Volcanic aerosol layer just above the tropopause was observed by a shipboard lidar over the tropical western Pacific in February 2005. The aerosol layer with vertical thickness less than 1 km was observed at the altitudes of about 19 km in the regions of 0-2°N and 7-9°N along about 156°E. The optical properties derived from the lidar data show the thin layer had optical thickness less than 0.01, small scattering ratio, moderate particle depolarization ratio, and large backscatter-related Ångström exponent compared to cirrus clouds below the tropopause. From the report on the volcanic eruptions and the wind profiles of the sonde data, it was concluded that the aerosol source was the 27-28 January large eruptions of Manam volcano in Papua New Guinea (4.10°S, 145.06°E). The volcanic emissions injected into the lower stratosphere were transported by tropical westerly and deposited at the altitudes of weak wind which is interpreted as the phase change of the Quasi-Biennial Oscillation (QBO).
The results of the empirical orthogonal function (EOF) analysis applied to the sea-level pressure (SLP) variability in the extratropical Northern Hemisphere is reexamined from viewpoint of statistical independence. For this purpose, “Independent Component Analysis” is introduced, whose effectiveness is demonstrated in its application to a set of simple hypothetical models. While its application to stratospheric variability confirms the relevance of the EOF analysis, its application to the SLP suggests that the two leading components of the EOF analysis are likely superpositions of two statistically independent signals. One of them represents a seesaw-like relationship in the intensities between the surface Aleutian and Icelandic lows, and the other is characterized by a significant long-term trend with the cold ocean-warm land pattern.
An example of tropical cyclone (TC) formation processes described by a 20-km grid model is presented. A numerical time integration is performed for more than 200 days under a simplified (idealized) condition. Among many TCs that form in this period, this paper describes a TC that forms from a band-shaped meso-α-scale convective system in a westerly flow to the south of the previous TC. The orientation of the mesoscale system is northeast-southwest at the early stage. This orientation is nearly perpendicular to the vertical shear vector of the environmental low-level flow. Strong latent instability is located at the northwestern side of the system, and lower temperatures near the surface (cold pool) is at the southeastern side. This system rotates cyclonically. A center of cyclonic flow, which is located far to the north of the convective system at the early stage, is forced to approach the system, and the wind is intensified by convective activity. When the orientation of the convective system becomes west-east, the system attains TC intensity. Behavior of mesoscale cells which constitute the mesoscale system is also described.
This paper investigated how atmospheric aerosol deposition into snowpack affects snow albedo reduction using radiation budget observation, atmospheric aerosol monitoring, and snow pit work during the winter of 2003/2004 in Sapporo, Japan. The mass concentration of snow impurities was less than 10 parts per million by weight (ppmw) in the core accumulation season and exceeded 100 ppmw in the melting season due to a heavy dust event on March 11 to 12, 2004. The relationship between the visible albedo and snow impurities suggested the considerable effect of snow impurities in reducing the visible albedo even during the accumulation season in Sapporo. After the dust event, high impurity concentrations of approximately 600 ppmw in the top snow layer were maintained, but the visible albedo still decreased because the increasing snow grain size continued to reduce the visible albedo. The size distribution of snow impurities measured from a snow sample with a Coulter counter during the dust event was compared with the size distributions of snow impurities calculated as wet and dry depositions from a laser optical particle counter for atmospheric aerosols. The result confirmed that the contribution from wet depositions was important except for giant particles with a radius larger than 2.5 µm.
A simple scheme to represent the marine stratocumulus clouds off the west coast of continents is implemented in the Global Spectral Model (GSM) at Japan Meteorological Agency (JMA). The parameterization is based on diagnostic cloud schemes where cloud fraction is diagnosed mainly as a function of inversion strength considering other parameters. The global distribution of marine stratocumulus clouds off the west coast of continents is improved remarkably with this new scheme. Low-level cloud amount shows reasonable agreement with the International Satellite Cloud Climatology (ISCCP). With the improved cloud amount, the radiation fields are also improved in comparison with the Earth Radiation Budget Experiment (ERBE). Seasonal and diurnal variations of marine stratocumulus cloud amount off the west coast of continents also show reasonable agreement with surface-based cloud amount data from Klein and Hartmann (1993) and other observations.
Tracks of typhoons are modeled through nonlinear interactions of vortices between a typhoon and the background winds, i.e., easterly and westerly, of which velocity profiles along latitude are approximated with Gaussian. An intense vortex filament, representing a typhoon, redistributes vortex filaments embedded in the background winds which carry the typhoon. Starting in the easterly, a typhoon track is found to be going on westward, turning eastward, or gyrating around the easterly-westerly boundary, in response to variations of vortex intensities of easterly and westerly as well as of a typhoon itself.
