ABSTRACT The evening transition between up and down slope/valley flows in complex terrain remains... more ABSTRACT The evening transition between up and down slope/valley flows in complex terrain remains one of the least understood aspects of mountain meteorology. Non-stationary and spatially inhomogeneous nature of processes involved and possible front formation have stymied the data analysis, delineation of physical processes and numerical modeling of evening transition. Following the theoretical, laboratory and observational insights gained from VTMX, we have conducted an evening transition experiment in the complex terrain of Phoenix airshed (TANSFLEX-2007) and participated in Meteo-Diffusion studies in the Biferno Valley of Italy, organized by ENEA. The results of former show that evening transition is associated with the formation of a front, immediately followed by its downward propagation to initiate the katabatic flow. Convective instabilities in the front also generated intense localized turbulence, thus causing strong aerosol entrainment. Once the downslope flow is established, the near surface stable layer cuts off the communication of ground with the upper layers, thus increasing the longevity of upslope flow aloft. On the other hand, the Biferno valley case is complicated by the contiguous ocean, and differential cooling rates between oceans and valley complicate the transition mechanism. The front formation may still be present, but the mean flow is dominated by the horizontal pressure gradients induced by the variation of land cover.
The Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program is a Multidiscipli... more The Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program is a Multidisciplinary University Research Initiative (MURI) designed to improve weather predictability in mountain terrain. Here we present some results from the first MATERHORN field experiment, a 30-day intense field campaign conducted during September 25-October 25, 2102 at the Granite Mountain Atmospheric Science Testbed (GMAST) of the US Army Dugway Proving Grounds (DPG). Although the thermal circulation at the Granite Peak is considered nominally simple, data from sonic anemometers as well as slow sensors mounted up to seven levels on 4 towers deployed along a main gentle slope (α ≈ 2-3 degrees) of GMAST show that canonical downslope flows existed only for short time, overshadowed by those arriving from nearby mountains and basins. Tethered balloon (traversed up to 400 m agl) and LiDAR measurements operated during Intense Operational Periods (IOPs) confirmed basin-scale interactions causing the dow...
This work focuses on the analysis of exchange processes in-out an urban-like canopy layer (UCL) a... more This work focuses on the analysis of exchange processes in-out an urban-like canopy layer (UCL) and their description in terms of morphometric parameters. Previous work has shown the potential of morphometric methods for the modelling of the mean flow and dispersion within the urban canopy layer. In this study we consider the dependence of flow characteristics from local building features in order to evaluate previous models for the in-canopy and exchange velocities. We use flow and turbulence measurements from four ultrasonic anemometers located in a large industrial site of a city in central Italy and detailed building data. This site has been chosen as an example of isolated neighborhoods of a real city, suitable for testing previous models for the in-canopy and exchange velocity derived from laboratory experiments.
The Sheppard formula (Q J R Meteorol Soc 82:528–529, 1956) for the dividing streamline height Hs ... more The Sheppard formula (Q J R Meteorol Soc 82:528–529, 1956) for the dividing streamline height Hs assumes a uniform velocity U∞ and a constant buoyancy frequency N for the approach flow towards a mountain of height h, and takes the form Hs/h=(1−F), where F=U∞/Nh. We extend this solution to a logarithmic approach-velocity profile with constant N. An analytical solution is obtained for Hs/h in terms of Lambert-W functions, which also suggests alternative scaling for Hs/h. A ‘modified’ logarithmic velocity profile is proposed for stably stratified atmospheric boundary-layer flows. A field experiment designed to observe Hs is described, which utilized instrumentation from the spring field campaign of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program. Multiple releases of smoke at F≈0.3–0.4 support the new formulation, notwithstanding the limited success of experiments due to logistical constraints. No dividing streamline is discerned for F≈10, since, if present, it is too close to the foothill. Flow separation and vortex shedding is observed in this case. The proposed modified logarithmic profile is in reasonable agreement with experimental observations
A field-scale remote soil moisture sensing technique that exploits polarization mode dispersion (... more A field-scale remote soil moisture sensing technique that exploits polarization mode dispersion (PMD) associated with radio frequency (RF) signal propagation is considered in this paper. Microwave polarization responses from rough surface scattering are quantified using a dual-polarized receiver system to estimate PMD responses. Changes in PMD response are elicited by changes in the dielectric properties due to soil moisture changes. The ability of PMD characterizations to remotely detect changes in soil moisture, with responses that exhibit good correlation to ground probe measurements, is demonstrated using a prototype with widely separated transmitter/receiver system deployed at the MATERHORN field experiments conducted at the US Army Dugway Proving Ground, Utah.
This paper first discusses the aerodynamic effects of trees on local scale flow and pollutant con... more This paper first discusses the aerodynamic effects of trees on local scale flow and pollutant concentration in idealized street canyon configurations by means of laboratory experiments and Computational Fluid Dynamics (CFD). These analyses are then used as a reference modelling study for the extension a the neighbourhood scale by investigating a real urban junction of a medium size city in southern Italy.
Emerging application areas such as air pollution in megacities, wind energy, urban security and o... more Emerging application areas such as air pollution in megacities, wind energy, urban security and operation of unmanned aerial vehicles have intensified scientific and societal interest in mountain meteorology. To address science needs and help improve the prediction of mountain weather, the US Department of Defense has funded a research effort – Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program – that draws the expertise of a multidisciplinary, multi-institutional and multinational group of researchers. The program has four principal thrusts, encompassing Modeling, Experimental, Technology and Parameterization components, directed at diagnosing model deficiencies and critical knowledge gaps, conducting experimental studies and developing tools for model improvements. The access to the Granite Mountain Atmospheric Sciences Test Bed of the US Army Dugway Proving Ground, as well as to a suite of conventional and novel high-end airborne and surface measurement platforms has provided an unprecedented opportunity to investigate phenomena of time scales from a few seconds to a few days, covering spatial extents of tens of kilometers down to millimeters. This article provides an overview of the MATERHORN and a glimpse at its initial findings. Orographic forcing creates a multitude of time-dependent sub-mesoscale phenomena that contribute to the variability of mountain weather at mesoscale. The nexus of predictions by mesoscale model ensembles and observations are described, identifying opportunities for further improvements in mountain weather forecasting.
