Proceedings with Extended Abstracts (single PDF file) - Radio ...

DETERMINATION OF THE TURBULENT FLUXES OF MOMENTUMAND VIRTUAL SENSIBLE HEAT WITH AN UHF RASS PROFILER.COMPARISON WITH IN SITU MEASUREMENTSB. Bénech, F Girard-Ardhuin, B. Campistron, F. Saïd, F. Lohou, and V. Puygrenier.Laboratoire d'Aérologie, UMR 5560 CNRS/UPS, Observatoire Midi-Pyrénées, Centre deRecherches Atmosphériques, 65300 Lannemezan, France.1. IntroductionUHF wind profiler equipped with five antenna has shown to be a very useful tool forthe investigation of the convective Atmospheric Boundary Layer (ABL) with a very goodtemporal and vertical resolution. Previous studies have demonstrated its ability to retrieve thewind velocity field and the mixing height. The Doppler spectral width contains anotherimportant information about the fine structure of the atmospheric dynamics, that is theturbulence intensity. For temperature measurements, the UHF profiler is equipped with aRASS system using acoustic sources located nearby the UHF antenna. From the verticalvelocity of this ‘acoustic echo’ measured by the UHF radar, air virtual temperature can beinferred. The spectral width of acoustic echo has been little used as thermal turbulentindicator.First, we have investigated one method to evaluate the momentum flux based on fourbeams measurements. The momentum flux is deduced from the combination of the spectralwidth measurements on two pairs of opposite beams. This estimation has been made in theboundary layer during windy and convective conditions. Some comparisons have been madewith aircraft data during two experiments.Second, we have used two methods to evaluate the vertical virtual sensible heat fluxusing the height of the boundary layer, the temperature, the spectral width of the verticalvelocity. These estimations are compared with in situ measurements made on mast and byaircraft during several field campaigns.2. UHF characteristicsThe Degréane UHF-RASS wind profiler is a 1.238 GHz radar, working with fivebeams (with 17° off-zenith angle) and four acoustic sources, the beamwidth is 8.5°. It giveswind parameters profiles every 5 minutes with a 75 m height vertical resolution, from 75 m upto 2 to 3 km, depending on atmospheric conditions. In RASS configuration, the radar providesvirtual temperature profiles up to 1 km. During the experiment with RASS system, arepetitive sequence made of 5 minutes of RASS mode followed by 10 minutes of windprofiling was used. At each range gate, an online consensus technique on the Doppler spectraselects the atmospheric peak, from which are inferred (for each beam) the radial velocity,reflectivity and spectral width. The virtual temperature is deduced from the measurement ofthe vertical velocity of the UHF signal backscattered on the acoustic front. The consensusprocessing works over a 30 minute running period for the dynamic and thermal data.3. Mean and turbulent parameters deduced from UHF/RASS profilerThe UHF-RASS provides an overview of the diurnal cycle of mean and turbulentcharacteristics in the ABL through dynamic and thermic measurements. The horizontal windhas been validated on different meteorological conditions by comparison with sodar and mastmeasurements (for example, see Jacoby-Koaly et al., 2002). UHF negative vertical velocitybias evidenced by Angevine (1987) in the convective boundary layer has been studied byLothon et al (2002). Figure 1 presents a composite day based on 13 days of measurement of310