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Stefano Serafin and Dino Zardi

Abstract

The daytime thermal structures of the valley boundary layer (VBL) and of the convective boundary layer (CBL) above a plain, as revealed by idealized large-eddy simulations, are compared. Simulations in the two environments consider similar thermal forcing, thus allowing an analysis of the atmospheric heating processes in the VBL and CBL in light of the volume-effect theory, traditionally invoked to explain the larger diurnal temperature ranges observed in valleys. It is found that, after an equal input of thermal energy, the atmospheric volumes affected by thermal perturbations in the CBL and in the VBL are comparable. Although the boundary layer top is higher in the VBL than in the CBL, the average VBL depth is approximately equal to the CBL depth, since the ground elevation is nonuniform in the valley. Accordingly, the volume-averaged potential temperature increments in the CBL and VBL are comparable. Nevertheless, surface air temperature variations are larger in the VBL, while differences in the thermal structures of the CBL and the VBL are found to be larger at elevated levels. These effects are related to the heat and mass transfer processes associated with upslope flows and midvalley subsidence. As far as the simulated CBL and VBL cases are representative of two asymptotic regions (respectively, far up valley and far over the plain) of a plain–valley system with a horizontal floor, their comparison provides insight in the mechanisms responsible for the generation of the pressure contrasts driving a daytime plain-to-valley wind at lower levels and possibly a valley-to-plain upper flow.

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Stefano Serafin and Dino Zardi

Abstract

The development of a morning upslope flow is studied by means of idealized numerical simulations. In particular, two cases are examined: a plane slope connecting a lower plain and an elevated plateau and a symmetric mountain in the middle of a uniform plain. The analysis examines various steepness cases and aims at understanding the processes occurring in the area of transition between the upslope flow region and the convective boundary layers (CBLs) growing nearby. A characteristic sequence of events is recognized in the simulations, and their relationship with the along-slope variability of the thermal energy and turbulent kinetic energy budgets is studied. Features occurring after the onset of the upslope wind include a transient depression in the boundary layer depth at the base of the slope and the formation of elevated turbulent layers above the CBL, caused by the divergence of turbulent flow from a thermal plume at the slope top. Numerical evidence agrees well with the results of previous experiments, including both field campaigns and water tank models. It is observed that the occurrence of streamwise inhomogeneities in the upslope flow field favors the occurrence of a multilayered vertical structure of the CBL near heated slopes. Multiple layering appears to be a transient feature, only persisting until sufficient heating causes the merging of the CBL with the overlying elevated turbulent layers. The analysis suggests that the slope steepness is an important factor in determining the speed at which the boundary layer structure near a slope evolves in time: in particular, the development of the wind system appears to occur faster in the vicinity of a steeper slope.

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Stefano Serafin and Dino Zardi

Abstract

The mechanisms governing the daytime development of thermally driven circulations along the transverse axis of idealized two-dimensional valleys are investigated by means of large-eddy simulations. In particular, the impact of slope winds and turbulent convection on the heat transfer from the vicinity of the ground surface to the core of the valley atmosphere is examined. The interaction between top-down heating produced by compensating subsidence in the valley core and bottom-up heating due to turbulent convection is described. Finally, an evaluation of the depth of the atmospheric layer affected by the slope wind system is provided.

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Gabriele Rampanelli, Dino Zardi, and Richard Rotunno

Abstract

The basic physical mechanisms governing the daytime evolution of up-valley winds in mountain valleys are investigated using a series of numerical simulations of thermally driven flow over idealized three-dimensional topography. The three-dimensional topography used in this study is composed of two, two-dimensional topographies: one a slope connecting a plain with a plateau and the other a valley with a horizontal floor. The present two-dimensional simulations of the valley flow agree with results of previous investigations in that the heated sidewalls produce upslope flows that require a compensating subsidence in the valley core bringing down potentially warmer air from the stable free atmosphere. In the context of the three-dimensional valley–plain simulations, the authors find that this subsidence heating in the valley core is the main contributor to the valley– plain temperature contrast, which, under the hydrostatic approximation, is the main contributor to the valley– plain pressure difference that drives the up-valley wind.

