High temporal resolution rainfall estimation from satellite is required for hydrology, especially for flood warning. Recently, rainfall estimation from passive microwave has been widely used. However, the sampling time is limited because the sensor is onboard low earth orbit satellite (LEO). Currently, the cloud motion vector derived from the infrared data of geostationary satellites is used to fill the observation gap by LEO, and a better cloud motion vector is essential for better rainfall estimation. Optically thick cloud area defined by the split window (11 µm and 12 µm) generally corresponds well to the rainfall area compared to the cloud area defined by single infrared data. We surveyed the effectiveness of useing split window data for deriving the cloud motion vector. We studied 30 rainfall cases during September 2003, and split window data indicated better score in 11 cases out of 30. The correlation values are improved by 45% at most for the 11 cases.
We report an application of the ensemble Kalman filter (EnKF) and smoother (EnKS) to an intermediate coupled atmosphere-ocean model of Zebiak and Cane, into which the sea surface height (SSH) anomaly observations by TOPEX/POSEIDON (T/P) altimetry are assimilated. Smoothed estimates of the 54,403 dimensional state are obtained from 1,981 observational points with 2,048 ensemble members. While assimilated data are SSH anomalies alone, the estimated sea surface temperature (SST) anomalies reproduce primary temporal characteristics of the actual SST. The smoothed estimate of the zonal wind anomalies is also consistent with the observation except for the westerly anomalies in the western Pacific.
In this study, the global circulation represented by JRA-25 is examined from the viewpoint of global energetics, compared with ERA-40 and NCEP/NCAR reanalysis. Although the peak and slope of the spectrum for JRA-25 are consistent with those of the other datasets, the magnitude of energy or energy conversion is generally ERA-40 > JRA-25 > NCEP. For short waves, a rapid decrease of amplitude with increasing wave number is shown for NCEP/NCAR reanalysis and ERA-40. The energy conversion from zonal available potential energy to zonal kinetic energy, caused by zonal-mean meridional circulation, is the largest for JRA-25, which is the result of a large amount of cancellation of the local conversion term in the Hadley cell.
Rapid transitions in the zonal wind (U) around the tropical tropopause within several days are investigated by using the global analysis data and their relation to the amplified Kelvin waves are considered. At 100 hPa, cases with a rapid increase in U (TypeU+ events) are concentrated in the eastern hemisphere, where such cases far outnumber cases that show a rapid decrease in U (TypeU- events). The difference in the number of the two types of events is greatest during the period November-March in the region 90°E-180°E. When only considering cases with a large zonal extent and eastward propagation, the dominance of TypeU+ events in the eastern hemisphere is much more distinct. The asymmetry between the two types is only weakly recorded at lower levels. An amplified and nonlinearly distorted Kelvin wave possibly accounts for predominance of these TypeU+ events at 100hPa. The dominance of TypeU+ events is detected in the basic easterly flow within the upper troposphere, which possibly facilitates the distortion by enhancing the upward propagation of energy and enabling the high intrinsic speed of the wave. In the distorted Kelvin wave, the latitudinal extent of the westerly signal is much smaller than that of the easterly signal.
We investigate the characteristics and potential future change of extreme climate phenomena based on daily statistical properties. To assess the fine-scale climate change projection over Korea, an ECHO-G B2 scenario simulation has been dynamically downscaled by using the RegCM3 nesting system. Two sets of multi-decadal simulations are performed for the present day (1971-2000) and future climate (2021-2050). We analyze four categories of weather spells in terms of the sequences of dry/wet and frost/hot days from the nested domain simulation of 20 km grid spacing. In the validation of the reference scenario, the model shows a reasonable performance of reproducing both observed climatological and distinct characteristics of each spell. Regarding the projected extreme climate events, the results indicate not only a significant change in frequency and intensity, but also substantial change in the fine scale detail of the regional distribution of temperature-based as well as precipitation-based extreme events. A substantial increase of hot spells is found along with increasing of maximum temperature. Longer-period frost spells are projected to decrease, whereas relatively short-period frost spells are likely to increase due to breaks of long spells by greenhouse warming. Wet spells of long periods tend to be more frequent, showing a distinct variation across regions.
