Global View of the Origin of Tropical Disturbances and Storms

A global observational study of atmospheric conditions associated with tropical disturbance and storm development is presented. This study primarily uses upper air observations which have become available over the tropical oceans in the last decade. Climatological values of vertical stability, low level wind, tropospheric vertical wind shear and other parameters relative to the location and seasons of tropical disturbance and storm development are discussed. Individual storm data are also presented in summary form for over 300 development cases (with over 1,500 individual observation times) for four tropical storm genesis areas. Results show that most tropical disturbances and storms form in regions equatorward of 20° lat. on the poleward side of doldrum Equatorial Troughs where the tropospheric vertical shear of horizontal wind (i.e., baroclinicity) is a minimum or zero. Storm development occurring on the poleward side of 20° lat. in the Northwest Atlantic and North-west Pacific takes place under significantly different environmental conditions, which are described. These latter developments make up but a small percentage of the global total. Observations are also presented which indicate that over the tropical oceans where disturbances and storms form, there is a distinct Ekman or frictional veering of the wind in the subcloud layer (surface to 600 m.) of approximately 10°. This produces or enhances synoptic-scale low level convergence and cumulus convection in regions of large positive relative vorticity which exist in the cyclonic wind shear areas surrounding doldrum Equatorial Troughs. Tropical disturbance and later storm development is viewed as primarily a result of large-scale Ekman or frictionally forced surface convergence (with resulting cumulus production and tropospheric heating), and a consequent inhibition of tropospheric ventilation by initially existing small vertical wind shear, and later inhibition of ventilation by cumulus up- and downdrafts acting to prevent increase of vertical shear as baroclinicity increases. The above processes produce the necessary condensation heating and allow for its concentration and containment in selective areas. Development is thus explained from a simple warming, hydrostatic adjustment point of view with the energy source analogous to Charney and Eliassen's proposed “conditional instability of the second kind.”