WMO/CAS/WWW
SEVENTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES
Topic 1: Tropical Cyclone Structure and Intensity Change
Topic Chair:
Robert Rogers
NOAA/AOML Hurricane Research Division
4301 Rickenbacker Causeway
Miami, FL 33149
E-mail:
Fax:
[email protected]
305-361-4528
Rapporteurs: J.L. Evans, E.A. Ritchie, L.K. Shay, J. Molinari, E. Fukada, A. Burton, J.R. Gyakum
Abstract
Multiple factors that govern tropical cyclone (TC) structure and intensity are discussed here. Largescale factors that impact TC structure and intensity are well-known, but new research has focused
conflicting impacts of the impact of the Saharan Air Layer on TC intensity. New studies are finding
that in some instances moderate and even strong vertical shear can be beneficial to intensification.
TC structure has focused on the size parameter, finding that TC’s tend to retain the size they have at
their inception, with possible modification based on outer-core humidity. Inner-core processes
include the relative importance of boundary-layer vs. lower-tropospheric convergence in amplifying
the TC primary circulation, as well as the structure of the eyewall and characteristics of the diabatic
heating. Convection is important in rapid intensification (RI) through subsidence warming,
generation of enhanced cores of cyclonic vorticity, and enhanced low-level updraft mass flux.
Eyewall structure using multiple airborne Doppler datasets found almost no relationship between
eyewall slope and intensity, but a relationship between eyewall slope and radius of maximum wind
(RMW). New observations of oceanic mixed layer (OML) thermal and current structure have shown
importance of deep mixed layer for limiting braking impacts of TC-induced cooling. Analyses of
recent field campaigns have yielded new formulations for both momentum and enthalpy exchange
coefficients, while research continues on sea spray and wave-breaking on TC structure and intensity
change.
While airborne data is fairly commonly-available in the Atlantic basin, and occasional airborne field
campaigns have occurred in the western Pacific basin, satellite data remains the primary tool for TC
structure and intensity analysis and forecasting around the globe. New techniques have been
developed to analysis this data, including automated Dvorak and satellite consensus techniques.
Microwave sensors are critical for assessing inner-core structure and evolution as well as outer-core
wind fields. The loss of the Quik-SCAT has been a big blow to forecast centers across the globe, but
it has been partially filled by ASCAT. New tools (e.g., MIMIC, ADT, SATCON) for intensity prediction
are based largely on satellite observations, highlighting their importance. The RI index is also based
on satellite data in conjunction with global model forecast fields. Statistical models continue to
show the greatest skill in forecasting intensity change and both the accuracy and availability of
statistical forecast aids has improved in the last four years. Recent advances in our understanding of
inner core processes, and our approaching capacity to achieve model resolutions fine enough to
describe those processes holds promise for a breakthrough in dynamical intensity forecasting skill.
Tools for structure prediction are lagging, but one scheme using climatology and persistence,
modified by factors known to influence size, has been developed, but data is lacking to verify these
1.0.1
predictions. Structure forecasting is considered a “poor man’s cousin” to track and intensity
forecasting.
There is a new recognition of subtropical and hybrid cyclones which involve a combination of
tropical and extratropical processes. They have a distinct geographic distribution that varies basinto-basin. One critical need is the development of a “universal” cyclone classification scheme.
1.0
Introduction
Tropical cyclone (TC) structure and intensity change processes span spatial and temporal scales of
many orders of magnitude. It therefore represents a significant challenge for forecasters and
researchers. Despite these challenges, significant advancements have occurred since the last IWTC
four years ago. These advancements have occurred in multiple facets of TC structure and intensity
change, including environmental impacts, inner-core impacts, air-sea interface and oceanic
influences, observation capabilities and opportunities, operational guidance, and subtropical and
hybrid systems.
1.1
Environmental Impacts
The dominant environmental factors that impact TC structure and intensity are well-known, i.e.,
vertical shear of the environmental wind, mid- and low-level humidity variations, SST and OHC
variations, and outflow-layer interactions with the environment. Much of the research that has
occurred on this topic since the last IWTC has focused on refining our knowledge of these impacts
through a combination of case studies, compositing studies, and modeling investigations.
Studies on the impact of the environment on TC intensity have investigated a variety of sources.
One potential mechanism for the negative impacts of a sheared environment on a TC is the
“ventilation effect” -- shear-induced development of mid-level, storm-relative flow imports low
entropy air from the environment into the TC core, suppressing the convection and so reducing the
surface-flux induced heating. Idealized modeling experiments have been performed to isolate the
effects of this shear-induced ventilation on TC intensity. The key findings are that ventilation
decreases the maximum steady-state intensity below the potential intensity, and so imposes a
minimum intensity below which a TC will unconditionally decay; in this framework, one can
determine an upper ventilation bound beyond which no steady TC can exist. Other modeling studies
have focused on the role of vertical shear in creating asymmetries that import low entropy air
toward the TC core, where it excites downdrafts that transport the low entropy air into the
boundary layer. These shear-induced downdrafts transport the low entropy air into the boundary
layer, where it is advected inward, reducing the flux of enthalpy from the sea to the air and so
limiting the TC intensity achievable through air-sea fluxes. While shear is generally considered to be
detrimental to TC intensity, there are situations where shear can generate a strong, long-lived
convective event that leads to a new surface circulation on the downshear flank of the initial TC, a
process known as down-shear reformation.
Another avenue of research has investigated the idea that interactions between a TC and its
environment are communicated via the TC outflow as it develops in a region of low environmental
inertial stability. In this environment the low inertial stability decreases environmental resistance to
TC outflow expansion allowing for multiple outflow jets to ventilate the storm. The asymmetric
outflow structure implied here may arise from direct interactions with another weather system (e.g.
