NATIONAL CENTERS FOR ENVIRONMENTAL PREDICTION Exercise – Constructing a best track from multiple data sources JACK BEVEN NATIONAL HURRICANE CENTER WHERE AMERICA’S CLIMATE AND WEATHER SERVICES BEGIN
Objectives • To construct two best tracks (wind/pressure every 6 hours) for a tropical cyclone (from the Atlantic) • The first uses only satellite data (subjective and objective Dvorak CI numbers/scatterometer) and surface obs • The second includes a full array of Atlantic aircraft data • Vertical black lines on the graphs denote landfalls • Note that there are data sources that could be used but were omitted (doppler radar/AMSU/others)
Notes on “conventional” data • Satellite intensity estimates marked TAFB and SAB are subjective Dvorak estimates from the NHC Tropical Analysis and Forecast Branch and the Satellite Analysis Branch. • The Objective T-Numbers are from the CIMSS ADT. • All Dvorak estimates use the Atlantic Dvorak wind/ pressure calibration. • The scatterometerob is from QuikScat. • Surface pressure obs plotted on the chart are not always central pressures. Sometimes they are near-center pressures used to calibrate the pressure curve.
Dvorak Technique Output Note: Other warning centers and basins use different pressures and wind averaging periods
Part 1 – No Aircraft Data Wind (kt) Pressure (hPa)
RECONNAISSANCE FLIGHT PATH Aircraft “ALPHA” Pattern
GPS Dropsondes Hurricane Mitch - Eyewall GPS Dropsonde Wind Profile 27 October 98 GPS sondes are used in the eyewall, elsewhere in the storm, and on G-IV missions
MBL WL150 Reducing Flight-Level Data to the SurfaceBased on GPS Dropsonde Data Winds increase downward from flight-level (10,000 ft) because the hurricane is “warm-core”. Friction decreases wind in the lowest 1500 ft of the eyewall. Wind averaging over the lowest layers of the sonde data
Stepped-Frequency Microwave Radiometer Relates microwave radiation from ocean to surface wind speed Can measure max surface winds in core of major hurricane Only provides data along line of flight First data from C-130s in 2007; on entire fleet in 2008
SFMR Issues Shoaling – breaking waves in areas of shallow water can artificially increase the SFMR retrieved wind speed Interaction of wind and wave field can introduce errors (~ 5 kt) Rain impacts not always properly accounted for (mainly < 50 kt). Calibration has been undergoing revision. Algorithms still under development, and forecaster don’t fully understand the error mechanisms.
Notes on aircraft central pressure data • Aircraft central pressures are either measured from dropsondes in/near the center or extrapolated from measured conditions at the flight-level center. • Dropsondes measure the surface pressure, but do not always hit the lowest pressure/lightest winds area at the surface. • Despite these issues, aircraft-measured pressures have a good track record compared to central pressures from surface obs. The aircraft data is usually good to within 1-3 hPa/mb.
Notes on aircraft wind data • Wind data has a lot of scatter, as the plots include winds from all quadrants of the radius of maximum winds – not just the strongest quadrant. • Aircraft surface winds are from the SFMR. • Flight-level winds have already had the appropriate reduction to the surface included on the plots. • Flight-level>Surface winds relationships can change depending on the organization of the cyclone. • The Dvorak Pressure>Wind data points use the Dvorak wind-pressure curve to convert central pressures to winds. Use these with caution! • The dropsonde surface winds are the unaveraged winds at the last report from the sonde. They may or may not be representative.
Part 2 – With Aircraft Data Wind (kt) Pressure (hPa)