Vertical Motions Prior to the Intensification of Simulated Typhoon Hagupit (2008)

Abstract

Signals prior to tropical cyclone (TC) intensification are important for improving TC intensity forecasts. The existence of such signals has been noted in previous studies of satellite-borne observations and lightning. Vertical motions can enhance TCs via the transportation of heat to the warm core; thus, we hypothesized they might signal TC intensification. This study separately examined the processes of updrafts and downdrafts, which are both concentrated within the eyewall surrounding the warm core. The analysis focused on the intensification of Typhoon Hagupit (2008). Numerical experiments were conducted using the Weather Research and Forecasting model to elucidate the changing structure of the vertical motions and their effects on the timing of TC intensification. A control (CTL) simulation and an idealized No-Land experiment both reproduced the storm structure well, whereas a Half-Bogus (Half-Moisture) experiment generated less-organized TC core structures (weakened moisture convection). Upward motions were found to signal TC intensification with lead times of 1.67-4.50 hr in all experiments. Downward motions apparently signaled TC intensification with lead times of 0.67-4.50 hr in the Half-Bogus and Half-Moisture experiments. However, the lead-lag relationship between downward motion and TC intensification was unclear in the CTL and No-Land experiments because of the large amount of water vapor in the enhanced eyewall. Thermodynamic analysis showed the latent heat consumed in the downdrafts in the CTL and No-Land experiments was much greater than in the two other experiments, which resulted in the smallest heating contribution in the downdrafts and a weak relationship with TC intensification.Plain Language Summary We believe that everyone can forecast the intensification of a tropical cyclone (TC) by the help of some prior signals, thus we tried to find these signals in this study. Vertical motions are the winds that moving upward or downward. They are able to become such prior signals, because they can strengthen the TC through transporting energy to the TC center. We found that the intensification of upward motions occurs <5 hr before the intensification of a TC. This proves the existence of such prior signals and might help to improve the forecast skill of TCs in future.

Assessment of Different Boundary Layer Parameterization Schemes in Numerical Simulations of Typhoon Nida (2016) Based on Aircraft Observations

“…Compared with other boundary schemes, the U-wind simulations in the eye area of the YSU and MYNN schemes were more similar to the observation/result of an ideal typhoon model (shown in Figure 9). Here, the ideal typhoon model means the vertical typhoon structure analyzed in Emanuel et al (1986) [26,27], Chen et al (2019) [20], and Fei et al (2021) [28], where the eye area sinks and both sides of the eye area rise. Under the action of guided wind circulation, the northeast to southwest of the U-wind boundary layer in the eye area showed an east-to-west wind transition, with an easterly jet near a height of 6000 m on the northeast side of the eye area and weak westerly winds on the southwest side of the upper levels.…”