mrs ch8xpol - engineeringece.uprm.edu/~pol/pdf/atmosphericconstituents.pdf · 4/18/17 5 microwave...

4/18/17

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• Ulaby• Long• Blackwell• Elachi• Fung• Ruf• Sarabandi• Zebker• Van Zyl

WIRELESS COMMUNICATION

MRS Ch 8 MICROWAVE INTERACTION WITH ATMOSPHERIC CONSTITUENTS

Why use a specific band to measure a specific Earth parameter (Ocean)

Earth Parameters above Ocean

Why use a specific band to measure a specific Earth parameter (Land)

Sensitivity

Earth Parameters above Land

Microwave Radar and Radiometric Remote Sensing, by Ulaby and Long

Chapter 8 Contents

u 8-1 Vertical profile of the atmosphereu 8-2 Absorption and emission by gasesu 8-3 Opacity of the clear atmosphereu 8-4 Emission by the clear atmosphereu 8-5 Extinction by hydrometeorsu 8-6 Dielectric properties of hydrometeorsu 8-7 Extinction and backscattering by clouds, fog, or hazeu 8-8 Extinction and backscattering by rainu 8-9 Radar equation for meteorologyu 8-10 Emission by clouds and rainu 8-11 Error sources and estimation statistics


Direct & Inverse Problems

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Standard Atmosphere


Atmospheric Composition

• Note that O2 is 21% by volume

• Also, N2 and O2 together account for 99%

Typical Atmosphere in %

78

210.93

NiO2Ar


Atm. CO2 Concentration 9


Height Profiles

8.1.4 Pressure Profile


Water Vapor Density Profile


Absorption and Emission by Gases

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Absorption Spectrum

It follows that:


Absorption Spectrum (cont.)


Millimeter-Wave Propagation Model (MPM)

MPM was developed over a 30-year period bythe Institute of Telecommunication Sciences

†MPM is contained in Computer Codes 8.1 to 8.12


Oxygen Spectrum

• 60 GHz complex (37 lines)• 118.75 GHz• Several significant lines at

frequencies higher than 300 GHz

Note the strong frequency behavior:• At 10 GHz, abs. coeff. is <0.01dB/km

99.8% transmission over 1km

• At 60 GHz, abs. coeff. is >20dB/km<1% transmission over 1 km


60-GHZ Complex Opens Up at Low Pressure Levels


Water Vapor Absorption

Water vapor density can vary from10−2 g/m3 in very cold, dry climates to as much as30 g/m3 in hot, humid climates.

Water vapor has lines centered at:• 22.235 GHz• 183.31 GFz• 325.15 GHz• And at higher frequencies in the IR region

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19

Relative Humidity

Air Temperature

T [F]

Vapor air can hold[g/kgdry]

Actual Vapor in the air

[gr per kg dry air]

Relative humidity

RH

86o 27.6 10.83 39%

77o 20.4 10.83 53%

68o 14.9 10.83 72%

59o 10.8 10.83 100%


Electromagnetic Spectrum

¨ Atmospheric Attenuation [dB]


Absorption/Emission spectrum


Examples of atmospheric gases

¨ * Green – oxygen atoms 60-93 miles up (100-150 km)* Red – oxygen atoms from 93-155 miles (150-250 km)* Purple – molecular nitrogen up to 60 miles (100 km)* Blue/purple – molecular nitrogen ions above 100 miles (160 km)


Why is the ocean blue (turquoise)

Least absorbance on blues and greens, therefore Turquoise


Just look at it! …Is not sky blue.

We’ll discuss why the sky is blue soon.

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Ex. Rincón, PR

Look at the color of the water and compare with the color of the sky.


Comparison of Model to Data


Atmospheric Opacity

Horizontally stratified model is applicable up to 70o in zenith angle


Zenith Opacity

T=293Krn=7.5 g/m3

P=1013 hPa

For various water densities


Opacity from various heights to 32km

The lowermost few kilometers account for most of the attenuation


Pressure Broadening

Oxygen lines at sea level (pressure broadened)and at a height of 20km (easily resolved).

