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Planetary Boundary Layer (planetary + boundary_layer)
Selected AbstractsAn introduction to the European Terrestrial Ecosystem Modelling ActivityGLOBAL ECOLOGY, Issue 6 2001Martin T. Sykes Abstract The objective of the European Terrestrial Ecosystem Modelling Activity (ETEMA) was to address some of the major challenges in developing generalized models to examine responses of natural and seminatural ecosystems to environmental change at the regional to European scale. The approach described herein was to break down the totality of ecosystem functioning into its key components, each with its characteristic spatial and temporal scales. A conceptual framework was developed describing the configuration of these components as modules within a generalized simulation model. The framework describes the key inputs, outputs and state variables, their spatial and temporal contexts, and information flows between modules. The ,backbone' of the model is a system of nested timing loops corresponding to the disparate time scales at which different ecosystem processes occur. The framework is a theoretical construct into which ecosystem models at levels of complexity ranging from the very general to the highly detailed can be mapped, and thus provides a guide for development of models for novel, particularly regional-scale, applications. A number of subsystem studies of the major components of ecosystem functioning, i.e. modules of the conceptual framework, are briefly introduced herein. The general aim of the subsystem studies was to identify the key alternative formulations (as opposed to minor variants) and test these against observational data. The various subsystem studies concern planetary boundary layer,ecosystem interactions, ecosystem CO2 and H2O fluxes, vegetation physiology and phenology, biogeography and vegetation dynamics, detritus and SOM dynamics, soil moisture and human and natural disturbances and, as individual papers, they complete this special ETEMA issue. [source] Integrating fluxes from heterogeneous vegetationGLOBAL ECOLOGY, Issue 6 2001F. Ian Woodward Abstract The vegetated landscape of Europe has been strongly impacted by human management to produce a heterogeneous patchwork of semi-natural and agricultural vegetation varying over a wide range of spatial scales. A model is described for averaging vegetation fluxes from a landscape of forest and grassland into the planetary boundary layer (PBL). At a scale of 1 km, model simulations indicate that vegetation heterogeneity exerts little effect on the PBL and regional fluxes will be simple areal averages of the different vegetation types. Above 5 km the model simulates significant effects of different vegetation types on the whole PBL. Averaging fluxes to the regional scale will therefore need to consider explicitly the nature, extent and behaviour of different vegetation types. [source] Europe's 2003 heat wave: a satellite view of impacts and land,atmosphere feedbacksINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 6 2006Benjamin F. Zaitchik Abstract A combination of satellite imagery, meteorological station data, and the NCEP/NCAR reanalysis has been used to explore the spatial and temporal evolution of the 2003 heat wave in France, with focus on understanding the impacts and feedbacks at the land surface. Vegetation was severely affected across the study area, especially in a swath across central France that corresponds to the Western European Broadleaf (WEB) Forests ecological zone. The remotely sensed surface temperature anomaly was also greatest in this zone, peaking at +15.4 °C in August. On a finer spatial scale, both the vegetation and surface temperature anomalies were greater for crops and pastures than for forested lands. The heat wave was also associated with an anomalous surface forcing of air temperature. Relative to other years in record, satellite-derived estimates of surface-sensible heat flux indicate an enhancement of 48,61% (24.0,30.5 W m,2) in WEB during the August heat wave maximum. Longwave radiative heating of the planetary boundary layer (PBL) was enhanced by 10.5 W m,2 in WEB for the same period. The magnitude and spatial structure of this local heating is consistent with models of the late twenty-first century climate in France, which predict a transitional climate zone that will become increasingly affected by summertime drought. Models of future climate also suggest that a soil-moisture feedback on the surface energy balance might exacerbate summertime drought, and these proposed feedback mechanisms were tested using satellite-derived heat budgets. Copyright © 2006 Royal Meteorological Society. [source] Impact of soil moisture on the development of a Sahelian mesoscale convective system: a case-study from the AMMA Special Observing PeriodTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue S1 2010Christopher M. Taylor Abstract Interactions between the land and atmosphere play an important role in the precipitation of the Sahel. The African Monsoon Multidisciplinary Analysis Special Observing Period provided observations with which to illuminate potential feedback mechanisms. This case-study highlights a major storm which developed over northern Mali in an area where a research aircraft was surveying the atmospheric response to soil moisture features. Soil moisture variability is characterized using satellite land-surface temperature data whilst cloud images illustrate the evolution of the storm and its relationship to the surface. Measurements in the planetary boundary layer (PBL) indicate mesoscale variations in pre-storm humidity and temperature consistent with high evaporation from wet soils. The storm developed above a dry surface within a wetter region with cells first appearing along a wet,dry soil boundary. This suggests that the storm was triggered in association with low-level convergence driven by the soil moisture pattern. A gravity wave propagating away from a remote mature storm also appears to have played an important role in the initiation, though only in the region of the soil moisture contrast did deep convection become established. Once organised into a Mesoscale Convective System, convection developed over wet areas as well as dry, and indeed at this stage, convection became more intense over wetter soils. This behaviour is consistent with the large gradients in PBL humidity. The study illustrates the complexity of soil moisture,convection feedback loops and highlights the mechanisms which may operate at different stages of a storm's life cycle. Copyright © 2009 Royal Meteorological Society [source] A consistent vertical Bowen ratio profile in the planetary boundary layerTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 620 2006M. Hantel Abstract It has recently been suggested that the integrand b=,,,,, of the subgrid-scale conversion rate between available and kinetic energy has a measurable impact upon the Lorenz energy cycle. Here we discuss a