Tropical Cyclogenesis Experiment

This document is divided into 3 sections:

Program Significance

The 1992 eastern North Pacific Tropical EXperiment in MEXico (TEXMEX) and the western North Pacific Tropical-Cyclone Motion experiments in 92 and 93 (TCM-92 and TCM-93) showed that the genesis of tropical cyclones (TC) is a result of a complex interactio n of phenomena on diverse scales and that mesoscale convective systems (MCS's) associated with mid-level vortices frequently accompany genesis. Large scale forcing superimposed on convective scale events is an important, if not crucial, ingredient in gene sis. 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) are important. Genesis occurred primarily in close proximi ty to tropical waves containing MCS's. A mid- and upper-level vortex spins up in the stratiform region of the MCS, near the melting level in response to diabatically forced descent below and ascent above the melting level. 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 frequently accompanied the genesis process. Frequently, the surface center was initially located some distance to the west of the cl oud system.

This 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, growth and u ltimately 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 scales. Thes e 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 response to south westerly surges and intense convection.

Objectives

The objectives of the experiment are:
  1. Determination of the linkage between the synoptic-scale systems and the mesoscale vortex spinup.
  2. Determination of the evolution of the mid-level and low-level vorticity centers. Do multiple vorticity centers merge during the spinup process or is vertical propagation of vorticity the more dominant mechanism?
  3. Determination of how the mid-level vortex builds downward to the surface to extract latent heat from the sea.
  4. Determination of the role surges in the equatorial southwesterlies play in the initiation and maintenance of convection.
  5. Determination of the role of convection in vorticity production.
  6. Determination of the significance in the spinup process of the transition from dynamics driven by convectively induced cold downdrafts to non-gust front dynamics.
  7. Determination of the relative role in the spinup process of external forcing versus enthalpy flux from the sea and elevation of mid-level qe.
  8. Development of criteria for the genesis of a TC from a tropical wave.

Mission Description

This plan calls for as many as four different kinds of aircraft sorties into incipient disturbances or a tropical wave in either the western Atlantic, Caribbean, or Eastern North Pacific. The basic mission involves the two NOAA P-3 aircraft to 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 would entail an Air Force WC-130 investigative mission in the same disturbance. When the new NOAA mid-level jet (200-300 mb) becomes available, it would enable the study of the upper tropospheric circulation as well.

The synoptic-scale aircraft should be a NOAA P-3 or the jet. It will fly a figure-4 survey pattern at 500 mb (200 mb for the jet) centered on the MCS (Fig. 1), but extending as far along the cardinal directions as available aircraft range allows (nomina lly 800 km).

Synoptic pattern

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 3 or 4 omega-dr opwindsondes (ODWs) and airborne expendable bathythermographs (AXBTs) on each leg, including the diagonal, to map the MCS atmospheric and oceanic environment (particularly accurate sea-surface temperature determination). The main purpose of this pattern is to map the synoptic-scale circulation encompassing the mid-level vortex embedded in the MCS. The east and northeast legs should attempt to penetrate past the 700-mb jet into the Saharan Air Layer so that ODWs can resolve some of the vertical structure of these features at lower levels. The south and southwest legs should penetrate past the low-level southeasterly jet, if it exists, in order to resolve its vertical structure with ODWs. This aircraft 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/aft scanning technique (F/AST).

The mesoscale aircraft should be a Doppler-equipped NOAA P-3. It will fly rotating figure-4 pattern at 600 or 700 mb under the anvil of the MCS (Fig. 2).

Mesoscale pattern

The leg lengths will be 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 ki nematic structure of the MCS. It may dispense ODWs 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 or 500 m, depending on the situation (Fig. 3).

Racetrack pattern

If a USAF WC-130 is available, they could be requested to fly a low-level investigative mission with their standard "racetrack" or "alpha" pattern. When the low-level aircraft is a NOAA P-3, it would fly a "racetrack" pattern oriented normal to the tropic al wave trough axis. This P-3 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 500 m is essential for comparis on with winds at upper levels in order to determine the vertical structure of the circulation features.

It would be highly desirable to construct on-board radar composites using the workstation for the purpose of properly positioning both aircraft with respect to the MCS. ODWs should be transmitted to NHC and NMC for inclusion in synoptic analyses.

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