Abstract
A set of numerical simulations is relied upon to evaluate the impact of air-sea interactions on the behaviour of tropical cyclone (TC) Bejisa (2014), using various configurations of the coupled ocean-atmosphere numerical system Meso-NH-NEMO. Uncoupled (SST constant) as well as 1D (use of a 1D ocean mixed layer) and 3D (full 3D ocean) coupled experiments are conducted to evaluate the impact of the oceanic response and dynamic processes, with emphasis on the simulated structure and intensity of TC Bejisa. Although the three experiments are shown to properly capture the track of the tropical cyclone, the intensity and the spatial distribution of the sea surface cooling show strong differences from one coupled experiment to another. In the 1D experiment, sea surface cooling (∼1 °C) is reduced by a factor 2 with respect to observations and appears restricted to the depth of the ocean mixed layer. Cooling is maximized along the right-hand side of the TC track, in apparent disagreement with satellite-derived sea surface temperature observations. In the 3D experiment, surface cooling of up to 2.5 °C is simulated along the left hand side of the TC track, which shows more consistency with observations both in terms of intensity and spatial structure. In-depth cooling is also shown to extend to a much deeper depth, with a secondary maximum of nearly 1.5 °C simulated near 250 m. With respect to the uncoupled experiment, heat fluxes are reduced from about 20% in both 1D and 3D coupling configurations. The tropical cyclone intensity in terms of occurrence of 10-m TC wind is globally reduced in both cases by about 10%. 3D-coupling tends to asymmetrize winds aloft with little impact on intensity but rather a modification of the secondary circulation, resulting in a slight change in structure.
Highlights
According to World Meteorological Organization’s Ninth International Workshop on Tropical Cyclones (IWTC-9, 2018), the recent deployment of new high space-time resolution geostationary and low-earth orbiting satellites over open oceans, together with ongoing improvements in numerical weather prediction (NWP) models have substantially improved the prediction of tropical cyclone (TC) tracks in all TC basins [1,2]
TC tracks simulated in the three experiments (Figure 3a) are relatively similar and are close to the best-track estimate analyzed by the regional specialized meteorologicval center (RSMC) La Réunion, despite a slight westward error in TC position
A translation speed of 5 m s−1 seems to be a threshold below which the differences in cooling between a 1D and a 3D ocean model are greater compared to the differences with a faster translation speed [11,16]
Summary
According to World Meteorological Organization’s Ninth International Workshop on Tropical Cyclones (IWTC-9, 2018), the recent deployment of new high space-time resolution geostationary and low-earth orbiting satellites over open oceans, together with ongoing improvements in numerical weather prediction (NWP) models have substantially improved the prediction of tropical cyclone (TC) tracks in all TC basins [1,2]. The energy required for genesis, sustenance and intensification of a TC primarily originates from the ocean through sea surface temperature (SST) and air-sea fluxes of heat and moisture e.g., [5]. These interactions have recently been shown to play a key role in the cold wake properties at the sea surface, which can impact the intensity of the cyclone itself, and influence the structure of the ocean-atmosphere coupled system for up to several weeks [6]. While the first (resp. second) process could be represented by using a slab ocean model (resp. a 1D mixed-layer model), a three-dimensional model is required to properly represent all three processes
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