During the winter monsoon, a convergence line called the Boso Front often appears in the area between the Kanto Plain and the Izu Islands, Japan. Two typical cases of high-level Precipitable Water Vapor (PWV) are observed along the convergence line in the lee of Chubu Mountains by the aid of latest techniques of Global Positioning System (GPS). Statistic analysis of GPS derived Precipitable Water Vapor (GPS-PWV) indicates that the prevailing wind direction controls the position of the high-level PWV in the lee of Chubu Mountains. A numerical model simulates the behavior of water vapor, namely, the temporal variation of simulated PWV and surface wind, which suggest the reasons for the high PWV near the convergence line. The convergence of surface wind gathers moisture, and trapped moisture by reduced wind velocity may also contribute the high-level PWV.
This study investigates some of the important characteristics of summer monsoon circulation and precipitation over East Asia in general and Korea in particular using the recent version of NCAR Regional Climate Model (RegCM3). RegCM3 has been integrated at 27 km horizontal resolution over the East Asian domain for the period from 1998 to 2002 (5 years). The important characteristics of temperature and wind at upper and lower levels and precipitation simulated by the RegCM3 over South Korea are examined in detail using different convective parameterization schemes namely, mass flux schemes, a simplified Kuo-type scheme and Emanuel (EMU) scheme. The monsoon circulation features (wind and temperature) simulated by RegCM3 are compared with those of the NCEP/NCAR reanalysis (Kalnay et al. 1996) and simulated summer precipitation in JJA (average of June to August) over Korea (KMR) is validated against the observation from Korea Meteorological Administration (KMA). Validation of simulated precipitation with KMA shows that the use of the Arakawa and Schubert (AS) and EMU convection schemes are more close to the KMA. The statistical analysis (area average precipitation, standard deviation and correlation analysis) also indicates that the use of EMU and AS schemes perform better over East Asia particularly over Korea.
In this study, we investigate the impact of Multi-Center Grand Ensemble (MCGE) forecasts, consisting of three operational ensemble forecasts by the Japan Meteorological Agency (JMA), the National Centers for Environmental Prediction, and the Canadian Meteorological Center. We verified the skill of MCGE forecasts in comparison with that of JMA ensemble forecast using root mean square error, anomaly correlation, and Brier skill score for 500 hPa geopotential height and 850 hPa temperature in the Northern Hemisphere (20°N-90°N) in September 2005. Our results show that MCGE forecasts are more skillful than single-center ensemble forecast without considering weight among ensemble members and bias corrections. This implies that considering weight or bias corrections may result in further improvement of MCGE forecasts, specifically in probabilistic forecasts.
The extratropical response to typhoon-related convective forcing over the western North Pacific in late summer is examined based on ECMWF global reanalysis (ERA-40) data during the 1958-2001 period. Typhoon activity is intimately associated with most of the major events in which an extratropical wavetrain structure prevails from the north of the Philippines through the central North Pacific. The vertical structure of the wavetrain pattern changes from baroclinic to a barotropic along the great circle. The analysis of the wave activity flux indicates that the extratropical wavetrain is stimulated by stationary Rossby waves. It was found that one or two typhoons, which are a synoptic-scale convective heat source over the western North Pacific, can induce the barotropic Rossby wavetrain and significantly influence the summer weather in the vicinity of Japan as remote forcing.
We studied the features of convective activity over the Bay of Bengal (BoB) in terms of cloud type and rainfall. In this paper, six cloud types were classified over the tropics using split window (11 and 12 µm) data measured by the Advanced Very High Resolution Radiometer (AVHRR) on board NOAA-9. The spatiotemporal structure of Dense Cirrus (D-Ci) and Cumulonimbus (Cb) type clouds is surveyed in this study. As for the summer monsoon season, D-Ci-type clouds higher than 10% can be seen over the entire BoB. The spatiotemporal structure of the D-Ci-type cloudiness coincides with those of outgoing longwave radiation (OLR) value of less than 230 W/m 2 over the BoB. The Cb-type cloudiness, which is less than D-Ci-type, concentrates over the northeastern part of the BoB in the boreal summer. The enhanced precipitation, derived from Tropical Rainfall Measurements Mission (TRMM) Precipitation Rader (PR), coincides with the D-Ci-type clouds area higher than 20% and the low OLR value less than 190 W/m 2 as well as large amount of Cb-type cloudiness in the boreal summer.
