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Dual-polarization radar often detects columnar regions of enhanced differential reflectivity (ZDR) extending vertically above the environmental 0 °C level. Indicative of supercooled liquid drops and wet ice particles lofted by strong updrafts, these ZDR columns are increasingly understood to be of use in predicting extreme rainfall. With the aim of achieving practical application of ZDR column measurements, this paper focuses on the relationship between the height of ZDR columns and rainfall intensity near the ground. All the data on ZDR columns analyzed in this study was collected from weather radar stations in Japan. The height of each column and rainfall rates at low levels were analyzed using an automated algorithm. A regression analysis result reveals peak column height to be positively correlated with maximum rainfall rate near ground level, and that rainfall intensity on the ground is likely to exceed 50 mm h-1 when radar identifies a ZDR column. Furthermore, extreme rainfall with an intensity of 180 mm h-1 or more is likely associated with a column over 3 km tall from the 0 °C level. These findings suggest that surveillance of ZDR columns can contribute to the reliability of very short-range forecasts or nowcasts as well as assist with the issue of early warnings of extreme rainfall and flash floods. [ABSTRACT FROM AUTHOR]

• Weather conditions often spoil outdoor acoustic mass notification system. • Long-term surveys of rainfall rate and outdoor noise at four different sites. • Rainfall increased the SPL of outdoor noise mainly above mid-frequencies. • A model for estimating the SPL of outdoor noise under rainy conditions is proposed. • The model indicates that heavy rainfall significantly reduces the system performance. In Japan, there are many natural disasters that often expose residents to the risk of serious damage. To minimize such damage, the Japanese government has built a disaster information network using a radio system, and as part of it, an outdoor acoustic mass notification system is routinely operated. However, its intelligibility may be reduced by outdoor noise, which is increased by rainfall. In the present study, long-term surveys of rainfall rate and outdoor noise were conducted at four sites with different surrounding environments and the relationship between them was analyzed to clarify the effects of the rainfall rate on the physical characteristics of outdoor noise. The results indicated that the median sound pressure level for each 1/3 octave band increased with increasing rainfall rate, mainly above mid-frequencies of around 500 Hz to 1 kHz, though the frequency at which the level began to rise depended on the time period and site. Furthermore, a newly proposed macroscopic model for estimating the sound pressure level of outdoor noise under rainy conditions indicated that the distance from the loudspeaker of the system to ensure a certain speech-to-noise ratio could be significantly reduced by heavy rainfall. [ABSTRACT FROM AUTHOR]

The Global Precipitation Measurement mission is a major U.S.–Japan joint mission to understand the physics of the Earth's global precipitation as a key component of its weather, climate, and hydrological systems. The core satellite carries a dual-precipitation radar and an advanced microwave imager which provide measurements to retrieve the drop size distribution (DSD) and rain rates using a Combined Radar-Radiometer Algorithm (CORRA). Our objective is to validate key assumptions and parameterizations in CORRA and enable improved estimation of precipitation products, especially in the middle-to-higher latitudes in both hemispheres. The DSD parameters and statistical relationships between DSD parameters and radar measurements are a central part of the rainfall retrieval algorithm, which is complicated by regimes where DSD measurements are abysmally sparse (over the open ocean). In view of this, we have assembled optical disdrometer datasets gathered by research vessels, ground stations, and aircrafts to simulate radar observables and validate the scattering lookup tables used in CORRA. The joint use of all DSD datasets spans a large range of drop concentrations and characteristic drop diameters. The scaling normalization of DSDs defines an intercept parameter NW, which normalizes the concentrations, and a scaling diameter Dm, which compresses or stretches the diameter coordinate axis. A major finding of this study is that a single relationship between NW and Dm, on average, unifies all datasets included, from stratocumulus to heavier rainfall regimes. A comparison with the NW–Dm relation used as a constraint in versions 6 and 7 of CORRA highlights the scope for improvement of rainfall retrievals for small drops (Dm < 1 mm) and large drops (Dm > 2 mm). The normalized specific attenuation–reflectivity relationships used in the combined algorithm are also found to match well the equivalent relationships derived using DSDs from the three datasets, suggesting that the currently assumed lookup tables are not a major source of uncertainty in the combined algorithm rainfall estimates. [ABSTRACT FROM AUTHOR]

The development and verification of a forecast method for localized, meso-γ-scale (2-20 km), extreme heavy rainfall ( MγExHR) is important, because it can cause urban flash flooding and inundation with accompanying damage and potential loss of life. Although previous studies have examined the predictability of precipitation at a very short range (≤1 h) using extrapolation-based nowcasts, they did not specifically focus on MγExHR. In this study, we examine the predictability of 23 selected events of MγExHR (1 h rainfall accumulation ≥50 mm) that occurred during the warm season of 2014 in Japan using High-Resolution Precipitation Nowcasts ( HRPNs) provided by the Japan Meteorological Agency, which are extrapolation-based nowcasts. Traditional grid-scale verification using the equitable threat score shows that the HRPNs usefully predict the heavy rainfall areas of ≥20 mm h−1 for at least 12 min at the grid scale of 1 km. Neighbourhood verification using fractions skill scores shows that HRPNs usefully predict the areas of ≥20 mm h−1 up to 29 min by tolerating 11 km displacement errors. After 30 min, a useful forecast cannot be obtained, even if the 11 km displacement errors are tolerated for the ≥20 mm h−1 areas. This result suggests that a numerical weather prediction ( NWP) model, whose accuracy is useful after ∼30 min, is necessary to seamlessly provide useful forecasts for heavy rainfall areas of ≥20 mm h−1 for MγExHR with ∼10 km displacement errors, by blending the extrapolation-based nowcast with NWP. [ABSTRACT FROM AUTHOR]

We used polybrominated diphenyl ether (PBDE) data in air at 38 sites across Japan (2009–2012), which were measured by the Japan Ministry of Environment (JMOE), to elucidate the time trend and seasonality of atmospheric PBDEs. In order to address few (7% for BDE-47) to many (63% for BDE-153 and 183) non-detect data, Tobit model, also called a censored regression model was used. The model revealed that the concentrations of PBDE congeners were influenced by a combination of year, temperature, rainfall rate, and population density. Greater declines were observed for BDE-47, -99, -153 and -183 (−21, −25, −17, −23%/year, p < 0.05) than for BDE-209 (−6%/year, p = 0.065). These trends were consistent with the estimated trends of penta-, octa- and deca-BDE contained in in-use products based on domestic demand for PBDEs in Japan and product lifespan. Seasonal patterns were opposite for light congeners (BDE-47 and -99), which increased with temperature, and heavy congeners (BDE-183, and -209), which decreased with temperature. Temperature-dependent emission (evaporation) for light congeners and temperature-independent emission (abrasion) for heavy congeners, coupled with seasonality of atmospheric boundary layer height, might explain these seasonal patterns. Human population density showed a positive correlation with all PBDE congener concentrations, whereas PBDEs showed negative correlation with rainfall rate. [ABSTRACT FROM AUTHOR]

