Global increase in rain rate of tropical cyclones prior to landfall

Domain: physics
Published: 2026-01-07
Editorial outcome: major_revision

Abstract
Most studies on tropical cyclone (TC) rain rate focus on long-term variability, yet the short-term (days or shorter) variations across the TC lifecycle, with a particular focus on the period before landfall, are most critical because they strongly influence flood risk. Using satellite data, we show that, globally, the mean rain rate of TCs increases by over 20% from 60 hours before landfall to the time of landfall. This increase occurs across hemispheres, ocean basins, intensity categories, and latitudes, although the magnitude varies. As a TC approaches the coast, land-sea thermal contrasts raise low-level humidity over land, while frictional differences enhance convergence, upward motion, and instability on the offshore side of the circulation. These conditions collectively promote increased convection and precipitation of TCs as they near landfall. Our findings critically strengthen the current understanding of TC precipitation dynamics and support more effective flood management.

Introduction
Heavy rainfall from landfalling tropical cyclones (TCs) often produces urban waterlogging, flash floods, and landslides; all pose significant threats to coastal communities worldwide 1–3. When the wind fields and outer rain bands of a TC first reach land, coastal communities feel the effects of the TC even before it makes landfall 4. The rain rate of a landfalling TC is one of key factors that determines the accumulated precipitation. In addition, when the TC stalls or passes over a given region, during the time it takes for the TC to pass, the accumulated precipitation increases. Because effective flood management and adaptation strategies must be prepared well in advance, it is essential that the position and precipitation of a TC are accurately predicted. Therefore, to accurately predict the precipitation, it is even more essential to know how the rain rate changes as the TC approaches the coastline.

Most studies have revealed that, in a warmer environment, global TCs move more slowly 5–7, and their annual-mean rain rates tend to increase 8–10. The stalling of a TC over coastal regions, combined with higher rain rates, lead to prolonged heavy rainfall and an increased risk of flooding 11. More recently, several studies have investigated how TC rainfall climatology varies across different surface types, such as over the ocean, near land, and over land 12,13. Collectively, these findings have advanced our understanding of how TC rainfall responds to long-term climate change 14, and how mean rain rates vary with surface conditions.

By contrast, short-term changes in rain rates during the lifecycle of individual TCs, especially at hourly to daily timescales as storms approach land, have received comparatively little attention 13,15. Yet these transient variations are more directly relevant to real-time emergency response and flood preparedness. Moreover, the physical mechanisms driving these short-term changes remain poorly understood. Here we show through both observational analyses and numerical simulations that TC rain rate prior to landfall increases by about 20% globally, which can be explained based on the changes in the relative humidity, convergence, vertical velocity at low-levels, and instability in this coupled land-sea system. Our findings advance our understanding of the dynamics of landfalling TCs and provide an important insight to effectively develop flood management and adaptation strategies.

Results
Rain rate increase in global landfalling TCs
Based on the TC center locations obtained from the available best-track datasets during the period 1980–2020, we calculate the mean rain rate of all the TCs at each time before they made landfall using the global TC rain rate data from the Multi-Source Weighted-Ensemble Precipitation (MSWEP) product. See Methods on the selection of the landfalling TCs and the extraction of the rain rates.

Globally, our observations show that there was a significant increase in the rain rate of the TCs as they approached land (Fig. 1a). During the 60 h before landfall, the mean rain rate of the TCs increases by over 20% from ~1.8 mm h−1 to ~2.2 mm h−1 (Supplementary Table 1). Markedly, the North and South Hemispheres had comparable increased rain rates (Fig. 1b, c). Overall, most of the increase in rain rate originates from the inner core of the TCs (0–200 km radius), while the outer region (200–500 km radius) contributes relatively little (Supplementary Fig. 2a, b). This is because inner-core rainfall is primarily driven by the TC’s intrinsic circulation, whereas rainfall in the outer region is more influenced by large-scale environmental conditions 16.

To quantitatively assess the contribution of land effects to enhanced TC precipitation, we further examined changes in the mean intensity of landfalling TCs and their relationship with mean rain rate. Globally, during the 60 h before landfall, TC rain rate and intensity increase at rates of 0.14±0.01 mm h⁻¹ per day and 0.29±0.18 ms⁻¹ per day, respectively, although the intensity trend is not statistically significant. During the same period, rain rate increases by 0.04±0.04 mm h⁻¹ per ms⁻¹ of intensity change. These results indicate that only about 0.012 mm h⁻¹ per day of the observed pre-landfall increase in rain rate can be attributed to TC intensification.

To confirm the results in Fig. 1, we compare the MSWEP rain rates with those from the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth generation atmospheric reanalysis (ERA5) and the Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) datasets. Given the relatively short temporal coverage of the TRMM dataset, we only compare TC rain rate records from MSWEP and ERA5 that overlap with the TRMM period. The increasing trend in landfalling TC rain rates is consistently evident across all three datasets (Supplementary Table 1 and Supplementary Fig. 4a). Although satellite- and reanalysis-derived rainfall datasets may exhibit spatial biases in rain rate estimates 17–19, particularly due to differences in observational coverage and algorithm performance between ocean and land regions 20–22, this consistency across independent data sources underscores the robustness and reliability of our finding that TC rain rates increase prior to landfall.

Rain rate increases irrespective of TC characteristics
Comparing the rain rates among different oceanic basins shows that the increase in the rain rate of the landfalling TCs exists in all six ocean basins, but the slope of the basin-averaged rain rate varies considerably across the different ocean basins, ranging from 0.10 to 0.18 mm h−1 per day. For instance, the southern Indian Ocean has the largest percentage change of ~28%, and the eastern North Pacific has the smallest percentage change with only a ~15% increase. The increase in rain rate of the TCs for the other ocean basins varies from ~15 to ~24%. We present the summary of the main inter-basin statistics in Fig. 1d–i and Supplementary Table 1.

We further divide the TCs into six latitudinal belts, ranging from 5° to 35° at 5° intervals based on their landfall locations. A significant increasing trend in the rain rate is clearly seen in every belt globally (Supplementary Fig. 4b and Supplementary Table 1). Overall, the increasing trends are larger and more evident near populated regions (10° to 25° global latitude belts), such as the mega-city clusters along the South China coast, where TC landfall effects would be more socially significant. The biggest increasing trend in the global latitudinal belts is in the 15°–20° belt, with a percentage change of 44.52%, whereas the increase in rain rate is less than 10% in the 25°–30° belts. The percentage change of other latitude belts ranges from 10% to 30%.

We also examine the changes in rain rate for each of the TC intensity categories based on the lifetime maximum intensities of the TCs 60 h before landfall at the global scale (see Methods). For all the intensity categories, the landfalling TC rain rates consistently increase, with the regression slope ranging from 0.11 to 0.40 mm h−1 per day (Supplementary Table 1 and Supplementary Fig. 4c). The increasing trend is more pronounced for Category 3 TCs (Cat3), with a percentage change of 40.94%, and smallest for Category 2 TCs (Cat2) (12.89%). The increases in the rain rates for the other categories range from ~18% to ~37%. Overall, the increase in rain rate before landfall does not depend on the intensity although the slopes of the regressions vary among the intensity categories.

Overall, our analyses of landfalling TCs at a global scale suggests that rain rates increase significantly as the TCs approach the coast, and all datasets show that this increase continues until landfall, regardless of hemisphere, ocean basin, latitude, or intensity category. It is noteworthy to point out that TCs typically undergo structural and intensity evolution prior to landfall, both of which can influence the rain rate. However, it is difficult to quantify the relative contribution of intensity and size to the pre-landfall increase in rain rate, given the limited availability and inherent uncertainty of TC size data, especially near coastlines. Therefore, the relationships between rain rate, intensity, and size, as well as the underlying physical mechanisms driving the short-term variation in TC rain rates, are explored below using numerical experiments.