‘Yamase’ clouds are typical maritime boundary-layer clouds, for which present state-of-the-art weather prediction models and general circulation models are currently unable to simulate adequately. Yamase clouds frequently appear over the western North-Pacific region, off the east coast of the Sanriku district, Japan. They occur during the summer season under cool easterly winds blown from Okhotsk anticyclones. Here we discuss optical and microphysical properties estimated from shipboard experiments and satellite remote sensing for Yamase clouds observed in June 2003. This was the first time we were able to observe time-varying features of the marine boundary-layer accompanied with Yamase clouds from formation to decay. At the ship site, the cloud-base height and liquid-water-path, time-averaged during the June Yamase event, were 270 m and 60 gm-2, respectively. At the same time it is revealed, from satellite remote sensing using the contemporary AVHRR/3 data on morning orbital NOAA-17, that the Yamase clouds were rather thin, stratiform low-level clouds, and their physical parameters gradually grew with time during the Yamase period. The optical thickness, effective particle radius, and liquid- water-path, averaged over the time and area, were estimated to be about 12, 12 µm, and 110 gm-2, respectively.
A remarkable decrease in water vapor was observed during daytime on calm days over coastal Thailand. Precipitable water vapor (PWV) and surface specific humidity (SSH) decreased by about 5 mm and 3 g kg -1, respectively, in Bangkok from the late morning to the late afternoon on sunny days with weak low-level winds between March 1998 and December 1999. A daytime moisture decrease occurs in the lower atmosphere over coastal areas near the Gulf of Thailand, where the sea breeze often prevails. The sea breeze is relatively strong near the northern gulf. Wind profile observed over Bangkok revealed strong descending flow in the lower atmosphere during daytime, suggesting divergence of the sea-breeze circulation. This descending flow explains the decrease in daytime moisture over coastal areas. A numerical experiment assuming calm conditions successfully simulated the daytime decrease in water vapor around Bangkok. The most remarkable drying associated with descending flow was simulated around the northern edge of the Bight of Bangkok, where the coastline is sheltered by inland areas. The shape of the coastline and the topography enhanced the sea-breeze circulation around Bangkok. In another model run assuming wet soil conditions, the diurnal range of water vapor was smaller than that simulated by the control run. Clear diurnal variation was observed on calm days, especially under dry soil moisture conditions. A large diurnal range in PWV tended to appear before the monsoon onset when the low-level wind was weak and the soil moisture was dry.
From three-member ensemble projections under the climatic change scenario, the Intergovernmental Panel on Climate Change (IPCC) Special Report Emissions Scenarios (SRES) A1B, regional impacts of global warming on near-surface permafrost are investigated for six analysis regions in the circum-arctic and highlands: Alaska, Alaskan Arctic, Canadian Arctic, Eastern Siberia, Russian Arctic, and Tibetan Plateau, using the Community Climate System Model version 3 (CCSM3). The projected results for the 21st century under the A1B scenario indicated that the ice volume in the deepest model soil layer at about 3 m depth, which had been completely frozen during the 1870s in the historical simulation, begins to melt abruptly at around 2000 in each region. Particularly in Alaska and Eastern Siberia which are more advanced than the other regions in the thawing of permafrost, more than 50% of the ice volume disappears by 2030. From a viewpoint of regional vulnerability, the Alaskan Arctic at around 2020 may suffer the most severe damage as it has the highest thawing rate. Owing to thawing of the frozen soil, subsurface runoff increases by 215.4% and soil moisture decreases by -19.3% in Eastern Siberia for the 1990s to the 2090s.
As a continuation of the previous study (Yamasaki 2006), another example of tropical cyclone (TC) formation processes described by a 20-km grid model is presented. This paper describes a TC that forms from two band-shaped mesoscale convective systems (MCSs) embedded in a large-scale convective system, which is produced by horizontal convergence due to northerly flow associated with the previous TC and southwesterly flow of air that crosses the equator. Among many MCSs, two MCSs are selectively enhanced. These two MCSs rotate cyclonically. The western MCS approaches the eastern one, and these become a single system. When the orientation of the major rainband becomes west-east, the system attains TC intensity, as in the previous study.
Seasonal predictions of rain or snowfall are usually too uncertain at regional scales. We suggest utilizing subsurface ocean measurements to improve long-term weather forecasts. The example we give is that regional snowfall in Japan can be predicted by a simple regression from an acoustic Doppler current profiler attached to a regular ferryboat to observe the transport of the Tsushima Warm Current. The lag correlation is shown to exceed 0.75 attributed to the simple underling marine meteorology and regional oceanography. The relationship certainly improves seasonal precipitation estimates led by the winter monsoon absorbing the latent heat from the Japan Sea. We predict there will be less snowfall this winter of 2005/2006 than in 2004/2005 despite the heavy snowfall event in the last December.
Future changes in extremes indices on precipitation were projected with a 20-km horizontal grid atmospheric general circulation model. At the end of the 21st Century, heavy precipitation was shown to increase enormously in South Asia, the Amazon, and West Africa, while a dry spell was shown to increase in South Africa, south Australia, and the Amazon, suggesting that the risk of water-related disasters will be higher in these regions. In the Asian monsoon region, heavy precipitation increases notably in Bangladesh and in the Yangtze River basin due to the intensified convergence of water vapor flux in summer. In the Amazon, a dry spell greatly increases due to a reduction in the Walker circulation caused by an El Niño-like change in SST prescribed as boundary condition.