TUTT, midlatitude trough), evolution in a sheared environment or convective asymmetries (which
may also arise from TC passage over a variable surface or the presence of dry air). Interaction of a
mature TC with an upper tropospheric vorticity center (e.g. upper-level trough) can result in TC
1.0.2
intensification as these “upper” and “lower” vortices interact, enhancing the primary TC circulation
directly or producing a secondary wind maxima. Other studies have looked at environmental
features such as precedent, high-amplitude ridging downstream of an advancing TC to explain how
some TC’s can retain their tropical characteristics even as they advance to high latitudes.
The early development of an incipient TC disturbance in the eastern North Atlantic can be
dramatically altered by the thermodynamic character of its environment. For example, a particularly
striking feature of the North Atlantic is the Saharan air layer (SAL). The SAL is an elevated layer of
warm, dry, dusty air that produces a very strong mid-tropospheric easterly jet. The thermodynamic
and dynamic impacts of the SAL on the formation and evolution of Atlantic hurricanes remains
unclear. The argument for a positive influence of the SAL on hurricane development is based upon
(1) a region of instability equatorward of the jet providing a preferred location for developing waves
that eventually form hurricanes and (2) rising motion south of the jet favoring development of deep
convection; the jet is typically stronger in storms that intensify. Potential negative impacts of the SAL
on TC include (1) low-level vertical wind shear associated with the jet; (2) increased thermodynamic
stability due to the elevated warm layer; and (3) evaporative cooling leading to cold dry downdrafts
in precipitating regions and weakening convection.
An extreme example of intensity change that has received considerable attention in recent years is
rapid intensification (RI). Many of the factors important in RI are similar to those that are known to
be favorable for intensification generally (e.g., low wind shear, warm SST/high OHC). Some cases of
RI in the presence of moderate or even strong shear have been documented, however. Composite
analyses of the environmental conditions favorable for RI vs. steady-state and weakening TCs in
multiple basins show that the relative importance of environmental factors has basin-to-basin
variability. However, there are few significant differences between the environments of intensifying
and rapidly intensifying TC, while there are clear differences between intensifying and
neutral/weakening TC. This indicates that in the composite average, the rate of TC intensification is
only weakly dependent on environmental conditions, provided the environment is reasonably
favorable.
Intensity forecast practices across ocean basins generally rely on the consideration of the same set
of environmental factors (upper level outflow, vertical wind shear and OHC), however the impacts of
these environmental modulators on TC intensity varies across basins. For example, in the western
North Pacific, tropical upper tropospheric troughs (TUTT) are most active during northern
hemisphere summer and provide either a favorable or unfavorable environment, depending on the
relative sizes and locations of the TUTT and TC. In the North Indian Ocean, the mid-summer Tropical
Easterly Jet in the upper levels is a major vertical wind shear-inducing feature that suppresses the
development and intensification of TC forming in the monsoon trough over the Bay of Bengal. The
Philippine Sea, Mozambique Channel and Gulf of Carpentaria are generally recognized as the most
favorable regions in the JTWC area of responsibility for RI due to their high potential for strong
upper level outflow and high SST (OHC). While dry air intrusion is a common consideration in TC
forecasting across all ocean basins, recent research has raised questions about the role of the SAL in
North Atlantic TC development and intensity change. The impacts of both the thermodynamics and
dynamics of the SAL environment on TC require further clarification before they can be incorporated
into NHC forecasts of Atlantic hurricane development. By contrast, dry air entrainment is rarely a
factor in forecasts for western North Pacific, Indian Ocean, and South Pacific cyclones except during
ET.
Structure research has focused on environmental impacts on the precipitation distribution, TC size,
and the radial distribution of TC winds. Vertical shear is a dominant mechanism for determining the
precipitation distribution, as has been well-documented. The precipitation distribution of a TC being
1.0.3
influenced by a mid-latitude trough (not resulting in ET) tends to be along the track or to the right of
track. By contrast, the precipitation of a TC undergoing ET tends to be to on the left side of the track
(for Northern Hemisphere TCs). Different-sized TCs tend to develop from different mechanisms -- in
general, small TCs form from an easterly wave synoptic pattern, while medium to large TCs tend to
form in monsoon-related patterns. Other studies hypothesize that the size of the outer TC wind field
is related to the intensity and extent of precipitation outside the inner core of the TC, which is in
turn dependent on the moisture in the TC environment. TCs tend to preserve their size throughout
their lifecycle, with the exception of ET.
While intensity forecasting remains a challenge, TC significant wind radii have possibly surpassed
intensity as the most difficult operational forecasts to produce. Forecasting tools for TC wind
structure have recently been developed based on climatological factors (translational speed,
intensity, latitude) and persistence (i.e. initial condition). Other tools use aircraft flight-level data
and coincident infrared satellite analyses to develop complementary methods to estimate tropical
cyclone winds using azimuthally averaged IR brightness temperature profiles. The lack of validation
of these forecasts – and the lack of appropriate data for validation – is a stumbling block to forecast
improvement. Variations of TC size (as measured by these critical threshold radii) and even radial
wind profiles are active and promising areas of research across the TC community, however these
results require further testing and scrutiny before they can be transitioned to operations.
1.2
Inner-core Impacts
A great deal of research has been conducted to look at the impact of inner-core processes on TC
structure and intensity change. The summary here will focus on the impact of inner-core processes
on TC intensification (normal), rapid intensification, super intensity, weakening, and structure.
The convergence of absolute angular momentum above the boundary layer (BL) has been shown to
spin up the outer circulation, which increases the vortex size but does little to the intensity of the
core. Instead, the inner core intensifies by radial convergence within the boundary layer, where the
flow becomes supergradient rather than being in gradient-wind balance. Other research has
considered the impact of static stability, eyewall slope, and tangential wind profiles on vortex
heating efficiency. The importance of the radial location of heating has been shown to be crucial -heating inside (outside) the radius of maximum wind is efficient (inefficient) at generating a localized
temperature tendency. This efficiency increased dramatically with storm intensity. Barotropic
instability that can arise as intense radial gradients of momentum, temperature, and moisture in the
vicinity of the eyewall can also play a role in modulating both TC inner-core structure and intensity.
