Observing the Tropics
ATMS 373
C.C. Hennon, UNC Asheville
Buoys
• Two types
– Drifter
• Attached to sub-surface anchor (“drogue”) 15
meters below surface
• Drogue can measure mixed layer currents
• Floats freely on water surface
• Average lifespan is 400 days
• Weigh between 44 and 100 lbs.
• Data transmitted to passing satellites
– ARGOS
ATMS 373
C.C. Hennon, UNC Asheville
Surface Float
Drogue
http://www.aoml.noaa.gov/phod/dac/gdp_drifter.html
ATMS 373
C.C. Hennon, UNC Asheville
http://www.aoml.noaa.gov/phod/graphics/dacdata/globpop.gif
ATMS 373
C.C. Hennon, UNC Asheville
ATMS 373
C.C. Hennon, UNC Asheville
Buoys
• Two types
– Moored
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ATMS 373
Anchored to sea floor
Generally located near coastlines
Provide important information for landfalling TCs
Data collected and monitored by National Data
Buoy Center (NDBC)
C.C. Hennon, UNC Asheville
Buoy regions controlled by the National Data Buoy Center
ATMS 373
C.C. Hennon, UNC Asheville
TAO Array – Detection and observation of ENSO events
- Consists of 70 buoys
ATMS 373
C.C. Hennon, UNC Asheville
ATMS 373
C.C. Hennon, UNC Asheville
http://www.oc.nps.navy.mil/webmodules/ENSO/TAO.html
ATMS 373
C.C. Hennon, UNC Asheville
Hurricane Katrina Passage over
moored buoy GDIL1
ATMS 373
C.C. Hennon, UNC Asheville
Katrina just 65 miles to the west (145 mph)
Significant wave height = 16.9 m (55.5 ft)
Maximum wave height (statistically derived)
= 32.1 m (109 ft)!!
SWH = Average
of highest 1/3
of waves
ATMS 373
C.C. Hennon, UNC Asheville
Reconnaissance Flights
• Aircraft penetrations of TCs
• Performed since 1944
• GPS Dropsonde most critical instrument
– measures temperature, humidity, pressure,
winds
– data available in near-real time at National
Hurricane Center
ATMS 373
C.C. Hennon, UNC Asheville
http://www.gsfc.nasa.gov/gsfc/earth/pictures/camex4/dropsonde.gif
ATMS 373
C.C. Hennon, UNC Asheville
“Hurricane Hunters”
• 53rd Weather Reconnaissance Squadron
– Keesler AFB, Biloxi MS
– 10 C-130 planes
ATMS 373
C.C. Hennon, UNC Asheville
ATMS 373
C.C. Hennon, UNC Asheville
Tropical Rainfall Measurement
Mission (TRMM)
• First space-borne precipitation radar
• Measures precipitation between 35°N and
35°S
• Used for a lot of purposes
– surface soil moisture
– lightning research
– detection of Madden-Julian Oscillation
– improving SST datasets
ATMS 373
C.C. Hennon, UNC Asheville
Tropical Rainfall Measurement
Mission (TRMM)
• “Active” sensors
– Precipitation
Radar (PR)
• “Passive”
sensors
– TRMM
Microwave
Imager (TMI)
ATMS 373
http://www.atmos.washington.edu/~houze/RAHouzeWebsite/houze_files/trmmsat.gif
C.C. Hennon, UNC Asheville
TRMM and Tropical Cyclones
• TMI used to
identify circulation
centers that may
be obscured in IR
imagery
• PR provides
information about
rain rate
ATMS 373
C.C. Hennon, UNC Asheville
TRMM pass over Tropical Cyclone Manou. The inner
swath (PR) shows rainfall rates (red = highest);
the outer swath (TMI) detects rainbands, and can also be
used to detect circulation centers.
TMI pass near landfall of the 2004 Brazilian hurricane.
Red areas represent strong convection.
The center of the circulation is noted with an arrow.
ATMS 373
C.C. Hennon, UNC Asheville
QuikSCAT
• Scatterometer
– Emits pulse of radiation (13.4 GhZ) toward
ocean surface
– Capillary waves (1 cm) on ocean surface
backscatter pulse
– More roughness = higher surface wind
speeds
– Wind direction can be determined by varying
the azimuthal “look angle”
• Observe same spot from at least 4 angles
ATMS 373
C.C. Hennon, UNC Asheville
QuikSCAT Operation
803 km
46 deg
53 deg
ATMS 373
C.C. Hennon, UNC Asheville
ATMS 373
C.C. Hennon, UNC Asheville
Uses of QuikSCAT
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Able to retrieve high resolution (25 or
12.5 km) 10-meter wind speed and
direction
Cross swath width of 1800 km
90% global coverage daily
Accurate within 2 m/s for wind speeds
of 2-20 m/s (~4 - 40 kt)
Data are assimilated in global weather
models and have made them more
accurate
Have helped TC forecasters identify
tropical depressions and maximum
extent of wind
Identify ITCZ and other tropical wind
features
ATMS 373
C.C. Hennon, UNC Asheville
TD-14 (2003). QuikSCAT resolves circulation center that is not apparent
via any other means.
ATMS 373
C.C. Hennon, UNC Asheville
Limitations of QuikSCAT
• Unrealistic wind directions for
TCs and other rainy areas
• Egregious wind speed errors
in rainy regions
• Areas of ocean missed in
between swaths
ATMS 373
C.C. Hennon, UNC Asheville
Rain Impacts on QuikSCAT Data
• Rain contaminates the QuikSCAT data:
– Attenuates beam (lowers retrieved wind
speed)
– Increases backscatter of the beam (raises
retrieved wind speed)
– Splash drops on surface enhance backscatter
(raises retrieved wind speed)
• What’s more important?
– Depends on rain rate and true surface wind
speed
ATMS 373
C.C. Hennon, UNC Asheville
QuikSCAT surface winds
Hurricane Isabel (2003)
NHC
ATMSWinds
373 = 140 kt.
MSLP = 932 mb
C.C. Hennon, UNC Asheville
Advanced Microwave Sounding
Unit (AMSU)
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Passive Radiometer
Onboard polar orbiting NOAA satellites
Senses microwave radiation
Can determine atmospheric temperatures
for many layers
– 15 different channel sensors
– Soundings
• Used to diagnose warm cores in TCs and
TC intensity
ATMS 373
C.C. Hennon, UNC Asheville
ATMS 373
C.C. Hennon, UNC Asheville