Measurements of small-scale turbulence made over the complex-terrain atmospheric boundary layer d... more Measurements of small-scale turbulence made over the complex-terrain atmospheric boundary layer during the MATERHORN Program are used to describe the structure of turbulence in katabatic flows. Turbulent and mean meteorological data were continuously measured at multiple levels at four towers deployed along the East lower slope (2-4 deg) of Granite Mountain. The multi-level observations made during a 30-day long MATERHORN-Fall field campaign in September-October 2012 allowed studying of temporal and spatial structure of katabatic flows in detail, and herein we report turbulence and their variations in katabatic winds. Observed vertical profiles show steep gradients near the surface, but in the layer above the slope jet the vertical variability is smaller. It is found that the vertical (normal to the slope) momentum flux and horizontal (along the slope) heat flux in a slope-following coordinate system change their sign below and above the wind maximum of a katabatic flow. The vertical momentum flux is directed downward (upward) whereas the horizontal heat flux is downslope (upslope) below (above) the wind maximum. Our study therefore suggests that the position of the jet-speed maximum can be obtained by linear interpolation between positive and negative values of the momentum flux (or the horizontal heat flux) to derive the height where flux becomes zero. It is shown that the standard deviations of all wind speed components (therefore the turbulent kinetic energy) and the dissipation rate of turbulent kinetic energy have a local minimum, whereas the standard deviation of air temperature has an absolute maximum at the height of wind-speed maximum. We report several cases where the vertical and horizontal heat fluxes are compensated. Turbulence above the wind-speed maximum is decoupled from the surface, and follows the classical local z-less predictions for stably stratified boundary layer.
Observations were taken on an east-facing sidewall at the foot of a desert mountain that borders ... more Observations were taken on an east-facing sidewall at the foot of a desert mountain that borders a large valley, as part of the MATERHORN field program at Dugway Proving Ground, Utah. A case study of nocturnal boundary-layer development is presented for a night in mid-May when tethered-balloon measurements were taken to supplement other MATERHORN field measurements. The boundary-layer development over the slope could be divided into three distinct phases during this night: (i) The evening transition from daytime upslope/up-valley winds to nighttime downslope winds was governed by the propagation of the shadow front. Because of the combination of complex topography at the site and the solar angle at this time of year, the shadow moved down the sidewall from approximately northwest to southeast, with the flow transition closely following the shadow front. (ii) The flow transition was followed by a 3–4 h period of almost steady-state boundary-layer conditions, with a shallow slope-parallel surface inversion and a pronounced downslope flow with a jet maximum located within the surface-based inversion. The shallow slope boundary layer was very sensitive to ambient flows, resulting in several small disturbances. (iii) After approximately 2300 MST the inversion that had formed over the adjacent valley repeatedly sloshed up the mountain sidewall, disturbing local downslope flows and causing rapid temperature drops.
The interaction of global climate change and urban heat island (UHI) is expected to have far-reac... more The interaction of global climate change and urban heat island (UHI) is expected to have far-reaching impacts on the sustainability of the world’s rapidly growing urban population centers. Considering that a wide range of spatiotemporal scales contributed by meteorological forcing and complex surface heterogeneity confounds UHI, a multi-model nested approach is used in this paper to study climate-change impacts on Chicago’s UHI covering a range of relevant scales. One-way dynamical downscaling is used with a model chain consisting of global climate (Community Atmosphere Model, CAM), regional climate (Weather Research and Forecasting, WRF), and microscale (ENVI-met) models. Nested mesoscale and microscale models are evaluated against the present-day observations (including a dedicated urban mini-field study), and the results favorably demonstrate the fidelity of the downscaling techniques used. A simple building-energy model is developed and used in conjunction with microscale model output to calculate future energy demands for a building, and a substantial increase (as much as 26% during daytime) is noted for future (~ 2080) climate. Although winds and lake-breeze circulation for future climate are favorable for reducing energy usage by 7%, the benefits are outweighed by such factors as exacerbated UHI and air temperature. An adverse change of human comfort indicators is also noted in future climate with 92% of the population experiencing thermal discomfort. The model chain used has general applicability to evaluate climate-change impacts on city centers, and hence for urban sustainability studies.
Over the past half century, burgeoning urban areas such as Chicago have experienced elevated anth... more Over the past half century, burgeoning urban areas such as Chicago have experienced elevated anthropogenic-induced alteration of local climates within urbanized regions. As a result, urban heat island (UHI) effect in these areas has intensified. Global climate change can further modulate UHI’s negative effects on human welfare and energy conservation. Various numerical models exist to understand, monitor, and predict UHI and its ramifications, but none can resolve all the relevant physical phenomena that span a wide range of scales. To this end, we have applied a comprehensive multi-scale approach to study UHI of Chicago.
The coupling of global, mesoscale, and micro-scale models has allowed for dynamical downscaling from global to regional to city and finally to neighborhood scales. The output of the Community Climate System Model (CCSM5), a general circulation model (GCM), provides future climate scenario, and its coupling with Weather Research and Forecasting (WRF) model enables studies on mesoscale behavior at urban scales. The output from the WRF model at 0.333 km resolution is used to drive a micro-scale model, ENVI-met. Through this coupling the bane of obtaining reliable initial and boundary conditions for the micro-scale model from limited available observational records has been aptly remedied. It was found that the performance of ENVI-met improves when WRF output, rather than observational data, is supplied for initial conditions. The success of the downscaling procedure allowed reasonable application of micro-scale model to future climate scenario provided by CCSM5 and WRF models. The fine (2 m) resolution of ENVI-met enables the study of two key effects of UHI at micro-scale: decreased pedestrian comfort and increased building-scale energy consumption. ENVI-met model’s explicit treatment of key processes that underpin urban microclimate makes it captivating for pedestrian comfort analysis. Building energy, however, is not modeled by ENVI-met so we have developed a simplified building energy model to estimate future cooling needs.
