Determining How Favorable an area is For Tropical Cyclogenesis
Before the first thunderstorms even begin to initiate, forecasters determine if an area
is favorable for tropical cyclogenesis in order to make weekly outlooks.
There are many resources that can be used to do this. First and
foremost, one must determine if the minimum sea surface temperature (SST) is
met across an area. The minimum SST required for tropical cyclone formation
is put at 26.5 C, which is 80 F. Many cases have been noted however
of cyclones strengthening or forming over cooler waters, but usually it
is due to an unusual atmospheric condition. My favorite source for SST data is
ORAD/ MAST
The next thing to look at is distance from the equator, due to lack of the Coriolis
effect as one gets too close to the equator. In almost all cases tropical cyclone
formation does not occur within 500 km of the equator, there have been a few
rare cases however where this was not true due to the interaction of pressure
systems on different sides of the equator.
The next important factor in determining how favorable an area is for tropical
cyclogenesis is the moistness of the low and mid-levels of the atmosphere. This
is usually determined with water vapor imagery, like the image below.
This was obtained from tropical ramsdis satellite server, which is by far my
favorite satellite imagery website for the tropics.
Tropical Ramsdis
Dry air is indicated by the values towards the left of the scale, and have dry colors
like brown. Moist air is indicated by values to the right of the scale and goes
from blue to purple. The dry air indicates an area of atmosphere that is not
favorable for tropical development as widespread thunderstorm activity cannot
be maintained within the dry air. Dry air goes hand in hand with the Saharian
Air Layer (SAL). The SAL is a major source for dry, dusty air. It originates from
the deserts of North Africa and comes in bursts commonly referred to as SAL
outbreaks. Tropical cyclones weaken when they entrain the SAL outbreaks, and
it is unlikely one will form within a SAL outbreak. The best source for SAL data is
from Cimss. CIMSS SAL Data.
The final factor usually used to determine how favorable an area is for tropical
cyclogenesis is wind shear and the wind shear forecast. This is most often
determined through very useful products provided by CIMSS. Below is a current
wind shear map for the Western Pacific. Areas with shear less than 20 kt
meet the shear requirement for tropical cyclone development. In this case
only areas between 0 and 10 north have low enough shear. Of course,
cyclones have formed in higher shear or have strengthened in higher shear, but
those are rare cases. Wind shear data can be obtained here: CIMSS Shear Data
One must also determine the wind shear forecast, as shear is constantly changing in
the atmosphere. This is partially determined by shear tendency plots which can
be found here: CIMSS Shear Tendency Data
With the shear tendency maps, it can be seen where shear is decreasing
and increasing as shown by the contours. When used in conjunction with
satellite imagery to see what systems are resulting in the changes,
a reasonable shear forecast can be made for 3-5 days out. This can be
tricky and requires quite a bit of concentration.
If all of the requirements for tropical cyclone genesis are met, an area can be
considered favorable for development. If some factors aren't met one must
decide how favorable that area is for development and classify it as moderately
favorable, slightly favorable, or unfavorable.
Forecasting During Tropical Cyclogenesis
Forecasting during tropical cyclogenesis is one of my favorite things to do, as it is
extremely tricky and always seems to throw a monkey wrench at you. First
off, the factors mentioned in the previous section have to be met to a certain
degree to allow tropical cyclogenesis. Second, a pre-existing disturbance is
needed with sufficient convergence and vorticity. Tropical cyclones do not just
form out of the blue. So, where do these tropical cyclones originate from? The
answer is many places, I will cover the Atlantic ones. The most famous place for
the origination of tropical cyclones is the Cape Verde area, as meso-scale
convective systems move out of the African Sahel (tropical waves). Another
common spawning point for tropical cyclones is on a stalled and dying front
which has left North America. This can be anywhere from the Gulf Of Mexico to
Canada really. The next area is the inter tropical convergence zone (ITCZ)
near South America. The ITCZ sends low pressure systems into the Carribean
occasionally which may form. The final spawning point for tropical
cyclones is on the eastern fringe of the Pacific monsoon. The Pacific monsoon is
a massive circulation that is common in early season, when a pocket of
thunderstorms spins up and separates from the monsoon it can form.
Now that we have a disturbance and proper conditions for formation, one must see if
a tropical cyclone indeed has formed. The most important factor for this is
determining if a low level circulation (LLC) is in existence under the convection.
