Dominique Ruffieux

The COST 720 temperature, humidity, and cloud profiling campaign : TUC

The international COST 720 Temperature, hUmidity, and Cloud (TUC) profiling experiment was organized over three months in winter 2003/2004 at Payerne, Switzerland. Various in-situ and active/passive groundbased remote sensing systems, including three microwave radiometers, a cloud radar and a wind profiler, were operated at the same location. The experiment has delivered a dataset for ground-based remote sensing measurements of winter conditions in the lower troposphere, including fog formation, development and erosion in the boundary layer. The data are being used to test atmospheric profiling products derived from integration of the various measurements. One example is fog/low cloud top and base derived from 78 GHz frequency-modulated continuous wave (FMCW) cloud radar and laser ceilometer measurements. The paper first describes the TUC experiment and the systems involved, including a brief analysis of the radiosoundings quality. An example is then used to show the ability of ground-based remote sensing systems to automatically determine the stratus base and top. Finally an overview of the publications related to the experiment is presented. Zusammenfassung Das internationale COST 720 Temperature, hUmidity and Cloud (TUC) profiling Experiment wurde wahrend drei Monaten im Winter 2003/2004 in Payerne, Schweiz, durchgefuhrt. Verschiedene in-situ- und bodengestutzte aktive/passive Fernerkundungssysteme, darunter drei Mikrowellenradiometer, ein Wolkenradar und ein Windprofiler, wurden am gleichen Ort betrieben. Das E xperiment lieferte einen Datensatz fur bodengestutzte Fernerkundungsmessungen von Winterbedingungen in der unteren Troposphare, welche die Bildung, die Entwicklung und die Auflosung von Nebel in der Gr enzschicht umfassen. Die Daten werden gebraucht, um atmospharische Profilmessungen, welche aus d er Kombination von verschiedenen Messsystemen hergeleitet werden, zu testen. Ein Beispiel ist die Bestimmung der Ober- und Untergrenze von Nebel oder tiefliegenden Wolken anhand von 78 GHz FMCW Wolkenradar und Laser Ceilometer Messungen. Der Artikel beschreibt zunachst das TUC Experiment und die darin verwendeten Messsysteme, sowie eine kurze Betrachtung zur Qualitat der Radiosondierungen. Anhand eines Beispiels wird dann die Moglichkeit gezeigt, mit bodengestutzten Fernerkundungssystemen die Nebelober- und -untergrenze automatisch zu bestimmen. Schlieslich folgt eine Ubersicht der auf das Experiment bezogenen Publikationen.

Detection of Fog and Low Cloud Boundaries with Ground-Based Remote Sensing Systems

The performance of the boundary determination of fog and low stratiform cloud layers with data from a frequency-modulated continuous-wave (FMCW) cloud radar and a Vaisala ceilometer is assessed. During wintertime stable episodes, fog and low stratiform cloud layers often occur in the Swiss Plateau, where the aerological station of Payerne, Switzerland, is located. During the international COST 720 Temperature, Humidity, and Cloud (TUC) profiling experiment in winter 2003/04, both a cloud radar and a ceilometer were operated in parallel, among other profiling instruments. Human eye observations (“synops”) and temperature and humidity profiles from radiosoundings were used as reference for the validation. In addition, two case studies were chosen to demonstrate the possibilities and limitations of such ground-based remote sensing systems in determining low clouds. In these case studies the cloud boundaries determined by ceilometer and cloud radar were furthermore compared with wind profiler signal-to-noise ratio time series. Under dry conditions, cloud-base and -top detection was possible in 59% and 69% of the cases for low stratus clouds and fog situations, respectively. When cases with any form of precipitation were included, performances were reduced with detection rates of 41% and 63%, respectively. The combination of ceilometer and cloud radar has the potential for providing the base and top of a cloud layer with optimal efficiency in the continuous operational mode. The cloud-top height determination by the cloud radar was compared with cloud-top heights detected using radiosounding humidity profiles. The average height difference between the radiosounding and cloud radar determination of the cloud upper boundary is 53 32 m.

Solar and Thermal Radiation Errors on Upper-Air Radiosonde Temperature Measurements

AbstractAtmospheric temperature and humidity profiles are important for weather prediction, but climate change has increased the interest in upper-air observations asking for very high-quality reference measurements. This paper discusses an experimental approach to determine the radiation-induced error on radiosonde air temperature measurements. On the one hand, solar shortwave and thermal longwave radiation profiles were accurately measured during radiosonde ascents from the surface to 35-km altitude. On the other hand, air temperature was measured with several thermocouples on the same flight, simultaneously under sun-shaded and unshaded conditions. The radiation experiments reveal that thermal radiation errors on the very thin thermocouple of the Meteolabor SRS-C34 radiosonde are similar during night- and daytime. They produce a radiative cooling in the lower troposphere and the upper stratosphere, but a radiative heating in the upper troposphere and lower stratosphere. Air temperature experiments with...

