In order to use the nationwide dense receiver network of the Global Positioning System (GPS) in Japan to contribute to water vapor monitoring and numerical weather prediction, a near real-time (NRT) analysis trial was performed at the Meteorological Research Institute. In this paper, the NRT analysis procedure is described along with some validation results. In view of the computational load involved in analyzing more than 1,300 GPS stations in Japan, the precise point positioning (PPP) procedure was adopted. The PPP procedure requires accurate information of GPS satellites’ positions and clock offsets. International GNSS Service (IGS) has been routinely providing ultra rapid ephemeris (IGU) that includes satellite orbits and clock offsets with latency of about 3 hours. We found the accuracy of satellite clock offsets in IGU was insufficient for the retrieval of precipitable water vapor (PWV) through the PPP procedure. Therefore, we applied correction to the IGU clock using the predicted clock offset at an IGS station “USUD”. The hydrogen maser atomic clock at USUD also had some differences with GPS time. However, we found it could be fitted and predicted by a linear equation for a period of several days. The resulting satellite clock offsets exhibited some biases toward the IGS final ephemeris, but the time constant biases of satellite clock offsets did not affect the PWV retrieval at all. The retrieved PWV data agreed well with those obtained from radio-sonde observations. The root mean square differences in summer and in winter were around 3.4 mm and 1.6 mm, respectively. The results were comparable with those obtained by preceding studies using the final ephemeris. The Retrieved spatial and temporal variation of PWV in a heavy rainfall case demonstrated the usefulness of the NRT PWV retrieval for weather monitoring. We could capture the water vapor increase that preceded torrential rain.
An observation of physical and optical properties of aerosols was carried out in an urban atmosphere of Nagoya, Japan from 23 July to 4 September 2004. Such observed data were analyzed to understand urban aerosol effects on atmospheric heat budget. Aerosols over our observation area were found to be highly light absorptive with a very low mean single scattering albedo (ω) of 0.72. The presence of such light absorbing aerosols was capable to significantly influence the diurnal cycle of ω, indicating important roles of such aerosols on atmospheric heat budget. An algorithm was developed to estimate the refractive index and density of dry aerosols, and such algorithm produced the mean values of refractive index and density of 1.53-0.038i and 1.7 gcm-3, respectively. Sub micron (d < 1.0 μm) aerosols were observed to contribute total aerosol mass concentration and scattering coefficient for aerosols less than 10.0 mm in diameter by ~81% and ~95%, indicating the important roles of sub micron aerosols on atmospheric radiative transfer process. The 24 hours mean values of radiative forcing efficiency (RFE) at surface and top of the atmosphere (TOA), atmospheric forcing efficiency (AFE), and heating rate at surface were -71.8 ± 12.3Wm-2, +2.5 ± 6.8Wm-2,74.4 ± 19.0Wm-2 and 2.46 ± 0.61 K/day, respectively. The study suggested that an increase of black carbon (BC) mass fraction could effectively increase the heating rate at surface by trapping more energy (or increasing AFE) in the atmosphere. It was observed that aerosols over our study area could have a positive RFE at TOA if BC mass fraction becomes greater than ~0.05 (or ω becomes smaller than ~0.75). Further, the study suggested that an increase of BC mass fraction by 0.1 could increase RFE at TOA by ~15.8 Wm-2, decrease RFE at surface by ~30.4 Wm-2 and increase heating rate at the surface ~1.39 K/day, respectively.
Large-eddy simulations (LESs) were performed for neutrally-stratified turbulent flows within and above a homogeneous plant canopy. 100 realizations of a three-dimensional turbulence field obtained from each of four LES runs, which differ in the driving force of the flow and the size of computational domain, were used in the present study. A conditional sampling technique was used to construct ensemble-averaged images of coherent eddies that induce predominant perturbations in the streamwise and vertical velocities near the canopy top. To reduce subjectivity, wavelet analysis was adopted for triggering the conditional sampling. Synthesis of the present study and numerous previous studies indicated that, in the canopy turbulence, the spatial scale of eddies that induce predominant perturbations in the streamwise velocity is generally three times larger than that of eddies that induce predominant perturbations in the vertical velocity, irrespective of whether concerning field observations, wind-tunnel experiments or numerical simulations. Therefore, scales of both eddies are mostly determined by a mechanism inherent in the roughness sublayer. An analysis of the ensemble-averaged results and each realization revealed several findings: (1) the smaller eddies that cause predominant perturbations in the vertical velocity are vortices that accord with the so-called mixing-layer (ML) analogy, which is widely accepted as a mechanism of coherent eddies developing near the canopy top; however, (2) the larger eddies inducing predominant perturbations in the streamwise velocity are not vortices and are much larger than expected from the ML analogy; (3) these eddies are streamwise-elongated motions of high-speed downdraft and low-speed updraft, having characteristic features such that the high-speed downdraft penetrates into the canopy and cross-streamwise spreads inside the canopy thus inducing low-speed updraft to the sides of the downdraft and that the low-speed updraft produces a lifted (higher than the canopy top) shear zone beneath an overriding high-speed motion thereby enhancing the shear instability in that area; (4) the high-speed and low-speed motions aligning side-by-side bear a close resemblance to streaky patterns observed in a near-surface region of planetary boundary layers, although the spatial scales are quite different.
