During the Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS), many isolated convective storms developed in the southern Kanto Plain on August 17, 2012. The aim of this study was to clarify the dynamics leading to the convection initiation of one of them using different remote sensing instruments.
Before the convection initiation, a southeasterly flow transported water vapor inland from Tokyo Bay and the well-mixed and a cumulus-cloud-topped convective boundary layer developed. A convergence line in the form of a sea breeze front (SBF) also moved inland from Tokyo Bay. A near-surface air parcel was lifted to its lifting condensation level (LCL) by an updraft in a convergence zone with a 3 km horizontal scale, which formed the west edge of the convergence line. The saturated air parcel at the LCL was then lifted to its level of free convection (LFC) by the updrafts associated with thermals below the cumulus cloud base. A Ku-band radar detected the first echo of hydrometeors about 6 minutes after the air parcel reached its LFC, then the convective cell developed rapidly. When an SBF arriving from Sagami Bay passed under the cell, the updraft over the nose of the SBF triggered a new precipitation cell, but no intensification of the preexisting cell was observed.
X-band dual-polarization (multi-parameter) radars were used to observe a supercell storm that generated an F3 tornado in Ibaraki Prefecture, Japan on 6 May 2012. The observed data collected for this storm clearly exhibited the typical polarimetric features of a supercell storm, such as the ZDR (differential reflectivity) arc, ZDR column, and the KDP (specific differential phase) column, as well as their time evolution. The ZDR arc emerged at 10 to 15 min before the tornadogenesis. The ZDR column appeared approximately 1 h before the formation of the ZDR arc and was intermittent until tornadogenesis. As the ZDR arc appeared, the column became tall and stable and lasted until the dissipation of the tornado. These ZDR signatures of the supercell storm persisted for approximately half an hour.
The relationships between the occurrence of intense rainfall and the convergence of surface winds and water vapor concentration for typical heavy-rainfall cases were examined using data from July to August in 2011-2013, obtained from high-density meteorological observations in Tokyo, Japan. Additionally, the temporal variations in wind convergence and water vapor between days with and without heavy rainfall events were compared. Corresponding to heavy-rainfall areas, the convergence of surface winds tended to increase for several tens of minutes prior to the heavy rainfall. The peak of convergence was observed 10-30 min before the heavy-rainfall occurrence, and convergence continued to increase for approximately 30 min until the convergence peak time. Around the heavy-rainfall area, the increase in the water vapor concentration index coincided with the increase in convergence. From these results, by monitoring the temporal variations and distributions of these parameters using a high-density observation network, it should be possible to predict the occurrence of heavy rainfall rapidly and accurately.
Convective storms are frequently initiated over mountains under weak synoptic forcing conditions. However, the initiation process of such convective storms is not well understood due to a lack of observations, especially the transition process from non-precipitating cumuli to precipitating convective clouds. To investigate the initiation process, we conducted observations around the mountains in the Kanto region, Japan on 18 August 2011 using a 35 GHz (Ka-band) Doppler radar and a pair of digital cameras. The evolution of convective clouds was classified into three stages: convective clouds visible but not detected by the Ka-band radar (stage 0), convective clouds detectable by the Ka-band radar with reflectivity below 15 dBZ (stage 1), and convective clouds accompanied by descending echoes corresponding to precipitation (stage 2). During the transition process from stage 1 to stage 2, weak radar echoes rose to the higher level and reflectivity rapidly increased. This phenomenon suggests that drizzle particles produced in a preexisting convective cloud were lifted by a newly developed updraft, and raindrops were formed rapidly by coalescence of the drizzle particles and cloud droplets. This hypothetical process explains the precipitation echo formation in the lower layer frequently observed in the mountainous area in the Kanto region.
Does the presence of cities enhance precipitation? It is an issue that needs to be addressed. This study focuses on the thermal excess in cities and attempts to determine the influence of this parameter on atmospheric instability, which constitutes the background condition for convective precipitation. A simple analytical approach involving the calculation of the daytime evolution of the mixed layer over a homogeneous ground surface was developed. The calculations were based on the data from the ensemble average of observations. Using these calculations, the convective available potential energy (CAPE) was obtained for two idealized geographic areas: one with urban land cover and the other with rural land cover. Urban heat excess, which was 200 W m−2 greater than rural area, increased CAPE value by 75 % comparing to the rural CAPE value of 513 J kg−1. Cities can produce stratification in the atmosphere that is favorable for convective precipitation.