Role of land–sea thermal contrasts in the enhancement of TC rain rate
We hypothesize that the increasing trend in the landfalling TC rain rates is associated with enhanced convection, primarily triggered by the contrast between the land and sea. To investigate the hypothesis, we perform a set of idealized simulations of TCs using the Weather Research and Forecasting (WRF 4.0) model 23. In all our experiments, the simulated TCs move from east to west across the domain under a typical 5 ms−1 easterly steering wind and a constant sea surface temperature. We configure each experiment with different land surface properties to the west of the initial vortex, with or without frictional differences, and turn on or off the shortwave and longwave radiation schemes. We aim to investigate how the frictional and thermal contrasts between the land and sea affected the TC rain rates. Detailed configurations of the model, the experiments and the physical parameterization schemes are presented in the Methods and Supplementary Table 2.

Experiment 1 (EXP1, Land-Rad) with land and radiation schemes is the control experiment. The simulated TC rain rate from Land-Rad experiment increases with statistical significance as the TC approaches land (Fig. 2a, b). The slope of the regression is 0.65±0.12 mm h-1 per day, which confirms the robustness of our earlier observations. Overall, the simulated TC rain rate increases with both intensity and size during the landfalling phase (Fig. 2c, d). Further analysis of the environmental parameters indicates that as the TC approaches the coast, the low-level relative humidity, convergence, vertical velocity, and instability all increase significantly (Fig. 2e–h; see Methods for details on the extraction of environmental parameters). The land–sea thermal contrast produces distinct diurnal variations in the local land–sea circulations, with stronger daytime warming-induced updrafts and nighttime cooling-induced downdrafts over land compared with those over the sea 24. This thermal contrast leads to higher low-level humidity over land than over the ocean during the simulation (Fig. 3a–d). As a result of the higher humidity, the water vapor moves from the land to the vortex circulation of TC as it moves closer to land (Fig. 3e). Meanwhile, the difference in the friction between the land and sea generates an asymmetric flow 25, increasing the instability on the offshore side of the TC vortex circulation 26,27 (Fig. 3f). We conclude that the frictional and thermal contrasts between the land and sea lead to increased humidity, strengthened convergent upward motion, and enhanced instability in the TC, all of which favor enhanced asymmetric convection and increased precipitation throughout the affected TC region.

To further investigate the relative importance of land surface roughness and thermal effects on changes in the rain rate of the TC, we conduct two additional experiments. We use the same radiation schemes in Experiment 2 (EXP2, noLand-Rad) as EXP1, but the simulated TC moves westward into a domain without any land surface, thereby removing any differential frictional effects between the land and sea. The simulated TC rain rate in the noLand-Rad experiment increases less than that in the Land-Rad experiment, with a regression slope of 0.34±0.12 mm h⁻¹ per day. This smaller increase is accompanied by weaker growth in both TC intensity and size, indicating that land effects not only enhance TC rain rates directly but also indirectly through their influence on TC intensity and size (Supplementary Fig. 5a–d). While the convergent upward motion continues to enhance the TCs development, there is no significant change in the instability, and more importantly, the low-level relative humidity decreases in noLand-Rad experiment (Supplementary Fig. 5e–h). Without the frictional difference between land and sea in noLand-Rad experiment, there is no enhanced asymmetric flow or instability developing on the offshore side of the vortex circulation of the TC. More notably, in the absence of land, the land–sea thermal contrast disappears, producing identical surface properties on both sides of the “imaginary coastline” and preventing the development of local atmospheric circulations. Without these circulations, the transport of near-surface moisture into the mid- to lower troposphere is suppressed, producing a horizontally uniform humidity field (Supplementary Fig. 6). As the TC approaches the “imaginary coastline”, the absence of external moisture supply leads to a progressive reduction in mid- to lower-tropospheric humidity in its environment. Even though low-level convergence and upward motion increase, the reduced water vapor content on the offshore side of the vortex still limit the available moisture for convection, being unfavorable for enhanced precipitation in the noLand-Rad experiment.

In Experiment 3 (EXP3, Land-noRad), the model uses the same land surface properties as the Land-Rad experiment, EXP1, but we turn off the radiation schemes. Without radiative effects, the simulated TC rain rate in the Land-noRad experiment shows no increase as the storm approaches land, despite the concurrent increases in intensity and size (Supplementary Fig. 7a–d). This behavior is associated with nearly unchanged convergence and vertical velocity, but with noticeable changes in relative humidity and atmospheric stability (Supplementary Fig. 7e–h). In the absence of radiation, no significant humidity contrast exists between land and sea (Supplementary Fig. 8a–d), consistent with the result from the noLand-Rad experiment. Due to the presence of land in Land-noRad experiment, the land-sea frictional differences produce instability similar to that in Land-Rad experiment (Figs. 2h, 3f). However, the Land-noRad experiment has less induced precipitation because of insufficient water vapor and less enhanced convergent upward motion (Supplementary Figs. 8e, 8f).

To summarize, we demonstrate that land-sea differences in surface friction and thermal effects lead to increases in low-level relative humidity, convergence, vertical velocity, and instability within the TC environment, all of which contribute to the enhanced asymmetric convection and increased rain rate of landfalling TCs. Figure 4 summarizes the various physical processes with a schematic diagram. Key controlling factors are the frictional and thermal contrasts between land and sea that influence the increasing trend in rain rate and the enhanced asymmetric convection, with the thermal contrasts being the most significant. In addition to the influence of land–sea thermal contrast on TC rainfall, land–sea frictional heterogeneity can induce asymmetric circulation and convection in landfalling TCs. Such asymmetries may generate localized potential vorticity anomalies, which can accelerate storm translation. The change in TC translation speed may further modulate regional rain rates.

Methods
Data
We use several types of datasets in this study: TC best-track data, global precipitation products, and atmospheric reanalysis. We use the TC best-track information provided by the International Best Track Archive for Climate Stewardship (IBTrACS, v0400) 33 from 1980 to 2020, including global TC records from various agencies. We use records from the World Meteorological Organization (WMO) that contain records on the latitude and longitude of TC locations, maximum wind speed (MWS), landfall and dist2land (distance to the nearest land) at 3-hourly intervals.

We extract the global TCs precipitation data from the most recent available Multi-Source Weighted-Ensemble Precipitation (MSWEP) dataset from 1980 to 2020. MSWEP combines rain gauge, satellite, and reanalysis data to produce high-quality rainfall estimates at 3-hourly intervals with 0.1° latitude and longitude resolutions 34. Precipitation of MSWEP dataset has been widely used because it is a long-term, continuous record with higher quality and high resolution in densely gauged and ungauged regions 35. We also use the rain rate from the Tropical Rainfall Measuring Mission (TRMM) TMPA 3B42 product 22, and the total precipitation from the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis (ERA5) dataset to verify our results 36. The TRMM and ECMWF data provide 3-hourly intervals and a 0.25° spatial resolution for their precipitation estimates. For consistency, we convert the total precipitation from the ERA5 dataset to rain rate at 3-hourly intervals.

In this study, we primarily use the MSWEP dataset from 1980 to 2020, rather than the TRMM dataset from 1999 to 2019 because the MSWEP dataset aligns with the TC best-track records from the post-satellite era, which are generally more homogeneous and reliable, and the MSWEP data provide us with enough samples to study. Once more, for consistency, we only use records of TC rain rates from the MSWEP dataset from 1999 to 2019 that overlap with the same period as the TRMM dataset when we validate the robustness of our observed results.

Selecting the landfalling TCs
We use the records of latitude, longitude, wind speed, landfall, and dist2land from the IBTrACS data to identify the landfalling TCs. The dist2land provides the distance to the nearest landfall point. A value of zero means the TC was over land, and the landfall time with a value of zero means that the TC crossed a coastline before the next record. We define landfall=0 and dist2land=0 as TC landfall. We define two selection criteria for landfalling TCs. Firstly, we only consider landfalling TCs with a lifetime maximum intensity of at least 35 knots and a lifespan of more than 60 h before landfall. Our focus was on the stage within the 60 h before landfall, because this period is the most crucial for disaster mitigation, prevention, and preparedness. During this 60-h stage, land surface properties gradually influence TCs, making those 60 h essential for accurate forecasting and mitigation efforts. Secondly, we filter out the TCs that underwent an extra-tropical transition.

