Weather radar data
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Weather Radar Data. Doppler Spectral Moments Reflectivity factor Z Mean Velocity v Spectrum width  v Polarimetric Variables Differential Reflectivity Z DR Specific Differential Phase Correlation Coefficient  hv Linear Depolarization Ratio L DR.

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Weather Radar Data

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Weather radar data

Weather Radar Data

  • Doppler Spectral Moments

    • Reflectivity factor Z

    • Mean Velocity v

    • Spectrum width v

  • Polarimetric Variables

    • Differential Reflectivity ZDR

    • Specific Differential Phase

    • Correlation Coefficient hv

    • Linear Depolarization Ratio LDR


Weather radar data

Contributors to Measurement Errors*1) Widespread spatial distribution of scatterers(range ambiguities)*2) Large velocity distribution (velocity ambiguities)3) Antenna sidelobes 4) Antenna motion*5) Ground clutter (regular and anomalous propagation)*6) Non meteorological scatterers (birds, etc.)*7) Finite dwell time 8) Receiver noise*9) Radar calibration *--- these can be somewhat mitigated


Weather radar data

Mitigation of Range Ambiguities

Uniform PRTs

Alternate batches of long (for Z) and short

(for velocity) PRTS.

El = 19.5o

Long PRTs (first PPI scan) for reflectivity ra>460 km;

Short PRTs (second PPI scan) for velocity, ra <200km; typically 150 km

7 Scans

= 5.25

= 4.3

5 Scans

= 2.4

= 1.45

4 Scans

= 0.5


Weather radar data

Reflectivity Field of Widespread Showers(Data displayed to 460 km)


Weather radar data

Velocity Field: Widespread Showers (5dB overlaid threshold; data displayed to 230 km)


Weather radar data

Spectrum Width Field: Widespread Showers (20 dB threshold)


Weather radar data

Echoes from Birds leaving a Roost; Spectrum Width Field


Weather radar data

Measurements of Rain

  • R(Z) relations

  • Error sources

  • Procedure on the WSR-88D


Weather radar data

Reflectivity Factor

Rainfall Rate Relations

Marshall-Palmer:

Z = 200 R1.6

Z(mm6 m-3); R(mm h-1)

For WSR-88D:

Z = 300 R1.4 - convective rain

Z = 200 R1.2 - tropical rain


Weather radar data

Rain Rate Error Sources

  • *1) Radar calibration

  • 2) Height of measurements

  • *3) Attenuation

  • 4) Incomplete beam filling

  • *5) Evaporation

  • *6) Beam blockage

  • 7) Gradients of rain rate

  • 8) Vertical air motions

  • *9) Variability in DSD


Weather radar data

DSDs, R(Z), and R(disdrometer)

Sep 11, 1999

Log(N)

Log(N)


Weather radar data

DSD’s, R(Z), and R(disrometer)

Dec 3, 1999

Log(N)


Weather radar data

Locations of Z Data used in the WSR-88D for Rain Measurement


Weather radar data

Applications of Polarization

  • Polarimetric Variables

  • Measurements of Rain

  • Measurements of Snow

  • Classification of Precipitation


Weather radar data

Polarimetric Variables

  • Quantitative - Zh, ZDR, KDP

  • Qualitative - |hv(0)|, , LDR, xv, hv

  • Are not independent

  • Are related to precipitation parameters

  • Relations among hydrometeor parameters allow retrieval of bulk precipitation properties and amounts


Weather radar data

Rainfall Relation R(KDP, ZDR)

R(KDP, ZDR) = 52 KDP0.96 ZDR-0.447

- is least sensitive to the variation of the median drop diameter Do

- is valid for a 11 cm wavelength


Weather radar data

Scatergrams: R(Z) and R(KDP, ZDR)vs Rain Gauge


Weather radar data

Sensitivity to Hail


Weather radar data

R(gauges)

Area Mean Rain Rate and BiasR(gauges)-R(radar)

R(gauges)-R(Z)

R(gauges)-R(KDPZDR)


Weather radar data

Fundamental Problems in Remote Sensing of Precipitation

  • Classification - what is where?

  • Quantification - what is the amount?


