Data impact experiments using IASI observations during the life cycle of Hurricanes Gustav, Hanna and Ike (2008)

The numerical forecast of the extratropical transition (ET) of a tropical cyclone (TC) is difficult because of the relatively small scales of the TC and the associated convection. This can result in large errors of the first guess. Hence, for such cases a good coverage of observations which are introduced in the data assimilation systems is important. Furthermore, the water vapor transport from the decaying TC into the midlatitudes is an important issue for the correct prediction of an ET. Vast improvements in the representation of the temperature and the humidity in numerical forecasts have been achieved by the use of the hyperspectral infrared instruments AIRS (advanced infrared sounder) and IASI (infrared atmospheric sounding interferometer). Our study is dedicated to the impact that the humidity sensitive infrared radiances of the IASI instrument have on the prediction of ET cases.

We performed experiments for a 20-day periode during which 3 TCs underwent ET in the Atlantic using the global weather prediction model ARPEGE of Meteo France. For one experiment the data density was enhanced by a factor of four. The global scores showed a significantly positive impact of the high data density on the northern hemisphere, the southern hemisphere and the tropics. The second experiment included 9 further IASI channels which are sensitive to water vapor. In the global scores the impact of the water vapor channels is positive for the southern hemisphere and slightly positive for the northern hemisphere and the tropics especially for humidity.

The ET cases were studied separately and it was found that in some cases, shortly before the respective ET event, the analysis error in the water vapor channel experiment was slightly smaller in comparison to that without the additional water vapor channels. In these cases, some errors over the Atlantic associated with synoptic systems with which the ETs interacted reduced strongly. Error reductions due to the water vapor channels over Europe could be found in the systems which developed downstream of the ET events.