The front of a sea breeze originating from Sagami Bay passed over Tokyo between 1300 LST and 1400 LST on August 10, 2006. Field observations of wind distributions over Tokyo by the use of a ground-based coherent Doppler lidar developed at the National Institute of Information and Communications Technology showed that a sharp multi-layered structure of vertical wind fields was gradually formed after the sea-breeze front passed the observation site. Numerical simulations using the Weather Research and Forecasting (WRF) model demonstrated that the multi-layered wind structure consisted of 1) the sea breeze at an altitude below 0.8 km above mean sea level (AMSL), 2) a layer of weak winds at an altitude of 0.8-1 km AMSL, 3) a return flow of the sea breeze, and 4) a northerly synoptic wind at an altitude above 3 km AMSL. This is the first direct observation of the formation of sharp multi-layered wind structure over Tokyo associated with sea-breeze circulation.
In order to elucidate meso-scale environmental characteristics contributing the generation of a typhoon-spawned significant tornado (F2) occurred in Nobeoka of Miyazaki prefecture, Japan in 2006, environmental and composite parameters in Miyazaki are examined in comparison with typhoon-spawned 3 tornadic events in Miyazaki and 16 nontornadic events using the forecast dataset of the operational nonhydrostatic mesoscale model of the Japan Meteorological Agency (JMA). It is found that the significant tornado in Nobeoka is accompanied by enhanced convectively unstable environment with large vertical wind shear, which is represented by a relatively large convective available potential energy based on mean parcel in the lowest 500 m layer above ground level (AGL) (mlCAPE) and significantly large shear parameters (e.g., storm relative helicity; SRH) compared with other tornadic and nontornadic events. We also discuss synoptic condition producing these characteristic environments along with the comparison of the environment for the tornadic supercells and hurricane-spawned tornadoes in the U.S.
The isentropic diffusion coefficient (Kyy) characterizes large-scale atmospheric mixing processes. Chemical constituents allow us to directly estimate Kyy from observational data sets. This study investigates general aspects of the mixing processes from a systematic survey of the common features and constituent dependency of Kyy. Analysis with chemical constituent data obtained from a global chemical transport model demonstrates that long-lived species whose chemical lifetimes (τ) are several years long have very common seasonal and latitudinal variations of Kyy, which is considered to represent actual atmospheric mixing. Kyy, estimated from chemical species with τ shorter than several weeks, becomes considerably greater than that from long-lived species and indicates the significance of nonlinear effects. Meridional transport analysis also investigates that shorter τ results in greater spatial constituent variations and larger contributions of the eddy compared to the mean motions in the constituent transport flux.
We evaluate a direct photographic method to measure droplet size distribution (DSD) in fog using an instrument, consisting of a digital camera and a stroboscope, designed for onboard use in a hot air balloon, which is safer than other aircrafts in fog. This method allows observation of fog droplets without mechanical suction or impaction. Drops of diameter 5-100 µ m could be imaged with a relative standard deviation of 20-21%. Extinction coefficients derived from the DSD show good agreement with those derived from the visibility. The feasibility of the instrument attached to a hot air balloon was validated in a field observation, demonstrating that the direct photographic method is feasible for determining fog DSD.
The long-term trend of precipitation at the central part of Tokyo was analyzed using digitized hourly data for 118 years (1890-2007). It was found that “no preceding precipitation” (NPP) cases, defined as not preceded by ≥ 1 mm precipitation for the last six hours, showed an increasing trend of precipitation as a rate of 30%/century or more from afternoon to early evening of the warm season. Analysis for spatial precipitation patterns for NPP cases, using hourly data on the AMeDAS network for the recent 30 years (1978-2007), also showed a positive anomaly in Tokyo in the afternoon of the warm season. These facts suggest the reality of the urban heat-island effect on the increase of warm season short-term precipitation in Tokyo.
Horizontal wind at 700 hPa over Sumatra (0°N, 100°E) analyzed by National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis was compared with wind observed by a VHF wind-profiling Doppler radar at Kototabang (0.20°S, 100.32°E) during 2001-2007. Radiosonde wind data at Padang (0.88°S, 100.35°E) were reported through Global Telecommunication System (GTS) for assimilation into NCEP/NCAR reanalysis. To examine if radiosonde observations at Padang improve wind quality of NCEP/NCAR reanalysis over Sumatra, dataset of radar wind, which was compared with NCEP/NCAR reanalysis, was divided into the group when radiosonde observations were reported though GTS (Group A) and when they were not reported (Group B). For each group, correlation coefficient and regression line were computed. In Group A, both zonal and meridional winds showed better correlation coefficients (0.90 and 0.77) than in Group B (0.79 and 0.63). Amplitude of NCEP/NCAR-reanalysis zonal and meridional winds showed a better agreement in Group A (82 and 96% of radar wind) than in Group B (66 and 80% of radar wind). These results suggest that wind observations in the Indonesian Maritime Continent contribute to improve wind quality of NCEP/NCAR reanalysis there.