With these two criteria, we select a total of 1,468 landfalling TCs during the 41-year period from 1980 to 2020. Based on the latitude and longitude of the landfall locations, we classify all the TCs into different geographic categories as being global or in different hemispheres (southern and northern hemispheres), basins (WNP: western North Pacific, ENP: eastern North Pacific, NA: North Atlantic, SI: South Indian Ocean, SP: South Pacific, NI: North Indian Ocean), latitudinal belts (≤10°, 10°–15°, 15°–20°, 20°–25°, 25°–30°, 30°–35°), and the Saffir-Simpson Hurricane Wind Scale (TS, Cat1, Cat2, Cat3, Cat4, Cat5).

Identifying the precipitation related to landfalling TCs
For each landfalling TC, we use the latitude and longitude of the center location at 3-hourly intervals to identify the TC rainfall from various precipitation products. Following the most common method 8,9, we define the TC rainfall as the rainfall within a fixed radius of 500 km from the storm center, corresponding to the estimated radius where the mean rain rate exceeds 0.5 mm h−1 for global TCs 37. Similarly, we define the TC inner core as the region within a radius of 0 to 200 km and the outer region as the region between 200 and 500 km. After extracting the TC rainfall, we project the grid precipitation as a function of the radial distance in 10-km intervals and the azimuth in 10° intervals relative to the storm center. Correspondingly, we construct the radius–time Hovmöller plots of the all-pixels average rain rate (with non-rainy pixels counted as zero) and the conditional rain rate (counting only rain pixels with a value exceeding 0.1 mm h−1) (Supplementary Fig. 1). The distributions of conditional and average rain rates are well-aligned. Because the conditional rain rate captures the realistic precipitation distribution better 38, we use the conditional rain rate to construct the time series of the average rain rate at 3-hourly intervals from the 60 h before landfall until the time of landfall. If not otherwise specified, the ‘average rain rate’ for all global TCs, for the two hemispheres, and for the other groupings, refers to the averaged conditional rain rate of the entire TC. Moreover, we also use the method described above to identify various variables of the TCs from the ERA5 dataset and to calculate the time series of all-pixels averaged values of each variable for the entire TC.

Statistics
We examine the trends in the mean rain rate of all the TCs or each group of TCs using linear regression with the full degrees of freedom in the time series determining the p value for the regression coefficients and correlation coefficient. To assess the robustness of the regression, we used the two-tailed Student’s t test. Dividing the difference between the final and initial points of the regression line by the value of the initial point determines the percentage change of the rain rate.

Configuring the model and designing the experiments
We use the Weather Research and Forecasting (WRF) version 4.0 23 to simulate an ideal TC on the double periodic f plane. We set the constant Coriolis parameter, f, to 5.0×10–5 s–1, centered at 20°N, and a constant sea surface temperature (SST) of 28.5°C throughout the simulation. The model adopts a double-nested configuration with horizontal grid spacing of 20 km (outermost domain size: 12000 × 12000 km) and 10 km (innermost domain size: 6000 × 6000 km). In addition, the model has 26 vertical levels, and the model top is set at 25 km. Our integration time step is 40 seconds. Physical parameterizations include the Yonsei University boundary layer scheme, the surface layer scheme by Dudhia et al 39, the WRF single-moment six-class microphysics 40, and the Kain-Fritsch (new Eta) cumulus scheme 41. Specifically, our choices of using the RRTMG shortwave 42 and longwave radiation schemes 43 depend on our experiment designs (Supplementary Table 2).

Similar to Tu et al. 37, we initialize the model at the domain center with an axisymmetric vortex in hydrostatic and gradient wind balance, with a maximum tangential wind speed of 15 ms−1 at a radius of 82.5 km and an outer radius of 412.5 km where the tangential wind speed drops to zero. We derive the basic-state potential temperature and humidity used for initializing the vortex from the mean hurricane season sounding of Jordan 44. This has been the most common model configuration used for ideal simulations in previous studies 12,45.

To investigate the physical processes influencing precipitation changes in landfalling TCs, we conduct three numerical experiments. In all the simulations, we subject the initial axisymmetric vortex to a 5 ms−1 easterly steering flow (u=5 ms−1, v=0 ms−1), directing it towards the western side of the domain. The magnitude of the steering flow is comparable to the usual translation speed of landfalling TCs globally 5. To isolate the respective roles of the land surface and radiation, three experiments are named Land-Rad, noLand-Rad, and Land-noRad. In Experiment 1 (EXP1, Land-Rad), referred to as the control experiment, we use the radiation schemes, and the domain configuration includes a combination of rectangular sea and land surfaces. Specifically, the model coastline is configured as a north-south oriented boundary with dry land, cropland, and pasture surface categories, positioned 1000 km east of the western boundary of the innermost domain. Consequently, even at the time of landfall, the TC center remains over 1000 km from the lateral boundary, a distance that well exceeds the 500 km radius used for rain-rate averaging. Thus, the lateral boundary is considered unlikely to introduce meaningful impacts on the simulated TC structure or the calculated mean rain rate.

In Experiment 2 (EXP2, noLand-Rad), to investigate the role of land surface friction in the changing landfalling TC precipitation, we only use the radiation schemes, and the domain configuration in this experiment does not include land. Experiment 3 (EXP3, Land-noRad) has the same domain configuration as the control experiment but without any radiation schemes so that we can assess the effect of the land-sea thermal contrast on landfalling TC precipitation. We integrate all the simulations for 240 h, with variables output at 6-hourly intervals on the model’s eta levels and then interpolate to pressure levels. Because our main interest is to determine the role of land-sea contrast in changing landfalling TC precipitation, we extract model outputs from 60 h before landfall to the time of landfall. Landfall time is defined as the moment the TC crosses a north–south oriented coastline in the Land-Rad and Land-noRad experiments. In the noLand-Rad experiment, where no actual coastline exists, landfall time is set to match that of the Land-Rad experiment. We determine the TC track based on the minimum center pressure at the surface, and we use the difference in accumulated total precipitation between the current and next time steps to calculate the rain rate.

