Tropical cyclone (TC) rainfall is increasingly important due to its potential for causing devastating floods and landslides. Theoretical models predict increased TC potential intensity with rising global mean sea surface temperatures. While the destructive power of TCs has seemingly increased, it remains debated whether the number and intensity of TCs have actually risen due to global warming, hampered by limitations in long-term homogenous TC records. Numerical models, however, consistently predict increased TC rainfall rates as a response to increased ocean evaporation and atmospheric moisture under global warming scenarios. The IPCC reports a projected increase of 5-20% in TC rainfall rates across all basins, with higher confidence for the Northern Hemisphere. However, validating these predictions globally with long-term, high-resolution data has been limited. Previous research, using lower-resolution data, found varied trends in total accumulated rain, highlighting the need for higher resolution data for accurate trend analysis. This study addresses this gap by analyzing high-resolution data to examine TC rainfall rate changes in all TC-prone basins, providing crucial validation for current climate models and projections.

Existing literature highlights the destructive potential of TC rainfall, linking it to increased mortality and economic damage. Theoretical models consistently predict that increasing sea surface temperatures will lead to more intense TCs. However, the observational evidence supporting these predictions, particularly on a global scale, is limited and inconsistent. Studies using lower-resolution datasets have reported inconsistent findings on TC rainfall trends. Some studies show an increase in the total accumulated rainfall of TCs in certain regions, while others report a decrease or no significant change. The lack of long-term, high-resolution data makes it difficult to determine whether these inconsistencies are due to limitations in data quality or actual variations in TC rainfall patterns. The lack of consensus in the literature underscores the need for this research using high-resolution datasets to provide a more complete understanding of global TC rainfall trends.

This study uses 3-hourly, 0.25° x 0.25° resolution data from the Tropical Rainfall Measuring Mission (TRMM) 3B42 and Global Precipitation Measurement (GPM) missions for the period 1998-2016. This provides a long-term, high-resolution dataset for assessing TC rainfall rates across all global TC-prone basins (excluding the Southern Atlantic due to insufficient data). TC characteristics (location, time, intensity) were obtained from the International Best Track Archive for Climate Stewardship (IBTRACS). To define the TC rainfall area, the study uses a method based on Tropical Cyclone Precipitation Features (TCPF), which offers a more accurate estimate of TC size compared to a simple 500 km radius truncation. Two statistical methods were employed to analyze the trends: linear regression and Mann-Kendall trend tests. The study defined TC inner-core and rainband regions using a storm-dependent radius of maximum azimuthal rain rate (RMR). Sea surface temperature (SST) and total precipitable water (TPW) data were extracted from the Statistical Hurricane Intensity Prediction Scheme (SHIPS) database to assess the environmental conditions surrounding the TCs. Data points with TC centers located beyond 44°N and 44°S were removed to ensure that the 3B42 extent covered the entire TC size. A minimum precipitation rate threshold of 0.1 mm/h was used to define rain cells. Sensitivity tests showed that different thresholds produced consistent global rising trend results.

The study found a significant increasing trend in global average hourly TC rainfall rate of 0.027 mm h⁻¹ year⁻¹ (approximately +1.3% yearly increase) from 1998-2016. This trend was more pronounced in the Northwestern Pacific and North Atlantic basins, reaching increases of up to 0.04 mm h⁻¹ year⁻¹. The Northern Indian Ocean showed increases around 0.03 mm h⁻¹ year⁻¹, while the East and Central Pacific showed increases near 0.018 mm h⁻¹ year⁻¹. In contrast, the Southern Pacific and Southern Indian Oceans exhibited minor trends. Across all TC intensity categories (Tropical Depression, Tropical Storm, and Hurricane Categories 1-5), a consistent increase in rainfall rate was observed, with more pronounced increases in the extreme categories (TD, TS, CAT4, and CAT5). Analysis of inner-core versus rainband rainfall revealed an inverse relationship: while the inner-core rainfall rate decreased, the rainband rainfall rate increased. This increase in rainband rainfall is the main driver of the overall global increase in TC rainfall rate. The study found a strong positive correlation between increased precipitation rates and increases in sea surface temperature (SST) and total precipitable water (TPW), particularly in the Northern Hemisphere. This suggests that warmer and moister environments are favoring increased rainfall in the rainband region. In contrast, the Southern Hemisphere showed less significant trends in SST and TPW, aligning with the smaller increase in rainfall rates observed in those basins. The study also found that the radius of maximum azimuthal rain rate (RMR) showed a slight outward expansion in CAT1-CAT5 hurricanes, although the significance level was lower than other trends. The magnitude of observed rainfall increase (+21% for a 0.21°C increase) was substantially higher than that projected by models (+14% for a 2°C increase). The discrepancy was explained by the dominating influence of rainband increases linked to increased atmospheric precipitable water. The decreasing inner-core rainfall rate contradicts some models but aligns with findings from another observational study by Kim (2020).

The findings of this study provide strong observational evidence supporting the prediction of increased TC rainfall rates in a warmer climate. The observed increase of 1.3% per year, although higher than initially projected by models, can be attributed to the dominant role of increased rainband precipitation driven by higher SST and TPW. The contrasting trends between the Northern and Southern Hemispheres highlight the regional variations in the response to climate change. The decrease in inner-core rainfall warrants further investigation. The study highlights the critical importance of environmental conditions in influencing TC rainfall beyond TC intensity alone. These results have substantial implications for understanding TC rainfall mechanisms and for refining climate models for future projections of TC rainfall intensity and their associated impacts. Future studies might focus on improving understanding of why the inner core rainfall is decreasing and exploring potential decadal oscillations.

This study provides compelling observational evidence of a global increase in TC rainfall rates, primarily driven by increases in rainband rainfall associated with higher SST and TPW. The observed increase is more pronounced in the Northern Hemisphere, particularly the Northwestern Pacific and North Atlantic basins. The study's findings contribute significantly to the understanding of TC rainfall mechanisms in a warming climate and highlight the need for further research to investigate the decrease in inner-core rainfall and refine climate models. Further work should also investigate the role of decadal climate variability and improve resolution of existing models.

The study's reliance on satellite data introduces potential uncertainties related to measurement errors and limitations in capturing rainfall in complex terrain. The analysis period is limited to 19 years, potentially insufficient to fully capture long-term climate trends. The exclusion of the Southern Atlantic due to limited data may limit the generalization of findings to all global basins. The study uses RMR as a proxy for RMW, a potential source of uncertainty in the analysis of inner-core rainfall.