Climate change represents an overwhelming challenge that demands urgent intervention for effective resolution. Among the devastating consequences of climate change, flash floods stand out as one of the most catastrophic repercussions. This research focuses on two primary objectives. Firstly, it aims to evaluate the existing state of flash flood intensity (FFI) in a specific area of Hamamatsu city, Japan, which frequently experiences flash flood incidents. Secondly, it seeks to develop a mitigation plan to alleviate the adverse impacts of flooding on buildings within the area. To accomplish these objectives, four parameters related to FFI (namely, runoff depth, runoff velocity, runoff duration, and affected portion) were selected and estimated through the implementation of hydrological and hydrodynamic models. Additionally, a hydrological model was employed, utilizing a storm event with a return period of 100 years as input. During this simulated storm event, FFI values were calculated and categorized into four distinct levels. The results revealed that more than one-tenth of the examined buildings encountered the highest scale of FFI (category 4), while categories 3 and 4 combined accounted for nearly three-quarters of all buildings in the study area. Moreover, two mitigation strategies were adopted to prevent flooding within the buildings' vicinity. Finally, this study provides a valuable framework and guidance for decision-makers and insurance companies, enabling them to assess the flood hazard status of buildings and make informed decisions accordingly. [ABSTRACT FROM AUTHOR]

A high-resolution, summit-to-sea unstructured-grid model was used to simulate two compound flooding events in different geomorphic settings in Japan: the July 2012 flood in Kumamoto city and July 2018 flood in Okayama City, both caused by a torrential rainfall during seasonal Meiyu front but otherwise exhibiting quite different compound flood characteristics. The model shows good performance in simulating flooding extent; e.g., the Hit Rate for the inundation event in 2018 exceeds 0.9. Sensitivity tests were conducted to determine the roles/significance of levee structures along major rivers in coastal regions. Our results indicate that riverine and oceanic factors as well as levees significantly contribute to the observed flooding extents, and we are able to quantify the contributions from each factor at different locations. The model has been implemented operationally as the first nationwide 3-day compound flooding forecast system for Japan. [ABSTRACT FROM AUTHOR]

Critical rainfall events are used in landslide early warning systems to predict the occurrence and severity of landslides. In the present study, historical critical rainfall events triggering landslides in Japan were analyzed. We mainly considered the amount of exceeding/nonexceeding rainfall (referred to as critical rainfall here) within an existing 1-km grid covering Japan where landslides occurred. Furthermore, this study could inform the LEWS operational performance. For this purpose, we used historical landslide records retrieved from a Japanese inventory, radar-based rainfall data (1-km grid resolution), and critical rainfall data collected over the past 17 years. Nearly equal numbers of rainfall events were identified with rainfall below and exceeding the critical rainfall level. The probability that a series of rainfall events could cause a landslide was approximately 1.15% when the critical rainfall level was exceeded and 0.09% when the critical rainfall level was not exceeded, with a difference of approximately 10 times. It was also found that even if critical rainfall was not exceeded, in the case of debris flows and slope failures, rainfall exceeding the critical rainfall level occurred one or two days before. In the case of landslides, there occurred rainfall exceeding the critical rainfall level one or two weeks before, and if critical rainfall was exceeded during a subsequent rainfall event, a landslide could occur. In addition, operational evaluation of the Japanese LEWS revealed a recall value of 0.486 in regard to the occurrence prediction accuracy, which was related to the fact that almost half of the rainfall events exhibited rainfall not exceeding the reference rainfall level. The nonoccurrence prediction accuracy reached 0.935, which was greatly influenced by true negative data of nonexceeding rainfall events, accounting for most of the data. [ABSTRACT FROM AUTHOR]

Interception loss (IL) by the forest canopy removes a substantial quantity of rainwater within forested ecosystems. The large-scale unmanaged Japanese coniferous plantations with high stand density (SD) in Japan raise concerns about an additional increasing IL as a result of a new influential factor of dead branches under canopies. Thus, evaluating the usage of IL estimation models is vital to regulating the water and environment in such coniferous plantations. This study aimed to examine the applicability of the reformulated Gash analytical model (RGAM) to unmanaged coniferous plantations with high SD laden with dead branches. We established two plots (P1 and P2) laden with dead branches under the same SD of 2250 stems ha−1 but with different numbers of dead branches (56 vs. 47 branches per tree) in an unmanaged Japanese coniferous plantation. Results demonstrated that a large difference was found in canopy storage capacity (S) in P1 and P2 (3.94 vs. 3.25 mm), which was influenced by the different number of dead branches; therefore, the IL ratio to gross rainfall differed considerably (32.7% in P1 and 26.7% in P2) regardless of the SD being the same. The difference in S enables the RGAM to reflect the influence of dead branch structures on IL, leading to an acceptable RGAM performance for both P1 and P2 ("fair" IL relative errors: −20.2% vs. −16.1%) in the present study of unmanaged coniferous plantations with high SD laden with dead branches. [ABSTRACT FROM AUTHOR]

Abstract: This research aims to investigate the role of wetland parameters in controlling the response of groundwater level (GWL) towards hydrologic condition and climate condition in Sphagnum mire of Sarobetsu Mire, northern Hokkaido, Japan. Hourly data of GWL, rainfall, and climate of summer for year 1993, 1994, 1997, 2003, 2008, and 2009 are used for modeling GWL fluctuation at point E on the observed area. The GWL modeling in this research adopts the philosophy of Sugawara''s tank model, which involved c1 as a function of soil porosity, c2 as a function of rainfall rate, and c3 as a function of soil hydraulic conductivity and slope. Parameter optimization of the model is undertaken by adjusting c1, c2, and c3. The model is quite successful in imitating the behavior of GWL on observed area indicated by average NS value of about 70%. The best-fit model of each year of simulation shows that the value of c1 and c3 is 1.3 and 0.0, respectively. The value of 0.0 for c3 indicates that there is no discharge coming out from the system. The value of c2 varied and depended on the rate of rainfall and evapotranspiration. The research also found that basically c2 is the ratio between rainfall and evapotranspiration. The model also indicates very small or almost no effect of lateral water balance such as lateral inflow from or to other sources on surrounding the observed area. [Copyright &y& Elsevier]