Convectively-generated vortical hot towers have been shown to be important in the intensification
of simulated tropical cyclones. Deep convective towers growing in the rotation-rich environment of
the incipient core amplify the local vertical rotation and were the basic coherent structures of the
tropical cyclone intensification process. TCs experiencing vertical shear can also have extreme
helicity values that can produce supercell-like convective structures. These cells may enable the TC
to resist the negative impact of strong vertical wind shear.
The impact of inner-core processes on RI has been investigated by a number of studies in the past
four years. There are a number of recent modeling and observational studies that support the
concept of a mixing of eye and eyewall air through strong convective hot towers and their associated
eyewall mesovortices. In this scenario there is transport and mixing of relatively high θe-air from the
low-level eye to the eyewall, which enhances the efficiency of the hurricane heat engine. A portion
of the low-level inflow of the hurricane can bypass the eyewall to enter the eye, and this air can
acquire enhanced entropy characteristics through interaction with the ocean beneath the eye.
1.0.4
Observational and modeling studies have documented the role of convection in RI. Observations
from Hurricane Dennis in the North Atlantic basin in 2005 showed convective bursts during the
mature stage and prior to a period of RI. Significant downdrafts occurred on the flanks of the
updrafts, with their accumulative effects hypothesized to result in the observed increases in the
warm core. It was hypothesized by the authors that the subsidence was transported toward the eye
by an inflow occurring over a deep layer from the convective core to the eye/eyewall interface. A
modeling study of this same storm emphasized the role of low-level updraft mass flux that was
driven by heating from the convective bursts. The total updraft mass flux enhancement resulted in
amplification of the secondary circulation and inertial stability, which lead to a rapid intensification
phase. The importance of the high θe reservoir in RI may be due not so much to an excess of energy
in the eye, but rather in a boost it can provide to episodic convection that may in turn initiate RI.
Recent studies have come to contradictory conclusions as to the important physical processes for
supporting superintensity. One modeling study emphasizes area-integrated latent heat fluxes as
being the important driver of storm intensity, also finding that surface fluxes radially far from the
eyewall have an important contribution to the storm’s θe budget. Another set of observational
studies showed a TC that maintained an intensity significantly above the theoretically predicted
maximum potential intensity on three separate days as it crossed the cold wake left behind by
previous TC. The presence of mesovortices in the eye and eyewall indicated that eye/eyewall mixing
was likely occurring during this time.
Recent studies have also documented the important physical processes associated with TC
weakening. These include the development of an asymmetric warm core structure, barotropic
instability, and asymmetrically-distributed convection.
In terms of inner-core structure, much recent research has focused on eye formation, secondary
eyewall formation, spiral bands, hub clouds, and the slope of the radius of maximum winds.
Detrainment of ascending air in the eyewall has been suggested as a mechanism for subsidence
warming and eye formation. Other studies have found that intense mesoscale convective systems
away from the vortex center, expected when there is vertical shear, can lead to multiple regions of
inner-core subsidence which can disrupt the eye formation process. However, other research has
found cases where shear-induced subsidence near the core appeared to support a short-lived,
hurricane-strength vortex.
Although the occurrence and behaviour of eyewall replacement cycles is well documented, the
formation mechanisms are still debated with widely diverse explanations available. One study found
that the secondary eyewall formed when there was an enhanced region of outer vorticity called a
beta skirt around the tropical cyclone. The formation of moat regions, which are present between
the inner and the outer eyewall, may be associated with rapid filamentation zones where the flow
was dominated by strain. In these regions, the development of deep moist convection could be
significantly distorted and even suppressed. Other work has suggested that the moat was mainly
controlled by the subsidence associated with the overturning flow from eyewall convection and
downdraft from the anvil stratiform outside of the eyewall.
The Sawyer-Eliassen transverse circulation equation has been used to examine the central hub cloud
and surrounding clear-air moat that are sometimes seen in the lower eye of a TC. The dimensional
dynamical eye size was found to be the crucial parameter in controlling the radial distribution of
subsidence in the eye. When the eye size was small, the subsidence was nearly uniform, with less
than 10% variation across the eye. When the eye size was large the subsidence rate at the edge of
1.0.5
the eye was more than twice that at the center. Such a distribution of subsidence should result in a
warm ring structure, rather than a warm core structure.
An examination of a series of airborne Doppler datasets from aircraft observations found that the
slope of the radius of maximum winds varies nearly linearly with radius so that a greater slope
outward of the radius of maximum wind occurred for larger radii. Also, there was almost no
relationship between intensity and radius of maximum wind. While a relationship between intensity
and radius of maximum wind often existed for an individual storm as its eyewall contracted, a strong
relationship was not apparent when looking across data from multiple storms.
Recommendations for inner-core impacts on TC structure and intensity change focus on continuing
research on a variety of fronts: the mechanisms that result in overall intensification of the tropical
cyclone primary circulation; mechanisms that either result in weakening of the tropical cyclone
primary circulation, or appear to appear to offset anticipated weakening because of environmental
conditions; mechanisms associated with rapid intensification; and mechanisms of structure change
associated with intensification or weakening of tropical cyclones.
1.3
Air-Sea Interface and Oceanic Influences
In the past 15 years, there has been significant progress made in understanding the basic oceanic
and atmospheric processes that occur during TC passage. The relevant physical processes span a
multitude of domains and spatial/temporal scales: from the background oceanic state (temperature,
current, and salinity profiles); to the oceanic thermal and current response to TC forcing; to
processes at the air-sea interface such as the surface wave field, surface winds, surface drag
coefficient, wind-wave coupling, enthalpy fluxes, and sea spray. The need is to isolate fundamental
physical processes involved in the interactions through detailed process studies using experimental,
empirical, theoretical, and numerical approaches. As demonstrated from new measurements, these
approaches are needed to improve predictions of TC intensity and structure.
