232 



MALKUS 



[chap. 4 



3. A much smaller central core consisting of: 



a. An "inner rain area" with pressures less than 1000 mb and hurricane 

 force winds. 



b. A relatively calm, clear central "eye" containing the lowest pressure. 



The key feature of the full hurricane, distinguishing it from the sub-hurricane 

 tropical storm, is the presence of a central core ( ~ 100-400 km in diameter) of 

 raging winds and tight radial pressure gradients, of the order of 30 mb per 

 60 km (Fig. 67). Thus the central pressures of a moderate hurricane (maximum 

 wind ~ 100 knots) must be around 960 mb, 4-5% below the sea-level mean. 



Fig. 66. Diagram showing low-level spiral trajectories (solid lines with arrow-heads) into 

 moderate strength tropical hurricane. (In part from Tropical Meteorology by H. Riehl. 

 Copyright, 1954. McGraw-Hill Book Co. Used by permission.) 



Distance co-ordinate degrees latitude (1° lat=lll km). Dashed circles are iso- 

 chrones (labelled in hours) which give the time an air parcel travelling horizontally 

 along the trajectories would take to reach a point 0.5° latitude from storm center, for 

 mean moderate storm. Co-ordinate system superposed is the natural co-ordinate 

 system used in model. The line s is everywhere tangential to the trajectories ; n is the 

 normal to the trajectories ; ^ is the "crossing angle" at which the trajectories intersect 

 lines of equal radius r. R denotes the radius of curvature of the trajectory. Shown 

 also is a standard polar co-ordinate framework with azimuth tfj, radial distance 

 positive outward. 



The new model relates core maintenance to mechanism, namely, cumulonimbus 

 convection and sea-air exchange. To sustain the required pressure gradients, two 

 coupled processes are necessary: first, a greatly magnified oceanic input of sensible 

 and- latent heat, and, secondly, the undilute release of the latter in concentrated hot 

 tower ascent, so that air of high heat content is pumped rapidly into the upper 

 troposphere. The quantitative establishment of these crucial relationships was 

 carried out in a joint analytic and observational framework (Malkus and 

 Riehl, 1960). 



