Effect of Flow-Dependent Unbalanced Background Error Variances on Tropical Cyclone Forecasting

Abstract

The background error variance in variational data assimilation can significantly affect a model’s initial field. Around extreme weather events, the variance of the unbalanced control variables have contributed highly to the total variance. This study investigates the effect of flow-dependent unbalanced variance on tropical cyclone (TC) forecasts using the ensemble of data assimilation (EDA) method. The analysis of TC Saudel (October 2020) shows that flow-dependent unbalanced variances can better represent the uncertainty in the background error, which is investigated in terms of magnitude and distribution. The vertical distribution of the temperature-explained variance ratio also shows that the contribution of the vorticity-balanced variance around Saudel is lower than the global average (in the troposphere). Single-observation experiments reveal that the structured flow-dependent errors of unbalanced control variables can also introduce corresponding structural information in analysis increments. As expected, the experiments in which the variances of all variables are flow-dependent in the one-month TC forecast performed better overall. Compared with the reference, these forecasts reduce the average absolute track and intensity errors by approximately 31% and 9%, respectively. The results demonstrate that EDA-based unbalanced variances can indeed improve the mean forecast skills of TC tracks and intensities despite instability at some lead times by improving the forecast of the circulation situation and providing a more appropriate balance relationship between variables.

Tropical cyclone Saudel impact on ocean dynamic over the South China Sea

“…The general characteristics of TC over the ocean, in addition to a minimum SST of 26.5°C, are also characterized by low AP around TC, so that it will affect the increase in SST and ultimately form a cyclonic storm [14]. SC storm as one of the TC over the ocean (western Pacific Ocean, especially the Philippine waters) and across the SCS until it reached the mainland of Vietnam during the period from October 18 to 25, 2020 [33]. The SC occurred over a period of more than 162 hours [17], with the intensity increasing slowly since pass into the waters of the SCS on October 21, 2020, then moving faster and getting stronger on October 22, 2020.…”