FIFTH INTERNATIONAL WORKSHOP ON TROPICAL CYCLONES
Topic 1.2: Environmental Effects
Rapporteur: E. A. Ritchie
University of New Mexico
Albuquerque, NM 87131
Working Group: Jenni Evans (U.S.A) Sarah Jones (Germany)
John Knaff (U.S.A.) John Molinari (U.S.A.)
Lloyd Shapiro (Germany) Chris Velden (U.S.A.)
Recent research to increase understanding, and techniques to improve forecasts, of the intensity and structural changes of a tropical cyclone due to interaction with the environment are summarized. The atmospheric environment is considered here, and the oceanic, and air-sea interface environments are summarized in Topic 1.4. Progress in understanding how a tropical cyclone interacts with its environment, and in developing techniques to forecast tropical cyclone intensity-change events, has been made over the past few years. Whereas any increase in skillful forecasts due to these research and techniques may take some years to develop, it is felt that improvement in tropical cyclone intensity forecasts is likely, due in part to the work described here.
The impacts of the environment on tropical cyclone structure and intensity have been studied for many years. Here, an update on progress in research and forecast techniques since the fourth IWTC is provided. It is well known that favorable environmental conditions (including minimum vertical wind shear) are required for tropical cyclone formation. Emanuel (1988) and Holland (1997) have developed separate relationships between the maximum potential intensity (MPI) and the sea-surface temperature (SST) and the environmental conditions, which include the static stability, upper-tropospheric conditions, and relative humidity. The wind structure (intensity) changes of a tropical cyclone from formation to maximum intensity to decay depend on a balance between favorable and inhibiting environmental conditions. Whereas, mostly atmospheric factors will be considered here, in a later topic (1.4) the sea-surface and oceanic forcing will be summarized. Environmental conditions summarized in the following sections include:
A study by Knaff et al. (2003a) highlights a small subset of tropical cyclones in the North Atlantic and eastern North Pacific basin that briefly developed unusual structural and intensity characteristics in low easterly vertical wind shear environments over constant or decreasing SSTs. As observed in infrared imagery, these tropical cyclones tended to have larger than average eye sizes, symmetrically distributed cold brightness temperatures in the eyewall, and little or no rainband features. In addition, these annular tropical cyclones were significantly stronger, maintained their peak intensities longer, and filled more slowly, than the average tropical cyclone in these basins. Knaff et al. (2003a) also note that average official forecast intensity errors for these types of tropical cyclones were 10 30 % larger than the 5-y mean official errors during the same period. It is interesting to note here that model simulations in low- or no-flow wind environments (i.e., f plane simulations) show a tendency for the tropical cyclone intensity to develop to the MPI calculated for the simulated environment (e.g., Peng et al. 1999; Frank and Ritchie 1999, 2001; Dengler and Keyser 2000).
Uniform-flow environments have mostly been considered in idealized model simulations of tropical cyclones. Whereas it would be unusual for the near-tropical cyclone environment (within 1000 km radius) to consist of a uniform flow because of the vertical wind shear associated with the beta gyres, it is educational to consider how a mean flow on a f plane and a beta plane affect tropical cyclone structure and intensity. In simulations on a f plane with fixed SST, a weak uniform flow imposed on a simulated tropical cyclone produced slightly varying results (Peng et al. 1999; Dengler and Keyser 2000; Frank and Ritchie 2001). Frank and Ritchie (2001) simulated a tropical cyclone in 3.5 m s-1of uniform background flow that intensified slightly more rapidly, and reached a slightly higher intensity, than the no-flow case. In this case of low uniform flow, the asymmetry in convection produced by frictional convergence in the front quadrant of the tropical cyclone (Shapiro 1983 also referred to as the motion asymmetry) produced an enhancement in the average precipitation of 2-5 cm (3 h)-1 in the inner 50 km of the tropical cyclone when compared to the no-flow case (Frank and Ritchie 2001). Dengler and Keyser (2000) also found that the tropical cyclone intensified more rapidly in a uniform background flow in their simulations, but the maximum intensity of the tropical cyclone was weaker than that for the no-flow case, particularly for strong uniform flow. In a separate experiment, Peng et al. (1999) found that a uniform flow of 5 m s-1 simulated a slightly weaker tropical cyclone, and 10 m s-1 uniform flow simulated a considerably weaker tropical than the no-flow case.