The multiparameter phased-array weather radar (MP-PAWR) was the first dual-polarized phased-array weather radar to be commissioned in Japan (2017). When conducting a volume scan, the MP-PAWR respectively uses electronic and mechanical scanning in the elevation and azimuth angles to achieve rapid scanning and high-density observations. Although the effectiveness of the MP-PAWR has been demonstrated in case studies, its observation accuracy is yet to be quantitatively analyzed. Therefore, this study compared data of MP-PAWR with that of an operational dual-polarized weather radar with a parabolic-type antenna [X-band multiparameter (X-MP) radar] using 2 347 097 data samples obtained over 14 h. The results showed that the observation accuracy of the MP-PAWR was approximately the same as that of the X-MP radar at low elevations. The correlations of observational parameters (radar reflectivity factor, differential resistivity, specific differential phase, and Doppler velocity) between the MP-PAWR and X-MP radar ranged from 0.77 to 0.99 when MP-PAWR data were recorded within 15 s of the X-MP radar observations. The correlation between the observational parameters of the two radars decreased as the observation time difference between the X-MP radar and MP-PAWR increased. In particular, the correlation coefficients between the specific differential phase and the differential reflectivity were considerably lower than the single-polarization parameter at observation time difference of 240–300 s. By providing high-frequency and high-density dual-polarization observations, the MP-PAWR can contribute to rainfall prediction in Japan and reduce the damage caused by localized, rapidly developing cumulonimbus clouds. Significance Statement: Data from the multiparameter phased-array weather radar—a newly developed Japanese dual-polarized phased-array radar—were quantitatively compared with those from an operational Doppler weather radar to determine the observation accuracy of the new radar and the effect of the observation time on observation accuracy. The results showed that the high temporal resolution of the multiparameter phased-array weather radar provides highly accurate precipitation observations developed in a short time. [ABSTRACT FROM AUTHOR]

Triggering of volcano seismic activity and eruptions by tides, atmosphere pressure, rainfall, and earthquakes have been in constant debate. However, there is limited evidence concerning the triggering of very‐long‐period signals (VLPs), which are closely linked to volcano conduit dynamics. Persistent and repetitive VLP event beneath the Aso volcano, historically termed long‐period tremor (LPT), manifests episodic pressurization and depressurization events in a crack‐like shallow conduit. Here we show that LPT activity display no appreciable spectral peaks associated with major diurnal/semidiurnal tidal constituents or barometric pressure. Instead, passing surface waves of ∼0.01 m/s from the 2011 Tohoku earthquake elevated LPT activity and preferentially increased the likelihood of depressurization events. We suggest that the hydrothermal reservoir near the LPT source behaves like a confined (unconfined) aquifer against short‐period (long‐period) stress. A high stress rate of ∼102 Pa/s is sufficient to enhance the permeability of the conduit plug/wall and preferentially promotes depressurization events. Plain Language Summary: At one of the most active volcanoes in Japan, Aso volcano, long‐period tremor (LPT) was first discovered in the 1930s by Prof. Sassa in Kyoto University and such LPT activity have persisted since. It offers a unique natural laboratory and controlled experiment to systematically explore how periodic loadings trigger or modulate LPT activity in a persistently degassing volcano equipped with a well‐developed hydrothermal system. After carefully considering the background noise level modulated by the local wind and meteorological condition, we find no evidence of modulation or triggering by tides, barometric pressure, or temperature. Systematic analysis of surface wave amplitudes from great earthquakes suggests a common triggering threshold (∼0.01 m/s) shared by LPT (pressure change events) in Aso and volcano‐tectonic earthquakes (brittle failure events) in some geothermal areas, supporting key processes of pressure oscillations and unclogging fractures by dynamic stress. Key Points: Repetitive very‐long‐period signals (VLP) in Aso volcano (aka long‐period tremor, LPT) do not show detectable response against major tidal constituents, barometric pressure and temperatureStrong surface waves from the 2011 Tohoku earthquake (> 0.01 m/s) preferentially modulate depressurization LPT events for several daysResponse of hydrothermal reservoir to pressure change induced by periodic loadings is likely stress‐rate dependent [ABSTRACT FROM AUTHOR]

Dynamics of the East Asian spring rainband are investigated with a reanalysis dataset and station observations. Here, it is revealed that the rainband is anchored by external forcings. The midtropospheric jet core stays quasi-stationary around Japan. It has two branches in its entry region, which originate from the south and north flanks of the Tibetan Plateau and then run northeastward and southeastward, respectively. The southern branch advects warm air from the Tibetan–Hengduan Plateau northeastward, forming a rainband over southern China through causing adiabatic ascent motion and triggering diabatic feedback. The rainband is much stronger in spring than in autumn due to the stronger diabatic heating over the Tibetan–Hengduan Plateau, a more southward-displaced midtropospheric jet, and the resulting stronger warm advection over southern China. The northern jet branch forms a zonally elongated cold advection belt, which reaches a maximum around northern China, and then weakens and extends eastward to east of Japan. The westerly jet also steers strong disturbance activities roughly collocated with the cold advection belt via baroclinic instability. The high disturbance activities belt causes large cumulative warm advection (CWA) through drastically increasing extremely warm advection days on its eastern and south flanks, where weak cold advection prevails. CWA is more essential for monthly/seasonally rainfall than conventionally used time-average temperature advection because it is shown that strengthened warm advection can increase rainfall through positive diabatic feedback, while cold advection cannot cause negative rainfall. Thus, the rainband is collocated with the large CWA belt instead of the warm advection south of it. This rainband is jointed to the rainband over southern China, forming the long southwest–northeast-oriented East Asian spring rainband. Increasing moisture slightly displaces the rainband southeastward. [ABSTRACT FROM AUTHOR]

