Tropical Cyclogenesis Experiment

This document is divided into 4 sections:


This multi-option, multi-aircraft experiment is designed to study one of the most important unanswered questions in tropical meteorology is: How does a tropical disturbance become a tropical depression with a closed surface circulation? This experiment se eks to answer the question through multilevel aircraft penetrations using dropsondes, flight-level data, and radar observations on the synoptic, meso, and convective spatial scales. It will focus particularly on both thermodynamic transformations in the m id-troposphere and lateral interactions between the disturbance and its synoptic-scale environment.

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Program Significance

The recent Tropical EXperiment in MEXico (TEXMEX) (1992) showed that the genesis of TCs in the east Pacific hurricane basin (EPAC) is a result of a complex interaction of phenomena on diverse scales. Analytical and numerical studies suggested that the principal mechanism for genesis is an enhanced latent heat flux from the sea surface that acts to elevate the mid-level equivalent potential temperature (qe) to the point where the downward flux of low qe air by evaporatively driven downdrafts is weakened and negative feedback due to cold downdrafts is reduced. A complete understanding of this process also requires a knowledge of environmental forcing of the initiating disturbance. Satellite and aircraft flight data suggest that the large scale forcing superimposed on convective scale events is an important, if not crucial, ingredient in genesis.

During TEXMEX, it appeared that the phase of the 40-50 day global oscillation, the year relative to El Nino, and the positions of both the long-wave, mid-latitude trough and associated mobile upper-level potential vorticity anomalies were important. No genesis occurred without a nearby tropical wave containing a mesoscale convective systems (MCS). The MCS convection was modulated by the diurnal cycle and by fast-moving squall lines. An easterly jet at 700 mb and a southwesterly jet near the surface fr equently accompanied the genesis process (similar to the synoptic-scale structure documented in the west Pacific hurricane basin (WPAC)). Intense convection occasionally developed at the southern end of rapidly moving squall lines generated near the exit region of the easterly jet. A small, intense vortex often spun up in mid-levels adjacent to the convection and built downward with time. Frequently, the surface center was initially located some distance to the west of the cloud system. Tropical-Cyclo ne Motion Experiments in 1992 and 1993 (TCM-92 and TCM-93) showed that MCSs associated with mid-level vortices frequently accompany genesis in WPAC, a relationship also shown by satellite climatologies. A mid- and upper-level vortex spins up in the strat iform region of the MCS, near the melting level in response to diabatically forced descent below and ascent above the melting level. This is consistent with the observations from TEXMEX, except in TEXMEX the area of interest was keyed to the 700-mb tropi cal wave trough axis. A number of researchers have speculated on the role of multiple interacting mid-level vortices in an incipient disturbance or wave.

The proposed experiment is designed to study incipient tropical systems which may ultimately develop into TCs. The importance of this study is not limited to TC investigations. The proposed experiment should yield useful insight into the structure, gro wth and ultimately the predictability of the systems responsible for the most tropical precipitation. The experiment focuses on features in the tropical atmosphere at several different levels in the vertical and on a wide range of spatial and temporal sc ales. These include: 1) the development of a mid-level vortex associated with MCSs at 500-700 mb, 2) role of the mid-level easterly jet in enhancing cyclonic vorticity and producing squall lines and surges at 700 mb, 3) low-level vortex spin-up in respon se to southwesterly surges and intense convection at cloud base. Portions of this experiment, mapping of low-level mesoscale structure and 500-mb synoptic structure, were flown successfully in dissipating Tropical-Storm Debby during the 1994 field progra m.

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Mission Description

This plan calls for as many as four different mission profiles for flights into incipient disturbances or tropical waves in the western Atlantic, Caribbean, or Eastern North Pacific. The two WP-3Ds anchor the experiment flying a high-level, synoptic-scale pattern and a mid- or low-level mesoscale pattern. Collection of separate low-, mid- and high-level observations simultaneously requires an additional investigative mission by an USAF WC-130. The G-IV, if available, could provide upper tropospheric (200- 300 mb) observations to study the upper tropospheric circulations.

The synoptic-scale aircraft will be a WP-3D or the G-IV. It will fly a figure-4 survey pattern at 500 mb (18,000 ft [5.5 km]) for the WP-3D or 200 mb (37,000 ft [11 km}) for the G-IV centered on the MCS (Fig. 9), but extending as far along the cardinal d irections as available aircraft range allows (nominally 430 nmi [800 km])

The approach leg should be east-west to facilitate location of the trough axis. Ideally, the diagonal leg should fall in the southeast quadrant, and the south-north leg should lie at an angle to the trough axis. This aircraft will dispense 5-10 GPS-sondes and airborne expendable bathythermographs (AXBTs, WP-3D only) on each leg, including the diagonal, to map atmospheric and oceanic environment of the MCS and mid-level vortex with particular emphasis on accurate sea-surface temperature determination. The east and northeast legs should attempt to fly past the 700-mb jet into the Saharan Air Layer to obtain GPS-sonde soundings to the surface through these features. The south and southwest legs should penetrate past the low-level southeasterly jet, if it exi sts, in order to resolve its vertical structure with GPS sondes. If the aircraft is a WP-3D, it should spend less than half its time under the anvil, but during that time it should collect microphysics observations and Doppler radar data using the fore/af t scanning technique (F/AST).

The mesoscale aircraft should also be a Doppler-equipped WP-3D. It will fly rotating figure-4 pattern at 600 or 700 mb (14,000 or 10,000 ft [4.2 or 3.0 km]) under the anvil of the MCS (Fig. 10)

The leg lengths will be 100-135 nmi (180-250 km), and the pattern will be approximately centered on the moving trough axis. The primary purpose of this aircraft is to collect Doppler radar data using F/AST throughout the mission in order to map the three- dimensional kinematic structure of the MCS. It may dispense GPS sondes and collect microphysics data on a target of opportunity basis.

A variation on the basic two-plane mission would add a third low-level aircraft. This aircraft flies a "racetrack" pattern at 850 mb (5,000 ft [1.5 km]) or 1,500 ft (500 m), depending on the situation (Fig. 11).

If a USAF WC-130 is available, the USAF could be requested to fly a low-level investigative mission with the standard "racetrack" or "alpha" pattern. When the low-level aircraft is a WP-3D, it would fly a "racetrack" pattern oriented normal to the tropica l wave trough axis. This WP-3D should have the C-band scatterometer (C-SCAT) and the stepped-frequency microwave radiometer (SFMR) for determination of the surface wind field. A low-level wind field, at either the surface or 1,500 ft (500 m) is essential for comparison with winds at upper levels in order to determine the vertical structure of the circulation features.

It would be useful to construct on-board radar composites using the workstation for more accurate positioning with respect to the MCS. GPS-sondes should be transmitted to TPC/NHC and NCEP for inclusion in synoptic analyses.

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