Power transmission networks are exposed to meteorological conditions that can often cause damage resulting in temporary loss of service. Because their reliability is of crucial importance to society, the ability to predict potential hazards in advance is of great interest. Studies conducted by the power transmission industry indicate that the majority of failures is caused by icing on overhead conductors and other components of power networks. This paper outlines an ice accretion forecasting system (IAFS) that uses a state-of-the-art, mesoscale, numerical weather prediction model, augmented by a precipitation type classifier, and an ice accretion model. The system is calibrated by hindcasting a well-documented freezing rain event in the United States, in December, 2002. Results indicate that IAFS can hindcast overall ice thicknesses as well as icing models can diagnose them using observed surface meteorological conditions.
Sounding data from the South China Sea Monsoon Experiment (SCSMEX) have provided a unique opportunity to document the variability of atmospheric mixed layers over the South China Sea (SCS) during the onset of the monsoon in this region. Six-hourly sounding data from two research vessels, deployed over the northern and southern SCS, are used to determine the mixed-layer depth and its thermodynamic properties. Results from the southern ship show the presence of mixed layers 83% of the time with a mean depth of 459 m, similar to other tropical oceanic locations. On the other hand, the northern ship exhibited mixed layers 48% of the time with a mean depth of 342 m, considerably less frequent and shallower than other tropical regions. This anomalous behavior, which was particularly evident after the monsoon onset, is likely due to northward advection of low-level warm, moist air over cooler waters, which results in very small or negative buoyancy fluxes over the northern SCS, and thus weak upward mixing of heat and moisture from the surface.
A study to compare Dvorak parameters (T-number and CI-number) for tropical cyclones over the western North Pacific in both JMA and JTWC datasets from 1987 to 2006 is presented to show if there is a difference between two datasets. The study shows that the Dvorak parameters by JTWC are generally higher than these by JMA during the period of 1992-1997 and 2002-2005. The major reasons for stronger cases in JTWC are “faster intensification before the mature stage and slow/delayed start of weakening after the mature stage”.
A simple algorithm for estimation of Photosynthetically Active Radiation (PAR) with satellite data is developed. The algorithm, which is based on a simple radiative transfer scheme, requires only one spectral channel (red) of a satellite. The algorithm was tested with satellite data taken by Terra/MODIS and Aqua/MODIS. The relative error of daily total PAR estimated by this algorithm was 27%. It can provide PAR maps with a resolution as fine as 250 m, with which the topographic influence on the PAR distribution (due to local clouds) is clearly seen.
To determine the spatial and temporal variations in the El Niño Southern Oscillation (ENSO)-rainfall relationship and seasonal variation in rainfall over northern Borneo, simultaneous and lag correlations between the December-February (DJF) average Niño-3.4 (5°S-5°N, 170°-120°W) sea surface temperature (SST) and rainfall were examined using rainfall data from 18 stations throughout Sarawak during 1963-2003. The lag correlations for June-August (JJA) rainfall and SST were significantly negative over southwestern Sarawak (SWS), but the simultaneous correlations for DJF rainfall and SST were significantly negative over northeastern Sarawak (NES). We categorized the 18 stations into four clusters (C1-C4) according to the variation of the seasonal fluctuation in rainfall. C1 and C2 are located in SWS along the convex and concave coastline, respectively, whereas C3 and C4 are located in NES along the convex and straight coastline, respectively. C2 and C4 are also located in the interior in SWS and NES respectively. From the composite of monthly rainfall for all clusters, a decrease in rainfall in JJA was found for all regions in El Niño years but only for C2, C3 and C4 in DJF during the warm phase of ENSO. The C3 region may be the most vulnerable to El Niño-induced droughts in Sarawak.
On 28 July, 2008, a local heavy rainfall occurred over the Hokuriku and Kinki districts, in central Japan. A stationary front existed near the heavy rainfall areas, and the atmospheric instability was increased by the inflow of the upper-level cold air. The operational mesoscale model (MSM) of the Japan Meteorological Agency (JMA) could not predict the heavy rainfall. The comparison of distributions of precipitable water vapor (PWV) between the initial condition of the MSM and the Global Positioning System (GPS) revealed that the MSM initial condition underestimated water vapor in the heavy rainfall areas because of a positional lag of a low-level convergence zone. Data assimilation experiments of GPS-derived PWV were conducted with JMA’s mesoscale 4 dimensional variational assimilation system (Meso 4D-Var). The PWV derived from the nationwide ground GPS network (GPS Earth Observation NETwork: GEONET) improved both the northward position error of the low-level convergence zone and the forecast of the observed rainfall. Moreover, further improvements were obtained when the PWV derived from the GPS stations of the International GNSS Service in East Asia were added.