The stable stratified atmospheric boundary layer continues to pose challenges for Numerical Weath... more The stable stratified atmospheric boundary layer continues to pose challenges for Numerical Weather Prediction and Climate models. Existing parameterizations cannot adequately capture the depth of the Planetary Boundary Layer (PBL), low-level jets and nocturnal near surface temperature because of their poor representation of turbulent fluxes, especially in mountainous terrain. In addition, small scale processes such as collisions between katabatic and valley flows which produce intense mixing, strong vertical velocities and a rapid drop in temperature are distributed in space and time and are unable to be captured. These interactions between flows of different scales, in general, can contribute significantly to sub-grid, short-lived, intense turbulence events, spasmodically producing high fluxes over short periods. Such vigorous mixing episodes need to be included in meso-scale models in order to improve their performance, especially in dealing with near surface flows. High-resolution numerical calculations were performed using the Weather Research and Forecasting (WRF) model to test its ability to predict mountain weather. Data from two comprehensive field experiments conducted under the aegis of Mountain Terrain Atmospheric Modelling and Observations (MATERHORN) Program (www.nd.edu/~dynamics/materhorn) were used in this study. Different PBL options available in the WRF model were tested and evaluated for stable conditions. The Yonsei University (YSU) PBL scheme was modified and implemented in WRF. The performance of the modified and original YSU scheme, were compared. Preliminary results show that the modified version is more capable of predicting the velocity of the observed low-level jet, which was consistently over-predicted by the original version. Further data processing and analysis is under way. The objective is to develop better parameterizations for turbulent fluxes in the complex terrain leading to improved predictability of local scale air flow. This work is a contribution in this direction and numerous applications follow from this research including air quality modelling and aviation
This paper discusses the performance of the temperature perturbation-type ADMS-Temperature and Hu... more This paper discusses the performance of the temperature perturbation-type ADMS-Temperature and Humidity Model (ADMS-TH) and the Computational Fluid Dynamics (CFD)-based model ENVI-met for the prediction of urban air temperature using measurements collected in the city of Lecce (IT) in summer 2012. The goal is to identify the most important factors influencing numerical predictions. Direct comparisons with measured data and statistical indices show that modelled results are within the range of acceptance. Daily trends are well captured although an underestimation of maximum temperature is observed. In ADMS-TH this is due to an underestimation of sensible heat fluxes during daytime, while in ENVI-met it can be attributed to an underestimation of turbulent momentum and thermal diffusivity. Overall, ADMS-TH did predict the temperature cycle with higher accuracy than ENVI-met and its performance was particularly good during the night. ENVI-met required an ad-hoc tuning of surface boundary conditions to predict nocturnal cooling, satisfactorily.
A simple conceptual model is presented to describe the near-surface flow of a long, partially urb... more A simple conceptual model is presented to describe the near-surface flow of a long, partially urbanized valley of slope β located normal to a coastline, considering forcing due to differential surface temperatures between the sea, undeveloped (rural) land and urban area. Accordingly, under weak synoptic conditions and when the coastal and urban (thermally induced pressure-gradient) forcing are in phase with that of the valley thermal circulation, the mean flow velocity U is parameterized by the cumulative effects of multiple forcing: U=Γw∗β1/3+C(gαΔTL)1/2 . This accounts for the coastal/urban forcing due to surface-air buoyancy difference gαΔT over a distance L . Here Γ and C are constants and w∗ the convective velocity. Comparisons with data of the Meteo-diffusion field experiment conducted in a coastal semi-urbanized valley of Italy (Biferno Valley) reveal that the inferences of the model are consistent with observed valley flow velocities as well as sharp morning and prolonged evening transitions. While the experimental dataset is limited, the agreement with observations suggests that the model captures essential dynamics of valley circulation subjected to multiple forcing. Further observations are necessary to investigate the general efficacy of the model.
Motivated by air quality and numerical modelling applications as well as recent theoretical advan... more Motivated by air quality and numerical modelling applications as well as recent theoretical advancements in the topic, a field experiment, dubbed transition flow experiment, was conducted in Phoenix, Arizona to study the evening transition in complex terrain (shift of winds from upslope to downslope). Two scenarios were considered: (i) the flow reversal due to a change of buoyancy of a cooled slab of air near the ground, and (ii) the formation of a transition front. A suite of in-situ flow, turbulence and particulate matter (PM) concentration sensors, vertically profiling tethered balloons and remote sensors were deployed, and a mesoscale numerical model provided guidance for interpreting observations. The results were consistent with the front formation mechanism, where it was also found that enhanced turbulence associated with the front increases the local PM concentration. During the transition period the flow adjustment was complex, involving the arrival of multiple fronts from different slopes, directional shear between fronts and episodic turbulent mixing events. The upward momentum diffusion from the incipient downslope flow was small because of stable stratification near the ground, and full establishment of downslope flow occurred over several hours following sunset. Episodic frontal events pose challenges to the modelling of the evening transition in complex terrain, requiring conditional parametrizations for subgrid scales. The observed increase of PM concentration during the evening transition has significant implications for the regulatory enforcement of PM standards for the area.
This paper first discusses the aerodynamic effects of trees on local scale flow and pollutant con... more This paper first discusses the aerodynamic effects of trees on local scale flow and pollutant concentration in idealized street canyon configurations by means of laboratory experiments and Computational Fluid Dynamics (CFD). These analyses are then used as a reference modelling study for the extension a the neighbourhood scale by investigating a real urban junction of a medium size city in southern Italy.
