Diurnal variation of cumulonimbus in typhoon inferred from negative brightness temperature difference validated by TRMM PR Data

Recently new algorithm of locating cumulonimbus (Cb) has been developed using negative differences between brightness temperature of infrared channel (TBir) and that of water vapour channel (TBwv). Using this method, we identified the area of dTB=TBir-TBwv<0 (ND: Negative Differences) with Cb. At first we compared ND obtained from GMS (Geostationary Meteorological Satellite) data with the precipitation area simultaneously observed by TRMM (Tropical Rainfall Measuring Mission satellite) PR (Precipitation Radar) in the case of Typhoon Rex (T9804) for the validation of ND. From this comparison we found that ND corresponded with the heavy precipitation area. And from the cross section of PR data, it was turned out that the heavy precipitation reached 6km in height and Cb brought it. We confirmed that ND represents Cb area.

Then we investigated the variation of the fractional coverage of ND in the rectangle (6 lat * 6 lon) moving with a typhoon. Analyzing this variation about Typhoon Lisa (T9612), we found the diurnal variation, which has a sharp maximum at 1100 LST and a minimum at 2100 LST on 6 - 11 August. And there was another diurnal variation, which has a maximum at 1700 LST on 12 - 13 August. On 6 - 11 August, there were no large islands around Lisa and its diurnal variation resembled the variation of convective clouds over the ocean. On the other hand, Lisa stayed near the Okinawa Island on 12 - 13 August. It was speculated that the land affected Lisa's diurnal variation. We also observed these diurnal variations in the case of other Typhoons (e.g. Winnie (T9713) and Rex (T9804)).

Cumulonimbus has a direct relation to typhoon intensity. So we think ND is very useful tool for objective estimation of typhoon intensity. But from this research considering the diurnal variation is necessary for the use of ND.