The impact of a foehn phenomenon on the generation of abnormally high surface air temperatures in the Tokyo metropolitan area was investigated through Observing Systems Simulation Experiments (OSSEs) by the use of the variational data assimilation system equipped on Weather Research and Forecasting model (WRF-VAR). First, an abnormally hot weather event on August 16, 2007 was simulated using the JMA’s operational meso-scale analysis data to produce “simulated observations”. Then, the vertical profiles of the simulated observations over several sites were assimilated into an initial field. Results of OSSEs demonstrated that northwesterly winds and high-temperature air associated with the foehn strongly affect the generation of the abnormally high surface air temperatures. Also, the results indicated that southerly sea breezes prevent the impact of the foehn from spreading to the southern part of the metropolitan area.
Some important recent findings in climate studies are the warming trend in the troposphere and the cooling trend in the stratosphere. However, the evidence for the cooling trend in the stratosphere may need to be revisited. This study presents evidence that the stratosphere is slightly warming since 1996. Using long-term Stratosphere Sounding Unit (SSU) measurements at channels 1 and 2 before 1996, we did obtain a cooling trend in the middle and upper parts of stratosphere similar to the findings for the lower stratosphere in literature (Ramaswamy et al. 2001; Fu et al. 2004). But, we also found that the temperature trend in the middle and upper stratosphere has alternated to warming since 1996. We also analyzed a time series of the Microwave Sounding Unit (MSU) channel 4 brightness temperatures between 1978 and 2006 and found the same reversing trend in the lower stratosphere. The reversing trend may relate to a possible recovery of stratospheric ozone concentration.
Energy Helicity Index (EHI), defined by the product of Convective Available Potential Energy (CAPE) and Storm Relative Environmental Helicity (SREH), is one of potential parameters to diagnose the possibility of tornado outbreak. In this study, probabilities that EHI exceed some criteria were examined with a mesoscale ensemble prediction system, whose grid interval was 15 km, in two tornado events in Japan (Nobeoka and Saroma tornado events). High probability regions (HPR, hereafter) of large SREH existed in the northeastern quadrants of a typhoon or a low-pressure system, while HPRs of large CAPE extended along the warm humid airflow from the Pacific Ocean. In the two events, the tornados were formed near HPRs of large EHI, where HPRs of large SREH and CAPE were overlapped. This result indicates the possibility of the probability forecast of the potential parameter for tornado outbreak.
The synoptic-scale climatology of cold frontal precipitation systems during the passage over central Japan was investigated for 19 years (1988-2006). Cold frontal precipitation events are classified into the following three types: Widespread, Hokuriku, and Jump type. Widespread-type events, which bring precipitation throughout Japan, mainly occur in spring and autumn, and the cyclones tend to move northeastward from the central part of the Sea of Japan. The central pressure of the Widespread-type cyclones is the deepest and this type has the most moisture out of the three types. Hokuriku-type events, which bring precipitation exclusively over the Hokuriku area often appear in winter, and the cyclones move eastward from the northern part of the Sea of Japan. As a result, the isobars form in an east-west orientation over mainland Japan as the cold front arrives in the Hokuriku area. The Hokuriku-type cyclones tend to be relatively weak and there is less moisture during the events. For the Jump-type events, in which the precipitation distribution appears as precipitation bands jumping over the Kanto area, cyclones develop rapidly due to the deep trough at the 500 hPa level, changing the isobars from east-to-west to northeast-to-southwest during the events.
A preconditioning of a typhoon genesis over the North Western Pacific by an Madden-Julian Oscillation (MJO) event is simulated in a boreal summer numerical experiment conducted using a global cloud-system-resolving model. The simulation highlights the importance of the following processes in the typhoon genesis that were not clearly captured in the conventional general circulation models: as the MJO propagates over that region, the easterly shear (with an enhanced low-level westerly) of the western branch of the MJO’s circulation moderates the climatological westerly shear that is otherwise persistent and inhibits cyclogenesis there. Meanwhile, its attendant cyclonic vorticity from the westerly region migrates northward, causing the genesis to occur. The trough in the upper troposphere at 15°-20°N is found to contribute to the genesis. The result demonstrates the performance of a global cloud-system-resolving model at predicting tropical cyclogenesis associated with the MJO in the North Western Pacific during the active typhoon season in boreal summer.
Influence of the Indian Ocean Dipole Mode (IOD) on the interannual variation of the activity of intraseasonal oscillation (ISO) on submonthly (6-30-day) time scales during boreal fall is studied using ECMWF reanalysis data from 1958 to 2001. There is high negative correlation between the IOD and ISO activity over the southeastern Indian Ocean. The disturbances that cause this high negative correlation propagate westward slowly while maintaining a symmetric structure with respect to the equator and have the first baroclinic vertical structure. These disturbances are identified as convectively coupled equatorial Rossby waves. It is also shown that the interannual variation of ISO activity has a high correlation coefficient (0.850) with maximum negative relative vorticity anomalies at 850 hPa produced by cyclonic disturbances over the southeastern Indian Ocean, while it has a low coefficient (0.119) with the number of cyclonic disturbances. It is concluded that whether the equatorial Rossby waves produce strong cyclonic disturbances is a key factor determining the interannual variation of ISO activity over the southeastern Indian Ocean. The reason strong cyclonic disturbances are produced and their relation with the IOD are discussed in this paper.
