1. Training Session: Satellite Applications on Tropical Cyclones
NOAA/NESDIS
ORA/CORP/RAMM
CIRA / Fort Collins, CO

2. Outline
Tracking and Intensity
Short Range Forecasting
Structure

3. Tracking and Intensity
Center location (fixing)
Intensity estimates
Short Range Forecasting
Intensity
Current trends
Vertical Shear (Assymetry)
Sea-surface Temperature
“Rapid” Intensification
Super Rapid Scan Images (SRSO)
SAL (Saharan Air Layer) – LW difference images
Track
Water vapor image applications / Recurvature
Unsmoothed tracks (oscillations and wobbles)
Structure
Outer Winds
Pressure-wind relationship
Surface Wind Analysis (RMW, Size)
Subtropical Cyclones – Subtropical Transition
Hybrid Tropical Cyclones
Upper-level forcing - Subtropical
Midgets
Monsoon Depressions
Extratropical Transition
Landfall

4. SATELLITE DATA TYPES: Geostationary: Microwave sounder (AMSU)IR
VIS/Ch2 WV
Split window
Microwave sounder (AMSU)
Microwave images
Satellite winds
Scatterometer winds (also Windsat)
Microwave winds (old SSMI algorithm)
SST images
High resolution multispectral images
MODIS
AVHRR
DMSP OLS
MSG
Super Rapid-scan Operations (SRSO) (1-min interval)

5. Tracking and Intensity
Center location (fixing)
Intensity estimates

6. Center location (fixing)
Center Location = surface center
Center of circulation
Lowest sea-level pressure
Visible and IR methods – Dvorak
Eye
Distinct and inferred center with shear pattern and low-level clouds
Spiral bands and curved cloud lines
Wedge method
Using animation
Low-level cloud motions
Deep layer cloud motions
Ignore cirrus layer cloud motions
Mid-level centers tilted from surface center
Using microwave images
Thick cirrus clouds in visible and IR images obscure features below, used for center location
Thick cirrus clouds in microwave images are more transparent, and the microwave images may often provide better views of features, for improved center locations
Using scatterometer winds
Problems encountered with cyclogenesis and early stages

7. Center Location Center Location = surface center
Center of circulation
Lowest sea-level pressure
Visible and IR methods – Dvorak
Eye
Distinct and inferred center with shear pattern and low-level clouds
Spiral bands and curved cloud lines
Wedge method

8. Center Location Using animation Low-level cloud motions
Deep layer cloud motions
Ignore cirrus layer cloud motions
Mid-level centers tilted from surface center

9. Center Location Using microwave images Using scatterometer winds
Thick cirrus clouds in visible and IR images obscure features below, used for center location
Thick cirrus clouds in microwave images are more transparent, and the microwave images may often provide better views of features, for improved center locations
Using scatterometer winds
Problems encountered with cyclogenesis and early stages

10. Intensity Estimates Dvorak Technique Objective Dvorak Technique
Advanced Microwave Sounding Unit (AMSU)
Using scatterometer winds
Other (low-level cloud motion vectors, microwave wind algorithms, in situ obs)

11. Tropical Cyclone Intensity
Intensity: highest surface wind speed
U.S. policy: 10-m, 1-min wind to nearest 5-knots (knot = n.mi./h, 60 n.mi. = 70 mi. = 111 km = 1 deg lat, 1 m/s = knots)
Alternate indicator of intensity is the central pressure, or minimum sea-level pressure (MSLP) in hPa (mb)

12. Dvorak Technique
The Dvorak technique uses patterns and measurements from satellite imagery to estimate the strength of a tropical cyclone.
Four basic types
Curved band pattern
Shear pattern
CDO pattern
Eye pattern

13. Dvorak Technique
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.

14. Dvorak Technique

15. Objective Dvorak Technique
Original version – Dvorak (1984) – “analysis using digital IR data”
Velden, Olander, Zehr (1998) – ODT
Computation used for hurricane intensities remains essentially unchanged
What is it? – Two IR temperature measurements, given a center location

16. Two IR temperature measurements
1) Surrounding temperature – Warmest pixel from those located on r=55 km circle
2) Eye temperature – Warmest pixel within the eye
Table assigns intensity to nearest 0.1 T-No.
Intensity increases as Surrounding T gets colder and as the Eye T gets warmer.

17. ODT - Improvements
Multi-radius computations of “surrounding temperature”
Time averaging (6-h running mean) of frequent (30-min) interval computations
Limits on rate of weakening
New computations for weak (pre-hurricane) intensities

18. Advanced Microwave Sounding Unit (AMSU)
Vertical temperature profiles retrieved from the AMSU multispectral radiances, give a 3-D measurement of the tropical cyclone warm core
Refinements to measurements of the warm core provide intensity estimates

19. Scatterometer winds
Surface wind vectors from the scatterometer at about 25 km resolution provide limited information for intensity estimates
The scatterometer wind speeds are not representative above minimal hurricane force, and give erroneously high winds in heavy rain areas