Heavy precipitation fell over Tokyo in the afternoon of 26 August 2011, leading to flooding and major disruptions for the population, businesses, and authorities. Over 150 mm of precipitation was observed over the city center on that day, with hourly accumulations reaching values as high as 90 mm in late afternoon. Numerical forecasts of this case were performed with a 250-m grid spacing version of the Global Environmental Multi-scale (GEM) model in the context of the Tokyo Metropolitan Area Convection Study (TOMACS). Although initialized only from a global 25-km upper-air analysis, results indicate that GEM can produce the intense precipitation over Tokyo at about the right location and time.
A sensitivity test in which the urban surface scheme is switched off and replaced with tall grass suggests that the urban environment might have had considerable impact on precipitation intensity, but not on its occurrence or its timing. Based on diagnostics from the GEM integrations, the increased intensity of precipitation seems more related to an enhancement of lateral inflow of low-level moist static energy from Tokyo Bay than to augmented surface fluxes of heat and humidity from the city itself. The existence of low-level bands with locally high values of equivalent potential temperature indicates that the additional moist energy is distributed unevenly through the Tokyo area, an aspect of the simulation which is speculated to have directly contributed to the increase in precipitation intensity over the city.
To elucidate the formation of localized rainfall in a basin with heat and aridity under a weak synoptic disturbance in summer, we described the characteristics of atmospheric conditions on the Kofu Basin preceding the appearance of primary precipitating cells from 23 localized rainfall events on the Kofu Basin on days of weak synoptic disturbance at the surface from 1 June to 30 September in 2012 to 2014. Furthermore, on the basis of a case study of an event on 25 July 2014, the formation of the atmospheric conditions was described from the standpoint of moisture behavior.
Owing to the thermal contrast between the Kofu Basin with heat and aridity and the outside environment, the south-component wind blowing in the valley connecting to the coastal region of Suruga Bay and the east-component wind blowing in the valley connecting to the Kanto Plain entered the basin as southwesterly wind and southeasterly wind, respectively, which caused an increase in the water vapor mixing ratio and a slight decrease in temperature at the surface. Thereafter, the amount of precipitable water vapor derived from the global navigation satellite system observations (GNSS-PWV) in the central region of the Kofu Basin increased abruptly after the moderate increase in GNSS-PWV at all the observation points in the basin. Finally, a cloud appeared over the local region between the southwesterly wind and the southeasterly wind and precipitating cells appeared at that location at 3.25 to 6.25 km above sea level.
We discussed the moisture transport into the Kofu Basin, the concentration of that moisture in a local region, and the appearance of precipitating cells. This is an example of the formation of atmospheric conditions leading to localized rainfall in a basin with heat and aridity.
The center for the Collaborative Adaptive Sensing of the Atmosphere (CASA) Dallas–Fort Worth (DFW) Urban Demonstration Network consists of a high-resolution X-band radar network and a National Weather Service S-band radar system (i.e., KFWS radar). On the basis of these radars, CASA has developed an end-to-end warning system that includes sensors, software architecture, products, data dissemination and visualization, and user decision-making modules. This paper presents a technical summary of the DFW radar network for urban weather disaster detection and mitigation from the perspective of the tracking and warning of hails, tornadoes, and floods. Particularly, an overview of the design trade-offs of the X-band radar network is presented. The architecture and associated algorithms for various product systems are described, including the real-time hail detection system, the multiple Doppler vector wind retrieval system, and the high-resolution quantitative precipitation estimation system. Sample products in the presence of high wind, tornado, hail, and flash flood are provided, and the system performance is demonstrated by cross-validation with ground observations and weather reports.