A phased array weather radar (PAWR) can complete one volume scan in 30 s, thus enabling us to obtain high spatiotemporal resolution echo intensities and wind fields of storms. Using its rapid scanning capability, we investigated the evolution of a convective storm in detail. To describe the evolution of convective storms, we used the following definitions. The precipitation cell is defined as a three-dimensionally contiguous region of 40 dBZ or greater. The precipitation core is defined by a threshold of positive deviation greater than 7 dBZ, which is a difference from the average reflectivity during the mature stage of the cell. An updraft core is defined as an updraft region of 1 m s-1 or stronger at a height of 2 km. An isolated convective storm was observed by two PAWRs on 7 August 2015 in the Kinki District, western Japan. The storm was judged as a single cell, according to the above definition. We identified nine precipitation cores and five updraft cores within 49 min in the mature stage of the cell. A long-lasting updraft core and its branches moved southwestward or southeastward. Around these updraft cores, the precipitation cores were gen- erated successively. The updraft core with the longest duration lasted 73.5 min; in contrast, the lifetimes of the precipitation cores were from 4.5 min to 14.5 min. The precipitation cell was maintained by the successive generations of updraft cores which lifted humid air associated with a low-level southwesterly inflow. The total amounts of water vapor inflow supplied by all the identified updraft cores were proportional to the volumes of the precipitation cell, with a correlation coefficient of 0.75. Thus, the extremely high spatiotemporal resolution of the PAWR observations provides us with new evidence that an isolated convective storm can be formed by multiple precipitation cores and updraft cores. [ABSTRACT FROM AUTHOR]

The Tropical Rainfall Measuring Mission (TRMM) satellite was launched in 1997, and the observations continued for more than 17 years. The features of TRMM observation were as follows: (a) it followed a non-sun synchronized orbit that enabled the diurnal variation of precipitation to be investigated; (b) it carried a precipitation radar and microwave and infrared radiometers, along with two instruments of opportunity in the form of a lighting sensor and a radiation budget sensor; and (c) it worked as a standard reference for precipitation measurements for other spaceborne microwave radiometers, which enabled global rain maps to be developed. For science, TRMM provided precise and accurate rain distributions over tropical and subtropical regions. The rainfall results are primarily essential for the study of precipitation climatologies, whereas the three-dimensional images of precipitation systems enabled the study of the global characteristics of precipitation systems. Technologically, the spaceborne rain radar onboard TRMM demonstrated the effectiveness of radars in space, whereas the combination with other rain observation instruments showed its effectiveness as a calibration source. Multi-satellite rain maps in which TRMM was the reference standard have been developed, and they became prototypes of the multi-satellite Earth observation systems. On the basis of the great success of TRMM, the Global Precipitation Measurement (GPM) was designed to expand TRMM's coverage to higher latitudes. The core satellite of GPM is equipped with a dual-frequency precipitation radar (DPR) and a microwave radiometer. DPR comprises a Kuband radar (KuPR) and a Ka-band radar (KaPR) and can discriminate solid from liquid precipitation. The period of the precipitation measurement with spaceborne radars extended to more than 23 years, which may make it possible to detect the change of precipitation climatology related to change in the global environment. Although TRMM's and GPM's accomplishments are very broad, this paper attempts to highlight Japan's contributions to the science of these missions. [ABSTRACT FROM AUTHOR]

The quantitative evaluation of the effects of bedrock groundwater discharge on spatial variability of stream dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) concentrations has still been insufficient. We examined the relationships between stream DOC, DIN and DIP concentrations and bedrock groundwater contribution to stream water in forest headwater catchments in warm‐humid climate zones. We sampled stream water and bedrock springs at multiple points in September and December 2013 in a 5 km2 forest headwater catchment in Japan and sampled groundwater in soil layer in small hillslopes. We assumed that stream water consisted of four end members, groundwater in soil layer and three types of bedrock groundwater, and calculated the contributions of each end member to stream water from mineral‐derived solute concentrations. DOC, DIN and DIP concentrations in stream water were compared with the calculated bedrock groundwater contribution. The bedrock groundwater contribution had significant negative linear correlation with stream DOC concentration, no significant correlation with stream DIN concentration, and significant positive linear correlation with stream DIP concentration. These results highlighted the importance of bedrock groundwater discharge in establishing stream DOC and DIP concentrations. In addition, stream DOC and DIP concentrations were higher and lower, respectively, than those expected from end member mixing of groundwater in soil layer and bedrock springs. Spatial heterogeneity of DOC and DIP concentrations in groundwater and/or in‐stream DOC production and DIP uptake were the probable reasons for these discrepancies. Our results indicate that the relationships between spatial variability of stream DOC, DIN and DIP concentrations and bedrock groundwater contribution are useful for comparing the processes that affect stream DOC, DIN and DIP concentrations among catchments beyond the spatial heterogeneity of hydrological and biogeochemical processes within a catchment. [ABSTRACT FROM AUTHOR]

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This study evaluates possible changes in tropical cyclone (TC) precipitation over Japan under a future warmer climate using an ensemble projection generated by a non-hydrostatic regional climate model with a resolution of 5 km (NHRCM05) under the RCP8.5 scenario. NHRCM05 reproduces TC precipitation and TC intensity more accurately than does a general circulation model with a resolution of 20 km. The number of TCs approaching Japan is projected to decrease under the future climate, while the TC precipitation rate increases. As these two effects cancel each other out, total TC precipitation, and the frequency of the moderate TC precipitation that is usual under the present climate, shows no significant change. On the other hand, the frequency of extreme TC precipitation increases significantly because the intensification of the TC precipitation rate outweighs the reduction in TC frequency. The increase in the TC precipitation rate is caused primarily by the increase in water vapor around the TCs, which in turn results from the increase in environmental water vapor. The intensification and structural changes to TCs also contribute to the enhanced TC precipitation. [ABSTRACT FROM AUTHOR]

The assimilation impact of high‐temporal volume scan data (1 min) on very‐short‐range (within 1 h) quantitative precipitation forecasts (QPFs) of a severe storm was investigated using a nudging data assimilation method. This study investigated (a) the assimilation impact of two observational parameters (potential temperature, retrieved using a traditional technique and pseudo‐water vapour, obtained from reflectivity observations), (b) the sensitivity to the temporal resolution of three‐dimensional radar observations, and (c) the assimilation method (re‐initialization at a single time or sequential nudging over 20 min), regarding the predictability of a tornadic storm observed on 2 September 2013 around Koshigaya City, Japan. Our results indicate that (a) data assimilation of both pseudo‐water vapour and potential temperature perturbation demonstrated the highest contribution for up to the first 30 min of the evaluation period (50 min in total), (b) finer temporal resolution provided a better forecast overall, and (c) a nudging scheme provided a better forecast in the first 30 min. Our results showed better predictability than traditional extrapolation‐based nowcasts in the latter 40 min of the evaluation period. The data assimilation with water vapour and potential temperature, determined from a 1 min volume scan, therefore has the potential to extend the cloud‐scale predictability in strong rainfall intensity situations (>20 mm/h) of very‐short‐range QPFs (within 30 min). [ABSTRACT FROM AUTHOR]

