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Dominik Michel, Rolf Philipona, Christian Ruckstuhl, Roland Vogt, and Laurent Vuilleumier

Abstract

Net radiation flux in correlation with surface energy budget, snowmelt, glacier ice balance, and forest or agricultural flux exchange investigations is measured in numerous field experiments. Instrument costs and energy consumption versus performance and uncertainty of net radiation instruments has been widely discussed. Here the authors analyze and show performance and uncertainty of two Kipp and Zonen CNR1 net radiometers, which were compared to high standard reference radiation instruments measuring individual shortwave and longwave downward and upward flux components. The intercomparison was aimed at investigating the performance of the radiometers under different climatological conditions and was made over one year at the midlatitude Baseline Surface Radiation Network (BSRN) station in Payerne, Switzerland (490 MSL). Of the two CNR1 radiometers tested, one was installed in a ventilation and heating system, whereas the other was mounted without ventilation and heating. Uncertainties of the different flux components were found to be larger for shortwave than longwave radiation and larger for downward than upward components. Using the single sensitivity coefficient provided by the manufacturer, which for CNR1 radiometers conditions using all four sensors, rather large root-mean-square differences between 2 and 14 W m−2 were measured for the individual components for hourly averages and between 2 and 12 W m−2 for daily averages. The authors then performed a field calibration, comparing each individual sensor to the reference instrument for one particular day. With the individual field calibration the uncertainty of hourly averages was reduced significantly for all components of the ventilated and heated instrument. For the unventilated CNR1 uncertainties could not be reduced significantly for all sensors. The total net radiation uncertainty of both CNR1 is rather large with up to 26% on daily averages (∼10 W m−2) for the original sensitivity coefficients and without field calibration. Only with the field calibration and for the ventilated and heated CNR1 net radiometer is an uncertainty of 10% of the daily totals of total net radiation reached, as claimed by the manufacturer.

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Robert Spirig, Christian Feigenwinter, Markus Kalberer, Eberhard Parlow, and Roland Vogt
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Roland Schön, Martin Schnaiter, Zbigniew Ulanowski, Carl Schmitt, Stefan Benz, Ottmar Möhler, Steffen Vogt, Robert Wagner, and Ulrich Schurath

Abstract

The imaging unit of the novel cloud particle instrument Particle Habit Imaging and Polar Scattering (PHIPS) probe has been developed to image individual ice particles produced inside a large cloud chamber. The PHIPS produces images of single airborne ice crystals, illuminated with white light of an ultrafast flashlamp, which are captured at a maximum frequency of ∼5 Hz by a charge-coupled device (CCD) camera with microscope optics. The imaging properties of the instrument were characterized by means of crystalline sodium hexafluorosilicate ice analogs, which are stable at room temperature. The optical resolving power of the system is ∼2 μm. By using dedicated algorithms for image processing and analysis, the ice crystal images can be analyzed automatically in terms of size and selected shape parameters. PHIPS has been operated at the cloud simulation chamber facility Aerosol Interaction and Dynamics in the Atmosphere (AIDA) of the Karlsruhe Institute of Technology at different temperatures between −17° and −4°C in order to study the influence of the ambient conditions, that is, temperature and ice saturation ratio, on ice crystal habits. The area-equivalent size distributions deduced from the PHIPS images are compared with the retrieval results from Fourier transform infrared (FTIR) extinction spectroscopy in case of small (<20 μm) and with single particle data from the cloud particle imager in case of larger (>20 μm) ice particles. Good agreement is found for both particle size regimes.

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C. David Whiteman, Manuela Lehner, Sebastian W. Hoch, Bianca Adler, Norbert Kalthoff, Roland Vogt, Iris Feigenwinter, Thomas Haiden, and Matthew O. G. Hills

Abstract

The successive stages of nocturnal atmospheric structure inside a small isolated basin are investigated when a katabatically driven flow on an adjacent tilted plain advects cold air over the basin rim. Data came from Arizona’s Meteor Crater during intensive observing period 4 of the Second Meteor Crater Experiment (METCRAX II) when a mesoscale flow above the plain was superimposed on the katabatic flow leading to a flow acceleration and then deceleration over the course of the night. Following an overflow-initiation phase, the basin atmosphere over the upwind inner sidewall progressed through three stages as the katabatic flow accelerated: 1) a cold-air-intrusion phase in which the overflowing cold air accelerated down the upwind inner sidewall, 2) a bifurcation phase in which the katabatic stable layer lifted over the rim included both a nonnegatively buoyant upper layer that flowed horizontally over the basin and a negatively buoyant lower layer (the cold-air intrusion) that continued on the slope below to create a hydraulic jump at the foot of the sidewall, and 3) a final warm-air-intrusion phase in which shear instability in the upper overflowing layer produced a lee wave that brought warm air from the elevated residual layer downward into the basin. Strong winds during the third phase penetrated to the basin floor, stirring the preexisting, intensely stable, cold pool. Later in the night a wind direction change aloft decelerated the katabatic wind and the atmosphere progressed back through the bifurcation and cold-air-intrusion phases. A conceptual diagram illustrates the first four evolutionary phases.

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Robert Spirig, Roland Vogt, Jarl Are Larsen, Christian Feigenwinter, Andreas Wicki, Joel Franceschi, Eberhard Parlow, Bianca Adler, Norbert Kalthoff, Jan Cermak, Hendrik Andersen, Julia Fuchs, Andreas Bott, Maike Hacker, Niklas Wagner, Gillian Maggs-Kölling, Theo Wassenaar, and Mary Seely

Abstract

An intensive observation period was conducted in September 2017 in the central Namib, Namibia, as part of the project Namib Fog Life Cycle Analysis (NaFoLiCA). The purpose of the field campaign was to investigate the spatial and temporal patterns of the coastal fog that occurs regularly during nighttime and morning hours. The fog is often linked to advection of a marine stratus that intercepts with the terrain up to 100 km inland. Meteorological data, including cloud base height, fog deposition, liquid water path, and vertical profiles of wind speed/direction and temperature, were measured continuously during the campaign. Additionally, profiles of temperature and relative humidity were sampled during five selected nights with stratus/fog at both coastal and inland sites using tethered balloon soundings, drone profiling, and radiosondes. This paper presents an overview of the scientific goals of the field campaign; describes the experimental setup, the measurements carried out, and the meteorological conditions during the intensive observation period; and presents first results with a focus on a single fog event.

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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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Manuela Lehner, C. David Whiteman, Sebastian W. Hoch, Erik T. Crosman, Matthew E. Jeglum, Nihanth W. Cherukuru, Ronald Calhoun, Bianca Adler, Norbert Kalthoff, Richard Rotunno, Thomas W. Horst, Steven Semmer, William O. J. Brown, Steven P. Oncley, Roland Vogt, A. Martina Grudzielanek, Jan Cermak, Nils J. Fonteyne, Christian Bernhofer, Andrea Pitacco, and Petra Klein

Abstract

The second Meteor Crater Experiment (METCRAX II) was conducted in October 2013 at Arizona’s Meteor Crater. The experiment was designed to investigate nighttime downslope windstorm−type flows that form regularly above the inner southwest sidewall of the 1.2-km diameter crater as a southwesterly mesoscale katabatic flow cascades over the crater rim. The objective of METCRAX II is to determine the causes of these strong, intermittent, and turbulent inflows that bring warm-air intrusions into the southwest part of the crater. This article provides an overview of the scientific goals of the experiment; summarizes the measurements, the crater topography, and the synoptic meteorology of the study period; and presents initial analysis results.

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