Coupled oceanic and atmospheric models to predict hurricane intensity and structure change will
eventually be used to issue forecasts to the public who increasingly rely on the most advanced
weather forecasting systems to prepare for landfall. For such models, it has become increasingly
clear over the past decade that ocean models will have to include realistic initial conditions to
simulate not only the oceanic response to hurricane forcing, but also to simulate the atmospheric
response to oceanic forcing. An important example of this later effect was observed during the
passages of hurricanes Katrina, Rita, and Wilma during the 2005 Atlantic Ocean hurricane season.
Favorable atmospheric conditions prevailed in the Northwest Caribbean Sea and Gulf of Mexico
(GOM) as the Loop Current (LC) extended several hundred kilometers north of the Yucatan Strait. As
these storms moved over the deep warm pools, all three hurricanes explosively deepened and were
more closely correlated with the ocean heat content (OHC) variations (and deep isotherms) than
with the sea-surface temperatures (SST) distributions, which were essentially flat and exceeded 30°C
over most of the region.
Upper-ocean mixing and cooling are a strong function of forced near-inertial current shears that
reduce the Richardson numbers below criticality, inducing entrainment mixing. The contributions to
the heat and mass budgets of the oceanic mixed layer (OML) by shear-driven entrainment heat
fluxes across the OML base are 60 to 85%, surface heat fluxes are between 5 and 15%, and
horizontal advection by ocean currents are 5 to 15%. Using satellite- and aircraft-based
measurements, TC impacts on the OML have been documented in TC’s in the Gulf of Mexico as well
as the western Pacific. Such impacts are evident on the thermal structure and evolution within the
OML as well as chlorophyll concentrations, which tend to rise during significant mixing event across
1.0.6
the OML base as nutrient-rich colder water below the thermocline is mixed into the OML by strong
current shears associated with TC passage. Observations of the ocean current response to TC
passage have been generally sparse over the global oceans as the community has had to rely on
fortuitous encounters with buoys and moorings deployed in support of other experiments or ships
crossing TC wakes. Nonetheless, some important current measurements have been made in
typhoons and hurricanes using coastal radars to document the surface current evolution.
Background oceanic flows that are set up by large horizontal pressure gradients due to temperature
and salinity may play a significant role in altering the development of strong wind-driven current
shears. Data assimilative ocean nowcasts are an effective method for providing initial and boundary
conditions to the ocean component of nested, coupled TC prediction models. The thermal energy
available to intensify and maintain a TC depends on both the temperature and thickness of the
upper ocean warm layer. The ocean model must be initialized so that ocean features associated with
relatively large or small OHC values are in the correct locations and T-S (and density) profiles, along
with the OHC, are realistic. Another important factor impacting the upper-ocean heat budget and
the fluxes to the atmosphere is the choice of entrainment mixing parameterizations. Recent
research has found that the three higher-order turbulent mixing schemes lead to a more accurate
ocean response simulation based on comparisons to the in-situ measurements. The surface drag
coefficient is also an important parameter that drives the current and its shear, leading to enhanced
shear instabilities and entrainment mixing.
If the upper-ocean warm layer is thick and has a large total OHC, the SST will decrease slowly during
TC passage, the negative feedback mechanism (or “brake”) will be weak, and the ocean will promote
TC intensification. Several studies over the globe have now shown that OHC relative to the depth of
the 26°C isotherm takes this into account and is a better indicator than a thin layer of high SST on TC
intensification. Studies linking OHC to TC intensification in the Atlantic, Indian, and West Pacific
basins have found a relationship between high OHC and TC intensification.
The effect of the underlying ocean has drawn attention towards gaining a better understanding of
the physical interaction between the atmosphere and ocean during TC’s. Due to limited observations
across the air-sea interface in high-wind conditions, the understanding has not progressed nearly
enough to significantly improve the parameterization of momentum and energy transfers between
the two fluids. The relationships of the transfer processes of small-scale roughness and surface-layer
stability are understood under low- to moderate-wind conditions, but additional phenomena not
typically observed such as the sea state maturity and sea spray have been shown to modulate the
heat and momentum exchange. These effects under TC winds have been primarily studied in
controlled laboratory experiments. In a TC environment, both young and mature waves are present
and impact air-sea fluxes and OML and atmospheric boundary layer processes.
The variability of the surface drag coefficient has received considerable attention over the last five
years including the Coupled Boundary Layer and Air-Sea Transfer (CBLAST) program sponsored by
the Office of Naval Research. Several treatments have come to the conclusion that there is a leveling
off or a saturation values at about 30 m s-1 +/- 3 m s-1. The ratio of the enthalpy coefficient and the
drag coefficient is central to air sea fluxes impacting the TC boundary layer. In this context, the
relationship between the coupled processes such as wave breaking and the generation of sea spray
and how this is linked to localized air-sea fluxes remains a fertile research area. A key element of this
topic is the atmospheric response to the oceanic forcing where there seems to be contrasting
viewpoints. One argument is that the air-sea interactions are occurring over surface wave (windwave) time and space scales and cause significant intensity changes by more than a category due to
very large surface drag coefficients. Yet empirical studies suggest the values to be between 2.5 to 3.4
x 10-3 compared to recent coupled model studies. While, these sub-mesoscale phenomena may
1.0.7
affect the enthalpy fluxes, the first-order balances are primarily between the atmospheric and
oceanic mixed layers.
Considerable ocean-atmosphere variability occurs over the storm scales that include fundamental
length scales such as the radius of maximum winds and another scale that includes the radius to
gale-force winds. Here, the fundamental science questions are how the two fluids are coupled
through OML and ABL processes, and what are the fundamental time scales of this interaction?
These questions are not easily answered as the interactions must be occurring over various
time/space scales. For example, one school of thought is that the only important process with
respect to the ocean is under the eyewall where ocean cooling has occurred. While it is at the
eyewall where the maximum wind and enthalpy fluxes occur, the broad surface circulation over the
warm OML also has non-zero fluxes that are contributing thermal energy to the TC. The deeper the
OML (and 26°C isotherm depth), more heat (OHC) is available to the storm through the enthalpy
fluxes. Notwithstanding, it is not just the magnitude of the OHC, since the depth of the warm water
is important to sustaining surface enthalpy fluxes. Process studies need to begin to look at these
multiple scale aspects associated with the atmospheric response to ocean forcing.