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Atmospheric Transmissivity

For Various Latitudes


Emission by the ClearAtmosphere

The high attenuation levels at and around the resonant freq. cause TDN to ‘saturate’ at a valueslightly lower thanthe surface temperature T=290K


SKY Radiometric Temperature


TSKY

For f<1GHz the contributionof TGAL may not be ignored.

This is a serious problem due to the wide range of values possible for TGAL depending on direction.In addition, RFI from man-made sources (radio and TV transmitters)

TSUN= is a function of frequency,also sunspots and flares vary.


Applicability of Stratified Model

It’s used to about 70o from the zenith.

How good is the Stratified Model?


Scattering from Hydrometeors

¨ Clouds¨ Snow, Hail, ¨ Rain, Fog

So far, we’ve only considered atmospheric gases, i.e. , clear-sky conditions.

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37

EM interaction with Single Spherical Particles

l Absorption¤ Cross-Section, Qa =Pa /Si¤ Efficiency, xa= Qa /pr2

l Scattered ¤ Power, Ps

¤ Cross-section , Qs =Ps /Si¤ Efficiency, xs= Qs /pr2

l Total power removed by sphere from the incident EM wave, xe = xs+ xa

l Backscatter, Ss(p) = Sisb/4pR2

Si


Extinction by Hydrometeors

Absorption:

Scattering:We first consider a SINGLE particle.


Backscattering Cross-Section


40

Mie Scattering: general solution to EM scattered, absorbed by dielectric sphere.

l Uses 2 parameters (Mie parameters)¤ Size wrt. l :

¤ Speed ratio on both media:

coλπrr e

lpc 2 2

p

==

oc

cb

cp

b kj

nn

n )( p abee

e -====


41

Mie Solution

l Mie solution

l Where am & bm are the Mie coefficients.

}Re{)12(2),(

)|||)(|12(2),(

12

2

1

22

mm

ma

mm

ms

bamn

bamn

å

å¥

=

¥

=

++=

++=

ccx

ccx

42

Mie coefficients

"'

1

1

1

1

cossin

}Re{}Re{

}Re{}Re{

jnnnjWwhere

WWmnA

WWmnAb

WWmnA

WWmnA

a

o

mmm

mmm

m

mmm

mmm

m

-=

+=

-÷÷ø

öççè

æ+

-÷÷ø

öççè

æ+

=

-÷÷ø

öççè

æ+

-÷÷ø

öççè

æ+

=

-

-

-

-

cc

c

c

c

c

coλπrr e

lpc 2 2

p

==

oc

cb

cp

b kj

nn

n )( p abee

e -====

Mie parameters

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Mie Scattering For Spherical Particles

For the atmosphere, background is air:

Computer Code 8.12.


Rayleigh Approximation

For backscattering:


Frequency Response

In Rayleigh region, absorption dominates over scattering


Role of Loss Tangent

Loss tangent determines absorption, so it is important in Rayleigh region


Backscattering by a Metal Sphere

Because backscattering efficiencyvaries as (r/λ)4, weather radar uses frequencies close to Χ=0.5 for the size distribution of rain, thereby having an efficiency close to 1, and avoiding the Mieresonance region.


Metal sphere for radar calibration

Known backscattering coefficient is used in Radar Equation

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49

Scattering from Hydrometeors

Rayleigh Scattering Mie Scattering

l >> particle sizel comparable to particle size--when rain or ice crystals are present.

33GHz (9mm) 95GHz (3mm)


50

Rayleigh scattering(λ >d)

Mie scattering(λ ~ d)

Rayleigh Approximation for ice crystals


51

Sizes for cloud and rain drops

coλπrr e

lpc 2 2

p

==

Mie Parameter for size determines which Region


Dielectric Properties ofHydrometeors

Ice is weak absorber at microwaves


53

For Rayleigh approximation

l the cross sectional areas of a scatterer.

24

652

322

24

652

||D

)Im(D

||3

D 2

wbb

waa

wss

Kr

KrQ

KrQ

lppxs

lppx

lppx

==

-==

==

D=2r =diameter

~f 4The sunlit sky is blue because air scatters high-frequency light more than low-frequency, f4.


54

Gas molecules = much smaller than visible l=> Rayleigh approx. is OK.

