Tropical Cyclone Intensity Forecasting National Hurricane Center

OUTLINE: TC INTENSITY FORECASTING INTENSITY DEFINITION HOW TO ESTIMATE CURRENT INTENSITY FACTORS THAT INFLUENCE INTENSITY CHANGE NUMERICAL GUIDANCE FOR INTENSITY FORECASTING OFFICIAL INTENSITY FORECASTS INTENSITY DEFINITION HOW TO ESTIMATE CURRENT INTENSITY FACTORS THAT INFLUENCE INTENSITY CHANGE NUMERICAL GUIDANCE FOR INTENSITY FORECASTING OFFICIAL INTENSITY FORECASTS

WHAT IS THE INTENSITY OF A TROPICAL CYCLONE? Maximum sustained surface wind: Maximum wind, averaged over a 1 minute interval at an altitude of 33 ft (10 m), associated with the circulation of the tropical cyclone at a given point in time. With very, very few exceptions, direct observations of the maximum sustained surface wind in a tropical cyclone are not available. Maximum sustained surface wind: Maximum wind, averaged over a 1 minute interval at an altitude of 33 ft (10 m), associated with the circulation of the tropical cyclone at a given point in time. With very, very few exceptions, direct observations of the maximum sustained surface wind in a tropical cyclone are not available.

HOW DO WE ESTIMATE INTENSITY? Satellites –Geostationary IR and VIS (Dvorak technique) –AMSU –QuikSCAT Surface observations –ships, buoys, land stations (limited) Satellites –Geostationary IR and VIS (Dvorak technique) –AMSU –QuikSCAT Surface observations –ships, buoys, land stations (limited) Aircraft reconnaissance –flight-level winds –GPS dropsondes –Stepped Frequency Microwave Radiometer (SFMR) –Airborne doppler radar Aircraft reconnaissance –flight-level winds –GPS dropsondes –Stepped Frequency Microwave Radiometer (SFMR) –Airborne doppler radar

SURFACE OBSERVATIONS CAN OCCASIONALLY BE USED TO ESTIMATE INTENSITY

Dvorak Technique Tropical cyclones have characteristic cloud patterns that correspond to stages of development and certain intensities. GOES-12 IR 1745 UTC 5 Sep 2003

AMSU Intensity Estimates Warm Core Observations from NOAA Polar Orbiting Satellites Hurricane Floyd 1999

AMSU Central Pressure Estimates vs Recon for Hurricane Isidore (2002)

QuikSCAT Tropical Storm Zeta ( ) QuikSCAT overpass 0840 UTC 1 Jan 2006 METEOSAT-8 IR image 0900 UTC 50 kt

Hurricane Georges Eyewall Photo: M Black HRD AIRCRAFT RECONNAISSANCE (WHEN THERE IS A THREAT TO LAND) AFRC/53WRS C-130 NOAA/AOC P-3

RECON FLIGHT-LEVEL WINDS HURRICANE GEORGES 9/20/ Z 105 kt 90 kt 95 kt RECON FLIGHT-LEVEL (10,000 FT) WINDS

GPS DROPWINDSONDE Developed in conjunction with the NOAA Gulfstream-IV jet aircraft. First systematic use for intensity was in 1998’s Hurricane Bonnie. GPS dropsondes provide direct measurements of the winds at low levels in the hurricane eyewall. Dropsonde data reveal that the structure of the eyewall is very complex, and can vary tremendously from storm to storm. Developed in conjunction with the NOAA Gulfstream-IV jet aircraft. First systematic use for intensity was in 1998’s Hurricane Bonnie. GPS dropsondes provide direct measurements of the winds at low levels in the hurricane eyewall. Dropsonde data reveal that the structure of the eyewall is very complex, and can vary tremendously from storm to storm.

Because the hurricane is “warm-core”, winds increase downward from flight-level (10000 ft). Friction decreases wind in the lowest 1500 ft of the eyewall.

ALTHOUGH THE NORMAL RATIO OF SURFACE TO FLIGHT WIND SPEED IS 90%, SOME HURRICANES HAVE TROUBLE BRINGING THEIR HIGH WINDS DOWN TO THE SURFACE.

THE SURFACE WINDS CAN EXCEED THE FLIGHT- LEVEL WIND AT TIMES.

New dropsonde wind speed record in a hurricane (Isabel in the Atlantic): 203 kt. NEW DROPSONDE WIND SPEED MEASUREMENT IN A HURRICANE (ISABEL IN THE ATLANTIC, 12 SEPTEMBER 2003): 203 KNOTS NEAR 4500 FEET.

ALTHOUGH CENTRAL PRESSURE IS APPROXIMATELY RELATED TO INTENSITY, THERE CAN BE BIG VARIATIONS…AS MUCH AS TWO SAFFIR/SIMPSON CATEGORIES FOR THE SAME CENTRAL PRESSURE!

FACTORS AFFECTING TROPICAL CYCLONE INTENSITY Sea surface temperature / upper ocean heat content. Environmental winds, esp. vertical wind shear. Trough interactions. Temperature and moisture patterns in the storm environment. Internal effects (eyewall replacement cycles). Interaction with land.

