The prediction of tropical cyclone (TC) intensity change remains one of the greatest challenges for forecasters. The Statistical Hurricane Intensity Prediction Scheme (SHIPS) is one of the most accurate models used in operational centers. The current version of the SHIPS uses predictors including climatology and persistence, environmental conditions, and infrared satellite information. One critical piece of information that is largely missing from the SHIPS is the rainfall and structural features of TCs. In this study, a novel Hurricane Convolutional Neural Network (HCNN) model is proposed to predict future TC intensity by using satellite rainfall images and existing SHIPS predictors. A 20-yr (2000-19) satellite rainfall dataset is obtained from the NASA Integrated Multi-satellitE Retrievals for the Global Precipitation Measurement (GPM) mission (IMERG) product for TCs from the Atlantic basin. The HCNN model is tested for three different radii of the IMERG data from the TC center and 200 km is selected. The model is trained using satellite images for TCs from 2000 to 2017 and tested using TCs from 2018 to 2019. Relative to a multiple linear regression model with SHIPS predictors trained using the same training sample and tested using the same test sample as used for the HCNN, the HCNN model with satellite rainfall input significantly improves forecasts by 9%-13%, 8%-18%, and 5%-9% for all TCs, major hurricanes, and intensifying TCs, respectively, at 6-24-h forecast intervals. Further experiments show that the HCNN can better utilize rainfall structural information than a multilinear regression model integrating SHIPS and rainfall predictors.

Using Tropical Rainfall Measuring Mission Microwave Imager observations of global tropical cyclones (TCs) from 1998 to 2013, relationships between TC intensification rate and inner -core convective and precipitation parameters are examined by decoupling the dependency of these parameters on TC intensity and that on TC intensification rate. A total of 16 TC intensity change-intensity categories are categorized based on the initial intensity and 24-h future intensity change. The results show that the TC inner -core mean rain rate, convective intensity, and stratiform rain occurrence, and axisymmetric index of convective intensity increase significantly with TC intensification rate for each TC intensity category. The symmetry of rain rate and stratiform rainfall occurrence also increase significantly with TC intensification rate for each intensity category, except from slowly intensifying (SI) to rapidly intensifying (RI) group when the initial intensity is major hurricane. The RI major hurricanes have significantly more asymmetric rainfall distribution and distribution of stratiform rainfall occurrence than those of SI major hurricanes. For TCs with initial intensity in tropical depression, tropical storm, and major hurricane categories, the RI group has a significantly more asymmetric pattern of shallow precipitation/convection occurrence in the inner core than the SI group, while it has a significantly more symmetric pattern of deep convection occurrence than the SI group. The inner -core size, as quantified by the radius of maximum azimuthal mean rainfall decreases with both TC intensification rate and TC intensity.

Estimating the magnitude of tropical cyclone (TC) rainfall at different landfalling stages is an important as-pect of the TC forecast that directly affects the level of response from emergency managers. In this study, a climatology of the TC rainfall magnitude as a function of the location of the TC centers within distance intervals from the coast and the per-centage of the raining area over the land is presented on a global scale. A total of 1834 TCs in the period from 2000 until 2019 are analyzed using satellite information to characterize the precipitation magnitude, volumetric rain, rainfall area, and axial-symmetric properties within the proposed landfalling categories, with an emphasis on the postlandfall stages. We found that TCs experience rainfall maxima in regions adjacent to the coast when more than 50% of their rainfall area is over the water. TC rainfall is also analyzed over the entire TC extent and the portion over land. When the total extent is considered, rainfall intensity, volumetric rain, and rainfall area increase with wind speed intensity. However, once it is quantified over the land only, we found that rainfall intensity exhibits a nearly perfect inversely proportional relation with the increase in TC rainfall area. In addition, when a TC with life maximum intensity of a major hurricane makes landfall as a tropical depression or tropical storm, it usually produces the largest spatial extent and the highest volumetric rain.SIGNIFICANT STATEMENT: This study aims to describe the cycle of tropical cyclone (TC) precipitation magni-tude through a new approach that defines the landfall categories as a function of the percentage of the TC precipitating area over the land and ocean, along with the location of the TC centers within distance intervals from the coast. Our central hypothesis is that TC rainfall should exhibit distinct features in the long-term satellite time series for each of the proposed stages. We particularly focused on the overland events due to their effects on human activities, finding that the TCs that at some point of their life cycle reached major hurricane strength and made landfall as a tropical storm or tropical depression produced the highest volumetric rain over the land surface. This research also presents key observa-tional evidence of the relationship between the rain rate, raining area, and volumetric rain for landfalling TCs.

