Other examples of signatures of mountain waves in radiosonde observations G. Romanens and D. Jacquemin (Report, MeteoSwiss Payerne) It could be a monochromatic wave with 1.8 km This questionable interpretation is valid if horizontal variation of wave field is neglected and if the wave is stationary! Interpretation of spatio-temporal variations in radiosonde data is difficult. Strong amplitudes in vertical wind would suggest horizontal small scales! DE$ Extreme case: Radiosonde observation of a stationary mountain wave Assumption:! z = ", true vertical wavelength (!* z = 1.8 km, apparent wavelength) updraft v asc = 6 m/s, ascent rate of red balloon u= 30 m/s, horizontal mean wind downdraft Intrinsic wave period: T=!* z / v asc = 300 s= 5 min Horizontal wavelength:! h = u T = 9 km Quite realistic values for a local mountain wave which is often seen in the Berner Oberland! This interpretation is supported by the large vertical wind perturbation observed by the balloon. DD$
Kitchen and Shutts (JGR, 1990) gave advices to derive wave parameters in case of nonstationary gravity waves with tilted phase fronts horizontal wind amplitudes of low-frequency inertia gravity waves would be larger and their vertical wind amplitudes would disappear DF$ Airborne measurement of Alpine mountain wave (Doyle and Smith, QJRMS, 2003) stream line of air flow showing a mountain wave with a horizontal wavelength of 10-20 km (by means of airborne measurement of vertical wind) orange/red/brown = backscatter from cloud layer (measured by airborne lidar) horizontal scales of 10-20 km are quite typical for mountain waves! (resonance of buoyancy oscillation?) topography of Hohe Tauern DG$
Wind measurement principles: 1. Drift methods: Time derivative of trajectory of balloon; Rocket+(parachute with GPS sonde, chemical release, or falling metal foils), meteor trails, airplane contrails, radar + drifting plasma irregularities, drifting clouds with infrared camera 2. Wind power: Inertia force of air stream molecules. Windmill and cup anemometers measure horizontal wind speed and wind direction (0 deg=southward (northerly wind), 90 deg=westward (easterly wind)) 3. Wind cooling: Hot-wire anemometer (cooling of wire increases with wind speed) 4. Doppler effect: Frequency shift of acoustic waves or electromagnetic waves (Radar, Lidar, Fabry-Perot interferometer (passive reception of air glow line emission,...) ultrasonic anemometer DH$ Wind measurement principles: 5. Dynamic pressure (with Bernoulli equation): stagnation pressure = static pressure + dynamic pressure p t = p s +! " v 2 v Pitot tube (e.g., at aircraft) # p t p s v DI$
How to measure wind with an airplane? airplane speed vector by GPS air flow vector by 5HP dynamic pressure sensor (u,v,w) = V airplane + M CS1 M CS2 U obs unknown atmospheric wind vector Matrices for coordinate system transformations van den Kroonenberg, et al.: First wind measurements with the meteorological UAV M2AV CAROLO. Available online: http://ams.confex.com/ams/ pdfpapers/139842.pdf. DJ$ Emission WIND PROFILER (1290 MHz RADAR) An electromagnetic pulse is emitted towards the zenith and at least 2 15deg-tilted directions (North and West for ex.) Courtesy of Dominique Ruffieux, MeteoSwiss, Payerne
Radar pulses are backscattered by irregularities of the atmospheric refractive index. The spectra of the radar echos from each altitude range received by the wind profiler are characterized by: Doppler shift #f / f 0 = -v los / c Spectral width Noise level Signal-to-noise ratio (SNR) line-of-sight wind velocity intensity 0 frequency Courtesy of Dominique Ruffieux, MeteoSwiss, Payerne Time-height cross section of Horizontal Wind: 6 November, 1999 Courtesy of Dominique Ruffieux, MeteoSwiss, Payerne
Chemical release experiment (trimethyl aluminium) of rocket (Chu et al., JGR, 2007): FE$ Altitude determination of chemiluminescent trail by means of spaced CCD cameras (Chu et al., 2007): FD$
Retrieved wind profiles (above Taiwan, Chu et al., JGR, 2007): Interpretation: upward propagating, inertio-gravity wave with a period of 11.2 hr and a vertical wavelength of 19.5 km FF$