At noon on April 20, 2006, a wind storm occurred in Fujisawa City, Kanagawa Prefecture, Japan with a F0-P1-P1 rank on the Fujita-Pearson-scale (FPP-scale). Damage to over 40 residential buildings was observed. The damage covered an area 2 km by 50 m. The strong winds were accompanied by the passage of a cold front. Eyewitness accounts of a dust column and the recording of a pressure drop (1.5 hPa) indicate the existence of a tornado vortex. A hook-shaped radar echo (5 km in diameter) was observed in the narrow cold frontal rainband, with strong horizontal wind shear above the damage area. The Doppler velocity pattern indicated a misocyclone, which had a diameter of 2 km and a vorticity on the order of 10-2 s-1. The wind storm may have caused the tornado, which formed in the cold frontal wind shear zone.
In this study, characteristics of the energy slope for the barotropic component of the atmosphere are examined in the framework of the 3D normal mode decomposition. The energy slope of E = ac2 was derived by Tanaka et al. (2004) based on the criterion of the Rossby wave breaking, where E is total energy, c is a phase speed of Rossby wave, and a is a constant. The wave breaking occurs when the local meridional gradient of potential vorticity is negative, i.e., ∂q/∂y < 0, somewhere in the domain. If the spectrum obeys the c2 law, it should obey the -4 power of the zonal wavenumber n, because the phase speed c is related to the total wavenumber by c = -β/k2, and if we assume the isotropy for zonal wind u and the meridional wind v over the range of synoptic to short waves, the energy spectrum can be expressed as a function of n instead of k. The theoretical inference of the energy slope is examined using JRA-25 data. According to the result of the analysis, the spectral slope agrees quite well with the -4 power law for the barotropic component of the atmosphere. It is, however, confirmed that the spectrum obeys the -3 power law as in previous studies for the baroclinic atmosphere. It is also found that the barotropic energy spectrum obeys the saturation theory where energy cascades up, but it does not obey where energy cascades down.
In this study, we investigate the daily forecast skill of Multi-Center Grand Ensemble (MCGE), consisting of three operational medium-range ensemble forecast data by the Japan Meteorological Agency (JMA), the National Centers for Environmental Prediction (NCEP), and the Canadian Meteorological Center (CMC). The skill is evaluated by comparison among the daily RMSE of ensemble mean forecasts for 500 hPa geopotential height over the Northern Hemisphere (20°N-90°N) from August 2005 to February 2006. It is found that MCGE with the same ensemble size as that of the JMA ensemble is more skillful than JMA ensemble for about 75% in frequency both in autumn and winter. Reduction of error with MCGE has little dependence on the atmospheric flow. The RMSE of MCGE can be reduced up to about 20% whether the atmospheric field is easily-predictable or not. Even for the case that MCGE is not more skillful than JMA, the RMSE is increased at most 10%. We argue that the major benefit of MCGE is to avoid the poorest forecast.
The sensitivity of optical properties of mineral dust aerosol to its direct radiative effect was investigated using four dust optical datasets (ADEC-1, ADEC-2, OPAC-MD and Dust-Like) with a global aerosol transport model. The annual and global mean direct aerosol radiative perturbation (DARP) by dust at the top of the atmosphere (TOA) can be positive or negative (-0.52 to +0.11 W m -2) depending on the absorptive property of dust aerosol dataset. The annual and global mean DARP at the surface is negative for all the dust optical datasets (-1.25 to -0.27 W m -2). If dust is strongly absorptive, DARP at the TOA over oceans at high latitude (> 40°) is positive due to the vertical configuration of dust and clouds. The longwave heating at the surface is strong and can exceed the shortwave cooling over desert regions if the absorption of dust is weak. Over a surface with high dust loading, the annual mean shortwave (longwave) DARP can reach -28 W m -2 (+16 W m -2). The global mean DARP values both at the TOA and surface are almost proportional to the imaginary part of the refractive index of dust. Seasonal variations of global mean DARP at the TOA significantly depend on the optical properties of dust.