Observations from a Doppler radar, a wind profiler, and a meteorological tower were used to study the evolution of two thin-line echoes that were observed in the radar reflectivity field over the Kanto Plain of Japan as Typhoon Higos (0221) passed in October 2002. Both thin-line echoes were accompanied by gusty winds and followed by cold airflows, and both passed the field site of the Meteorological Research Institute (MRI) in Tsukuba, Japan. Data from the MRI instrument array and from a surface observation network revealed that the first thin-line echo was caused by a gust front, and the second thin-line echo was caused by a solitary wave. Although it was demonstrated that the first thin-line echo was due to a gust front, this gust front could not be categorized as a so-called thunderstorm gust front because it had no parent thunderstorm; rather, Doppler radar and wind profiler observations suggested that both relatively strong (∼4ms-1 downdrafts and large momentum transported by the downdrafts from aloft (∼4 km) to lower levels produced and enhanced near-surface winds behind the gust front. In contrast, the solitary wave associated with the second thin-line echo likely developed as another gust front that generated from an outer rainband to the south of the typhoon center met a stable layer formed by the cold outflow behind the gust front associated with the first thin-line echo. The ducting mechanism that enabled the solitary wave to propagate was also revealed from the wind profiler and radiosonde measurements.
The time evolution of the circulation change at the end of the Baiu season is investigated using ERA40 data. An end-day is de?ned for each of the 23 years based on the 850 hPa θe value at 40˚Nin the 130?140˚E sector exceeding 330 K. Daily time series of variables are composited with respect to this day. These composite time-series exhibit a clearer and more rapid change in the precipitation and the large-scale circulation over the whole East Asia region than those performed using calendar days. The precipitation change includes the abrupt end of the Baiu rain, the northward shift of tropical convection perhaps starting a few days before this, and the start of the heavier rain at higher latitudes. The northward migration of lower tropospheric warm, moist tropical air, a general feature of the seasonal march in the region, is fast over the continent and slow over the ocean. By mid to late July the cooler air over the Sea of Japan is surrounded on 3 sides by the tropical air. It is suggestive that the large-scale stage has been set for a jump to the post-Baiu state, i.e., for the end of the Baiu season. Two likely triggers for the actual change emerge from the analysis. The first is the northward movement of tropical convection into the Philippine region. The second is an equivalent barotropic Rossby wave-train, that over a 10-day period develops downstream across Eurasia. It appears likely that in most years one or both mechanisms can be important in triggering the actual end of the Baiu season.
An extremely heavy precipitation event occurred in the mountainous Kii Peninsula in Japan, associated with Typhoon Meari in 2004. A marked characteristic of this heavy precipitation was its extreme rainfall rate, more than 100 mm h-1. Another feature was that the area of precipitation was far from the storm center, more than 500 km. From radar and surface observations, it was found that the heavy rains were composed of a stationary precipitation system and two moving precipitation systems. In order to evince a physical mechanism for the precipitation systems, the event was analyzed in detail using data from cloud-resolving simulations and observations. The results demonstrated that the heavy precipitation was produced in a synergistic manner from the three precipitation systems. The following are identified as the key factors for the formation and maintenance of each precipitation system: a) elimination of vertical convective instability in a low-level warm and moist easterly on the eastern slope of the mountainous region, b) moisture supply due to a low-level confluent flow along the boundary between the low-level easterly and south-easterly flow, and lifting of a slightly warmer south-easterly flow, and c) low-level convergence due to cold pool, running nearly parallel to the rainband axis and located slightly to the southwest of the band, acting as an obstacle to the low-level inflow into the system. Precipitation efficiency revealed that precipitation was enhanced when moving precipitation systems merged with the stationary precipitation system. The enhancement was attributed to the greater rate of conversion of cloud water to rainwater via accretion of cloud water by rain, under the condition of intense water vapor flux convergence. The moving precipitation systems provided raindrops for the accretion of cloud droplets in the stationary precipitation system. Based on our findings, extremely heavy precipitation in the present case is caused by the enhancement of the accretion process due to the merger of precipitation systems in addition to precipitation in each system.
