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Tropical cyclogenesis is a continuous process that may span several days, rather than a sudden event. At the beginning of this process a tropical cyclone does not exist and at the end one clearly does. During this time, the incipient tropical cyclone acquires a low-level mesoscale circulation and associated organised convection and the two (together with the surface heat fluxes) begin to co-operate to amplify the system further. The forecaster's task during this period is to monitor the area of responsibility for regions of collocated persistent convection and low-level cyclonic circulation(s). Such suspect areas are intensively watched for evidence of organisation in the convection. Formation forecasts are made by evaluating large-scale factors, which are known to inhibit or enhance development.
Each ocean basin has operational criteria for tropical cyclone formation. These must be monitored and appropriate administrative and procedural steps taken when they are reached. However, the forecaster should always keep in mind the idea of a process over several days, rather than an instantaneous event when making meteorological assessments.
The Dvorak (1984) analysis is the generally accepted means of monitoring tropical cyclone formation and intensity changes using satellite imagery (Section 2.3.2). This section contains additional material to identify the various stages of tropical cyclone formation and assess the likelihood of further development using both satellite imagery and conventional observations and analyses. Much of this additional material is adapted from recently completed research on western North Pacific tropical cyclogenesis by Zehr (1992). The procedures should be applicable to other basins as well.
Zehr (1992) identifies two pre-genesis stages in his conceptual model. Two distinct increases (superimposed on the diurnal cycle) of deep convection occur in a developing disturbance. The first convective increase within a persistent tropical disturbance is the beginning of Stage One. As the convection subsides, a small vortex (less than a few hundred kilometres across) remains. This vortex will probably go undetected in conventional observations unless they are dense or well-placed. A subsequent increase in convection in the vicinity of the small vortex marks the beginning of Stage Two, during which the systems acquires all of the characteristics of a minimal tropical cyclone, including maximum winds of 17 ms-1 (34 kt, 63 km h-1) or more concentrated near the vortex center, a warm core, and associated deep convection.
A suggested procedure is provided for formation monitoring and forecasting. The procedure involves assigning each tropical disturbance to one of three stages using satellite and in-situ data. The stages are patterned after Zehr (1992), and are not intended to be rigid classes based on wind speed or pressure criteria. Rather, the stages are intended to focus the forecaster's attention on those trends or conditions associated with development.
Stage 0 ("Pre-genesis") is initially assigned to any cloud mass containing convection that:
a) persists for at least 24 h or
b) fails to diminish from morning to evening according to the typical diurnal pattern.
Once Stage 0 has been assigned, it should be maintained until convection has been absent for an entire 24-h period. The system should be monitored for an increase in convection beyond the diurnal cycle and the associated formation of a Low-Level Circulation Centre (LLCC), which has a scale of a few hundred kilometres.
Stage I ("Suspect Area") is assigned to systems with current or recent (12-24 h) evidence of an LLCC, but with diminishing or steady convection not closely associated with the LLCC.
The distinguishing mark of an LLCC is low-level concentration; especially in monsoon trough regions, cyclonic curvature and shear may be present but only on scales of 500-1000 km or more. The LLCC may go undetected in conventional data and infrared imagery but is often first evident in visible satellite imagery, especially loops at full resolution. If convection persists, the existence of an LLCC may also become apparent through the development of curved bands. Care is required, however, as it is possible for thick cirrus to appear highly organised without any surface circulation at all.
Convection may be less extensive than during Stage 0. Stage I systems should be monitored for an increase in convection beyond the diurnal cycle, particularly within a few hundred kilometres of the LLCC.
Stage II ("Incipient Tropical Cyclone") is assigned to systems with current or recent evidence of an LLCC that has increasing convection relative to the diurnal cycle in its vicinity. Commencement of this stage often is associated with organisation of convection into curved bands. Stage II systems should be monitored using the intensity guidelines in Section 2.3.
Figure 2.2 contains a work sheet for monitoring cloud masses and direct observations for indications of tropical cyclogenesis. The procedure is designed to reconcile the Dvorak method and whatever synoptic observations are available. It also accounts for the convective burst that often occurs during the formation process Zehr (1990).
