Analysis of the Factors that Led to an uncertainty of track forecast of Typhoon Krosa (2019) by 101-member ensemble forecast experiments using NICAM

Abstract

Typhoon Krosa (2019) formed in the eastern part of the Philippines Sea and ~1400 km east of another typhoon Lekima on August 6th and made a landfall in the western part of Japan’s mainland on August 15th. The operational global model forecasts, which were initialized just after Krosa’s formation, showed a very large uncertainty and totally failed to predict the actual track of Krosa. In this study, we investigated the causes of this large uncertainty through 101-member ensemble forecast experiments by using a 28-km mesh global nonhydrostatic model. The experiments initialized at 12 UTC, August 6th, showed a large uncertainty. An ensemble-based sensitivity analysis indicated that the western North Pacific Subtropical High (WNPSH) retreated further east in the members with large track forecast errors than in the members with small errors. The members with a large track forecast error for Krosa, Krosa and Lekima approached by 250 km and Krosa propagated northward faster than the observation in 36 hours from the initialization time. For the members with a small track forecast error for Krosa, two typhoons approached by only 50 km, and the northward propagation speed was comparable with that of the observation. The typhoon relative composite analysis exhibited that at the initialization time, the members with a large Krosa track forecast error had a larger horizontal size of Krosa and higher moisture in the east of Krosa’s center. The difference in Krosa’s size was kept during the forecast period, and precipitation was larger in the outer region for the members with a large Krosa’s track error. This difference led to a stronger interaction between the two typhoons, thus resulting in a fast northward propagation speed for the members with a large Krosa track error.