Global View of the Origin
of
Tropical Disturbances and Storms
by
William M. Gray
Technical Paper No. 114
Department
of
Atmospheric Science
Colorado State University
Fort Collins, Colorado
GLOBAL
VIEW
OF
THE
ORIGIN
OF
TROPICAL
DISTURBANCES
AND
STORMS
by
William
M.
Gray
Department
of
Atmospheric
Science
Colorado
State
University
Fort
Collins.
Colorado
October
1967
Atmospheric
Science
Paper
No.
114
TABLE
OF
CONTENTS
Abstract
1.
Introduction.........·········
2.
Global
Climatology
of
Storm
Development.
Location
and
Frequency
of
Initially
Observed
Disturbances
and
Storms
Climatology
of
Equatorial
Trough
(Eq.
T.)
Association
of
Equatorial
Trough
with
Initial
Disturbances
and
Storms
Climatology
of
Potential
Buoyancy
in
the
Lower
Half
of
the
Troposphere
Climatology
of
Tropospheric
Vertical
Wind
Shear
Variability
from
Climatology
1
. 3
3.
Statistics
of
Individual
Storm
Development.
..•
38
Disturbance
Intensification
Equatorward
of
20°
Latitude
Disturbance
Intensification
Poleward
of
20°
Latitude
Hybrid
Type
of
Storm
Three
Types
of
Disturbance
Intensification
4.
Statistical
Characteristic
of
Ekman
or
Frictional
Veering
of
Wind
Over
the
Tropical
Oceans.
• . • • . . . . . 56
5.
Importance
of
Vertical
Momentum
Transport
by
the
Cumulus
Up-
and
Downdrafts
....•......
6.
Idealized
Portrayal
of
Conditions
Associated
with
Tropical
• 63
Type
Disturbance
Intensification.
. . . . . . . . . . . . . 70
7.
Characteristics
of
Development
in
Each
Genesis
Area.
. 76
Region
I
Northeast
Pacific
Region
II
Northwest
Pacific
Region
III
and
IV
North
Indian
Ocean
Region
V
South
Indian
Ocean
Region
VI
and
VII
Area
to
the
Northwest
of
Australia
and
South
Pacific
Region
VIII
North
Atlantic
Regions
of
Southwest
Atlantic
and
North
Central
Pacific
8.
Summary
Discussion.
Acknowledgements
..
Appendix--Data
References.
References.
. • . . . . . • . .
.83
.
•.
90
•
91
.98
ABSTRACT
A
global
observational
study
of
atmospheric
conditions
associated
with
tropical
disturbance
and
storm
development
is
presented.
This
study
primarily
uses
upper-air
observations
which
have
become
available
over
the
tropical
oceans
in
the
last
decade.
Climatological
values
of
vertical
stability,
low-level
wind,
tropospheric
vertical
wind
shear
and
other
parameters
relative
to
the
location
and
seasons
of
tropical
disturbance
and
storm
development
are
discussed.
Indi-
vidual
storm
data
is
also
presented
in
summary
form
for
over
two
hundred
development
cases
(with
over
one
thousand
individual
obser-
vation
times)
for
three
genesis
areas
in
the
Northern
Hemisphere.
Results
show
that
most
tropical
disturbances
and
storms
form
in
regions
equatorward
of
20°
latitude
on
the
poleward
side
of
doldrum
Equatorial
Troughs
where
the
tropospheric
vertical
shear
of
hori-
zontal
wind
(i.
e.,
baroclinicity)
is
a
minimum
or
zero.
Storm
devel-
opment
occurring
on
the
poleward
side
of
20°
latitude
in
the
NW
At-
lantic
and
NW
Pacific
takes
place
under
significantly
different
environ-
mental
conditions
which
are
described.
These
latter
developments
make
up
but
a
small
percentage
of
the
global
total.
Observations
are
also
presented
which
indicate
that
over
the
tropical
oceans
where
disturbances
and
storms
form,
there
is
a
distinct
Ekman
or
frictional
veering
of
the
wind
in
the
sub-cloud
layer
(surface
to
600
m)
of
approximately
10°.
This
produces
or
enhances
synoptic-scale
low-level
convergence
and
cumulus
convection
in
re-
gions
of
large
positive
relative
vorticity
which
exist
in
the
cyclonic
wind
shear
areas
surrounding
doldrum
Equatorial
Troughs.
Tropical
disturbance
and
later
storm
development
is
viewed
as
primarily
a
result
of
large-scale
Ekman
or
frictionally
forced
surface
convergence
(with
resulting
cumulus
production
and
tropospheric
heating),
and
a
consequent
inhibition
of
tropospheric
ventilation
by
initially
existing
small
vertical
wind
shear,
and
later
inhibition
of
ventilation
by
cumulus
up-
and
downdrafts
acting
to
prevent
increase
of
vertical
shear
as
baroclinicity
increases.
The
above
processes
produce
the
necessary
condensation
heating
and
allow
for
its
con-
centration
and
containment
in
selective
areas.
Development
is
thus
explained
from
a
simple
warming,
hydrostatic
adjustment
point
of
view
with
the
energy
source
analogous
to
Charney
and
Eliassen's
(1964b)
proposed
"conditional
instability
of
the
second
kind.
"
iv