Weather radar data

Weighting Functions


Weather radar data

Partitions in the Zh, ZDR Space into Regions of Hydrometeor Types


Weather radar data

Weighting Function forModerate Rain WMR(Zh, ZDR)


Weather radar data

Scores for hydrometeor classes

Ai= multiplicative factor 1

Wj = weighting function of two variables assigned to the class j

Yi= a variable other than reflectivity (T, ZDR, KDP, hv, LDR)

j = hydrometeor class, one the following: light rain,

moderate rain, rain with large drops, rain/hail mixture,

small hail, dry snow, wet snow, horizontal crystals,

vertical crystals, other

Class j for which Sj is a maximum is chosen as the correct one


Weather radar data

Florida


Weather radar data

Florida


Weather radar data

Florida


Weather radar data

Florida


Weather radar data

Florida


Weather radar data

Fields of classified Hydrometeors - Florida


Weather radar data

Fields of classified Hydrometeor - Florida


Weather radar data

Fields of classified Hydrometeors - Florida


Suggestions

Suggestions

  • Data quality - develop acceptance tests

  • Anomalous Propagation - consider “fuzzy logic” scheme

  • Classify precipitation into type (snow, hail, graupel, rain, bright band) even if only Z is available

  • Calibrate the radar (post operationally, use data, gauges, ..anything)


Weather radar data

Specific Differential Phase at short wavelengths (3 and 5 cm)

  • Overcomes the effects of attenuation

  • Is more sensitive to rain rate

  • Is influenced by resonant scattering from large drops


Suggestions for polarimetric measurements at 3 and 5 cm

Suggestions for Polarimetric measurements at =3 and 5 cm

  • Develop a classification scheme

  • Develop a R(KDP, ZDR) or other polarimetric relation to estimate rain

  • Correct Z for attenuation and ZDR for differential attenuation (use DP)

  • Use KDP to calibrate Z


Weather radar data

Radar Echo Classifier

  • Uses “fuzzy logic” technique

  • Base data Z, V, W used

  • Derived fields (“features”) are calculated

  • Weighting functions are applied to the feature fields to create “interest” fields

  • Interest fields are weighted and summed

  • Threshold applied, producing final algorithm output


Weather radar data

AP Detection Algorithm

  • Features derived from base data are:

    • Median radial velocity

    • Standard deviation of radial velocity

    • Median spectrum width

    • “Texture” of the reflectivity

    • Reflectivity variables “spin” and “sign”

      • Similar to texture

  • Computed over a local area


Weather radar data

Clutter

mean V

Clutter

texture Z

Weather

mean V

Weather

texture Z

Investigate data “features”

  • Feature distributions

    • AP Clutter

    • Precipitation

  • Best features have good separation between echo types


Weather radar data

AP Weighting Functions

Median Radial Velocity

Median Spectrum Width

1

1

“Texture” of Reflectivity

Standard Deviation of Radial Velocity

0

0

“Reflectivity Spin”

F) Spin

100

0 50

G) Sign

“Reflectivity Sign”

-10 -0.6 0 0.6

10


Weather radar data

Field of Weights for AP Clutter

AP Clutter

AP Clutter

For median velocity field,

the weighting function is:

1

Interest Field

Radial Velocity

0

Radial Velocity

+3 m/s

+3 m/s

-2.3

0

2.3

Weighting functions are

applied to the feature field

to create an “interest” field

Values scaled between 0-1


Weather radar data

Reflectivity

Radial Velocity

Example of APDA

using S-Pol data

from STEPS

Polarimetric truth field

given by the

Particle Identification

(PID) output

APDA is thresholded

at 0.5

Good agreement between PID clutter

and APDA

PID

APDA

Clutter

Rain

20 June 2000, 0234 UTC 0.5 degree elevation


Weather radar data

Storm-Scale Prediction

  • Sample 4-hour forecast from the Center for Analysis and Prediction of Storms’ Advanced Regional Prediction System (ARPS) – a full-physics mesoscale prediction system

  • For the Fort Worth forecast

    • 4-hour prediction

    • 3 km grid resolution

    • Model initial state included assimilation of

      • WSR-88D reflectivity and radial velocity data

      • Surface and upper-air data

      • Satellite and wind profiler data


Weather radar data

7 pm

8 pm

6 pm

4 hr

3 hr

2 hr

Forecast w/Radar

Radar


Weather radar data

7 pm

8 pm

6 pm

Radar

4 hr

3 hr

2 hr

Fcst w/o Radar


Weather radar data

R(Z) for Snow and Ice Water Content

Snow fall rate:

Z(mm6m-3) =75R2 ; R in mm h-1 of water

Ice Water Content:

IWC(gr m-3)= 0.446 (m)KDP(deg km-1)/(1-Zv/Zh)


Weather radar data

Vertical Cross Sections

Z

ZDR

KDP

hv


Weather radar data

In Situ and Pol Measurements

T-28 aircraft


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