Strong vertical wind shear produces asymmetries in the eyewall structure of a tropical cyclone (TC) and is generally a hostile environment for TC intensification. Typhoon Noul (2015), however, reintensified and formed a closed eyewall despite 200-850-hPa vertical shear in excess of 11 m s-1. Noul's reintensification and eyewall formation in strong shear were examined by using Doppler radar and surface observations. The evolution of the azimuthal-mean structure showed that the tangential wind at 2-km altitude increased from 30 to 45 m s-1 in only 5 h. During the first half of the reintensification, the azimuthal-mean inflow penetrated into the ~40-km radius, well inside the radius of maximum wind (RMW), at least below 4-km altitude, and reflectivity inside the RMW increased. As for the asymmetric evolution, vigorous convection, dominated by an azimuthal wavenumber-1 asymmetry, occurred in the downshear-left quadrant when shear started to increase and then moved upshear. A mesovortex formed inside the convective asymmetry on the upshear side. The direction of vortex tilt between the 1- and 5-km altitudes rotated cyclonically from the downshear-left to the upshear-right quadrant as the vortex was vertically aligned. In conjunction with the alignment, the amplitude of the wavenumber-1 convective asymmetry decreased and a closed eyewall formed. These features are consistent with the theory that a vortex can be vertically aligned through upshear precession. The analysis results suggest that the vortex tilt, vigorous convection, and subsequent intensification were triggered by the increase in shear in a convectively favorable environment. [ABSTRACT FROM AUTHOR]

The precipitation characteristics of extreme events in August determined from 13 years of satellite data around Japan in the TRMM observation region and their relationship with large-scale environmental conditions are examined. Two types of extreme events, extreme rainfall and extreme convective events, are defined in each analysis grid box using maximum near-surface rainfall and maximum 40-dBZ echo-top height in each event, respectively. There are clear differences in precipitation characteristics between the two types of extreme events. Extreme rainfall events are more organized precipitation systems than the extreme convective events, with relatively lower echo-top heights and very low lightning activity. There are also clear differences in the related environmental conditions, where the environments related to the extreme rainfall events are somewhat convectively stable and very humid in almost the entire troposphere. These facts are consistent with our previous studies and reinforce the importance of warm-rain processes in extremely intense precipitation productions. The environments related to the extreme rainfall events exhibit a zonally extended moist anomaly in the free troposphere from southern China to the east of Japan, indicating that the excessive moisture transported from the west by a large-scale flow may partially play a role in producing environmental conditions favorable for extreme rainfall. On the other hand, the environments related to extreme convective events are not associated with free-tropospheric moisture inflow. The relationships with the tropical cyclones and upper-tropospheric dynamical fields are also examined, and are found to be clearly different between the extreme rainfall events and extreme convective events. [ABSTRACT FROM AUTHOR]

A ground-based Doppler radar observed the rapid intensification (RI) of Typhoon Goni (2015) for 24 h immediately after it completed an eyewall replacement cycle. Goni's RI processes were examined by using radar reflectivity and wind fields retrieved by the ground-based velocity track display (GBVTD) technique. The maximum wind at 2-km altitude increased by 30 m s-1 during the first 6 h of RI, and it further increased by 20 m s-1 during the subsequent 12 h. Around the onset of RI, relatively strong outflow (>2 m s-1) was present both inside and outside the radius of maximum wind (RMW) above the boundary layer (BL), suggesting the existence of supergradient flow in and just above the BL. Despite this outflow, angular momentum increased inside the RMW. The low-level RMW contracted rapidly from 50 to 33 km, causing the RMW to slope greatly outward with height. The radius of maximum reflectivity was a few kilometers inside the RMW. A budget analysis of absolute angular momentum showed that the outflow contributed to the contraction of the tangential wind field. During RI, eyewall convection was enhanced, and a well-defined eye appeared. The low-level outflow changed into inflow immediately outside the RMW. Then the tangential wind field and high inertial stability region expanded radially outward, followed by the formation of an outer reflectivity maximum at twice the RMW. The contraction speed of the low-level RMW slowed down. [ABSTRACT FROM AUTHOR]

A tornadic supercell and associated low-level mesocyclone (LMC) observed on the Kanto Plain, Japan, on 6 May 2012 were predicted with a nonhydrostatic mesoscale model with a horizontal resolution of 350 m through assimilation of surface meteorological data (horizontal wind, temperature, and relative humidity) of high spatial density and C-band Doppler radar data (radial velocity and rainwater estimated from reflectivity and specific differential phase) with a local ensemble transform Kalman filter. With assimilation of both surface and radar data, a strong LMC was successfully predicted near the path of the actual tornado. When either surface or radar data were not assimilated, however, the LMC was not predicted. Therefore, both surface and radar data were essential for successful LMC forecasts. The factors controlling the strength of the predicted LMC, defined as a low-level maximum vertical vorticity, were clarified by an ensemble-based sensitivity analysis (ESA), which is a new approach for analyzing LMC intensification. The ESA showed that the strength of the LMC was sensitive to low-level convergence forward of the storm and to low-level relative humidity in the rear of the storm. Therefore, the correction of these low-level variables by assimilation of dense observations was found to be particularly important for forecasting and monitoring the LMC in the present case. [ABSTRACT FROM AUTHOR]

The characteristics and environmental properties of warm-season quasi-stationary convective clusters (QSCCs) in Japan were statistically investigated using operational weather radar and upper-air sounding data from May to October during 2005-2012. The characteristics of the environmental conditions for the development of QSCCs were described through a comparison with those for no-rain cases. We identified 4133 QSCCs over the Japanese major islands. By compiling numerous QSCC samples, the horizontal scales of QSCCs with a circular shape are about 20 km on average with a maximum of 72 km, indicating that warm-season QSCCs in Japan are meso- β-scale phenomena. The analyses of the environmental conditions for the QSCC and no-rain cases showed that the amount of moisture in the lower layer controls the stability condition for the development of the QSCCs, and that the magnitudes of the wind shear and the helicity in the lower troposphere distinguish the kinematic environments for the development of the QSCCs. An increase in the middle-level moisture leads to a larger amount of precipitable water vapour in the QSCC environments, suggesting that atmospheric moistening before the development stage of convection plays an important role in the development of the QSCCs. Additionally, the precipitation intensity has a higher correlation with the convective instability, whereas the precipitation area correlates with the shear intensity. A comparison between slower- and faster-moving CCs indicated that the precipitation intensity of the slower-moving CCs is stronger. This feature is related to a higher convective instability for the slower-moving ones. [ABSTRACT FROM AUTHOR]

