ABSTRACT

Under task from Meteorological Services Canada, the NRC Convair 580 turboprop aeroplane was flown through extratropical Hurricane Juan immediately prior to landfall at Halifax, Nova Scotia, on the 28^th September 2003. In-situ wind data was gathered during two passes: a north-south core-crossing and a southwest-northeast core crossing at 60 and 25 nautical miles from landfall, respectively, both at 20,000 feet altitude. The core wind-flow field has been used to estimate the vortex strength of the primary vortex of the hurricane, size of the core radius and position of the core centre, for each of the core-crossings. The aeroplane passed within 11-12% of the derived core centre on each occasion. In spite of the low vortex-axis height to radius ratio, the core mean-flow followed the ‘solid-body’ nature of two-dimensional line vortex cores, wherefore the magnitude of the induced tangential mean-flow is directly proportional to radius from the core-centre. Based upon the model, core characteristics indicated primary vortex strength was between 6x10⁶ and 8x10⁶m²/s. Likewise, the estimated core radii were quite different for the two crossings, about 29 and 40 km, respectively. In part, the differences were attributable to indefinite core edges (measurement uncertainty), but possibly also elliptical elongation of the core (physical characteristics of the core). The core edge velocity distributions were quite rounded and showed peak wind velocities substantially less than that induced by a line vortex of the model-estimated core strength. It would appear that the high shear associated with such velocity magnitudes would have lead to a re-distribution of vortex strength inside and outside of the core edges: when referred to a moving origin of the identified hurricane core centre, the tangential velocity distribution showed evidence of flight passage through several smaller vortex cores inside/outside of the primary core, notably at r/r_C values of about 1.5, 2 and 4. It is considered probable that the smaller vortices resided in annular distribution around the primary core. Some vortices, as could be expected by flow topological reasoning, were opposite in sign to the strength of the primary vortex. However, the integral along a radial outbound spoke showed a net addition to the strength of the primary vortex. Thus, the velocity distribution had lower magnitude near the core edges and larger magnitudes (‘filling-in’ of the distribution) away from the edges. From this observation, the concept of equivalent vortex strength is proposed, defined as the integrand of the ‘filled-in’ distribution, G_eq=2pr_C∫₁⁸Vd(r/r_C)/ln(r/r_C). G_eq is about 13.6x10⁶m²/s. Turbulent vorticity estimations, from high frequency wind velocity fluctuations (32Hz, equivalent to a Nyquist eddy scale-size of about 7.5m) showed turbulent vorticity peaks in the vicinity of the vortex cores traversed during inbound and outbound tracks to the hurricane centre, in addition to the immediate vicinity of the hurricane core centre.

INFLIGHT DATA

Instrumentation on the NRC Convair, Fig.1, included sensing of air data (wind and turbulence determination), inertial data, LWC, aerosol, cloud droplet and ice particle and precipitation sensors and imagers. In addition, several dropsondes were released. The location of the hurricane centre during the two passes is shown in Fig.2.

Figure 1 – Test aeroplane, CV580 C-FNRC

Figure 2 – Derived Hurricane Juan vortex core centre locations, NRC Convair closet points of approach

VORTEX PARAMETER IDENTIFICATION

The horizontal windfield of Hurricane Juan has been modelled, assuming the simple free line vortex characteristics of ‘solid-body’ rotational mean core flow and hyperbolic tangential velocity magnitude outside the core. The vortex parameters are strength, G, and core radius, r_C. Outside the core, the induced velocity magnitude is taken to be V_t=G/2pr; inside the ‘solid-body’ vortex core, the magnitude is taken to be V_t=(G/2pr_c)(r/r_c).

For identification of the [G r_c] vortex parameter vector, a [V_N-S V_E-W] resolution of horizontal wind components provides time-gradients of velocity magnitude during core traverses:

_{and ,}

whilst, at the edges of the core:

Figure 3 – First core crossing, on-board derived wind field and fitted singular line vortex model (the climbing aircraft achieved level flight at 20,000 feet at the inbound edge of the core penetration).

Figure 4 – Second core crossing on-board derived wind field and fitted singular line vortex model.

Figure 5 - Tangential velocity, showing symmetrical distribution

RESULTS AND DISCUSSION

For the first crossing, the results of the modelling, indicated a primary vortex strength of about 6x10⁶m²/s and a core radius of about 29 km, Fig.4. For the second crossing, the parameters were about 8.4x10⁶m²/s and 40 km respectively, Fig.5. The peak velocity magnitude shortfall and the ‘filling-in’ outside the core edges are evident. Thereafter, the windfield has been transformed into tangential and radial components with respect to the moving vortex core centre, Fig.6. Also shown is the residual tangential velocity waveform, obtained when the line vortex induced velocity is removed. This waveform highlights the other discrete vortex contributions. The strengths, core radii and positions of some of the other vortices have been estimated, using a similar process to before, Fig.7. A satellite view of the hurricane, approaching Nova Scotia (Fig.8), shows imaged structure, which would appear to have evidence of embedded secondary vortices and an elongated core shape, with a major to minor radii ratio of about 1.8:1. The secondary vortices ‘fill-in’ the velocity profile, suggesting the concept of equivalent vortex strength, defined as the integrand of the ‘filled-in’ distribution, G_eq=2pr_C∫₁⁸Vd(r/r_C)/ln(r/r_C)=13.6x10⁶m²/s for the second pass.

Figure 6 – radial flow is asymmetric, possibly due to with spiral flow.

Figure 7 – Estimated placement and core sizes of other significant discrete vortex cores traversed during the second pass, determined by a similar process.

Figure 8 – Satellite visible image of Hurricane Juan, about 420 nm offshore during approach to Nova Scotia, showing evidence of secondary vortices, to the southwest at about r/r_C=2, to the northeast at about r/r_C=2, 4 and 5, within spiral flows.

[" Meteorological Services Canada]