The long-term variation of precipitation around Lake Balkhash since 1872 was studied. CDIAC TR051 (1872-1989 in the study area) was compared with GHCN Gridded Precipitation Products (1900-2007, GHCN_GRD) and CRU TS2.1 (1901-2002), the latter two being used in IPCC AR4 for assessing the global annual land precipitation anomalies in the 20th century. From 1901 to 1989, these datasets had no statistically significant differences, and exhibited similar variations. CDIAC TR051 was, therefore, combined with GHCN_GRD for 1925 without any inhomogeneities found in the combined time series. Around Lake Balkhash, negative precipitation anomalies prevailed from 1872 to 1950 with a significant recovery found around 1900. IPCC AR4 indicated that precipitation increased in Central Asia throughout the 20th century, whereas this study clarified that the increasing trend has been presented since 1872 in the study area. Moreover, the integral curve of the precipitation anomalies corresponded well with variation of the water level of Lake Balkhash, i.e., negative precipitation anomalies from 1872 onward resulted in the continuous decrease of the water level until 1950. This coincides with other proxy data such as a sediment core of Lake Balkhash dug up in 2007.
Simulations of 10-day backward trajectories of air masses from Mount Wrangell, an Alaskan ice core site, were calculated for 11 years on a daily basis. Results were analyzed statistically in order to interpret monthly air mass contributions over the ice core site and to discuss implications for ice core studies in the North Pacific Region (NPR). Increases in tropospheric air mass transport from EA in spring suggest favorable transport conditions for Asian dust during this season. The stratospheric air mass (< 300 hPa) over the ice core site increases in winter and that from East Asia (EA) to the North Pacific Ocean in late spring. The tritium peaks observed in the ice core in late spring were discussed in the context of the present results with two possibilities on the time lag of tritium transportation in the stratosphere and the seasonal variations of water vapor amount in the troposphere. Increases in air masses originating from Siberia, Alaska and Canada in summer-fall favor the transport of black carbon due to forest fires over the ice core site. These results allow advanced interpretation of the origin and transport processes of materials in the ice core proxies in the NPR.
In order to detect microscale influences on observed surface air temperature changes, 29 years’ data on the AMeDAS network were used to identify the relation between temperature trends and wind speed trends, which were regarded as reflecting the changes of site exposure. The results were found to differ according to time of the day and the mean wind speed of the site. For stations with relatively high mean wind speed (≥ 1.5 m s-1), a significant negative correlation is found between daytime temperature trends and wind speed trends. For stations with low wind speed (< 1.0 m s-1), evening temperature trends are negatively correlated with wind speed trends, whereas daytime temperature trends tend to be positively correlated with them. These facts give statistical evidence of the dependence of temperature trends on microscale environmental changes, with complications according to the degree of site exposure.
Multiple equilibrium states in the Venusian atmosphere were investigated using a Venus-like atmospheric general circulation model (AGCM) with dayside/nightside heating contrast. Results showed that the super-rotation state and slow zonal wind state can exist for identical external parameters. Thermal tides appeared because of the heating contrast. They redistribute the zonal momentum and reduce the zonal jet thickness. Rossby-type waves are found in addition to thermal tides in the case of a super-rotation type state.
Localized heavy rainfall that occurred in the Toshima, Chiyoda, Shinjuku, and Bunkyo wards of Tokyo, Japan on 5 August 2008, causing 5 deaths and the flooding of 34 houses above floor level, was observed by the National Research Institute for Earth Science and Disaster Prevention (NIED) X-band polarimetric radar (MP-X). Preliminary analyses of the radar data reveal the following characteristics of the rainfall event. Heavy rainfall occurred in a small area over a short period. The extents of the areas over which 1-hour accumulated rainfall was greater than 50 and 80 mm were about 20 and 5 km2, respectively. The maximum total rainfall amount (123 mm) from 11:30 to 13:30 LST was observed by MP-X over Edogawa Elementary School. The thunderstorm initially developed rapidly. Only 15 minutes after the onset of rainfall, a precipitation rate in excess of 100 mm h-1 was observed, continuing for 35 minutes from 11:55 to 12:30 LST. The maximum rainfall rate (153 mm h-1) was observed by a rain-gauge over Edogawa Elementary School at around 12:10 LST. A comparison of MP-X data with surface rain-gauge data reveals that MP-X provides more accurate rainfall information than conventional radar, making it useful in monitoring localized heavy rainfall.
