The focus of this study is to investigate the effect of upper-level troughs in the westerlies on the intensity change of tropical cyclones. Conventional wisdom states that increased shear associated with these troughs is detrimental to tropical cyclones. Hanley et al. (1997) showed in several case studies that favourable interactions are characterized by relatively small-to-moderate values of vertical wind shear associated with shallow potential vorticity (PV) anomalies. When the PV anomaly is restricted to the outflow layer of the tropical cyclone, diabatic erosion can weaken the anomaly and thereby diminish the vertical shear in the vicinity of the tropical cyclone. In contrast, unfavourable interactions involve large values of vertical shear associated with deep PV anomalies that extend below the layer where diabatic heating can reduce the strength of the PV anomaly. This work extends these results to a larger number of cases.
The present study uses 12 hourly 1.125 degree uninitialized ECMWF analyses. All named tropical cyclones in the Atlantic basin during 1985 through 1996 are considered. As in Hanley et al. (1997), a trough interaction is defined when the eddy flux convergence of angular momentum (EFC) at 200 hPa over a 300-600 km storm-centred radial band is greater than 10 (m/s)/d and the total pressure change is greater than 10 hPa. Three times per composite are chosen; t=0 is taken to be the time immediately before the pressure began rising or falling. Times 12 hours before and 12 hours after this time are also included. Cases are identified and composited for six different situations. In composites 1-4 the tropical cyclone intensifies, while in composites 5 and 6 the tropical cyclone weakens. The categories of composites are: (1) superposition (upper tropospheric PV anomaly within 400 km of storm centre), (2) distant interaction (PV anomaly between 400 and 1000 km from the centre), (3) extratropical transition (intensification after transition), (4) favourable no trough (intensification with EFC less than 5 (m/s)/d for at least 3 consecutive periods), (5) unfavourable interaction (weakening with enhanced EFC), and (6) unfavourable no trough (weakening with EFC less than 5 (m/s)/d for at least 3 consecutive periods). Using storm-centred cylindrical grids, PV on theta surfaces is calculated for each composite. Standard variables are composited for 20 pressure levels on a storm-centred 1.125 degree grid. Eliassen-Palm fluxes (Molinari et al. 1995) and vertical wind shear and ageostrophic circulations (Loughe et al. 1995) are calculated for each composite.
Results from composites 1 and 5 support the findings of Hanley et al. (1997) for selected case studies. In general, composites 1, 2 and 3 consist of a divergent outflow directed in part toward the trough. The corresponding minimum of the divergent velocity potential is centred approximately 200-300 km to the east or northeast of the storm centre. In the case of composite 4, the divergent circulation is more symmetric and stronger than that for composites 1, 2 and 3 but the minimum in divergent velocity potential is located over the storm centre. Unfavourable interactions (composite 5) resemble favourable interactions but have stronger divergent circulations, i.e. very strong outflow, and much larger shear than in the favourable cases. Work is still ongoing for composite 6.
REFERENCES:
Hanley, D. E., J. Molinari and D. Keyser, 1997: The Effect of Trough Interactions on Tropical Cyclone Intensity Change. Preprints, 22nd Conference on Hurricanes and Tropical Meteorology, Ft. Collins, CO, Amer. Meteor. Soc., 354-355.
Loughe, A. F., C.-C. Lai, and D. Keyser, 1995: A Technique for Diagnosing Three-Dimensional Ageostrophic Circulations in Baroclinic Disturbances on Limited-Area Domains. Mon. Wea. Rev., 123, 1476-1504.
Molinari, J., S. Skubis, and D. Vollaro, 1995: External Influences on Hurricane Intensity. Part III: Potential Vorticity Structure. J. Atmos. Sci., 52, 3593-3606.