Theoretical models of the potential intensity of tropical cyclones (TCs) suggest that TC rainfall rates should increase in a warmer environment but limited observational evidence has been studied to test these hypotheses on a global scale. The present study explores the general trends of TC rainfall rates based on a 19-year (1998–2016) time series of continuous observational data collected by the Tropical Rainfall Measuring Mission and the Global Precipitation Measurement mission. Overall, observations exhibit an increasing trend in the average TC rainfall rate of about 1.3% per year, a fact that is contributed mainly by the combined effect of the reduction in the inner-core rainfall rate with the increase in rainfall rate on the rainband region. We found that the increasing trend is more pronounced in the Northwestern Pacific and North Atlantic than in other global basins, and it is relatively uniform for all TC intensities. Further analysis shows that these trends are associated with increases in sea surface temperature and total precipitable water in the TC environment.

This study examines the shear-relative rainfall spatial distribution of tropical cyclones (TCs) during landfall based on the 19-year (1998–2016) TRMM satellite 3B42 rainfall estimate dataset and investigates the role of upper-tropospheric troughs on the rainfall intensity and distribution after TCs make a landfall over the six basins of Atlantic (ATL), eastern and central Pacific (EPA), northwestern Pacific (NWP), northern Indian Ocean (NIO), southern Indian Ocean (SIO), and South Pacific (SPA). The results show that the wavenumber 1 perturbation can contribute ∼ 50% of the total perturbation energy of total TC rainfall. Wavenumber 1 rainfall asymmetry presents the downshear-left maxima in the deep-layer vertical wind shear between 200 and 850 hPa for all the six basins prior to making a landfall. In general, wavenumber 1 rainfall tends to decrease less if there is an interaction between TCs and upper-level troughs located at the upstream of TCs over land. The maximum TC rain rate distributions tend to be located at the downshear-left (downshear) quadrant under the high (low)-potential vorticity conditions.

Based on 19 years of precipitation data collected by the Tropical Rainfall Measuring Mission (TRMM) and the Global Precipitation Measurement (GPM) mission, a comparison of the rainfall produced by tropical cyclones (TCs) in different global basins is presented. A total of 1789 TCs were examined in the period from 1998 to 2016 by taking advantage of more than 47,737 observations of TRMM/GPM 3B42 multi-satellite derived rainfall amounts. The axisymmetric component of the TC rainfall is analyzed in all TC-prone basins. The resulting radial profiles show that major hurricanes in the Atlantic basin exhibit significantly heavier inner-core rainfall rates than those in any other basins. To explain the possible causes of this difference, rainfall distributions for major hurricanes are stratified according to different TC intensity and environmental variables. Based on the examination of these parameters, we found that the stronger rainfall rates in the Atlantic major hurricanes are associated with higher values of convective available potential energy, drier relative humidity in the low to middle troposphere, colder air temperature at 250hPa, and stronger vertical wind shear than other basins. These results have important implications in the refining of our understanding of the mechanisms of TC rainfall.

Based on 35-yr (1982–2016) best track and Statistical Hurricane Intensity Prediction Scheme data, this study examined climatology of rapidly intensifying (RI) and slowly intensifying (SI) events as well as their time evolutions of storm-related and environmental parameters for tropical cyclones (TCs) in both North Atlantic (AL) and eastern North Pacific (EP) basins. Major hurricanes were intensified mainly through RI while tropical depression and tropical storms were intensified through SI. The percentage of TCs that underwent RI peaks in the late hurricane season whereas the percentage of TCs that underwent SI peaks early. For the first time in the literature, this study found that RI events have significantly different storm-related and environmental characteristics than SI events for before-, during-, and after-event stages. In both AL and EP basins, RI events always intensify significantly faster during the previous 12 h, are located farther south, and have warmer sea surface and 200-hPa temperatures, greater ocean heat content, larger 200-hPa divergence, weaker vertical wind shear, and weaker 200-hPa westerly flow than SI events for all event-relative stages. In the AL basin, RI events have larger low-level and midlevel relative humidity and larger 850-hPa relative vorticity than SI events for all event-relative stages in the AL and most event-relative stages in the EP. RI events are associated with more convectively unstable atmosphere and are farther away from their maximum potential intensities than SI events for most event-relative stages in the AL and for all event-relative stages in the EP.

