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 Linear Circular Elliptical ** DESCRIBES DE DIRECTION OF THE ELECTRICAL FIELD VECTOR LINEAR VERTICAL HORIZONTAL CIRCULAR Left Hand (LHC)-Counter Clockwise Right Hand (RHC) - Clockwise ELLIPTICAL

Weather Radars: Why is it important? MECHANISM… HOW? (Tx) Transmit Power (S) Power is Scattered over its path (Rx) Scattered Power towards radar is measured 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 Lineal Typical Horizontal Vertical ZHH ZVV ZHV ZVH

CSU-CHILL Radar Dual Polarized Doppler S-band V port H port Towards reflector 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)

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

How are things done? 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 pulse Backscattered 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

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 RAIN Wet Ice Differential Phase doesn’t say anything by itself BUT ITS CHANGE OVER SPACE and TIME DOES!!!!

Negative KDP observed in thunderstorm anvil For vertically-oriented particles, Svv > Shh; KDP negative

RAIN Dual-polarized Radars DP-based methods: Simple Attenuation Correction RAIN

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

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 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

rHV summary Typical Values 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

Linear Depolarization Ratio (LDR) 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)

Frozen hydrometeors, especially with high bulk density and water coatings, typically generate more depolarization than rain drops. Red line ~upper LDR limit for rain Tropical” (ice-free) rain LDR observations: Upper LDR limit ~-24 to -25 dB. 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

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

Hail areas present variable LDR levels 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

Cyril Zh(X) Corr. for Rain CASA: June 10th, 2007 PPI at 12.25 in elevation Cyril Zh(X) Corr. for Rain HID

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 hail High ZH collocates with HDR areas above 10dB

Reflectivity and HDR Movie

NO questions… Estoy saturada de Polarimetría!