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Sofia Farina
,
Mattia Marchio
,
Francesco Barbano
,
Silvana di Sabatino
, and
Dino Zardi

Abstract

This paper investigates the surface-layer processes associated with the morning transition from nighttime downslope winds to daytime upslope winds over a semi-isolated massif. It provides an insight into the characteristics of the transition and its connection with the processes controlling the erosion of the temperature inversion at the foot of the slope. First, a criterion for the identification of days prone to the development of purely thermally driven slope winds is proposed and adopted to select five representative case studies. Then, the mechanisms leading to different patterns of erosion of the nocturnal temperature inversion at the foot of the slope are analyzed. Three main patterns of erosion are identified: the first is connected to the growth of the convective boundary layer at the surface, the second is connected to the descent of the inversion top, and the third is a combination of the previous two. The first pattern is linked to the initiation of the morning transition through surface heating, and the second pattern is connected to the top-down dilution mechanism and so to mixing with the above air. The discriminating factor in the determination of the erosion pattern is identified in the partitioning of turbulent sensible heat flux at the surface.

Significance Statement

The purpose of this study is to improve our understanding of the thermally driven slope circulations with a focus on the unsteady processes associated with the morning transition and the erosion patterns of the nocturnal temperature inversion, so far in the literature less investigated and understood than the evening transition. Understanding this diurnal process will advance our abilities to model it and to improve the accuracy of weather forecasting in complex terrain.

Open access
Silvana Di Sabatino
,
Laura S. Leo
,
Rosella Cataldo
,
Carlo Ratti
, and
Rex E. Britter

Abstract

A morphometric analysis of a southern European city and the derivation of relevant fluid dynamical parameters for use in urban flow and dispersion models are explained in this paper. Calculated parameters are compared with building statistics that have already been computed for parts of three northern European and two North American cities. The aim of this comparison is to identify similarities and differences between several building configurations and city types, such as building packing density, compact versus sprawling neighborhoods, regular versus irregular street orientation, etc. A novel aspect of this work is the derivation and use of digital elevation models (DEMs) for parts of a southern European city. Another novel aspect is the DEMs’ construction methodology, which is low cost, low tech, and of simple implementation. Several building morphological parameters are calculated from the urban DEMs using image processing techniques. The correctness and robustness of these techniques have been verified through a series of sensitivity tests performed on both idealized building configurations, as well as on real case DEMs, which were derived using the methodology here. In addition, the planar and frontal area indices were calculated as a function of elevation. It is argued that those indices, estimated for neighborhoods of real cities, may be used instead of the detailed building geometry within urban canopy models as those indices together synthesize the geometric features of a city. The direct application of these results will facilitate the development of fast urban flow and dispersion models.

Full access
Luigi Brogno
,
Francesco Barbano
,
Laura Sandra Leo
,
Harindra J. S. Fernando
, and
Silvana Di Sabatino

Abstract

In the realm of boundary layer flows in complex terrain, low-level jets (LLJs) have received considerable attention, although little literature is available for double-nosed LLJs that remain not well understood. To this end, we use the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) dataset to demonstrate that double-nosed LLJs developing within the planetary boundary layer (PBL) are common during stable nocturnal conditions and present two possible mechanisms responsible for their formation. It is observed that the onset of a double-nosed LLJ is associated with a temporary shape modification of an already-established LLJ. The characteristics of these double-nosed LLJs are described using a refined version of identification criteria proposed in the literature, and their formation is classified in terms of two driving mechanisms. The wind-driven mechanism encompasses cases where the two noses are associated with different air masses flowing one on top of the other. The wave-driven mechanism involves the vertical momentum transport by an inertial–gravity wave to generate the second nose. The wave-driven mechanism is corroborated by the analysis of nocturnal double-nosed LLJs, where inertial–gravity waves are generated close to the ground by a sudden flow perturbation.

Open access
Manuela Lehner
,
C. David Whiteman
,
Sebastian W. Hoch
,
Derek Jensen
,
Eric R. Pardyjak
,
Laura S. Leo
,
Silvana Di Sabatino
, and
Harindra J. S. Fernando

Abstract

Observations were taken on an east-facing sidewall at the foot of a desert mountain that borders a large valley, as part of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) field program at Dugway Proving Ground in 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: 1) 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. 2) 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. 3) After approximately 2300 mountain standard time, the inversion that had formed over the adjacent valley repeatedly sloshed up the mountain sidewall, disturbing local downslope flows and causing rapid temperature drops.

Full access
Janet Barlow
,
Martin Best
,
Sylvia I. Bohnenstengel
,
Peter Clark
,
Sue Grimmond
,
Humphrey Lean
,
Andreas Christen
,
Stefan Emeis
,
Martial Haeffelin
,
Ian N. Harman
,
Aude Lemonsu
,
Alberto Martilli
,
Eric Pardyjak
,
Mathias W Rotach
,
Susan Ballard
,
Ian Boutle
,
Andy Brown
,
Xiaoming Cai
,
Matteo Carpentieri
,
Omduth Coceal
,
Ben Crawford
,
Silvana Di Sabatino
,
Junxia Dou
,
Daniel R. Drew
,
John M. Edwards
,
Joachim Fallmann
,
Krzysztof Fortuniak
,
Jemma Gornall
,
Tobias Gronemeier
,
Christos H. Halios
,
Denise Hertwig
,
Kohin Hirano
,
Albert A. M. Holtslag
,
Zhiwen Luo
,
Gerald Mills
,
Makoto Nakayoshi
,
Kathy Pain
,
K. Heinke Schlünzen
,
Stefan Smith
,
Lionel Soulhac
,
Gert-Jan Steeneveld
,
Ting Sun
,
Natalie E Theeuwes
,
David Thomson
,
James A. Voogt
,
Helen C. Ward
,
Zheng-Tong Xie
, and
Jian Zhong
Open access