Introduction to Radar Meteorology

Leyda V. León-Colón, PhD

Electrical and Computer

Engineering Department

Types of radar Ground Based

Airborne Based

Satellite Based

Mobile

Radar Equation and Radar Reflectivity

Clear Air

Electromagnetic Waves and Polarization

** DESCRIBES DE DIRECTION OF THE ELECTRICAL FIELD VECTOR

LINEAR VERTICAL HORIZONTAL

CIRCULAR Left Hand (LHC)-Counter

Clockwise Right Hand (RHC) - Clockwise

ELLIPTICAL

Linear Circular Elliptical

Weather Radars: Why is it important?

MECHANISM…

(Tx) Transmit Power

(S) Power is Scattered over its path

(Rx) Scattered Power towards radar is measured

HOW?

Linear Tx Horizontal Rx Vertical Or any combination ZHH, ZHV,ZVV, ZVH

Circular For Spheres: Tx RHC &

Rx LHC For Irregular: Tx and Rx

same power, i.e. Police Radars

CDR: Circular Depolarization Ratio

Targets

Dual Polarization in Weather Radars

Dual polarization radars can estimate several return signal properties beyond those available from conventional, single polarization Doppler systems.

Hydrometeors: Shape, Direction, Behavior, Type, etc…

Events: Development, identification, extinction

ZHH ZVV

ZHV ZVH

Lineal Typical•Horizontal•Vertical

V port

H port

Towards

reflector

CSU-CHILL Radar

Dual Polarized Doppler S-band

CP2 Radar

Located at Brisbane, Australia

Single Polarized Doppler X-band

Dual polarized Doppler S-band

SPOL & XPOL

NCAR’s SPOL Dual Polarized,

ZH

NOA’s XPOL (transportable) Dual Polarized

CASA and TropiNet Radars vs. NEXRAD

Dual polarized Doppler X-band

WSR-88D: NEXRAD, all around the US Single Polarized,

Doppler

KOUN: NSSL’S Dual polarized Prototype

Backscattered electric field from an individual scatterer is described by the scattering matrix. “S” values are complex numbers that depend on the scatterer shape, orientation and dielectric constant

Incident field due to transmitted radar pulseBackscattered electric

field; contains both H and V components

Here, subscripts are transmit, receive from the particle viewpoint

Largest terms are “co-polar” (repeated subscript) matrix elements

How are things done?

Some useful quantities that such a radar can measure are:

Ratio of the H and V signal powers (ZDR)

Phase difference between the H and V returns (fDP)

Degree of correlation between the H and V returns (rHV)

Ratio of orthogonal to “on channel” signal power (LDR)

Inherent difference in Zdr characteristics of raindrops vs. hailstones

Zdr observations in rain and hail

Hail (~random orientation) dominates Z-weighted mean axis ratio: Zdr decreases to ~0 dB

Differential Phase ΦDP vs. Specific Differential Phase KDP

Differential Phase doesn’t say anything by itself

BUT ITS CHANGE OVER SPACE and TIME DOES!!!!

RAIN Wet Ice

Negative KDP observed in thunderstorm anvil

For vertically-oriented particles,

Svv > Shh; KDP negative

RAIN

Dual-polarized Radars

DP-based methods: Simple Attenuation Correction

Numerator:

Decreases when Shh and Svv are not uniformly correlated among the scatterers; (i.e., Svv is not always = .5 Shh for all scatterers in the pulse volume. When this uniformity does exist, rHV goes to 1.0)

Denominator:

Normalizes the ratio into 0 to 1 range

Factors that Reduce rHV (Balakrishnan and Zrnic 1990):

Radar pulse volume variations in the distribution of scatterer:

1.Shapes, 2.Sizes, 3. d magnitudes (d is Mie-related differential phase shift on scattering)

4. canting angles 5. hydrometeor types (example: both liquid and frozen present)

6. hydrometeor shape irregularities (some rough aggregates, etc.)

Co-polar H,V return signal correlation (rhv or rco)

rHV reduced in hail area:

Mixed precip types; rHV especially reduced when Zrain=Zice

Diverse shapes

Melting level / bright band readily recognized by local rHV minimum. Reflectivity maximizes as frozen particles initially develop an outer water coating. With further descent / warming, smaller particles completely melt. Mixed frozen and completely

melted layer gives lowest rHV values. (Enhanced Z is a few 100 m higher up)

Blue contours are 20 and 40 dBZ

Primarily useful to characterize variability of scatterer characteristics within the pulse volume.

Drizzle / light rain > ~0.98

Convective (but no ice) rain > ~0.96

Hail / rain mixtures ~0.90

Bright band mixed rain and snow ~0.75

Tornado debris ~0.50 or less

rHV summary Typical Values

• Is the ratio of the cross-polar to co-polar backscattered signal powers. Here the HV subscripts represent the receive and transmit polarizations respectively.

• For cloud and precipitation targets, the cross polar signal level is typically only 10-2 – 10-3 of the co-polar level (LDR~ -20 to -30 dB)

Linear Depolarization Ratio (LDR)

Red line ~upper LDR limit for rain

Frozen hydrometeors, especially with high bulk density and water coatings, typically generate more depolarization than

rain drops.

Note small LDR magnitudes. Snow LDR of -30 dB implies that cross polar signal from 30 dB snow echo is 0 dB. Noise can bias / obliterate such weak cross polar channel signals

Tropical” (ice-free) rain LDR observations: Upper LDR limit ~-24 to -25 dB.

As with rHV, LDR maximizes in the melting level region where wet, non-spherical, gyrating ice particles exist.

Hail areas present variable LDR levels. In this storm, the dBZ core area is characterized by LDR levels that are virtually all below -22 dB.

Note also how LDR increases in clutter, noise, and many echo edge areas.

Hydrometeor identification (HID)Radar data values are used to develop a numerical score for each designated

particle type. Identification is based on the highest-scoring type.

Hydrometeor classifications at 5.5 km MSL in a thunderstorm complex

CASA: June 10th, 2007

PPI at 12.25 in elevation

HIDCyril Zh(X) Corr. for

Rain

HID after attenuation correction

DP-based SRT-modified

Hail Event on March 23rd, 2012 on SW Puerto Rico

ENDI News Report Differential Hail Signal (HDR)

Dependent on ZH and Polarimetric Variable ZDR

Su, et al 2010, Bringi and Chandrasekar 2001

Detection on TropiNet: Cornelia

HDR>10dB detect areas with hailHigh ZH collocates with HDR areas above 10dB

Reflectivity and HDR Movie

NO questions… Estoy saturada de Polarimetría!