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.

A STUDY OF THE INFLUENCE OF MICROPHYSICAL PROCESSES ON TYPHOON NIDA(2016) USING A NEW DOUBLE-MOMENT MICROPHYSICS SCHEME IN THE WEATHER RESEARCH AND FORECASTING MODEL

The basic structure and cloud features of Typhoon Nida(2016) are simulated using a new microphysics scheme(Liuma) within the Weather Research and Forecasting(WRF) model. Typhoon characteristics simulated with the Liuma microphysics scheme are compared with observations and those simulated with a commonly-used microphysics scheme(WSM6). Results show that using different microphysics schemes does not significantly alter the track of the typhoon but does significantly affect the intensity and the cloud structure of the typhoon. Results also show that the vertical distribution of cloud hydrometeors and the horizontal distribution of peripheral rainband are affected by the microphysics scheme. The mixing ratios of rain water and graupel correlate highly with the vertical velocity component and equivalent potential temperature at the typhoon eye-wall region. According to the simulation with WSM 6 scheme,it is likely that the very low typhoon central pressure results from the positive feedback between hydrometeors and typhoon intensity. As the ice-phase hydrometeors are mostly graupel in the Liuma microphysics scheme, further improvement in this aspect is required.

EXPERIMENTAL STUDY ON A DYNAMIC ASYMMETRICAL TYPHOON INITIALIZATION SCHEME BASED ON 4D-VAR

Axisymmetric bogus vortexes at sea level are usually used in the traditional bogus data assimilation （BDA） scheme. In the traditional scheme, the vortex could not accurately describe the specific characteristics of a typhoon, and the evolving real typhoon is forced to unreasonably adapt to this changeless vortex. For this reason, an asymmetrical typhoon bogus method with information blended from the analysis and the observation is put forward in this paper, in which the impact of the Subtropical High is also taken into consideration. With the fifth-generation Penn State/NCAR Mesoscale Model （MM5） and its adjoint model, a four-dimensional variational data assimilation （4D-Var） technique is employed to build a dynamic asymmetrical BDA scheme to assimilate different asymmetrical bogus vortexes at different time. The track and intensity of six surmner typhoons much influenced by the Subtropical High are simulated and the results are compared. It is shown that the improvement in track simulation in the new scheme is more significant than that in the traditional scheme. Moreover, the periods for which the track cannot be simulated well by the traditional scheme can be improved with the new scheme. The results also reveal that although the simulated typhoon intensity in the new scheme is generally weaker than that in the traditional scheme, this trend enables the new scheme to simulate, in the later period, closer-to-observation intensity than the traditional scheme. However, despite the fact that the observed intensity has been largely weakened, the simulated intensity at later periods of the BDA schemes is still very intensive, resulting in overly development of the typhoon during the simulation. The limitation to the simulation effect of the BDA scheme due to this condition needs to be further studied.

NUMERICAL SIMULATION STUDY ON THE RAPID INTENSIFICATION OF TYPHOON HAIKUI(1211) OFF THE SHORE OF CHINA

Forecasting the rapid intensification of tropical cyclones over offshore areas remains difficult. In this article,the Weather Research and Forecast(WRF) model was used to study the rapid intensification of Typhoon Haikui(1211)off the shore of China. After successful simulation of the intensity change and track of the typhoon, the model output was further analyzed to determine the mechanism of the rapid change in intensity. The results indicated that a remarkable increase in low-level moisture transportation toward the inner core, favorable large-scale background field with low-level convergence, and high-level divergence played key roles in the rapid intensification of Typhoon Haikui in which high-level divergence could be used as an indicator for the rapid intensity change of Typhoon Haikui approximately 6 h in advance. An analysis of the typhoon structure revealed that Typhoon Haikui was structurally symmetric during the rapid intensification and the range of the eyewall was small in the low level but extended outward in the high level. In addition, the vertically ascending motion, the radial and tangential along wind speeds increased with increasing typhoon intensity, especially during the process of rapid intensification. Furthermore, the intensity of the warm core of the typhoon increased during the intensification process with the warm core extending outward and toward the lower layer. All of the above structural changes contributed to the maintenance and development of typhoon intensity.

Statistical analysis of intensity variations in tropical cyclones in the East China Sea passing over the Kuroshio

Intensity variations of the SE-NW-oriented tropical cyclones(TC)in the East China Sea(ECS)passing over the Kuroshio are studied using multi-year high-resolution sea surface temperature data,the tropical cyclone data,and the global reanalysis dataset.The statistical results show that there are 81 TCs passing over the Kuroshio from the southeast in the East China Sea,over 68 years from 1949 to 2016.In terms of the change of the atmospheric pressure in the center of the TC,there are three categories:31 TCs are intensified,28 maintain their intensities,and 22 weakened.Significant seasonal differences are presented in the distribution.In the analysis on the intensified TCs,it is found that the TCs in the range where the center translational speed is from 1 m/s to 8 m/s and the distance from the Kuroshio main axis is from 10 km to 75 km are intensified more significantly.The analysis of three specific examples of TCs show that the stronger characteristics of the Kuroshio warm water in the ECS,the greater the intensity increase of such tropical cyclones.

Western North Pacific Tropical Cyclone Intensity Guidance Evaluations Using an Alternative Verification Technique

In this study,six intensity forecast guidance techniques from the East China Regional Meteorological Center are verified for the 2008 and 2009 typhoon seasons through an alternative forecast verification technique.This technique is used to verify intensity forecasts if those forecasts call for a typhoon to dissipate or if the real typhoon dissipates.Using a contingency table,skill scores,chance,and probabilities are computed.It is shown that the skill of the six tropical cyclone intensity guidance techniques was highest for the 12-h forecasts,while the lowest skill of all the six models did not occur in 72-h forecasting.For both the 2008 and 2009 seasons,the average probabilities of the forecast intensity having a small error(6 m s-1) tended to decrease steadily.Some of the intensity forecasts had small skill scores,but the associated probabilities of the forecast intensity errors > 15 m s-1 were not the highest.