Changes in temperature-based extremes over land due to global warming estimated by a global 20-km-mesh atmospheric model are analyzed using Frich’s five extremes indices. At the end of the 21st Century, under the Intergovernmental Panel on Climate Change (IPCC) SRES A1B scenario, the model projects that the total number of frost days (Fd) decreases by more than 20 days per year and the length of the growing season (GSL) increases by about 14-34 days in northern mid- and high latitudes. The heat wave duration index (HWDI) and the percentage of time with a daily minimum temperature above the present-day 90th percentile (Tn90) increase worldwide. The intra-annual extreme temperature range (ETR) decreases in northern high latitudes, east Asia, and eastern North America by 1.3-3.9°C, but it increases by 1.0°C in the Amazon. The high-resolution simulation reveals that changes in these indices are influenced by regional properties, such as the altitude and distance from the coast.
The Improved Limb Atmospheric Spectrometer (ILAS)-II frequently observed polar stratospheric clouds (PSCs) in the Southern Hemisphere (SH) throughout the winter of 2003. Simultaneous observations of the aerosol extinction coefficient (AEC) at 780 nm, nitric acid, and water vapor data were analyzed to investigate the ambient thermodynamic conditions associated with observed PSCs. PSCs were first observed with ILAS-II at the end of May, and observed most frequently in August/September as temperatures cooled. At approximately 20 km late in the PSC season, however, PSCs were less likely to occur, despite cold temperatures, because of the lower concentration of nitric acid due to denitrification caused by sedimentation of previously occurring PSCs. The probability of PSC occurrence and the probability of ambient temperatures colder than nitric acid trihydrate (NAT) saturation temperature (TNAT) were well correlated below -20 km throughout the winter. In contrast, PSC frequency at -22 km from late August to early September was low even when temperatures were sufficiently colder than TNAT; this is, at least, partly because of the decrease in background aerosol particles in the atmosphere.
The influence of sea ice thickness on the winter Arctic atmosphere has been investigated using an atmospheric general circulation model, focusing on heat fluxes at the sea ice surface. Due to heat conduction from the sea ice bottom, the sea ice surface temperature increases when the sea ice becomes thin. However, the heat balance among net longwave cooling, sensible and latent heat fluxes, and heat conduction changes with sea ice thickness. When the sea ice is thick, longwave cooling is balanced with the heating of heat conduction and downward sensible heat flux. When the sea ice is sufficiently thin, cooling by sensible and latent heat fluxes plays a large role in heat balance at the sea ice surface, canceling the increase of downward longwave heating associated with cloud change. It is suggested that thinner sea ice leads to warming of a large part of the troposphere in the Arctic region, causing a weakening of upper westerly wind in the sub-arctic region. The magnitude of such a wind response to possible sea ice thickness variability can be 10-20% of interannual variability.
Upper-tropospheric vortices having a horizontal wavelength of 300-400 km were observed on water vapor images of the Japanese geostationary satellite (MTSAT-1). Grid point values predicted by the Regional Spectral Model of the Japan Meteorological Agency show that the vortices were located along a zonal belt with strong cyclonic shear and horizontal convergence. A quasi-geostrophic linear stability analysis of the basic flow having horizontal and vertical shear shows that the fastest growing mode has a horizontal wavelength, a phase speed and a growth rate that reasonably agree with those of the satellite observation. The amplitude of the fastest growing mode is confined to a region having a meridional width of 2 degrees and a vertical depth of 2 km. An energy budget analysis shows that barotropic instability is the dominant generation mechanism for the growing mode.
The meridional cross section of the long-term ozone losses between the two periods 1968-1975 (pre-ozone hole era) and 1990-2002 (mature ozone hole era) are estimated by using operational ozonesonde observations over Japan. The results are projected on the bins defined by potential temperature (θ) and equivalent latitude (φ eq) coordinate system to estimate the midlatitude and polar ozone losses separately. The ozone decrease between the two periods are decomposed of 0.3 ppmv (∼5-20%) decline widely distributed in midlatitude stratosphere and 1.0 ppmv (∼30-50%) depletion in the focused region defined by 400 ≤ θ ≤ 500 K and φ eq > 70°N during February-April season. The latter gives an estimate of the enhanced ozone loss in the Northern polar vortex, while the former represents general ozone loss in midlatitude lower stratosphere. Atmospheric circulation changes possibly induced by perturbed ozone are discussed in terms of mean φ eq.