An X-band radar system was deployed in Santa Clara, CA from February through May 2016 to support the National Weather Service in the event of potential flooding during one of the largest El Niños on record and to provide a better understanding of rainfall processes occurring in the Bay Area. The system was also used to provide high-quality precipitation estimation (quantitative precipitation estimation—QPE) for Santa Clara's urban hydrologic modeling system. Although the Bay Area has coverage from the Next-Generation Weather Radar (NEXRAD) operational radar network, the combination of topographic influences and proximity to a maritime environment provide unique QPE challenges in this urban region. The X-band radar provided high-quality rainfall estimates that performed better than NEXRAD, demonstrating the added value of the X-band system. High-resolution rainfall monitoring systems in urban regions also provide a host of benefits across different sectors of the economy, including flood damage mitigation, water quality, water supply, and transportation.
In the preparation for polarimetric radar data assimilation, it is essential to examine the accuracy of forward operators based on different formulations. For this purpose, four forward operators that focus on warm rain conditions are compared both with each other and actual observations with respect to their performance for C-band dual polarimetric radars. These operators mutually consider radar beam broadening and climatological beam bending. The first operator derives polarimetric parameters assuming an exponential raindrop size distribution obtained by the models and is based on fitting functions against scattering amplitudes. The other three converters estimate the mixing ratio of rainwater from the measured polarimetric parameters. The second converter uses both the horizontal reflectivity (ZH) and the differential reflectivity (ZDR), the third uses the specific differential phase (KDP), and the fourth uses both KDP and ZDR, respectively. Comparisons with modeled measurements show that the accuracy of the third converter is superior to the other two. Another evaluation with actual observations shows that the first converter has slightly higher fractions skill scores than the other three. Considering the attenuation effect, the fitting function and the operator only with KDP are found to be the most suitable for data assimilation at C-band.
A local heavy rainfall of about 100 mm h−1 occurred in Tokyo and Kanagawa Prefecture on 26 August 2011. This rain was brought by a mesoscale convective system (MCS) that developed near a stationary front that slowly moved southward. In an analysis using geostationary multi-purpose satellite rapid scan images and dense automated weather station networks, development of the MCS occurred after the merging of sea breezes from the east (Kashima-nada) and the south (Tokyo Bay).
Numerical experiments by the Japan Meteorological Agency (JMA) nonhydrostatic model (NHM) with horizontal resolutions of 10 and 2 km using mesoscale 4D-VAR analysis of JMA for initial conditions tended to predict the position of intense rainfall areas west of the observed positions. In the mesoscale ensemble forecast using perturbations from JMA's one-week global ensemble prediction system (EPS) forecast, some ensemble members showed enhanced precipitation around Tokyo, but false precipitation areas appeared north of the Kanto and Hokuriku districts.
As an attempt to improve the model forecast, we modified the model, reducing the lower limit of subgrid deviation of water vapor condensation to diagnose the cloudiness for radiation. In the modified model simulation, surface temperatures around Tokyo increased by about 1°C, and the position of the intense precipitation was improved, but the false precipitation areas in the Hokuriku district were also enhanced in the ensemble member which brought a better forecast than the control run.
We also conducted an ensemble prediction using a singular vector method based on NHM. One of the ensemble members destabilized the lower atmosphere on the windward side of the Kanto district and suppressed the false precipitation in the Hokuriku district, and observed characteristics of the local heavy rainfall were well reproduced by NHM with a horizontal resolution of 2 km.
A conceptual model of the initiation of deep convection by the formation of a low-level convergence zone succeeding merging of the two sea breezes from the east and south is proposed based on observations, previous studies, and numerical simulation results. In this event, the northerly ambient wind played an important role on the occurrence of the local heavy rainfall around Tokyo by suppressing the northward intrusion of the sea breeze from the south.
