Tropical cyclogenesis as revealed by the NOGAPS analysis

Recent improvements in satellite retrievals, data assimilation schemes and numerical models have all contributed to a significant improvement in the quality of global analyzed fields, rendering them capable of representing realistic atmospheric states in data sparse regions such as open oceans. To understand the processes associated with tropical cyclogenesis, the Naval Operational Global Atmosphere Prediction System (NOGAPS) analysis fields are used to study tropical synoptic-scale disturbances pre-exist hurricanes occurred in the North Atlantic summer season of 2004. A 3-8 day band pass filter is applied to the analyzed fields to extract out the synoptic-scale disturbances. We trace these tropical waves upstream and prior to the genesis of tropical depressions as defined by the National Hurricane Center. For the 12 tropical cyclones we examined, most of them can be identified as being originated from Easterly waves out of Africa. The characteristics of these pre-storm waves and no-storm waves are investigated. Large-scale convergence, shear, upper level divergence and other environmental features favorable for tropical cyclogenesis and their relations with the pre-storm easterly waves are examined.