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SEMINAR NCAR
Hurricanes that Stressed the
System: Catarina (2004) and Alex (2004)
Lance F. Bosart and Ron McTaggart-Cowan
Department of Earth and Atmospheric Sciences
University at Albany/SUNY
Christopher A. Davis
Mesoscale and Microscale Meteorology Division
National Center for Atmospheric Research
This talk will highlight the tropical transitions (TTs)
of South Atlantic Hurricane Catarina (2004) and North
Atlantic Hurricane Alex (2004). Both
hurricanes were noteworthy storms. Catarina arguably represents the only tropical development
in the South Atlantic
Ocean since the
advent of the satellite era and therefore presented significant operational
prediction challenges to forecasters.
Alex became a Category 3 storm, defying the expectations of forecasters
who expected that it would not reach hurricane status.
Hurricane Catarina began as a weak baroclinic cyclone over southeastern Brazil on 19 March 2004. It
moved slowly southeastward and intensified only slightly until 23 March. As the upper-level flow pattern evolved into
a Rex block a significant reduction in the deep-layer shear over the developing
center to below classical threshold values for tropical cyclogenesis
was observed. Between
23-25 March the cyclone underwent a TT as deep convection eroded the
remnants of the cold trough at mid-levels and further reduced the shear over
the center through vertical momentum transports. Deepening as a tropical cyclone, Catarina tracked slowly westward and made landfall in southeastern
Brazil on 28 March as a nominal Category 1
hurricane with satellite-estimated winds between 75 and 85 kts.
This repeated injection of low potential vorticity
(PV) air poleward of transient troughs passing over
the Andes mountains resulted in a long-lived Rex-block that favored the TT of Catarina over 24-25 C sea surface temperatures. Covective outflow
from Catarina also likely helped to reinforce the poleward ridge member of the Rex block. The resulting
reduction in westerly flow, and reversal to easterly flow, allowed Catarina to reverse direction and accelerate toward the
Brazilian coast.
Alex
appeared to be the result of multiple PV anomaly interactions. Between 26-29 July 2004
one PV anomaly became isolated over the Bahamas at the western of a long PV tail. A second PV anomaly fractured from a midlatitude trough over the Mississippi Valley and deepened southward into Georgia and South Carolina. A low-level
circulation appeared over the northwestern Bahamas on 30 July as the midlatitude
PV anomaly approached the PV anomaly over the northern Bahamas. Alex gained
tropical storm status on 1-2 August as both PV anomalies interacted with one
another in the presence of a narrow coastal ridge that enveloped Alex from the
north. Alex rapidly achieved hurricane
intensity by 1200 UTC 3 August as a second midlatitude
PV anomaly approached the storm from the northwest. Subsequently, Alex continued to intensify
while it moved east northeastward into the open Atlantic
Thursday, 23 June 2005, 3:30 PM
Refreshments 3:15 PM
NCAR-Foothills
Laboratory
3450
Mitchell Lane
Bldg 2 Auditorium
(Rm1022)