Research On Climatic Characteristics And Wind Field Calculatiion Of Tropical Cyclones Over Western North Pacific

Based on tropical cyclones data and NCEP reanalysis data, the basic climatic characteristics, such as frequency, intensity, genesis region and period of tropical cyclones (TC) over Western North Pacific (WNP) are analyzed, as well as the effect of large-scale circulation factors on long-term tendencies of tropical storm activity. The relationship between ENSO and the frequency, duration, strength of different-level TC are investigated adopting TC accumulated cyclone energy (ACE), which shows the relationship between super typhoon(TY) and ENSO cycle. The products of two new-generation Coupled Ocean-Atmospheric Global Circulation Models (CGCM) are used to study how the increase of CO2 concentration affects the climatic characteristics of model TC. At last, the numerical calculation method of the maximum wind speed radius and asymmetric wind speed distribution within the domain of a TC on sea are derived and presented. The main conclusions are shown as follows.1. The reanalysis of the basic climatic characteristics of Tropical Storms (TSs) over WNPDuring 1946 to 2005, there occurred 1559 TCs over WNP, including 1280 TSs and 781 typhoons, with an annual average of 34 TCs,27.8 TSs and 17 typhoons respectively. The occurrences of TCs change evidently with season. The highest frequency is in August, and the least is in February. More than 70 percent TCs, including TSs and typhoons, occurred mainly in July, August, September and October. The central air pressure of most TCs is between 990 and 1000hPa. The number of TCs decreases rapidly with the increase of their intensity. Most TCs generated mainly in the South China Sea, east of the Philippine Islands and the Marianas Islands, whereas most super typhoon generated in east of the Mariana Islands and almost none in the South China Sea. The genesis region of tropical cyclones also has a significant seasonal variation, which extends northward in May, reaches the most northern position near 30°N in August and September, then moves to south after October. Tropical cyclone, tropical storm and typhoon activities have 3-4a and 5-6a short period cycle with significant period energy, especially in mid-1960s to mid 1970s with high TC occurrence frequency, which spectrum energy passed 95% significance level. 2. Impacts of large-scale circulation factors on long-term variation trend of TS activityThere are 4 stages of TSs long-term activities during 1960 to 2005, i.e. the first high frequency period between 1960 and 1974 (HFP1), the first low frequency period between 1975 and 1986 (LFP1), the second high frequency period between 1987 and 1995 (HFP2), and the second low frequency period between 1996 and 2005(LFP2). TS activity increased again from 2002, which indicated that a new high frequency period starts possibly. Large-scale circulation factors affect long-term activity trend of TSs in some ways. During the high frequency period, there is a lower sea level pressure, lower tropospheric vertical wind shear, northward West Pacific subtropical high, weaker South Asia high and stronger easterly jet on 500hPa, and vice versa during the low frequency period. Sea surface temperature and total precipitation have no obvious relation to the long-term trend of TS activity. During the high frequency period, high-level divergence and low-level relative vorticity are large and vice versa during the the low frequency period.3. Impacts of ENSO cycle on climatic characteristics of Super TyphoonImpacts of ENSO events on frequency, lifetime, and cumulative duration of the super typhoon are much greater than other levels of TCs, with significant relative time sustaining near one year. In El Nino years, the frequency, lifetime and SuperTY Days increase significantly and vice versa in the La Nina years. ENSO events mainly affect the entire ACE index of super TY through changing their frequency. SuperTYs have significant seasonal activities, and most of them occurs in the second half of year (July～December). During 1951 to 2006, the total amount of SuperTY is 494 in the second half years, with an average of 9 per year. Comparatively, there is only 82 in the first half year (January～June), with an average of 1.5 per year. The frequency and genesis region of super typhoon in ENSO years are obvious different from those in usual years. In July and August, Super TY frequencies in both warm and cold events of ENSO years are significantly more than those in usual years. In September and October, all warm and cold ENSO events may cause the evident increase of super TY frequency. In November and December, the frequencies decrease significantly both in warm and cold events. Comparing the large-scale circulation factors of the key areas in the second half years, it is found that:relative vorticity and sea-surface temperature relate closely with the genesis region and frequency