Oceanic responses to relatively strong Madden-Julian Oscillations (MJOs) and background winds controlled by El Niño-Southern Oscillation (ENSO) are examined. The MJO’s arrival excites dominant downwelling and upwelling Kelvin waves during El Niño developing (pre-El Niño: PEN) and other (non-PEN) phases, respectively. These opposite signals come from background wind directions under different ENSO phases and exert opposite impacts on SST. In addition, MJO convection itself develops accompanied by larger surface wind variations during PEN phases, which can be related to the interactive amplifications of synoptic- and planetary-scale disturbances when westerly wind bursts occur. Consequently, the strength of westerly forcing and its oceanic response during PEN phases are larger than that of the corresponding easterly forcing and its response during non-PEN phases. These results suggest that modulations of MJO amplitude and structure under the background westerly and easterly winds associated with ENSO phases exert opposite but asymmetric impacts on the ocean.
We performed an experiment under ideal conditions using a general circulation model and found that aerosols can play an important role in altering the precipitation in Asia. Emissions of anthropogenic aerosols are increasing in Asia, affecting the global and regional climates through their direct and indirect effects. However, the link between aerosol loading and climate change due to these effects is still difficult to understand. The present study was based on a three-dimensional aerosol transport model coupled with a mixed-layer ocean model; it focused on the effects of anthropogenic aerosols on boreal summer precipitation over Asia. We investigated these effects by taking differences between the results of equilibrium runs with pre-industrial and current fossil fuel burning aerosol (FFA) conditions. In this model simulation, aerosol loading changed the circulation field through changes in the radiation budget. The indirect effects of aerosols include not only the first and second indirect effects but also the effects of these changes. Our results show that an increase in anthropogenic aerosols reduces precipitation due to weaker convection with both surface cooling and sea surface temperature (SST) changes. This mechanism is the most effective way for aerosols to impact precipitation in this model simulation.
The permafrost distribution under Quaternary climate conditions is investigated by a frozen ground classification method based solely upon the near surface air thermal indices as a diagnosing analysis of the Paleoclimate Model Intercomparison Project 2 (PMIP2) output. The methodology is constructed using the present-day distribution of the frozen ground and the thermal (freeze and thaw) index, and showed reasonable capability for large-scale mapping. The frozen ground distribution was reconstructed for 0 ka (pre-industrial), 6 ka (mid-Holocene), and 21 ka (the last glacial maximum; LGM) conditions. The Holocene simulations (0 ka and 6 ka) produced largely similar results. The LGM outputs showed substantial increase of the permafrost area by 49.6% relative to the pre-industrial conditions in median among the models, but also showed insufficient cooling during the cold season in some regions. Across-model variations illustrate the regions that need careful examination in using PMIP2 outputs for further subsurface thermal regime calculations.
An analysis of the low-level jet (LLJ) observed near the center of Typhoon Higos (0221) over the Kanto plain is presented by using upper soundings and a dual analysis of Doppler radars. The LLJ in the rear of Higos was observed near the surface in a cool airflow that flowed out of the airmass on the cold side of the frontal zone, considered to conserve high angular momentum relative to Higos moving rapidly on the warm side of the front. The LLJ was also observed in front of Higos; the wind maximum was in a warm airmass elevated over a cool surface air associated with relatively weak surface winds.
A method of correction for radiosonde pressure and temperature data by using simultaneous global positioning system (GPS) ellipsoidal height (zGPS) is proposed. The correction is made by adjusting the observed pressure and temperature so that the ellipsoidal height (zPTU) calculated from integrating the hypsometric equation by using the latitude- and altitude-dependent gravity together with the observed pressure, temperature and humidity (PTU) agrees with zGPS. The temperature bias is assumed to arise only from inaccurate radiation correction so that there is no temperature bias in the nighttime data. Under this assumption, the deviations of zPTU from zGPS in the nighttime data result only from observational errors in pressure. The pressure adjustment required to remove these deviations is applied also to the daytime data. The daytime temperature bias can then be estimated from the difference between zPTU and zGPS during the day. The biases in Vaisala RS80 pressure and temperature measurements are estimated using the Soundings of Ozone and Water in the Equatorial Region campaign data. The estimated pressure bias is positive below ∼7 km and negative above it. The bias above 15 km is statistically significant. The daytime temperature bias lacks statistical significance due to fluctuations in the results.
This study assesses the forecasting performance of operational medium-range ensemble forecasts: BOM, CMA, CMC, CPTEC, ECMWF, JMA, KMA, NCEP, and UKMO, in terms of atmospheric blocking during DJF (December-January-February) of 2006/07, 2007/08, and 2008/09. The state-of-the-art medium-range ensemble forecasts performed well in simulating the frequencies of Euro-Atlantic (EA) and Pacific (PA) blockings, even after 216-hr lead time, whereas they did not simulate well the frequencies of Greenland and Ural blockings, even in the middle of the forecast range. The ensemble forecasts are not always able to capture the blockings with high probability in the latter half of the forecast range. During this latter half, blocked flows were frequently predicted with low probability during the active blocking period, whereas they were seldom predicted with similar probability during the non-active blocking period. This result might suggest that the active blocking period is more chaotic than the non-active blocking period. In addition, it was more difficult to predict an onset of EA blocking than to predict an onset of PA blocking, and probabilistic blocking forecasting over the PA sector was more skillful than that over the EA sector. These results suggest that PA blocking has a higher predictability than does AT blocking.