A four-dimensional local ensemble transform Kalman filter (4D-LETKF) is applied to the JMA global model (GSM) at a TL159/L40 resolution to assimilate real observations including satellite radiances. It turns out that the vertical error covariance localization plays an essential role in assimilating satellite radiances. This study proposes and tests a new approach of the vertical localization, where the normalized sensitivity function of satellite sensors is used as the localization weights. With the vertical localization, AMSU-A, AMSU-B, SSM/I, TMI, and AMSR-E satellite channels are assimilated appropriately and indicate clear positive impacts. Thus, the proposed localization technique is a promising way of assimilating satellite radiances within the LETKF system.
Tropospheric column ozone derived from Global Ozone Monitoring Experiment (GOME) measurements (GOME-O3) was validated using operational ozonesonde measurements over Japan and compared with Tropospheric Ozone Residual (TOR) derived from Total Ozone Mapping Spectrometer and Solar Backscatter Ultraviolet measurements. The GOME-O3 validation showed positive biases with a magnitude of less than 3 DU (∼10%) and random errors of 5-9 DU (∼15-30%) at collocated sonde stations. GOME-O3 showed better agreement with ozonesonde measurements over Japan than TOR did. The direct comparison between GOME-O3 and TOR showed the underestimation of TOR during winter and spring in the southern part of Japan (around 30°N).
A four-dimensional local ensemble transform Kalman filter (4D-LETKF) is applied to the AFES (AGCM for the Earth Simulator) at a T159/L48 resolution to perform an experimental reanalysis called “ALERA” since May 2005 until October 2006, without using satellite radiances. The medium-term reanalysis confirms for the first time that the 4D-LETKF system is stable for more than a year with a realistic AGCM and real observations. ALERA generates reasonable reanalysis of the nature atmosphere, which is shown by comparing with the NCEP/NCAR reanalysis. ALERA is disadvantageous in the areas where satellite radiances play an essential role such as upper atmosphere and Southern Hemisphere high latitudes, although ensemble spreads capture the large errors. ALERA has large departures from the NCEP/NCAR reanalysis at upper levels above 30 hPa where the departures are larger than what is expected from the ensemble spreads. The ensemble reanalysis provides not only analysis itself but also samples for the analysis error statistics, which offers various potential uses for further studies.
This study investigates the capability of the regional climate model REMO to simulate the summer monsoonal rainfall distribution and evolution on the Tibetan Plateau. The model is performed for summer 1998 driven by “perfect boundaries” in a one-way double-nesting system, with a 55 km grid point intermediate mother domain encompassing East Asia and a 18 km grid point spacing nested domain covering almost the whole Tibetan Plateau. By comparison with in situ observations, the model results demonstrate that the higher resolution run out-performs the lower one. At a horizontal resolution of 18 km, REMO could reproduce the major precipitation events at daily scale and simulate the spatial differences. The evolution and spatial distribution of the monsoonal rainfall in the model are reasonable and comparable to observations.
Daily precipitation data for 105 years (1901-2005) at 51 stations in Japan were analyzed to evaluate the trends in the spatial concentration of precipitation distribution. The index of spatial concentration was defined by the departure of precipitation amount from the average over the surrounding area. It is found that the degree of spatial concentration has an increasing trend, mainly for weak and moderate precipitation. A similar feature is found for precipitation over 5-31 days as well.
Numerical experiments are performed to simulate and understand Niigata-Fukushima heavy rainfall in July 2004. The initial condition is adopted from global analysis (GANAL) data of JMA, and another numerical experiment is made with modified initial moisture field to better simulate the rainfall. A mesoscale-convection-resolving model is used, and the horizontal grid size is taken to be 1/18 degrees (about 5 km). It is emphasized that latent instability (positive buoyancy of rising air or CAPE) is important to explain convective activity and the rainfall, in contrast to a previous study which suggested the importance of convective instability. Latent instability is weak or absent in the GANAL data, but the observed heavy rainfalls and satellite images suggest that latent instability should be present at the initial time. A slight modification of the moisture field from the GANAL data gives rise to rainfall patterns which are more similar to those indicated by Radar-AMeDAS data in the early 10 hours. The rainwater distribution is discussed, compared with latent instability distribution. It is also suggested that neither convective instability nor equivalent potential temperature in the boundary layer are a good measure to explain the observed rainfall distributions.