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Lorenzo Giovannini, Dino Zardi, and Massimiliano de Franceschi

Abstract

The temperature contrasts typically marking urban heat island (UHI) effects in the city of Trento, Italy, located in an Alpine valley and inhabited in its inner urban area by a population of about 56 000, are investigated. Time series of air temperature data, collected at an urban weather station, in the city center, and at five extraurban stations are compared. The latter are representative of rural and suburban areas, both on the valley floor and on the valley sidewalls. It is found that the extraurban weather stations, being affected by different local-scale climatic conditions, display different temperature contrasts with the urban site. However, the diurnal cycle of the UHI is characterized by similar patterns of behavior at all of the extraurban weather stations: the UHI intensity is stronger at night, whereas during the central hours of the day an “urban cool island” is likely to occur. The diurnal maximum UHI intensity turns out to be typically of order 3°C, but under particularly favorable conditions it may be higher than 6°C. An urban cool island effect is also detected, which is probably caused by the compactness of the inner urban area, and displays typical diurnal maximum intensities of order 1.5°C. As to the seasonal dependence, at the extraurban weather stations on the valley floor the UHI intensity tends to be slightly stronger during dry months, whereas on the valley sidewalls it is mainly influenced by the seasonal lapse-rate changes. Further weather factors, such as wind speed and cloud cover, also affect urbanization effects, making them weaker with stronger winds and cloudier skies.

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Lorenzo Giovannini, Dino Zardi, and Massimiliano de Franceschi

Abstract

The results of measurement campaigns are analyzed to investigate the thermal structure in an urban canyon and to validate a simplified model simulating the air and surface temperatures from surface energy budgets. Starting from measurements at roof-top level, the model provides time series of air and surface temperatures, as well as surface fluxes. Two campaigns were carried out in summer 2007 and in winter 2008/09 in a street of the city of Trento (Italy). Temperature sensors were placed at various levels near the walls flanking the canyon and on a traffic light in the street center. Furthermore, the atmosphere above the mean roof-top level was monitored by a weather station on top of a tower located nearby. Air temperatures near the walls, being strongly influenced by direct solar radiation, display considerable contrasts between the opposite sides of the canyon. On the other hand, when solar radiation is weak or absent, the temperature field remains mostly homogeneous. Moreover, air temperature inside the canyon is generally higher than above roof level, with larger differences during summertime. Air temperatures from the above street measurements are well simulated by the model in both seasons. Furthermore, the modeled surface temperatures are tested against a dataset of wall surface temperatures from the Advanced Tools for Rational Energy Use Towards Sustainability–Photocatalytic Innovative Coverings Applications for Depollution (ATREUS–PICADA) experiment, and a very good agreement is found. Results suggest that the model is a reliable and convenient tool for simplified assessment of climatic conditions occurring in urban canyons under various weather situations.

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Elena Tomasi, Lorenzo Giovannini, Dino Zardi, and Massimiliano de Franceschi

Abstract

The paper presents the results of high-resolution simulations performed with the WRF Model, coupled with two different land surface schemes, Noah and Noah_MP, with the aim of accurately reproducing winter season meteorological conditions in a typical Alpine valley. Accordingly, model results are compared against data collected during an intensive field campaign performed in the Adige Valley, in the eastern Italian Alps. In particular, the ability of the model in reproducing the time evolution of 2-m temperature and of incoming and outgoing shortwave and longwave radiation is examined. The validation of model results highlights that, in this context, WRF reproduces rather poorly near-surface temperature over snow-covered terrain, with an evident underestimation, during both daytime and nighttime. Furthermore it fails to capture specific atmospheric processes, such as the temporal evolution of the ground-based thermal inversion. The main cause of these errors lies in the miscalculation of the mean gridcell albedo, resulting in an inaccurate estimate of the reflected solar radiation calculated by both Noah and Noah_MP. Therefore, modifications to the initialization, to the land-use classification, and to both land surface models are performed to improve model results, by intervening in the calculation of the albedo, of the snow cover, and of the surface temperature. Qualitative and quantitative analyses show that, after these changes, a significant improvement in the comparability between model results and observations is achieved. In particular, outgoing shortwave radiation is lowered, 2-m temperature maxima increased accordingly, and ground-based thermal inversions are better captured.

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Dino Zardi, Marco Falocchi, Lorenzo Giovannini, Werner Tirler, Elena Tomasi, Gianluca Antonacci, Enrico Ferrero, Stefano Alessandrini, Pedro A. Jimenez, Branko Kosovic, and Luca Delle Monache

Capsule

A tracer-based experiment in the Bolzano basin (Italian Alps) reveals peculiar features of orographic advection and turbulent dispersion processes in mountain valleys, and provides a remarkable dataset of atmospheric and concentration measurements for calibration and testing of numerical models over complex terrain.

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