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Andreas Stohl
,
Kathrin Baumann
,
Gerhard Wotawa
,
Matthias Langer
,
Bruno Neininger
,
Martin Piringer
, and
Herbert Formayer

Abstract

This paper describes a simple method, based on routine meteorological data, to produce high-resolution wind analyses throughout the planetary boundary layer (PBL). It is a new way to interpolate wind measurements. According to this method, high-frequency information from surface wind measurements is extrapolated to greater heights by assuming that the vertical shear of the horizontal wind, that is, the differential vertical wind profile, is horizontally more homogeneous than the wind profile itself. Under this assumption, it is sufficient to combine high-resolution surface wind measurements with low-resolution vertical profiles of differential winds—for which high-resolution measurements usually do not exist—to yield high-resolution wind analyses throughout the PBL. The method can thus be viewed as a diagnostic downscaling of large-scale wind fields. Downscaling works best during daytime within a homogeneous air mass and in flat terrain. A validation against sodar wind measurements demonstrates that downscaling actually improves large-scale wind fields. A comparison of trajectories calculated from large-scale wind fields, from downscaled wind fields, and from wind fields produced by a conventional diagnostic wind field model, with daytime constant level balloon flights, again shows that the downscaled wind fields are most accurate.

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Markus Furger
,
Philippe Drobinski
,
AndréS. H. Prévôt
,
Rudolf O. Weber
,
Werner K. Graber
, and
Bruno Neininger

Abstract

Measurements of the horizontal and vertical wind component by a crosswind scintillometer during foehn, the chinooklike downslope windstorm in the Alps, are presented. Because of the sparsity of vertical velocity measurements in the immediate vicinity, the scintillometer calibration is checked mainly with horizontal wind measurements. Then it is assumed that the calibration is the same for both components. The concept was tested during the Mesoscale Alpine Programme field campaign in the autumn of 1999, during which two scintillometers were deployed. Strong, long-lasting, quasi-stationary downward motions on the order of 5 m s−1 and horizontal wind speeds of over 30 m s−1 were detected during strong foehn phases within the valley. Aircraft measurements of various transects near the light paths are compared with two crosswind evaluation techniques. One of them, the slope method, tends to overestimate the actual wind speed by about 20%, whereas the peak technique gives values that are about 10% too low for high wind speeds. The peak method also fails to measure meaningful vertical crosswind speeds. The scintillometer data of one particular foehn storm are compared with nearby Doppler lidar data. The agreement of the horizontal measurements is reasonable. Discrepancies are attributed to topographic and dynamic effects that cause significant spatial inhomogeneities in the wind field. The applicability of continuous scintillometer vertical crosswind measurements in mountainous terrain is demonstrated.

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Mathias W. Rotach
,
Pierluigi Calanca
,
Giovanni Graziani
,
Joachim Gurtz
,
D. G. Steyn
,
Roland Vogt
,
Marco Andretta
,
Andreas Christen
,
Stanislaw Cieslik
,
Richard Connolly
,
Stephan F. J. De Wekker
,
Stefano Galmarini
,
Evgeny N. Kadygrov
,
Vladislav Kadygrov
,
Evgeny Miller
,
Bruno Neininger
,
Magdalena Rucker
,
Eva Van Gorsel
,
Heidi Weber
,
Alexandra Weiss
, and
Massimiliano Zappa

During a special observing period (SOP) of the Mesoscale Alpine Programme (MAP), boundary layer processes in highly complex topography were investigated in the Riviera Valley in southern Switzerland. The main focus was on the turbulence structure and turbulent exchange processes near the valley surfaces and free troposphere. Due to the anticipated spatial inhomogeneity, a number of different turbulence probes were deployed on a cross section through the valley. Together with a suite of more conventional instrumentation, to observe mean meteorological structure in the valley, this effort yielded a highly valuable dataset. The latter is presently being exploited to yield insight into the turbulence structure in very complex terrain, and its relation to flow regimes and associated mean flow characteristics. Specific questions, such as a detailed investigation of turbulent exchange processes over complex topography and the validity of surface exchange parameterizations in numerical models for such surfaces, the closure of the surface energy balance, or the definition and meaning of the “boundary layer height,” are investigated using the MAP-Riviera dataset. In the present paper, we provide details on sites and their characteristics, on measurements and observational strategies, and on efforts to guarantee comparability between different instrumentation at different sites, and we include an overview of the available instrumentation. On the basis of preliminary data and first results, the main research goals of the project are outlined.

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Jason Beringer
,
Jorg Hacker
,
Lindsay B. Hutley
,
Ray Leuning
,
Stefan K. Arndt
,
Reza Amiri
,
Lutz Bannehr
,
Lucas A. Cernusak
,
Samantha Grover
,
Carol Hensley
,
Darren Hocking
,
Peter Isaac
,
Hizbullah Jamali
,
Kasturi Kanniah
,
Stephen Livesley
,
Bruno Neininger
,
Kyaw Tha Paw U
,
William Sea
,
Dennis Straten
,
Nigel Tapper
,
Richard Weinmann
,
Stephen Wood
, and
Steve Zegelin

Savannas are highly significant global ecosystems that consist of a mix of trees and grasses and that are highly spatially varied in their physical structure, species composition, and physiological function (i.e., leaf area and function, stem density, albedo, and roughness). Variability in ecosystem characteristics alters biophysical and biogeochemical processes that can affect regional to global circulation patterns, which are not well characterized by land surface models. We initiated a multidisciplinary field campaign called Savanna Patterns of Energy and Carbon Integrated across the Landscape (SPECIAL) during the dry season in Australian savannas to understand the spatial patterns and processes of land surface–atmosphere exchanges (radiation, heat, moisture, CO2, and other trace gasses). We utilized a combination of multiscale measurements including fixed flux towers, aircraft-based flux transects, aircraft boundary layer budgets, and satellite remote sensing to quantify the spatial variability across a continental-scale rainfall gradient (transect). We found that the structure of vegetation changed along the transect in response to declining average rainfall. Tree basal area decreased from 9.6 m2 ha−1 in the coastal woodland savanna (annual rainfall 1,714 mm yr−1) to 0 m2 ha−1 at the grassland site (annual rainfall 535 mm yr−1), with dry-season green leaf area index (LAI) ranging from 1.04 to 0, respectively. Leaf-level measurements showed that photosynthetic properties were similar along the transect. Flux tower measurements showed that latent heat fluxes (LEs) decreased from north to south with resultant changes in the Bowen ratios (H/LE) from a minimum of 1.7 to a maximum of 15.8, respectively. Gross primary productivity, net carbon dioxide exchange, and LE showed similar declines along the transect and were well correlated with canopy LAI, and fluxes were more closely coupled to structure than floristic change.

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