Numerical simulations of the genesis of hurricane Diana (1984)

Hurricane Diana formed within 100 km of the east coast of Florida on 10 September, 1984 in an environment featuring marked baroclinicity for that location at that time of year. Classical baroclinic cyclogenesis gave way to tropical storm genesis as convection became more prominently associated with the cyclonic circulation on 8 September. The storm intensity remained nearly constant for about 12 h until 1200 UTC 10 September when explosive deepening commenced.

The genesis of tropical storm Diana and later, hurricane Diana, is simulated with the PSU/NCAR mesoscale model (MM5). We initialize the model at 1200 UTC 7 September using only the NCEP/NCAR reanalyses and rawinsonde and surface data. No vortex bogussing is performed. The model configuration consists of interactive nests with a minimum of 9 km resolution. Three-phase microphysics and the Kain-Fritsch cumulus schemes are used on all domains in our control simulation. Sea surface temperatures (SSTs) are obtained from hand analyses of ship and buoy reports. Simulations show considerable sensitivity of the storm intensification to the choice of cumulus scheme and to the SST analysis.

Diagnostics reveal a two-stage process of intensification. In the first stage, the spinup of the nascent tropical storm, widespread convection is initiated as a piece of the closed upper low moves across the weak baroclinic zone and over the warm SSTs (29-30 C). The pattern of convection consists of convective bursts with each burst being associated with a local maximum of vorticity and potential vorticity perhaps 20-30 km across. These vorticity anomalies are swept toward the developing larger-scale (200-300 km) cyclonic disturbance, first by the ambient northeasterlies, then by the swirling flow itself. Some of the anomalies appear to merge with the central PV maximum as the circulation slowly intensifies. Vorticity flux calculations indicate an inward directed flux below about 2 km AGL between a radius of 50 km and 200 km during the initial intensification.

Following the burst of convection and the development of the incipient tropical storm, a 12-hour period of relative quiesence ensues, during which there is only a slight deepening of the storm, and heavy precipitation is nearly absent. During this period, precipitation appears to organize into spiral bands for the first time. Eventually, deep convection initiates near the center of the tropical storm and rapid intensification into a hurricane follows. It is believed that this second deepening represents the onset of the classical air-sea interaction instability. Mechanisms determining the timing of this intensification are being considered, but three potential factors stand out; (1) the storm nearing the maximum SSTs; (2) attainment of some critical strength for the air-sea mechanism to operate and; (3) attainment of near saturation over the lower troposphere within 100 km of the center of the storm.