Atmospheric stability
Atmospheric stability that accounts for both temperature and moisture is quantified by moist static stability (MSS). It is defined as the difference in equivalent potential temperature θe between 700 and 850 hPa 26, which is given by
MSS=θe700 − θe850 (1)
The equivalent potential temperature 46 θe is defined as:
θe = T (P0/P) Rd/cp exp (Lv q / cp T) (2)
where T, q and P, are the absolute temperature (K), the specific humidity (kg/kg), and the atmospheric pressure (hPa), respectively. Rd is the gas constant for dry air, and cp is the specific heat capacity at a constant pressure. Typically, Rd/cp=0.286 for air. P0 is the reference pressure, usually taken as 1000 hPa, and Lv is latent heat of vaporization, with a constant value of 2.5 × 106 J/kg.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Global Increase in Rain Rate of Tropical Cyclones Prior to Landfall by Zhong et al.\nThis study investigates short-term variations in tropical cyclone (TC) rainfall, particularly around landfall. Using global satellite data, the authors find that TC mean rain rates increase by more than 20% from 60 hours before landfall to the time of landfall, regardless of basin, hemisphere, or intensity. The enhancement is linked to land–sea thermal contrasts, increased low-level humidity over land, and frictional convergence offshore, which together intensify convection and precipitation.\nThis manuscript is engaging, clearly written, and makes an excellent contribution in terms of scientific importance and societal relevance. In particular, understanding rainfall processes near TC landfall is highly valuable for disaster risk reduction. However, before publication, additional analysis and discussion regarding other factors influencing the increase in rain rate are necessary. My detailed comments are as follows:\nMany TCs typically undergo structural development prior to landfall. During this period, intensity and size (e.g., R17, radius of 17 m s⁻¹ wind) often increase, which can directly influence rain rate. Consequently, part of the observed 60-hour increase in rain rate may be attributable to changes in intensity and size, independent of landfall itself. Although quantifying this contribution may be challenging, the authors should at least discuss how such changes occur within the 60-hour window and evaluate their potential impact on the observed rain rate increase. It would also be valuable to examine whether non-landfalling TCs exhibit similar rain rate increases, since intensification and size growth alone could enhance precipitation without land interaction. For non-landfalling TCs, one could determine the time interval (T1) between the lifetime maximum intensity (LMI_time) and landfall for landfalling TCs, and then apply this mean T1 to the non-landfalling cases. By adding this mean T1 to each non-landfalling TC’s LMI_time, an equivalent reference time can be established, allowing the rain rate during the preceding 60 hours to be calculated. This would enable a fairer comparison between landfalling and non-landfalling TCs.\nThe same concern applies to the numerical experiments. As TCs propagate, their intensity and size typically increase, which could contribute to enhanced rain rates. This may partly explain the rain rate increase observed even in EXP2 (without land included). The fact that EXP1 (with land) shows a greater increase than EXP2 suggests that this excess can be attributed to land effects. However, to interpret this robustly, the manuscript should present how TC characteristics (intensity and size) evolve in both experiments. For instance, if land interaction alters intensity or size, this could also contribute to rain rate changes, and such effects should be disentangled from the pure land–sea contrast mechanism.", "reviewer_2": "Review of \"Global Increase in Rain Rate of Tropical Cyclones Prior to Landfall\" by Zhong et al. for Nature Communications\n(#NCOMMS-25-51671)\nRecommendation: Minor Revision\nGeneral comments:\nUsing 40-yr global rainfall data, this study found that rain rates of tropical cyclones (TCs) increase significantly as the TCs\napproach the coast from 60 h prior to landfall to the landfall time, and all datasets show the increase continues until landfall,\nregardless of hemisphere, ocean basin, latitude, or TC intensity category. It was hypothesized that the increasing trend in the\nlandfalling TC rain rates is associated with enhanced convection, primarily triggered by the contrast between the land and\nsea. To investigate the hypothesis, a set of idealized simulations of TCs using the Weather Research and Forecasting\n(WRF4.0) model was performed. It was found that land-sea differences in surface friction and thermal effects lead to\nincreases in low-level relative humidity, convergence, vertical velocity, and instability within the TC environment, all of which\ncontribute to the enhanced asymmetric convection and increased rain rate of landfalling TCs.\nThe paper is well-written and easy to follow. The results are interesting. Although the general phenomenon of rain rate\nincreasing as TCs approach to coast/land from ocean has been shown by previous studies (Jiang et al. 2008 and Guzman\nand Jiang 2023, see my minor comment below), the authors did a great job in presenting this increase as a fund of time from\n60 h prior to landfall to the landfall time. I also applause the authors’ effort of testing the hypothesis using idealized\nsimulations. The physical explanation of the short-term increasing of TC rainfall prior to landfall is scientifically sound and\nimportant for advancing our knowledge in the field. I only have one minor comment below. I believe that the paper is suitable\nfor publishing in Nature Communications after addressing the comment.\nMinor Comment:\nLine 68-69: Please add the following two references here. An early study by Jiang et al. (2008) using TRMM TMPA data\nshowed that TC rain rates over land and coast are larger than those over ocean (their Fig. 1). More clearly, using 20 years of\nIMERG satellite rainfall data for global TCs, Guzman and Jiang (2023)’s Fig. 2 showed an increase of TC rain rates from\nocean to off-shore then to near-land stages, very similar to what was found in this study.\nJiang, H., J. B. Halverson, J. Simpson, and E. J. Zipser 2008: Hurricane \"Rainfall Potential\" Derived from Satellite\nObservations Aids Overland Rainfall Prediction. J. Appl. Meteor. Climatol., 47, 944-959.\nGuzman, O. and H. Jiang, 2023: Climatology of Tropical Cyclone Rainfall Magnitude at Different Landfalling Stages: An\nEmphasis on After Landfall Rain. J. Appl. Meteor. Climatol., 62, 801–815. https://doi.org/10.1175/JAMC-D-22-0055.1"}, "author_response": "Responses to Referee\nManuscript number: NCOMMS-25-51671\nTitle: Global Increase in Rain Rate of Tropical Cyclones Prior to Landfall\nAuthor(s): Quanjia Zhong et al.\nRevision on October 30, 2025\nResponses to Reviewer #1\nOverall summary of the main contribution of the paper\nThis study investigates short-term variations in tropical cyclone (TC) rainfall,\nparticularly around landfall. Using global satellite data, the authors find that TC mean\nrain rates increase by more than 20% from 60 hours before landfall to the time of\nlandfall, regardless of basin, hemisphere, or intensity. The enhancement is linked to\nland–sea thermal contrasts, increased low-level humidity over land, and frictional\nconvergence offshore, which together intensify convection and precipitation.\nThis manuscript is engaging, clearly written, and makes an excellent\ncontribution in terms of scientific importance and societal relevance. In particular,\nunderstanding rainfall processes near TC landfall is highly valuable for disaster risk\nreduction. However, before publication, additional analysis and discussion regarding\nother factors influencing the increase in rain rate are necessary.\nResponse: Thanks to the reviewer’s time and valuable suggestions for the\nimprovement of the manuscript. We revised the manuscript carefully to the reviewer’s\ncomments. More details and point-to-point responses to the reviewer’s comments are\nlisted as follows (reviewers’ comments in black font, our replies in blue font). Particularly,\nthe red font in the manuscript indicates some important changes in comparison with the\nprevious version.\nMain Questions/Comments\nMy detailed comments are as follows:\n1. Many TCs typically undergo structural development prior to landfall. During this\nperiod, intensity and size (e.g., R17, radius of 17 m s⁻¹ wind) often increase, which\ncan directly influence rain rate. Consequently, part of the observed 60-hour increase\nin rain rate may be attributable to changes in intensity and size, independent of\nlandfall itself. Although quantifying this contribution may be challenging, the\nauthors should at least discuss how such changes occur within the 60-hour window\nand evaluate their potential impact on the observed rain rate increase. It would also\nbe valuable to examine whether non-landfalling TCs exhibit similar rain rate\nincreases, since intensification and size growth alone could enhance precipitation\nwithout land interaction. For non-landfalling TCs, one could determine the time\ninterval (T1) between the lifetime maximum intensity (LMI_time) and landfall for\nlandfalling TCs, and then apply this mean T1 to the non-landfalling cases. By\nadding this mean T1 to each non-landfalling TC’s LMI_time, an equivalent\nreference time can be established, allowing the rain rate during the preceding 60\nhours to be calculated. This would enable a fairer comparison between landfalling\nand non-landfalling TCs.