radar observations of the utls region geraint vaughan university of manchester, uk
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Radar observations of Radar observations of the UTLS regionthe UTLS region
Geraint VaughanGeraint Vaughan
University of Manchester, UK
TopicsTopics
Introduction to radarIntroduction to radar
VHF radarVHF radar
What do they measure and how?What do they measure and how?
Mesoscale structure of stormsMesoscale structure of storms
Inertia-gravity wavesInertia-gravity waves
Cloud radarCloud radar
Basics of radar methodBasics of radar method
Pulses of EM radiation ~ Pulses of EM radiation ~ 1 µs long1 µs longHeterodyne detection Heterodyne detection (Local Oscillator)(Local Oscillator)Doppler spectrum allows Doppler spectrum allows velocity of target to be velocity of target to be measuredmeasuredPolarisation of radar Polarisation of radar beam can reveal target beam can reveal target shapeshapeHeight resolution for Height resolution for distributed target distributed target z=z=½c½c
Pulse length
TIME
Height
z
= 1 s z = 150 m
= 0.1 s z = 15 m
Doppler methodDoppler methodDoppler shift from a moving target:
= 2V/
When return signal is mixed with local oscillator, the Doppler shift of the signal is obtained.
To measure the spectrum, the signal is sampled at intervals t (several return pulses combined). A Fourier transform of N points then gives the spectrum.
t determines the maximum unambiguous velocity (Nyquist frequency):
max = 1/(2t)
Vmax = ½ max = /(4t)
e.g. = 6m, t = ⅓s Vmax = 4.5 ms-1
Number of points in FT, N, determines separation of points in spectrum
Let T be the length of the FT; T=Nt
V = /(2T)
e.g. = 6m, T = 10 s V = 0.3 ms-1
0
0.5
1.0
1.5
2.0
-5.0 -2.5 0 2.5 5.0
Mean Doppler shift
Spectral width
Frequency shift (velocity)
Po
we
r sp
ect
ral d
en
sity
Radar targetsRadar targets
Particles: raindrops, ice particles. In UTLS, Particles: raindrops, ice particles. In UTLS, best observed with short wavelength best observed with short wavelength radar, e.g. 78 GHz (4 mm)radar, e.g. 78 GHz (4 mm)Clear air: Inhomogenieties in refractive Clear air: Inhomogenieties in refractive index on scale of radar wavelength:index on scale of radar wavelength: a) a) in troposphere, variations in humidityin troposphere, variations in humidity b) b) in lower stratosphere, variations in in lower stratosphere, variations in c) c) in mesosphere, variations in electron in mesosphere, variations in electron
densitydensity
In the early days of radar, clear-air echoes were called ‘angels’!
Types of radar used for UTLSTypes of radar used for UTLS
VHF radars (50 – 70 VHF radars (50 – 70 MHz, vertically MHz, vertically pointing – clear air pointing – clear air radarsradars
S-band (~3 GHz) – S-band (~3 GHz) – operational weather operational weather radarsradars
35, 78, 95 GHz – cloud 35, 78, 95 GHz – cloud radars for cirrus radars for cirrus observations observations
Frequency and Wavelength of the IEEE Radar Band designation
300-3000 kHz 1 km-100 m ...MF 3-30 MHz 100-10 m ........HF
30-300 MHz 10-1 m ..........VHF 300-3000 MHz 1 m-10 cm .. UHF
1-2 GHz ............30-15 cm ....L Band 2-4 GHz ...........15-7.5 cm.....S Band 4-8 GHz ........7.5-3.75 cm.....C Band 8-12 GHz ......3.75-2.50 cm... X Band 12-18 GHz ......2.5-1.67 cm...Ku Band 18-27 GHz .....1.67-1.11 cm....K Band 27-40 GHz 1.11 cm-7.5 mm .Ka Band 40-75 GHz...............................V Band 75-110 GHz............................W Band 110-300 GHz ......................mm Band 300-3000 GHz...................u mm Band
Lower frequencies used for mesospheric observations e.g HF, MF
The UK MST radarThe UK MST radar
46.5 MHz coded pulses (6 m wavelength)46.5 MHz coded pulses (6 m wavelength)Runs continuously (24/7)Runs continuously (24/7)Typical height resolution 300m, time resolution 2 minTypical height resolution 300m, time resolution 2 minMeasures echo power, winds, turbulenceMeasures echo power, winds, turbulence
http://mst.nerc.ac.uk/
What does the MST measure?What does the MST measure?
