Bias Analysis and Correction of ERA5 Reanalysis in the Context of Tropical Cyclones

A detailed analysis of temperature and relative humidity biases in ERA5 reanalysis under tropical cyclone (TC) conditions is conducted using a composite analysis method based on more than 17,000 dropsonde observations. The statistical results indicate that ERA5 underestimates the warm-core intensity of TCs. It shows a moist bias at the tropopause and a cold-moist bias in the boundary layer beyond four times the radius of maximum winds (RMW). In contrast, within 4 RMW, a warm-dry bias in the boundary layer was observed. The temperature bias is more pronounced at the TC center, and relative humidity exhibits a similar pattern, primarily in the mid and upper troposphere. The bias distribution is asymmetric, most prominent in the right front quadrant of the TC's motion direction. Besides, bias correction is performed using an eXtreme Gradient Boosting (XGBoost) model and the biases can be reduced by approximately 60%. The dependence relationships between relative humidity and the influencing factors are investigated through SHapley Additive exPlanations (SHAP) values. The results indicate that low-level relative humidity increases and instability intensifies in TCs at night. Additionally, the findings suggest that ERA5 may inadequately capture the diurnal variation of relative humidity in TCs. The SHAP analysis also shows that relative humidity tends to be stronger in TC's motion direction.