Dl. Zhang et N. Bao, OCEANIC CYCLOGENESIS AS INDUCED BY A MESOSCALE CONVECTIVE SYSTEM MOVING OFFSHORE .2. GENESIS AND THERMODYNAMIC TRANSFORMATION, Monthly weather review, 124(10), 1996, pp. 2206-2225
The genesis of intense cyclonic vorticity in the boundary layer and th
e transformation of a low-level cold pool to a warm-core anomaly assoc
iated with the long-lived mesoscale convective systems (MCSs), which p
roduced the July 1977 Johnstown flash flood and later developed into a
tropical storm, are examined using a 90-h real-data simulation of the
evolution from a continental MCS/vortex to an oceanic cyclone/storm s
ystem. It is shown that the midlevel vortex/trough at the end of the c
ontinental MCS's life cycle is characterized by a warm anomaly above a
nd a cold anomaly below. The mesovortex, as it drifts toward the warm
Gulf Stream water, plays an important role in initiating and organizin
g a new MCS and a cyclonic (shear) vorticity band at the southern peri
phery of the previously dissipated MCS. It is found from the vorticity
budget that the vorticity band is amplified through stretching of abs
olute vorticity as it is wrapped around in a slantwise manner toward t
he cyclone center. Then. the associated shear vorticity is converted t
o curvature vorticity near the center, leading to the formation of a '
'comma-shaped'' vortex and the rapid spinup of the surface cyclone to
tropical storm intensity. Thermodynamic budgets reveal that the vertic
al transfer of surface fluxes from the warm ocean and the convectively
induced grid-scale transport are responsible for the development of a
high-theta(e) tongue, which is wrapped around in a fashion similar to
the vorticity band, causing conditional instability and further organ
ization of the convective storm. Because the genesis occurs at the sou
thern periphery of the vortex/trough, the intensifying cyclonic circul
ation tends to advect the pertinent cold air in the north-to-northwest
erly flow into the convective storm and the ambient warmer air into th
e cyclone center, thereby transforming the low-level cold anomaly to a
warm-cored structure near the cyclone core. It is shown that the tran
sformation and the evolution of the surface cyclone are mainly driven
by the low-level vorticity concentrations. It is found that many of th
e cyclogenesis scenarios in the present case are similar to those note
d in previous tropical cyclogenesis studies and observed at the early
stages of tropical cyclogenesis from MCSs during the Tropical Experime
nt in Mexico. Therefore, the results have significant implications wit
h regard to tropical cyclogenesis from MCSs.