We examined the connection between the Asian summer monsoon and stratosphere-troposphere circulation over the Asian region during a 25-year period (1980-2004) in boreal summer (June-August) using NCEP/ NCAR reanalysis data. Strong monsoon years (6 years) were extracted from the 25-year sample using the monsoon index of Webster and Yang (1992), and composite maps of selected variables were created. Analyses revealed significant positive geopotential height anomalies over Japan and over Iran around 100 hPa in strong monsoon years. Both stretching and horizontal advection terms were dominant in the vorticity budget of high pressure anomalies over Japan, whereas only horizontal advection term was dominant over Iran. Wave activity flux was calculated for the strong monsoon years. Upward wave fluxes occurred over the Aral Sea and over Mongolia, in association with precipitation in northern India and in the Philippines. These correspondences are likely to show the Rossby wave response to diabatic heating in the Asian monsoon regions. Energy conversion was estimated in both East Asia and Central Asia using an equation for the rate of change of perturbation total energy. In East Asia, barotropic energy conversion was positive in the low-latitude lower troposphere. In mid-latitudes, baroclinic energy conversion was positive in the troposphere. These results correspond to northward wave fluxes in the tropical lower troposphere and upward wave fluxes in mid-latitudes. In Central Asia, barotropic energy conversion was more dominant than baroclinic energy conversion in mid-latitudes. Upward fluxes occur in mid-latitudes, whereas northward fluxes do not occur in the tropics, unlike in East Asia. The estimation of the total wavenumber of stationary Rossby waves indicates that precipitation in the Philippines is strongly connected with high pressure anomalies over Japan through internal Rossby waves. In contrast, high pressure anomalies over Iran do not seem to be deeply related to precipitation in India. These results suggest that lower stratospheric high pressure anomalies over Japan are connected with both Rossby waves generated through precipitation in the Philippines and baroclinic energy conversion over Mongolia, whereas high pressure anomalies over Iran are controlled profoundly by internal dynamical processes near the Aral Sea.
In July 2004, torrential rainfalls caused significant damages in parts of Japan, followed by heat waves. Our data analysis shows that both rainfall and heat wave events in late Baiu season were caused by the intensification of the subtropical anticyclone near Japan (Bonin high) and that intensity of the Bonin high was significantly influenced by propagation of Rossby waves along the subtropical jet. Hindcast experiments from 15 July were conducted to study the mechanisms and predictability of these high-impact weather events. On 17-18 July, localized rainfalls at a few locations along the coast of the Sea of Japan including Sakata and Fukui were successfully simulated in a high-resolution (21-km mesh) global hindcast simulation. These rainfall events were found to occur near the leading edge of a filament of moist and warm air advected clockwise. On 20 July, anomalously high temperature was reproduced in the high-resolution hindcast simulation. With a moderate resolution of 83 km, the intensification of the subtropical anticyclone was reproduced although the föhn was much weaker. This result indicates that temperature distribution associated with föhn requires a resolution high enough to resolve major mountains. In order to investigate the predictability of propagation of Rossby waves and intensification of the Bonin high, 25-member ensemble experiments from 1 July 2004 were conducted using the moderate-resolution model. It is shown that the region along the Asian jet has twice as long predictability as the entire Northern Hemisphere. This case study suggests that the intensification of the Bonin high associated with the propagation of Rossby waves along the Asian jet could be predicted a few weeks in advance with an ensemble forecast at a moderate resolution.
There is a diurnal cycle of systematic cloud migration over Sumatera Island, i.e., cloud systems developing in the mountainous area in the afternoon migrate westward and/or eastward for several hundreds of kilometers (about 500 km) from night to morning. The regional characteristics and internal structure of migratory cloud systems with a diurnal cycle over Sumatera Island during CPEA-I were examined using data from an X-band Doppler radar (XDR), a VHF wind pro?ler (Equatorial Atmosphere Radar (EAR)), rawinsondes, and Geostationary Operational Environmental Satellite (GOES9). During CPEA-I, the cloud system had a horizontal scale of several hundred kilometers and migrated both westward and eastward over nearly all of Sumatera Island except for the southernmost part. The cloud system migrated only westward over southernmost Sumatera Island during CPEA-I. From a case study on April 17 and 18, 2004, precipitation systems with horizontal scales of several tens of kilometers were observed in a cloud system by XDR, and they migrated in a direction similar to that of the cloud system at a speed of about 3 m s-1, which roughly corresponded to the wind direction and speed in the lower troposphere. Convective precipitation was observed mainly in the forward region of the precipitation systems, and stratiform precipitation was observed in the rearward region. The convective precipitation successively generated new convective cells in front of old convective cells. These results suggest that the migratory mechanism of the precipitation systems is self-replication of convective cells and the advection of background wind in the lower troposphere.