In addition, the direction of environmental uniform flow has also been found to be a factor in modeling studies when the beta effect is included. Peng et al. (1999) and Dengler and Keyser (2000) both found that uniform westerly flow was more favorable for tropical cyclone intensification than uniform easterly flow. Peng et al. (1999) concluded that westerly (easterly) uniform flow partially cancelled (enhanced) the northwesterly tropical cyclone motion induced by the beta gyres and thus reduced (increased) any motion asymmetry resulting in more symmetrically (asymmetrically) organized convection. Dengler and Keyser (2000) concluded that the higher drift speed in the easterly flow cases resulted in less boundary-layer moistening ahead of the vortex, and thus less convective instability.
1.2.4 Environmental vertical wind shear
The effects of vertical wind shear on the intensity, and to a lesser extent the structure, of a tropical cyclone is qualitatively well known. Strong vertical wind shear has an inhibiting, and even weakening, effect on tropical cyclone intensification. In strong shear, the low-level center of the tropical cyclone will often become exposed, with the convection and cloud shield shifted downshear of the exposed center. However, the relationship between weak to moderate shear and tropical cyclone structure and intensity change is less clear. More recent studies have begun to elucidate more details regarding the effects of different strength, and structure, of vertical wind shear and how this affects the intensity and structure of tropical cyclones of different intensities.
126.96.36.199 Intensification and weakening trends in tropical cyclones
The strength of the environmental vertical wind shear has been related to the amount of weakening or intensification that occurs in a tropical cyclone. Observational (Gallina and Velden 2002) and model studies (Frank and Ritchie 2001) have found an essentially linear relationship between lower (higher) vertical wind shear magnitudes and the mean deepening (filling) effect on tropical cyclones. A lag between the onset of shear and response in the pressure tendency of the affected tropical cyclone was also observed in both studies. In particular, using vertical wind shear calculated from satellite-derived cloud drift winds, Gallina and Velden (2002) found that under the same vertical shear strength environment, the lag time was longer for stronger tropical cyclones compared with weak tropical cyclones. Frank and Ritchie (2001) found that for a comparable strength tropical cyclone, the simulated lag time was longer for weaker shear (e.g., 5 m s-1 (200- 1000 hPa)), compared with stronger shear (e.g., 15 m s-1). An initial finding from Gallina and Velden (2002) is that for Atlantic tropical cyclones, the critical shear value where the tendency changes from intensifying to weakening tropical cyclones occurs at about 7-8 m s-1 (200 850 hPa) of vertical wind shear. For the western North Pacific basin this critical shear value is 9-10 m s-1 (200 850 hPa). No mention was made whether this critical value was different for weaker/stronger tropical cyclones.
A study by Black et al. (2002) of two eastern North Pacific tropical cyclones found that in 13-20 m s-1 of easterly shear Jimena (1991) either maintained intensity or weakened slightly. Olivia (1994) continued to intensify in 8 m s-1 of easterly shear, and when the shear reversed to > 15 m s-1 of westerly shear Olivia weakened, although it was also moving over lower SSTs and into drier air at the time. Frank and Ritchie (2002) found that there was sensitivity to the direction of the imposed shear in simulated tropical cyclones. They found that the beta-plane-induced shear vector was to the southeast, and so partially cancelled (enhanced) the effects of easterly (westerly) environmental vertical wind shear. Thus, a simulated tropical cyclone embedded in westerly (easterly) shear weakened more (less), which is consistent with the tendencies in Black et al. (2002).
