This study investigates impacts of sea spray-mediated latent heat in forecasts of rapidly intensifying typhoons. Sea spray-mediated latent heat, expected to improve the forecast performance of typhoon intensification, must evaluate complex wind-wave interactions. To simply incorporate its effects, a regression equation was developed using YJ22 algorithm of Lan et al. (2022). The experiments with sea spray effect accurately predicted the tracks and intensities of category 5 typhoons, but overestimated the intensities and deviated from actual tracks for category 3 typhoons. Our results suggest that including spray-induced latent heat flux can notably improve the forecast precision of category 5 RI typhoons.
The recent increase in Rapidly Intensifying (RI) typhoons, defined as an increase in the maximum sustained wind speed of a tropical cyclone exceeding 30 kt in 24 hours, is causing significant damage to estuaries and coastal seas. RI typhoons accounted for 21.5% of the Northwest Pacific (NWP) from 1977 to 2016, with 34.9% of these RI typhoons occurring in the eddy-rich NWP (Kim et al., 2019). Kim et al. (2019) reported a rapid increase in the number of very strong typhoons and RI typhoons after 2011, even though the total number of typhoons was unchanged. The increase in the intensification rate of typhoons between 1974 and 2013 was also the subject of a report by Guan et al. (2018).
The realistic prediction of RI typhoons is a crucial challenge to prevent damage to human and social resources. Even if the typical typhoon intensity can be predicted in advance, predicting the timing of rapid intensification has been unsatisfactory. In recent decades, wind-wave interaction and sea spray at extreme wind speeds have been noticed to have an important effect on heat exchange, and many researchers have proposed parameterization methods that take these effects into account. However, most current typhoon prediction models do not include the sea spray effect in calculating latent heat flux.
One of the leading studies to quantify the heat flux mediated by sea spray is Andreas et al. (2015). They proposed a parameterization to calculate spray-mediated heat flux by introducing a sea spray generation function depending on the radius of spray droplets. In addition, Andreas et al. (2015) presented a more simplified equation, i.e., a fast spray-flux algorithm, by selecting the droplet radius of 50 and 100 μm, which have large peaks in latent and sensible heat fluxes, respectively. The spray-mediated heat flux exceeded the interfacial flux, controlled by molecular processes at the air-sea interface, when wind speed reached approximately 25 m/s (Andreas et al., 2015). In particular, the latent heat induced by sea spray reached more than three times the sensible heat at a wind speed of 40 m/s.
