Satellite data assimilation is transitioning from clear‐sky to all‐sky approach at some operational forecasting centers; the all‐sky approach directly assimilates observations under clear, cloudy, and precipitating conditions and shows a positive impact on medium‐range forecasts. Although there has been progress in the all‐sky technique for the forecasting of high‐impact weather processes, such as typhoon, these studies mainly focused on a single typhoon case, and the influence of all‐sky assimilation on the forecasts of binary typhoons, which have tracks and intensities that are more complex, is less investigated. In this study, a binary typhoons case (i.e., Typhoons Haitang and Nesat), which occurred in the western Pacific in July 2017, is selected, and the Weather Research and Forecasting data assimilation system is applied to examine the impact of all‐sky assimilation of the Microwave Humidity Sounder‐2 data from FengYun‐3C on the analysis and forecasting of the binary typhoons for the first time. The results show that with more cloud‐ and precipitation‐affected data being assimilated, the vertical structures of temperature and humidity around the core areas of the two typhoons and the forecasts of wind, temperature, and humidity are improved significantly in the all‐sky assimilation. These improvements result in the simulated tracks being more similar to the observations, and an error reduction in intensity is achieved in the all‐sky assimilation. The heavy rainfall (>250 mm) caused by Nesat over Taiwan is also improved. These encouraging results suggest that all‐sky assimilation is able to improve the forecasts of binary typhoons.
e binary interaction is one of the most challenging factors to improve the forecast accuracy of multiple tropical cyclones (TCs) in close vicinity. e effect of binary interaction usually results in anomalous track and variable intensity of TCs. A typical interaction type, one-way influence mode, has been investigated by many studies which mainly focused on the anomalous track and recordbreaking precipitation, such as typhoons Morakot and Goni. In this paper, a typical case of this type, typhoons Tembin and Bolaven, occurred in the western North Pacific in August 2012, was selected to study how one typhoon impacts the track and intensity of the other one. e vortex of Tembin or Bolaven and the monsoon circulation were removed by a TC bogus scheme and a low-pass Lanczos filter, respectively, to carry out the numerical experiments. e results show that the presence of monsoon made the binary interaction more complex by affecting the tracks and the translation speeds of the TCs. e influence of Bolaven on the track of Tembin was more significant than the influence of Tembin on the track of Bolaven, and the looping track of Tembin was also affected by the external surrounding circulation associated with Bolaven. e absence of Tembin was not conducive to the development of Bolaven due to stronger vertical wind shear condition and the less kinetic energy being transported to upper troposphere. Note that the above analysis demonstrates the interacting processes between TCs and sheds some light on the prediction of binary TCs.
The dynamic emissivity retrieved from window channels of the microwave humidity sounder II (MWHS-2) onboard the China Meteorological Administration’s FengYun (FY)-3C polar orbiting satellite can provide more realistic emissivity over lands and potentially improve the numerical weather prediction (NWP) forecasts. However, whether the assimilation with the dynamic emissivity works for the precipitation forecasts over the complex geography is less investigated. In this paper, a typical precipitating case generated by the Southwest Vortex is selected and the Weather Research and Forecasting data assimilation (WRFDA) system is applied to examine the impacts of assimilating MWHS-2/FY-3C with the uses of the emissivity atlas and the dynamic emissivity on the forecasts. The results indicate that the use of the dynamic emissivity retrieved from the 89 GHz channel of MWHS-2/FY-3C apparently increases the used data number for assimilation and does improve the initial fields and the 24-hour forecasts (from 0000 UTC 24 June 2016 to 0000 UTC 25 June 2016) of precipitation distribution and intensity except for the rainfall over 100 mm. But these positive impacts are not evidently better than those with the emissivity atlas. In general, these results still suggest that the future use of the dynamic emissivity in the assimilation over the complex terrain is promising.
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