A comparison with previous investigations shows that street-level concentrations crucially depend on the wind direction and street canyon aspect ratio W/H (with W and H the width and the height of buildings, respectively) rather than on tree crown porosity and stand density. It is usually assumed in the literature that larger concentrations are associated with perpendicular approaching wind. In this study, we demonstrate that while for tree-free street canyons under inclined wind directions the larger the aspect ratio the lower the street-level concentration, in presence of trees the expected reduction of street-level concentration with aspect ratio is less pronounced.
Observations made for the idealized street canyons are re-interpreted in real case scenario focusing on the neighbourhood scale in proximity of a complex urban junction formed by street canyons of similar aspect ratios as those investigated in the laboratory. The aim is to show the combined influence of building morphology and vegetation on flow and dispersion and to assess the effect of vegetation on local concentration levels. To this aim, CFD simulations for two typical winter/spring days show that trees contribute to alter the local flow and act to trap pollutants. This preliminary study indicates that failing to account for the presence of vegetation, as typically practiced in most operational dispersion models, would result in non-negligible errors in the predictions.
ABSTRACT The evening transition between up and down slope/valley flows in complex terrain remains... more ABSTRACT The evening transition between up and down slope/valley flows in complex terrain remains one of the least understood aspects of mountain meteorology. Non-stationary and spatially inhomogeneous nature of processes involved and possible front formation have stymied the data analysis, delineation of physical processes and numerical modeling of evening transition. Following the theoretical, laboratory and observational insights gained from VTMX, we have conducted an evening transition experiment in the complex terrain of Phoenix airshed (TANSFLEX-2007) and participated in Meteo-Diffusion studies in the Biferno Valley of Italy, organized by ENEA. The results of former show that evening transition is associated with the formation of a front, immediately followed by its downward propagation to initiate the katabatic flow. Convective instabilities in the front also generated intense localized turbulence, thus causing strong aerosol entrainment. Once the downslope flow is established, the near surface stable layer cuts off the communication of ground with the upper layers, thus increasing the longevity of upslope flow aloft. On the other hand, the Biferno valley case is complicated by the contiguous ocean, and differential cooling rates between oceans and valley complicate the transition mechanism. The front formation may still be present, but the mean flow is dominated by the horizontal pressure gradients induced by the variation of land cover.
The Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program is a Multidiscipli... more The Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program is a Multidisciplinary University Research Initiative (MURI) designed to improve weather predictability in mountain terrain. Here we present some results from the first MATERHORN field experiment, a 30-day intense field campaign conducted during September 25-October 25, 2102 at the Granite Mountain Atmospheric Science Testbed (GMAST) of the US Army Dugway Proving Grounds (DPG). Although the thermal circulation at the Granite Peak is considered nominally simple, data from sonic anemometers as well as slow sensors mounted up to seven levels on 4 towers deployed along a main gentle slope (α ≈ 2-3 degrees) of GMAST show that canonical downslope flows existed only for short time, overshadowed by those arriving from nearby mountains and basins. Tethered balloon (traversed up to 400 m agl) and LiDAR measurements operated during Intense Operational Periods (IOPs) confirmed basin-scale interactions causing the dow...
This work focuses on the analysis of exchange processes in-out an urban-like canopy layer (UCL) a... more This work focuses on the analysis of exchange processes in-out an urban-like canopy layer (UCL) and their description in terms of morphometric parameters. Previous work has shown the potential of morphometric methods for the modelling of the mean flow and dispersion within the urban canopy layer. In this study we consider the dependence of flow characteristics from local building features in order to evaluate previous models for the in-canopy and exchange velocities. We use flow and turbulence measurements from four ultrasonic anemometers located in a large industrial site of a city in central Italy and detailed building data. This site has been chosen as an example of isolated neighborhoods of a real city, suitable for testing previous models for the in-canopy and exchange velocity derived from laboratory experiments.
The Sheppard formula (Q J R Meteorol Soc 82:528–529, 1956) for the dividing streamline height Hs ... more The Sheppard formula (Q J R Meteorol Soc 82:528–529, 1956) for the dividing streamline height Hs assumes a uniform velocity U∞ and a constant buoyancy frequency N for the approach flow towards a mountain of height h, and takes the form Hs/h=(1−F), where F=U∞/Nh. We extend this solution to a logarithmic approach-velocity profile with constant N. An analytical solution is obtained for Hs/h in terms of Lambert-W functions, which also suggests alternative scaling for Hs/h. A ‘modified’ logarithmic velocity profile is proposed for stably stratified atmospheric boundary-layer flows. A field experiment designed to observe Hs is described, which utilized instrumentation from the spring field campaign of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program. Multiple releases of smoke at F≈0.3–0.4 support the new formulation, notwithstanding the limited success of experiments due to logistical constraints. No dividing streamline is discerned for F≈10, since, if present, it is too close to the foothill. Flow separation and vortex shedding is observed in this case. The proposed modified logarithmic profile is in reasonable agreement with experimental observations
A field-scale remote soil moisture sensing technique that exploits polarization mode dispersion (... more A field-scale remote soil moisture sensing technique that exploits polarization mode dispersion (PMD) associated with radio frequency (RF) signal propagation is considered in this paper. Microwave polarization responses from rough surface scattering are quantified using a dual-polarized receiver system to estimate PMD responses. Changes in PMD response are elicited by changes in the dielectric properties due to soil moisture changes. The ability of PMD characterizations to remotely detect changes in soil moisture, with responses that exhibit good correlation to ground probe measurements, is demonstrated using a prototype with widely separated transmitter/receiver system deployed at the MATERHORN field experiments conducted at the US Army Dugway Proving Ground, Utah.