SEA SURFACE TEMPERATURE UPPER OCEANIC HEAT CONTENT

VERTICAL WIND SHEAR EXPOSED CENTER DEEP CONVECTION ft ft ft ft 5000 ft 1000 ft

TROUGH INTERACTION: HURRICANE BERTHA, JULY UTC

1200 UTC BERTHA STRENGTHENED JUST BEFORE LANDFALL

IN ADDITION TO LARGE-SCALE INFLUENCES, TROPICAL CYCLONE INTENSITY CHANGE CAN BE CAUSED BY INNER- CORE PROCESSES, SUCH AS EYEWALL REPLACEMENT CYCLES. AFTER A PERIOD OF SIGNIFICANT INTENSIFICATION, AN INNER EYEWALL IS OFTEN REPLACED BY AN OUTER EYEWALL. FIRST WE SEE CONCENTRIC EYEWALLS, AND THEN THE INNER EYEWALL DISSIPATES LEAVING AN OUTER EYEWALL (LARGER DIAMETER). SOME WEAKENING USUALLY OCCURS DURING THIS REPLACEMENT PROCESS. IN SOME CASES, THE OUTER EYEWALL CONTRACTS AND THE HURRICANE RE-STRENGTHENS TO COMPLETE THE CYCLE. THIS CYCLE COULD REPEAT SEVERAL TIMES IN THE LIFETIME OF THE HURRICANE.

CONCENTRIC EYEWALL CYCLE HURRICANE FLOYD 13 / 1122Z 14 / 2228Z 14 / 1110Z 14 / 0104Z 13 / 2240Z 13 / 1347Z 13 / 0116Z

CONCENTRIC EYEWALL CYCLE HURRICANE FLOYD 13 / 1122Z 14 / 2228Z 14 / 1110Z 14 / 0104Z 13 / 2240Z 13 / 1347Z 13 / 0116Z

EYEWALL REPLACEMENT

CENTRAL PRESSURE VS. TIME FOR HURRICANE ALLEN, 1980: LARGE FLUCTUATIONS LARGELY DUE TO EYEWALL REPLACEMENT CYCLES

50% 30% 10% DRY AIR IN THE MIDDLE LEVELS OF THE ATMOSPHERE PROBABLY CAUSED THE STORM TO WEAKEN

INLAND WIND DECAY MODEL: EXPONENTIAL DECREASE OF INTENSITY WITH TIME

TROPICAL CYCLONE INTENSITY FORECAST MODELS Statistical Models: SHIFOR (Statistical Hurricane Intensity FORecast). Based solely on historical information - climatology and persistence. (Analog to CLIPER.) Statistical/Dynamical Models: SHIPS (Statistical Hurricane Intensity Prediction Scheme): Based on climatology, persistence, and statistical relationships to current and forecast environmental conditions. Dynamical Models: GFDL, GFS, UKMET, NOGAPS. Based on the present and the future by solving the governing equations for the atmosphere (and ocean).

TROPICAL CYCLONE INTENSITY STATISTICAL FORECAST MODELS SHIFOR (Statistical Hurricane Intensity FORecast): Based solely on climatology and persistence. SHIPS (Statistical Hurricane Intensity Prediction Scheme): Based on climatology, persistence, and current/predicted environmental conditions. DSHIPS (Decay SHIPS): same as SHIPS except when track forecast points are over land – when a decrease in intensity following an inland decay model is included. DSHIPS modified to include information about oceanic heat content and inner core convection (using infrared satellite imagery). SHIFOR (Statistical Hurricane Intensity FORecast): Based solely on climatology and persistence. SHIPS (Statistical Hurricane Intensity Prediction Scheme): Based on climatology, persistence, and current/predicted environmental conditions. DSHIPS (Decay SHIPS): same as SHIPS except when track forecast points are over land – when a decrease in intensity following an inland decay model is included. DSHIPS modified to include information about oceanic heat content and inner core convection (using infrared satellite imagery).

THE SHIPS MODEL (+) SST POTENTIAL (VMAX-V): Difference between the maximum potential intensity (depends on SST) and the current intensity. (-)VERTICAL ( MB) WIND SHEAR: Current and forecast. (+) PERSISTENCE: If it’s been strengthening, it will probably continue to strengthen, and vice versa. (-) UPPER LEVEL (200 MB) TEMPERATURE: Warm upper-level temperatures inhibit convection (+) THETA-E EXCESS: Related to buoyancy (CAPE); more buoyancy is conducive to strengthening (+) MB LAYER AVERAGE RELATIVE HUMIDITY: Dry air at mid-levels inhibits strengthening (+) SST POTENTIAL (VMAX-V): Difference between the maximum potential intensity (depends on SST) and the current intensity. (-)VERTICAL ( MB) WIND SHEAR: Current and forecast. (+) PERSISTENCE: If it’s been strengthening, it will probably continue to strengthen, and vice versa. (-) UPPER LEVEL (200 MB) TEMPERATURE: Warm upper-level temperatures inhibit convection (+) THETA-E EXCESS: Related to buoyancy (CAPE); more buoyancy is conducive to strengthening (+) MB LAYER AVERAGE RELATIVE HUMIDITY: Dry air at mid-levels inhibits strengthening Statistical multiple regression model relating tropical cyclone intensity change to various climatological, persistence, and environmental predictors.

