Convection associated with an equatorial westerly wind burst was
first observed late November during the strong El Niņo of 1997 at
approximately 2000 km southwest of Hawaian Islands. This region of
convection led to the formation of twin tropical cyclones, one in the
southern hemisphere named Pam and the other in the northern hemisphere named Paka. During the first week in December, tropical cyclone Paka, the system of concern, reached tropical storm stage as it moved rapidly westward at relatively low latitudes. During 10-12 December, Paka rapidly developed into a typhoon.
This initial explosive growth that took place during 10-12 December occurred when Paka passed south of Majuro and wajalein Atolls. During this event, Paka was monitored by the eosynchronous Meteeorological Satellite (GMS) infrared (IR) window and water vapor channels; the F-11, F-13, and F-14 Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave/Imager (SSM/Is) and the Optical Transient Detector (OTD) lightning observer; the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), Precipitation Radar (PR), and Lightning Imagering Sensor (LIS) lightning dectector; and the Kwajalein weather radar (DWSR-935). Sea surface temperatures and atmospheric diagnostics were also
employed and obtained, respectively, from the Goddard Flight Space Center's Data Assimilation Office and the European Centre for Medium-Range Weather Forecasting (ECMWF) model.
Preliminary results from the GMS IR, DMSP SSM/I and TRMM TMI observations indicated that Paka's inner core (i.e., 55 km from the center) region went though a period of rapidly increasing rainrates (i.e., 15 mm/h increase between 9-11 December) and convective activity. This was followed by an increase of maximum winds from 22 m/s on the 10 December to 55 m/s on the 12 December. Hourly cloud top temperature observation from GMS suggested that the rapid growth of convection occurred within the inner core region at approximately 0600 UTC 10 December and slowly dissipated by
approximately 0000 UTC 12 December.
Events that led to this convective burst episode appeared to be
associated with the SST variations and upper-tropospheric forcing
mechanisms. In regards to SSTs, the increasing surface temperature that Paka traversed, while moving westward, appeared to provide the additional moist static energy flux to support the enhanced convection. In addition, tropospheric conditions that favored convective growth were the increasing strong inward flux of moisture in the northeastern octant of Paka and weak 850 - 200 mb vertical wind shear of less 12 m/s.
Two upper-tropospheric troughs that Paka encountered appear to contain a triggering mechanism which initiated this convective burst. The first trough interacted with Paka at 1200 UTC 10 December, while the second trough crossed Paka at 1200 UTC 11 December. During theses encounters, an outfow channel was formed creating a region of strong horizontal divergence north of Paka and an influx of eddy relative angular momentum (ERFC) into
the tropical system. Paka's ERFC values at this time were characteristic of a moderate environmental-tropical cyclone interaction. Elevated ERFC values suggested that the gradient wind adjustment processes (associated with the thermally direct circulation at the entrance region of Paka's outflow channel) was a possible mechanism that could have enhanced Paka's inner core precipitation and, therefore, its intensity.
Another possible strong upper-tropospheric triggering mechanism,
was the merging of positive zones of potential vorticity associated with the upper-tropospheric troughs and the inner core region of Paka. As the two troughs approached Paka, cross sectional analyses through the center of Paka revealed that the positive potential vorticity anomalies associated with Paka and the troughs merged. It has been suggested that the merging of the positive potential vorticity increased the inner core convection by either: 1) initiating the evaporation-wind feedback instability (i.e., Wind-Induced Surface Heat Exchange mechanism [WISHE]); 2) reducing the depth of the upper-tropospheric trough and, thereby, helping to reduce the
vertical wind shear; or 3) initiating an outer core convective ring cycle