This paper first discusses the aerodynamic effects of trees on local scale flow and pollutant con... more This paper first discusses the aerodynamic effects of trees on local scale flow and pollutant concentration in idealized street canyon configurations by means of laboratory experiments and Computational Fluid Dynamics (CFD). These analyses are then used as a reference modelling study for the extension a the neighbourhood scale by investigating a real urban junction of a medium size city in southern Italy.
Emerging application areas such as air pollution in megacities, wind energy, urban security and o... more Emerging application areas such as air pollution in megacities, wind energy, urban security and operation of unmanned aerial vehicles have intensified scientific and societal interest in mountain meteorology. To address science needs and help improve the prediction of mountain weather, the US Department of Defense has funded a research effort – Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program – that draws the expertise of a multidisciplinary, multi-institutional and multinational group of researchers. The program has four principal thrusts, encompassing Modeling, Experimental, Technology and Parameterization components, directed at diagnosing model deficiencies and critical knowledge gaps, conducting experimental studies and developing tools for model improvements. The access to the Granite Mountain Atmospheric Sciences Test Bed of the US Army Dugway Proving Ground, as well as to a suite of conventional and novel high-end airborne and surface measurement platforms has provided an unprecedented opportunity to investigate phenomena of time scales from a few seconds to a few days, covering spatial extents of tens of kilometers down to millimeters. This article provides an overview of the MATERHORN and a glimpse at its initial findings. Orographic forcing creates a multitude of time-dependent sub-mesoscale phenomena that contribute to the variability of mountain weather at mesoscale. The nexus of predictions by mesoscale model ensembles and observations are described, identifying opportunities for further improvements in mountain weather forecasting.
Measurements of small-scale turbulence made over the complex-terrain atmospheric boundary layer d... more Measurements of small-scale turbulence made over the complex-terrain atmospheric boundary layer during the MATERHORN Program are used to describe the structure of turbulence in katabatic flows. Turbulent and mean meteorological data were continuously measured at multiple levels at four towers deployed along the East lower slope (2-4 deg) of Granite Mountain. The multi-level observations made during a 30-day long MATERHORN-Fall field campaign in September-October 2012 allowed studying of temporal and spatial structure of katabatic flows in detail, and herein we report turbulence and their variations in katabatic winds. Observed vertical profiles show steep gradients near the surface, but in the layer above the slope jet the vertical variability is smaller. It is found that the vertical (normal to the slope) momentum flux and horizontal (along the slope) heat flux in a slope-following coordinate system change their sign below and above the wind maximum of a katabatic flow. The vertical momentum flux is directed downward (upward) whereas the horizontal heat flux is downslope (upslope) below (above) the wind maximum. Our study therefore suggests that the position of the jet-speed maximum can be obtained by linear interpolation between positive and negative values of the momentum flux (or the horizontal heat flux) to derive the height where flux becomes zero. It is shown that the standard deviations of all wind speed components (therefore the turbulent kinetic energy) and the dissipation rate of turbulent kinetic energy have a local minimum, whereas the standard deviation of air temperature has an absolute maximum at the height of wind-speed maximum. We report several cases where the vertical and horizontal heat fluxes are compensated. Turbulence above the wind-speed maximum is decoupled from the surface, and follows the classical local z-less predictions for stably stratified boundary layer.
Observations were taken on an east-facing sidewall at the foot of a desert mountain that borders ... more Observations were taken on an east-facing sidewall at the foot of a desert mountain that borders a large valley, as part of the MATERHORN field program at Dugway Proving Ground, Utah. A case study of nocturnal boundary-layer development is presented for a night in mid-May when tethered-balloon measurements were taken to supplement other MATERHORN field measurements. The boundary-layer development over the slope could be divided into three distinct phases during this night: (i) The evening transition from daytime upslope/up-valley winds to nighttime downslope winds was governed by the propagation of the shadow front. Because of the combination of complex topography at the site and the solar angle at this time of year, the shadow moved down the sidewall from approximately northwest to southeast, with the flow transition closely following the shadow front. (ii) The flow transition was followed by a 3–4 h period of almost steady-state boundary-layer conditions, with a shallow slope-parallel surface inversion and a pronounced downslope flow with a jet maximum located within the surface-based inversion. The shallow slope boundary layer was very sensitive to ambient flows, resulting in several small disturbances. (iii) After approximately 2300 MST the inversion that had formed over the adjacent valley repeatedly sloshed up the mountain sidewall, disturbing local downslope flows and causing rapid temperature drops.
The interaction of global climate change and urban heat island (UHI) is expected to have far-reac... more The interaction of global climate change and urban heat island (UHI) is expected to have far-reaching impacts on the sustainability of the world’s rapidly growing urban population centers. Considering that a wide range of spatiotemporal scales contributed by meteorological forcing and complex surface heterogeneity confounds UHI, a multi-model nested approach is used in this paper to study climate-change impacts on Chicago’s UHI covering a range of relevant scales. One-way dynamical downscaling is used with a model chain consisting of global climate (Community Atmosphere Model, CAM), regional climate (Weather Research and Forecasting, WRF), and microscale (ENVI-met) models. Nested mesoscale and microscale models are evaluated against the present-day observations (including a dedicated urban mini-field study), and the results favorably demonstrate the fidelity of the downscaling techniques used. A simple building-energy model is developed and used in conjunction with microscale model output to calculate future energy demands for a building, and a substantial increase (as much as 26% during daytime) is noted for future (~ 2080) climate. Although winds and lake-breeze circulation for future climate are favorable for reducing energy usage by 7%, the benefits are outweighed by such factors as exacerbated UHI and air temperature. An adverse change of human comfort indicators is also noted in future climate with 92% of the population experiencing thermal discomfort. The model chain used has general applicability to evaluate climate-change impacts on city centers, and hence for urban sustainability studies.