Intercomparison of integrated water vapour measurements

Measurements of tropospheric integrated water vapour (IWV) made with two microwave radiometers (ASMUWARA, TP/WVP-3000), GPS, and radiosondes (SRS 400) during the Temperature, hUmidity, and Cloud (TUC) profiling campaign under mid-latitude conditions in Payerne, Switzerland, in winter 2003/2004 are compared. All methods provide robust IWV retrievals in clear sky and cloudy situations. The mean difference between radiometric and radiosonde IWV is less than 0,15 kgm -2 being not significant with respect to the standard deviation and to the theoretical accuracy. The GPS IWV measurements have a persistent significant dry bias of approx: 0,5 kgm -2 with respect to radiometers and radiosondes. The different temporal and spatial resolutions of the instruments were found to have a strong influence on the standard deviation. A characteristic diurnal cycle of the GPS and radiometric IWV was observed.

Influence of Radiation on the Temperature Sensor Mounted on the Swiss Radiosonde

Abstract The Swiss radiosonde (SRS400) measures the air temperature with a very thin copper–constantan thermocouple. The influence of the visible and infrared radiation, as well as the dependency of the air pressure on the measured temperature, is analyzed. After a brief review of the heat transfer by convection, diffusion, and radiation, two independent ways of estimating the difference of temperature between the sensor of the sonde and its environment are presented: 1) laboratory experiments followed by 2) a statistical analysis of aerological soundings. Good agreement between theory, laboratory experiments, and statistical analyses (based on day–night differences) was found. The overall influence of radiation amounts to about 0.8 K at 100 hPa (1.8 K at 10 hPa). At high altitude (low pressure), the heat transfer by diffusion equals the one by convection. Therefore, the diffusion term should not be neglected, as it is often reasonable for the larger sensors or at atmospheric pressure close to ground. As ...

Stratospheric ozone profiles over Switzerland measured by SOMORA, ozonesonde and MLS/AURA satellite

Since January 2000, continuous information on the vertical distribution of ozone over Payerne in Switzerland has been derived from measurements taken by the stratospheric ozone monitoring radiometer SOMORA. A new retrieval of ozone profiles from the measured ozone line spectra has been developed using the inversion software ARTS/Qpack. To validate the new retrieval method it has been compared to ozone profiles measured by ozonesonde and satellite (MLS/AURA). Before May 2001 and after June 2005, the relative difference to ozonesonde is within −5 to 15% depending on the altitude. However, SOMORA and ozonesonde profiles show a difference of up to 30% below 23 km between May 2001 and June 2005, after technical interventions on the instrument, i.e. mixer and front end changes. The new retrieval still has some problems with the instrumental change of 2001 below 23 km but is satisfactory for the change of 2005 where the difference to ozonesonde has been significantly reduced compared to the previous retrieval algorithm. A high difference is shown in summer 2006 which can be related to low tropospheric transmittance during this period. The comparison to MLS on AURA satellite is good before May 2007 with values of −5 to 15% depending on the altitude. The relative difference to satellite ozone profiles show a seasonal cycle with higher values in summer correlated to low tropospheric transmittance.

Wind profiler as a tool to check the ability of two NWP models to forecast winds above highly complex topography

Wind observations from a wind profiler located next to the crest of the Alps are used to test the ability of two Numerical Weather Prediction (NWP) models to forecast the air flow from the surface up to 3000 m above the complex topography. A statistical analysis of the Mesoscale Alpine Programme Special Observing Periods (MAP-SOP) data grouped into three main weather types is first presented. Then a foehn case study is used to illustrate the ability of NWP models to forecast strong wind events over the Alps.

Use of Computed Trajectories in Aerobiology and Air Pollution Studies

Trajectories can be considered as a powerful tool to study transport of aerosols and tracer gases into the atmosphere. The TRAJEK Model (Fay et al. 1994) developed by the DWD (Deutscher Wetterdienst) is used twice daily by MeteoSwiss to compute hundreds of trajectories. Forward trajectories can be used to forecast the transport of nuclear or chemical pollutants in case of civil or military accidents. They are computed at various levels from locations corresponding to existing nuclear installations. Forecasters can use these results any time to predict direction and speed of nuclei transport in case of accidental releases.