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Training Session: Satellite Applications on Tropical Cyclones

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Presentation on theme: "Training Session: Satellite Applications on Tropical Cyclones"— Presentation transcript:

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) Detection of “Rapid” Intensification 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 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

18 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

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

20 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)

21 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

22 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 th edeep cloud and cirrus assymetry 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.

23 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

24 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

25 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

26 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

27 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

28 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

29 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.

30 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

31 Surface Wind Analysis

32 Subtropical Cyclones

33 Hybrid Tropical Cyclones

34 Extratropical Transition

35 Landfall


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