Journal of the Meteorological Society of Japan. Ser. II Advance Publications

 As an alternative approach to the previous multisensor satellite evaluation method of cloud system resolving models, a method is presented using combined infrared and microwave channels for precipitation clouds in cloud system resolving models over the ocean. This method determines characteristics of cloud-top temperatures and ice scatterings for clouds using infrared 11-μm and microwave high frequencies (89.0 GHz) brightness temperatures (TBs). The threshold of the TB at low frequencies (18.7 GHz) is also used to identify precipitation regions. This method extends the previous approach via the wider swath of the passive microwave sensor and sensitivities to ice clouds compared to the previous Tropical Rainfall Measuring Mission (TRMM)-based analysis method using the narrower coverage of the Precipitation Radar.

 The numerical results of the non-hydrostatic icosahedral atmospheric model (NICAM) with two cloud microphysics schemes are evaluated over the tropical open ocean using this method. The intensities of the scatterings in the two simulations at 89.0 GHz are different due to the parameterizations of the snow and graupel size distributions. A bimodal size distribution of the snow improved the underestimation of the TBs at 89.0 GHz. These results have a similar structure to the joint histograms of cloud-top temperatures and precipitation-top heights in the previous method: the overestimated intensity of scattering and the frequencies of high precipitation-top heights above 12 km in the control experiment. We find that the change in the snow size distribution in the cloud microphysics scheme can lead to better agreements of simulated TBs at 89.0 GHz with observations. We further investigate impacts of non-spherical assumptions for snow using a satellite simulator. The effect of a non-spherical shape of snow in the radiative transfer model causes a smaller change of TBs at 89.0 GHz compared to the difference between the TBs of the two simulations without non-spherical assumptions.

 An observation operator to assimilate satellite radiances with the Non-hydrostatic Icosahedral Atmospheric Model (NICAM)-based Local Ensemble Transform Kalman Filter (LETKF) is newly developed using the radiative transfer model RTTOV (Radiative Transfer for the TOVS (TIROS Operational Vertical Sounder)) version 11.1. Here we assimilate the Advanced Microwave Sounding Unit-A (AMSU-A) brightness temperature observations which are known to bring a large improvement to global numerical weather prediction. We apply the online estimation of bias correction for both airmass and scan biases, or the biases originating from the atmospheric state and scan position. Comparing the two experiments with and without the AMSU-A radiances, we find that the adaptive bias correction methods work appropriately, and that the analysis is significantly improved by assimilating the AMSU-A radiances. This is an important step toward assimilating different types of satellite radiances with NICAM-LETKF.

  An algorithm for retrieving the macroscopic, physical and optical properties of clouds from thermal infrared measurements is applied to the Himawari-8 multiband observations. A sensitivity study demonstrates that the addition of the single CO₂ band of Himawari-8 is effective for the estimation of cloud top height. For validation, retrieved cloud properties are compared systematically with collocated active remote sensing counterparts with small time lags. While retrievals agree reasonably for single-layer clouds, multilayer cloud systems with optically thin upper clouds overlying lower clouds are the major source of error in the present algorithm. Validation of cloud products is critical for identifying the characteristics, advantages and limitation of each product and should be continued in the future.

 As an application example, data are analyzed for eight days in the vicinity of the New Guinea to study the diurnal cycle of the cloud system. The present cloud property analysis investigates cloud evolution through separation of different cloud types and reveals typical features of diurnal cycles related to the topography. Over land, middle clouds increase from 0900 to 1200 local solar time (LST), deep convective clouds develop rapidly during 1200–1700 LST with a subsequent increase in cirrus and cirrostratus cloud amounts. Over the ocean near coastlines, a broad peak of convective cloud fraction is seen in the early morning. The present study demonstrates the utility of frequent observations by Himawari-8 for life cycle study of cloud systems, owing to the ability to capture their continuous temporal variations.

 Eddy transport of atmospheric water vapor from the tropics is important for rainfall and related natural disasters in the middle latitudes. Atmospheric rivers (ARs), intense moisture plumes typically associated with extratropical cyclones, often produce heavy precipitation upon encountering topography on the west coasts of mid-latitude North America and Europe. ARs also occur over the northwestern Pacific and sometimes cause floods and landslides over East Asia, but the climatological relationship between ARs and heavy rainfall in this region remains unclear. Here, we evaluate the contribution of ARs to the hydrological cycle over East Asia using high-resolution daily rainfall observations and an atmospheric reanalysis during 1958–2007. Despite their low occurrence, ARs account for 14–44% of total rainfall and 20–90% of extreme heavy-rainfall events during spring, summer and autumn seasons. AR-related extreme rainfall is especially pronounced over western-to-southeastern slopes of terrains over the Korean Peninsula and Japan, owing to strong orographic effects and a stable direction of low-level moisture flows. A strong relationship between warm-season AR heavy rainfall and preceding-winter El Niño is identified since the 1970s, suggesting the potential of predicting heavy-rainfall risk over Korea and Japan at seasonal leads.

