Figures 4 and 5 show the NDCI value changes with longitude (latitude) with the hatched lines based on the latitude (longitude) of the center of the typhoon at 03:00 UTC (mild-strength stage) and 21:00 UTC (moderate-strength stage) on September 26, respectively. According to Figs. 4 and 5, the NDCI values within the range of the 250 km radius of Typhoon Longwang center were mostly , and the values closer to the center of the circulation were smaller. Yet, when the typhoon eye was formed at 19:00 UTC on September 27 (severe-strength stage), the NDCI values within the eye suddenly increased (as shown in Fig. 6). The reason was that near the eye, the cloud layers were rather thin and the air was relatively dry. As a result, the IR1 and WV channel TBs were close. This result was similar to the result obtained by Olander and Velden using the cross-section analysis with difference values ().14 However, in this study, in the cross-section analysis of the NDCI values, the area of near-zero or negative pixels covered both the stratiform cloud systems and the convective cloud systems with deep vertical development or the overshooting areas. The latter cannot be depicted clearly and separately. Here we first used the IR1 TBs between 192.8 and 208.8 K of the MB curve as the criterion for determining the overshooting areas.40 According to Fig. 7(a), when Typhoon Longwang was formed, there was a round area around the center of the typhoon, which could be considered as the overshooting area, and the main deep convection area was at the south of the center of the typhoon. When the wind speed of Typhoon Longwang reached the standard for a moderate typhoon [shown in Fig. 7(b)], the overshooting area expanded significantly. Deep convective cloud systems were significant, especially at all the sides of the center of the typhoon. Thus, the overshooting areas across the center of the typhoon were symmetric with rotation. When the wind speed of Longwang reached the standard of the severe typhoon stage [as shown in Fig. 7(c)], in addition to the area where the typhoon eye was located, the overshooting cloud systems also distributed around the eye with the apparent rotation. According to the JTWC wind speed data, when Typhoon Longwang became a severe typhoon, because it was still above the sea, so there was sufficient energy supplied to Longwang for development. Therefore, its maximum sustained wind continued to increase. The typhoon kept on moving west with severe intensity. When this study conducted the tracking of the hourly data of the overshooting areas (figures not shown), it was found that the coverage of the overshooting areas did not necessarily increase as the intensity of the typhoon increased (compared with JTWC’s maximum sustained wind); on the contrary, it sometimes even decreased. For example, according to the distribution at 06:00 UTC on September 29, the overshooting areas even contracted and the coverage decreased [as shown in Fig. 7(d)]. Yet, during the development of the typhoon, the overshooting areas were all around the typhoon center. This may be considered a significant feature to be noted about the distributions of overshooting areas.