technique to estimate this quantity within the lower part of an atmospheric column by relating b to the subgrid-scale fluxes of sensible and latent heat in form of their sum (the total convective heat flux, c, to be diagnosed from the pertinent energy law) and their ratio (a generalized Bowen ratio, ,, to be specified a priori). We focus on the frequently observed case that c vanishes at or above the top of the boundary layer, which implies that , must be minus unity at the same level (referred to as ,critical pressure'). , at the earth's surface is taken as measured. Observations suggest that the vertical curvature of the , profile is negative in the boundary layer. We specify an analytic vertical profile ,(,) that interpolates these pieces of information; , is a non-dimensional vertical coordinate. The pertinent thermodynamic energy law from which the column profile c(,) is gained (referred to here as convection equation) is driven by the (observed) grid-scale budget; the solution c is over most of the boundary layer quite insensitive to ,. It is only in the immediate vicinity of the critical pressure that c(,) becomes sensitively dependent upon ,(,); it actually turns infinite at this level (a ,pole' of the convection equation). We remove the pole through adjusting the critical pressure by a uniquely determined (and actually quite small) amount. This makes the , profile consistent with the convection equation and with the other convective flux profiles, across the entire boundary layer. The remaining open parameter that cannot be fixed by our method is the curvature of the Bowen ratio profile. This exercise has implications for about a third of all atmospheric columns over the globe and thus may be relevant for the quantification of the global energy cycle. Copyright © 2006 Royal Meteorological Society [source] The impact of urban areas on weatherTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 614 2006C. G. Collier Abstract The industrial revolution led to a rapid development of urban areas. This has continued unremittingly over the last 200 years or so. In most urban areas the surface properties are heterogeneous, which has significant implications for energy budgets, water budgets and weather phenomena within the part of the earth's atmosphere that humans live. In this paper I discuss the structure of the planetary boundary layer, confining our analysis to the region above the rooftops (canopy layer) up to around the level where clouds form. It is in this part of the atmosphere that most of the weather impacting our lives occurs, and where the buildings of our cities impact the weather. In this review, observations of the structure of the urban atmospheric boundary layer are discussed. In particular the use of Doppler lidar provides measurements above the canopy layer. The impact of high-rise buildings is considered. Urban morphology impacts energy fluxes and airflow leading to phenomena such as the urban heat island and convective rainfall initiation. I discuss in situ surface-based remote sensing and satellite measurements of these effects. Measurements have been used with simple and complex numerical models to understand the complexity and balance of the interactions involved. Cities have been found to be sometimes up to 10 degC warmer than the surrounding rural areas, and to cause large increases in rainfall amounts. However, there are situations in which urban aerosol may suppress precipitation. Although much progress has been made in understanding these impacts, our knowledge remains incomplete. These limitations are identified. As city living becomes even more the norm for large numbers of people, it is imperative that we ensure that urban effects on the weather are included in development plans for the built environment of the future. Copyright © 2006 Royal Meteorological Society [source] The role of stability and moisture in the diurnal cycle of convection over landTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 604 2004J.-P. Chaboureau Abstract The diurnal cycle of convection over land is investigated by a cloud-resolving model simulation. Three regimes of convection,dry, shallow, and deep,successively take place during daytime under the presence of substantial convective available potential energy. The convective inhibition (CIN) and the normalized saturation deficit (NSD) in the cloud-base layer are identified as the major two variables that characterize the cycle of the convective regimes. The surface heating during daytime leads to the development of a quasi-dry well-mixed convective planetary boundary layer (PBL). This yields a decrease of CIN while NSD remains steady. Shallow convection is initiated as soon as the CIN becomes lower locally than the vertical kinetic energy in the PBL. This timing also marks the minimum of CIN, both in local and in domain-mean senses. Then, detrainment of moisture from the cloud layer gradually moistens the low free troposphere, resulting in a NSD decrease. Finally, deep convection is triggered when sufficient moistening is realized, as measured by a NSD minimum. During deep convection, NSD rapidly increases and CIN increases. Once CIN has exceeded the vertical kinetic energy in the PBL, deep convection ceases. Copyright © 2004 Royal Meteorological Society [source] On the accuracy of retrieved wind information from Doppler lidar observationsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 587 2003F. Davies Abstract A single pulsed Doppler lidar was successfully deployed to measure air flow and turbulence over the Malvern hills, Worcester, UK. The DERA Malvern lidar used was a pulsed Doppler lidar. The lidar pulse repetition rate was 120 Hz and had a pulse duration of . The system was set up to have 41 range gates with range resolution of 112 m. This gave a theoretical maximum range of approximately 4.6 km. The lidar site was 2 km east of the Malvern hill ridge which runs in a north,south direction and is approximately 6 km long. The maximum height of the ridge is 430 m. Two elevation scans (Range,Height Indicators) were carried out parallel and perpendicular to the mean surface flow. Since the surface wind was primarily westerly the scans were carried out perpendicular and parallel to the ridge of the Malvern hills. The data were analysed and horizontal winds, vertical winds and turbulent fluxes were calculated for profiles throughout the boundary layer. As an aid to evaluating the errors associated with the derivation of velocity and turbulence profiles, data from a simple idealized profile was also analysed using the same method. The error analysis shows that wind velocity profiles can be derived to an accuracy of 0.24 m s,1 in the horizontal and 0.3 m s,1 in the vertical up to a height of 2500 m. The potential for lidars to make turbulence measurements, over a wide area, through the whole depth of the planetary boundary layer and over durations from seconds to hours is discussed. Copyright © 2003 Royal Meteorological Society [source] | ||||||||||