Dual Polarimetric RadarWalt Petersen, NASA-MSFC

• Take away: A versatile and useful tool for research and operations

Outline:• Local network in the Tennessee Valley• Basic variables• A sample of research and operational

applications

Why does NASA care about ground-based dual-polarimetric radar?

NASA Precipitation Science

• Dual-pol radar provides the means to remotely sense precipitation processes over volumes of atmosphere not practically accessible by other means.

• Advancement of physically-based satellite retrieval algorithm development.

Basic Polarimetric Radar Variables

1. Reflectivity factor Z at horizontal (Zh) or vertical (Zv) polarization [Conventional radar measure]

- Measure of drop size and concentration;

• Most sensitive to drop SIZE (D6)

2. Differential reflectivity ZDR (Zh/Zv)

- Measure of median drop diameter→ SIZE/SHAPE

- Useful for rain / hail / snow discrimination→ SIZE/SHAPE

3. Propagation differential phase KDP (kh – kv)

- Measure of water content and drop size→ NUMBER/SHAPE

- Immune to radar calibration, attenuation, partial beam blockage

4. Correlation coefficient ρhv

- Indicator of mixed precipitation → SHAPE/PHASE/CANTING (Depolarization)

- Useful for identifying non-meteorological scatterers too!

Advantages: Better description of various particle types/shapes in a given volume

• Determine size distribution- more accurate rain rates (improved QPE)

• Hydrometeor ID and non-meteorological scatterers (clutter!)

• Consistent calibration

Zh, kh

Zv, kv

We need the measurement in H and V directions!

Walter A. Petersen NASA MSFC VP-61

Variables……..

Some Tools: N. Alabama Dual-Pol Radars and Supporting Infrastructure

Instruments/Infrastructure

• Radars:

• C and X-band dual-polarimetric

• VHF Lightning mapping array

• Mobile Integrated Profiling System (MIPS)

• Surface met./Sounding

• Network of Disdrometers, Rain gauges

• Model/Forecast R&D (NASA SPoRT)

Science/Application:

• Operational: Severe weather, QPE, lightning, WRF model; NOAA-HWT, NWS-COMET, TVA

• Basic/Applied Research: QPE, Cloud Physics and kinematics, cloud electrification, boundary Layer and convective initiation

• Satellite: NASA satellite precipitation retrieval algorithms, NOAA-GLM risk reduction

Technology Transfer :

• UAH: Graduate student education

• NWS-Huntsville: Ops/Training

• TVA: Radar applications

• WHNT-TV (other markets): Dual-pol applications, public awareness

KBMX

RSA

68 km

KGWX

UAH/NSSTC THOR Center and Hazardous Weather Testbed

MIPS/NSSTC

ARMOR

KHTX

75

DD lobe

1 km Res.

1.5 km Res.

LMA 100-500 m

LMA Antenna

NEXRAD

ARMOR

MIPS Profiler

MAX ?

MAX

RESEARCH + TECHNOLOGY TRANSFER

Academic, Govt./Public, Private Sectors

Jumping right in with both feet…………..

Microphysics: 25 July 2007 Heavy rain/hail mix,large drops

Z hv ZDR

Vert. Develop/Mixed Phase Ext.Mixed phase Glaciating Glaciated

Demonstrating the Microphysical sequence in a T-storm: Antiquity of application.

Proprietary content: Walter A. Petersen NASA MSFC VP-61

Using DP Variables for Thunderstorm /Lightning Remote Sensing

Project: NASA MSFC Support of launch operations at KSC/CCAFS

•Question: Can we exploit the dual-pol characteristics for lightning initiation and cessation nowcasting/

Phase shifts due to vertically-aligned ice crystals

1.E-field increase2.Vertical alignment3.Lightning4.E-field decrease5.Crystal relaxation

Proprietary content: Walter A. Petersen NASA MSFC VP-61

Cool season applications: Melting level asymmetry……….

Veering in a pre-frontal zone

Note asymmetries in ZDR and RHOHV “bright bands”

Polarimetric variables, in particular RHOHV, are much more sensitive to presence of mixed phase

VR

ZDR

hv

Melting level ID is a big problem in traditional radar QPE

ARMOR 1/29/2010: Cold Season Mixed Phase w/Complex FL heightProblem: • Temperatures around or just above freezing…..

• Where is it liquid, freezing and frozen?

• Z has only limited information

• In this case, RHOHV adds considerable information

• We could interpret individual variables (takes time/practice)

or..........?

Combine Pol variables into “easy to digest” information: Hydrometeor ID

Vivekanandan et al. (1999, BAMS)

Vivekanandan et al., 1999 Liu and Chandrasekar, 2001

Originally (90’s) we used simple tables….But boundaries between categories in nature are soft……move to “fuzzification”

ARMOR: 1/29/10 Cold Season Mixed Phase: Now add HID- easier to interpret

Dual-Pol HID: N. AL Downburst Case and Lightning

Ze

• First precip echo 1639 UTC

• Low level rain w/ strong development at T< 0oC

• Mixed phase development 1648-1652 UTC

• First Lightning 1655 UTC

• Whole mixed phase core falls by 1702 UTC, last CG lightning 1703 UTC, last IC 1705

First IC 1655

First CG 1702

QPE: Dual-Pol Selling point- Hybrid Polarimetric Rainfall Algorithms

Premise: Combined variables account for clutter, DSD variability, and phase ID

Application for TVA : ARMOR Distributed Rainfall Products (AREPS)1-hr Accum.

6-hr Accum.

6-hr Basin Mosaic

Text file for 6-hour accumulation

Accumulated and Generated Every 5-minutes

•Also transmitted to TVA:

• Basin 1-hour accumulations• Gauge-location max, min and mean

Operational Products: Image and Text

Walter A. Petersen NASA MSFC VP-61

AREPS QPE Product Verification: ARMOR vs. TVA rain gauges (October 2007 – June 2008)

Point Comparisons

Bias = -10% (-0.99 mm)

Error = 12%

Critical: For operational applications a constant monitoring of calibration maintains precision and accuracy of product.

Walter A. Petersen NASA MSFC VP-61

CHILL: Rainfall AccumulationOptimization Algorithm

CHILL: Rainfall AccumulationNEXRAD Z-R Algorithm

ColoradoCSU-CHILL

RadarICE-Algorithm

(R. Cifelli)

Tornado Debris (2/6/2008)

KDP, RHOVH used to map debris trace

Tropical Cyclone Rita Tornadoes

Size sorting and spatial separation of drops-microphysics feedbacks on dynamics?

Dual Pol radar is an outstanding tool for exploring and detecting many different manifestations of weather.

When implemented in network form, it will be the next leap in both research and operational applications.

Having said that…………….

We should also admit that dual-pol radar is not the “pot of gold at the end of the precip remote sensing rainbow”……

It won’t completely replace our best rain gauges…………

It won’t solve all of our rainfall-related issues………….

In the end…..it will be the combination of Dual-pol radar with other observations and our ability to assimilate that ensemble of information into human and automated analysis/modeling systems that will be the key!

DSD

Particle types/phase

Rain rate

3-D Precip structure and evolution

Microphysical feedbacks to dynamics

Hydrology and water budgets

And more……….

W. Petersen, NASA-MSFC