In recent years, Moso bamboo ( Phyllostachys pubescens) forests have rapidly expanded in Japan by replacing surrounding coniferous and/or broadleaved forests. To evaluate the change in water yield from forested areas because of this replacement, it is necessary to examine evapotranspiration for Moso bamboo forests. However, canopy interception loss, one of the major components of evapotranspiration in forested areas, has been observed in only two Moso bamboo forests in Japan with relatively high stem density (~7000 stems/ha). There are, in fact, many Moso bamboo forests with much lower stem density. Thus, we made precipitation ( Pr), throughfall ( Tf) and stemflow ( Sf) observations for 1 year in a Moso bamboo forest with stem density of 3611 stems/ha and quantified canopy interception loss ( Ic). Pr and Ic for the experimental period were 1636 and 166 mm, respectively, and Ic/ Pr was 10%. The value was approximately the same as values for the other two Moso bamboo forests and lower than values for coniferous and broadleaved forests. On the other hand, Tf/ Pr and Sf/ Pr for our forest (86% and 4%, respectively) were approximately 10% of Pr larger and smaller than values for the other two Moso bamboo forests. These results suggest that the difference in stem density greatly affects precipitation partitioning (i.e. Tf/ Pr and Sf/ Pr) but does not greatly change Ic/ Pr. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The number of tropical cyclone (TC) genesis over the South China Sea and the Northwest Pacific Ocean in 2009 is significantly less than the average (27.4). However, the number of landfall TC over mainland China and its associated rainfall is more than the average. This paper focuses on the performance of numerical weather prediction (NWP) of landfall TC precipitation over China in 2009. The China Meteorological Administration (CMA) and Japan Meteorological Agency (JMA) models are compared. Although the schemes of physical processes, the data assimilation system and the dynamic frame are entirely different for the two models, the results of forecast verification are similar to each other for TC rainfall and track except for TC Goni. In this paper, a day with daily rainfall amount greater than 50 mm was selected as a storm rain day when there was a TC affecting the mainland. There are 32 storm rain days related to the landing of typhoons and tropical depressions. The rainfall forecast verification methods of National Meteorological Centre (NMC) of CMA are selected to verify the models' rainfall forecast. Observational precipitation analyses related to TCs in 2009 indicate a U-shape spatial distribution in China. It is found that the rain belt forecasted by the two models within 60 hours shows good agreement with observations, both in the location and the maximum rainfall center. Beyond 3 days, the forecasted rainfall belt shifts northward on average, and the rainfall amount of the model forecasts becomes under-predicted. The rainfall intensity of CMA model forecast is more reasonable than that of JMA model. For heavy rain, the JMA model made more missing forecasts. The TC rainfall is verified in Guangdong, Guangxi, Fujian and Hainan where rainfall amount related to TCs is relatively larger than in other regions. The results indicate that the model forecast for Guangdong and Guangxi is more skillful than that for Hainan. The rainfall forecast for Hainan remains difficult for the models because of insufficient observation data and special tropical ocean climate. [ABSTRACT FROM AUTHOR]

A short-baseline weather radar network consisting of Ku-band broadband radars (BBR) for meteorological application is under development. The BBR is a short-range and high-resolution Doppler radar for meteorological application, whose range and temporal resolution are several meters and 1 min per volume scan, respectively and coverage is from 50 m to 15 or 20 km. A radar network consisting of BBRs is useful for rapidly and accurately detecting and analyzing small-scale weather phenomena such as localized scattered thunderstorms, tornadoes, and downbursts in locations with a complex topography such as urban or mountainous area. The BBR network project plans to deploy three BBRs in the north Osaka, Japan, area and to observe and analyzing small-scale weather phenomena in detail. At present, two functioning BBRs have been deployed in the Osaka area. This paper presents a description of the BBR network project and its preliminary assessment. Observations of the two-BBR test bed derive expected properties of the three-BBR network, which efficiently covers troposphere in the overlapped area with a minimum observable altitude of tens of meters and a sensitivity of 10.2 dBZ (spatial average at a 5000-km altitude). Integrated reflectivity and preliminary algorithm for multiradar products in the two-BBR test bed, presenting a promising fine structure of precipitation cells, is also shown. [ABSTRACT FROM AUTHOR]

To determine the effects of the characteristics of litter and surface soil on the infiltration capacity in bamboo groves, infiltration rates were measured on a plot-scale using an oscillating-nozzle rainfall simulator at seven Moso bamboo ( Phyllostachys pubescens) sites and one Madake ( P. bambusoides) site in Japan. The maximum final infiltration rate (FIR), which was defined in this study as the infiltration capacity, was calculated, and the correlation between FIR and earth surface properties was analyzed. The FIR ranged from 9 to 308 mm h, showing little correlation with litter as surface cover, and a weak correlation with bulk density (0.3-0.9 g cm) and organic matter content (10.4-45.9%). These results suggest that the bulk density and organic matter content of surface soil, rather than surface cover, can be the controlling factors of infiltration capacity in bamboo groves. The bulk density of surface soil correlated significantly with the soil organic matter content, represented by bamboo rhizomes, suggesting that the infiltration capacity can increase with the amount of root networks present, related to macropore formation. The stand density showed a weak positive correlation, although not significant, with the soil organic matter content and a positive correlation with FIR. Thus, the infiltration capacity of bamboo groves can increase as the stand density of culms and bamboo rhizomes increase. [ABSTRACT FROM AUTHOR]

