The Effect of Typhoon-Induced SST Cooling on Typhoon Intensity: The Case of Typhoon Chanchu (2006)

In order to investigate air-sea interactions during the life cycle of typhoons and the quantificational effects of typhoon-induced SST cooling on typhoon intensity, a mesoscale coupled air-sea model is developed based on the non-hydrostatic mesoscale model MM5 and the regional ocean model POM, which is used to simulate the life cycle of Typhoon Chanchu （2006） from a tropical depression to a typhoon followed by a steady weakening. The results show that improved intensity prediction is achieved after considering typhoon-induced SST cooling; the trend of the typhoon intensity change simulated by the coupled model is consistent with observations. The weakening stage of Typhoon Chanchu from 1200 UTC 15 May to 1800 UTC 16 May can be well reproduced, and it is the typhoon-induced SST cooling that makes Chanchu weaken during this period. Analysis reveals that the typhoon-induced SST cooling reduces the sensible and latent heat fluxes from the ocean to the typhoon＇s vortex, especially in the inner-core region. In this study, the average total heat flux in the inner-core region of the typhoon decrease by 57.2%, whereas typhoon intensity weakens by 46%. It is shown that incorporation of the typhoon-induced cooling, with an average value of 2.17℃, causes a 46-hPa weakening of the typhoon, which is about 20 hPa per 1℃ change in SST.

The Impact of Storm-Induced SST Cooling on Storm Size and Destructiveness: Results from Atmosphere-Ocean Coupled Simulations

In this study, both an atmospheric model [Weather Research and Forecasting(WRF) model] and an atmosphere(WRF)–ocean(Princeton Ocean Model;POM) coupled model are used to simulate the tropical cyclone(TC) Kaemi(2006). By comparing the simulation results of the models, effects of oceanic elements, especially the TC-induced sea surface temperature(SST) cooling, on the simulated TC size and destructiveness are identified and analyzed. The results show that there are no notable differences in the simulated TC track and its intensity between the uncoupled and coupled experiments;however, there are large differences in the TC size(i.e., the radius of gale-force wind)between the two experiments, and it is the TC-induced SST cooling that decreases the TC size. The SST cooling contributes to the decrease of air–sea moisture difference(ASMD) outside the TC eyewall, which subsequently leads to the decreases in surface enthalpy flux(SEF), radial sea-level pressure gradient, absolute vorticity advection, and wind speed outside the TC eyewall. As a result, the TC size and size-dependent TC destructive potential all decrease remarkably.

Parameterizing sea surface temperature cooling induced by tropical cyclones using a multivariate linear regression model

Combining a linear regression and a temperature budget formula, a multivariate regression model is proposed to parameterize and estimate sea surface temperature（SST） cooling induced by tropical cyclones（TCs）. Three major dynamic and thermodynamic processes governing the TC-induced SST cooling（SSTC）, vertical mixing, upwelling and heat flux, are parameterized empirically using a combination of multiple atmospheric and oceanic variables：sea surface height（SSH）, wind speed, wind curl, TC translation speed and surface net heat flux. The regression model fits reasonably well with 10-year statistical observations/reanalysis data obtained from 100 selected TCs in the northwestern Pacific during 2001–2010, with an averaged fitting error of 0.07 and a mean absolute error of 0.72°C between diagnostic and observed SST cooling. The results reveal that the vertical mixing is overall the pre dominant process producing ocean SST cooling, accounting for 55% of the total cooling. The upwelling accounts for 18% of the total cooling and its maximum occurs near the TC center, associated with TC-induced Ekman pumping. The surface heat flux accounts for 26% of the total cooling, and its contribution increases towards the tropics and the continental shelf. The ocean thermal structures, represented by the SSH in the regression model,plays an important role in modulating the SST cooling pattern. The concept of the regression model can be applicable in TC weather prediction models to improve SST parameterization schemes.