Track of Super Typhoon Haiyan Predicted by a Typhoon Model for the South China Sea

Super Typhoon Haiyan was the most notable typhoon in 2013. In this study, results from the operational prediction of Haiyan by a tropical regional typhoon model for the South China Sea are analyzed. It is shown that the model has successfully reproduced Haiyan’s rapid passage through the Philippines and its northward deflection after its second landfall in Vietnam. However, the predicted intensity of Haiyan is weaker than the observed. An analysis of higher-resolution model simulations indicates that the storm is characterized by an upper-level warm core during its mature stage and a deep layer of easterly flow. Sensitivity experiments are conducted to study the impact of certain physical processes such as the interaction between stratus and cumulus clouds on the improvement of the typhoon intensity forecast. It is found that appropriate boundary layer and cumulus convective parameterizations, and orographic gravity-wave parameterization, as well as improved initial conditions and increased horizontal grid resolution, all help to improve the intensity forecast of Haiyan.

Lightning Activity and Its Relationship with Typhoon Intensity and Vertical Wind Shear for Super Typhoon Haiyan (1330)

Super Typhoon Halyan （1330）, which occurred in 2013, is the most powerful typhoon during landfall in the meteorological record. In this study, the temporal and spatial distributions of lightning activity of Haiyan were analyzed by using the lightning data from the World Wide Lightning Location Network, typhoon intensity and position data from the China Meteorological Administration, and horizontal wind data from the ECMWF. Three distinct regions were identified in the spatial distribution of daily average lightning density, with the maxima in the inner core and the minima in the inner rainband. The lightning density in the intensifying stage of Haiyan was greater than that in its weakening stage. During the time when the typhoon intensity measured with maximum sustained wind speed was between 32.7 and 41.4 m s-1, the storm had the largest lightning density in the inner core, compared with other intensity stages. In contrast to earlier typhoon studies, the eyewall lightning burst out three times. The first two eyewall lightning outbreaks occurred during the period of rapid intensification and before the maximum intensity of the storm, suggesting that the eyewall lightning activity could be used to identify the change in tropical cyclone intensity. The flashes frequently occurred in the inner core, and in the outer rainbands with the black body temperature below 220 K. Combined with the ECMWF wind data, the influences of vertical wind shear （VWS） on the azimuthal distribution of flashes were also analyzed, showing that strong VWS produced downshear left asymmetry of lightning activity in the inner core and downshear right asymmetry in the ralnbands.

Impact of Cloud Microphysical Processes on the Simulation of Typhoon Rananim near Shore. Part II： Typhoon Intensity and Track

The impact of cloud microphysical processes on the simulated intensity and track of Typhoon Rananim is discussed and analyzed in the second part of this study. The results indicate that when the cooling effect due to evaporation of rain water is excluded, the simulated 36-h maximum surface wind speed of Typhoon Rananim is about 7 m s-1 greater than that from all other experiments; however, the typhoon landfall location has the biggest bias of about 150 km against the control experiment. The simulated strong outer rainbands and the vertical shear of the environmental flow are unfavorable for the deepening and maintenance of the typhoon and result in its intensity loss near the landfall. It is the cloud microphysical processes that strengthen and create the outer spiral rainbands, which then increase the local convergence away from the typhoon center and prevent more moisture and energy transport to the inner core of the typhoon. The developed outer rainbands are supposed to bring dry and cold air mass from the middle troposphere to the planetary boundary layer （PBL）. The other branch of the cold airflow comes from the evaporation of rain water itself in the PBL while the droplets are falling. Thus, the cut-off of the warm and moist air to the inner core and the invasion of cold and dry air to the eyewall region are expected to bring about the intensity reduction of the modeled typhoon. Therefore, the deepening and maintenance of Typhoon Rananim during its landing are better simulated through the reduction of these two kinds of model errors.

Application of Lightning Data Assimilation to Numerical Forecast of Super Typhoon Haiyan (2013)

Previous observations from World Wide Lightning Location Network(WWLLN) and satellites have shown that typhoon-related lightning data have a potential to improve the forecast of typhoon intensity. The current study was aimed at investigating whether assimilating TC lightning data in numerical models can play such a role. For the case of Super Typhoon Haiyan in 2013, the lightning data assimilation(LDA) was realized in the Weather Research and Forecasting(WRF) model, and the impact of LDA on numerical prediction of Haiyan’s intensity was evaluated.Lightning data from WWLLN were used to adjust the model’s relative humidity(RH) based on the method developed by Dixon et al.(2016). The adjusted RH was output as a pseudo sounding observation, which was then assimilated into the WRF system by using the three-dimensional variational(3DVAR) method in the cycling mode at 1-h intervals. Sensitivity experiments showed that, for Super Typhoon Haiyan(2013), which was characterized by a high proportion of the inner-core(within 100 km from the typhoon center) lightning, assimilation of the inner-core lightning data significantly improved its intensity forecast, while assimilation of the lightning data in the rainbands(100–500 km from the typhoon center) led to no obvious improvement. The improvement became more evident with the increase in LDA cycles, and at least three or four LDA cycles were needed to achieve obvious intensity forecast improvement. Overall, the improvement in the intensity forecast by assimilation of the inner-core lightning data could be maintained for about 48 h. However, it should be noted that the LDA method in this study may have a negative effect when the simulated typhoon is stronger than the observed, since the LDA method cannot suppress the spurious convection.