 We investigated the predictability of plume advection in the lower troposphere and the impact of AMeDAS surface wind data assimilation using the radioactive cesium emitted by the Fukushima nuclear accident in March 2011 as an atmospheric tracer. We conducted two experiments of radioactive plume predictions over eastern Japan for March 15, 2011 with a 3-km horizontal resolution using the Japan Meteorological Agency non-hydrostatic weather forecast model and local ensemble transform Kalman filter (JMANHM-LETKF) data assimilation system. The assimilated meteorological data were obtained from the standard archives collected for the JMA operational numerical weather prediction and the AMeDAS surface wind observations. The standard archives do not contain land surface wind observations. The modeled radioactive cesium concentrations were examined for plume arrival times at 40 observatories. The mean error of the plume arrival times for the standard experiment (assimilating only the standard archives) was 82.0 minutes with a 13-hour lead-time on average. In contrast, the mean error of the AMeDAS experiment (assimilating both the standard archives and AMeDAS surface wind observations) was 72.8 minutes, which was 9.2 minutes (11 %) better than that of the standard experiment. This result indicates that the plume prediction has a reasonable accuracy for the environmental emergency response and the prediction can be significantly improved by the surface wind data assimilation.

 A mesoscale convective system (MCS) is organized thunderstorms with connected anvils, which has significant impact to the global climate. Focusing on MCS over the Maritime Continent of Indonesia, this study aims to gain a better understanding on the properties of MCSs over the study area. The “Grab 'em Tag 'em Graph 'em” (GTG) tracking algorithm is applied to hourly Multi-functional Transport Satellite-1R (MTSAT-1R) data for two years period to observe the distribution of MCSs and the evolution of MCSs along their lifetime. The result of MCS identification is combined with CloudSat data products to study the vertical structure of the MCSs at various MCS life stages: developing, mature, and dissipating.

 The distribution of MCSs in Indonesia has a seasonal variation and distinct diurnal cycle. The life stages of observed MCSs are characterized by distinct cloud microphysics at each stage. In developing stage, the upper level of the MCS raining region shows the presence of precipitating ice particles. As the MCS matures, the proportion of raining area becomes smaller and the intensity of rain is reduced, accompanied by larger occurrence of smaller-sized ice particles at the upper level. In dissipating stage, large hydrometeors no longer exist at the upper part of raining region. Within the MCS anvils, the dissipating stage shows a more uniform distribution of ice-particle effective radius compared to developing and mature stages.

 MCS characteristics over the land and ocean are also clarified to differ on the minimum brightness temperature, the equivalent radius, the maximum rain rate, and the rain fraction which vary along MCS evolution.

 This study examines analysis and forecast impacts in the Navy Global Environmental Model (NAVGEM) from direct assimilation of temperature and wind “pseudo-raob” profiles derived from analysis fields of the ECWMF-IFS (Integrated Forecast System). The pseudo-raob profiles are provided on eight vertical levels from 250 hPa to 1000 hPa on a 1°latitude /longitude grid and are assimilated as synthetic observation data by NAVGEM at 0000 UTC and 1200 UTC for an experimental time-period of 48 days. The pseudo-raob observations are assumed in these experiments to have observation errors identical to temperature and wind data provided by conventional radiosonde observations.

 Assimilation of pseudo-raob profiles, in this diagnostic context, significantly reduces temperature and height biases in the NAVGEM analysis and provides general improvements to forecast skill, verifying against both self-analysis and rawinsondes. Reduction of NAVGEM temperature bias is most evident in southern hemisphere high-latitudes, where assimilation of pseudo-raob information mitigates NAVGEM temperature bias and indicates sub-optimal bias correction of radiance data in the NAVGEM Control analysis. Despite the revisiting of assimilated observation information when assimilating pseudo-raobs from the IFS analysis into the NAVGEM analysis, improvement to the NAVGEM analyses and forecasts is both statistically significant and consistent across several verification techniques. This suggests that there are likely small effects from any correlations between pseudo-raob data and the NAVGEM background. Assimilation of pseudo-raob data also reduces total observation impact in NAVGEM as estimated by the adjoint model, which is an indicator of general improvement to analysis and forecast quality.

