So what must occur - for a tropical disturbance consisting of an unorganized complex of thunderstorms to evolve into a more coherent structure of a tropical storm and then a highly organized hurricane-strength system?
As we learned (or re-learned) so clearly last week from Isaac, the answer to the question, “to be or not to be?” can be frustrating days anticipating whether a tropical storm will merely attain category 1 hurricane status.
In other cases, (e.g., Felix in 2007) an unimpressive appearing cluster of thunderstorms developed in 2-3 days into a major catgory 4 hurricane with winds over 140 mph. As we saw Wednesday, Michael explosively developed from a tropical storm to a major hurricane in 12 hours.
Despite an array of theories, the truth is a succinct and verifiable explanation of hurricane genesis and subsequent development to hurricane status remains an enigma at best.
Veteran hurricane research Ed Zipser says understanding what causes some storms to explosively develop is the “holy grail” of hurricane research. This includes why many more tropical disturbances do not develop into tropical storms than do, even when it the basic pre-conditions appear satisfied (e.g., water temperatures exceeding 80 deg F, little vertical shear - change in wind velocity with height – abundant moisture supply, etc).
The pivotal transformation in hurricane development is consolidation of thunderstorm downdrafts, which must compensate for the rising air within the updrafts of individual thunderstorms into a single intense downdraft that defines the eye and center of circulation.
Although scientists understand many of the ingredients needed for this to happen, it remains unknown what processes kick start and then govern this transformation.
To explain further, it is necessary to clarify an oft-misunderstood aspect of hurricane development: namely, the mechanisms which transfer the heat extracted from the underlying ocean to what drives the formation and intensification of a storm.
But the heat released actually produces little, if any, net warming of the rising air column that would lead directly to surface pressure falls and strengthening of an incipient hurricane.
Surprised? Almost all the heat (from the ocean) goes toward offsetting the (“adiabatic”) cooling that otherwise would occur as air rises vertically within updrafts.
In effect, the (ocean) heating releases the brake on both the volume of moist air rising within updrafts and, therefore, on the sinking motion required to achieve a balance between what’s going up and going down.
Most of the deepening (intensification) of a developing tropical system results from warming (and drying) of descending air as it becomes increasingly concentrated above the surface low in what becomes the warm core “eye of the storm”.
It’s important to recognize that this relatively simplistic conceptual model can help explain some of the vagaries of actual hurricane development beyond just the fundamentals. For example, not all of the compensating descent need be concentrated in the eye. Downdrafts between and within rainbands could offset the otherwise necessary descent (and warming) within the eye. The extent to which this occurs can modulate the degree and rate of intensification.
But understanding and forecasting the evolution of this sinking air and if/how it will concentrate at the eye of the storm has proven elusive. It is a major reason why predicting hurricane genesis and intensity changes remain so difficult.