Typhoon Vicente(2012) underwent rapid intensification(RI) within 24 h before landfall in China's Mainland. Analysis of the large-scale environment and characteristics of Vicente identifies the aforementioned intensification as classic RI. The process occurred in an environmental flow with a deep-layer shear ranging from 5 ms-1 to 8 ms-1. Convection caused by persistent vertical shear forcing of the vortex was observed primarily in the downshear left quadrant of the storm. However, radar and satellite observations indicate that the northern convection of the inner core of Vicente quickly developed in the down-shear right three hours near landfall.
In this paper,the effects of sea spray on tropical cyclone(TC)structure and intensity variation are evaluated through numerical simulations using an advanced sea-spray parameterization from the National Oceanic and Atmospheric Administration/Earth System Research Laboratory(NOAA/ESRL),which is incorporated in the idealized Advanced Research version of the Weather Research and Forecast (WRF-ARW)model.The effect of sea spray on TC boundary-layer structure is also analyzed.The results show that there is a significant increase in TC intensity when its boundary-layer wind includes the radial and tangential winds,their structure change,and the total surface wind speed change.Diagnosis of the vorticity budget shows that an increase of convergence in TC boundary layer enhances TC vorticity due to the dynamic effect of sea spay.The main kinematic effect of the friction velocity reduction by sea spray produces an increment of large-scale convergence in the TC boundary layer,while the radial and tangential winds significantly increase with an increment of the horizontal gradient maximum of the radial wind, resulting in a final increase in the simulated TC intensity.The surface enthalpy flux enlarges TC intensity and reduces storm structure change to some degree,which results in a secondary thermodynamic impact on TC intensification.Implications of the new interpretation of sea-spray effects on TC intensification are also discussed.
When Typhoon Toraji(2001)reached the Bohai Gulf during 1-2 August 2001,a heavy rainfall event occurred over Shandong province in China along the gulf.The Advanced Research version of the Weather Research and Forecast(WRF-ARW)model was used to explore possible effects of environmental factors,including effects of moisture transportation,upper-level trough interaction with potential vorticity anomalies,tropical cyclone(TC)remnant circulation,and TC boundary-layer process on the re-intensification of Typhoon Toraji,which re-entered the Bohai Gulf after having made a landfall.The National Centers for Environmental Prediction(NCEP)global final(FNL)analysis provided both the initial and lateral boundary conditions for the WRF-ARW model.The model was initialized at 1200 UTC on 31 July and integrated until 1200 UTC on 3 August 2001,during which Toraji remnant experienced the extratropical transition and re-intensification.Five numerical experiments were performed in this study,including one control and four sensitivity experiments.In the control experiment,the simulated typhoon had a track and intensity change similar to those observed.The results from three sensitivity experiments showed that the moisture transfer by a southwesterly lower-level jet,a low vortex to the northeast of China,and the presence of Typhoon Toraji all played important roles in simulated heavy rainfall over Shandong and remnant re-intensification over the sea,which are consistent with the observation.One of the tests illustrated that the local boundary layer forcing played a secondary role in the TC intensity change over the sea.
Tropical cyclones(TCs) Lionrock,Kompasu,and Namtheun were formed successively within 40 hours in 2010.Over the next several days afterwards,these TCs exhibited unusual movements which made operational prediction difficult.Verifications are performed on the forecasts of the tracks of these TCs with six operational models,including three global and three regional models.Results showed that the trends of TC tracks could be reproduced by these models,whereas trajectory turning points and landfall locations were not simulated effectively.The special track of Lionrock should be associated with its direct interaction with Namtheun,according to a conceptual model of binary TC interaction.By contrast,the relation between Kompasu and Namtheun satisfied the criteria for a semi-direct interaction.Numerical experiments based on the Global and Regional Assimilation and Prediction System-Tropical Cyclone forecast Model(GRAPES-TCM) further confirmed the effect of Namtheun on the unusual tracks of Lionrock and Kompasu.Finally,the physical mechanism of binary TC interaction was preliminarily proposed.
This study utilized the MM5 mesoscale model to simulate the landfalling process of Typhoon Talim.The simulated typhoon track,weather patterns,and rainfall process are consistent with the observation.Using the simulation results,the relation of the second type thermal helicity(H2) to rainfall caused by the landfalling typhoon Talim was analyzed.The results show that H2 could well indicate the heavy inland rainfall but it did not perform as well as the helicity in predicting rainfall during the beginning stage of the typhoon landfall.In particular,H2 was highly correlated with rainfall of Talim at 1-h lead time.For 1-5-h lead time,it also had a higher correlation with rainfall than the helicity did,and thus showing a better potential in forecasting rainfall intensification.Further analyses have shown that when Talim was in the beginning stage of landfall,1) the 850-200-hPa vertical wind shear around the Talim center was quite small(about 5 m s-1);2) the highest rainfall was to the right of the Talim track and in the area with a 300-km radius around the Talim center,exhibiting no obvious relation to low-level temperature advection,low-level air convergence,and upper-level divergence;3) the low-level relative vorticity reflected the rainfall change quite well,which was the main reason why helicity had a better performance than H2 in this period.However,after Talim moved inland further,1) it weakened gradually and was increasingly affected by the northern trough;2) the vertical wind shear was enhanced as well;3) the left side of the down vertical wind shear lay in the Lushan and Dabieshan mountain area,which could have contributed to triggering a secondary vertical circulation,helping to produce the heavy rainfall over there;hence,H2 showed a better capacity to reflect the rainfall change during this stage.
This study introduces a new dynamical quantity, shear gradient vorticity (SGV), which is defined as vertical wind shear multiplying the horizontal component of vorticity gradient, aiming to diagnose heavy precipitation induced by some strong convective weather systems. The vorticity gradient component can be used to study the collision or merging process between different vortexes or the deformation of a vortex with a sharp vorticity gradient. Vertical wind shear, another contributed component of SGV, always represents the environmental dynamical factor in meteorology. By the combined effect of the two components, overall, SGV can represent the interaction between the environmental wind shear and the evolution of vortexes with a large vorticity gradient. Other traditional vorticity-like dynamical quantities (such as helicity) have the limitation in the diagnosis of the convection, since they do not consider the vorticity gradient. From this perspective, SGV has the potential to diagnose some strong convective weather processes, such as Extratropical Transition (ET) of tropical cyclones and the evolution of multicell storms. The forecast performance of SGV for the numerical ET case of Typhoon Toraji (0108) has been evaluated. Compared with helicity, SGV has shown a greater advantage to forecast the distribution of heavy precipitation more accurately, especially in the frontal zone.