As indicated by the previous discussion, the critical events of development are establishment of a small-scale low-level vortex and collocated persistent convection. Satellite imagery (especially visible loops) is the most useful data source. Where available and trustworthy, in situ reports can assist in detecting a low-level "centre of action" within a convective mass. Pressures are especially useful in trade-wind areas. Unusually low pressures or 24 h pressure falls (2 hPa or more) at reliable stations should be noted, especially if concentrated in a relatively small area.
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Surface winds are more difficult to interpret because of local circulations (land-sea breezes and the like) or nearby rain showers; time-sections should be kept for stations in suspicious areas to help detect persistent trends. Turning of the wind, between stations or with time at a single station, is especially noteworthy. Strong winds are not necessarily significant; they may be part of a large scale feature such as a trade or monsoon surge, the disturbance may be moving rapidly, or there may be a local squall. Subtracting the velocity of the disturbance to obtain disturbance-relative winds can be helpful; for example, the 15 ms-1 (30 kt) trades poleward of a disturbance moving west at 10 ms-1 (20 kt) are only 5 ms-1 (10 kt) in a relative sense. Local maxima in wind speed that are embedded in the curved flow defining the circulation and are close to its centre are highly significant.
Rawinsonde and jet aircraft reports, when available, can provide valuable indirect evidence of the development of the deep, warm-core structure associated with a tropical cyclone. Upper-tropospheric warm anomalies and low- to mid-tropospheric cyclonic curvature are supporting evidence of tropical cyclogenesis.
The internal dynamics of incipient tropical cyclones are difficult to observe in most situations and the relationship between internal processes and large-scale conditions are poorly understood, so forecasting is usually reduced to "watchful waiting." The environmental influences that are easiest to measure are those that inhibit formation, and clear cases of non-development are relatively frequent. Cases where formation can definitely be forecast to occur are much less common; often the large-scale conditions do not appear adverse, yet an apparently vigorous disturbance fails to progress. A work sheet for formation forecasting based on satellite and conventional observations of the environment is presented in Fig. 2.3.
Satellite signatures: For evaluating the satellite signature and shear, the disturbance centre should be placed in order of priority: at the LLCC; at the CSC (Cloud System Centre, Section 2.3.2, Step 1); or at the overall centre of the most persistent convection during the last 24 h if no circulation is evident. As noted in Section 2.2.1, the degree of organisation (presence of an LLCC or curved convective bands) is an indication of the stage of formation. Future trends are best inferred from the tendency (relative to the diurnal trend) of the area covered by cloud tops colder than about -65° C. Emphasis should be given to deep convection in the vicinity of an existing circulation.
Satellite imagery can also indicate adverse environmental conditions. The cloud-band rule (last on the list of inhibiting factors) has been extensively used in the Northern Hemisphere as part of Dvorak's (1984) procedures, but its utility has been questioned in the Australian region. It should therefore be applied tentatively in the Southern Hemisphere until more convincing evidence of its usefulness is obtained.
Figure 2.3: Forecasting formation; the work sheet should be evaluated at 6-12 h intervals for each system being monitored. |
Weldon and Holmes (1991) have recently published guidelines for using water vapor (6.7 micron) imagery in identifying favourable environments for tropical cyclone formation. These rules have not been incorporated in this Guide but appear promising for basins viewed by geostationary satellites fitted with a water vapor channel.
Environmental Factors: Vertical shear should be computed from the vector difference of horizontally averaged winds in a ring 500-1000 km around the disturbance. An alternate method for computing the horizontal averages is described in an appendix at the end of this chapter.
Climatology: Climatology can help estimate the probability that a given disturbance will progress to tropical cyclone stage, especially when large-scale synoptic information is scarce. Disturbances in favourable regions and seasons should be regarded as a greater threat unless the synoptic conditions are clearly adverse; the converse is true of disturbances in areas and at times where tropical cyclones seldom form. Care needs to be taken with climatology, since rare and potentially dangerous conditions are neglected. This can be alleviated to some extent by keeping additional climatological information of the important rare events, such as rapid development.
Maps of tropical cyclone formation by month or other periods can provide a useful forecast aid. The archiving procedures suggested below will make these maps more useful with each passing year. Track records from the pre-satellite era should be used with caution; the beginning of a best track may only indicate where the tropical cyclone was known to exist based on conventional data. Formation may have actually occurred well prior to that.
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