1.4
Observation Capabilities and Opportunities
The primary tools for observing TCs are manned aircraft and satellites. Since the vast majority of
aircraft observations occur in the Atlantic basin, the forecast agencies in other parts of the world by
necessity rely on satellite data to collect information on TC structure and intensity change. For this
reason the majority of this summary will focus on satellite capabilities, with limited discussion of
aircraft capabilities.
Responsibility for regional data coverage by geostationary meteorological satellites is shared
amongst the USA, EUMETSAT (for Europe), Japan and Russia. India maintains its INSAT satellite for
national use and PRC is developing its FY-2 series of experimental geostationary satellites.
Multiple operational microwave imagers have been launched since 2006 and are available for TC
monitoring using near real-time digital data sets. The Defense Meteorological Satellite Program
(DMSP) has successfully launched two satellites (F-17 and F-18), both including the Special Sensor
Microwave Imager Sounder (SSMIS). The SSMIS is similar to one on F-16, with hardware
modifications intended to mitigate temperature anomalies experienced with the F-16 sensor. F-17
and F-18’s SSMIS data are now being reviewed for inclusion in the TC intensity algorithms as these
sensors provide important temporal sampling not possible with the NOAA/MetOp-A orbits. The
Advance Microwave Sounding Unit (AMSU), which was on many previous NOAA satellites, was
launched on the NOAA N-19 (Feb 09) and EUMETSAT MetOp-A (Oct 06). These cross track scanners
cover a large swath (2200 km+), though not all of the swath data can be used for TC monitoring due
to ground spatial resolution (pixels) becoming too large near the edge of the scan.
The current constellation of microwave imager satellites occupies four different orbital planes; a) an
early morning 0530 orbit with a DMSP satellite, b) one mid morning 0800 orbit with another DMSP
satellite, c) a second mid morning 0930 orbit via EUMETSAT’s MetOp-A spacecraft, and d) an
afternoon 1330 orbit via NOAA spacecraft. Each has a descending path beginning 12 hours later. This
arrangement provides temporal sampling intended to effectively monitor TC structure and intensity
changes, especially rapid intensification or decay, which often occurs within a few hours.
The primary methods for surface wind estimation involve aerial reconnaissance data and satellitebased scatterometer estimates. Reconnaissance data in the Atlantic Basin are obtained from C130
aircraft flown by the U.S. Air Force Reserve, and by NOAA P-3 and Gulfstream IV aircraft. A major
1.0.8
improvement in surface wind estimation from aircraft since 2006 arose from use of the steppedfrequency microwave radiometer (SFMR) on all reconnaissance aircraft in the Atlantic Basin. This
instrument provides direct estimates of surface wind speed. The U.S. National Hurricane Center uses
these estimates, along with dropwindsonde data and reduction of flight-level wind speeds to the
surface, to determine maximum surface wind speed and the radii of 34, 50, and 64 knot winds,
though errors still arise due to sampling limitations of the aircraft. Limited aerial reconnaissance has
also been conducted in the Western North Pacific Ocean around Taiwan since 2003 using an ASTRA
aircraft collecting dropwindsonde data.
Satellite-based scatterometer winds provide a critical alternative to the accurate but limited aerial
reconnaissance data and are used by all international tropical cyclone forecast centers.
Scatterometers cannot retrieve the maximum wind values for most TCs, but can represent a
“minimum maximum” or the lowest max wind to be used with all other sources (in-situ and remote
sensing) in determining the max winds. The two scatterometers primarily used for this purpose are
the AMI on the ERS-1 spacecraft and the Advanced Scatterometer (ASCAT). These platforms have
assumed significant importance since the failure of QuikSCAT in November 2009 after more than a
decade of stellar service providing a 1800 km swath of increasingly reliable ocean surface wind
vectors useful for TC applications.
All operational centers have expressed concern over the loss of QuikScat in November 2009. The
ASCAT scatterometer has filled some of the gap, but it has only 60% of the coverage of QuikScat. In
addition, the optimum use of ASCAT remains uncertain. The U.S. National Hurricane Center has
noted a negative bias for surface wind speeds with ASCAT. RSMC New Delhi uses ASCAT to estimate
the center location and intensity only for weak systems. Similarly, RSMC Tokyo uses ASCAT primarily
for the determination of tropical storm intensity. TCWC Perth and RMSC Nadi use ASCAT for
guidance on the radii of 34 and 50-knot wind speeds. JTWC also uses ASCAT winds, but for 34, 50,
and 64-knot wind radii they also make use of the CIRA multi-platform wind distribution (CIRW)
available on the Regional and Mesoscale Meteorology Branch (RAMMB) web site. This latter tool has
been added since 2006.
Another developing TC reconnaissance asset is the U.S. Navy’s R&D WindSat polarimetric radiometer
onboard the Coriolis spacecraft (2003 launch) which provides near real-time ocean surface wind
vectors over its 1000 km swath. The 25 km ground spatial resolution enables WindSat to assist in
defining TC gale force winds when rain contamination does not cause wind retrieval issues.
The advantages inherent to microwave data have made it useful for intensity estimation, despite the
gaps in temporal coverage. Operational centers all expressed a desire to more formally add
microwave methods to their process of intensity estimation. A number of tools are being developed
in research settings that are becoming available to operational centers.
One key tool that has facilitated the development of these intensity estimation algorithms is Google
Earth. The use of georeferenced images allows for the overlay of multiple datasets, including a
synthesis of satellite, aircraft, and model data.