The present study has used reflectivity data from 8-year observations of the TRMM precipitation radar to investigate diurnal variations of the Meiyu/Baiu rain belt associated with the Meiyu/Baiu frontal zone and their relations to diurnal variations on both sides of the frontal zone. A front-relative coordinate system has been designed for analysis. It is found that reflectivity in the frontal zone has a remarkable diurnal cycle that peaks in the early morning. Meanwhile, reflectivity both south and north of the frontal zone possesses a distinct diurnal cycle with a different phase from that in the frontal zone and has a peak in the late morning and afternoon. Based on the analysis, a possible regulation of the diurnal cycle of the Meiyu/Baiu rain belt through the thermally forced diurnal evolution of rainfall systems on both sides of the frontal zone is suggested.
Using the Japanese long-term Re-Analysis project (JRA-25) data, we investigated the seasonal dependence on dynamical links between the Pacific-Japan (PJ) pattern and typhoon activity. It was found that the PJ patterns tend to prevail soon after typhoons to the east of the Philippines migrating northward across the 20°N line during the period from July to October. Especially in early summer the low-level southwesterly jet formed by the penetration of the Asian summer monsoon westerlies into the Philippine Sea enables the typhoons to generate the PJ pattern, whereas in autumn the upper-level Asian jet contributes to the appearance of PJ as an alternative waveguide. The seasonal differences in latitudinal position and strength between the two different waveguides influence the vertical tilt structure of PJ in the vicinity of Japan. It also turns out that even though the typhoons are far away from Japan, they have the potential to indirectly and remotely activate stationary fronts around Japan during the rainy season through the dominance of PJ.
Analysis of a recent 6-year period (2000-2005) revealed a high frequency of dust outbreaks in the Tarim Basin in 2001 and 2002 and a strong positive correlation between dust outbreaks and strong surface winds; however, this trend was not observed in 2003, when a low frequency of dust outbreaks was recorded despite frequent strong surface winds. The threshold wind speed for 2003 was higher than that for 2002, which had a high frequency of dust outbreaks and a high index of dust storms. The NDVI between 2002 and 2003 was analyzed as a proxy for land surface characteristics. Variations in soil wetness were clearly observed for April and May, and the soil wetness for 2003 was higher than that for 2002. These findings suggest that a wet land surface in 2003 caused an increase in the threshold wind speed.
The assimilation of surface precipitation data with the ‘JMA nonhydrostatic model(JMA-NHM)’-based four-dimensional variational data assimilation system (JNoVA) is presented. The tangent linear and the adjoint codes of the 2-ice bulk microphysics scheme of Rutledge and Hobbs (1983) have been recently developed in nearly straightforward manner and the accuracy of the tangent linear model has been investigated. We succeed to assimilate the Radar-Raingauge analyzed precipitation data with the JNoVA using this cloud microphysics with horizontal grid spacing of 10 km and improve the precipitation of the analysis. The same precipitation data is successfully assimilated with the JNoVA using the simplified moist physics. The comparison of both analyses revealed that the analysis by the JNoVA using the 2-ice bulk scheme is not optimized as much as that by the JNoVA using the simplified physics. From the investigation of the cost functions, the tangent linear approximation of 2-ice bulk scheme is not accurate enough to use in the variational data assimilation system with the horizontal grid spacing of 10 km.
SeaWiFS measurements of Chlorophyll-a (Chl-a) and Normalized Difference Vegetation Index (NDVI) are analyzed for understanding the coupling between the oceanic and terrestrial biospheres at interannual timescale. Principal component analysis (PCA) of NDVI and Chl-a jointly suggests existence of stationary/dipole modes of variability between several neighbouring land-ocean regions. PC time series for NDVI and Chl-a separately are found to be anticorrelated during the period of this analysis (Autumn 1997-Summer 2006). Other satellite and analyzed products, e.g., sea-surface temperature, precipitation, absorbing aerosols suggest that the observed land-ocean biospheres are coupled beyond that can be explained by purely dynamical mechanisms.
