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.