 The high temporal and spatial resolutions of geostationary satellite observations achieved by recent technological advances have facilitated derivation of atmospheric motion vectors (AMVs), even in a tropical cyclone (TC) where the winds abruptly change. This study used TCs in the western North Pacific basin to investigate the ability of upper tropospheric AMVs to estimate TC intensity and structure. We first examined the relationships between the cloud-top wind fields captured by 6-hourly upper tropospheric AMVs derived from images of the Multi-functional Meteorological Satellite (MTSAT) and the surface maximum sustained wind (MSW) of the Japan Meteorological Agency best-track data for 44 TCs during 2011–2014. The correlation between the maximum tangential winds of the upper tropospheric AMVs (UMaxWinds) and MSWs was high, about 0.73, the suggestion being that the cyclonic circulation near the cloud top was intensified by the upward transport of absolute angular momentum within the TC inner core. The upper tropospheric AMVs also revealed that the mean radii of UMaxWinds and the maximum radial outflows shifted inward as the TC intensification rate became large, the implication being that low-level inflow was strong for TCs undergoing rapid intensification. We further examined the possibility of estimating the MSW by using 30-min-interval UMaxWinds derived from Himawari-8 target observations, which have been used to track TCs throughout their lifetimes. A case study using Typhoon Lionrock (1610) showed that the UMaxWinds captured changes of the cyclonic circulation near the cloud top within the inner core on a time scale shorter than one day. It was apparent that the increase of UMaxWind was associated with intensification of the TC warm core and shrinkage of UMaxWind radius. These results suggest that the Himawari-8 AMVs include useful information on TC intensification and related structural changes to support the TC intensity analysis and structure monitoring.

 By comparison with satellite and field observations, the comprehensive performance and potential utility of near real-time forecasts using Nonhydrostatic Icosahedral Atmospheric Model (NICAM) are demonstrated exploiting the Cooperative Indian Ocean Experiment on Intraseasonal Variability in the Year 2011 (CINDY2011)　/　Dynamics of the Madden–Julian Oscillation (DYNAMO) campaign. A week-long forecast was run each day using a regionally stretched version of NICAM, with the finest mesh size of 14 km over the tropical Indian Ocean (IO), throughout the intensive observation period (IOP).
 The simulated precipitation time series fairly represented the evolution and propagation of the observed Madden-Julian Oscillation (MJO) events, although a 30% overprediction of precipitation over the IO domain (60º—90ºE, 10ºS—10ºN) was found on average. Frequencies of strong (>40 mm day^-1) precipitation were overpredicted, while those of weak precipitation were underpredicted against satellite observations. Compared with the field observations at Gan Island, the biases in precipitation frequency were less obvious, whereas the growth of lower to middle tropospheric dry (~1 g kg^-1) and warm (~1 K) biases were found. Despite these mean biases, temporal variations of the moisture and zonal wind profiles including the MJO events were reasonably simulated.

Using the forecast data the moisture and energy budgets during the IOP were investigated. The diagnosis using the 7–day–mean fields captured the observed features of the MJO events. Meanwhile, significant upward transport of moisture by the grid-resolved high-frequency variability throughout the IOP. The relationship between this high-frequency effects and the simulated MJO or mean biases is also discussed.

 X-band dual-polarization (multi-parameter) radars observed the supercell storm that generated an F3 tornado in Ibaraki Prefecture, Japan on 6 May 2012. The observational data collected for the storm clearly show typical polarimetric features of a supercell storm, such as the Z_DR (differential reflectivity) arc, Z_DR column, and K _DP (specific differential phase) column, and their time evolution. The Z_DR arc emerged 10 or 15 min before tornadogenesis. The Z_DR column appeared about 1 hour before Z_DR arc formation and developed intermittently until tornadogenesis. Just when the Z_DR arc appeared, the column was becoming taller and stable, and lasted until the dissipation of the tornado. These Z_DR signatures of the supercell storm lasted around 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 the heavy-rainfall area, 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 of the transition process from non-precipitating cumuli to precipitating convective clouds. In order 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 were visible but not detected by the Ka-band radar (stage 0), convective clouds were detectable by the Ka-band radar with reflectivity below 15 dBZ (stage 1), and convective clouds were 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 pre-existing 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.

 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. The first echo of hydrometeors was detected by a Ku-band radar 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.