In the period from June 2006 to December 2008, we measured the tropospheric nitrogen dioxide (NO2) column by ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) at an urban site in China (Tai’an) and three sites in Japan, covering urban (Yokosuka), suburban (Tsukuba), and remote areas (Hedo). This robust dataset is used to characterize Ozone Monitoring Instrument (OMI) tropospheric NO2 column data (the standard product, version 3). Correlations between MAX-DOAS and OMI data, both of which show very low NO2 at Hedo and moderate/high levels at the other sites, have correlation coefficients (R2) as high as 0.64, indicating that relative changes in OMI NO2 data are reliable. However, OMI data have a negative bias of 31% on average. Assuming that these results are valid for OMI data taken over China, we find an increasing trend in tropospheric column NO2 at about 5% per year on average in the industrial areas of China (30°-40°N and 110°-123°E) over 2005-2008, but its spatial distribution is highly inhomogeneous.
In this study, we apply the local ensemble transform Kalman filter (LETKF) to the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) to develop the NICAM-LETKF. In addition, an algorithm to adaptively estimate the inflation parameter and the observational errors is introduced to the LETKF. The feasibility and stability of the NICAM-LETKF are investigated under the perfect model scenario. According to the results, we confirm that the converged analysis errors of the NICAM-LETKF are smaller than the observational errors, and the magnitude and distribution of the root mean square errors (RMSEs) are comparable to those of the ensemble spreads. In our experiments, we find that the inflation parameter is optimally tuned and the observational errors are close to the true value. It is concluded that the NICAM-LETKF works appropriately and stably under the perfect model scenario even if the inflation parameter and the observational errors are adaptively estimated within the LETKF.
This study proposes a combined use of spaceborne cloud profiling radar, shortwave and microwave measurements provided by the A-Train constellation of satellites to investigate the warm-rain collection process in low clouds. The continuous collection model is employed to interpret the relationship between the observables of CloudSat, MODIS and AMSR-E measurements. The theoretical formula representing the collection process involves the collection efficiency factor, which is then inferred from these observations by exploiting the differing sensitivities of these sensors to cloud drop sizes. The collection efficiency implied ranges from the order of 0.001 to that of 1.0, and is systematically larger than estimated by traditional methods when updraft velocity is ignored in the continuous collection model. The collection efficiency is also found to increase with columnar effective particle radius in a manner consistent with classical relationships. These results suggest that the new observations provide a gross measure of the real collection efficiency factor on the global scale.
The climatological features of warming events over Toyama Plain and the heating mechanisms are investigated by using observation and the non-hydrostatic regional climate model NHRCM. Warming events are frequently observed in March and April. The 20km-NHRCM can reproduce the seasonal variation of the frequency of warming events. In spite of the large diversity of heating mechanisms in the warming events, typical warming factors are apparent, and the ratio of contribution differs case by case. The results of the 10km-NHRCM and backward trajectory analysis indicate that these factors are moist-adiabatic processes, dry-adiabatic processes, and sunshine insolation. The frequency of the typical insolation event is quite low, but it often accelerates warming with other factors. The NHRCM demonstrates high potential for the application of local climate studies as a dynamical downscaling tool.
When an averaging method is used for model future projection with weights determined according to the model performance in the present climate, generally, the stationarity of the model performance between present and future is implicitly assumed. Here we investigate this assumption using multi-model data. We consider the correlation between inter-model similarities in the spatial pattern for the present-day climate and future climate change for surface air temperature, precipitation and sea level pressure on global and zonal domains in the seasonal time scale. We further extend previous work by devising a bootstrap method to estimate the statistical significance of all correlations, which have previously not been estimated. Most of the correlation coefficients for precipitation were significant, but moderate or low in the absolute value. Many of those for the other variables were not significant. Also, we discuss the magnitude of the inter-model similarity used in this work.
A daily gridded precipitation dataset for 1961-2004 was created by collecting rain gauge observation data across Asia through the activities of the Asian Precipitation—Highly Resolved Observational Data Integration Towards the Evaluation of Water Resources (APHRODITE) project. Our number of valid stations was between 5000 and 12,000, representing 2.3 to 4.5 times the data available through the Global Telecommunication System network, which were used for most daily grid precipitation products. APHRODITE’s daily gridded precipitation (APHRO_V0902) is the only long-term (1961 onward) continental-scale daily product that contains a dense network of daily rain gauge data for Asia including the Himalayas and mountainous areas in the Middle East. The product contributes to studies such as the evaluation of Asian water resources, diagnosis of climate change, statistical downscaling, and verification of numerical model simulation and high-resolution precipitation estimates using satellites. We released APHRO_V0902 datasets for Monsoon Asia, Russia and the Middle East (on 0.5° × 0.5° and 0.25° × 0.25° grids) at http://www.chikyu.ac.jp/precip/. Herein, we show the algorithm and input data of APHRO_V0902.