Many products have been developed for intensity estimation. Examples include a scheme that uses
multi-channel microwave imager data and one where TC central pressure estimations are made
from AMSU data. The CIMSS Advanced Dvorak Technique (ADT) is a well-known technique that
addresses one of the traditional areas of difficulty in assessing TC intensity with IR-based Dvorak
techniques, the Central Dense Overcast (CDO) scene type. Changes in TC structure can occur
beneath the cold and blanketing cirrus of the CDO, leading to changes in intensity. This cirrus
overcast masks changes in the TC structure and intensity, primarily in the development stages of the
1.0.9
TC eyewall. The resulting impact on the ADT is that the estimated intensity will plateau during the
CDO phase (when IR temperatures change little) until an eye becomes visible in the IR imagery, at
which time the ADT logic uses the eye scene to intensify the TC. In order to address the limitation in
the IR-based identification of an eye, microwave imagery is used with an approach called the CIMSS
Automated Rotational Center Hurricane Eye Retrieval (ARCHER). This scheme uses passive
microwave data in the 85-92 GHz range to refine the intensity estimate based on characteristics of
the inner core. The SATellite CONsensus (SATCON) algorithm combines estimates from three
sources: the CIMSS ADT and the CIMSS and CIRA AMSU algorithms. SATCON provides the TC
forecaster with the ability to quickly reconcile differences in objective intensity methods thus
decreasing the amount of time spent on the analysis of current intensity.
Other algorithms have been developed using satellite data to predict changes in TC structure, e.g.,
eyewall replacement cycles. One technique predicts secondary eyewall formation using
geostationary satellite data plus information on the surroundings of the storm from the Statistical
Hurricane Intensity Prediction Scheme to provide forecasters with a probability of imminent
secondary eyewall formation. This provides an additional tool for evaluating tropical cyclone
structure and its role on future intensity.
Future plans for microwave imagers have as a big uncertainty the plans for the availability of imagers
planned to be launched by China and Russia. The major US effort is geared to the launch of two
Advanced Temperature and Moisture Sounders (ATMS) onboard the NPOESS Preparatory Program
(NPP) satellite in late 2011 and the first satellite in the newly created Joint Polar Satellite System
(JPSS) which represents the NOAA portion of the redirected NPOESS project. ATMS holds promise
for continuing the TC intensity applications now applied using AMSU data. The future also holds
promise with two non sun-synchronous R&D satellites planned for the next several years: a) MeghaTropiques (MT), and b) GPM-Core. Megha Tropiques has a non sun synchronous orbit like TRMM
and will provide ~ 10 km 89 GHz brightness temperature imagery over the entire swath. This
represents a major TC monitoring asset. In addition, the Global Precipitation Mission (GPM) will
carry two sensors when launched in 2013; 1) GPM Microwave Imager (GMI) and the Dual frequency
Precipitation Radar (DPR). The GMI will have a 904 km swath (larger than TMI) with frequencies
ranging from 10-89 GHz for standard imagery and include channels at 165 and 183 for enhanced rain
retrievals while the DPR will operate at 13.6 and 35.5 GHz with swaths of 245 and 120 km
respectively (in between the swath range for the TRMM PR).
Recommendations include the possible use of unmanned aircraft to augment the resource
limitations of manned aircraft (though unmanned aircraft can be quite resource-intensive as well),
additional scatterometer information, holding the planned microwave launches to their current
schedule given the likely continued degradation in the current constellation of microwave satellite
coverage, the development and enlargement of forecast training and collaborations among
operational centers, basin-specific Dvorak validation, and the further development of an objective
technique for intensity and intensity change determination using microwave data.
1.5
Operational Guidance
Operational analysis of tropical cyclone structure continues to be focused on the radius of maximum
winds and the radial extent of winds exceeding various thresholds. Objective analytical methods
have been developed but analysis of structure continues to be hampered by a lack of suitable
observations. Prior to the real-time availability of scatterometer winds gale radii were often inferred
from conventional (IR and VIS) satellite imagery by correlating the size and asymmetry of the central
dense overcast with the gale force wind field. Estimates made via this method are likely to have
some gross skill in discerning between large and small circulations but they are associated with low
1.0.10
confidence and significant uncertainty, and verification has been considered impossible. Low level
atmospheric motion vectors (AMVs) can also be used to infer the near surface wind field by making
assumptions about the reduction of winds to the surface; however most of the gale wind field is
obscured by high cloud making it impossible to determine low level AMVs.
Microwave imagers have provided significant additions to the capabilities for operational
assessment of TC structure and intensity. Passive microwave imagery (36-37 and 85-91 GHz) is
particularly useful for diagnosing changes in the cyclone’s inner structure, such as the formation of
an eye feature and the occurrence of eyewall replacement cycles. In addition to determining the
structure of the wind field, tropical cyclone analysts often need to understand the thermal structure
of systems, particularly those that develop from subtropical cyclones and those undergoing extra
tropical transition. Since 1998 the Advanced Microwave Sounding Unit (AMSU) has provided direct
objective measurement of the thermal structure of tropical cyclones. Scatterometers have proven
to be a vital tool, in particular for describing the outer wind structure of tropical cyclones. The
launch of Seawinds on QuikSCAT in 1999 saw a quantum improvement in coverage, with 90%
coverage of the tropical oceans in a 24 hour period, and QuikSCAT soon became a mainstay of
tropical cyclone analysis. Following the demise of QuikSCAT in November 2009, ASCAT continues to
provide near surface winds through active microwave sensing albeit with roughly half the coverage
and a reduction in effective wind range.
To address the variable temporal gaps in microwave coverage, CIMSS has developed a family of
algorithms called Morphed Integrated Microwave Imagery at the Cooperative Institute for
Meteorological Satellite Studies (MIMIC) to create synthetic “morphed” images that utilize the
observed imagery to fill in the time gaps and present time-continuous animations of tropical
cyclones and their environment. MIMIC-TC is a product that presents a storm-centered 15-minresolution animation of microwave imagery in the ice-scattering range (85–91 GHz), which can be
interpreted very much like a ground-based radar animation. A second product, MIMIC-IR, animates a
tropical cyclone–retrieved precipitation field layered over geostationary infrared imagery. These
tools allow forecasters and analysts to use microwave imagery to follow trends in a tropical cyclone's
structure more efficiently and effectively, which can result in higher-confidence short-term intensity
forecasts. Another product developed by CIMSS, the Automated Rotational Center Hurricane Eye
Retrieval (ARCHER), estimates TC structure and position information from available passive
microwave channels in the 85-92 GHz frequency range.
