A Rare Tropical Cyclone Necessitating the Issuance of Gale or Storm Wind Warning Signal in Hong Kong in Late Autumn in 2022—Severe Tropical Storm Nalgae

Abstract

The invaluable meteorological observations for a late autumn tropical cyclone that came very close to Hong Kong, namely, Nalgae in November 2022, are documented in this article. In particular, the dropsonde data for two consecutive days for this late season storm close to Hong Kong are presented. Meteorological data revealed that while Nalgae appeared to be rather weak from the meteorological satellite image under the cool sea surface water and the cool and dry northeast monsoon, it still maintained considerable intensity near the lower-boundary layer and managed to bring gale to storm force 10 min mean winds over many places in Hong Kong, necessitating the issuance of a gale or storm wind signal in November since 1972. The consideration in the issuance of the warning signal in Hong Kong and the difficulty in numerical weather prediction (NWP) model in forecasting Nalgae are also discussed in this article.

Weather, Vol. 79, no. 2, February 2024, pp. 38‐68

“…However, both to calculate the standardised anomaly and to estimate the event's rarity, one must assume that the observations from which we estimate it are representative of a much longer time period, and to convert it into a probability, we must further assume that the values fall into a OSI-450-a (v3.0, 2022;1978-2020 and OSI-430-a (v3.0, 2022;2021-2023 normal (Gaussian) distribution. These may not be sound assumptions.…”

“…In recent years, following the installation of a dropsonde system in the aircraft, it is rather rare to collect data directly in the atmospheric boundary layer by flying the aircraft into the TC. Instead, dropsondes provide vital information on the vertical profiles of cyclones, for example as stated in He et al (2021) regards the strengthening and weakening of TCs, and in Chan et al (2023) about the structure of a rare cyclone necessitating the issuance of gale to storm warning in Hong Kong during a late season (November) typhoon in this region.…”

“…Sea ice and atmospheric conditions over ocean grid points in February 2023 (left) and July 2023 (right). Panels (a) and (b): Sea ice concentration anomalies from the climate data recordsOSI-450-a (v3.0, 2022;1978-2020 andOSI-430-a (v3.0, 2022;2021-2023. The black line indicates the climatological median monthly sea ice edge (>15% concentration), the green line shows the monthly mean ice edge in 2023, and red and blue colours indicate negative and positive anomalies, respectively, compared to climatology for 1981-2010.…”

Challenges in the Forecasting of Severe Typhoon Koinu in 2023

“…Weak monsoon flow, with slightly cooler and drier air from the continent, may occasionally reach the coast of Southern China, but this is not yet strong enough to dominate in this region. The interaction between the northeast monsoon and a TC has long been a problem for TC warning in Hong Kong (e.g., in the case of Tropical Cyclone Nalgae in 2022, as documented in Chan et al [4]). On the one hand, cooler and drier continental air might cause weakening of the TC as it gets close to the South China coast.…”

Aircraft observations of Typhoon Nesat (2022) over the northern part of South China Sea

“…(2021) regards the strengthening and weakening of TCs, and in Chan et al . (2023) about the structure of a rare cyclone necessitating the issuance of gale to storm warning in Hong Kong during a late season (November) typhoon in this region.…”