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Satellite Applications in Tropical Weather Forecasting

Mark DeMaria

Regional and Mesoscale Meteorology Team

NESDIS/CIRA

Colorado State University, Ft. Collins CO

Satmet 99-2Tuesday, 27 April 1999

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Acknowledgments• RAMMT

– Roger Phillips, Ray Zehr, Jack Dostalek, John Knaff, Bernadette Connell, Stan Kidder

• TPC– Jiann-Gwo Jiing (SOO), Richard Pasch,

Michelle Huber, Bill Frederick

• CIMSS– Chris Velden

• NESDIS ORA– Roger Weldon

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Outline

• General Circulation in the Tropics– ITCZ– Subtropical Ridge– Tropical Upper Tropospheric Trough

• Synoptic-Scale Weather Systems– Upper-level lows– Tropical Waves

• Tropical Cyclones– Dvorak method– Environmental interactions– Measurements from Polar orbiting satellites

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Terminology

• Channel 1 - Visible - .6 m ( .52- .72)

• Channel 2 - Shortwave IR - 3.9 m (3.78- 4.03)

• Channel 3 - Water Vapor - 6.7 m (6.47-7.02)

• Channel 4 - Longwave IR - 10.7 m (10.2-11.2)

• Channel 5 - Split Window - 12.0 m (11.5-12.5)

• Microwave frequencies 20-90 Ghz (1.5-0.3 cm)

1 2 3 4 5 1 2 3 4 5

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Hadley Cell

Walker Cells

P (mb)

-90 -60 -30 0 30 60 90-90 -60 -30 0 30 60 90

DJF JJA

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1979-1996 January Average 200 mb Streamlines

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1979-1996 July Average 200 mb Streamlines

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1979-1996 January Average 850 mb Streamlines

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1979-1996 July Average 850 mb Streamlines

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GMS, GOES, METEOSAT IR Imagery Composite

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GMS, GOES, METEOSAT IR Imagery Composite

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Tropical Upper-Level Lows

• Typically form within TUTT

• Cold-core systems

• Shallower circulation than mid-latitude lows (little circulation below 500 mb)

• Often produce precipitation

• Can influence intensity and track of tropical cyclones

• Can be tracked using water vapor imagery

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300 mb 700 mb

Cold-low in NCEP Analysis 8/19/96 0000 UTC

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Perturbation Temperature Cross-Section from NCEP Analysis 8/26/96 0000 UTC

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Schematic Representation of Cold Low Structure

(from Whitfield and Lyons, WF, 1992)

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Radiation Analysis of WV Imager Channel for Idealized Sounding

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16-Frame Water Vapor Imagery Loop7/26/98 02:45 to 7/29/98 20:45

Formation of a Cold-Low

Presentation

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TPC Tropical Analysis and Forecast BranchCold-Low Study

• Period of Study: Aug. 12-Oct. 1, 1996

• Domain: 0-35 N, 110-20 N

• Cold-low centers tracked using GOES WV imagery

• 47 lows during 50 days

• Average Duration of 3 days, Max of 12 days

• Up to 8 lows in domain at once (average of 3)

• Vorticity center present in NCEP 200 mb analyses for nearly all cold lows identified by satellite observations– Average NECP analysis location error of 100 nm

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NCEP Aviation Model Cold-Low Average Track Error

Extrapolation

Persistence

Cold Lows

1996 Tropical Cyclones

200

400

600

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Tropical Waves

• Formation near western Africa

– Barotropic/baroclinic instability, PV gradient changes sign

– Secondary instability region in Western Caribbean

• Maximum amplitude near 700 mb

• Period of 2-5 days, wavelength 2-3000 km, May-Dec

• Precipitation associated with waves

• About 2/3 of Atlantic TC genesis associated with waves

• Role in east Pacific tropical cyclogensis

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Seasonal Mean PV at 750 mb (Molinari et al, 1997)

Mean meridional wind fromGATE wave composite(Reed et al, 1977)

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TAFB Methods for Tracking Waves

• TAFB tropical surface analysis generated 4 times per day, includes surface wave positions

• Rawindsone time series• Surface data when available • Satellite analysis over west Africa

– Animation of channels 1, 2 and 4 indicates rotation

• Hovmoller satellite diagrams for continuity across tropical Atlantic

• NCEP aviation model analyses and forecasts

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Dakar Sounding (15 N, 18 W) Time Series August 1995-97

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96

97

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20 W 40 E20 W40 E

08/01/96

08/15/96

08/16/96

08/31/96

Tropical Strip Time Series40 E-40 W 0 N-15 N

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Tropical Cyclone Classification

• NHC has responsibility for Atlantic and east Pacific basins– Atlantic: 10 storms, 6 hurricanes

– East Pacific: 16 storms, 10 hurricanes

• Aircraft recon available only for Atlantic west of 55 W

• Majority of center and intensity estimates from GOES satellite data

• TAFB, SAB, AFWA provide classifications

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Overview of the Dvorak Technique

• Visible and Infrared Technique• Simplified Visible Technique given here (See

Technical Report for full details)• Uses patterns and measurements as seen on

satellite imagery to assign a number (T number) representative of the cyclone’s strength.

