Abstract
The westward moving Soudano-Sahelian mesoscale convective systems (MCS) frequently reach and cross the Atlantic Coast. At the end of their continental route, most MCS weaken and vanish over the ocean, near the coast, while others strengthen. The latter play an important part in the genesis of some Atlantic tropical cyclones. In the present paper, following the work of Gray (1977, 1979) [Gray, W.M., 1977. Tropical cyclone genesis in the western North Pacific. J. Meteorol. Soc. Jpn. 55, 465–482; Gray, W.M., 1979. Hurricanes: their formation, structure and likely role in the tropical circulation. Meteorology over the Tropical Oceans, D.B. Shaw, (Ed.), Roy. Meteorol. Soc., 155–218] and Gray et al. (1994, 1999) [Gray, W.M., Landsea, C.W., Mielke Jr., P.W., Berry, K.J., 1994. Predicting Atlantic seasonal tropical cyclone activity by 1 June. Weather Forecast. 9, 103–115; Gray, W.M., Landsea, C.W., Mielke Jr., P.W., Berry, K.J., 1999. Forecast of Atlantic seasonal hurricane activity for 1999. Dept. of Atmos. Sci. Report, Colo. State Univ., Ft. Collins, CO, released on 4 June, 1999], an index liable to be associated with the coast-crossing MCS cyclonic evolution is built. The data used in this work are observations by the Dakar-Yoff radar, reanalyses of NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research), outgoing long wave radiation at the top of the atmosphere, and the resources of the National Hurricane Center data base. Several terms describing the variation of individual meteorological parameters are first analysed and then combined into an index of cyclogenesis or ICY. Combination of vertical vorticity at 925 hPa and potential vorticity at 700 hPa is notably found to be a good factor to discriminate between strengthening and weakening MCS over the near Atlantic. A good correlation between the ICY maximum and the beginning of the MCS cyclogenesis is observed. This index enables discrimination of the simultaneous presence of two separate cyclonic perturbations over the Atlantic. These results show that the sole variable ICY is useful to detect a cyclogenesis process in progress in a Sahelian MCS.
Highlights
In the present paper, the concept of cyclogenesis refers to the growth of a tropical depression, a tropical cyclone, from a pre-existing mesoscale convective storm (MCS—Frank, 1987)
Humidity plays a central part in the convective system development and a high humidity concentration at low and middle tropospheric levels is favourable to cyclogenesis (e.g. Houze and Betts, 1981)
From the difference between temperature inside a convective system and the environmental temperature, McBride (1981) shows that, over the Atlantic, horizontal temperature gradient is strong in the upper layer, around 300 hPa for systems where cyclogenesis is in progress and lower elsewhere
Summary
The concept of cyclogenesis refers to the growth of a tropical depression, a tropical cyclone, from a pre-existing mesoscale convective storm (MCS—Frank, 1987). It shows that many westward moving Sahelian convective storms and MCSs reach the Atlantic Coast and that a significant fraction cross this line and travel over the sea. The reanalysis provides air temperature, geopotential altitude, as well as the three components of wind velocity for 17 pressure levels in a grid mesh of 2.58 Â 2.58 It provides the relative humidity between 1000 and 300 hPa. Information on the characteristics of the tropical disturbances, notably their classification in the Saffir-Simpson code of intensity level (Simpson, 1974), provided by the NHC (National Hurricane Center) were used to follow and identify the time evolution of the systems over Atlantic
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