Other tools have been developed to produce analyses of the 2-D wind field. These techniques use
aircraft flight-level data and coincident infrared imagery. These techniques are described in the
previous section.
The analysis of TC intensity, primarily using satellite data, includes the Dvorak technique, Automated
Dvorak technique, and SATellite CONsensus (SATCON) methods. These methods have been
discussed in the previous section.
In the past the Dvorak technique dominated intensity estimation, at times being the sole method for
determining intensity. Although subjective Dvorak estimates continue to be an important
consideration in intensity analysis, the current state of the art in operational centres is to take a
consensus approach: subjectively weighting all available guidance to determine the final intensity
estimate. The trend toward an increasing number of objective aids with skill comparable to
subjective methods is encouraging. However data platforms and intensity estimation methods often
give conflicting information and despite the increasing skill of objective consensus methods such as
SATCON, the analyst often has to rely on a thorough knowledge of the strengths and weaknesses of
each technique and data platform to arrive at an official intensity estimate.
1.0.11
In terms of forecasting, much less guidance is available for forecasting structural changes in the
tropical cyclone wind field compared with track and intensity forecasting. Consequently the forecast
process is very subjective, and varies more strongly between operational centers and even between
forecasters. In the cyclic routine of tropical cyclone forecasting the structure forecast usually follows
the track and intensity forecast. The estimation of future structure is therefore often performed
under increasing time pressure as the issue time for warnings approaches. Forecasts of tropical
cyclone structure are rarely verified in the same way that track and intensity forecasts routinely are,
largely due to the limitations of current observational systems. Together with the paucity of
available guidance these factors have tended to establish structure as the “poor cousin” of track and
intensity. However the critical dependence of storm surge on structure creates a strong imperative
for improvement and there is clearly a need to improve the quality of both observing systems and
guidance in this area.
The general approach is to use a combination of persistence and climatological values modified by
factors known to influence size. For example, some growth in the wind field with increasing latitude
will generally be assumed, or a forecast increase in wind shear may be used to infer increasing
asymmetry in the wind field. One recent study has found that eyewall replacement cycles and the
interaction of TCs with synoptic environmental features such as warm air advection or vertical wind
shear result in tropical cyclone growth. An algorithm has been developed that predicts the variation
of significant wind radii based on climatological factors (translational speed, intensity, latitude) and
persistence (i.e. initial condition).
A number of recent advances in the analysis of intensity are noted and progress has been made in
providing more skilful operational guidance on intensity change, though rapid intensification
remains an area of poor skill. Statistical models continue to show the greatest skill in forecasting
intensity change and both the accuracy and availability of statistical forecast aids has improved in
the last four years. Further work is required to ensure that recent developments in the Atlantic and
North East Pacific are made available in all tropical cyclone basins. Recent advances in our
understanding of inner core processes, and our approaching capacity to achieve model resolutions
fine enough to describe those processes holds promise for a breakthrough in dynamical intensity
forecasting skill. An increasing emphasis on employing consensus methods for intensity forecasting
is noted. These efforts will need to be maintained to maximize gains in forecast skill as statistical and
dynamical forecast aids improve.
1.6
Subtropical and Hybrid Systems
Since the IWTC in 2006, there has appeared increasing documentation and evidence in the refereed
literature of extratropical processes occurring in association with named tropical cyclones. Such
cases are typically classified as either subtropical (STC) or hybrid cyclones. Because of the proximity
of STC and hybrid storms to important features of the midlatitude flow (upstream troughs, blocks,
cutoffs, etc.), the influence of the TC - and in particular the region of diabatically reduced PV in the
upper-level outflow - is readily amplified. This interaction mechanism is similar to that observed
during extratropical transition, but can occur over an extended period of time since the STC or
hybrid vortex may not yet have undergone recurvature into the midlatitude westerlies.
Hybrid cyclones have energetics and structures of both tropical and baroclinic cyclones. Many TCs
form outside of the main development region in the North Atlantic via baroclinic processes. In fact,
studies from the 1990’s have shown that baroclinic processes were a factor in the development of
almost 50% of all North Atlantic TCs. Three distinct baroclinic genesis pathways existed for North
Atlantic TCs.
1.0.12
There is another category of storms known as subtropical cyclones (hereafter, STC) or hybrid
cyclones that possess attributes of both TCs (e.g., they have low-level warm cores) and ordinary
baroclinic cyclones (e.g., they are governed by QG processes) with attendant frontal structures. STCs
differ from ordinary baroclinic cyclones in that they develop in environments with little low-level
baroclinicity in conjunction with diabatic processes. The distinction between a STC that goes
through its life cycle in a sheared environment and a TC that forms via the tropical transition (TT)
process is often subtle.
There are some basic and frequent characteristics common to this variety of systems and
meteorological situations. Highlights include:
•
These cyclones can develop in regions where SST values are colder than the typical 26°C
needed for a tropical storm to form in the North Atlantic basin.
•
Usually there is an interaction between a baroclinic disturbance, e. g., an upper level trough
or a cut-off low, and the preexisting subtropical or tropical system. This process produces an
intensification of the system and changes in the internal and external cyclone structure. In the case
of the ET affecting Spain they use to produce a significant impact with strong winds over larger areas
than those affected by the previous tropical cyclone or STC.
•
Another feature that is common to this kind of systems is a low level shallow warm core that
clearly appears in some stage of the storm life cycle.