of SuperTY, which are the important ways of ENSO to affect SuperTY. SuperTY occurrs mainly when the vertical wind shear in the absolute value is less than 8 m/s between 850 and 200hPa, has no significant correlation with relative humidity of the middle troposphere and neither the lower nor higher level troposphere horizontal wind shear.4. Impacts of CO2 concentration increasing on the climatic characteristics of model tropical stormsConsidering the characteristic of ECHAM5 and GFDL model, the criterias for identifying model tropical storm (MTS) are proposed reasonably. The identified MTS has a very good structural similarity with the NCEP tropical storm in radial wind, tangential wind, warm core structure, sea level pressure,850hPa relative vorticity,200hPa divergence, the underlying flow field and the average precipitation. The experiments of 20C3M show that there are certain similarity in the path of tropical storms, seasonal distribution, inter-annual change between MTS identified from the two models and the observed tropical storms, but genesis region and path of MTS identified from GFDL model are in lower latitude than the observed tropical storms, and which from ECHAM5 model are biased further to higher latitudes than the observed tropical storms. The 2X and the 4X tests show that frequencies of MTS decreases with the increase of CO2 concentration. Compared with 20C3M test, the MTS frequencies of the 2X test decrease from 15.1 to 34.1%, and 4X test reduce from 13.5 to 89.7%, precipitation increased, main genesis regions and activities region have a westward movement trend, lifetime and seasonal distribution of MTS has no significantly changes, and their active season remains from July to October. MTS intensities from different models are not the same. With central sea level pressure and warm core of MTS to measure MTS intensity, Greenhouse effect enhances MTS intensity and strong MTS frequency in the GFDL model, while in ECHAM5 model MTS strength do not change significantly, strong MTS reduces. The large-scale circulation factor changes are analyzed during the period of the high and low CO2 concentration, the result shows that the MTS frequency is reduced, when sea level pressure gets higher and tropospheric vertical wind shear increases. On the contrary, when the sea level pressure and tropospheric vertical wind shear reduced, then the frequency increases. Enhancing sea surface temperature and increasing relative humidity of the middle Troposphere do not play major role to MTS frequency and intensity.5. The calculation program of wind field within the domain of a tropical cyclone area on seaBased on gradient wind equations including frictional force, and considering the effect of the movement of a tropical cyclone on wind speed, the Fujita Formula is improved and further simplified, and the numerical scheme for calculating the maximum wind speed radius and wind velocity distribution of a moving tropical cyclone is derived. In addition, the effect of frictional force on the internal structure of the tropical cyclone is discussed. Results show that when the environmental air pressure and friction are given, the structure of a motionless tropical cyclone is axially symmetrical. When the frictional coefficient is given, the smaller the clockwise departure of friction from the opposite direction of the wind vector is, the larger the wind direction inner deflection angle and the maximum wind speed radius are. When the direction of friction is given, the larger the frictional coefficient (friction), and the smaller the wind direction inner deflection angle and maximum wind speed radius. Supposing the environment air pressure and friction are evenly distributed, then the structure of the moving tropical cyclone is asymmetrical. The wind direction inner deflection angle is symmetrical in relation to the direction of movement, and the maximum wind speed radius on the straight right side is smaller than that on the straight left side. The maximum wind speed occurs on the straight right side of the moving direction of the tropical cyclone. The value and direction of friction have basically little effect on the value of maximum wind speed of a tropical cyclone. However, they can cause huge impact on the maximum wind speed radius. This will affect the structure of the tropical cyclone and thereby influence the wind speed distribution within the domain of the tropical cyclone. The example of calculating the maximum wind speed showed that it is reasonable to assume the environment air pressure to be the numerical value of the near circular closed isobar in the outermost periphery of a tropical cyclone. The calculated peripheral wind field of a tropical cyclone is relatively weak, and its synthetic result with the reanalyzed field corresponds perfectly with observation. Therefore, it reflects rationally the distributional characteristics of wind speed within the domain of a tropical cyclone.