Red 700nm

Violet 400nm

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55

Mie Scattering

¨ Mie scatt. is almost independent of frequency¨ Cloud droplets ~20mm compare to 500nm¨ Microwaves have l~cm or mm (large) – Rayleigh for most atmospheric

constituents¨ Laser have l~nm - Mie

[l dependent] [almost l independent]

56

Observe scattering in Visible EM; forward scattering vs. backscattering

Mie scattering by dust particles and aerosols

Rayleigh scattering by water vapor molecules and gases.

57 58

Mie forward scattering nos impide ver bien a menos que haya alto contraste.

59Microwave Radar and Radiometric Remote Sensing, by Ulaby and Long

Snowflakes

Real part

Imaginary part

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Size Distributions

for several cloud types.

Similar-form expressions apply to the absorption, extinction, and backscattering coefficients


Cloud Models

Drop-Size Distribution

The coefficients and exponents are positive constants related to the type of cloud and its physical parameters:the water content and mode radius.


Efficiency Plots vs. particle radius

3 GHz 30 GHz 300 GHz

Rayleigh applicable to all cloudsAnd rain

Rayleigh applicable to cloudsand low rainfall

Rayleigh applicable to Fair-Weather cloud only


Cloud Attenuation Below 50 GHz


Cloud Attenuation Above 50 GHz

Above 50 GHz, Mie expressions should beused for clouds because Rayleigh approximation is

no longer accurate sufficiently.


Microwave attenuation vs Optical visibility

Mathematically, the visibility range Rv is defined as the range over which the total attenuation in the visible region ( λ = 0.4–0.7 μm) is approximately 18 dB, which corresponds to a transmission coefficient of 1%.

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Drop-Size Distribution for Rain


Extinction by Rain


Rain vs. Snow


Rain Reflectivity

Volume backscattering coefficient:

l in cmZ=radar reflectivity


71

Raindrops symmetry

Differential Reflectivity

Zdr


72

Reflectivity Factor, Z

l Is defined as

so that

l and sometimes expressed in dBZ to cover a wider dynamic range of weather conditions.

l Z is also used for rain and ice measurements.

ò= dDDNZ )(D6 ZKwo

vc2

4

5

|| lps =

ZdBZ log10=

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73

Reflectivity in other books

36

1-

24

512

/mmmin expressed is and cmin is where

|| 10

Z

ZKwo

h

lph -=


74

Reflectivity & Reflectivity Factor

Refle

ctiv

ity, h

[cm

-1]

dBZ

for

1g/m

3

Reflectivity and reflectivity factor produced by 1g/m3 liquid water Divided into drops of same diameter. (from Lhermitte, 2002).

svZ (in dB)

ZKwo

vc2

4

5

|| lps =


Downward Emission


Homework ProblemsSend by email in PDF

For Next Thursday Apr6: Ch8. ¨ Prob.1, 2 (for 2 km), 3 (for 2km), 4(25km), ¨ 5(for 307 K, and 35 g/m3 of water vapor ), ¨ 6 (Use To=307K)For Next Tuesday (April 18): Ch8. ¨ Prob. 7 (use rho=5g/m3), ¨ 8(use standard Atmosphere with To=300K), ¨ 11(for sphere with permittivity of 45-j7),¨ 14(cloud at 13oC), 16, ¨ 17(rho= 35 g/m3)


Projects:

¨ Daniel= Radiometer design & paper presentation¨ Carlos Wah- papers & Presentation on antenna design

for polarimetric radiometer¨ Abel- Papers & PPT on microwave radiometric

measurements of rainfall¨ Alvin -¨ Andres-¨ Jhon-1. Send proposed papers by this Thursday.2. Draft essay and PPT by Tuesday after Holi week.


Codes

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Error Sources and EstimationStatistics

g=geophysical parameter

ga = the actual (true) value of g.

x = the in-situ measured value of g.

Ns=Number of spatial samplesy = the sensor-estimated value of g.

(using inversion algorithm)

The in-situ errorIn-situ variance

The algorithm error

Algorithm variance

Total variance


Model Validation

mrs ch8xpol - engineeringece.uprm.edu/~pol/pdf/atmosphericconstituents.pdf · 4/18/17 5 microwave...

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