A new database, the tropical cyclones passive microwave brightness temperature (TCsBT) database including 6273 overpasses of 503 tropical cyclones (TC) was established from 6-year (2011–2016) Fengyun-3B (FY-3B) Microwave Radiation Imager (MWRI) Level-1 brightness temperature (TB) data and TC best-track data. An algorithm to estimate the TC intensity is developed using MWRI TB’s from the database. The relationship between microwave TB and the maximum sustained surface wind (Vmax) of TCs is derived from the TCsBT database. A high correlation coefficient between MWRI channel TB and Vmax is found at the radial distance 50–100 km near the TC inner core. Brightness temperatures at 10.65, 18.70, 23.8, and 36.5 GHz increase but 89 GHz TB’s and polarization corrected TB at 36.5 GHz (PCT36.50) and PCT89 decrease with increasing TC intensity. The TCsBT database is further separated into the 5063 dependent samples (2010–2015) for the development of the TC intensity estimation algorithm and 1210 independent samples (2016) for algorithm verification. The stepwise regression method is used to select the optimal combination of storm intensity estimation variables from 12 candidate variables and four parameters (10.65h, 23.80v, 89.00v and PCT36.50) were selected for multiple regression models development. Among the four predictors, PCT36.50 contributes the most in estimating TC intensity. In addition, the errors are lower for estimating 6-h and 12-h future Vmax than estimating the current Vmax.

Using a 13-year dataset of Atmospheric Infrared Sounder (AIRS) retrieved temperature profiles including 5019 AIRS overpasses in 1061 tropical storm through category-2 tropical cyclones (TCs) in global basins during 2002-2014, this study examines the relationship between the warm-core structure and TC intensity change with a focus on rapid intensification (RI). The AIRS TC overpasses are classified into RI, slowly intensifying (SI), neutral (N), and weakening (W) categories. The effect of the warm-core structure upon TC intensification is entangled with that upon TC intensity. It is necessary to exclude the weakening category in order to single out the relationship between TC intensification and warm-core structure from a statistical method. The composite warm-core maximum temperature anomaly is the strongest in RI storms (similar to 7 K), followed by W (similar to 6 K), SI (similar to 5 K) and N (similar to 4 K) storms. RI storms have the highest equivalent potential temperature (theta(e)) and CAPE in the eye among all intensity change categories. The warm-core structure of RI storms is asymmetric relative to shear, with the higher temperature anomaly and convective available potential energy (CAPE) located in the down-shear quadrant. When only considering samples with intensification rates >= 0, a significant and positive correlation is found between the warm-core strength and TC intensification rate. The warm-core height is also positively correlated with the TC intensification rate at a high confidence level. The AIRS-derived warm-core temperature anomaly greater than 4 K and weighted warm-core height higher than 450 hPa are the necessary conditions for RI.

A statistical passive microwave intensity estimation (PMW-IE) algorithm for estimating the intensity of tropical cyclones (TCs) in the North Atlantic and northeastern and central Pacific basins is developed and tested. The algorithm is derived from Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) 85-GHz brightness temperatures and near-surface rain-rate retrievals to provide objective estimates of current maximum sustained surface winds (Vmax) and 6-h future Vmax of TCs. The full record of TRMM data (1998-2013) including 2326 TMI overpasses of 503 TCs is separated into dependent samples (1998-2010) for model development and independent samples (2011-13) for model verification. The best track intensities are used as dependent variables in a stepwise multiple-regression approach. Separately for each basin, three regression models are derived using selected 1) 85-GHz-only variables, 2) rain-rate-only variables, and 3) combined 85-GHz and rain variables. The algorithms are evaluated using independent samples and those with contemporaneous aircraft-reconnaissance measurements. Rain-only and combined models perform better than the 85-GHz-only model. Lower errors are found for estimating the 6-h future Vmax than estimating the current Vmax using all three models. This suggests that it is optimal to use passive-microwave-retrieved rain variables observed a few hours earlier to estimate TC intensity. The MAE (RMSE) of 6-h future Vmax is 9 (12) kt (1 kt approximate to 0.51 m s(-1)) when testing the combined models with ATL and EPA independent samples. Aircraft-reconnaissance-based independent samples yields a MAE of 9.6 kt and RMSE of 12.6 kt for estimating 6-h future Vmax.