\nResponse:\nWe thank the reviewer for this insightful comment. We agree that TCs typically\nundergo structural and intensity evolution prior to landfall, both of which can\ninfluence the rain rate. As the reviewer correctly noted, it is difficult to quantify the\nrelative contribution of TC intensity and size to the pre-landfall increase in rain rate,\ngiven the limited availability and inherent uncertainty of TC size data. This is\nespecially the case for TCs near coastlines because observational data for the part of\na TC over land are likely either not available or very uncertain. To address this issue,\nwe attempt to evaluate the potential impacts of TC intensity and size on rain rate\nusing sensitivity numerical experiments, as described in our response to Major\nComment 2. Meanwhile, we have also added a brief discussion in the subsection\ntitled “Rain rate increases irrespective of TC characteristics” in the revised\nmanuscript to clarify this point further. The relevant details are as follows:\n“It is noteworthy to point out that TCs typically undergo structural and intensity\nevolution prior to landfall, both of which can influence the rain rate. However, it is\ndifficult to quantify the relative contribution of TC intensity and size to the pre-\nlandfall increase in rain rate, given the limited availability and inherent uncertainty\nof TC size data, especially near coastlines. Therefore, the relationships between rain\nrate, intensity, and size, as well as the underlying physical mechanisms driving the\nshort-term variation in TC rain rates, are explored below using numerical\nexperiments.”\nRegarding the suggestion to compare landfalling and non-landfalling TCs, we\nbelieve that such a comparison could be affected by sampling differences between\nthe two TC groups, rather than land interaction itself. Since our primary objective is\nto investigate landfalling TCs, our dataset naturally focuses on those that actually\nmake landfall.\nIn line with the reviewer’s suggestion, however, we further examined the rain-rate\nevolution of landfalling TCs as a function of their distance to land. This approach\navoids the sampling bias between different TC populations and allows the direct\nidentification of land effects. As shown in Figure A1 (Extended Data Fig. 3 in\nrevised manuscript), when a TC is more than 800 km away from land, it is mainly\nover the open ocean without any influence from land, and its rain rate does not\nchange significantly. In contrast, within 800 km of land, the rain rate increases\nmarkedly as the TC approaches the coast. This result is consistent with the result\nshown in Figure 1 of the main text. From 60 hours before landfall to the landfall\ntime (0 hours), the rain rate exhibits a continuous and significant rise, likely\ninfluenced by land-induced effects. These results reinforce the robustness of our\nmain finding that the TC rain rate tends to increase as TCs approach land.\nWe appreciate the reviewer’s constructive suggestions. The new Extended Data Fig.\n3 and corresponding text describing the two-phase comparison have been added to\nthe revised manuscript. The relevant details are as follows:\n“To further assess the effect of land, we examined the changes in rain rate of\nlandfalling TCs as a function of their distance to land (Extended Data Fig. 3). When\na TC is more than 800 km away from land, it is over the open ocean without any\nland influence, and its rain rate shows no significant change. However, within 800\nkm of land, the rain rate increases markedly as the TC approaches the coast, likely\ndue to land effects. This pattern, consistent with the time-based analysis shown in\nFig. 1, clearly indicates a negligible trend in the former phase but a significant\nincrease in the latter. These results reinforce the robustness of the observed pre-\nlandfall increase in TC rain rate and highlight the possible contribution of land-\ninduced processes.”\nFigure A1 Rain rate of tropical cyclones (TCs) as a function of distance to land.\nThe mean rain rate of all TCs is shown at 25 km intervals from 1600 km to 0 km from\nland (solid line with black dots), along with their linear trends (dashed lines) for the two\nphases: TCs more than 800 km away from land and those within 800 km of land. Light\nshading represents the two-sided 95% confidence range of the linear trend. The linear\nregression coefficients with uncertainty estimates (unit: mm h⁻¹ per 800 km) are\nindicated at the bottom, and the corresponding R² and p values are shown at the top.\nDistance is expressed in kilometers relative to land (0 km), with positive values\nindicating positions prior to landfall. The vertical black dashed line marks the distance\nof 800 km from landfall.\n2. The same concern applies to the numerical experiments. As TCs propagate, their\nintensity and size typically increase, which could contribute to enhanced rain rates.\nThis may partly explain the rain rate increase observed even in EXP2 (without land\nincluded). The fact that EXP1 (with land) shows a greater increase than EXP2\nsuggests that this excess can be attributed to land effects. However, to interpret this\nrobustly, the manuscript should present how TC characteristics (intensity and size)\nevolve in both experiments. For instance, if land interaction alters intensity or size,\nthis could also contribute to rain rate changes, and such effects should be\ndisentangled from the pure land–sea contrast mechanism.\nResponse:\nWe appreciate this valuable comment. Following the reviewer’s suggestion, we have\nexamined the evolution of TC intensity and size (radius of 17 m s⁻¹ wind) in both\nnumerical experiments (EXP1: Land-Rad and EXP2: noLand-Rad). The results\nshow that both intensity and size generally increase during the 60-hour period, and\nthe rates of increase are greater in EXP1 than in EXP2 (see Figures A1 and A2). This\nindicates that land–sea contrasts may not only directly enhance TC rain rates but\nmay also indirectly contribute through modifications of TC intensity and size.\nWe have added these analyses and corresponding discussions in the Results section\nof the revised manuscript to clarify this mechanism. Figure 2 and Extended Data\nFigures. 5 and 7 have been replotted with additional analyses of TC intensity and\nsize. The relevant details are as follows:\n“The simulated TC rain rate in the noLand-Rad experiment increases less than that\nin the Land-Rad experiment, with a regression slope of 0.34 ± 0.12 mm h⁻¹ per day.\nThis smaller increase is accompanied by weaker growth in both TC intensity and\nsize, indicating that land effects not only enhance TC rain rates directly but also\nindirectly through their influence on TC intensity and size (Extended Data Figs. 5a–\nd).”\nFigure A2 Track and variations in rain rate, intensity, and size of the simulated\nTC in the control experiment (EXP1, Land-Rad). (a) 60-hour track, (b) rain rate\n(mm h⁻¹), (c) maximum wind speed (m s⁻¹), and (d) radius of 17 m s⁻¹ wind (R17, km)\nof the simulated TC at 6-hour intervals (solid lines with black dots). Different line\ncolors and shaded areas represent different variables. In panel (a), the starting point\ndenotes the TC location 60 hours before landfall (red dot). Light shading in panels (b)–\n(d) indicates the 95% confidence interval of the linear trend. Dashed lines denote the\nlinear regression of rain rate and environmental parameters before landfall. Linear\nregression coefficients with error estimates are shown at the bottom, and R² and p values\nare provided in the top-right corner. Time is given in hours relative to landfall (0 h),\nwith negative values indicating hours before TC landfall.\nFigure A3 As in Fig. A2, except for Experiment 2 (EXP2, noLand-Rad).\nResponses to Reviewer #2\nGeneral comments:\nUsing 40-yr global rainfall data, this study found that rain rates of tropical\ncyclones (TCs) increase significantly as the TCs approach the coast from 60 h prior to\nlandfall to the landfall time, and all datasets show the increase continues until landfall,\nregardless of hemisphere, ocean basin, latitude, or TC intensity category. It was\nhypothesized that the increasing trend in the landfalling TC rain rates is associated\nwith enhanced convection, primarily triggered by the contrast between the land and\nsea. To investigate the hypothesis, a set of idealized simulations of TCs using the\nWeather Research and Forecasting (WRF4.0) model was performed. It was found that\nland-sea differences in surface friction and thermal effects lead to increases in low-\nlevel relative humidity, convergence, vertical velocity, and instability within the TC\nenvironment, all of which contribute to the enhanced asymmetric convection and\nincreased rain rate of landfalling TCs.\nThe paper is well-written and easy to follow. The results are interesting. Although\nthe general phenomenon of rain rate increasing as TCs approach to coast/land from\nocean has been shown by previous studies (Jiang et al. 2008 and Guzman and Jiang\n2023, see my minor comment below), the authors did a great job in presenting this\nincrease as a fund of time from 60 h prior to landfall to the landfall time. I also applause\nthe authors’ effort of testing the hypothesis using idealized simulations. The physical\nexplanation of the short-term increasing of TC rainfall prior to landfall is scientifically\nsound and important for advancing our knowledge in the field. I only have one minor\ncomment below. I believe that the paper is suitable for publishing in Nature\nCommunications after addressing the comment.