Mathieu Rouault et al: Top

Conclusion
The storm resulted from interaction between a continental heat low, an anticyclone located over the South West Indian Ocean, and a midlevel trough approaching from the west. Unlike previously documented heavy precipitation events in South Africa, most of the moisture appears to have originated from the Agulhas Current region. This led us to assess the potential role of the greater Agulhas Current system in the storm development. It is quite possible that the significance of the warm Agulhas Current as a moisture source for this storm may be substantially underestimated by the NCEP re-analyses examined here. As revealed by a weekly composite of NOAA AVHRR SST , the core of the Agulhas Current (waters of about 23º C and warmer) was only about 100 km wide. This narrow core cannot be adequately resolved by the NCEP re-analyses or by current forecast models. Even some of the best SST products available do not adequately take into account the Agulhas Current. TRMM TMI SST offers one possibility to help remedy this situation as it can be used in cloudy as well as cloud-free areas. TRMM SST are in good agreement with the AVHRR SST in the cloud-free areas. This result suggests that TRMM SSTs can help overcome the problem of clouds above the Agulhas Current. It is important to obtain accurate SST data since underestimation will lead to a subsequent underestimation of the surface specific humidity, latent and sensible heat flux by NCEP and other models. In fact, data collected at sea have shown that the NCEP surface latent heat flux is significantly underestimated over the core of the Agulhas Current. As a result of these underestimates, we believe that the contribution of the moisture source from the Agulhas Current region to the development of this storm may have been even larger than our analyses have suggested. The results of this study therefore reinforce the need to take into account more carefully the Agulhas Current in operational forecast and other models applied to the South African region. Not only should high resolution SST products, such as TRMM or AVHRR, be used in the models but special provisions need to be made for the high latent heat fluxes produced by the core of the current. Finally, this study has demonstrated that even in the absence of radar or dense radiosonde networks, satellite remote sensing and model data can be extremely useful for case study research.

Conclusion

The storm resulted from interaction between a continental heat low, an anticyclone located over the South West Indian Ocean, and a midlevel trough approaching from the west. Unlike previously documented heavy precipitation events in South Africa, most of the moisture appears to have originated from the Agulhas Current region. This led us to assess the potential role of the greater Agulhas Current system in the storm development. It is quite possible that the significance of the warm Agulhas Current as a moisture source for this storm may be substantially underestimated by the NCEP re-analyses examined here. As revealed by a weekly composite of NOAA AVHRR SST , the core of the Agulhas Current (waters of about 23º C and warmer) was only about 100 km wide. This narrow core cannot be adequately resolved by the NCEP re-analyses or by current forecast models. Even some of the best SST products available do not adequately take into account the Agulhas Current. TRMM TMI SST offers one possibility to help remedy this situation as it can be used in cloudy as well as cloud-free areas. TRMM SST are in good agreement with the AVHRR SST in the cloud-free areas. This result suggests that TRMM SSTs can help overcome the problem of clouds above the Agulhas Current. It is important to obtain accurate SST data since underestimation will lead to a subsequent underestimation of the surface specific humidity, latent and sensible heat flux by NCEP and other models. In fact, data collected at sea have shown that the NCEP surface latent heat flux is significantly underestimated over the core of the Agulhas Current. As a result of these underestimates, we believe that the contribution of the moisture source from the Agulhas Current region to the development of this storm may have been even larger than our analyses have suggested. The results of this study therefore reinforce the need to take into account more carefully the Agulhas Current in operational forecast and other models applied to the South African region. Not only should high resolution SST products, such as TRMM or AVHRR, be used in the models but special provisions need to be made for the high latent heat fluxes produced by the core of the current. Finally, this study has demonstrated that even in the absence of radar or dense radiosonde networks, satellite remote sensing and model data can be extremely useful for case study research.

Conclusion| References Thanks]

Email: rouault@physci.uct.ac.za

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