Estimating Tropical Cyclone Vertical Gradient Parameter and Its Relationship with TC Intensity

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 (F_TC-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 F_TC-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, F_TC-VGP time series of all TC samples are calculated, and while the relationship between F_TC-VGP and TC intensity evolution are inspected, results can be divided into three main parts.One of results demonstrates that F_TC-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 F_TC-VGP. That is, F_TC-VGP abnormal inflection point indicates TC strength mutation. The second, intensity analysis result is about the relationship between F_TC-VGP and environmental circulation. When the variation between TC intensity and F_TC-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 F_TC-VGP has a fast adaptive adjustment process to contribute to TC intensity, for example, during the TC strengthening process, the F_TC-VGP increasing and decreasing generate the TC development. The result also presents the intensity changes with their internal cloud structure. In brief, F_TC-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.