Precipitation is a critical component of the Earth's hydrological cycle. Launched on 27 November 1997, TRMM is a joint U.S.-Japan satellite mission to provide the first detailed and comprehensive dataset of the four-dimensional distribution of rainfall and latent heating over vastly undersampled tropical and subtropical oceans and continents (40°S-40°N). Over the past 14 years, TRMM has been a major data source for meteorological, hydrological, and other research and application activities around the world. This short article describes how the NASA Goddard Earth Sciences Data and Information Services Center (GES DISC) provides TRMM archive and near-real-time precipitation datasets and services for research and applications. TRMM data consist of orbital data from TRMM instruments at the sensor's resolution, gridded data at a range of spatial and temporal resolutions, subsets, ground-based instrument data, and ancillary data. Data analysis, display, and delivery are facilitated by the following services: (1) Mirador (data search and access); (2) TOVAS (TRMM Online Visualization and Analysis System); (3) OPeNDAP (Opensource Project for a Network Data Access Protocol); (4) GRADS Data Server (GDS); and (5) Open Geospatial Consortium (OGC) Web Map Service (WMS) for the GIS community. Precipitation data application services are available to support a wide variety of applications around the world. Future plans include enhanced and new services to address data-related issues from the user community. Meanwhile, the GES DISC is preparing for the Global Precipitation Measurement (GPM) mission, which is scheduled for launch in 2014. [ABSTRACT FROM AUTHOR]

This study proposes an algorithm to identify streamflow source areas through the interpretation of the predictive uncertainty analysis of streamflow generation. The method interprets model-free variance-based sensitivity indices and response surfaces to track the spatiotemporal evolution of streamflow source areas that define the components of a single rainfall-runoff event-driven hydrograph. Under a Monte Carlo framework, a distributed model was used to investigate (at daily resolution) a mountainous catchment with simple terrain in Japan and (at monthly resolution) a mountainous Andean watershed with complex terrain. Results suggest that for simple terrains, integrative river discharge observations at the outlet describe, to a great extent, most states of the catchment runoff response. However, for the time steps during which rainfall strikes, the validity of the observations at the outlet is restricted to the main stream. In the watershed with simple terrain, permanently saturated areas were found in the low lands and the riparian areas (slope gradients from 1% to 10%). The storage zones are on the headwaters (slope gradients up to 23%). Transitional zones cover the remaining areas. In complex terrains, the role of the topographic slope gradients is more relevant than the topology for the streamflow source areas delineation. In the Andean watershed, the dominating processes are in the headwaters, where the slope gradients are gentle (from 3% to 20%), and wetlands act as transient water reservoirs. In conclusion, the transferability of information at different spatiotemporal scales between catchments with heterogeneous topography and complex terrain seems unlikely. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Understanding the effects of severe human induced forest disturbances with soil loss on rainfall-runoff responses is important for future forest management. However, few studies have addressed this issue, which is methodologically difficult compared with the hydrological assessments of the effects of logging. In this study, several small catchments in Japan with different soil and geological conditions were compared using the runoff model HYCYMODEL to reveal their runoff characteristics. The results were then examined on the basis of runoff mechanisms to demonstrate the possible ranges of the effects derived from human disturbances for each geological type. For granite mountains, bare land can be considered the severest case of disturbances leading to high stormflow peaks, although a large baseflow remains because of the water storage fluctuation in weathered bedrock. For sedimentary rock mountains, the severest case may be a forest on the clayey soil without brown forest soil producing flashy runoff characteristics including a large stormflow volume with a sensitive response to the antecedent dryness and a low baseflow rate. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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The article presents a study that examines the hydrological process and other factors that affect nutrient discharge in the Todoroki watershed in Ishigaki Island, Japan. It says that nutrient discharges affect coral reefs because of an increase in macroalgae. It states that nutrient analyzer was used at the Todoroki River to measure dissolved nutrients. It adds that nutrient discharge is a stressor for coral reefs, which stresses the need to adopt ways to protect reef ecosystems.

The article explores spatial and temporal variations in summer precipitation between 1976 and 2007 in mountain areas in Japan. It notes that positive correlations between the amount of precipitation and the mean depth of precipitation were collected from 28 stations. It mentions that a temporal distribution pattern, based on the data collected, contributed to the determination of hydrological cycles in Japanese mountain areas.

The article discusses a study which examines the role of macropores in the process of water flow within sloping soils. Field studies were conducted at the Hitachi Ohta Experimental Watershed in Japan to investigate the effects of the disconnected macropores on various runoff generation processes such as subsurface flow, saturation overland flow and subsurface return flow. The results show that hydraulic gradients may result to subsurface erosion although macropores are disconnected.

We measured the effects of rain attenuation on 120-GHz band wireless link during the heavy rainy period (from July to September, 2008) and annual period (from March to December, 2008). The heavy rainy period data are used as a basis for the wireless link design, and the annual data are used to calculate the reliability of the wireless link. The 120-GHz band wireless link with V polarization was operated on a 400-m-long path from March to December 2008 in Atsugi, Japan. The rain rate distribution for Atsugi is in good agreement with the conditional M distributions. As for rain attenuation, the annual data coincide with the ITU and Laws and Parsons models. Cumulative distributions of attenuation and bit-error-rate deterioration due to rain were obtained. [ABSTRACT FROM AUTHOR]

In this study, an experimental 12-h lead-time flood forecasting methodology that combines the fifth generation mesoscale model (MM5) of the U.S. National Center of Atmospheric Research with the physically based, spatially distributed watershed environmental hydrology (WEHY) model is described and applied to the Shiobara Dam watershed in Japan in order to explore its utility. The Shiobara-Dam watershed is a mountainous steep-sloped watershed that has an area of 123 km2. Meanwhile, the routine atmospheric assimilation data that are provided by the Japan Meteorological Agency (JMA) over Japan, have spatial resolution of 20 km and are at 12-h time intervals. In order to utilize the JMA’s atmospheric data at 12-h intervals as initial and boundary conditions for 12-h lead-time hourly precipitation forecast inputs to the WEHY model of Shiobara-Dam watershed for runoff forecasts, the MM5 nonhydrostatic atmospheric forecast model was chosen and nested inside the JMA’s data domain. The JMA’s atmospheric data were used as initial and boundary conditions for MM5 which then produced detailed forecasted precipitation fields for the WEHY model of Shiobara-Dam watershed for its hourly runoff forecasts for a 12-h lead time. The results of an application of this forecasting scheme to a 48-h rainfall-runoff event over Shiobara-Dam watershed are presented and discussed. [ABSTRACT FROM AUTHOR]