In the development of both the Statistical Hurricane Intensity Prediction Scheme (SHIPS) (DeMaria and Kaplan 1999) and the Statistical Typhoon Intensity Prediction Scheme (STIPS) (J. Knaff personal communication), vertical wind shear has been shown to be a skillful predictor of tropical cyclone future intensity change. For the SHIPS (STIPS) schemes, vertical wind shear is calculated from NCEP (NOGAPS) fields using a 200-800 km radius annular average from the center of the tropical cyclone. Statistical analysis using data from 1989-2001 for SHIPS, and 1997-2001 for STIPS indicates that for the Atlantic, East Pacific, and western North Pacific basins, vertical wind shear magnitudes exceeding 8.5 m s-1 (200-850 hPa), 5.9 m s-1, and 7.9 m s-1 respectively, result in future weakening of the tropical cyclone. The greatest impact of the vertical wind shear occurs at 24 h, 24 h, and 36 h respectively in these basins. The results also indicate sensitivity to latitudinal location of the tropical cyclone with tropical cyclones at lower latitudes being more susceptible to vertical wind shear that tropical cyclones at higher latitudes.
188.8.131.52 Tropical cyclone convective asymmetries
Previous modeling studies that investigated dry, adiabatic processes in tropical cyclone-like vortices in vertical wind shear found that a consistent wave number 1 asymmetry in the vertical motion occurred when the vortex tilted away from the vertical (Jones 1995; DeMaria 1996; Frank and Ritchie 1999). Ascent occurred to the right of an observer facing in the direction of the vortex tilt and descent to the left. Jones (2000b) showed that the relationship between the direction of tilt and pattern of vertical motion was robust even when the direction of tilt changes with height. Frank and Ritchie (1999) related the vertical motion pattern to direction of shear and noted that the pattern was generated in the downshear right quadrant of the vortex. Again, the relationship between direction of shear, and pattern of vertical motion was robust for different directions of shear. The vertical motion dipole was attributed to flow along sloped isentropes within the tilted vortex (Jones 1995) and it was postulated that this vertical motion pattern would modulate convection in a real tropical cyclone (Jones 1995; Frank and Ritchie 1999). Recently, model simulations that include diabatic effects (Frank and Ritchie 1999, 2001; Kimball and Evans 2002) and several observational studies (Reasor et al. 2000; Corbosiero and Molinari 2002a, 2002b; Black et al. 2002; Zehr 2003) have established and verified the existence of persistent patterns of asymmetric convection and rainfall that develop in the downshear-left quadrant of the storm. The model study of Frank and Ritchie (2001) found that the asymmetries developed due to the storms response to imbalances caused by the shear, but differ from the prior adiabatic simulations because saturation in the eyewall leads to a different lifting mechanism. Interestingly, an observational study (Corbosiero and Molinari 2002a) that used lightning flash density in 35 tropical cyclones found that in the inner core region (< 100 km radius) the flashes occurred preferentially in the downshear left quadrant, which is consistent with the predictions of Frank and Ritchie (2001). In the outer rainbands (100300 km of the center) the preference for the lightning was for downshear right, similar to the adiabatic model studies of Jones (2000b) and Frank and Ritchie (1999). These lightning distributions were valid both over land and water, and for depression, storm and hurricane stages.
Corbosiero and Molinari (2002b) also noted a relationship between direction of storm motion and the azimuthal distribution of electrified convection with a preference for the right front quadrant. They found a systematic relationship between the flash distribution due to vertical wind shear and to storm motion, because the storm motion vector in their sample was predominantly left of (i.e., counter clockwise from) the shear vector. These results supported the importance of a downshear displacement of the upper anticyclone, which consistently produces motion left of shear (Wu and Emanuel 1993). The results (Corbosiero and Molinari 2002b) were further broken down by direction of shear, and it was shown that the beta effect also played a significant role in the relationship between motion and vertical wind shear. These results also suggested that a substantial downshear tilt of the cyclonic part of the tropical cyclone vortex was uncommon, because that alone would produce motion right of shear. Corbosiero and Molinari (2002b) determined the relative importance of asymmetric friction and vertical wind shear on the azimuthal asymmetry of convection and found that, without exception, the influence of vertical wind shear dominated the distribution.
A case study of Hurricane Olivia of 1994 (Reasor et al. 2000) using aircraft dual-Doppler radar data investigated the effects of increasing vertical wind shear. It was found that an increase in the westerly shear to > 15 m s-1 was accompanied by an increased eastward tilt with height of the storm center. In addition, they found an increased asymmetry in the convection with the largest upward motion observed downshear left, with weaker upward motion downshear right consistent with both the lightening studies of Corbosiero and Molinari (2002b) and the model findings of Jones (2000b) and Frank and Ritchie (2001).