This study investigates the major sources of potential predictability and associated regulating processes for summer (JJA) low-level tropical circulations using a 1979?2003 ensemble hindcast made by the Central Weather Bureau (CWB) Global Forecast System (GFS) model. This hindcast is conducted with a two-tier system using the external SST forcing so that it lacks the processes of air-sea interactions. This study focuses on three tropical regions: the eastern Pacific Niño (EPN; 160°E-80°W, 30°S-30°N), the western Pacific monsoon (WPM; 100°E-160°E, 30°S-30°N), and the Indian Ocean monsoon (IOM; 40°E-100°E, 30°S-30°N). The WPM and IOM circulations are found to have different predictability sources and should be examined separately. The predictability source for the former is primarily from SST anomalies in the tropical eastern Pacific, while SST anomalies in the tropical central Indian Ocean (IO) for the latter. Strong SST anomalies tend to induce persistent and large-amplitude circulation anomalies and by so doing enhance potential predictability. Circulation predictability is generally higher over the WPM and EPN regions than over the IOM region. The eastern Pacific SST anomalies induce a pair of low-level divergence-convergence anomalies over the Pacific to modulate the EPN and WPM rotational circulations simultaneously via Rossby-wave response. The predictability of these two circulations tends to be temporally coherent. For the IOM circulation, its predictability is regulated in two different ways. Strong in situ SST anomalies in the tropical IO may directly affect the IOM circulation via Rossby-wave response. In the absence of strong SST anomalies, the IOM circulation is mainly maintained by a local SST dipole pattern via changing local Walker circulation cells.
Aerosol optical parameters obtained from sky radiometer instrument are important not only for studying aerosol effects on climate change, but also for validating several results obtained from satellite retrievals and numerical simulations. However, the greatest challenge is to separate cloud-affected and cloud free data from data measured by sky radiometer. In this study, we present an algorithm to separate such cloud-affected and cloud free data. The proposed algorithm is comprehensively tested with observational data. The algorithm consists of three tests: (i) test with global irradiance data, (ii) spectral variability test, and (iii) statistical analyses test. Though the test with the global irradiance data is the most powerful test, our study shows that it has some limitations, which can sometimes cause some clear sky data to be detected as cloud-affected data. In order to cope with this problem, a modified version of spectral variability algorithm is proposed. As the second test, the modified spectral variability algorithm is applied to filter clear sky data from data detected as cloud-affected by the first test. Finally, statistical analyses tests are performed to remove any outlier, if exists, from clear sky data detected by the first and second tests. It is shown that our proposed algorithm can screen cloud-affected data more effectively in comparison to other cloud screening algorithms. An application of this algorithm to screen observation data of one year collected in Chiba, Japan produces the seasonal means of optical thickness at 500 nm (Angstrom exponent) as ∼0.17(∼1.42), ∼0.38(∼0.98), ∼0.53(∼1.21), and ∼0.21(∼1.28) for winter, spring, summer and autumn seasons, respectively. Depending on the season, the initial seasonal mean optical thicknesses at 500 nm decrease by ∼0.07 to ∼0.16 and mean Angstrom exponents increase by ∼0.087 to ∼0.162 due to cloud screening. An application of this algorithm to dust-loaded atmospheres is also discussed. The proposed algorithm can be applied to any sky radiometer observation site as long as global irradiance data are available.
In the horizontally 2-dimensional quasi-geostrophic system, we construct analytical solutions of transversely propagating Rossby waves across the basic zonal flow. On the assumption that the basic potential vorticity is piece-wise constant in the meridional direction, the solution with a meridionally localized initial disturbance can be obtained analytically. The analytical solutions show, for example, the following. In the case of a uniform basic zonal flow, the Rossby wave propagates diagonally due to the planetary potential vorticity gradient. In the case of a jet-like basic zonal flow, the oscillating Rossby wave is almost trapped along the jet-axis. However, for the growing Rossby wave, the amplitudes on the jet-axis and on the jet-flanks eventually become comparable.