Over the past half century, burgeoning urban areas such as Chicago have experienced elevated anth... more Over the past half century, burgeoning urban areas such as Chicago have experienced elevated anthropogenic-induced alteration of local climates within urbanized regions. As a result, urban heat island (UHI) effect in these areas has intensified. Global climate change can further modulate UHI’s negative effects on human welfare and energy conservation. Various numerical models exist to understand, monitor, and predict UHI and its ramifications, but none can resolve all the relevant physical phenomena that span a wide range of scales. To this end, we have applied a comprehensive multi-scale approach to study UHI of Chicago.
The coupling of global, mesoscale, and micro-scale models has allowed for dynamical downscaling from global to regional to city and finally to neighborhood scales. The output of the Community Climate System Model (CCSM5), a general circulation model (GCM), provides future climate scenario, and its coupling with Weather Research and Forecasting (WRF) model enables studies on mesoscale behavior at urban scales. The output from the WRF model at 0.333 km resolution is used to drive a micro-scale model, ENVI-met. Through this coupling the bane of obtaining reliable initial and boundary conditions for the micro-scale model from limited available observational records has been aptly remedied. It was found that the performance of ENVI-met improves when WRF output, rather than observational data, is supplied for initial conditions. The success of the downscaling procedure allowed reasonable application of micro-scale model to future climate scenario provided by CCSM5 and WRF models. The fine (2 m) resolution of ENVI-met enables the study of two key effects of UHI at micro-scale: decreased pedestrian comfort and increased building-scale energy consumption. ENVI-met model’s explicit treatment of key processes that underpin urban microclimate makes it captivating for pedestrian comfort analysis. Building energy, however, is not modeled by ENVI-met so we have developed a simplified building energy model to estimate future cooling needs.
The stable stratified atmospheric boundary layer continues to pose challenges for Numerical Weath... more The stable stratified atmospheric boundary layer continues to pose challenges for Numerical Weather Prediction and Climate models. Existing parameterizations cannot adequately capture the depth of the Planetary Boundary Layer (PBL), low-level jets and nocturnal near surface temperature because of their poor representation of turbulent fluxes, especially in mountainous terrain. In addition, small scale processes such as collisions between katabatic and valley flows which produce intense mixing, strong vertical velocities and a rapid drop in temperature are distributed in space and time and are unable to be captured. These interactions between flows of different scales, in general, can contribute significantly to sub-grid, short-lived, intense turbulence events, spasmodically producing high fluxes over short periods. Such vigorous mixing episodes need to be included in meso-scale models in order to improve their performance, especially in dealing with near surface flows. High-resolution numerical calculations were performed using the Weather Research and Forecasting (WRF) model to test its ability to predict mountain weather. Data from two comprehensive field experiments conducted under the aegis of Mountain Terrain Atmospheric Modelling and Observations (MATERHORN) Program (www.nd.edu/~dynamics/materhorn) were used in this study. Different PBL options available in the WRF model were tested and evaluated for stable conditions. The Yonsei University (YSU) PBL scheme was modified and implemented in WRF. The performance of the modified and original YSU scheme, were compared. Preliminary results show that the modified version is more capable of predicting the velocity of the observed low-level jet, which was consistently over-predicted by the original version. Further data processing and analysis is under way. The objective is to develop better parameterizations for turbulent fluxes in the complex terrain leading to improved predictability of local scale air flow. This work is a contribution in this direction and numerous applications follow from this research including air quality modelling and aviation
This paper discusses the performance of the temperature perturbation-type ADMS-Temperature and Hu... more This paper discusses the performance of the temperature perturbation-type ADMS-Temperature and Humidity Model (ADMS-TH) and the Computational Fluid Dynamics (CFD)-based model ENVI-met for the prediction of urban air temperature using measurements collected in the city of Lecce (IT) in summer 2012. The goal is to identify the most important factors influencing numerical predictions. Direct comparisons with measured data and statistical indices show that modelled results are within the range of acceptance. Daily trends are well captured although an underestimation of maximum temperature is observed. In ADMS-TH this is due to an underestimation of sensible heat fluxes during daytime, while in ENVI-met it can be attributed to an underestimation of turbulent momentum and thermal diffusivity. Overall, ADMS-TH did predict the temperature cycle with higher accuracy than ENVI-met and its performance was particularly good during the night. ENVI-met required an ad-hoc tuning of surface boundary conditions to predict nocturnal cooling, satisfactorily.
A simple conceptual model is presented to describe the near-surface flow of a long, partially urb... more A simple conceptual model is presented to describe the near-surface flow of a long, partially urbanized valley of slope β located normal to a coastline, considering forcing due to differential surface temperatures between the sea, undeveloped (rural) land and urban area. Accordingly, under weak synoptic conditions and when the coastal and urban (thermally induced pressure-gradient) forcing are in phase with that of the valley thermal circulation, the mean flow velocity U is parameterized by the cumulative effects of multiple forcing: U=Γw∗β1/3+C(gαΔTL)1/2 . This accounts for the coastal/urban forcing due to surface-air buoyancy difference gαΔT over a distance L . Here Γ and C are constants and w∗ the convective velocity. Comparisons with data of the Meteo-diffusion field experiment conducted in a coastal semi-urbanized valley of Italy (Biferno Valley) reveal that the inferences of the model are consistent with observed valley flow velocities as well as sharp morning and prolonged evening transitions. While the experimental dataset is limited, the agreement with observations suggests that the model captures essential dynamics of valley circulation subjected to multiple forcing. Further observations are necessary to investigate the general efficacy of the model.
