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 ZDR (differential reflectivity) arc, ZDR column, and K DP (specific differential phase) column, and their time evolution. The ZDR arc emerged 10 or 15 min before tornadogenesis. The ZDR column appeared about 1 hour before ZDR arc formation and developed intermittently until tornadogenesis. Just when the ZDR arc appeared, the column was becoming taller and stable, and lasted until the dissipation of the tornado. These ZDR 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.
A working group is studying the feasibility of a future Japanese space-borne coherent Doppler wind lidar (CDWL) for global wind profile observation. This study is composed of two companion papers: an instrumental overview of the space-borne CDWL for global wind profile observation (Part 1) and the wind measurement performance (error and bias) investigated using a full-fledged space-borne CDWL simulator (Part 2). The objective of this paper is to describe the future space-borne CDWL in terms of technical points and observation user requirements. The future mission concept is designed to have two looks for vector wind measurement with vertical resolutions of 0.5 (lower troposphere: 0-3 km), 1(middle troposphere: 3-8 km), and 2 km (upper troposphere: 8-20 km) and horizontal resolution of < 100 km along a satellite. The altitude and orbit of the satellite are discussed from a scientific viewpoint. The candidate altitude and orbit of the satellite are 220 km and an inclination angle of 96.4° (polar orbit) or 35.1° (low-inclination-angle orbit), respectively. The technical requirements of the space-borne CDWL are a single-frequency 2-μm pulse laser with an average laser power of 3.75 W, two effective 40-cm-diameter afocal telescopes, a wide-bandwidth (> 3.4 GHz) detector, a high-speed AD converter, and a systematic lidar efficiency of 0.08. The space-borne CDWL looks at two locations at a nadir angle of 35° at two azimuth angles of 45° and 135° (225° and 305°) along the satellite track. The future space-borne CDWL wind profile observation will fill the gap of the current global wind observing systems and contribute to the improvement of the initial conditions for NWP, the prediction of typhoons and heavy rain, and various meteorological studies.
A feasibility study of tropospheric wind measurements by a coherent Doppler lidar aboard a super-low-altitude satellite is being conducted in Japan. The considered lidar uses a 2.05 μ m laser light source of 3.75 W. In order to assess the measurement performances, simulations of wind measurements were conducted. The mission definition is presented in a companion paper (Part 1) while, in this paper, we describe the measurement simulator and characterize the errors on the retrieved line-of-sight (LOS) winds. Winds are retrieved from the Doppler-shift of the noisy backscattered signal with a horizontal resolution of 100 km along the orbit track and a vertical resolution between 0.5 and 2 km. Cloud and wind fields are the pseudo-truth of an Observing System Simulation Experiment while aerosol data are from the Model-of-Aerosol-Species-IN-the-Global-AtmospheRe (MASINGAR) constrained with the pseudo-truth wind. We present the results of the analysis of a full month of data in summer time for a near-polar orbiting satellite and a LOS nadir angle of 35°. Below ≈ 8 km, the ratio of good retrievals is 30—55 % and the median LOS wind error is better than 0.6 m s-1 (1.04 m s-1 for the horizontal wind). In the upper troposphere, the ratio is less than 15 % in the southern hemisphere and high-latitudes. However the ratio is still 35 % in the northern Tropics and mid-latitudes where ice-clouds frequently occur. The upper-tropospheric median LOS-wind measurement error is between 1-2 m s-1 depending on the latitude (1.74-3.5 m s-1 for the horizontal wind). These errors are dominated by uncertainties induced by spatial atmospheric inhomogeneities.
Northern Xinjiang (NX), China, located at middle latitude of Northern Hemisphere, has abundant snowfall and a long period of snow cover. To assess the impacts of climate change in this region and to provide scientific knowledge for the resources and contingency plans, analysis of the spatial and temporal variations in extreme snowfall events (ESEs) in NX was carried out based on five defined ESE indices in this study, i.e. days of heavy snowfall, maximum 1-day snowfall amounts, maximum 1-event snowfall amounts, maximum consecutive snowfall days, and frequency of heavy snowfall events. To reconstruct the snowfall dataset, the relationship between air temperature and snowfall events were compared, and it was found that daily minimum air temperature below 0℃ is the best indicator to select snowfall days. ESEs in NX were taking an increasing proportion in snow events, though snowfall days were decreasing. Consistent increasing trends in all ESE indices were found for the whole NX, while different changes in these ESE indices were found for subregions. With high increasing trends in these ESE indices in most of subregions, Daxigou-Xiaoquzi area and Qitai area were the hot spots for ESEs. Since these hotspots are likely to be influenced by airflow from the Arctic Ocean, the changes in Arctic Ocean and associated atmospheric circulation as a consequence of climate change might be the main reason for the detected increasing trends of the ESEs in NX.
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.
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