184.108.40.206 Tropical cyclone potential vorticity asymmetries
In addition to convective asymmetries, Reasor et al. (2000) observed interesting wave number two asymmetries in the inner-core vorticity structure of Hurricane Olivia (1994) that were found to be partially consistent with vortex Rossby wave behaviour. Although the cause of these vorticity perturbations was not elucidated, some speculation that the vorticity perturbations enhanced convection in their vicinity was offered.
Frank and Ritchie (2001) also simulated wave number two potential vorticity asymmetries after shear had been imposed on the simulated tropical cyclone for a period of time. They concluded that the initially circular bulls-eye distribution of potential vorticity in the core of simulated diabatic tropical cyclone vortices broke down to higher wave numbers initially wave numbers one and two - and then dissipated for even weak values of vertical wind shear. They concluded that the time lag between onset of shear and subsequent filling reported in subsection 220.127.116.11 above was because of internal adjustments that occurred in the tropical cyclone core after onset of vertical shear. Whereas the initial response of the vortex to vertical shear was to develop a persistent wave number one asymmetry in the convection, the subsequent vortex response to the asymmetric convection was to breakdown into higher order wave numbers at the expense of the symmetric circulation. This effect was most dramatic at upper levels and resulted in a weakening of the upper-level inertial stability and advection downstream of the upper-level warm core (Frank and Ritchie 2001; Ritchie and Elsberry 2001). As the upper atmosphere began to cool, the surface pressure began to rise. As time progressed, the vortex breakdown progressed down in the atmosphere until balance was re-established between the environment and vortex (Frank and Ritchie 2001; Ritchie and Elsberry 2001).
In addition to the low- to mid-level structural asymmetries that develop in the inner core of a tropical cyclone under the influence of vertical wind shear, development of potential vorticity asymmetries have been simulated away from the inner core (Jones 2000a, b). Jones (2000a, b) found that the potential vorticity asymmetries influenced the motion and the vertical tilt of the vortex. The development of the potential vorticity asymmetries was attributed to the distortion of the initially symmetric vortex by the horizontally sheared flow associated with the vertical projection of the tilted potential vorticity anomaly (Jones 2000a, b).
Probably the principle reason why a tropical cyclone eventually will weaken under the influence of strong environmental vertical wind shear is because the tropical cyclone upper-level warm core cannot be maintained at a level that will continue to support the surface low pressure. Model studies (e.g., Frank and Ritchie 2001; Ritchie and Elsberry 2001) indicate that environmental vertical wind shear would impact the tropical cyclone at the upper-levels initially, which is where the inertial stability associated with the tropical cyclone primary circulation would be a minimum. Ritchie and Elsberry (2001) simulated an initial advection downstream of the upper-level warm core of the tropical cyclone, and thus a reduction in the magnitude of the warm core aloft. This resulted in a reduction in the height of the maximum warm core, an enhancement of the warm core at lower levels (due to subsidence into the core forced by convergence between the environmental winds and the cyclonic flow of the tropical cyclone), an associated rise in the sea-level pressure, and a reduction in the cyclonic flow aloft, which further reduced the inertial stability aloft. Consequently the vortex became more susceptible to the vertical wind shear and thus more of the warm core was advected downstream. Although this negative feedback could lead to continued erosion of the deep convection and upper-tropospheric warm core, and thus finally a dissipation of the tropical cyclone, Ritchie and Elsberry (2001) found that an eventual balance between the environment and (weaker, shallower) tropical cyclone was established.