20. Low-level cloud motion vectors
0.8 x (low-level sat wind speed)
= Surface wind speed estimate

21. Short Range Forecasting
Intensity
Current trends
Vertical Shear (Assymetry)
Detection of “Rapid” Intensification
SAL (Saharan Air Layer) – LW difference images
Track
Water vapor image applications / Recurvature
Unsmoothed tracks (oscillations and wobbles)

22. Intensity Trends
Short-term changes in satellite images following Dvorak intensity measurements
Recognizing peak intensity
Short range intensity forecasts often based on current trends shown in images

23. Vertical Shear Intensity changes are often due to:
Vertical wind shear of the environmental deep layer in which the tropical cyclone circulation is embedded
The direction and magnitude of the vertical shear is indicated by the deep cloud and cirrus asymetry with respect to the TC center
Vertical shear is quantified by the vertical profile of the wind averaged over a large TC centered circle. The vector difference of wind at different levels is the vertical shear.

24. Sea Surface Temperature (SST)
The SST encountered by a TC can be important to the intensity forecast
SSTs provide an upper bound on intensity
Ocean heat content (OHC) is important in the same way, particularly for slow moving TCs that will mix out the shallow water measured by the SST
Satellite data is essential for timely and accurate analyses of SST and OHC

25. Rapid Intensification
Most large errors in intensity forecasts are due to “rapid intensification” events
Rapid intensification occurs in low vertical shear environments over very warm oceans
Satellite image characteristics associated with rapid intensification

26. Super Rapid-scan Operations (SRSO)
Animations of 1-minute interval visible images
Comparison with 30-minute interval
Meso-vortices within the hurricane eye

27. SAL (Saharan Air Layer)
The SAL is often observed in association with dust that is transported large distances across the Atlantic from its source region in Africa
The SAL is characterized by very dry mid-level air and a stable air layer
The SAL inhibits deep convection and TC development
The long wave IR difference can be displayed as an image product to track the SAL

28. Water vapor images
Tropical cyclones track according to the deep layer mean wind
Water vapor images depict upper level cloud motions, and mid-level motions in cloud free areas
Features that force abrupt changes in TC track are often identified in water vapor images

29. Track oscillations and wobbles
Hurricanes with well defined eyes can be very accurately tracked with satellite images
The center may exhibit short term oscillations or wobbles about the track representing the longer term motion
Center relative animated images are useful in depicting short term motion

30. Structure Outer Winds (Size) Pressure-wind relationship
Surface Wind Analysis (RMW, Size)
Subtropical Cyclones – Subtropical Transition
Hybrid Tropical Cyclones
Upper-level forcing - Subtropical
Midgets
Monsoon Depressions
Extratropical Transition
Landfall

31. Outer Winds
The TC maximum winds are usually km from the center. The outer winds generally decrease away from the center but are not well related to the maximum
TC size can be very different and has important implications
TC size
Radius of gale force (34 kt / 17.5 m/s) winds
Outer closed isobar
Radius of zero tangential wind

32. Pressure-wind relationship
Central pressure, i.e. minimum sea-level pressure (MSLP) is well correlated with maximum surface wind speed (Vmax)
An average pressure-wind relationship is used to assign intensity as MSLP and Vmax, in the absence of additional observations, such as aircraft data.

33. Pressure-wind relationship
Aircraft observations reveal deviations from the average pressure-wind relationship
Environmental and structure characteristics influence the pressure wind relationship
Environmental pressure
Latitude
Size
Intensity trend
TC Motion
Radius of Maximum Wind
Landfall

34. Surface Wind Analysis
The main objectives of operational tropical cyclone satellite applications have been primarily….
center location and 2) intensity estimate
-- With today’s improved satellite data…
Those two objectives can be incorporated into a satellite-derived surface wind analysis, that portrays additional useful information.

35. Surface Wind Analysis Isotachs and streamlines depict:
1) Intensity (Vmax)
2) Location of Vmax
3) Center (wind speed minimum)
4) Size (radial extent of strong winds)
5) Asymmetry of wind field

36. Subtropical Cyclones
Definition: A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones
Definition: a manifestation of “cut-off low” at the surface
definition -- cut-off low -- cold low displaced equatorward of westerly flow

37. Transition from Subtropical
A few Tropical Cyclones originate from subtropical cyclones
A few Subtropical Cyclones maintain subtropical characteristics but intensify to produce Storm Force winds (> 34 kt) (Subtropical Storm)
During transition, a “hybrid” tropical storm has characteristics of both types

38. Hybrid Tropical Cyclones
The term “hybrid” is used to refer to a tropical cyclone that has originated not solely from “latent heat release” with a typical pre-existing tropical disturbance. This may involve baroclinic processes, as with extratropical cyclones, subtropical lows, etc.
Other tropical weather systems have been identified which may at times resemble or evolve into a “hybrid” cyclone
Monsoon Depressions
West African Cyclones
Arabian Sea Cyclones
Midget Tropical Cyclones

39. Extratropical Transition
A term used in advisories and tropical summaries to indicate that a cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes. It is important to note that cyclones can become extratropical and still retain wind of hurricane or tropical storm force.

40. Landfall
Wind speeds decrease when a tropical cyclone moves over a land mass. This is due both to the reduced latent and sensible heat transfer from the land compared to the ocean, and the additional friction from the land surface.