Several tools are used to determine cyclone type, include hybrid storms and STCs. These are: visible
and infrared satellite imagery; AMSU temperature anomaly cross sections; 1000-500 hPa thickness
and 850 hPa temperature analyses and forecasts; cyclone phase space diagrams; sea surface
temperature; scatterometer and multi-source satellite wind data; vertical soundings; 500 to 300 hPa
heights; total precipitable water; and vertical wind shear.
The description of the tropical transition (TT) development pathway has spurred new interest in TC
initiation that does not adhere to traditional tropical norms. During TT, a precursor vortex of
midlatitude origin interacts with an upper-level trough, providing large-scale QG forcing for ascent
that enhances deep moist convection near the center of the developing storm. Vertical transport of
PV and momentum reduces shear and eventually weakens the interacting trough as the lower-level
system acquires a symmetric structure with a pronounced outflow at upper levels. This warm-core,
vertically stacked tropical cyclone intensifies through the air-sea interaction process. Prior to and
during the transformation period, the vortex often displays characteristics of a subtropical or hybrid
system whose evolution can be considered generally analogous to that of a midlatitude occlusion
occurring over warm waters with an ample supply of lower-level moisture.
A composite analysis has identified North Atlantic TC development events between 1948 and 2004.
From this study six pathways were identified: non-baroclinic (40%), low-level baroclinic (13%),
transient trough interaction (16%), trough induced (3%), weak TT (13%) and strong TT (16%).
Different development pathways are preferred in different areas of the basin. Of the primary hybrid
pathways, transient trough, weak TT and strong TT developments occur at progressively higher
latitudes along an axis extending from the Gulf of Mexico along the eastern seaboard and into the
central North Atlantic. This climatology has been extended to all basins. The preferred development
pathways are dramatically different in other basins, with a much larger fraction of traditional tropical
pathways (non-baroclinic and low level baroclinic) in general. The western North Pacific shows a
peak in trough-induced and weak TT events that may be associated with transient uppertropospheric trough (TUTT) cells, while the South Pacific displays a secondary maximum in hybrid TTtype developments.
1.0.13
The impacts of hybrid or STCs are similar to those of tropical storms - high winds, heavy rains, and
storm surges in coastal areas. In most cases, these systems transition into TCs at or before the winds
reach hurricane force. Systems that remain hybrid are typically not of hurricane intensity, and the
wind and surge impacts are generally not severe. However, a few cases have developed hurricaneforce winds without developing full TC structure, such as the December 1994 storm in the western
Atlantic. The rainfall – and resulting flooding impacts – of a hybrid cyclone can be as large as that of
a TC.
The Unites States TC warning/response process is triggered by NHC products on STCs and TCs, and it
often involves significant response by emergency managers (EM). The warning and response system
for non-tropical cyclones is not as centralized, and it normally includes a lower level of EM response.
This difference adds a potential non-meteorological factor to any NHC decision to designate a
transitioning cyclone as a STC or TC, as the warning/response process changes significantly when the
NHC initiates advisories on a system. This can make a cyclone undergoing TT near land especially
challenging. The black/white dichotomy of the warning/response process stands in contrast to the
shade-of-gray continuum of cyclone types and their ability to change types.
JTWC forecasts track, intensity, and wind radii for tropical cyclones, but not for subtropical or hybrid
cyclones. Cyclones deemed subtropical or hybrid are instead described on JTWC “significant tropical
cyclone advisories” (ABPW covering the western North and South Pacific Ocean and ABIO covering
the Indian Ocean) and closely monitored for transition into tropical cyclones. When transition into a
tropical cyclone is expected to occur within 24 hours and maximum sustained wind speeds will meet
warning criteria (discussed in the background section above) a Tropical Cyclone Formation Alert
(TCFA) will be issued. Because JTWC does not currently warn on subtropical or hybrid systems, the
challenges of forecasting track and intensity change are delayed until TT has occurred. However,
satellite analysts and forecasters must still analyze and describe (on tropical weather advisories and
TCFAs) the structural characteristics and dynamical mechanisms driving STC and hybrid cyclone
development and characterize the potential for these cyclones to undergo TT.
Hybrid systems have, it seems, notoriously low predictability in numerical models and the forecast
setting in general. The structural evolution of the hybrid system is often a consequence of a tug-ofwar between forcing mechanisms seeking dominance of the evolution through their respective
forcing. This predictability of hybrid systems has been researched in several ways by the author’s
group since IWTC6, including composite analysis and numerical simulation.
Subtropical and other hybrid cyclones often occur outside of the climatological areas for TCs, as
shown by the March 2010 South Atlantic cyclone, and at times distinct from those expected from
climatology. In addition, the marginal tropical character of these systems raises questions about
their inclusion in seasonal TC counts. Currently, the National Hurricane Center (NHC) only includes
them if they clearly become STCs or TCs. Any change in this practice would impact TC climatology. It
is possible that older storms in the Atlantic TC tracks database were hybrid for some or all of their
life, with the contemporary observations insufficient to distinguish the various phases.
Recommendations for the IWTC include continued research of TC development pathways, since the
ability of traditional analysis techniques and numerical models to forecast such events may depend
significantly on the subtropical or hybrid natures of the systems themselves during the development
process. There is presently no single set of objective criteria that, if applied, would irrefutably
support a forecaster’s analysis of cyclone type (subtropical, hybrid or tropical). In light of the
ambiguities that often arise when classifying cyclones, the IWTC working group could make a
substantial contribution to the operational TC forecasting community by recommending a
“universal” cyclone classification methodology based on the latest research, operational forecasting
capabilities, and real-time data availability. Once an accepted definition of STC is established, then
1.0.14
an accepted, and maintained, database of STCs (based on reanalysis data) would allow for needed
analysis and diagnosis of such STCs. Enhanced communication of subtropical and hybrid system
impacts to emergency managers is necessary. In the absence of a more appropriate warning system,
it may be preferable to continue to use TC warning systems for subtropical/hybrid systems. Finally,
further research on model forecasts of STC and hybrid system evolution is needed.
1.0.15