We tracked fast moving mesoscale convective systems (MCSs) over the Bay of Bengal, counted their numbers, and analyzed the characteristics of wind and geopotential fields of when such MCSs propagate southward over the Bay. We used hourly TBB data of GMS5 IR1 (1996-2001, April-October) for the tracking, and used 6-hourly ECMWF 40 year re-analysis data to analyze the characteristics of wind and geopotential fields. The numbers of fast moving MCSs were as follows: April 11/56 (20%); May 46/103 (45%); June 50/173 (29%); July 33/160 (21%); August 27/125 (22%); September 8/61 (13%); October 8/67 (12%); total 183/745 (25%). The fractions represent (number of southward propagating MCSs)/(total number of MCSs), and the numbers inside the parentheses are the percentages of the southward propagating MCSs. We concluded that fast southward propagation of MCSs tends to occur when a trough exists over the Bay at the height of 600 hPa, and the wind shear between the surface and the 600 hPa height has a southward component. We also found that propagation towards other directions is related with eddy disturbances over the Bay.
A robust dealiasing method was developed and evaluated that can extend unambiguous Doppler velocity limits for a pulsed Doppler weather radar with multiple pulse repetition interval (PRI). The dealiasing method can correct a sparsely distributed velocity field with large random noise and/or large velocity gradients. The method includes areal multiple PRI dealiasing and dealiasing with spatial continuity. This method makes the simple assumption that Doppler velocities can be spatially approximated by linear polynomials of the coordinates in each small area. The dealiasing method was evaluated using simulated data that included uniform wind, Rankine vortices, axisymmetric divergence, and wind shifts. The simulated fields also included missing data and random noise. A test with real observations was also conducted. The results show that the method can successfully dealias most folded velocities. The processing time to dealias one PPI data in the evaluation is at most a few seconds using a 2.4-GHz CPU workstation, short enough for real-time radar observations.
Trends of heavy-precipitation events represented by available global observations such as the Global Precipitation Climatology Project (GPCP) and four global reanalysis datasets are examined using two indices, OPC (occurrence of rainy days within precipitation class) and NPC (normalized precipitation within precipitation class), mainly over the tropics. These indices are defined by normalization with the total integrated precipitation amount for the analysis period, 1979-2001. Over land, the observational datasets exhibit larger decreasing trends in the heavier classes. All but one of the reanalysis datasets reproduce these tendencies. Significant correlations for the heaviest class between the GPCP and the other datasets can be found. Over sea, there was a large discontinuity around 1987 in the two indices for the GPCP pentad data, which was probably caused by the introduction of Special Sensor Microwave Imager (SSM/I) data. Restricted to data after 1988, all of the reanalysis datasets are seen to have positive and more increasing tendencies in the heavier classes, while the observational results show the opposite tendencies, although they are statistically insignificant. To resolve this discrepancy in order to enable the use of reanalysis data in extreme-event studies, additional improvements will be needed, not only in reanalysis but also in observational datasets.
A yellow sand dust (Asian dust; Kosa) event, which took place over Western Kyushu, Japan on 13 April 2003, was investigated using multiple observational data sets such as optical particle counters, sky radiometer, Mie-scattering lidar and intensive airplane observational equipments. The results of the present study show that the three-dimensional structure and the manner of transportation of dust particles on this day was very complicated especially in the vertical direction, and suggest that the spatial distribution of atmospheric aerosols over East Asia is occasionally very heterogeneous both in the horizontal and vertical directions.
Variability in tropical cyclone (TC) days in the western North Pacific (WNP) since the late 1970s is investigated based on two datasets. As an overall behavior, the intense TC days have increased for the last 30 years from both the Japan Meteorological Agency (JMA) dataset and the Joint Typhoon Warning Center (JTWC) dataset. Both datasets show that TC days with an intensity of Saffir-Simpson category 2 or higher have increased by 15-30% over the past 30 years. In terms of the detailed behavior of this increase in intense TC days, the contribution obtained from the JMA dataset is different from that of the JTWC. The JMA dataset reveals that the increase in moderately intense TC days contributes to the overall increase in intense TC days, while the JTWC dataset shows that the increase in extremely intense TC days has the dominant contribution. The difference between the two datasets becomes significant after 1987 when aircraft reconnaissance by the US Air Force was deactivated. The difference between the assessed contributions is due to different implementations of the Dvorak technique, the basis for TC intensity estimation at the JMA and the JTWC after the deactivation of aircraft reconnaissance.
A three dimensional wave activity flux applicable to non-hydrostatic inertio-gravity waves in a time mean flow in a Boussinesq fluid is derived. It is shown that the flux gives the wave-action density flux relative to the local time mean flow, and additive terms in residual circulation are equal to Stokes drift under the WKB limit. The flux is a generalization of three dimensional wave activity fluxes that have been used to analyze quasi-geostrophic wave disturbances. A flux under the log-pressure coordinate system on spherical geometry applicable to global hydrostatic inertio-gravity waves is also derived.
