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Dominique Ruffieux and Juerg Joss

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 a result of applying the experiments in the laboratory to , the influence of longwave radiation is negligible compared to other influences.

Based on the results herein, a second-degree polynomial fit was calculated for correcting the bias caused by radiation on the measured temperatures. This correction is operationally applied with success to the daytime soundings performed with the SRS400.

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Dominique Ruffieux, Daniel E. Wolfe, and Catherine Russell

Abstract

Denver's Continuous Air Monitoring Program (CAMP) site, typically recording the highest carbon monoxide levels in the metropolitan area; lies within a large region of downtown Denver shadowed by tall buildings. Two studies conducted during the winters of 1987/88 and 1988/89 indicated several possible scenarios leading to the high-pollution episodes often reported at CAMP. Sodar records and stability calculations at CAMP indicate that building shadows may be a contributing factor. The building shadowing was simulated by a computer model and its effects were examined from 2 days of detailed vertical temperature profiles taken in the vicinity of CAMP. The vertical temperature structure was mapped both spatially and temporally as it pertains to the shadowed and unshadowed regions. Results show that shadowing at CAMP is quickly followed by the formation of a shadow surface-based inversion and a local rise in carbon monoxide concentrations. Strength of the inversion depends on the meteorology and surface albedo and relates to a difference in solar radiation intensity of >100 W m−2 between shadowed and unshadowed regions.

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Daniela Nowak, Dominique Ruffieux, Judith L. Agnew, and Laurent Vuilleumier

Abstract

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.

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F. Martin Ralph, Paul J. Neiman, and Dominique Ruffieux

Abstract

Correlations between range-corrected signal power S rc and radial vertical velocity Vr, from the vertical beam of a UHF wind profiler can be used to distinguish between air- and precipitation-dominated echoes using an S rcVr correlation diagram. While there is no clear correlation between vertical air motions and S rc, there is a strong correlation between the precipitation fall velocity and S rc in snow, and to a lesser extent, in rain. This is illustrated through intercomparison of three types of precipitation events, and two types of clear-air events.

Using a histogram of Vr, from an event where there is evidence of precipitation in its S rcVr correlation diagram, and from other information, it is possible to objectively determine a threshold value of Vr, referred to as VT, that approximately identifies which measurements are dominated by Rayleigh scattering from precipitation in that event. A method is introduced that uses the histogram of observed Vr, from that event to provide an estimate of how many measurements are incorrectly attributed to Bragg scattering or Rayleigh scattering as a function of VT. The error estimates can be used to select VT on a case-by-case basis and according to the needs of the particular application. An objective dual-optimization technique results in an estimated overall error of less than 6%, averaged over three case studies. In addition, it is shown that inclusion of velocity variance from the vertical beam in the S rcVr, correlation diagrams can help distinguish between rain and snow, and between convective and stratiform precipitation.

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