We evaluated the ozone distribution over the Kanto region in Japan, calculated by a one-way nested global-regional air quality forecasting (AQF) system. This AQF system consists of global and regional chemistry-transport models (CTMs). The global CTM is based on CHASER (Chemical Atmospheric General Circulation Model for the Study of Atmospheric Environment and Radiative Forcing) model, whereas the regional CTM is based on the WRF (Weather Research and Forecasting)/ Chem model. The lateral boundary of the regional CTM domain is updated every 3 hours from the global CTM output. An experimental phase for this model system began in July 2006 and has provided 15-hour forecasts of the distribution of ozone concentrations over the Kanto region four times daily. The time evolution and horizontal distribution of chemical species calculated by this AQF system were compared to ground-based observations. Values of statistical measures are within the range suggested by the U.S. Environmental Protection Agency (EPA). By changing the initial value of the meteorological field from 6-hour forecast values to an analysis meteorological field, the average value of the correlation coefficient increased from 0.763 to 0.773.
A meso-scale model simulation is sometimes conducted with a saturation adjustment scheme with respect to ice in the upper troposphere. In such a simulation, the super-saturation with respect to ice is not permitted in upper tropospheric clouds, which is contradicting with natural situation. In this study, we evaluate the influence of the application of ice-saturation adjustment to upper tropospheric clouds in a meso-scale model. When the ice-saturation adjustment is applied, all of the excess water vapor in upper troposphere is consumed to produce large amount of cloud ice and subsequently snow. Small amount of excess water vapor remain for the depositional growth of snow. Since the fall velocity of snow becomes smaller, the residence time of snow in the atmosphere becomes longer. Consequently, the surface rainfall intensity, on domain-average, becomes slightly smaller through the integration time, compared to the simulation without ice-saturation adjustment.
An efficient implementation of the local ensemble transform Kalman filter (LETKF) with the error covariance localization by physical distances is introduced and assessed in this study. Instead of using local patches uniform in the model grid space to localize the error covariance, accurate physical distances are computed and used for the localization, so that the problem of analysis discontinuities in the Polar Regions is solved. Data assimilation cycle experiments with real observations are performed, which indicate less discontinuity in the Polar Regions. Moreover, the computational time is shorter and more robust for various localization scales. Thus, the implementation introduced in this study is a promising choice of future LETKF systems.
Torrential rains that repeatedly occurred over Java Island causing widespread floods in late January and early February 2007 coincided with a strong and persistent trans-equatorial monsoon flow from the Northern Hemisphere. While convections develop frequently over the island’s mountainous areas in the afternoon, convections over the northern plains are active during the night and morning hours. The strong trans-equatorial monsoon flow with an upper southeasterly wind produces a strong low-level vertical shear of wind and dry mid-level environment over the island. These conditions allow the severe convections to occur repeatedly for days and to sustain for an extended period of time. The results suggest that the trans-equatorial monsoon flow plays a principal role in the formation of the repeated torrential rains. The probability of occurrence of a strong and persistent trans-equatorial monsoon flow that causes torrential rains and widespread floods over Java Island is estimated to be once every 5-10 years.
The effect of the assimilation of dropsonde data over the tropical western Pacific was investigated in an objective analysis. In June 2005, 30 dropsondes were released on four separate flight days. The data impact was assessed using the objective analysis dataset of “ALERA.” The analysis of the zonal wind field over the tropical western Pacific in ALERA revealed large errors corresponding to active convection. These errors were reduced by 1-3 m s-1 due to the assimilation of the dropsondes. The impact signal due to the assimilation of the dropsondes propagated northward and appeared significantly around Japan. The phase and group speeds of the impact signal at 700 hPa were approximately 3 m s-1 and 12 m s-1, respectively. The former speed was consistent with the mean meridional wind speed, and the latter speed roughly corresponded to the meridional group speed of Rossby waves with wavelengths of a few thousand kilometers.
Characteristics of the water-vapor field in relation to thunderstorms on summer days over the Kanto district in Japan were studied, using precipitable water-vapor (PWV) derived from GPS during 2001-2005. PWV averaged on the active thunderstorm days showed distinct diurnal variation. In general, hourly data showed that PWV maximum appeared 1∼2 hours prior to the maximum thunderstorm activity (precipitation or cloud to ground stroke). The increase of PWV in the mountainous region of northern Kanto coincided well with the increase of low-level wind toward the mountains, which was observed by Wind profiler Network and Data Acquisition System (WINDAS). Using the 5 minute data, we further examined the time lag between the PWV and cloud to ground (CG) stroke related to individual thunderstorms in detail. The PWV maxima preceded that of CG stroke by 15∼30 minutes, for about 40% of the thunderstorms. In many cases, both PWV and its increment in 30 minutes showed large values within one hour before the CG stroke occurrence. This suggested that GPS derived PWV appears to reflect well the local variations associated with a thunderstorm.