There is uncertainty as to whether the typical warm-core structure of tropical cyclones (TCs) is featured as an upper-level warm core or not. It has been hypothesized that data from the satellite-borne Advanced Microwave Sounding Unit (AMSU) are inadequate to resolve a realistic TC warm-core structure. This study first evaluates 13 years of Atmospheric Infrared Sounder (AIRS) temperature retrieval against recent dropsonde measurements in TCs. AIRS can resolve the TC warm-core structure well, comparable to the dropsonde observations, although the AMSU-A retrievals fail to do so. Using 13-yr AIRS data in global TCs, a global climatology of the TC warm-core structure is generated in this study. The typical warm-core height is at the upper level around 300–400 hPa for all TCs and increases with TC intensity: 400 hPa (~8 km) for tropical storms, 300 hPa (~10 km) for category 1–3 hurricanes, 250–300 hPa (~10–11 km) for category 4 hurricanes, and 150 hPa (~14 km) for category 5 hurricanes. The range of warm-core height varies with TC intensity as well. A strong correlation between TC intensity and warm-core strength is found. A weaker but still significant correlation between TC intensity and warm-core height is also found.

The tropical cyclone (TC) webpage developed by the U.S. Naval Research Lab (NRL) has been widely used by the community for real‐time TC monitoring and forecasting. Using 14 years of Tropical Rainfall Measuring Mission Microwave Imager and Precipitation Radar data in TCs, precipitation/convection types are quantitatively classified based on vertical radar profiles corresponding to seven different color regions in the NRL's 37 GHz color composite product. The bright cyan and all pink (including pure pink, bright cyan/pink, and weak cyan/pink, and green/pink) regions are identified as nearly 100% precipitative regions. The bright cyan and pure pink regions are shallow and deep convective precipitation, respectively, while the green/pink, weak cyan/pink, and bright cyan/pink regions are mainly stratiform rain. Eighty‐four percent of pixels in the weak cyan region are precipitating, including either shallow convective, stratiform, or anvil precipitation. Seventy‐six percent of pixels in the green region are nonprecipitating pixels, and 24% of them are very lightly precipitating pixels, which could be contaminated from clouds with high liquid water content. The ability to perform TC precipitation‐type analysis of this detail (convective versus stratiform and deep versus shallow) was previously only possible using radar‐based classification algorithms. Using the 37 GHz precipitation‐type analysis, a significant increase of coverage of stratiform precipitation and shallow convection in the inner core is found between 3 and 21 hr before the onset of TC rapid intensification.

The properties of stratiform and the convective precipitation of tropical cyclones (TCs) over the northwest Pacific are examined using the Tropical Rainfall Measuring Mission data for 1998–2013. TCs are classified into inner core (IC), inner rainband (IB), and outer rainband (OB) regions, and the results show that TCs are dominated by stratiform precipitation, which accounts for more than 78% of the total raining area. The highest fraction of the stratiform raining area exists in the IB region and increases as the TC intensity increases (from 80% to 93%). Strong convective signatures generally occur in the IC region, less often in the IB region, and least often in the OB region. Stratiform precipitation in the IC region generally has comparable or even stronger ice scattering signatures and higher 20 dBZ radar echo heights than the convective precipitation in the IB and OB regions. Weak convection decreases significantly as the TC intensity increases, which leads to increased convective intensity. Stratiform (convective) precipitation accounts for 61% (39%) of the total TC volumetric rain and 25% (75%) of the total TC lightning flash, respectively. Moreover, stratiform precipitation's contribution to the total TC volumetric rain and lightning flash increases as the TC intensity increases, which indicates that stronger TCs are favorable for maintaining more stratiform precipitation. The stratiform and convective precipitation properties in different TC regions and intensities cooperatively change with the enhanced ascending branch in the IC region and the radial outflow at the upper levels of the secondary circulation.

In preparation for the Fourth International Workshop on Tropical Cyclone Landfall Processes (IWTCLP-IV), a summary of recent research studies and the forecasting challenges of tropical cyclone (TC) rainfall has been prepared. The extreme rainfall accumulations in Hurricane Harvey (2017) near Houston, Texas and Typhoon Damrey (2017) in southern Vietnam are examples of the TC rainfall forecasting challenges. Some progress is being made in understanding the internal rainfall dynamics via case studies. Environmental effects such as vertical wind shear and terrain-induced rainfall have been studied, as well as the rainfall relationships with TC intensity and structure. Numerical model predictions of TC-related rainfall have been improved via data assimilation, microphysics representation, improved resolution, and ensemble quantitative precipitation forecast techniques. Some attempts have been made to improve the verification techniques as well. A basic forecast challenge for TC-related rainfall is monitoring the existing rainfall distribution via satellite or coastal radars, or from over-land rain gauges. Forecasters also need assistance in understanding how seemingly similar landfall locations relative to the TC experience different rainfall distributions. In addition, forecasters must cope with anomalous TC activity and landfall distributions in response to various environmental effects.