This can be done with visible satellite imagery, QUIKSCAT, surface
observations, and of course reconnaissance flights.
When one looks at visible satellite imagery, they are seeing 3 cloud levels. Upper,
mid, and low. It takes some practice, but it becomes easy to see if a circulation
is in fact present at the low level with visible satellite imagery even if
the center is obscured. The first thing to look for is curvy bands that are rotating
about a center like in the cyclone below. Also, if one can see clouds
moving north, south, east, and west around convection it is likely an LLC has formed.
An extremely valuable tool in determining if an LLC has formed is QUIKSCAT.
QUIKSCAT determines the strength and direction of surface winds. If a north, south,
east and west wind exist in a QUIKSCAT image as below, an LLC has formed.
QUIKSCAT data can be found here: QUIKSCAT
Sadly, QUIKSCAT is nearing the end of its life, so
lets enjoy this resource for the time being.
If there is a suspicion that a disturbance has an LLC with convection over it, a
reconnaissance flight from the air force or hurricane research division (HRD) will
head out to confirm. All recon data is obtained from here: HRD Recon Page
The plan of the day is the schedule, which outlines the flights planned and what
disturbances they are going to. RECCO observations are observations about every 15
minutes from the plane, there is a decoder guide on the page for those who
wish to obtain data from the RECCO observations. There are also dropsonde
observations, they are much harder to read. If an LLC is found a vortex data
message is issued, which usually leads to classification but not always.
Finally, a surface map with observations on it can be used to determine if an LLC has
formed. Usually the observations come from ships transiting the area, or
buoys. The surface map is most useful off the East coast, in the Gulf of Mexico,
and in the Carribean, as other areas lack buoys and ships. Tropical
Ramsdis includes surface obs in its satellite imagery creating a nice overlay.
As has been mentioned, an LLC must be under some sort of centralized convection to
be considered a tropical cyclone. Also, winds must be 30 mph and up for a
disturbance with an LLC to be considered a tropical cyclone. Winds can be
determined from QUIKSCAT, surface obs, and recon.
Forecasting Strength Once A Tropical Cyclone Has Formed
Strength forecasting for tropical cyclones is a gray area in tropical meteorology.
There is alot to grasp when forecasting the future strength of a tropical cyclone.The
same factors come in here that were mentioned in the cyclogenesis
section, and then some. First off, an SST of 80 degrees must be
met. The warmer the SST the more a tropical cyclone can strengthen. Also,
shear above 20 kt tends to disrupt any organized convection over a tropical
cyclone, and a cyclone can take 20 kt of shear for only so long before it collapses.
One must look however to see if the cyclone is moving with the
shear, like hurricane Wilma. If a cyclone can move fast enough with the shear,
the relative shear becomes zero. Same goes for if a cyclone moves against the
shear, the shear will be stronger and weaken the cyclone more. As for dry air
and the SAL, when dry air begins to entrain into a tropical cyclone (which can be
seen from water vapor imagery) then weakening of the cyclone can be expected.
Outflow
Another factor to determine strength of a tropical cyclone is the outflow pattern and
development. A desirable situation for a tropical cyclone is outflow around
the entire circulation. Outflow appears on visible imagery as high clouds
ejecting from the storm, and on water vapor as a moisture pocket
emanating from the storm. If this pocket is restricted, as seen in the image
below, it means the outflow is restricted. This is usually the result of shear.
Outflow restriction results in less venting of convection in the center of the storm
and thus, a weaker system, or less strength being obtained. A cyclone
can very well function with half its outflow restricted, but once more than half is
restricted collapse can be imminent. It all depends.
Rapid Intensification And Eyewall Replacement Cycles
One of the new and perhaps least understood area of forecasting tropical cyclone
strength forecasting is rapid intensification (RI). Rapid intensification is
only possible if an eyewall exists. This can be determined from recon reports,
radar, and if the storm is already well developed, satellite imagery.
When the storm is maintaining itself well in a favorable environment, and has an
eyewall, the eyewall may contract causing a sudden and extremely violent
increase in wind speeds, as well as a major drop in central pressure. This has
been seen countless times, from Andrew to Wilma, and is the biggest concern
when a tropical cyclone is approaching shore. Many factors contribute to RI and
it is not well understood, so the best way to forecast the possibility of RI is
by studying the eyewall replacement cycles of a storm as well as meso-vortices
if they do exist in a storm. Eyewall replacement cycles occur as
new eyewalls develop and old ones are destroyed, at the end (and start) of RI.