Motivated by air quality and numerical modelling applications as well as recent theoretical advan... more Motivated by air quality and numerical modelling applications as well as recent theoretical advancements in the topic, a field experiment, dubbed transition flow experiment, was conducted in Phoenix, Arizona to study the evening transition in complex terrain (shift of winds from upslope to downslope). Two scenarios were considered: (i) the flow reversal due to a change of buoyancy of a cooled slab of air near the ground, and (ii) the formation of a transition front. A suite of in-situ flow, turbulence and particulate matter (PM) concentration sensors, vertically profiling tethered balloons and remote sensors were deployed, and a mesoscale numerical model provided guidance for interpreting observations. The results were consistent with the front formation mechanism, where it was also found that enhanced turbulence associated with the front increases the local PM concentration. During the transition period the flow adjustment was complex, involving the arrival of multiple fronts from different slopes, directional shear between fronts and episodic turbulent mixing events. The upward momentum diffusion from the incipient downslope flow was small because of stable stratification near the ground, and full establishment of downslope flow occurred over several hours following sunset. Episodic frontal events pose challenges to the modelling of the evening transition in complex terrain, requiring conditional parametrizations for subgrid scales. The observed increase of PM concentration during the evening transition has significant implications for the regulatory enforcement of PM standards for the area.
This paper first discusses the aerodynamic effects of trees on local scale flow and pollutant con... more This paper first discusses the aerodynamic effects of trees on local scale flow and pollutant concentration in idealized street canyon configurations by means of laboratory experiments and Computational Fluid Dynamics (CFD). These analyses are then used as a reference modelling study for the extension a the neighbourhood scale by investigating a real urban junction of a medium size city in southern Italy.
A comparison with previous investigations shows that street-level concentrations crucially depend on the wind direction and street canyon aspect ratio W/H (with W and H the width and the height of buildings, respectively) rather than on tree crown porosity and stand density. It is usually assumed in the literature that larger concentrations are associated with perpendicular approaching wind. In this study, we demonstrate that while for tree-free street canyons under inclined wind directions the larger the aspect ratio the lower the street-level concentration, in presence of trees the expected reduction of street-level concentration with aspect ratio is less pronounced.
Observations made for the idealized street canyons are re-interpreted in real case scenario focusing on the neighbourhood scale in proximity of a complex urban junction formed by street canyons of similar aspect ratios as those investigated in the laboratory. The aim is to show the combined influence of building morphology and vegetation on flow and dispersion and to assess the effect of vegetation on local concentration levels. To this aim, CFD simulations for two typical winter/spring days show that trees contribute to alter the local flow and act to trap pollutants. This preliminary study indicates that failing to account for the presence of vegetation, as typically practiced in most operational dispersion models, would result in non-negligible errors in the predictions.
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Papers by Laura S Leo
variability is smaller. It is found that the vertical (normal to the slope) momentum flux and horizontal (along the slope) heat flux in a slope-following coordinate system change their sign below and above the wind maximum of a katabatic flow. The vertical momentum flux is directed downward (upward)
whereas the horizontal heat flux is downslope (upslope) below (above) the wind maximum. Our study therefore suggests that the position of the jet-speed maximum can be obtained by linear interpolation between positive and negative values of the momentum flux (or the horizontal heat flux) to derive the height where flux becomes zero. It is shown that the standard deviations of all wind speed components (therefore the turbulent kinetic energy) and the dissipation rate of turbulent kinetic energy have a local minimum, whereas the standard
deviation of air temperature has an absolute maximum at the height of wind-speed maximum. We report several cases where the vertical and horizontal heat fluxes are compensated. Turbulence above the wind-speed maximum is decoupled from the surface, and follows the classical local z-less predictions for stably stratified boundary layer.
The coupling of global, mesoscale, and micro-scale models has allowed for dynamical downscaling from global to regional to city and finally to neighborhood scales. The output of the Community Climate System Model (CCSM5), a general circulation model (GCM), provides future climate scenario, and its coupling with Weather Research and Forecasting (WRF) model enables studies on mesoscale behavior at urban scales. The output from the WRF model at 0.333 km resolution is used to drive a micro-scale model, ENVI-met. Through this coupling the bane of obtaining reliable initial and boundary conditions for the micro-scale model from limited available observational records has been aptly remedied. It was found that the performance of ENVI-met improves when WRF output, rather than observational data, is supplied for initial conditions. The success of the downscaling procedure allowed reasonable application of micro-scale model to future climate scenario provided by CCSM5 and WRF models. The fine (2 m) resolution of ENVI-met enables the study of two key effects of UHI at micro-scale: decreased pedestrian comfort and increased building-scale energy consumption. ENVI-met model’s explicit treatment of key processes that underpin urban microclimate makes it captivating for pedestrian comfort analysis. Building energy, however, is not modeled by ENVI-met so we have developed a simplified building energy model to estimate future cooling needs.
Prediction and Climate models. Existing parameterizations cannot adequately capture the depth of the Planetary
Boundary Layer (PBL), low-level jets and nocturnal near surface temperature because of their poor representation of
turbulent fluxes, especially in mountainous terrain. In addition, small scale processes such as collisions between
katabatic and valley flows which produce intense mixing, strong vertical velocities and a rapid drop in temperature
are distributed in space and time and are unable to be captured. These interactions between flows of different scales,
in general, can contribute significantly to sub-grid, short-lived, intense turbulence events, spasmodically producing
high fluxes over short periods. Such vigorous mixing episodes need to be included in meso-scale models in order to
improve their performance, especially in dealing with near surface flows.
High-resolution numerical calculations were performed using the Weather Research and Forecasting (WRF) model to
test its ability to predict mountain weather. Data from two comprehensive field experiments conducted under the
aegis of Mountain Terrain Atmospheric Modelling and Observations (MATERHORN) Program
(www.nd.edu/~dynamics/materhorn) were used in this study. Different PBL options available in the WRF model
were tested and evaluated for stable conditions. The Yonsei University (YSU) PBL scheme was modified and
implemented in WRF. The performance of the modified and original YSU scheme, were compared. Preliminary
results show that the modified version is more capable of predicting the velocity of the observed low-level jet, which
was consistently over-predicted by the original version.