THE SHIPS MODEL (cont.) (+)850 MB ENVIRONMENTAL RELATIVE VORTICITY: Vorticity is averaged over a large area, about 10° radius. Intensification is favored when the storm is in an environment of cyclonic low-level vorticity. (-)ZONAL STORM MOTION: Intensification is favored when TCs are moving west (-)STEERING LEVEL PRESSURE: intensification is favored for storms that are moving more with the upper level flow. This predictor usually only comes into play when storms get sheared off and move with the flow at very low levels (in which case they are likely to weaken). (+)200 MB DIVERGENCE: Divergence aloft enhances outflow and promotes strengthening (-)CLIMATOLOGY: Number of days from the climatological peak of the hurricane season (+)850 MB ENVIRONMENTAL RELATIVE VORTICITY: Vorticity is averaged over a large area, about 10° radius. Intensification is favored when the storm is in an environment of cyclonic low-level vorticity. (-)ZONAL STORM MOTION: Intensification is favored when TCs are moving west (-)STEERING LEVEL PRESSURE: intensification is favored for storms that are moving more with the upper level flow. This predictor usually only comes into play when storms get sheared off and move with the flow at very low levels (in which case they are likely to weaken). (+)200 MB DIVERGENCE: Divergence aloft enhances outflow and promotes strengthening (-)CLIMATOLOGY: Number of days from the climatological peak of the hurricane season Statistical multiple regression model relating tropical cyclone intensity change to various climatological, persistence, and environmental predictors.

Satellite/Oceanic Predictors have been added to SHIPS 1.GOES cold IR pixel count 3. Oceanic heat content from 2.GOES IR T b standard deviation satellite altimetry (TPC/UM algorithm)

A STATISTICAL TECHNIQUE TO AID IN THE FORECAST OF RAPID INTENSIFICATION: The 7 predictors used to estimate the probability of Rapid Intensification (defined as an increase in maximum wind speed of at least 25 kt over 24 h): PredictorDefinition PER Previous 12 h intensity change SHR mb vertical shear SST Observed sea-surface temperature at T=0 POT Maximum Potential Intensity –Current Intensity RHLO mb relative humidity STDIR Standard deviation of IR brightness temperature PIX Percentage of GOES pixels colder than -50°C

VERIFYING: 160 KNOTS

TROPICAL CYCLONE INTENSITY DYNAMICAL FORECAST MODELS GFDL, NCEP Global Model (GFS), UKMET (U.K. Met Office), NOGAPS (U.S. Navy), ECMWF (European) These models are of limited use, because of… sparse observations sparse observations inadequate resolution (need to go down to a few km grid spacing; the GFDL, our highest-resolution operational hurricane model, is currently about 18 km) inadequate resolution (need to go down to a few km grid spacing; the GFDL, our highest-resolution operational hurricane model, is currently about 18 km) incomplete understanding and simulation of basic physics of intensity change incomplete understanding and simulation of basic physics of intensity change biases in upper-level wind forecasts. biases in upper-level wind forecasts.

GFDL INTENSITY FORECASTS FOR KATRINA DID SHOW STRENGTHENING TO A MAJOR HURRICANE OVER THE GULF

Early on 19 October, Wilma deepened at a rate of ~ 10 mb/hr! GFDL FORECAST FROM 10/17/05 18Z OBSERVED GFDL MODEL DID CAPTURE SOME OF WILMA’S RAPID DEEPENING

65 55 observed observed: dissipated

NHC OFFICIAL INTENSITY FORECAST Based on statistical guidance from SHIPS and SHIFOR, qualitative guidance from dynamical models. Persistence is used quite a bit! Obvious signs in the environment, i.e. cooler waters, increasing upper-level winds, are taken into account Generally corresponds to what is normal for a storm in any particular situation (e.g. the standard Dvorak development rate). Tends to be conservative; extreme events are almost never forecast. For forecasts 24 h and beyond, the average error is roughly 1 SSHS Category (15-20 knots). Based on statistical guidance from SHIPS and SHIFOR, qualitative guidance from dynamical models. Persistence is used quite a bit! Obvious signs in the environment, i.e. cooler waters, increasing upper-level winds, are taken into account Generally corresponds to what is normal for a storm in any particular situation (e.g. the standard Dvorak development rate). Tends to be conservative; extreme events are almost never forecast. For forecasts 24 h and beyond, the average error is roughly 1 SSHS Category (15-20 knots).

Intensity Forecasts

No progress with intensity?

Intensity Guidance

CONCLUDING REMARKS (INTENSITY FORECASTING) Intensity forecasting is not as advanced as track forecasting. There is less skill for intensity forecasting than there is for track forecasting. Current guidance is provided mainly by statistical models. While some improvements are still being sought, statistical models by their nature have difficulties capturing the outliers. The main hope for the future lies in improved dynamical models, coupled with enhanced observations and understanding of the hurricane’s inner core. This is a long-term project.