Tropical cyclone (TC) rainfall results in damages to properties and loss of lives. It is also a significant source of freshwater in the Philippines. This study describes a method in forecasting accumulated TC rainfall using analogous TCs from historical datasets. A TC rainfall database where precipitation within 5∘ of TC centers was created for all landfalling TCs from 1951 to 2015. To predict TC rainfall, the mean rainfall of all past TCs with similar tracks included in the database, referred to as analog TCs, is calculated. Landfalling TCs from 2016 to 2018 are used to optimize the selection of past analog TCs. Each past TC member was also adjusted according to a target TC’s intensity and movement speed. The optimized analog method is then applied to landfalling TCs from 2019 to November 2020. Results show that the composite rainfall from past TCs within 1.8∘ of the forecast TC yields the best hit rate of intense rainfall. The analog TC rainfall forecast generally has a similar spatial distribution as the observed TC rain. However, this method tends to miss extreme rainfall values due to a “smoothing” effect caused by the variability of extreme rain locations of each TC member and constraints in the rainfall data used in the database. Nevertheless, forecast assessment results show that analog TC rainfall forecasting performed better than the WRF model in predicting intense and inland rainfall. In addition to it being computationally inexpensive, it can complement the inherent biases of dynamical models.

Analog forecasting of tropical cyclone rainfall in the Philippines

Accurate prediction of rainfall distribution in landfalling tropical cyclones (LTCs) is very important to disaster prevention but quite challenging to operational forecasters This chapter will describe the rainfall distribution in LTCs, including both axisymmetric and asymmetric distributions and their major controlling parameters, such as environmental vertical wind shear, TC intensity and motion, and coastline In addition to the composite results from many LTC cases, several case studies are also given to illustrate the predominant factors that are key to the asymmetric rainfall distribution in LTCs Future directions in this area and potential ways to improve the operational forecasts of rainfall distribution in LTCs are also discussed briefly

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Rainfall Distribution in Landfalling Tropical Cyclones

Tropical cyclone extreme rainfall (TCER) causes devastating floods and severe damage in China and it is therefore important to determine its long-term climatological distribution for both disaster prevention and operational forecasting. Based on the tropical cyclone (TC) best-track dataset and TC precipitation data from 1960 to 2019, the spatiotemporal distribution of TCER affecting China is analyzed. Results showed that there were large regional differences in the threshold for TCER in China, which decreased from the southeastern coast to the northwest inland, with the maximum precipitation (578 mm) about 35 times higher than the minimum (16.1 mm). TCER occurred infrequently in northern China, but had a high intensity and was highly localized. TCER was characterized by group occurrence. The frequency and intensity of TCER showed slightly increasing trends over time and was most likely to occur in August (41.0%). Most of the TC precipitation processes with extreme rainfall lasted for four to six days, with TCER mainly occurring on the third to fourth days. TC with wide areas of extreme rainfall showed two types of prevailing landfalling tracks: a northwestward track and a westward track. On average, stronger (weaker) TC corresponded to more (less) extreme precipitation. However, there were exceptions and some weak TCs led to strong extreme rainfall. A total of 64.7% (35.3%) of the TCER samples occurred when the TC was centered over the land (sea). TCER≥250 mm was located within 3° of the center of the TC.

Climatology of Tropical Cyclone Extreme Rainfall over China from 1960 to 2019

The tropical cyclone (TC) size is a crucial parameter in determining the destructiveness of a TC, but the relationship between the inner-core size and rainfall distribution in landfalling TCs is little known. This study presents an observational analysis of the relationship between the radius of maximum wind (RMW) and the rainfall in landfalling TCs over China during 2001–2020. It is found that small TCs have higher rainrate with higher axisymmetry than large TCs. Though both small and large TCs have rainfall within a radius of 5° latitudes, the rainfall occurs up to 10 times of the RMW in small TCs compared to within 5 times of the RMW in large TCs. Small TCs have a 6-hr delayed rainfall decrease compared to large TCs near the landfall. Higher TC intensity may partly contribute to higher rainrate during landfall in smaller TCs than in larger TCs. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2021GL097576

The Relationship Between the Inner‐Core Size and the Rainfall Distribution in Landfalling Tropical Cyclones Over China

The tropical cyclone (TC) size is a crucial parameter in determining the destructiveness of a TC, but the relationship between the inner‐core size and rainfall distribution in landfalling TCs is little known. This study presents an observational analysis of the relationship between the radius of maximum wind (RMW) and the rainfall in landfalling TCs over China during 2001–2020. It is found that small TCs have higher rainrate with higher axisymmetry than large TCs. Though both small and large TCs have rainfall within a radius of 5° latitudes, the rainfall occurs up to 10 times of the RMW in small TCs compared to within 5 times of the RMW in large TCs. Small TCs have a 6‐hr delayed rainfall decrease compared to large TCs near the landfall. Higher TC intensity may partly contribute to higher rainrate during landfall in smaller TCs than in larger TCs.

Ying Jun Chen

Papers

Benchmark rainfall verification of landfall tropical cyclone forecasts by operational access-TC over China.