This paper reports the development of a very-short-range nowcast system, VIL Nowcast, which aims to provide precise forecasts of imminent rainfall, and in particular, heavy and localized events. The system is based on the vertically integrated liquid water content (VIL), which is estimated from three-dimensional radar observations as well as the 1-minute-resolution rainfall map obtained from the X-band polarimetric (multi-parameter) RAdar Information Network (XRAIN), to predict rainfall amounts over 10 minutes periods that extend to 10-60 minutes into the future. The spatial resolution of VIL Nowcast was 500 m, and nowcasts were produced at a temporal resolution of 5 minutes. Three precipitation events, of which two were isolated storms and one was a synoptic storm, were used as case studies to verify the model. The performance of VIL Nowcast was evaluated against the XRAIN radar rainfall data and an existing rainfall-rate nowcast system using the same advection scheme. The scope of the evaluation was limited mainly to the first prediction for 10 minutes ahead. It was found that VIL Nowcast showed a small, statistically significant improvement over the entire precipitation event, although its skill decreased at longer lead times and at higher thresholds. The key findings of this study are: (1) VIL Nowcast appears capable of generating skillful forecasts at short lead times, even for very localized heavy rainfall; (2) VIL Nowcast can reduce the time lag in the rainfall-rate nowcast system at initiation and peak precipitation; and (3) this system may improve the accuracy of heavy rainfall alerts provided for public activities and emergency alarms.
During the intensive observation period (IOP) of the Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS) in the summer months of 2011-2013, the atmospheric environment of several heavy rainfalls was observed by radiosonde soundings in the Tokyo metropolitan area. We investigated the formation and development processes of an extremely developed thunderstorm (Case 1 on August 26, 2011) and a moderately developed thunderstorm (Case 2 on July 18, 2013) during the TOMACS IOP by utilizing radiosonde sounding data. Compared with Case 2, Case 1 featured a lower level of free convection and a deeper layer of easterly flow in the mesoscale environment of the severe storm. We performed numerical simulations to investigate the formation processes of the convective systems in the two cases by using the nonhydrostatic model of the Japan Meteorological Agency incorporating the square prism urban canopy scheme. Model results fairly represented the spatial distribution and amounts of the rainfall in both cases. In Case 1, the formation of a distinct convergence zone between easterly and southerly flows was the likely trigger of active convective systems around Tokyo. To further examine the urban impact on precipitation, we performed two comparative simulations: one using realistic current urban surface conditions (CRNT experiment) and the other using less-urbanized surface conditions (LURB experiment). The CRNT experiment yielded more rainfall than the LURB experiment in the central urban area. It appears that the higher temperatures caused by urbanization can lead to increased rainfall in Tokyo by intensifying convergence and ascending motion.
The Tokyo Metropolitan Area Convection Study (TOMACS) for extreme-weather-resilient cities is a research and development project (RDP) of the World Weather Research Programme (WWRP). TOMACS provided a multiplatform and high spatiotemporal resolution dataset for the present research on three episodes of deep convection in the Tokyo Metropolitan Area (TMA) under its heat island effect and sea breeze circulations. Heavy rainfall episodes of August 26, 2011, and July 23 and August 12, 2013, were simulated with (and without) the tropical town energy budget (T-TEB) model coupled with the advanced regional prediction system (ARPS). The T-TEB/ARPS system used initial and boundary conditions from the Japan Meteorological Agency (JMA) mesoscale analysis data for 24-hour integration runs at 5-km resolution over Japan and at 1-km resolution over TOMACS area. The 1-km resolution hourly rainfall field simulations were verified against the respective automated meteorological data acquisition system (AMeDAS) rain gauge network measurements. Statistics of the Contingency tables were obtained to estimate the critical success index (CSI), probability of detection (POD), and false alarm rate (FAR) as well as the root mean square error (RMSE). The T-TEB/ARPS simulations improved the south and east sea breeze circulations of TMA and its urban heat island effect. The time evolution of CSI scores improved within the advective time scale, whereas dissipation (phase) errors on precipitation RMSE increased with the integration time and were larger than the dispersion (amplitude) errors. The initial and boundary conditions of JMA greatly improved the simulations as compared to the previous ones performed with the outputs of NCEP's global forecast system as indicated by the TOMACS datasets. Thus, the results represent the temporal and spatial evolutions of the atmospheric conditions leading to the development of a deep convection within TOMACS region. Furthermore, TMA is a good testbed to evaluate the urban surface schemes, such as T-TEB in this study.