Estimating Tropical Cyclone Vertical Gradient Parameter and Its Relationship with TC Intensity
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摘要: 该文给出了热带气旋暖心垂直倾斜度参数FTC-VGP(TC vertical gradient parameter) 的定义。使用卫星微波遥感的温度垂直分布以及热带气旋最佳路径数据等资料,对2009—2011年典型热带气旋的暖心垂直倾斜度进行计算,并研究其与气旋强度变化的关系。结果表明:暖心垂直倾斜度参数FTC-VGP变化与气旋自身强度变化有较好的对应性,该参数的变化能够明显地指示气旋强度增强、减弱以及强度突变, 能够指示环境场变化 (冷空气侵入) 等;其与气旋强度变化的周期不同,在强度增强或减弱阶段,倾斜度参数是经过快速适应性调整才完成了对热带气旋变化过程的贡献。因此,FTC-VGP参数的计算和研究结果实现了对于热带气旋内部三维结构特征的定量描述,可以准确把握热带气旋强度和变化趋势,并对热带气旋的监测、预报具有一定业务参考价值。Abstract: There are many challenges in tropical cyclone (TC) intensity theoretical study and forecasting, and diagnostic analysis and mechanism study will help improve the understanding and predication of TC intensity evolution. And revealing TC internal structure is one of efficient methods. It is satisfying to find that TC structure and its environmental field can even be described by satellite measurements. Especially, for the applications with some microwave instruments can provide TC vertical temperature profiles to help detecting the TC internal structure, such as NOAA/AMSU-A (advanced microwave sounding unit-A) on board the polar satellites. By using NOAA/AMSU-A retrieved temperature data, more details of TC vertical structure evolution could be presented, Therefore, an objective method describing the vertical symmetry is originally proposed, which focuses on how to calculate a vertical structure index with the combined information from observations, which including the TC track and strength from CMA-STI data.In this method, the tropical cyclone vertical gradient parameter (FTC-VGP) is defined, and several prominent features for the TC upper warm-core variation in aspects of altitude, temperature anomaly and the warm core center location are considered for calculation. This parameter of FTC-VGP describe the TC upper warm-core declined from their low level circulation, it reflects the three-dimensional thermal structural information. And for this calculating method of TC internal structure, typical TCs occurs in the South China Sea and the Northwest Pacific during 2009—2011 are selected as the examples, FTC-VGP time series of all TC samples are calculated, and while the relationship between FTC-VGP and TC intensity evolution are inspected, results can be divided into three main parts.One of results demonstrates that FTC-VGP is well fitted to the TC intensity and indicating the TC intensity evolution process, anomalous and abrupt points of TC intensity in the time sequence can be found by FTC-VGP. That is, FTC-VGP abnormal inflection point indicates TC strength mutation. The second, intensity analysis result is about the relationship between FTC-VGP and environmental circulation. When the variation between TC intensity and FTC-VGP is found opposite trend, the intensity evolution cause is considered from other influences. It shows that sometimes intensity decrease is not caused by vertical structure changes, but by environmental cold air. Another important conclusion shows that FTC-VGP has a fast adaptive adjustment process to contribute to TC intensity, for example, during the TC strengthening process, the FTC-VGP increasing and decreasing generate the TC development. The result also presents the intensity changes with their internal cloud structure. In brief, FTC-VGP can give a quantitative description of TC internal three-dimensional structure characteristics, showing important reference value for grasping TC intensity and trends accurately, as well as TC monitoring and forecasting.
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图 1 卫星反演的热带气旋温度距平
(a) 沿19°N台风灿都 (1003) 上层暖心和螺旋雨带纬向垂直剖面图,(b) 沿124°E台风鲇鱼 (1013) 双暖心结构经向垂直剖面图,(c) 沿120.9°E台风鲇鱼 (1013) 减弱阶段的暖心结构经向垂直剖面图
Fig. 1 Tropical cyclone vertical temperature anomaly retrieved from satellite data
(a) cross-section of upper warm-core and rainbands of typhoon Chanthu (2010) along 19°N, (b) cross-section of double warm-core of typhoon Megi (2010) along 124°E, (c) cross-section of the warm-core evolution of typhoon Megi (2010) in the weakening stage along 120.9°E
图 2 2011年7月25日18:09台风洛坦 (1108) 的极轨卫星云图及微波资料表征的垂直倾斜结构
(a) 极轨卫星云图,(b) 沿14°N湿度距平纬向垂直剖面,(c) 沿123.4°E温度距平经向垂直剖面
Fig. 2 Typhoon Nock-ten (2011) cloud image and the unsymmetrical structure at 1809 UTC 25 July 2011
(a) the cloud image from polar satellite, (b) cross-section of temperature anomaly along 14°N, (c) cross-section of temperature anomaly along 123.4°E
表 1 2009—2011年部分典型热带气旋微波数据
Table 1 The part of tropical cyclones samples captured by microwave from 2009 to 2011
热带气旋生成区域 热带气旋编号 名称 微波数据覆盖热带气旋时间长度/h 西北太平洋 1108 洛坦 6 1013 鲇鱼 6 0908 莫拉克 11 南海 1003 灿都 4 0907 天鹅 8 表 2 2009年8月热带风暴天鹅 (0907) 的FTC-VGP与强度的变化过程描述
Table 2 Description of tropical storm Goni (2009) FTC-VGP and intensity evolution in August 2009
序号 时间 中心最低气压/hPa 相对于上一时次的强度发展趋势 FTC-VGP/(rad) 指示的热带气旋结构对称程度 1 04T06:00 980 略有增强 1.01 热带气旋强度略有增强或基本维持,垂直倾斜度减小 2 04T18:00 980 基本维持 0.72 3 05T06:00 985 正在减弱 (于08-04T22:00在广东台山登陆) 0.52 热带气旋登陆后,结果不对称性有一个突变,垂直倾斜度开始增大 4 05T18:00 985 基本维持 (在广东境内继续西行) 0.56 受地形影响,垂直倾斜度增大 5 06T06:00 985 基本维持 0.67 受地形影响,垂直倾斜度增大 6 06T18:00 983 强度略有增强 0.54 高层云系已经进入洋面,倾斜度减小 7 07T06:00 972 强度再次加强 (进入北部湾海面) 0.40 中心进入洋面,倾斜度继续减小,垂直结构趋于较对称 8 07T18:00 985 强度再度减弱 (在北部湾打转,受到周围地形影响) 1.00 倾斜度再次增大,垂直结构再次向不对称发展 -
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