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.

Water Vapor,Cloud,and Surface Rainfall Budgets Associated with the Landfall of Typhoon Krosa(2007):A Two-Dimensional Cloud-Resolving Modeling Study

Water vapor, cloud, and surface rainfall budgets associated with the landfall of Typhoon Krosa on 6-8 October 2007 are analyzed based on a two-dimensional cloud-resolving model simulation. The model is integrated with imposed zonally-uniform vertical velocity, zonal wind, horizontal temperature, and vapor advection from NCEP/Global Data Assimilation System （GDAS） data. The simulation data that are validated with observations are examined to study physical causes associated with surface rainfall processes during the landfall. The time- and domain-mean analysis shows that when Krosa approached the eastern coast of China on 6 October, the water vapor convergence over land caused a local atmospheric moistening and a net condensation that further produced surface rainfall and an increase of cloud hydrometeor concentration. Meanwhile, latent heating was balanced by advective cooling and a local atmospheric warming. One day later, the enhancement of net condensation led to an increase of surface rainfall and a local atmospheric drying, while the water vapor convergence weakened as a result of the landfall-induced deprivation of water vapor flux. At the same time, the latent heating is mainly compensated the advective cooling. Further weakening of vapor convergence on 8 October enhanced the local atmospheric drying while the net condensation and associated surface rainfall was maintained. The latent heating is balanced by advective cooling and a local atmospheric cooling.

Impacts of TRMM SRR assimilation on the numerical prediction of tropical cyclone

Impacts of the four-dimensional variation （4DVAR） assimilation of Tropical Rainfall Measuring Mission （TRMM）/TMI surface rainfall rate （SRR） on the initialization and simulation of TC Danas in 2001 are studied. A number of sensitivity experiments are designed to evaluate the performance of assimilation during the particular stage when Danas developed from a tropical depression towards a typhoon, as well as an abrupt turning. Under the dynamic and thermodynamic constraints of meso-scale model and the 4DVAR, the TRMM SRR data are directly assimilated in high horizontal resolution （ 18 kin）. On the basis of the adjustment of initial model field, 4DVAR of TRMM SRR considerably improved the depiction of TC＇ s structure and rainfall. The same explanation can also be given to the enhancement of simulation on TC＇ s intensity and track. In comparison with the popular idea of twice steps of SRR assimilation, namely, 4DVAR pins 1DVAR, the retrieval error is expected to be avoided in the procedure of direct 4DVAR. In addition, the TRMM SRR 4DVAR within a high-resolution model can take into account as much information as observed and is expected to be performed in operational use if the observed rainfall data are routinely available.

A Modified Moist Ageostrophic Q Vector

The quasi-geostrophic Q vector is an important rainfall associated with large-scale weather systems diagnostic tool for studying development of surface and is calculated using data at single vertical level. When ageostrophic Q vector was introduced, it required data at two vertical levels. In this study, moist ageostrophic Q vector is modified so that it can be calculated using data at a single vertical level. The comparison study between the original and modified moist ageostrophic Q vectors is conducted using the data from 5 to 6 July 1991 during the torrential rainfall event associated with the Changjiang-Huaihe mei-yu front in China. The results reveal that divergences of original and modified moist ageostrophic Q vectors have similar horizontal distributions and their centers are almost located in the precipitation centers. This indicates that modified moist ageostrophic Q vector can be used to diagnose convective development with reasonable accuracy.

IMPACT OF SEA SPRAY ON TROPICAL CYCLONE STRUCTURE AND INTENSITY CHANGE

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.

NUMERICAL SIMULATION ON RE-INTENSIFICATION OF TROPICAL REMNANT RE-ENTERING THE SEA:A CASE STUDY

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.

EVALUATING THE PERFORMANCE OF WESTERN NORTH PACIFIC TROPICAL CYCLONE INTENSITY GUIDANCE. PART Ⅱ:INTENSITY FORECAST ACCURACY IN DIFFERENT LIFE STAGES

The accuracy of eight tropical cyclone(TC) intensity guidance techniques currently used in the East China Regional Meteorological Center during the 2008 and 2009 western North Pacific seasons has been evaluated within two intensity phases:intensification and decay.In the intensification stage,the majority of the techniques indicated > 60% probabilities of the errors of forecast 12-h intensity change within ±5 ms-1 from the 12- to 60-h forecast intervals,while none had capability to predict the rapid intensification and most of them had a bias toward smaller 48-h intensity changes at the beginning of the stage.The majority of the guidance techniques showed > 70% probabilities of the errors of forecast 12-h intensity change within ±5 ms-1 through 60 h during the decay phase,and the techniques had little capability of predicting rapid decay events.It is found that the evaluated statistical models had difficulty in predicting the strongest cases of decay 36 h after peak intensity,whereas the dynamical and official forecasts were seemingly able to produce some large decay rates.

Q Vector Analysis of Torrential Rainfall from Meiyu Front Cyclone:A Case Study

Following similar derivation of quasi-geostrophic Q vector （Q^C）, a new Q vector （Q^N） is constructed in this study. Their difference is that the geostrophic wind in quasi-geostrophic Q vector is replaced by the wind in Q^N vector. The diagnostic analysis of Q^N vector is compared with that of Q^G vector in the case study of a typical Meiyu front cyclone （MYFC） occurred over Changjiang-Huaihe regions during 5-6 July 1991. The results show that the Q^N vector has more diagnostic advantages than Q^G vector does. Convergence of Q^N vector at 700 hPa is found to be a good indicator to mimic the horizontal distribution of precipitation. Q^N vector is further partitioned into four components： Q^Nalst （along-stream stretching）,Q^Ncurv （curvature）,Q^Nshdv （shear advection）, and Q^Ncrst （cross-stream stretching） in a natural coordinate system with isohypse （PG partitioning）. The application of Q^N PG partitioning in the MYFC torrential rain indicates that PG partitioning of Q can identify dominant physical processes. The horizontal distribution of 2V·Q^Nalst is similar to that of 2V·Q^N and mainly accounts for 2V·Q^N during the entire period of Meiyu. The effects of Q^Ncurv on rainfall enhancement fade from the mature stage to decay stage. Qshdv enhances precipitation significantly as the MYFC develops, and the effect weakens rapidly when the MYFC decays during its eastward propagation. Q^Ncrst shows little impacts on rainfall during the onset and mature phases whereas it displays significant role during the decay phase.Q^N alst and Q^Nshdv and Q^Ncrst show cancellation only during the decay period.