Further data processing and analysis is under way. The objective is to develop better parameterizations for turbulent
fluxes in the complex terrain leading to improved predictability of local scale air flow. This work is a contribution in
this direction and numerous applications follow from this research including air quality modelling and aviation
A comparison with previous investigations shows that street-level concentrations crucially depend on the wind direction and street canyon aspect ratio W/H (with W and H the width and the height of buildings, respectively) rather than on tree crown porosity and stand density. It is usually assumed in the literature that larger concentrations are associated with perpendicular approaching wind. In this study, we demonstrate that while for tree-free street canyons under inclined wind directions the larger the aspect ratio the lower the street-level concentration, in presence of trees the expected reduction of street-level concentration with aspect ratio is less pronounced.
Observations made for the idealized street canyons are re-interpreted in real case scenario focusing on the neighbourhood scale in proximity of a complex urban junction formed by street canyons of similar aspect ratios as those investigated in the laboratory. The aim is to show the combined influence of building morphology and vegetation on flow and dispersion and to assess the effect of vegetation on local concentration levels. To this aim, CFD simulations for two typical winter/spring days show that trees contribute to alter the local flow and act to trap pollutants. This preliminary study indicates that failing to account for the presence of vegetation, as typically practiced in most operational dispersion models, would result in non-negligible errors in the predictions.
variability is smaller. It is found that the vertical (normal to the slope) momentum flux and horizontal (along the slope) heat flux in a slope-following coordinate system change their sign below and above the wind maximum of a katabatic flow. The vertical momentum flux is directed downward (upward)
whereas the horizontal heat flux is downslope (upslope) below (above) the wind maximum. Our study therefore suggests that the position of the jet-speed maximum can be obtained by linear interpolation between positive and negative values of the momentum flux (or the horizontal heat flux) to derive the height where flux becomes zero. It is shown that the standard deviations of all wind speed components (therefore the turbulent kinetic energy) and the dissipation rate of turbulent kinetic energy have a local minimum, whereas the standard
deviation of air temperature has an absolute maximum at the height of wind-speed maximum. We report several cases where the vertical and horizontal heat fluxes are compensated. Turbulence above the wind-speed maximum is decoupled from the surface, and follows the classical local z-less predictions for stably stratified boundary layer.
The coupling of global, mesoscale, and micro-scale models has allowed for dynamical downscaling from global to regional to city and finally to neighborhood scales. The output of the Community Climate System Model (CCSM5), a general circulation model (GCM), provides future climate scenario, and its coupling with Weather Research and Forecasting (WRF) model enables studies on mesoscale behavior at urban scales. The output from the WRF model at 0.333 km resolution is used to drive a micro-scale model, ENVI-met. Through this coupling the bane of obtaining reliable initial and boundary conditions for the micro-scale model from limited available observational records has been aptly remedied. It was found that the performance of ENVI-met improves when WRF output, rather than observational data, is supplied for initial conditions. The success of the downscaling procedure allowed reasonable application of micro-scale model to future climate scenario provided by CCSM5 and WRF models. The fine (2 m) resolution of ENVI-met enables the study of two key effects of UHI at micro-scale: decreased pedestrian comfort and increased building-scale energy consumption. ENVI-met model’s explicit treatment of key processes that underpin urban microclimate makes it captivating for pedestrian comfort analysis. Building energy, however, is not modeled by ENVI-met so we have developed a simplified building energy model to estimate future cooling needs.
Prediction and Climate models. Existing parameterizations cannot adequately capture the depth of the Planetary
Boundary Layer (PBL), low-level jets and nocturnal near surface temperature because of their poor representation of
turbulent fluxes, especially in mountainous terrain. In addition, small scale processes such as collisions between
katabatic and valley flows which produce intense mixing, strong vertical velocities and a rapid drop in temperature
are distributed in space and time and are unable to be captured. These interactions between flows of different scales,
in general, can contribute significantly to sub-grid, short-lived, intense turbulence events, spasmodically producing
high fluxes over short periods. Such vigorous mixing episodes need to be included in meso-scale models in order to
improve their performance, especially in dealing with near surface flows.
High-resolution numerical calculations were performed using the Weather Research and Forecasting (WRF) model to
test its ability to predict mountain weather. Data from two comprehensive field experiments conducted under the
aegis of Mountain Terrain Atmospheric Modelling and Observations (MATERHORN) Program
(www.nd.edu/~dynamics/materhorn) were used in this study. Different PBL options available in the WRF model
were tested and evaluated for stable conditions. The Yonsei University (YSU) PBL scheme was modified and
implemented in WRF. The performance of the modified and original YSU scheme, were compared. Preliminary
results show that the modified version is more capable of predicting the velocity of the observed low-level jet, which
was consistently over-predicted by the original version.
Further data processing and analysis is under way. The objective is to develop better parameterizations for turbulent
fluxes in the complex terrain leading to improved predictability of local scale air flow. This work is a contribution in
this direction and numerous applications follow from this research including air quality modelling and aviation
A comparison with previous investigations shows that street-level concentrations crucially depend on the wind direction and street canyon aspect ratio W/H (with W and H the width and the height of buildings, respectively) rather than on tree crown porosity and stand density. It is usually assumed in the literature that larger concentrations are associated with perpendicular approaching wind. In this study, we demonstrate that while for tree-free street canyons under inclined wind directions the larger the aspect ratio the lower the street-level concentration, in presence of trees the expected reduction of street-level concentration with aspect ratio is less pronounced.
Observations made for the idealized street canyons are re-interpreted in real case scenario focusing on the neighbourhood scale in proximity of a complex urban junction formed by street canyons of similar aspect ratios as those investigated in the laboratory. The aim is to show the combined influence of building morphology and vegetation on flow and dispersion and to assess the effect of vegetation on local concentration levels. To this aim, CFD simulations for two typical winter/spring days show that trees contribute to alter the local flow and act to trap pollutants. This preliminary study indicates that failing to account for the presence of vegetation, as typically practiced in most operational dispersion models, would result in non-negligible errors in the predictions.