Stratospheric sudden warming (SSW) in association with projected global warming (GW) in the Northern Hemisphere was investigated by 20th-century and 21st-century simulations (20CS and 21CS) using the atmosphere-ocean coupled general circulation model. Composite analysis revealed that the model simulated 20th-century dynamical aspects of SSW well. Although SSW events in early winter are missing, considering difficulties in reasonable SSW simulation, we regard the 20CS results as reference values. The 21CS suggests that forcing by GW increases the number of both major and minor SSW events in Dec, Jan, and early Feb, in correlation with larger eddy heat flux (EHF) at 100 hPa in Nov, Dec, and early Jan. The increase of the flux mainly originates from tropospheric stationary wave response. The greater flux continues until early Jan and then the polar night jet is much more weakened in Feb. This background state then effectively blocks wave propagation into the stratosphere and the EHF at 100 hPa slightly decreases though the difference is not statistically significant. The major SSW events decreases in Mar.
A case of aircraft triggered lightning, which occurred below the cloud base, was observed at Komatsu Airbase along the coast of the Sea of Japan during the intensive observation period using Doppler radar. The aircraft triggered lightning was accompanied by winter thunderclouds with graupel precipitation in the dissipating stage. The aircraft triggered lightning was initiated by a bi-directional leader. Lightning channels from the aircraft propagated simultaneously both upward and downward with return strokes. The surface electrical field was relatively weak (10 kV/m) compared with that of natural CG lightning.
In this study, the frequency of surface cyclogenesis and the surface cyclone track in East Asia are investigated using high-resolution and long-term reanalysis data, which are ERA40 data in a reduced Gaussian grid system. A cyclone center was identified in the surface pressure anomaly using a modified method after Serreze (1995). Surface cyclogenesis frequently occurs in several specific areas in all seasons. The broadening of the frequent areas is mostly narrower than those shown in previous studies. The areas of high-frequency cyclogenesis are distributed in the lee of mountains, basins, the Pacific Ocean to the east of Japan, the Japan Sea, the Kuroshio Current region to the south of Japan, and the East China Sea. In particular, this study showed newly several findings; the inactive cyclogenesis in the lee of Tahsinganling in summer; the frequent cyclogenesis in the Hebei Plain except for summer; and the cyclogenesis maxima around the mouth of the Yangtze River and in the East China Sea to the northeast of Taiwan in winter.
The time evolution of two major chlorine reservoirs, HCl and ClONO2, in the polar lower stratosphere for both hemispheres in late spring and early summer was analyzed by utilizing satellite measurements. Analysis of the collocated ClONO2 observed with ILAS, and HCl observed with HALOE indicated that Cly was mostly in the form of HCl in the Antarctic vortex in November 1996, while more than half was ClONO2 in the Arctic vortex in March 1997. The analysis of N2O indicates that there was strong subsidence of the air inside the Antarctic vortex. The vortex remnant with HCl-rich air persisted in December 1996 even after the vortex weakened, suggesting slow mixing in the boundary. The ClONO2/Cly ratio was about 0.3, which was derived from analysis of correlation with N2O, as a representative value for the extravortex air and upper air inside the vortex (with potential temperature greater than ∼600 K) in both hemispheres in spring.
Relationship between Asian dust frequency observed in Japan (ADF) and dust storm frequency (DSF) in northern China was examined with up-to-date long-term observational data. Possible climatic conditions associated with variations of ADF were also explored. It was found that DSF at stations in Inner Mongolia and its vicinity (IMV) and ADF behaved nearly parallel to each other from the mid 1970s. Significant positive correlation between DSF in IMV and ADF suggests that IMV has a stronger influence on ADF than other dust source regions in northern China. Composite maps of Sea Level Pressure and 700 hPa wind fields for extreme ADF years exhibited a spatial pattern with cyclonic anomalies over the area from Northeast China to north Japan and anticyclonic anomalies around the Gobi desert in high ADF years. This situation corresponds to enhanced cyclonic activities in Northeast China and intensified cold air outbreaks in IMV, which are favorable for formation of dust storms in IMV.
