# Scattering from Hydrometeors: Clouds, Snow, Rain

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Scattering from Hydrometeors: Clouds, Snow, Rain
Microwave Remote Sensing INEL 6069 Sandra Cruz Pol Professor, Dept. of Electrical & Computer Engineering, UPRM, Mayagüez, PR

Outline: Clouds & Rain Single sphere (Mie vs. Rayleigh)
Sphere of rain, snow, & ice (Hydrometeors) Find their ec, nc, sb Many spheres together : Clouds, Rain, Snow a. Drop size distribution b. Volume Extinction= Scattering+ Absorption c. Volume Backscattering Radar Equation for Meteorology TB Brightness by Clouds & Rain

Clouds Types on our Atmosphere

Cirrus Clouds Composition
% Cirrus Clouds Composition

EM interaction with Single Spherical Particles
Absorption Cross-Section, Qa =Pa /Si Efficiency, xa= Qa /pr2 Scattered Power, Ps Cross-section , Qs =Ps /Si Efficiency, xs= Qs /pr2 Total power removed by sphere from the incident EM wave, xe = xs+ xa Backscatter, Ss(p) = Sisb/4pR2

Mie Scattering: general solution to EM scattered, absorbed by dielectric sphere.
Uses 2 parameters (Mie parameters) Size wrt. l : Speed ratio on both media:

Mie Solution Mie solution
Where am & bm are the Mie coefficients given by eqs 5.62 to 5.70 in the textbook.

Mie coefficients

Non-absorbing sphere or drop (n”=0 for a perfect dielectric, which is a non-absorbing sphere)
Rayleigh region |nc|<<1

Conducting (absorbing) sphere

Plots of Mie xe versus c Four Cases of sphere in air : n=1.29 (lossless non-absorbing sphere) n=1.29-j0.47 (low loss sphere) n=1.28-j1.37 (lossy dielectric sphere) n= perfectly conducting metal sphere As n’’ increases, so does the absorption (xa), and less is the oscillatory behavior. Optical limit (r >>l) is xe =2. Crossover for Hi conducting sphere at c =2.4 Weakly conducting sphere is at c =.06

Rayleigh Approximation |nc|<<1
Scattering efficiency Extinction efficiency where K is the dielectric factor

Absorption efficiency in Rayleigh region
i.e. scattering can be neglected in Rayleigh region (small particles with respect to wavelength) |nc|<<1

Scattering from Hydrometeors
Rayleigh Scattering Mie Scattering  >> particle size comparable to particle size --when rain or ice crystals are present.

Single Particle Cross-sections vs.c
Scattering cross section Absorption cross section In the Rayleigh region (nc<<1) =>Qa is larger, so much more of the signal is absorbed than scattered. Therefore For small drops, almost no scattering, i.e. no bouncing from drop since it’s so small.

Rayleigh-Mie-GeometricOptics
Along with absorption, scattering is a major cause of the attenuation of radiation by the atmosphere for visible. Scattering varies as a function of the ratio of the particle diameter to the wavelength (d/l) of the radiation. When this ratio is less than about one-tenth (d/l<1/10), Rayleigh scattering occurs in which the scattering coefficient varies inversely as the fourth power of the wavelength. At larger values of the ratio of particle diameter to wavelength, the scattering varies in a complex fashion described by the Mie theory; at a ratio of the order of 10 (d/l>10), the laws of geometric optics begin to apply.

Mie Scattering (d/l1), Mie theory : A complete mathematical-physical theory of the scattering of electromagnetic radiation by spherical particles, developed by G. Mie in 1908. In contrast to Rayleigh scattering, the Mie theory embraces all possible ratios of diameter to wavelength. The Mie theory is very important in meteorological optics, where diameter-to-wavelength ratios of the order of unity and larger are characteristic of many problems regarding haze and cloud scattering. When d/l neither Rayleigh or Geometric Optics Theory applies. Need to use Mie. Scattering of radar energy by raindrops constitutes another significant application of the Mie theory.

Backscattering Cross-section
From Mie solution, the backscattered field by a spherical particle is Observe that perfect dielectric (nonabsorbent) sphere exhibits large oscillations for c>1. Hi absorbing and perfect conducting spheres show regularly damped oscillations.

Backscattering from metal sphere
Rayleigh Region defined as For conducting sphere (|n|= ) where,

Scattering by Hydrometeors
Hydrometeors (water particles) In the case of water, the index of refraction is a function of T & f. (fig 5.16) @T=20C For ice. For snow, it’s a mixture of both above.

Liquid water refractivity, n’

Sphere pol signature Co-pol Cross-pol

Sizes for cloud and rain drops

Snowflakes Snow is mixture of ice crystals and air
The relative permittivity of dry snow The Kds factor for dry snow

Volume Scattering Two assumptions:
particles randomly distributed in volume-- incoherent scattering theory. Concentration is small-- ignore shadowing. Volume Scattering coefficient is the total scattering cross section per unit volume. [Np/m]

Total number of drops per unit volume
in units of mm-3

Volume Scattering It’s also expressed as or in dB/km units, Using...
[Np/m] [s,e,b stand for scattering, extinction and backscattering.] [dB/km]

For Rayleigh approximation
Substitute eqs. 71, 74 and 79 into definitions of the cross sectional areas of a scatterer. D=2r =diameter

Noise in Stratus cloud image -scanning Ka-band radar

Volume extinction from clouds
Total attenuation is due to gases,cloud, and rain cloud volume extinction is (eq.5.98) Liquid Water Content LWC or mv ) water density = 106 g/m3

Relation with Cloud water content
This means extinction increases with cloud water content. where and wavelength is in cm.

Raindrops symmetry

Volume backscattering from Clouds
Many applications require the modeling of the radar return. For a single drop For many drops (cloud)

Reflectivity Factor, Z Is defined as so that
and sometimes expressed in dBZ to cover a wider dynamic range of weather conditions. Z is also used for rain and ice measurements.

Reflectivity in other references…

Reflectivity & Reflectivity Factor
h Z (in dB) Reflectivity, h [cm-1] dBZ for 1g/m3 Reflectivity and reflectivity factor produced by 1g/m3 liquid water Divided into drops of same diameter. (from Lhermitte, 2002).

Cloud detection vs. frequency

Rain drops

Precipitation (Rain) Volume extinction where Rr is rain rate in mm/hr
[dB/km] and b are given in Table 5.7 can depend on polarization since large drops are not spherical but ~oblong. [dB/km] Mie coefficients

W-band UMass CPRS radar

Rain Rate [mm/hr] If know the rain drop size distribution, each drop has a liquid water mass of total mass per unit area and time rainfall rate is depth of water per unit time a useful formula

Volume Backscattering for Rain
For many drops in a volume, if we use Rayleigh approximation Marshall and Palmer developed but need Mie for f>10GHz.

Rain retrieval Algorithms
Several types of algorithms used to retrieve rainfall rate with polarimetric radars; mainly R(Zh), R(Zh, Zdr) R(Kdp) R(Kdp, Zdr) where R is rain rate, Zh is the horizontal co-polar radar reflectivity factor, Zdr is the differential reflectivity Kdp is the differential specific phase shift a.k.a. differential propagation phase, defined as

Snow extinction coefficient
Both scattering and absorption ( for f < 20GHz --Rayleigh) for snowfall rates in the range of a few mm/hr, the scattering is negligible. At higher frequencies,the Mie formulation should be used. The is smaller that rain for the same R, but is higher for melting snow.

Snow Volume Backscattering
Similar to rain

Radar equation for Meteorology
For weather applications for a volume

Radar Equation For power distribution in the main lobe assumed to be Gaussian function.

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