Dropsonde observations and numerical simulations of intensifying/weakening tropical cyclones over the northern South China Sea

Introduction Since 2016, the Hong Kong Observatory (HKO) has been arranging dropsonde flights by a fixed-wing aircraft of the Government Flying Service (GFS) for the survey of tropical cyclones over the northern South China Sea (Chan et al., 2018). The aircraft has to fly within the Hong Kong Flight Information Region and due to this spatial limitation, it is not always possible to launch dropsondes on all quadrants of any tropical cyclone. The information obtained by the dropsondes is useful in the real-time assessment of the intensity of a cyclone and the spatial distribution of its winds, for example, wind radii near the surface. The data obtained are exchanged internationally for use in Numerical Weather Prediction (NWP) models worldwide. This paper examines the intensification and/or weakening of selected tropical cyclones over the northern South China Sea based on dropsonde measurements. The dropsonde data are three-dimensional and, as well as recording wind data, they also contain thermodynamic variables. In this paper, the inflow/outflow at various heights of the tropical cyclones are considered, and the vertical profiles of potential temperature and equivalent potential temperature are calculated. Three typical tropical cyclone cases are considered, namely, intensifying cyclones like Mangkhut in 2018 and Higos in 2020, as well as the weakening tropical cyclone Khanun in 2017. It is hoped that dropsonde data could be used more extensively in the real-time assessment of the development and intensity change of future tropical cyclones. This paper is a follow-up of the previous case study by He et al. (2020) with the novelty of the consideration of thermodynamic profiles and the case study of the rapidly intensifying Cyclone Higos in 2020. Dropsondes have been used extensively for the study and forecasting of tropical cyclones over the Atlantic Ocean as conducted by the US and over the western North Pacific as conducted by Taiwan. For instance, the boundary layer structure of hurricanes in Atlantic was studied extensively in Zhang et al. (2013) and Ren et al. (2020), and axisymmetric tropical cyclone boundary layer structure was analysed by compositing dropsonde data from multiple tropical cyclones. However, these studies concern the Atlantic basin only. In this work, for the first time, we study the boundary layer of tropical cyclones over the South China Sea, which is an active tropical cyclone region (Yuan et al., 2007; Yang et al., 2015; Chow et al., 2019). Dropsondes are expected to provide far more information than, say, a land-based wind profiler, especially in terms of the strengthening and weakening of tropical cyclones, such as studied in Liao et al. (2019). The relationship between tropical cyclone intensification and the equivalent potential temperature profile has been reported in some numerical experiments before, for example in Raju et al. (2011). Some previous studies also used dropsonde observations to evaluate the numerical model performance, for example Nolan et al. (2009), Zhu et al. (2014), and Zhang et al. (2020), but most of them have focused on Atlantic hurricanes. The present paper supplements such studies by providing observations from the dropsondes for tropical cyclones over the South China Sea.