The potential predictability of seasonal mean river discharge at a river mouth is investigated based on a dynamical ensemble approach with an atmospheric general circulation model. The difference in predictability between river discharge and net water flux into the ground (P−E) tends to increase with an increase in basin area because the collection of total runoff through the river routing network reduces unpredictable variability. The travel time through the river channel network shifts the phase of the predictability of river discharge and inverts the magnitude relation of the concurrent predictability between river discharge and P−E. Snow accumulation and the melting process also delay the phase in mid- and high latitude river basins. The effects of land surface hydrological processes on river discharge include improvement, phase shift, and smoothing of the predictability, leading to unique features in the potential predictability of seasonal mean river discharge, different from those of P−E.
The cloud particle size distribution of water clouds was simulated by a non-hydrostatic spectral bin microphysics cloud model. The result shows two different correlation patterns in the simulated optical thickness and effective radius; positive and negative correlations for non-drizzling and drizzling clouds, respectively, similar to the observed features reported by past remote sensing works. The correlation pattern in a pristine condition is mainly composed by negative correlation with small fragment of positive correlation, similar to the satellite-observed characteristics over the FIRE region off California. In the case of polluted condition, on the contrary, only a positive correlation is simulated, similar to the satellite observation over the ASTEX region of North Atlantic Ocean. Satellite-observed contrast of correlation patterns between FIRE and ASTEX regions can be explained by the difference in the aerosol burden of airmasses over the two regions.
We assessed the spectral imaginary part of refractive index (ni) of Asian dust aerosol in the visible wavelengths (0.412-0.865 µm) by comparing simulated and satellite-observed top of the atmosphere (TOA) reflectances. The approach employed in this study combines the results obtained from SeaWiFS ocean color sensor, the vertical profiles provided by CFORS mesoscale transport model, and the size distribution observed concurrently by Aerosol Robotic Network (AERONET) sites. Analyses for four dust instances over the Sea of Japan during 2001 and 2002 showed that the ni values of Asian dust are significantly lower than previously published, as low as about 0.001 at 0.5 µm. Although the backward trajectories suggest that the four events have different sources, all events show very similar ni values. The corresponding averaged coarse-mode dust single-scattering albedo (SSA) is found to be 0.98 ± 0.002 at 0.555 µm, which is consistent with the results of several contemporaneous observations conducted during Asian Aerosol Characterization Experiment (ACE-Asia). These results indicate that Asian dust particles itself may cool the climate system more strongly than is generally considered and a weakly absorbing model of Asian dust is more appropriate for use in climate modeling than the already used dust model.
A new lightning formation scheme to simulate electrical space charge density and lightning is introduced into a 3-dimensional cloud-resolving model (JMANHM: Japan Meteorological Agency NonHydrostatic Model, Saito et al. 2006). A tripole structure in the electrical space charge distribution in cumulonimbi, clarified previous observational and numerical idealized studies (e.g., Williams 1989; Takahashi 1984), is well reproduced in numerical simulations with this new scheme. The numerical simulations with the horizontal resolution of 1.5 km are performed to reproduce the thunderstorm observed over the Kanto area, the central part of the Japan Islands, on 08 July 2004. In these simulations, four electrical charge transfer methods are applied to this event. Among them, the modified method from Helsdon and Farley (1987) (mHF-method) predicts cloud-ground lightning polarity most successfully in comparison with the observations. The electrical space charge structures simulated with the mHF-method agree with the previous observational and idealized numerical studies.
A four-dimensional local ensemble transform Kalman filter (4D-LETKF) is applied to the Japan Meteorological Agency (JMA)’s nonhydrostatic model (NHM) with explicit cloud microphysics to enable mesoscale ensemble prediction and data assimilation. Convective-scale data assimilation experiments in a perfect model scenario with 5-km grid spacing are performed, which indicates that the 4D-LETKF system works appropriately. Observations are taken every 10 minutes and every 2 × 2 × 2 grid points for horizontal winds, temperature, relative humidity, surface pressure, and precipitation rate. Although fixed lateral boundary conditions cause error reduction even without data assimilation, the advantages of 4D-LETKF are clear. When precipitation-rate observations are assimilated, some convective systems are better captured, although the impact is not always positive. Overall, 4D-LETKF shows encouraging results; it would be a tool adopted in future researches in convective-scale data assimilation and ensemble prediction.