\nResponse:\nWe thank the reviewer for his/her helpful suggestions, which helps improving this\nmanuscript. In revised paper, we have addressed the major concern about the prior work\nand carefully revised the manuscript according to the comments and suggestions raised\nby the reviewer. Please find below a detailed point-by-point response to all comments\n(reviewers’ comments in black font, our replies in blue font). In addition, the red font\nin the revised manuscript indicates some changes in comparison with the previous\nversion.\nMinor Comment:\n1. Line 68-69: Please add the following two references here. An early study by Jiang\net al. (2008) using TRMM TMPA data showed that TC rain rates over land and\ncoast are larger than those over ocean (their Fig. 1). More clearly, using 20 years\nof IMERG satellite rainfall data for global TCs, Guzman and Jiang (2023)’s Fig.\n2 showed an increase of TC rain rates from ocean to off-shore then to near-land\nstages, very similar to what was found in this study.\nJiang, H., J. B. Halverson, J. Simpson, and E. J. Zipser 2008: Hurricane \"Rainfall Potential\"\nDerived from Satellite Observations Aids Overland Rainfall Prediction. J. Appl. Meteor.\nClimatol., 47, 944-959.\nGuzman, O. and H. Jiang, 2023: Climatology of Tropical Cyclone Rainfall Magnitude at\nDifferent Landfalling Stages: An Emphasis on After Landfall Rain. J. Appl. Meteor.\nClimatol., 62, 801–815. https://doi.org/10.1175/JAMC-D-22-0055.1.\nResponse:\nWe thank the reviewer for the helpful comment. Relevant references have been\nadded to the Introduction section and the reference list in the revised manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thank you for the authors’detailed responses. However, despite their clarifications,\nseveral key concerns remain unresolved and, in some respects, have become more\nsignificant.\nMy main point in the previous comments was based on the well-established relationship that TC rain rate increases with TC\nintensity. Therefore, the observed increase in rain rate as TCs approach land could partly result from the concurrent\nintensification of TCs rather than land effects alone. In the idealized experiments, the authors clearly demonstrated that even\nwithout land, rain rate increases as TC intensity increases. While I agree that land interaction can further enhance rainfall, it\nis essential to discuss how much of the rain rate increase shown in Figure 1 is attributable to intensity changes. To evaluate\nthis, I suggest plotting the same sample data from Figure 1 using TC intensity instead of rain rate. If intensity increases along\nthe same timeline, it would indicate that part of the rain rate increase is indeed due to TC strengthening.\nAdditionally, when TCs move poleward from lower to higher latitudes, they encounter cooler and drier air masses, which can\noften enhance precipitation due to increased baroclinicity. Thus, rain rate may also increase with latitude. It would therefore\nbe informative to calculate the mean latitude of the TCs in Figure 1. If latitude increases alongside rain rate, this would\nsuggest that TC position (latitude change) also contributes to the rain rate increase.\nI also believe that non-landfalling TCs can show rain rate increases associated with changes in intensity and latitude. The\nauthors should carefully analyze how these factors affect rain rate for non-landfalling cases. Without such analysis, it is\ndifficult to attribute the observed rain rate increase in landfalling TCs solely to land effects.\nThe additional Figure A1 presented by the authors does not adequately address this concern, as it does not include\ninformation on TC intensity or latitude. I strongly encourage the authors to provide an additional analysis showing how TC\nintensity and latitude variations influence rain rate in non-landfalling TCs.\nRegarding the comparison between EXP1 and EXP2, I also have several questions:\n1) It is generally understood that as a TC intensifies, mid- to lower-tropospheric relative humidity increases due to enhanced\nconvection, condensation, and latent heat release. However, in EXP2, TC intensity increases while relative humidity does\nnot. Could the authors explain this discrepancy?\n2) The TC track comparison between EXP1 and EXP2 shows that the TC in EXP2 moves considerably more slowly, despite\nboth experiments using an easterly steering flow of 5 m s⁻¹. What causes such a large difference in translation speed? This\ndiscrepancy could significantly influence TC structure and rain rate. Could the experimental design be modified to minimize\nthe effects of such translation speed differences?\nFinally, I am particularly concerned about the limited model domain size. In EXP1, when the TC reaches land, its center is\nonly about 400 km from the domain boundary. Given that the rain rate is averaged within a 500 km radius, the outer 100 km\nportion would be excluded, potentially artificially inflating the mean rain rate near landfall. The authors should carefully\nassess and discuss how this boundary limitation might affect the quantitative results and conclusions."}, "author_response": "Responses to Referee\nManuscript number: NCOMMS-25-51671A\nTitle: Global Increase in Rain Rate of Tropical Cyclones Prior to Landfall\nAuthor(s): Quanjia Zhong et al.\nRevision on November 16, 2025\nResponses to Reviewer #1\nOverall summary of the main contribution of the paper\nThank you for the authors’detailed responses. However, despite their clarifications,\nseveral key concerns remain unresolved and, in some respects, have become more\nsignificant.\nResponse: We sincerely thank the reviewer for the time and thoughtful comments\nprovided. We have carefully revised the manuscript in response to all concerns. More\ndetails and point-to-point responses to the reviewer’s comments are listed as follows\n(reviewers’ comments in black, our replies in blue). In the revised manuscript, text highlighted\nin red indicates substantial modifications made following the reviewer’s suggestions.\n1. My main point in the previous comments was based on the well-established\nrelationship that TC rain rate increases with TC intensity. Therefore, the observed\nincrease in rain rate as TCs approach land could partly result from the concurrent\nintensification of TCs rather than land effects alone. In the idealized experiments, the\nauthors clearly demonstrated that even without land, rain rate increases as TC intensity\nincreases. While I agree that land interaction can further enhance rainfall, it is\nessential to discuss how much of the rain rate increase shown in Figure 1 is attributable\nto intensity changes. To evaluate this, I suggest plotting the same sample data from\nFigure 1 using TC intensity instead of rain rate. If intensity increases along the same\ntimeline, it would indicate that part of the rain rate increase is indeed due to TC\nstrengthening.\nResponse:\nFollowing the reviewer’s suggestion, we re-examined the evolution of TC intensity and\nmean rain rate during the 60 hours before landfall and further quantified the\ncontribution of intensity changes to the increase in rain rate.\nOur results show that the mean rain rate increases by more than 20%, from\napproximately 1.8 mm h⁻¹ to 2.2 mm h⁻¹, corresponding to an increase rate of 0.14 ±\n0.01 mm h⁻¹ per day (Figure A1a). During the same period, TC intensity also increases,\nbut only marginally, at 0.29 ± 0.18 m s⁻¹ per day, and this trend is statistically\ninsignificant (Figure A1b).\nTo isolate the contribution of intensity to the change in rain rate, we further examined\ntheir relationship and found that rain rate increases by 0.04 ± 0.04 mm h⁻¹ per m s⁻¹ of\nintensity change. Combining these results indicates that of the observed pre-landfall\nrain-rate increase of 0.14 ± 0.01 mm h⁻¹ per day, only about 0.012 mm h⁻¹ per day\n(0.04 mm h⁻¹ per day0.29 m s⁻¹ per day) can be attributed to TC intensification, and\nthis contribution is likewise statistically insignificant.\nThese findings have now been incorporated into the revised manuscript to clarify that\nalthough TC intensity does increase slightly before landfall, its contribution to the\nenhancement of rain rate is small compared to the land-induced effects. The relevant\ndetails are as follows:\n“To quantitatively assess the contribution of land effects to enhanced TC precipitation,\nwe further examined changes in the mean intensity of landfalling TCs and their\nrelationship with rain rate. Globally, during the 60 hours before landfall, TC rain rate\nand intensity increase at rates of 0.14 ± 0.01 mm h⁻¹ per day and 0.29 ± 0.18 m s⁻¹ per\nday, although the intensity trend is not statistically significant. During the same period,\nrain rate increases by 0.04 ± 0.04 mm h⁻¹ per m s⁻¹ of intensity change. These results\nindicate that only about 0.012 mm h⁻¹ per day of the observed pre-landfall increase in\nrain rate can be attributed to TC intensification.”\nFig. A1 Changes in the mean rain rate and mean intensity of landfalling TCs\nglobally, and their relationship. In panels a and b, the mean rain rates and intensities\nof all TCs from 60 hours before landfall to the time of landfall at 3-hour intervals are\nshown by the solid line with black dots, and the linear trends are shown by the dashed\nline. Light shading in all panels represents the two-sided 95% confidence interval of\nthe linear trend. The x-axis represents time in hours relative to landfall (00 h), where\nnegative values indicate hours before landfall. In panel c, the red line indicates the linear\nregression between the mean rain rate and the mean intensity before landfall. The linear\nregression coefficients with their error estimates (unit: mm h⁻¹ per day) are shown at\nthe bottom. The R² and p values are in the top right corner.