The introduction of the Advanced Microwave Sounding Unit (AMSU) has allowed the routine examination of tropical cyclone thermal structure. While the AMSU soundings lack the horizontal resolution to resolve the warm core of the tropical cyclone eye, the broader-scale warm core envelope can be measured. The strength of this broad-scale warm signature has been related to intensity (Brueske and Velden 2003) and the horizontal extent of the warm core along with an estimate of maximum intensity can be related to surface wind structure (DeMuth et al. 2003). As described earlier, vertical wind shear has been related to a lowering of the maximum warm core anomaly to lower levels of the atmosphere resulting in a corresponding surface pressure rise (Ritchie and Elsberry 2001). The variations in the level of the warm core in the atmosphere, and the strength of the maximum warm anomaly can be observed in the AMSU soundings and related to effects of vertical wind shear on tropical cyclone intensity (DeMaria, personal communication).
1.2.5. Upper-level trough interactions
The precise manner and degree to which upper-level troughs weaken or intensify a tropical cyclone's circulation is not yet well understood. Although an upper-level trough in close proximity increases the vertical wind shear over the tropical cyclone, studies have demonstrated that complicated dynamic processes occur during the interaction between an upper-level trough and a tropical cyclone that then affect the core dynamics of the tropical cyclone in ways that are only just beginning to be investigated. A trough interaction has been defined by Hanley et al. (2001) to occur when the eddy momentum flux convergence calculated over a 300-600 km radial range is greater than 10 m s-1 d-1. In the following discussion, not all studies adhere to this definition. Bosart et al. (2000) suggest that an important factor in determining whether a storm-trough interaction is favorable or unfavorable for intensification is how far a storm is from its maximum potential intensity (MPI).
18.104.22.168 Favorable trough interactions
A favorable factor for intensification of a tropical cyclone has been characterized as a good trough interaction and two types of good troughs have been identified and described using composite analysis (Hanley 1999; Hanley et al. 2001). In this scenario, an upper troposphere trough becomes juxtaposed with the warm outflow from the tropical cyclone to cause: (i) a positive eddy momentum flux convergence that contributes to a cyclonic spinup of the inner vortex; and/or (ii) an enhancement of the jet streak that contributes to a larger outflow from the tropical cyclone, and consequently a spinup of the vortex (Hanley et al. 2001; Hanley 1999).
In the case of (i), the adjacent cold trough and warm outflow are arranged such that the outflow (inflow) within 600 km of the tropical cyclone transports anticyclonic (cyclonic) wind components, so that the eddy momentum flux convergence is positive. A key to this process is the relatively low inertial stability conditions aloft in the mature tropical cyclone such that the response to this momentum flux forcing can extend into the tropical cyclone core. In the composites of Hanley et al. (2001), as upper-level trough approaches the tropical cyclone center, the associated potential vorticity anomaly is weakened, most likely through diabatic processes, reducing the vertical wind shear associated with the upper-level trough. Thus, there is little or no unfavorable vertical shear due to the upper-level trough to inhibit the associated tropical cyclone deepening as the (weakened) upper-level trough passes over the tropical cyclone.
In the jet streak scenario of (ii) above, the upper-level potential vorticity maximum associated with the cold trough remains well to the west of the tropical cyclone center, and intensification is not due to superposition. Instead, the temperature gradient between the warm tropical cyclone outflow and the cold trough is enhanced, which accelerates the intervening jet streak. If the center of the intensifying tropical cyclone is located in the right-entrance region of the jet, upward motion is favored. A net effect is a more favorable outflow channel from the tropical cyclone. With favorable small inertial stability conditions aloft, this additional outflow will contribute to low-level convergence and spinup of the tropical cyclone vortex (Hanley et al. 2001).
In their idealized simulations, Kimball and Evans (2002) note that a merger between the upper-level trough and tropical cyclone leads to reduced vertical wind shear from the trough over the tropical cyclone. Rapid intensification of the tropical cyclone followed in conjunction with contraction of the radius of maximum winds.
