Thursday, 2 August 2001: 10:30 AM
A preliminary investigation of the genesis of Atlantic tropical storms in the 2000 Hurricane season using the Operational Global data
Recently Karyampudi and Pierce (2000, hereafter KP) suggested that tropical cyclogenesis over the E. Atlantic region is influenced by the dynamical forcing exerted by the Saharan Air Layer (SAL), which is frequently carried by African wave. KP have shown that tropical cyclogenesis, in general, occurs when the sign reversal of the meridional gradient of PV (850-700 mb) exists across the midtropospheric jet, in accordance with the Charney and Stern (1962) criterion for barotropic and baroclinic instability, first applied by Burpee (1972) to explain the growth of African waves over North Africa. KP have also shown that from vorticity budget analysis that PVA on the south and leading edge of the midtropospheric jet contributes to the growth of wave disturbances whereas the vortex stretching dominates the enhancement of low-level vortices. In this study, a preliminary investigation of the all the tropical storms that occurred in the year 2000 hurricane season has been conducted using the operational analysis of the AVN global model forecasts. The purpose of this study is not only to validate the results of KP but also to develop diagnostic parameters within the Pcgridds software system currently used at the tropical desk of the NWS/NCEP/HPC, which is also available to WMO IV users, to help forecast the genesis of the Atlantic storms even before they are classified as tropical depressions. Therefore, Pcgridds software has been applied to obtain diagnostic variables such as the meridional gradient of PV, horizontal vorticity advection, vortex stretching, 850-700 mb layer static stability etc. A preliminary assessment of these results show that a majority (65%) of the 18 storms that formed over the Atlantic not only originated from African wave disturbances, in consistent with the previous studies, but are also in close proximity to the SAL both over the eastern and western Atlantic regions. All these 12 storms are accompanied by a midtropospheric jet that appears to exist to the south of the SAL but to the north-northeast of the disturbance center. Furthermore, in agreement with the results of KP, the combined barotropic and baroclinic instability criterion appears to be satisfied for a majority of the storms including the existence of PVA at 700 mb level and vortex stretching at 1000 mb level. A majority of these storms formed at least one to three days prior to their depression classification by the NWS/NCEP/TPC within a deep moist environment characterized by high equivalent potential temperature air, which enhanced the potential for further development. Therefore, these preliminary results not only validate the findings of KP but also suggest that dynamic diagnostic parameters can be used to detect the genesis of the storms using Pcgridds software applied to the operational global model data. However, a thorough investigation of the relationship between SAL and tropical cyclone formation needs to be conducted over the western Atlantic region, possibly with in-situ observations, since there is some uncertainty in the identification of the SAL in the global model analysis due to the general weakening of the SAL in this region.