\n2. Additionally, when TCs move poleward from lower to higher latitudes, they\nencounter cooler and drier air masses, which can often enhance precipitation due to\nincreased baroclinicity. Thus, rain rate may also increase with latitude. It would\ntherefore be informative to calculate the mean latitude of the TCs in Figure 1. If\nlatitude increases alongside rain rate, this would suggest that TC position (latitude\nchange) also contributes to the rain rate increase.\nResponse:\nWe appreciate the reviewer’s constructive comments. Following the reviewer’s\nsuggestion, we examined the change in mean latitude of landfilling TCs using the same\nsamples and time window as in Figure 1a. The results show that the mean latitude\nincreases linearly before landfall and exhibits no notable variability (Fig. A2). This\nfeature is consistent with the general poleward movement of most TCs after genesis,\nwhich is driven by the beta effect and the large-scale environmental flow (Chan and\nGray, 1980; Wang and Li, 1992).\nAdditionally, we agree that as TCs move toward the mid- and high latitudes, they may\nencounter cooler and drier air masses, and the associated increase in baroclinicity may\nenhance precipitation, especially with some storms even undergoing extratropical\ntransition. However, in our dataset of 1,468 TCs, the vast majority remain\nequatorward of 30° latitude, and only 117 TCs made landfall at higher latitudes,\naccounting for less than 10% of the total sample. Also, all records involving\nextratropical transition were removed during the landfalling TCs selection\nprocess.\nTo further assess the influence of latitude, we divided the TCs into six 5° latitudinal\nbelts from 5° to 35° based on their landfall locations. A significant increase in rain rate\nis evident in every belt (Fig. A3 and Table A1). However, within the same 5° latitude\nrange, the increasing trends do not strengthen with landfall latitude and are not\nlarger in the 30°–35° belt compared to lower latitudes.\nMoreover, for the non-landfalling TCs, although their mean latitude still increases\nlinearly prior to reaching the “imaginary coastline”, their rain rate does not\nincrease (Fig. A5). More details can be found in our response to Major Comment 3.\nOverall, although the mean latitude of the TCs does increase prior to landfall, these\nfindings suggest that this upward latitudinal shift is more likely a manifestation of\nthe poleward motion of the TCs, and that its contribution is relatively small compared\nto land effects, even though it may contribute to the rain rate increase.\nReference\nChan, J. C. L., and W. M. Gray, 1982: Tropical Cyclone Movement and Surrounding Flow\nRelationships. Mon. Weather Rev., 110, 1354-1374.\nWang, B., and X. Li, 1992: The beta drift of three-dimensional vortices: A numerical study. Mon.\nWeather Rev., 120, 579-593.\nFig. A2 As in Figs. A1a, but for changes in the latitude of landfalling TCs.\nFig. A3 Slopes of the mean rain rate for landfalling tropical cyclones (TCs) 60\nhours before landfall and their error estimates. The mean rain rate of TCs is at 3-\nhourly intervals. Slopes of rain rate obtained from different latitudinal belts (5°-10°,\n10°-15°, 15°-20°, 20°-25°, 25°-30°, and ＞=30°). 5°-10° represents the landfalling\nlatitudinal belts of 5°-10°S and 5°-10°N, and similarly for 10°-15°, 15°-20°, 20°-25°,\n25°-30°, and ＞=30°.\nTable A1 | Statistics of landfalling tropical cyclones (TCs) rain rate with time for\nvarious latitudinal belts.\nNumber of TCs Trend (mm h-1 per day) Increment (%)\nGlobal 1468 0.14±0.01 20.42\n5º- 10º 55 0.09±0.02 17.61\n10º- 15º 267 0.19±0.01 31.82\n15º- 20º 386 0.28±0.02 44.52\n20º- 25º 279 0.18±0.02 23.67\n25º- 30º 95 0.04±0.02 5.58\n＞=30º 117 0.11±0.03 12.72\n3. I also believe that non-landfalling TCs can show rain rate increases associated with\nchanges in intensity and latitude. The authors should carefully analyze how these\nfactors affect rain rate for non-landfalling cases. Without such analysis, it is difficult\nto attribute the observed rain rate increase in landfalling TCs solely to land effects.\nThe additional Figure A1 presented by the authors does not adequately address this\nconcern, as it does not include information on TC intensity or latitude. I strongly\nencourage the authors to provide an additional analysis showing how TC intensity and\nlatitude variations influence rain rate in non-landfalling TCs.\nResponse:\nWe appreciate the reviewer’s constructive comments. We appreciate the reviewer’s\nconstructive suggestions. We examined the changes in the mean rain rate, mean\nintensity, and mean latitude of non-landfalling TCs globally as they approach an\nimaginary coastline located 800 km from the real coast, following our previous study\n(Zhong et al., 2025). This imaginary coastline allows all non-landfalling TCs to “make\nlandfall’’ on the same reference line, which helps isolate the influence of land effects\non rain rate. Here, an example of the selected non-landfalling TCs over the western\nNorth Pacific is provided in Fig. A4. These TCs include only cases that remain at least\n800 km from any coastline, fully meeting the reviewer’s request to ensure that they\nare not approaching the actual coast.\nIn this response, we further show that for non-landfalling TCs across all ocean basins\nthat remain far from land, no significant increase in rain rate or intensity occurs,\nalthough their mean latitude still increases linearly with time during the 60 hours\nbefore reaching the imaginary coastline (Fig. A5). These results also indicate that the\nrain rate increase observed in landfalling TCs is mainly attributable to land–sea\ncontrast, consistent with the increasing trend demonstrated in our model simulations.\nFinally, it is important to note that although land effects provide the primary\ncontribution to the observed rain rate increase in landfalling TCs, they are not the only\ncontributing factor. We have clearly clarified this point in the “Role of land–sea thermal\ncontrasts in the enhancement of TC rain rate” section of the revised manuscript.\n“We hypothesize that the increasing trend in landfalling TC rain rates is associated\nwith enhanced convection, primarily triggered by the contrast between land and sea.”\nFig. A4: Tracks of non-landfalling tropical cyclones crossing an imaginary\ncoastline 800 km from the real coast. The 800 km imaginary coastline represents the\ndistance from the hypothetical landfall points of non-landfalling TCs to the nearest land\npoint, based on the ' dist2land ' variable in the IBTrACS dataset.\nFig. A5 Changes in the mean rain rate, mean intensity, and mean latitude of\nlandfalling TCs globally, and their relationship. In all panels, the mean rain rate,\nmean intensity, and mean latitude of all TCs from 60 hours before landfall to the time\nof landfall at 3-hour intervals are shown by the solid line with black dots, and the linear\ntrends are shown by the dashed line. Light shading in all panels represents the two-\nsided 95% confidence interval of the linear trend. The x-axis represents time in hours\nrelative to landfall (00 h), where negative values indicate hours before landfall. In panel\nThe linear regression coefficients with their error estimates (unit: mm h⁻¹ per day) are\nshown at the bottom. The R² and p values are in the top right corner.\nReferences\n1. Zhong Q, Chan J C L, Duan W, et al. Land-sea contrast leading to a speedup of landfalling tropical\ncyclones [J]. Nature Geoscience, 2025, Acceptance in principle.\n4. Regarding the comparison between EXP1 and EXP2, I also have several questions:\n4.1. It is generally understood that as a TC intensifies, mid- to lower-tropospheric\nrelative humidity increases due to enhanced convection, condensation, and latent heat\nrelease. However, in EXP2, TC intensity increases while relative humidity does not.\nCould the authors explain this discrepancy?\nResponse:\nWe thank the reviewer for this insightful comment. We agree that the mid- to lower-\ntropospheric relative humidity of TCs is generally associated with their intensity\nevolution. In the Land-Rad experiment (EXP1), the relative humidity of simulated TC\nincreases as it intensifies and approaches the real coastline. However, in the noLand-\nRad experiment (EXP2), the TC relative humidity does not increase with its intensity\n(Figure A6). This discrepancy mainly arises from the presence or absence of land–\nsea thermal contrasts.\nIn EXP1 (Land-Rad), the large difference in thermal capacity between land and\nocean facilitates the development of local land–sea circulations. During daytime,\nthe land surface heats more rapidly than the ocean, inducing low-level convergence and\nupward motion over land (X-distance from –2400 to 2000 km), which helps transport\nnear-land moisture into the mid- to lower troposphere. As a result, the relative humidity\nover land becomes higher than that over the ocean. When the TC approaches the\ncoastline, moisture from the land side is advected into the storm circulation, increasing\nthe relative humidity (Figure A7).\nIn contrast, in EXP2 (noLand-Rad), the absence of land removes the land–sea\nthermal contrast, resulting in identical surface properties on both sides of the\n“imaginary coastline” and preventing the formation of local atmospheric\ncirculations. The transport of near-surface moisture into the mid- to lower troposphere\nis therefore suppressed, producing a horizontally uniform humidity field (Figure A8).