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.
To examine effects of sea spray evaporation and dissipative heating on structure and intensity of a real tropical cyclone, the sea spray flux parameterization scheme was incorporated into the fifth-generation Penn- sylvania State University National Center for Atmospheric Research Mesoscale Model （MM5）. Sensitivity tests were performed with varying the spray source function intensities and with and without dissipation heating. The numerical results indicate that sea spray evaporation increases the interfaeial sensible heat flux, which is increased by 16% for the moderate spray and 47% for the heavy spray, but has little effect on the interfaeial latent heat flux. The net effect of sea spray evaporation is to decrease the total sensible heat flux and to increase the total latent heat flux. The total enthalpy flux is increased by 1% and 12% with moderate and strong spray amounts, respectively. Consistent with these results, the intensity of the tropical cyclone is increased by 5% and 16% in maximum 10-m wind speed, respectively, due to sea spray evapora- tion. Sea spray evaporation and dissipative heating modify the tropical cyclone structure in important but complex ways. The effect of sea spray on the near-surface temperature and moisture depends on the spray amounts and its location within the tropical cyclone. Within the high-wind region of a tropical cyclone, the lower atmosphere becomes cooler and moister due to the evaporation of sea spray. However, the dissipative heating offsets the cooling due to sea spray evaporation, which makes the lower atmosphere warmer.
Traditional variational data assimilation(VDA) with only one regularization parameter constraint cannot produce optimal error tuning for all observations. In this paper, a new data assimilation method of "four dimensional variational data assimilation(4D-Var) with multiple regularization parameters as a weak constraint(Tikh-4D-Var)" is proposed by imposing different regularization parameters for different observations. Meanwhile, a new multiple regularization parameters selection method, which is suitable for actual high-dimensional data assimilation system, is proposed based on the posterior information of 4D-Var system. Compared with the traditional single regularization parameter selection method, computation of the proposed multiple regularization parameters selection method is smaller. Based on WRF3.3.1 4D-Var data assimilation system, initialization and simulation of typhoon Chaba(2010) with the new Tikh-4D-Var method are compared with its counterpart 4D-Var to demonstrate the effectiveness of the new method. Results show that the new Tikh-4D-Var method can accelerate the convergence with less iterations. Moreover, compared with 4D-Var method, the typhoon track, intensity(including center surface pressure and maximum wind speed) and structure prediction are obviously improved with Tikh-4D-Var method for 72-h prediction. In addition, the accuracy of the observation error variances can be reflected by the multiple regularization parameters.
An integrated vertical-slantwise convective parameterization scheme, based on the vertical Kuo-Anthes and the slantwise Nordeng convective parameterization schemes, is introduced into the MM5 model. By employing the MM5 model with the proposed scheme, numerical simulations of a snowstorm event that occurred over southern China on 28-29 January 2008 and of Typhoon Haitang （2005） are conducted. The results indicate that during the snowstorm event, the atmosphere was convectively stable in the vertical direction but with conditional symmetric instability （CSI） in the lower troposphere, and when the area of CSI developed and extended to upper levels, strong rising motion occurred and triggered the release of large amount of energy, producing enhanced convective precipitation with the total precipitation much closer to the observation. The development and strengthening of CSI corresponded to changes in the intensity of snowfall, convergence, and ascending motions of air, revealing that CSI was responsible for the initiation and growth of the snowstorm. The results from a 72-h explicit simulation of Typhoon Haitang indicate that CSI occurred mainly at lower levels with a well-defined spiral structure, and it tended to have a larger impact on the intensity of typhoon than on its track. The minimum pressure at the typhoon center for the 72-h runs with the integrated vertical-slantwise convective parameterization scheme was on average 3 hPa （maximum 8 hPa） lower than that from the runs with only the vertical cumulus parameterization scheme. Introducing the influence of CSI into the model has improved the warm core structure at the middle and upper levels of the typhoon, with stronger and persistent upward motions causing increased precipitation, and the latent heat released through convection in turn made the typhoon develop further.
Significant anomalous tracks were observed when the severe tropical storm Goni （0907） and typhoon Morakot （0908） in September 2009 were evaluated in short distances. The relationship between the two is regarded as a case of binary interaction. Based on an MM5 model （fifth=generation mesoscale model of Pennsylvania State University-National Center for Atmospheric Research）, in this study a series of sensitivity experiments were designed to determine the binary interaction between them. The sensitivity of the storm characteristics to the binary interaction was demonstrated through modeling experiments with different TC intensities and sizes based on the bogus vortices initialization. Furthermore, the contributions of large-scale environmental flow and the effects of interaction between the motions of the cyclones were distinguished by numerical experiments using only one of the TC vortices. Results from these experiments show that Morakot （0908） had a greater impact on the motion of Goni （0907）, whereas Goni （0907） had a relatively limited impact on Morakot （0908）. At the upper level, the northeasterly jet flow in the third quadrant of Morakot （0908） enhanced the upper-level divergence of Goni （0907） and had an important role in maintaining and increasing Goni＇s （0907） intensity. And at the lower level, Morakot （0908）, with strong convergence and ascending airflow, made a stable transport channel of southwesterly warm and wet flow, thus supporting the lower-level water vapor convergence of Goni （0907）. Goni （0907）, which was located upriver of the southwesterly flow, intercepted part of the water vapor transportation in the southwesterly flow, causing the water vapor convergence to strengthen while the water vapor convergence of Morakot （0908） weakened.
