Observational Analysis of Tropical Cyclone Formation. Part II: Comparison of Non-Developing versus Developing Systems

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  • 1 Department of Atmospheric Science, Colorado State University, Fort Collins 80523
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Abstract

The thermodynamic and dynamic fields surrounding the composite tropical weather systems described in Part I (McBride, 1981a) are examined for differences between non-developing and developing systems. The main findings are as follows: (i) Both non-developing and developing systems are warm core in the upper levels. The temperature (and height) gradients are more pronounced in the developing system, but the magnitudes are so small that the differences would be difficult to measure for individual systems. (ii) The developing or pre-typhoon cloud cluster exists in a warmer atmosphere over a large horizontal scale, for example, out to 8° latitude radius in all directions. (iii) There is no obvious difference in vertical stability for moist convection between the systems. (iv) There is no obvious difference in moisture content or moisture gradient. (v) Pre-typhoon and pre-hurricane systems are located in large areas of high values of low-level relative vorticity. The low-level vorticity in the vicinity of a developing cloud cluster is approximately twice as large as that observed with non-developing cloud clusters. (vi) Mean divergence and vertical motion for the typical western Atlantic weather system are well below the magnitudes found in pre-tropical storm systems. (vii) Once a system has sufficient divergence to maintain 100 mb or more per day upward vertical motion over a 4° radius area, there appears to be no relationship between the amount of upward vertical velocity and the potential of the system for development. (viii) Cyclogenesis takes place under conditions of zero vertical wind shear near the system center. (ix) There is a requirement for large positive zonal shear to the north and negative zonal shear close to the south of a developing system. There is also a requirement for southerly shear to the west and northerly shear to the east. The scale of this shear pattern is over a 10° latitude radius circle with maximum amplitude at ∼6° radius.

Under the assumption of a symmetric disturbance, these findings can be synthesized into one parameter for the potential of a system for development into a hurricane or typhoon: Daily Genesis Potential (DGP) = ζ900 mb − ζ200 mb, when applied over 0-6° radius.

Wind fields are examined surrounding 79 individual weather systems in the tropical Atlantic and it is shown that the composite findings are present on a case by case basis. The individual case analysis also reveals that the high values of DGP must be made up of fairly equal contributions from all directions around the disturbance. This is consistent with the requirement for the existence of zero lines in both zonal and meridional vertical shear.

Abstract

The thermodynamic and dynamic fields surrounding the composite tropical weather systems described in Part I (McBride, 1981a) are examined for differences between non-developing and developing systems. The main findings are as follows: (i) Both non-developing and developing systems are warm core in the upper levels. The temperature (and height) gradients are more pronounced in the developing system, but the magnitudes are so small that the differences would be difficult to measure for individual systems. (ii) The developing or pre-typhoon cloud cluster exists in a warmer atmosphere over a large horizontal scale, for example, out to 8° latitude radius in all directions. (iii) There is no obvious difference in vertical stability for moist convection between the systems. (iv) There is no obvious difference in moisture content or moisture gradient. (v) Pre-typhoon and pre-hurricane systems are located in large areas of high values of low-level relative vorticity. The low-level vorticity in the vicinity of a developing cloud cluster is approximately twice as large as that observed with non-developing cloud clusters. (vi) Mean divergence and vertical motion for the typical western Atlantic weather system are well below the magnitudes found in pre-tropical storm systems. (vii) Once a system has sufficient divergence to maintain 100 mb or more per day upward vertical motion over a 4° radius area, there appears to be no relationship between the amount of upward vertical velocity and the potential of the system for development. (viii) Cyclogenesis takes place under conditions of zero vertical wind shear near the system center. (ix) There is a requirement for large positive zonal shear to the north and negative zonal shear close to the south of a developing system. There is also a requirement for southerly shear to the west and northerly shear to the east. The scale of this shear pattern is over a 10° latitude radius circle with maximum amplitude at ∼6° radius.

Under the assumption of a symmetric disturbance, these findings can be synthesized into one parameter for the potential of a system for development into a hurricane or typhoon: Daily Genesis Potential (DGP) = ζ900 mb − ζ200 mb, when applied over 0-6° radius.

Wind fields are examined surrounding 79 individual weather systems in the tropical Atlantic and it is shown that the composite findings are present on a case by case basis. The individual case analysis also reveals that the high values of DGP must be made up of fairly equal contributions from all directions around the disturbance. This is consistent with the requirement for the existence of zero lines in both zonal and meridional vertical shear.

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