Large-Scale Influences on the Genesis of Hurricane Karl (2010)

The events leading up to the genesis of Hurricane Karl (2010) pose a unique opportunity to examine the continuing problem of understanding tropical cyclogenesis. A precursor disturbance to the genesis of Karl was well sampled by ongoing field campaigns, particularly the PRE-Depression Investigation of Cloud-systems in the Tropics (PREDICT), led by NSF, and the Genesis and Rapid Intensification Processes (GRIP) project, led by NASA. Dropsonde observations obtained in these two field campaigns provide a means to examine the pre-genesis environment of Karl in great detail, including improved gridded analyses. Although NHC best track data for Karl begins less than a day prior to the time of genesis (1200 UTC 14 September), a precursor disturbance can be tracked for up to a week beforehand. The purpose of this presentation is to examine the evolution of the precursor disturbance from a combined synoptic and equatorial wave perspective. From the synoptic perspective, one of the major players in the initial spin-up of pre-Karl is a surge of southerly winds from northern South America on 8-9 September, leading to the formation of a nearly closed earth-relative circulation. This circulation weakened late on 10 September and remained weak through 13 September, during which time the disturbance lacked vertical organization due to an increase in vertical wind shear and only sporadically produced convective bursts. During 13-14 September, more persistent convection developed as the precursor disturbance became vertically stacked and pre-Karl was classified as a tropical cyclone.

From the equatorial wave perspective, both the initial spin-up and subsequent shearing of pre-Karl can be tied to the passage of a convectively coupled Kelvin wave (CCKW). The observed formation of the nearly closed circulation on 9 September is well timed with the passage of the convectively active phase of a CCKW, an event found to be associated with a reduction in climatological westerly shear. Similarly, the convectively suppressed phase of a CCKW served to temporarily suppress convection in the vicinity of the pre-Karl disturbance on 10-13 September. After the passage of the convectively suppressed phase of the CCKW, more persistent convection develops and the pre-Karl disturbance undergoes genesis.

In order to utilize a gridded dataset that assimilated the unique dataset provided by PREDICT and GRIP dropsondes while maintaining a high spatial resolution, operational ECMWF analyses and Climate Forecast System Reanalysis data were employed for the synoptic perspective. ERA-Interim reanalysis data are used in conjunction with filtered outgoing longwave radiation observations to track CCKW features for the equatorial wave perspective. The evolution of the vertical wind shear over pre-Karl and the convective activity associated with the disturbance will be examined from both of these perspectives in order to provide a comprehensive perspective of the role of CCKWs in the modulation of synoptic features commonly associated with the genesis process.