mainly to limited sampling rate. Mixing height derived from the turbulence, and aerosol layer height from the backscatter profiles, showed similar diurnal cycles ranging from c. 300 to 800 m, increasing to c. 200 to 850 m under clear skies. The aerosol layer height was sometimes significantly different to the mixing height, particularly at night under clear skies. For convective and neutral cases, the scaled turbulence profiles resembled canonical results; this was less clear for the stable case. Lidar observations clearly showed enhanced mixing beneath stratocumulus clouds reaching down on occasion to approximately half daytime boundary layer depth. On one occasion the nocturnal turbulent structure was consistent with a nocturnal jet, suggesting a stable layer. Given the general agreement between observations and canonical turbulence profiles, mixing timescales were calculated for passive scalars released at street level to reach the BT Tower using existing models of turbulent mixing. It was estimated to take c. 10 min to diffuse up to 190 m, rising to between 20 and 50 min at night, depending on stability. Determination of mixing timescales is important when comparing to physico-chemical processes acting on pollutant species measured simultaneously at both the ground and at the BT Tower during the campaign. From the 3 week autumnal data-set there is evidence for occasional stable layers in central London, effectively decoupling surface emissions from air aloft

Uncertainty of eddy covariance flux measurements over an urban area based on two towers

The eddy covariance (EC) technique is the most direct method for measuring the exchange between the surface and the atmosphere in different ecosystems. Thus, it is commonly used to get information on air pollutant and greenhouse gas emissions, and on turbulent heat transfer. Typically an ecosystem is monitored by only one single EC measurement station at a time, making the ecosystem-level flux values subject to random and systematic uncertainties. Furthermore, in urban ecosystems we often have no choice but to conduct the single-point measurements in non-ideal locations such as close to buildings and/or in the roughness sub-layer, bringing further complications to data analysis and flux estimations. In order to tackle the question of how representative a single EC measurement point in an urban area can be, two identical EC systems - measuring momentum, sensible and latent heat, and carbon dioxide fluxes - were installed on each side of the same building structure in central Helsinki, Finland, during July 2013-September 2015. The main interests were to understand the sensitivity of the vertical fluxes on the single measurement point and to estimate the systematic uncertainty in annual cumulative values due to missing data if certain, relatively wide, flow-distorted wind sectors are disregarded. The momentum and measured scalar fluxes respond very differently to the distortion caused by the building structure. The momentum flux is the most sensitive to the measurement location, whereas scalar fluxes are less impacted. The flow distortion areas of the two EC systems (40-150 and 230-340 degrees) are best detected from the mean-wind-normalised turbulent kinetic energy, and outside these areas the median relative random uncertainties of the studied fluxes measured by one system are between 12 % and 28 %. Different gap-filling methods with which to yield annual cumulative fluxes show how using data from a single EC measurement point can cause up to a 12 % (480 g C m(-2)) underestimation in the cumulative carbon fluxes as compared to combined data from the two systems. Combining the data from two EC systems also increases the fraction of usable half-hourly carbon fluxes from 45 % to 69 % at the annual level. For sensible and latent heat, the respective underestimations are up to 5 % and 8 % (0.094 and 0.069 TJ m(-2)). The obtained random and systematic uncertainties are in the same range as observed in vegetated ecosystems. We also show how the commonly used data flagging criteria in natural ecosystems, kurtosis and skewness, are not necessarily suitable for filtering out data in a densely built urban environment. The results show how the single measurement system can be used to derive representative flux values for central Helsinki, but the addition of second system to other side of the building structure decreases the systematic uncertainties. Comparable results can be expected in similarly dense city locations where no large directional deviations in the source area are seen. In general, the obtained results will aid the scientific community by providing information about the sensitivity of EC measurements and their quality flagging in urban areas.Peer reviewe

Regional nutrient decrease drove redox stabilisation and metazoan diversification in the late Ediacaran Nama Group, Namibia

The late Ediacaran witnessed an increase in metazoan diversity and ecological complexity, marking the inception of the Cambrian Explosion. To constrain the drivers of this diversification, we combine redox and nutrient data for two shelf transects, with an inventory of biotic diversity and distribution from the Nama Group, Namibia (similar to 550 to similar to 538 Million years ago; Ma). Unstable marine redox conditions characterised all water depths in inner to outer ramp settings from similar to 550 to 547Ma, when the first skeletal metazoans appeared. However, a marked deepening of the redoxcline and a reduced frequency of anoxic incursions onto the inner to mid-ramp is recorded from similar to 547Ma onwards, with full ventilation of the outer ramp by similar to 542Ma. Phosphorus speciation data show that, whilst anoxic ferruginous conditions were initially conducive to the drawdown of bioavailable phosphorus, they also permitted a limited degree of phosphorus recycling back to the water column. A long-term decrease in nutrient delivery from continental weathering, coupled with a possible decrease in upwelling, led to the gradual ventilation of the Nama Group basins. This, in turn, further decreased anoxic recycling of bioavailable phosphorus to the water column, promoting the development of stable oxic conditions and the radiation of new mobile taxa.Peer reviewe