Characteristics of the thermal and ozone tropopauses and the tropopause inversion layer (TIL) in the Antarctic and Arctic are investigated using high vertical resolution ozonesonde data. The ozone tropopause is clearly defined even in the Antarctic winter and spring in which the thermal tropopause is ambiguous. The Antarctic ozone tropopause shows a smaller annual variation than the thermal tropopause. While the ozone tropopause is located below the thermal tropopause in the Arctic throughout the year, the ozone tropopause in the Antarctic summer is located at the same height as or slightly above the thermal tropopause. A description of the vertical structure around the tropopause in the ozone tropopause based coordinate presents a sharp and continuous transition from the troposphere to the stratosphere, while the transition in the thermal tropopause based coordinate is sharp but discontinuous. The TIL defined in the ozone tropopause based coordinate almost disappears in the Antarctic winter and spring unlike in summer and fall. In the Arctic, the TIL is observed even in winter, but its maximum value of Brunt-Väisälä frequency squared (N2) is smaller than summer. The value of N2 in the TIL is larger in the Antarctic summer than in the Arctic summer.
An estimation method for temporal variation of refractive index (RI) using phase data observed by a C-band Doppler radar equipped with a magnetron transmitter is presented. Temporal variation of RI was estimated from phase increments that were caused by temporal atmospheric variations. Because the wavelength of the C-band radar is as short as 5.7 cm, phase wrapping frequently occurs. Thus, number of phase wrapping was counted by monitoring the phase increments at the interval of 30 seconds. Effect of frequency drift of the magnetron transmitter was removed by using phase increments of a reference target and atmospheric delay between the radar site and the reference target. The method was applied to a case study of daytime sea-breeze front in the Kanto plain. The estimated variation of RI, which has the information of water vapor, showed an agreement with the position of the cloud band that was generated in the surface convergence zone.
Using satellite-derived surface current products, we examined annual current variations in the equatorial Pacific Ocean and associated zonal displacement of the Pacific warm pool. Annual current variations, generated mainly by annual equatorial Rossby waves, displaced the warm pool eastward (westward) during boreal spring-summer (winter), with accompanying increase (decrease) in the zonal heat advection at a rate of 0.2-0.4 °C mon-1 in the warm pool region in normal years when neither El Niño nor La Niña event take place; similar seasonal increases (decreases) have also been observed in El Niño years. Annual zonal currents variations attributable to oceanic Rossby waves provide more (less) favorable background sea surface conditions for the onset of El Niño during boreal summer (winter) by weakening (strengthening) the zonal sea surface temperature gradient through eastward (westward) heat transport near the equator in the warm pool region. Annual zonal currents variations might play a key role in seasonally locking the onset of El Niño events in the warm pool region.
Both wet and dry deposition fluxes of Aeolian dust were analyzed for over one year (from March 2007 to May 2008) including two dust seasons in Tsukuba, Japan. The annual wet and dry deposition fluxes of dust were 5.5 g m-2 and 3.8 g m-2, respectively. The largest wet deposition flux associated with the one heavy Asian dust event was 0.95 g m-2, which accounted for 10% to the annual total deposition. A typical feature of the deposition fluxes marked seasonal variation with higher deposition in spring. Furthermore, the wet deposition fluxes in the spring (March-May) 2007 were larger than those in 2008. The temporal variations of wet depositions were agreed well with the model simulation. Analysis of model simulation showed that the seasonal and year to year variations of the wet deposition fluxes in Tsukuba could be closely connected with the variation of dust emission over the Asian continent associated with stronger wind. For dry deposition fluxes of dust, local dust near the observation region also contributed to the deposition fluxes in addition to the Asian dust.
Linear regression analysis was applied to the time series of hourly rainfall data between June and September for 31 years to elucidate the features of weak rainfall over Japan. Many stations exhibiting a significant positive trend in the frequency of weak rainfall (1-5 mm h-1) were localized in the Hokkaido district. In contrast, there were only a few stations with a significant positive trend in weak rainfall over regions other than Hokkaido; however, many stations exhibited a significant positive trend in heavy rainfall exceeding 25 mm h-1. Thus, there existed a clear geographical contrast between the two regions.