The structure of the Baiu frontal zone over the East China Sea on 27 June 2004 was investigated using data from aircraft observations. During the period of 09-12 JST (Japanese Standard Time: UTC + 9 hours), three cloud zones appeared over the East China Sea. The Baiu frontal zone is conveniently divided into three zones on the basis of the three cloud zones. The zones on the southern side, in the middle, and on the northern side are called FZ1, FZ2, and FZ3, respectively. The position of FZ2 corresponded with that of a stationary front on the surface weather map. Remarkable variations of winds, potential temperature, and moisture corresponding to the three zones were detected using in situ measurements at 500 m MSL (Mean Sea Level). In FZ1, a decreased southerly wind area associated with a remarkable moisture gradient was seen, although the potential temperature gradient was unclear. FZ2 had remarkable gradients of potential temperature and moisture associated with a drastic change from southwesterly to northwesterly winds. FZ3 also had remarkable gradients of potential temperature and moisture. This study shows observational evidence of the fact that multiple frontal structures exist inside the Baiu frontal zone.
The reason for the record-breaking typhoon landfall on Japan in 2004 was examined by analyzing the behavior of tropical intraseasonal variation associated with the Madden-Julian Oscillation (MJO, Madden and Julian 1972). There were three active phases of MJO in June, August and early October over the western North Pacific during the typhoon season in 2004. The monsoon trough was enhanced during the active phase of the MJO over the western North Pacific and generated most of the typhoons over the region. The steering flow anomaly persisted, due to the long duration of the active phase of the MJO over the western Pacific, and directed westward-northwestward from the TC genesis region with both anti-cyclonic (cyclonic) centers over east of Japan (east of the Philippines), and we suggest that the persisted steering flow anomaly provided a favorable condition for typhoon landfall on Japan in 2004.
The analysis of the brightness temperature differences (BTD) between 11 µm and 12 µm channels of Moderate Resolution Imaging Spectroradiometer (MODIS) aboard Aqua/NASA satellite for a Yellow Sand event on 8 April 2006 reveals that the BTD on this event cannot be explained by the theoretical simulation using the complex refractive index of ‘Sahara’ or ‘Afghan’ dust. The refractive indexes at 11 µm and 12 µm so determined as to fit the observed BTD are similar to those of quartz. In a visible wavelength, the observed reflectance of 0.65 µm channel of MODIS over the most dense portion of Yellow Sand on this day is much higher than that expected by the conventional complex refractive index of mineral dust. The weaker absorption, like the quartz refractive index, is required to explain the observed reflectance. The optical thickness at infrared and visible wavelengths using a small value of imaginary part of refractive index in visible wavelength show a smooth and monotonic relationship between them.
We developed a method to retrieve the optical thickness (τ) and effective particle radius (re) of water clouds using the split-window channels and the 8.7-µm channel of Meteosat-8. Valid ranges are approximately from 1 to 9 for τ and smaller than 18 µm for re. Water clouds were first identified using 8.7-µm and 11-µm data. The retrieval used the brightness temperature (TBB) and brightness temperature difference (BTD) between the split windows, as computed with the radiation code RSTAR5b for various properties of water clouds and vertical profiles of temperature and water vapor. Arch-shaped curves in the TBB and BTD domains resembled those for ice clouds, depending on τ and re. The retrieved cloud parameters were then compared to those retrieved by the solar reflection method, which uses the 0.6-, 3.9-, and 11-µm channels of Meteosat-8. Comparison of the two methods revealed that the split-window technique could capture spatial features for both τ and re, showing some agreement with re for the solar reflection but needing more improvement for τ in the current case study.
The quality of climate data is critical for reliable simulations of spatial and temporal variations in the carbon budget. To examine the quality of the new dataset produced by the JMA reanalysis project (JRA-25), we applied JRA-25 data as well as data from the established climate datasets NCEP-R1, NCEP-R2, and ERA40 to the terrestrial ecosystem model Sim-CYCLE and compared the result. Using each of the datasets, we conducted global simulations for the period 1979-2001 and investigated the spatial and temporal patterns of plant uptake and the net ecosystem budget of carbon. The JRA-25 dataset provided pertinent land surface conditions with respect to solar radiation, temperature, and precipitation, resulting in moderate simulation results of terrestrial productivity and the carbon budget. Unusual conditions of the JRA-25 resulted in different regional carbon budgets such as the Amazon basin and global budgets in a few anomalous years.
Detection/attribution analyses of temperature extremes were carried out by comparing a new gridded observational dataset of daily maximum/minimum temperatures (HadGHCND) and the simulation of MIROC3.2. It was shown that significant anthropogenic warming is detectable in the annual warmest night, and the coldest day and night from 1950 to 1999, while human influence was not detected in the warmest day. These findings are in agreement with a previous study that examined the simulation of HadCM3. Human influence is also identified in the decrease in the number of frost days, but not with the increase in the number of summer days. Furthermore, it was suggested that half of the warming trend due to rising greenhouse gas concentrations is canceled out by other factors, predominantly aerosol cooling. It is expected that a rapid decline of aerosol emissions coupled with rising greenhouse gas concentrations would induce larger changes in temperature extremes in the future.
