Other examples of signatures of mountain waves in radiosonde observations

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1 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$

2 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 km (by means of airborne measurement of vertical wind) orange/red/brown = backscatter from cloud layer (measured by airborne lidar) horizontal scales of km are quite typical for mountain waves! (resonance of buoyancy oscillation?) topography of Hohe Tauern DG$

3 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$

4 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: pdfpapers/ 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

5 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

6 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$

7 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$