This is because as an eyewall becomes very contracted from RI, an outer
eyewall forms around the contracted one and suffocates it with subsidence. This
leads to a weakening of the max winds at the time, but sets up the opportunity
for further RI strengthening if conditions remain favorable. The timing of this
cycle is extremely crucial as a storm approaches shore, as it can mean the
difference between a Cat 1 and Cat 5, as seen with the Labor Day hurricane,
Charley is another example.
Meso-Vortices
As for meso-vortices, they are an unknown but do have a significant effect on
hurricanes. Meso-vortices, from all research obtained so far, are based in the
eyewall and either spin into the eye or out to the storm. Meso-vortices for all
intensive purposes represent entropy, as the friction from the surface tries to
destroy the perfect balance of centrifugal and centripetal forces in the eyewall.
Meso-vortices at times strengthen the max winds of a storm, as they in
themselves become 50 mph systems or so and then add on to the max wind in
the eyewall. At other times though, they weaken a system as they slow the
eyewall overall. It was observed with hurricane Isabel that once environemntal
conditions became unfavorable, the meso-vortices helped in rapidly weakening
the whole system. Thus, if meso-vortices are present in a hurricane, be ready
for wild swings in intensity. Loop of mesovortices, warning java: Isabel Loop
Diurnal Cycles
Another often discussed factor in tropical cyclone forecasting is the diurnal maximum.
This occurs at night, more towards morning, as the upper
atmosphere cools from lack of solar radiation. This stimulates convection to rise
into the cool layer, creating a noticeable burst of convection in the system.
The diurnal maximum can be observed throughout the tropics. The diurnal
minimum during peak heating of the day has the opposite effect of the
maximum. Also, the diurnal cycle is accompanied by pressure waves of a few
millibars, some are confused by this and think the storm is gradually
strengthening or weakening when its really just the entire environment changing pressure.
One more note, dvorak satellite intensity estimates are also used by forecasters to
determine the strength of a cyclone. These estimates are based on a variety of
things including spiral banding and the look of the eyewall. These estimates can
be found here: Satellite Intensity Estimates
Tropical Cyclone Track Forecasting
Although forecasting the track of a tropical cyclone is less difficult than forecasting
the strength, it still is a tricky thing to do.
The best way to determine the track of a tropical cyclone is to first understand a
tropical cyclone is like a bubble. The bubble will take the path of least resistance
on its trip to the surface (polar region). The bubble will encounter
areas of higher density fluid and lower density fluid. These of course are highs and lows.
A common feature of the North Atlantic is the Bermuda high. This high can stretch
from Mexico across to the whole Atlantic. Storms caught under the
Bermuda high keep on going in an almost straight line around it until they hit a
pocket of little resistance, and then shoot north to the pole. So, forecasting a
cyclone on the southern periphery of a high is easy until it encounters this
vacuum of sorts. The vacuum can be a gap between the Azores high and
Bermuda high or a weakness in the high caused by an incoming front from
North America. When there is no forcing mechanism however the cyclone will
just drift aimlessly, causing forecasters to draw a large circle around the storm
and call it the cone of probability. When a storm is near shore this can be very
nerve wracking for all. I have made a figure below showing steering patterns
common in the North Atlantic.
A good resource to use for determining the steering patterns that result from all this
is the CIMSS steering level analysis
The different millibar ranges are for different strengths of storms, as
that influences what steering currents actually impact the storm.
Now, the issue in determining track is knowing where the pressure systems will move
and how they will change in strength. Satellite imagery can be used to see
where the pressure systems are, where their headed, and how their
strength is changing. Surface obs can also be used to see how the pressure
systems are changing strength. From this, a forecaster can make a get a
general idea of a where a cyclone will go. This general idea can then be made
into a cone of error which shows all possible points for the movement of a
tropical cyclone. This part of forecasting requires alot of practice which will
eventually build instinct.
Forecast Models
Forecast models are a valuable tool in determining the path of a tropical cyclone, but
become much less reliable in regards to intensity forecasts and
cyclogenesis. My favorite model site is this one courtesy of FSU:
Experimental Tropical Cyclone Genesis Potential Fields
It gives a good idea of the fluidity of the tropics. Models can be used in conjunction
with the forecasting techniques above to create a better forecast, if one knows how to use them.
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