Both low and higher wavenumber asymmetries of tropical cyclone (TC) precipitation are quantified from 16 years of Tropical Rainfall Measuring Mission (TRMM) microwave imager rainfall estimates using Fourier decomposition. The energy spectrum of rainfall perturbation and impacts of the storm motion and vertical wind shear to the rainfall asymmetry are analyzed. It is found that the wavenumber 1 perturbation only contributes 37% of the total perturbation energy of total TC precipitation, while over 70% of the total perturbation energy is from the sum of wavenumbers 1 through 6. Most of the total precipitation asymmetry is from the heavy precipitation, and contributions for light and moderate precipitation types are small. The maximum motion‐relative precipitation asymmetry is generally located down motion but shifts cyclonically after adding wavenumber 2–6 components to wavenumber 1. The asymmetry index of wavenumber 1 and 1–6 asymmetries relative to the vertical wind shear is about 2 to 3 times as large as that relative to the storm motion. The vertical wind shear is a more important factor than the storm motion in producing precipitation asymmetry, especially for the wavenumber 1. A dominant downshear left wavenumber‐1 asymmetry maximum is found regardless of the shear vector orientation to the TC motion, the magnitude of the shear value, and the speed of TC motion. However, the maximum asymmetry of wavenumber 1–6 asymmetry becomes downshear right for the subgroup with low shear, fast motion, and the shear vector left to the motion.

Because surface wind speeds within tropical cyclones are important for operational and research interests, it is vital to understand surface wind structure in relation to various storm and environmental influences. In this study, global rain-corrected scatterometer winds are used to quantify and evaluate characteristics of tropical cyclone surface wind asymmetries using a modified version of a proven aircraft-based low-wavenumber analysis tool. The globally expanded surface wind dataset provides an avenue for a robust statistical analysis of the changes in structure due to tropical cyclone intensity, deep-layer vertical wind shear, and wind shear's relationship with forward storm motion. Apresentation of the quantified asymmetry indicates that wind shear has a significant influence on tropical storms at all radii but only for areas away from the radius of maximum wind in both nonmajor and major hurricanes. Evaluation of a shear's directional relation to motion indicates that a cyclonic rotation of the surface wind field asymmetry from downshear left to upshear left occurs in conjunction with an anticyclonic rotation of the directional relationship (i.e., from shear direction to the left, same, right, or opposite of the motion direction). It was discovered that in tropical cyclones experiencing effects from wind shear, an increase in absolute angular momentum transport occurs downshear and often downshear right. The surface wind speed low-wavenumber maximum in turn forms downwind of this momentum transport.

The structural evolution of the inner core and near-environment throughout the life cycle of Hurricane Edouard (2014) is examined using a synthesis of airborne and satellite measurements. This study specifically focuses on differences in the distribution of deep convection during two periods: when Edouard intensified toward hurricane status, and when Edouard peaked in intensity and began to weaken. While both periods saw precipitation maximized in the downshear-left and upshear-left quadrants, deep convection was only seen from the aircraft during the intensifying period.Deep convection was located farther inside the radius of maximum winds (RMW) during the intensifying period than the weakening period. This convection is traced to strong updrafts inside the RMW in the downshear-right quadrant, tied to strong low-level convergence and high convective available potential energy (CAPE) as the storm remained over warm water in a moist environment. Strong updrafts persisted upshear left and were collocated with high inertial stability in the inner core. During weakening, no deep convection was present, and the precipitation that was observed was associated with weaker convergence downshear right at larger radii, as CAPE was reduced from lower sea surface temperatures, reduced humidity from subsidence, and a stronger warm core. Weak updrafts were seen upshear left, with little coincidence with the high inertial stability of the inner core.These results highlight the importance of the azimuthal coverage of precipitation and the radial location of deep convection for intensification. A more symmetrical coverage can occur despite the presence of shear driven azimuthal asymmetries in both the forcing and the local environment of the precipitation.