\nAs the TC approaches this region, the lack of external moisture supply causes a\nreduction in mid- to lower-tropospheric humidity in the TC environment and decreases\nthe overall area-averaged relative humidity, even though the TC intensity increases\nslightly.\nIn summary, the land–sea thermal contrast enhances local circulations that\ntransport moisture upward and toward the TC’s circulation, thereby increasing\nmid- to lower-tropospheric humidity and rainfall as the storm approaches land.\nWe have clearly clarified this point in the “Role of land–sea thermal contrasts in the\nenhancement of TC rain rate” section of the revised manuscript. The relevant details\nare as follows:\n“More notably, in the absence of land, the land–sea thermal contrast disappears,\nproducing identical surface properties on both sides of the “imaginary coastline” and\npreventing the development of local atmospheric circulations. Without these\ncirculations, the transport of near-surface moisture into the mid- to lower troposphere\nis suppressed, producing a horizontally uniform humidity field (Extended Data Fig. 6).\nAs the TC approaches the “imaginary coastline”, the absence of external moisture\nsupply leads to a progressive reduction in mid- to lower-tropospheric humidity in its\nenvironment. Even though low-level convergence and upward motion increase, the\nreduced water vapor content on the offshore side of the vortex still limits the available\nmoisture for convection, being unfavorable for enhanced precipitation in the noLand-\nRad experiment.”\nFigure A6 Changes in relative humidity of the simulated TCs. (a) Experiment 1\n(EXP1, Land-Rad) and Experiment 2 (EXP2, noLand-Rad). Different colored lines at\n6-hour intervals (solid lines with black dots) and the shading represent relative humidity\nacross the entire TC (0–500 km radius). Light shading denotes the 95% confidence\ninterval of the linear trend. Dashed lines indicate the linear regression during the period\nbefore landfall. Linear regression coefficients with error estimates are shown at the\nbottom, and R² and p values are shown in the top-right corner. Time is presented in\nhours relative to landfall (0 h), with negative values indicating hours before TC landfall.\nFigure A7 Spatial and temporal variations of wind vectors (m s⁻¹), relative\nhumidity (shading, %) in experiment 1 (EXP1, Land-Rad). (a), (b) Radius–pressure\nHovmöller diagrams of wind and relative humidity at y = 0 km for the TC during the\n48 hours of daytime and 36 hours of nighttime before landfall. (c), (d) horizontal\ndistributions of wind and relative humidity fields at 850 hPa during daytime and\nnighttime. (e) azimuthal–time Hovmöller diagrams of respective relative humidity at\n850 hPa. In panels (a) to (d), the time in the top left corner is in hours relative to landfall\n(00 h), with negative values indicating hours before landfall. Day and night refer to\nhorizontal and vertical distributions of wind vectors and relative humidity (shading)\nduring the day and night, respectively. In panels (e), we averaged the azimuthal–time\nHovmöller diagram over the TC radius from 0 to 500 km. The blue dashed line indicates\nthe coastline, with land to the west and the sea to the east. In panels (a) to (e), the radius\nvalues on the x- and y-axes are presented relative to the model domain center, with\nnegative values indicating positions west (south) of the domain center and positive\nvalues indicating positions east (north) of the domain center. In panels (a), (b), and (e),\nthe typhoon symbol corresponds to the x-axis values of the TC center location.\nFigure A8 | Same as in Figure A6, except for Experiment 2 (EXP2, noLand-Rad).\nIn panels (a)–(e), the blue dashed line indicates the “imaginary coastline,” with the sea\nlocated to the west and east, respectively.\n4.2 The TC track comparison between EXP1 and EXP2 shows that the TC in EXP2\nmoves considerably more slowly, despite both experiments using an easterly steering\nflow of 5 m s⁻¹. What causes such a large difference in translation speed? This\ndiscrepancy could significantly influence TC structure and rain rate. Could the\nexperimental design be modified to minimize the effects of such translation speed\ndifferences?\nResponse:\nWe thank the reviewer for this insightful comment. The difference in TC translation\nspeed between EXP1 and EXP2 primarily arises from the land–sea contrast in surface\nroughness. Our previous studies have demonstrated that land–sea surface contrast can\ninduce asymmetric flow and convection in landfalling TCs which may generate\nlocalized potential vorticity anomalies and subsequently speedup storm motion (\nWong and Chan, 2006; Zhong et al., 2025).\nSpecifically, in EXP1 (Land-Rad), land–sea surface contrasts enhance surface friction\nand induce asymmetric convection, leading to accelerated TC translation. In EXP2\n(noLand-Rad), the absence of land and the horizontally homogeneous surface\nproperties prevent the development of asymmetric flow and instability on the offshore\nside of the TC circulation (Extended Data Fig. 6), and no notable TC acceleration is\nfound. Consequently, although both experiments are driven by the same easterly\nsteering flow, the TC in EXP1 translates considerably faster (Figure A9). More\ndetailed observational analyses, numerical experiments, and mechanistic diagnostics\nare provided in Zhong et al. (2025), so we do not elaborate on them further in this study.\nIt is also important to note that land–sea contrast not only modulates TC motion but\nalso exerts a direct influence on the rain rate. Therefore, rather than being an artifact of\nmodel design, the translation speed difference reflects a physically consistent response\nto surface property heterogeneity, which is intrinsic to the process of TC landfall and\nrelevant to this study’s focus. We have now clarified this point in the section ‘Role of\nland–sea thermal contrasts in the enhancement of TC rain rate’ in the revised manuscript.\nThe relevant details are provided below:\n“In addition to the influence of land–sea thermal contrast on TC rainfall, land–sea\nfrictional heterogeneity can induce asymmetric circulation and convection in\nlandfalling TCs. Such asymmetries may generate localized potential vorticity\nanomalies, which can accelerate storm translation (Zhong et al., 2025). The change in\nTC translation speed may further modulate regional rain rates.”\nReferences\n2. Wong M L M, Chan J C L. Tropical cyclone motion in response to land surface friction[J]. Journal\nof the Atmospheric Sciences, 2006, 63(4): 1324-1337.\n3. Zhong Q, Chan J C L, Duan W, et al. Land-sea contrast leading to a speedup of landfalling tropical\ncyclones [J]. Nature Geoscience, 2025, Acceptance in principle.\nFigure A9 Comparison of the mean translation speed of the simulated TCs 60 h\nbefore landfall in three experiments and their error estimates.\n5. Finally, I am particularly concerned about the limited model domain size. In EXP1,\nwhen the TC reaches land, its center is only about 400 km from the domain boundary.\nGiven that the rain rate is averaged within a 500 km radius, the outer 100 km\nportion would be excluded, potentially artificially inflating the mean rain rate near landfall.\nThe authors should carefully assess and discuss how this boundary limitation might\naffect the quantitative results and conclusions.\nResponse: In this study, the model employs a two-way double-nested configuration,\nwith an outer domain of 12,000 × 12,000 km and an inner domain of 6,000 × 6,000 km.\nThe north-south oriented coastline is positioned 3,000 km and 1,000 km from the\nwestern boundaries of the outer and inner domains, respectively. Therefore, even when\nthe TC makes landfall, its center remains approximately 1,000 km from the\ninnermost domain boundary, which is substantially larger than the 500 km radius\nused for computing the mean TC rain rate (Fig. A10). As a result, the full averaging\narea remains well within the model domain, and the influence of the lateral\nboundary on both the simulated TC structure and the calculated rain rate is\nexpected to be minimal. We have added a discussion of this point in “Methods”\nsection of the revised manuscript to clarify that the boundary limitation does not\nmaterially affect our quantitative conclusions. The relevant details are as follows:\n“Specifically, the model coastline is configured as a north-south oriented boundary with\ndryland, cropland, and pasture surface categories, positioned 1,000 km east of the\nwestern boundary of the innermost domain. Consequently, even at the time of landfall,\nthe TC center remains over 1,000 km from the lateral boundary, a distance that well\nexceeds the 500 km radius used for rain-rate averaging. Thus, the lateral boundary is\nconsidered unlikely to introduce meaningful impacts on the simulated TC structure or\nthe calculated mean rain rate.”\nFigure A10 Horizontal distributions of wind vectors in Experiment 1 (EXP1,\nLand-Rad). Panels a and b show the horizontal wind vectors (arrows, m s⁻¹) and their\nmagnitudes (shading, m s⁻¹) during daytime and nighttime, respectively. In both panels,\nthe x and y coordinates represent the distance from the model domain center, where\nnegative values indicate positions west or south of the domain center, and positive\nvalues indicate positions east or north of the domain center."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors were successfully addressed all major comments raised by me and therefore I accept this work to be published\nin Nature Communucations in the present form."}, "author_response": ""}]

Decision letter