On clear, calm summer days, cloud lines are often observed above the expanding urban area along the railroads from Tokyo. The cloud lines were simulated using a numerical model with a simplified urban surface parameterization. The horizontal distributions of the simulated clouds agree well with the observed clouds in satellite images. Some sensitivity tests indicate that the urban thermal effect enhances cloud formation but suppresses clouds in the adjacent rural area. The simulated clouds are consistent with the observations when the urban maximum sensible heat flux is substantially larger than the rural one. With a reduction of the thermal contrast, cloud suppression in the rural area is gradually weakened. The mechanisms of the cloud contrast are as follows: 1) Thermals form in the mixed layer above the land surface. 2) Because of the larger sensible heat flux, the mixed layer tends to be higher in the urban area; thus, thermals reach the lifting condensation level more easily than they do in the rural area. 3) The wide compensating downdrafts of the strong urban thermals cover the entire rural area and suppress thermals and clouds there.
We examined interannual variation in snow-accumulation events in Tokyo and the relationship of this variation to large-scale atmospheric patterns. Years when snow-accumulation events occurred tended to coincide with a west-to-east wavetrain pattern in the middle and upper troposphere over Eurasia. The pattern, which includes cyclonic anomalies over Europe and East Asia and anticyclonic anomalies over Siberia, is identical to the negative phase of the Eurasian (EU) pattern, which is the leading mode of an empirical orthogonal function (EOF) analysis applied for the Eastern Hemisphere in the 500-hPa height field. Anomalous cold air associated with the negative EU pattern widely covers East Asia and Japan in the lower troposphere. No significant relation to storm-track activities around Japan was found except for extremely deep snowfall years. The cold atmospheric anomaly associated with the negative EU pattern possibly lowers the surface air temperature over Tokyo, creating an environment favorable to snowfall and snow accumulation. In the extremely deep snow years, the signature of the EU pattern was weak, and storm tracks over the ocean to the south of Japan were significantly active. No clear long-term trend in snow-accumulation events was found, although a downward trend due to anthropogenic effects was expected.
We evaluated the ability of a regional air quality (AQ) model, the Weather Research and Forecast chemistry (WRF/Chem), to reproduce vertical profiles of summertime ozone (O3) pollution over the Kanto region of Japan using lidar-observed O3 data collected over Tsukuba on 27-29 July and 16-21 August 2005. WRF/Chem well reproduced the high O3 concentrations > 2.5 × 1018 molec. m-3 (∼100 ppbv below 1.5 km above sea level ASL) observed on 28 July and 20 August, and the low concentrations on the other days below 1.5 km ASL, as well as day-to-day variations above 1.5 km ASL. The formation mechanism of the observed O3 layer in the typical northward transport case on 28 July was also examined by the model. The O3 plumes entered the free troposphere through vertical transport and mixing near the sea breeze front and southward transport by the return flow in the afternoon. In the evening, the O3 layer formed at 0.7-1.5 km ASL possibly because of less transport between the sea breeze and return flow and longer photochemical lifetime than at the surface.
To examine the characteristics of polar stratospheric clouds (PSCs) in the Antarctic, we have analyzed short-time (≤ 5 days) changes in nitric acid (HNO3) and aerosol extinction coefficient (AEC) at 780 nm, focusing near 20 km altitude in June 2003 as observed by the Improved Limb Atmospheric Spectrometer (ILAS)-II. The Match technique based on the air parcel trajectory was applied to the ILAS-II data. The several Match pairs have revealed decreased HNO3 values with increased AEC values within short times, indicating “temporary” denitrification. It is also suggested that the observed PSCs could be nitric acid trihydrate (NAT) particles, considering that the temperatures were above existence temperatures for supercooled ternary solution, but below those for NAT. Given appropriate size distributions for NAT particles, it is suggested that the median radius of particles was less than 3 µm.
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