65th IHC Booklet/Program (pdf - 4.9MB) - Office of the Federal ...

Is hurricane rapid intensification possible
from the energy released inside of a convective burst?
Owen A. Kelley 1 , Jeffrey B. Halverson, 2 and John Stout 1
(Owen.Kelley@nasa.gov)
1 NASA Goddard Space Flight; 2 University of Maryland Baltimore County
When a hurricane intensifies, the NHC forecast discussion sometimes mentions particularly
vigorous convection being observed in the eyewall. When Hurricane Earl (2010) was
intensifying to category 4 on its approach toward the U.S., the NHC discussion stated: "THE
EYE HAS CONTINUED TO CLEAR AND IS SURROUNDED BY CLOUD TOPS TO -70
DEGREES CENSIUS OR COLDER" [5 PM AST 30 Aug 2010]. Forecasters may mention such
bursts of eyewall convection because they are common in infrared satellite animations of
intensifying hurricanes. Or perhaps forecasters mention convective bursts because some of the
intensity forecast schemes that forecasters rely on take into account the vigor of eyewall
convection through various empirical parameterizations. It is still a subject of debate, however,
what physical mechanisms connect convective bursts with energy flowing into the hurricane's
tangential wind field.
Even before there is scientific consensus on these physical mechanisms, it would be helpful to
estimate what energy conservation and other thermodynamic considerations permit in terms of a
convective burst's effect on hurricane intensity. This study provides such a thermodynamic
estimate, incorporating recently collected satellite datasets, modeling studies, and analyses of
multi-instrument wind observations.
First, satellite observations provide estimates of the net amount of condensation occurring inside
of a vigorous eyewall cell. Net condensation is proportional to net latent heat release. Making
reasonable assumptions, a convective burst (a series of vigorous cells) could release 6e17 J of
latent heat into the eyewall every 12 hours, in excess of the amount of latent heat that would
have been released in the eyewall in the absence of a convective burst. Convective bursts have
been observed to persist for 9 to 48 hours. Second, modeling studies provide estimates of the
efficiency with which latent heat released in the eyewall may be transformed into increased
kinetic energy of the hurricane's tangential wind. A 5% to 7% efficiency is consistent with
recent modeling studies. Third, an examination of wind analyses provides a mapping between
changes in kinetic energy and changes in surface wind intensity. An experimental extension of
the H*wind analysis to three dimensions suggests a power-law relation between initial and final
kinetic energy (KE) and initial and final surface wind intensity (I): KE f / KE i = ( I f / I i ) 1.34 .
Putting these three pieces together, this study estimates that intensification at the rate of 11−19 m
s -1 (21−37 kt) in 12 hours could result from the extra amount of latent heat that the convective
burst releases inside of the eyewall. As a point of reference, rapid intensification is often
considered ≥15 m s -1 (30 kt) in 24 hours, and some convective bursts persist for several days.
Poster Session – Page 30