The Greenhouse Gases Observing Satellite (GOSAT) was launched on January 23, 2009, to monitor global atmospheric levels of CO2 and CH4 from space. GOSAT started initial operation of its instruments after an initial satellite system check. Although the radiant data obtained by the GOSAT instruments are currently in the preliminary stages of calibration and validation, the spectral absorption features of CO2 and CH4 are clearly identifiable. An initial retrieval of these gaseous concentrations was performed for measurement scenes of cloud-free conditions over land. These results showed that column-averaged dry air mole fractions of both CO2 and CH4 in the northern hemisphere were higher than those in the southern hemisphere. These latitudinal differences agree with data obtained from ground-based sources and other satellite observations; however, the absolute values of the gaseous concentrations from GOSAT data seem to have been underestimated. Calibrations as well as validation should be conducted to improve the quality of GOSAT retrievals.
In this report, we present the results from our recent experiments using 20 km-mesh and 60 km-mesh atmospheric general circulation models with prescribed sea surface temperatures (SST). The results of the experiments consistently show a reduction in the global tropical cyclone frequency due to global warming. By the experiments with the models of different resolution and with different SST changes, we find that the reduction in the global tropical cyclone frequency due to global warming is a very robust feature. In contrast, the regional tropical cyclone frequency change varies a lot among the experiments with different SST change distribution. We find that the regional tropical cyclone frequency change is sensitive to relative SST change distribution. This suggests that the regional change is strongly affected by the change in tropical circulation and convective activity which is dominated by relative SST distribution patterns, and therefore, for a reliable projection of the regional change, a reliable projection of the pattern of SST change is vitally important.
The authors investigate trends in precipitation extremes using daily precipitation data from Southeast Asian countries during 1950s to 2000s. Number of wet days, defined by a day with at least 1 mm of precipitation, tends to decrease over these countries, while average precipitation intensity of wet days shows an increasing trend. Heavy precipitation indices, which are defined by precipitation amount and percentile, demonstrate that the number of stations with significant upward trend is larger than that with significant downward trend. Heavy precipitation increases in southern Vietnam, northern part of Myanmar, and the Visayas and Luzon Islands in the Philippines, while heavy precipitation decreases in northern Vietnam. Annual maximum number of consecutive dry days decreases in the region where winter monsoon precipitation dominates. Decrease of precipitation event in the dry season is suggested in Myanmar.
The central Tibetan Plateau contains numerous lakes, the largest and deepest of which is Nam Co. Heat and water budgets over Nam Co were estimated throughout the year. Latent and sensible heat fluxes from the lake are very small from spring to summer and larger from fall to early-winter. Although the land surface over the Tibetan Plateau supplies heat energy to the atmosphere during the pre-monsoon and monsoon periods, a deep lake such as Nam Co releases a large amount of heat in the post-monsoon period. Annual evaporation from the lake was 658 mm, much larger than the annual rainfall of 415 mm and also more than two times the annual evaporation from the land surface. The diurnal variation of surface-air temperature difference over the lake showed a reverse phase of that over land in the monsoon period, a condition that likely affects the convective activity over the lake.
Physical processes involved in basin-wide warming (BW) in the Indian Ocean (IO) and the IO “Dipole” mode (IOD) are investigated, using solutions to an air-sea coupled general circulation model (CGCM). In this study, we pay attention to the seasonal difference of the IO SST in response to the ENSO. A suite of time-slice CGCM experiments, by imposing the El Niño-related SST anomalies in the Pacific for the boreal summer and fall, are conducted to study the El Niño impact on the IO SST anomalies. The impacts of El Niño largely depend on the monsoon seasonality, and significantly contribute to the IOD and IO BW. Additional experiments demonstrate the salient seasonal difference in the SST sensitivity to the ocean dynamics.
In this study, characteristics of the energy spectrum in the zonal wavenumber domain are examined for the cloud resolving global model NICAM. A series of numerical experiments are conducted for NICAM with various horizontal resolutions from 224 km (glevel-5) to 7.0 km (glevel-10) using the T2K-Tsukuba System and 3.5 km (glevel-11) using the Earth Simulator (ES). The energy spectra of most of horizontal resolutions obey k-3 power law in synoptic and sub-synoptic scales (wavenumbers k = 5 to 30). However, the energy slope for glevel-5 becomes much steeper around zonal wavenumber k = 10. Nastrom et al. (1985) explained that the energy spectrum near the tropopause at wavelengths below 400 km appears to follow the k-5/3 power. This scale corresponds to about k = 70 near 45°N. It is found that the energy spectra for k > 30 for glevel-10 and 11 follow the k-5/3 power law. These results agree quite well with the observational studies. It is also found that the kinetic energy of the vertical wind is white noise spectrum.
Using an objective method to calculate the saltation threshold friction velocity u*t based on measurements using a sand particle counter, the values of u*t were determined to be 0.48 and 0.56 m s-1 under dry (θv = 0.002 m3 m-3) and wet (θv = 0.009 m3 m-3) conditions, respectively, at a gobi site in the Taklimakan Desert. This result shows that u*t increased when soil moisture increased resulting from snowfall in March 2003. A simple relationship was found between the saltation flux q(z) at a reference height z and friction velocity u* and u*t.
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