22.214.171.124 Unfavorable trough interactions
In the contrasting bad trough scenario, the strong winds on the leading side of an approaching upper-level trough produce a strong vertical wind shear that is concentrated in the upper portions of the troposphere over the tropical cyclone. One composite pattern was identified in association with the bad trough scenario (Hanley et al. 2001) as a distant trough interaction. In this distant trough case, strong vertical wind shear was identified as the major factor that weakened the tropical cyclone. The interaction between the shear associated with the upper-level trough and the tropical cyclone causes: (i) ventilation of the warm core of the tropical cyclone; or (ii) an asymmetry in the cloud/precipitation distribution that is less favorable for intensification of the tropical cyclone than is a symmetric cloud/precipitation distribution. Recent observational (e.g., Reasor et al. 2000; Corbosiero and Molinari 2002a; Black et al. 2002) and modeling studies (e.g., Frank and Ritchie 2001; Ritchie and Elsberry 2001) have demonstrated that the favored ascent region is shifted to the downshear left quadrant of the tropical cyclone. Clearing of the deep convection and formation of a dry slot can occur on the upshear side in response to forced subsidence (Ritchie and Elsberry 2001). Whereas a symmetric vortex would in principle spin up to its MPI given sufficient time in quiescent environmental conditions, the asymmetric cloud pattern leads to less effective eye warming, and a rising of the surface central pressure (e.g., Ritchie and Elsberry 2001; Kimball and Evans 2002) possibly due to asymmetric subsidence in the eye (Kimball and Evans 2002).
In the strong-shear ventilation scenario of (i), the numerical models demonstrate that the advection downstream of the warm core would occur first at the top of the tropical cyclone (e.g., Frank and Ritchie 2001; Ritchie and Elsberry 2001), which is where the inertial stability would be a minimum. A corresponding rise in the sea-level pressure, reduction in cyclonic winds, weakening of upper-level inertial stability, and continued advection of the warm core downstream would occur as described in subsection 126.96.36.199. This negative feedback could finally lead to a dissipation of the tropical cyclone (Ritchie and Elsberry 2001).
In addition, Kimball and Evans (2002) note that in their model simulations, the deformed trough inhibits outflow on the east side of the tropical cyclone, which hampers future intensification.
188.8.131.52 Observational case studies
Although composite and model studies of an upper-level trough, and its associated vertical shear interaction have provided insight into the mechanisms of trough interaction, it has proved particularly difficult to apply these insights to individual cases. In recent years, Hurricane Opal of 1995 has become one of the most intensely studied hurricanes ever. However, the cause of the hurricane's rapid intensification over the Gulf of Mexico is still a matter of controversy. Several studies (e.g., Bosart et al. 2000; Persing et al. 2001; Möller and Shapiro 2002; Shapiro and Möller 2002) used a range of techniques to elucidate the role of the upper-level trough in the intensification of Hurricane Opal. Some of the insights from these studies are provided here to help illustrate just how difficult these cases are to understand, let alone forecast.
Bosart et al. (2000) found that an increase of upper-level divergence preceded convective growth in the eyewall of Opal and the onset of rapid intensification. This upper-level divergence was attributed to a jet-trough-hurricane interaction in a low-shear environment. Eddy fluxes of heat and momentum associated with the trough were derived from European Centre for Medium-Range Weather Forecasts (ECMWF) analyses to deduce the symmetric balanced vortex outflow. It was found that the maximum deduced near-vortex 200-mb outflow was largest at the time of Opal's rapid deepening, which implied a large contribution from environmental forcing. Since the induced tangential wind tendency was not evaluated, Bosart et al. (2000) could not establish the actual contribution of the environmental forcing to the intensification of Opal.
Geophysical Fluid Dynamics Laboratory (GFDL) hurricane forecast model results were used in two other studies of Hurricane Opal to evaluate: (i) the contributions of mean and eddy vorticity fluxes, mean and eddy vertical advection, and friction to the tangential momentum budget of Hurricane Opal during simulated rapid intensification (Persing et al. 2001); and (ii) the contributions of eddy fluxes of heat and momentum, and asymmetric as well as symmetric heating and friction to the symmetric secondary circulation and thereby Opal's evolution (Möller and Shapiro 2002). Persing et al. (2001) found little difference in the contributions of the eddy vorticity flux to the tangential wind tendency during the intensification times compared with a period of non-intensification, and so concluded that the hurricane intensification was not due to a trough interaction. Möller and Shapiro (2002) found that asymmetric eddy forcing made a small contribution to Opal's lower-tropospheric near-core spinup, and made a conclusion similar to Persing et al. (2001). However, using the method of piecewise potential vorticity inversion to diagnose the asymmetric features that contribute to tropical cyclone intensification, Shapiro and Möller (2002) found that the eddy fluxes associated with the upper-level trough were not confined locally to the upper-level trough, but instead were found to extend into Opal's inner-core region. Thus, whereas the upper-level trough may make more of a contribution to Opal's evolution than suggested by Persing et al. (2001), its influence on intensification may not be as positive as suggested by Bosart et al. (2000).