Echoes mainly from clear air - Echoes mainly from clear air - precipitation echoes are precipitation echoes are possible but unusualpossible but unusualWinds from Doppler shift of Winds from Doppler shift of returned echoreturned echo3 components of wind by beam 3 components of wind by beam swinging (6swinging (6º off zenith) – º off zenith) – achieved by changing the achieved by changing the phasephase of the EM wave across of the EM wave across the arraythe arrayTurbulence from spectral width Turbulence from spectral width of returned echoof returned echo
6º6º
Beam width ~ 2.3º
MST Echo powerMST Echo power
Power is proportional to Power is proportional to potential refractivity Mpotential refractivity M22::
So, high echo power So, high echo power denotes: denotes:
- high static stability OR - high static stability OR negative humidity negative humidity
gradientgradient
M z
qT
qz
pT
- 7800
T1
1 15600
Fresnel scatter is the most common echo: anisotropic, partial reflection at small steps in the θ or q profile
Strong turbulence gives Bragg scatter. This is isotropic: EM wave scattered off corresponding wave vector in turbulent field
And anything in between..
Tropopause observed by MST radarTropopause observed by MST radar
Definite Tropopause Indefinite
Tropopause
Passage of cold front observed Passage of cold front observed by MST radarby MST radar
Sting Jet observed by MST radarSting Jet observed by MST radar
CH
Comparison with UK Unified model mesoscale fieldsComparison with UK Unified model mesoscale fields
Surface chart, midnight 27/10/02 NOAA IR, 0300 27/10/02
MST radar (colour) and UM (contour) zonal windsMST radar (colour) and UM (contour) zonal winds
MST radar power with UM RHMST radar power with UM RH
Inertia-gravity wavesInertia-gravity waves
Long-period gravity waves, affected by Earth’s rotation.
Frequency ~ f
Horizontal Wavelength > 100 km
Vertical wavelength ~2 km
Wind vector rotates elliptically with time or ht.
Wave packet = ? km
Group velocity
Phase velocity
Phase front
Path traced by wind vector over time
z
The case of July 1999 The case of July 1999
Eastward wind component measured over 4 days, 7-11 July 1999
Echo power (dB), showing that wave modulates static stability
Spectral width, indicating (weak) turbulence
LINES DENOTE EASTWARD WIND MAXIMA
Wave sourcesWave sources
Strong deceleration at jet stream level Strong deceleration at jet stream level (e.g. jet exits or highly curved jets)(e.g. jet exits or highly curved jets)
Baroclinic instabilityBaroclinic instability
Instability of a horizontal shear layerInstability of a horizontal shear layer
ConvectionConvection
Orographic forcing Orographic forcing
Instability of shear layerInstability of shear layerMeteosat water vapour images every 12 hrs from 06h 7 March 1997
Courtesy Heini Wernli
Radar data, 8-9 March 1997
Chilbolton ObservatoryChilbolton Observatory
3 GHz radar for precipitation measurements
95 GHz cloud radar (left) and measurements of cirrus cloud, 2 June 2000.
35 GHz cloud radar
http://www.chilbolton.rl.ac.uk/
SummarySummaryVHF radars have been the main radar tool to date for UTLS studies.
They measure winds, turbulence and vertical structure and are very good for gravity waves, tropopause height and mesoscale structures
There are about a dozen research radars around the world and several more used operationally
Mm wave radar technology has now advanced sufficiently that cloud radars (10s of GHz in frequency) are routinely used for cirrus measurements in the UTLS
Some bibliography:Doviak, R. J. and D. S. Zrnic. Doppler radar and weather observations. Academic Press, 1993. G. Vaughan. The UK MST radar. Weather, 57, 67-73, 2002. H. J. Reid and G. Vaughan. Convective mixing in a tropopause fold. Quart. J. Roy. Meteorol.
Soc., 130, 1195-1212, 2004. E. Pavelin, J. Whiteway and G. Vaughan. Observation of a long-period gravity wave in the lower
stratosphere. J. Geophys. Res., 106, 5153-5179, 2001. G. Vaughan and R. M. Worthington. Break-up of a stratospheric streamer observed by MST
radar. Quart. J. Roy. Met. Soc., 126, 1751-69, 2000.