This study updates the projection of global warming impact on rice production in Japan using five coupled Atmosphere-Ocean General Circulation Model (AOGCM) products under the SRES A1B scenario. Projection of daily maximum and minimum temperatures and daily total solar radiation for each AOGCM product are fed to a regional-scale rice model by which the rice heading day and yield are simulated. Most climate model results show that while the heading day becomes significantly earlier, the yield mostly remains in the range of inter-annual variability of the present climate. Projected future yield shows a distinct pattern of increase in the northern Japan and decrease in the southwestern Japan, with exception in parts of Kyushu. Projected yield change in the central Japan differs depending on the location of the prefecture. The trend of change in yield is consistent among simulation years in the northern and central Japan whereas it varies year-to-year in the southwestern Japan. The variance in yield over southwestern Japan possibly becomes larger due to heat stress as a result of global warming.
To further understand the characteristics of the convection of medium depth (CMD) that was introduced by Zhang et al. (2006), the dynamic and thermodynamic aspects of the CMD were examined with the use of a cloud-resolving numerical model. A typical case of the CMD south of the Meiyu front over the east part of continental China, which occurred between 00 UTC and 12 UTC 22 July 2002, was simulated and analyzed. Based on realistic simulation, results of a case study indicated that the CMD possessed the following features: 1) dominated by warm rain process, 2) rapid formation of raindrops and their rapid falling down to the surface, by comparison with those of deep convection. It is expected that the CMD south of the Meiyu front over the east part of continental China shares such characteristics.
Many coupled atmosphere-ocean general circulation models (AOGCMs) have difficulty reproducing the El Niño/Southern Oscillation (ENSO) phenomenon. The AOGCM MIROC underestimates ENSO variability. This paper investigates the dependence of the simulated ENSO amplitude on the treatment of ocean subsurface processes. Excessive vertical diffusivity in the subsurface ocean interferes the reproduction of a tightly stratified thermocline because the MIROC model includes numerical instability in the vertical diffusion computation and uses a large background diffusivity. The introduction of a stable numerical scheme and small background diffusivity helped improve the stratification. The improved thermocline enhanced the ENSO amplitude because the vertical heat exchange between above and below the thermocline was enhanced during the El Niño phase.
The influence of rainfall distribution on NDVI (Normalized Difference Vegetation Index) anomaly was qualitatively investigated over the arid region in Mongolia using Ulaanbaator Airport radar data from 2003 to 2005. As for the steppe vegetation zone where annual rainfall amount was 100-200 mm, when considerable (little) rainfall was observed in June to July, a positive (negative) NDVI anomaly occurred in July to August and the timing that NDVI reached the maximum was 20-40 days earlier (later) than the 8-year mean. Furthermore, the southern limit of the large rainfall area observed by weather radar corresponded well to the southernmost limit of the positive NDVI anomaly. However, these features were not apparent over the forest steppe vegetation zone where annual rainfall amount was more than 200 mm. It was found that the rainfall distribution obtained by weather radar had a potential for the prediction of the NDVI anomaly and the timing that NDVI reached the maximum over the arid region of Mongolia.
In ten years of 1994 to 2003, January air transports in the Hokkaido, Kanto, and Kyushu regions were analyzed calculating 7day backward air parcel trajectories with the meteorological wind analysis data set (NCEP-DOE AMIP-II Reanalysis). As a new result, an anomalous air parcel route was found out over the Hokkaido region: In January climatology as has been well-known so far, air parcels transported from the Eurasian continent sweep out the entire Japan Islands. In several years interval, however, only the Hokkaido region happens to be invaded by air parcels moving westwards from the Kamchatka Peninsula, although this route is not seen in the other Januarys.
Interannual fluctuations of the convective activity around the Philippines are highly correlated with the east-west gradient of SST between the North Indian Ocean (NIO) and the western North Pacific (WNP). We conducted a set of experiments by use of an atmospheric general circulation model (AGCM) to assess the relative importance of the remote (NIO) versus in situ (WNP) SST anomalies in determining the WNP monsoon rainfall as ocean-to-atmosphere feedback. The solutions indicate that both in situ and remote SST anomalies regulate precipitation around the Philippines in the early summer. This result implies that the WNP monsoon rainfall is sensitive to the spatial distribution of the NIO SST anomalies as well as the in situ anomalies. These physical interpretations suggest further increase of the predictability in the WNP monsoon.
There was a request of corrections in the published paper
by the author S. Miyahara. The request was approved by the Editorial Committee to publish in Errata.
Wrong:The wrong equations (19-3) and (22), p110
Right:The correct equations (19-3) and (22), p110
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