Eyewall Vertical Motion Structure Experiment

This document is divided into 4 sections:

Summary

This single-option, dual-aircraft experiment is designed to map the three-dimensional spatial structure of the hurricane eyewall up- and downdrafts and to use dual-Doppler analysis to relate the vertical motion structure to the effects of environmental shear through the eyewall. It utilizes both NOAA WP-3D aircraft flying highly coordinated flight patterns to map the three-dimensional structure of eyewall vertical motions. The target storm must have an eyewall (or a developing one) with significant areas of deep convection.

[Return to table of contents above]

Program Significance

The eyewall of the TC is the primary region for organized vertical motions. Updrafts typically cover a large portion of the eyewall's area and may extend several kilometers in the vertical. The deep, organized convection in the hurricane eyewall is necess ary to maintain or to increase the storm's intensity. Knowledge of the three-dimensional structure of vertical motions in the hurricane eyewall are crucial for understanding the internal processes that govern intensification. The remote-sensing capability of the Doppler radar on the WP-3D aircraft, combined with the accuracy of Global Positioning Satellite (GPS) navigation, allows for the study of eyewall vertical winds in greater detail than was formerly available.

Previously, the study of vertical motions in hurricanes was limited to data collected by research aircraft at flight levels in the lower troposphere. More recently, utilization of airborne Doppler data from vertically pointing radar rays (vertical incide nce) allowed researchers to estimate vertical motions throughout the depth of the troposphere. The Doppler data were available in vertical planes along the aircraft track, providing a two-dimensional (radius-height) analysis of hurricane vertical wind structure. These analysis confirmed the results of the flight-level study in that the eyewall contained the strongest and largest updrafts and which were capable of transporting air with large amounts of moist static energy from the boundary layer to the upper troposphere. These vertical transports of mass are necessary for the maintenance and intensification of the hurricane. Updrafts in the eyewall, some of which appeared to extend throughout the depth of the eyewall, exhibited a pronounced radially-out ward slope with height.

While the persistent and organized two-dimensional spatial structure of eyewall updrafts was revealed in the Doppler studies, questions remain concerning the asymmetric distribution and structure of eyewall vertical motions. Eyewall updrafts not only slope radially outward with height, but because of the strong horizontal winds and large vertical shear of the horizontal wind, updrafts undoubtedly have a large slope in the azimuthal plane as well. Pseudo-dual Doppler analysis suggested this type of structure but because of limitations in both time and spatial resolutions, the actual structure remains uncertain. Additionally, large variations in the magnitude and size of eyewall vertical motions have been observed among different hurricanes and appea r to be related to intensity and intensity changes. Furthermore, large asymmetries in eyewall vertical motions are related to the precipitation structure and may be a result of the environmental shear through the eyewall.

With the advent of GPS navigation, both dual-doppler analysis and vertical incidence data from coordinated, parallel flight tracks of both WP-3D aircraft can be used to study, in detail, the three-dimensional structure of eyewall vertical motions. The GPS navigation provides accurate positioning of the aircraft, relative to the storm center, resulting in smaller errors in the total wind field, including vertical velocity estimates. Data collected simultaneously from both aircraft in two adjacent ra dius-h eight profiles through the eyewall can be used to infer the azimuthal continuity of the largest up- and downdrafts. The dual-doppler analysis may confirm the highly organized nature of these drafts. The data collected from this experiment will be used to expand knowledge of the relation between vertical motion structure and intensity change and to provide a basis for use in numerical modeling efforts of hurricane eyewall processes that lead to intensification or weakening.

[Return to table of contents above]

Objectives

[Return to table of contents above]

Mission Description

The Eyewall Vertical Motion Structure Experiment (EVMSE) will use both NOAA WP-3D aircraft flying highly coordinated flight patterns to map the three-dimensional structure of eyewall vertical motions. The primary requirement is for the target storm to hav e an eyewall (or a developing one) with significant areas of deep convection. Both aircraft must have fully operational tail radar systems and at least one aircraft must have a working lower fuselage radar. Recording of cloud physics data is desired but not necessary. The aircraft will fly at two altitudes, one at either 6,000 ft (1.8 km) or 12,000 ft (3.6 km) and the other at 8,000 ft (2.4 km) or 14,000 ft (4.2 km). The lower of the two aircraft should have up to 6 GPS sondes available for drops in the eye and outside of the eyewall. The first and last portions of the mission include coordinated "figure 4" patterns (Fig. 27a) with leg lengths nominally set at 75 nmi (140 km).

The length may vary depending on the size of the eye. After completing the initial "figure 4", the aircraft will rendezvous in a relatively clear area outside of the eyewall to coordinate an inbound leg into the eye (Fig. 27b). The aircraft should fly at the same ground speed so as to be parallel to each other along the radial leg. The horizontal spacing between aircraft can vary from 1,500 ft (0.5 km) to 6,500 ft (2.0 km) and the vertical separation can be 2,000 ft (600 m) or greater, depending on safety considerations. The dual vertical-incidence module (Fig. 27a) consists of coordinated radial legs into and out of the eye with downwind legs flown outside of the eyewall between the outbound and inbound legs. The radial legs will typically be 40-60 nmi (70-110 km) long, depending on the eye size. Coordination between aircraft should be done in clear air in the eye and outside of the eyewall at the end of the downwind legs. If the eye diameter is too small to maneuver the aircraft, straight legs through the eye and eyewall may be used. The series of radial legs should be repeated so as to maximize the areal coverage of the eyewall, but to allow time for a coordinated "figure 4" pattern at the end of the flight.

[Return to table of contents above]

Back to the Hurricane Field Program Experiment page.