Reliable prediction of precipitation by numerical weather prediction (NWP) models depends on the appropriate representation of cloud microphysical processes and accurate initial conditions of observations of atmospheric variables. Therefore, to retrieve reasonable cloud distributions, a 1-D variational Ice Cloud Microphysics Data Assimilation System (IMDAS) is developed to improve the predictability of NWP models. The general framework of IMDAS includes the Lin ice cloud microphysics scheme as a model operator, a four-stream fast microwave radiative transfer model in the atmosphere as an observation operator, and a global minimization method that is known as the shuffled complex evolution. IMDAS assimilates the satellite microwave radiometer data set of the Advanced Microwave Scanning Radio- meter for the Earth Observing System (AMSR-E) and retrieves integrated water vapor and integrated cloud liquid water content. This new method successfully introduces heterogeneity into the initial state of the atmosphere, and the modeled microwave brightness temperatures agree well with the observations of the Wakasa Bay Experiment 2003 in Japan. It has significantly improved the performance of the cloud microphysics scheme by the intrusion of heterogeneity into the external global reanalysis data, which resultantly improved atmospheric initial conditions. [ABSTRACT FROM AUTHOR]

A shallow landslide triggered by rainfall can be forecast in real-time by modeling the relationship between rainfall infiltration and decrease of slope stability. This paper describes a promising approach that combines an improved three-dimensional slope stability model with an approximate method based on the Green and Ampt model, to estimate the time–space distribution of shallow landslide hazards. Once a forecast of rainfall intensity and slope stability-related data, e.g., terrain and geology data, are acquired, this approach is shown to have the ability to estimate the variation of slope stability of a wide natural area during rainfall and to identify the location of potential failure surfaces. The effectiveness of the estimation procedures described has been tested by comparison with a one-dimensional method and by application to a landslide-prone area in Japan. [ABSTRACT FROM AUTHOR]

The Watershed Environmental Hydrology (WEHY) model was applied to the Shiobara-Dam watershed for model validation for four historical events. Based on the analysis of the established geographic information system (GIS) database, which consists of the digital elevation model map, the land use map, the soils map, and the Landsat TM images, along with information on soil depths and soil properties, the modules of the WEHY model for the Shiobara-Dam watershed were selected and configured. A system of model computational units and stream reaches formed the basic building blocks for the watershed model. Using the model parameters, which were estimated from the established GIS database of the Shiobara-Dam watershed directly, good results were obtained when the WEHY model watershed runoff predictions were compared against their observed counterparts for several historical rainfall-runoff events. Interactions among the hydrologic flow processes at the watershed were examined in some detail. [ABSTRACT FROM AUTHOR]

On 25 February 1999, on the western side of the central mountain range of Japan, orographic snow clouds had formed under conditions of weak cold advection due to a winter monsoon after a cyclonic storm. The data from Ka-band Doppler radar, microwave radiometer, hydrometeor videosondes, and 2D Grey imaging probe provided unique datasets that were used to analyze the evolution of meso- and microscale structures, especially ice and supercooled liquid water (SLW) evolutions associated with variations in surrounding conditions. In the present case, four stages were identified in the evolution of the clouds: stratiform (I), transition (II), shallow convective (III), and dissipating (IV). During stage I, substantial blocking of the low-level flow occurred. The echo top was relatively flat and the echo pattern was stratiform with a bright band. The clouds were considered to be almost glaciated, primarily by the deposition growth of ice crystals. The wind speed up the slope gradually increased in the latter half of stage I and reached maximum intensity at stage II. Simultaneously, the LWP increased and the hydrometeors observed at the surface and aloft indicated heavy riming by accretion of supercooled droplets. During stage III, the wind up the slope weakened and the clouds became shallower and more convective. Snowflakes and aggregated snow images detected frequently at the surface suggested that some of the mass in the precipitation was due to depositional growth, which is consistent with the clouds of low LWP [high ice water path (IWP)] during stage III. In stage IV, the clouds dissipated as a relatively warm/dry airflow from the south dominated during stable conditions. These results indicate that SLW was associated primarily with orographic lifting but not with convection. The dominance of SLW was reflected in the enhanced flow up the slope that should increase the upward air motion. The convective clouds showed only small amounts of SLW and an abundance of ice crystals. [ABSTRACT FROM AUTHOR]

During 19–20 October 2004, a series of spectacular arc-shaped rainbands developed south or southeast of southwestern Japan when Typhoon Tokage (TY0423) approached the region from the southwest. As the typhoon moved closer and the upstream Froude number (Fr) continued to increase, these rainbands first remained quasi-stationary but eventually retreated backward. Using the Nagoya University Cloud-Resolving Storm Simulator (CReSS) at 1-km grid size, these rainbands were successfully simulated, and their behavior during the transition period from a relatively low-Fr to a high-Fr regime was investigated and compared with idealized two-dimensional (2D) model results from theoretical studies. In the present case, the rainbands were found to develop along a low-level frontal convergence zone between the southerly flow associated with the typhoon and the northerly flow from the Sea of Japan. The northeasterly winds accelerated through gaps between topography and fed the offshore flow at the backside of the rainbands, producing a strong resistance that allowed the rainbands to remain stationary under significantly higher Fr values (at least 1.2) than predicted by 2D simulations (of about 0.3–0.5) for the retreat to occur in conditionally unstable flow with a convective available potential energy of about 1300 J kg−1. Typically ≤ 500 m in depth with a potential temperature (θ) deficit of 2–4 K across the rainband, the cooler offshore flow was also found to be enhanced by evaporative cooling as in some other events. The cooling effect helped the rainbands to hold their position until Fr of the upstream flow became too large, and the rainband with stronger cooling behind was able to withstand a higher Fr before retreat. Once the retreat started, the offshore layer became thinner and the θ deficit also reduced, and the rainbands were washed back by the strengthening upcoming flow. [ABSTRACT FROM AUTHOR]

The concentrations and the concentration ratios of individual short-lived (222)Rn decay products ((214)Pb and (214)Bi) in rainwater were measured at Kumatori village (34.39 degrees N, 135.35 degrees E, approximately 70 m above sea level) in Osaka, Japan, by gamma-ray spectrometry using a low-background Ge detector. The dependence of the time variations of the concentrations and their ratios on rainfall rate was investigated. It was observed that the concentrations were negatively correlated with the rainfall rate in some rainfall events, and that there was no clear correlation in other rainfall events. The changes in the dependence of the concentration on the rainfall rate occurred after the passage of a cold front during a single rainfall event. The concentration ratios showed a weak negative correlation with the rainfall rate for most of the observed rainfall events. A scavenging model was designed in this study in order to explain the observation results. Based on the relationship between the concentrations of (214)Pb and (214)Bi in the rainwater and the rainfall rate for an individual rainfall event, the increase in the environmental gamma-ray dose rate from (214)Pb and (214)Bi deposited on the ground was calculated, and the calculated increase agreed well with that observed by the in situ measurement on flat ground.