Transcript Wind profiler observations of small

Radar observations of
the UTLS region
Geraint Vaughan
University of Manchester, UK
Topics
Introduction to radar
VHF radar
What do they measure and how?
Mesoscale structure of storms
Inertia-gravity waves
Cloud radar
Basics of radar method
Pulses of EM radiation ~
1 µs long
Heterodyne detection
(Local Oscillator)
Doppler spectrum allows
velocity of target to be
measured
Polarisation of radar
beam can reveal target
shape
Height resolution for
distributed target z=½c
 = 1 s  z = 150 m
 = 0.1 s  z = 15 m
z
H
e
i
g
h
t
Pulse length 
TIME
Doppler method
Doppler shift from a moving target:
Number of points in FT, N, determines
separation of points in spectrum
 = 2V/
Let T be the length of the FT; T=Nt
When return signal is mixed with local
oscillator, the Doppler shift of the signal
is obtained.
t determines the maximum
unambiguous velocity (Nyquist
frequency):
max = 1/(2t)
Vmax = ½ max = /(4t)
e.g.  = 6m, t = ⅓s  Vmax = 4.5 ms-1
e.g.  = 6m, T = 10 s  V = 0.3 ms-1
Power spectral density
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.
V = /(2T)
2.0
Mean Doppler shift
1.5
Spectral width
1.0
0.5
0
-5.0
-2.5
0
2.5
Frequency shift (velocity)
5.0
Radar targets
Particles: raindrops, ice particles. In UTLS,
best observed with short wavelength
radar, e.g. 78 GHz (4 mm)
Clear air: Inhomogenieties in refractive
index on scale of radar wavelength:
a) in troposphere, variations in humidity
b) in lower stratosphere, variations in 
c) in mesosphere, variations in electron
density
In the early days of radar, clear-air echoes were called ‘angels’!
Types of radar used for UTLS
VHF radars (50 – 70
MHz, vertically
pointing – clear air
radars
S-band (~3 GHz) –
operational weather
radars
35, 78, 95 GHz – cloud
radars for cirrus
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 radar
46.5 MHz coded pulses (6 m wavelength)
Runs continuously (24/7)
Typical height resolution 300m, time resolution 2 min
Measures echo power, winds, turbulence
http://mst.nerc.ac.uk/
What does the MST measure?
Echoes mainly from clear air precipitation echoes are
possible but unusual
Winds from Doppler shift of
returned echo
3 components of wind by beam
swinging (6º off zenith) –
achieved by changing the
phase of the EM wave across
the array
Turbulence from spectral width
of returned echo
6º
Beam width ~ 2.3º
6º
MST Echo power
Power is proportional to
potential refractivity M2:
M 
p 1  
q
7800 q 
1

15600




T  z 
T
T z 
So, high echo power
denotes:
 - high static stability OR
 negative humidity
gradient
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 radar
Definite
Tropopause
Indefinite
Tropopause
Passage of cold front observed
by MST radar
Sting Jet observed by MST radar
Surface chart, midnight 27/10/02
NOAA IR, 0300 27/10/02
C
H
Comparison with UK Unified model mesoscale fields
MST radar (colour) and UM (contour) zonal winds
MST radar power with UM RH
Inertia-gravity waves
Long-period gravity waves,
affected by Earth’s rotation.
Phase velocity
Group velocity
Frequency ~ f
Horizontal Wavelength > 100
km
z
Vertical wavelength ~2 km
Wind vector rotates
elliptically with time or ht.
Wave packet = ? km
Phase front
Path traced
by wind
vector over
time
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 sources
Strong deceleration at jet stream level
(e.g. jet exits or highly curved jets)
Baroclinic instability
Instability of a horizontal shear layer
Convection
Orographic forcing
Instability of shear layer
Meteosat water vapour images every 12 hrs from 06h 7 March 1997
Courtesy Heini Wernli
Radar data, 8-9
March 1997
Chilbolton Observatory
35 GHz cloud radar
95 GHz cloud radar (left)
and measurements of cirrus
cloud, 2 June 2000.
http://www.chilbolton.rl.ac.uk/
3 GHz radar for precipitation
measurements
Summary
VHF 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.