• The T number scale runs from 0 to 8 in increments of 0.5.

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Overview of the Dvorak Technique Cont’d

• In the following examples, only the Data T Number (DT) will be calculated, the final (official) T number assigned to a tropical cyclone includes further considerations.

• DT computations familiarize one to various tropical cyclone patterns.

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Four Basic Patterns

• Curved Band Pattern

• Shear Pattern

• Central Dense Overcast (CDO) Pattern

• Eye Pattern– Pattern is not always obvious– Pattern typically varies with time

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Patterns and Associated T Numbers

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Empirical relationship between T number and wind speed

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Finding the Cloud System Center (CSC)

• First step in the Dvorak technique• From Dvorak (1985):

“The cloud system center is defined as the focal point of all the curved lines or bands of the cloud system. It can also be thought of as the point toward which the curved lines merge or spiral.”

• Center not always obvious, especially at night• TPC technique combines channel 2 and 4

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T.S. Lisa Channel 4

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T.S. Lisa

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Curved Band PatternTS Ivan 9/23/98 11:15 UTC

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Curved Band Pattern Cont’d

1.0 2.0 2.5 3.0 3.5 4.0 4.5

DT Number

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Shear PatternHurricane Bertha 7/11/96 2015 UTC

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Shear Pattern DT Numbers

1° latitude = 60 nautical miles (nmi) = 111 km

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Central Dense Overcast (CDO)Hurricane Georges 9/21/98 1545 UTC

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CDO

• No eye

• DT number determined by CF+BF=DT– CF=CENTRAL FEATURE– BF=BANDING FEATURE– DT=DATA T NUMBER

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CDO Central Feature (CF)• Measure Diameter of CDO in degrees latitude• For a well defined CDO

– 3/4 ° CF=2– 1 1/4 ° CF=3– 1 3/4 ° CF=4– >2 1/4 ° CF=5

• For an irregular CDO– 1° to 1 1/2 ° CF=2– >1 1/2 ° CF=3

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CDO - Banding Feature (BF)

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Eye PatternHurricane Georges 9/19/98 1345 UTC

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Eye Pattern

• DT number determined by CF+BF=DT– CF=CENTRAL FEATURE– BF=BANDING FEATURE– DT=DATA T NUMBER

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Banding Eye Hurricane Bonnie 8/25/98 2131 UTC

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Infrared (IR) Technique

• Can be used during night as well as during day

• At times, more objective than visible technique

• Fully objective version developed at CIRA

• Updated objective technique from CIMSS

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Example Digital IR: Hurricane Erika 1515 UTC 8 September 1997

• Warmest eye pixel 16 °C

• Warmest pixel 30 nmi (55

km) from center -71 °C• Nomogram gives Eye no.

=7

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Operational Dvorak Technique Verificationfor 1997-98 Atlantic Seasons

Position (nm)

x-bias (nm)

y-bias (nm)

Max wind (kt)

Wind bias (kt)

TAFB 97 18.8 -3.8 -0.4 4.6 -1.2

SAB 97 19.7 -3.9 -3.0 5.9 -2.2

TAFB 98 21.1 -1.6 0.3 6.5 0.7

SAB 98 21.3 0.0 -2.1 6.5 -1.7

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Input for NHC Track and Intensity Forecasts

• Track– 70 % Numerical model guidance– 15 % Synoptic reasoning– 15 % Recent trends

• Intensity– 50 % Recent trends– 40 % Synoptic reasoning– 10 % Numerical model guidance

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Applications of Satellite Data to TC Forecasting

• Track– Inclusion of remotely sensed data in NWP models– Diagnosis of model initial state– Evaluation of synoptic situation

• Intensity– Evaluation of factors affecting intensity

• SST changes

• vertical shear (especially cloud track winds)

• trough interaction

– Inclusion in NWP models

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300 mb AVN Winds and WV Image 19 Sept 1998

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NEW GOES WINDS PRODUCTS ARE BEING PRODUCED BY CIMSS/NESDIS:

- HIGH - DENSITY WINDS DERIVED FROM IR AND HIGH-RESOLUTION VISIBLE CLOUD MOTIONS AS WELL AS WATER VAPOR MOTIONS, USING AUTOMATED ALGORITHMS

- DISPLAYS OF THESE WINDS FOR UPPER- & LOWER-LEVEL LAYERS OVER THE TROPICS ARE ROUTINELY AVAILABLE VIA THE INTERNET

-UWISC/CIMSS TC WEB SITE: http://cimss.ssec.wisc.edu/tropic/tropic.html

NEW GOES WINDS PRODUCTS ARE BEING PRODUCED BY CIMSS/NESDIS:

- HIGH - DENSITY WINDS DERIVED FROM IR AND HIGH-RESOLUTION VISIBLE CLOUD MOTIONS AS WELL AS WATER VAPOR MOTIONS, USING AUTOMATED ALGORITHMS

- DISPLAYS OF THESE WINDS FOR UPPER- & LOWER-LEVEL LAYERS OVER THE TROPICS ARE ROUTINELY AVAILABLE VIA THE INTERNET

-UWISC/CIMSS TC WEB SITE: http://cimss.ssec.wisc.edu/tropic/tropic.html

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Vertical Wind Shear Analysis from GOES High Density Winds

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Hurricane-Trough PV Interaction During Hurricane Elena 1985 (Molinari et al 1995)

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8-Frame Water Vapor Imagery Loop 9/20/98 23:45 to 9/21/98 23:45

Hurricane/Trough Interaction

Presentation

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Storm-Scale Structure

• Convective transients, asymmetries– Velden and Olander (1998) technique

• IR BT usually warmer than WV

• Deep convection transport into stratosphere

• WV BT warmer than IR

• Concentric eye walls, eye wall cycles

• Mesovortices within the eye– High spatial and time resolution imagery (RSO, SRSO)– Extra-high density satellite winds

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Channel 3, 4 Convective Parameter (CP) for Hurricane Opal 1995 (From Bosart, et al 1999)

Min P P (mb)CP

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GOES-East Scanning Strategies

• Routine Scanning– Conus hr+01,31– Extended NH hr+15,45– (2 or 4 per hr)

• Rapid Scan– Conus, NH 5-10 min– (8 per hr)

• Super Rapid Scan– Selected Sector 1-5 min– (22 per hr)

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10-frame Rapid-Scan Visible Imagery Loop

9/21/98 19:02 to 20:10Hurricane Georges Approaching Puerto Rico

Presentation

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Hurricane Luis 9/6/95 1345 UTC

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TC Measurements from Microwave Frequencies

• Special Sensor Microwave Imager (SSM/I)– DMSP polar orbiting satellites

– 19, 22, 37, 85 GHz, 25 km resolution

• Advanced Microwave Sounding Unit (AMSU)– NOAA polar orbiting satellites (NOAA 15 +)

– 15 channels 23-89 GHz, 50 km resolution

• Can see “through” clouds• Depicts rainband, eye structure• Rainfall and surface wind algorithms• AMSU temperature retrievals

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Hurricane Jeanne

9/23/981021 UTC

From NRL web site

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AMSU Retrieved Temperature Anomaly and Gradient Wind for Hurricane Bonnie 8/25/98 1200 UTC

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AMSU Retrieved Temperature Anomaly and Gradient Wind for Hurricane Bonnie 8/25/98 1200 UTC (Adjusted to remove low-level cold anomaly)

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-20

-10

0

10

20

30

40

50

0 100 200 300 400 500

Radius (km)

Tan

ge

nti

al W

ind

(m

/s)

Recon

AMSU

AMSU(adj)

AMSU and Aircraft Reconnaissance 700 mb Tangential Winds

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IR Imagery from Bonnie 8/25/98 1145 UTC (Blue Rings at 100 and 350 km Radius)

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IR ImageryMarch 1, 1999

AMSU TempertatureRetrieval (570 mb)

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Summary

• Multispectral GOES imagery provides synoptic overview– ITCZ, Subtropical Ridges, TUTT and cold lows, waves, TCs

• WV Imagery is especially useful for tracking cold lows

• Continuity in imagery is primary tool for tropical waves

• Dvorak method provides quantitative intensity estimates– Also provides framework for operational forecasting

– Shortwave IR aids center location, especially at night

• Sat. winds: environmental interactions, model intialization

• WV- IR difference isolates tropical deep convection

• Rapid-scan imagery: storm-scale fluctuations

• Microwave imagery is useful for low-level storm structure

• New AMSU data shows promise for hurricane analysis