These studies point to the sensitivity of results based on two data sets: the ECMWF analyses; and GFDL model forecasts. Studying the same hurricane (Opal) with three different analysis techniques, three different conclusions were reached. It is worth noting here that other studies of Hurricane Opal examine the oceanic forcing during the rapid intensification episode of Hurricane Opal. Although this topic is covered in detail elsewhere (Topic 1.4), the conclusions of Hong et al. (2000) and Shay et al. 2000) are that the oceanic warm core ring that Opal passed over during the period of rapid intensification had significant impact on the hurricanes heat budget and thus also impacted its intensification.
Additional case studies of Supertyphoon Flo (Titley and Elsberry 2000; Wu and Cheng 1999) and Typhoon Gene (Wu and Cheng 1999) during1990 found that whereas STY Flo intensified due to internal processes, TY Gene intensified due to an interaction with a nearby trough. In addition, Hanley (2002) has used water vapor imagery with some success to identify a tropical cyclone-trough interaction, which gives some hope that continued investment in remote sensing technology may help with this forecast challenge.
Environmental moisture has been shown to be positively correlated to future tropical cyclone intensity trends (Emanuel 1988; Holland 1997; DeMaria and Kaplan 1999; Knaff et al. 2003b) although their affects are secondary to the effects of SST, ocean heat content, and vertical wind shear. The advection of Saharan dust over the tropical Atlantic is symptomatic of an increased low-level (~700 hPa) easterly jet that propagates westward from the northwest African continent. This low-level dry-air surge can cause a marked increase in vertical wind shear and dry air entrainment that can act to influence tropical cyclones that encounter it (Dunion and Velden 2002a). It has been observed that as the air masses overtake tropical cyclones the convective intensity and organization is reduced resulting in a general weakening and that as the tropical cyclones emerge from the air mass intensification can subsequently occur (Dunion and Velden 2002b).
Clearly significant challenges exist in forecasting tropical cyclone structure and intensity change during interaction with dynamic environments. A tropical cyclone moving into an exceptionally low shear environment may cause increased intensity forecast errors. A weakening of the tropical cyclone in response to an unexpected encounter with an enhanced vertical wind shear environment can cause over-forecasts of intensity increase. In addition, an encounter between a tropical cyclone and a midlatitude trough presents many different challenges: is it a good trough or a bad trough, and does the good part of a good trough interaction depend on the difference between the tropical cyclones current intensity and its MPI? It is also difficult, at times, to diagnose the current intensity and wind structure associated with tropical cyclones. In addition, current intensity forecast models are seldom able to outperform forecasts derived from climatology and persistence (Gross 1999; JTWC 2002), and only recently has a systematic way to forecast and verify wind radii information been developed (McAdie 2002).
Studies involving model simulations show promise in fundamental understanding of the physical processes occurring during intensity and structural changes of tropical cyclones that occur due to environmental forcing. However, much work clearly still is needed in order to translate this understanding to useful guidance for forecasters.
Studies using satellite remote sensing products to enhance general understanding of the effects of the environment on future tropical cyclone intensity and structure change also show much promise and may prove to be a fruitful way to bridge the gap between theoretical knowledge and practical guidance for forecasters. Currently there are several efforts underway to transition fruitful research results and proven techniques to operational forecast centers. Examples of these efforts include but are not limited to: the operational use of AMSU-derived intensities and wind radii estimates; the implementation of STIPS to the western North Pacific; the addition of ocean heat content and Geostationary IR satellite information to the SHIPS model; the use of a coupled ocean in the GFDL hurricane forecast model (Bender 2002); the H-wind algorithm (Powell, 2002); and a rapid intensification probability index for the Atlantic Basin (Kaplan and DeMaria 2002).
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