Air-sea Interaction of Typhoon Sinlaku (2002) Simulated by the Canadian MC2 Model

Three experiments for the simulation of typhoon Sinlaku （2002） over the western North Pacific are performed in this study by using the Canadian Mesoscale Compressible Community （MC2） atmospheric model. The objective of these simulations is to investigate the air-sea interaction during extreme weather conditions, and to determine the sensitivity of the typhoon evolution to the sea surface temperature （SST） cooling induced by the typhoon. It is shown from the three experiments that the surface heat fluxes have a substantial influence on the slow-moving cyclone over its lifetime. When the SST in the East China coastal ocean becomes 1℃ cooler in the simulation, less latent heat and sensible heat fluxes from the underlying ocean to the cyclone tend to reduce the typhoon intensity. The cyclone is weakened by 7 hPa at the time of its peak intensity. The SST cooling also has impacts on the vertical structure of the typhoon by weakening the warm core and drying the eye wall. With a finer horizontal resolution of （1/6）° × （1/6）°, the model produces higher surface wind, and therefore more surface heat fluxes are emitted from the ocean surface to the cyclone, in the finer-resolution MC2 grid compared with the relatively lower resolution of 0.25° × 0.25° MC2 grid.

Effect of wind-current interaction on ocean response during Typhoon KAEMI(2006)

The Weather Research and Forecasting （WRF） model, the Princeton Ocean Model （POM）, and the wave model （WAVEWATCH III） are used to develop a coupled atmosphere-wave-ocean model, which involves different physical pro- cesses including air-forcing, ocean feedback, wave-induced mixing and wave-current interaction. In this paper, typhoon KAEMI （2006） has been examined to investigate the effect of wind-current interaction on ocean response based on the coupled atmosphere-ocean-wave model, i.e., considering the sea surface currents in the calculation of wind stress. The results show that the wind-current interaction has a noticeable impact on the simulation of 10 m-winds. The model involving the effect of the wind-current interaction can dramatically improve the typhoon prediction. The wind-current interaction prevents excessive momentum fluxes from being transferred into the upper ocean, which contributes to a much smaller turbulence kinetic energy （TKE）, vertical diffusivity, and horizontal advection and diffusion. The Sea Surface Temperature （SST） cooling induced by the wind-current interaction during the initial stage of typhoon development is so minor that the typhoon intensity is not very sen- sitive to it. When the typhoon reaches its peak, its winds can disturb thermocline, and the cold water under the thermocline is pumped up. However, this cooling process is weakened by the wind-current interaction, as ocean feedback delays the decay of the typhoon. Meanwhile, the temperature below the depth of 30 m shows an inertial oscillation with a period about 40 hours （-17°N） when sudden strong winds beat on the ocean. Due to faster currents, the significant wave height decreases as ignoring the wind-current interaction, while this process has a very small effect on the dominant wave length.

Study of the Air-Sea Interaction During Typhoon Kaemi (2006)

The high-resolution Weather Research and Forecasting （WRF） model is coupled to the Princeton Ocean Model （POM） to investigate the effect of air-sea interaction during Typhoon Kaemi that formed in the Northwest Pacific at 0000 UTC 19 July 2006. The coupled model can reasonably reproduce the major features of ocean response to the moving tropical cyclone （TC） forcing, including the deepening of ocean mixed layer （ML）, cooling of sea surface temperature （SST）, and decaying of typhoon. Due to the appearance of maximum SST cooling to the left of the simulated typhoon track, two points respectively located to the left （16.25 N, 130.1 E, named as A, the maximum SST cooling region） and right （17.79 N, 130.43 E, named as B） of the typhoon track are taken as the sampling points to study the mechanisms of SST cooling. The low temperature at point A has a good correlation with the 10-m winds but does not persist for a long time, which illustrates that the temperature dropping produced by upwelling is a quick process. Although the wind-current resonance causes oscillations to the left of typhoon track at point A, the fluctuation is not so strong as that at point B. The thin ML and upwelling produced by the Ekman pumping from strong 10-m winds are the main reason of maximum SST cooling appearing to the left of the typhoon track. Due to weaker 10-m winds and thicker and warmer ML at point B, the colder water under the thermocline is surpressed and the temperature dropping is not dramatic when the strongest 10-m winds occur. Afterwards, the temperature gradually decreases, which is found to be caused by the inertial oscillations of the wind-current system.