A shallow snowpack, such as that seen in Mongolia, easily melts through heating by the atmosphere. In turn, a snow-free surface without much meltwater heats the atmosphere quickly through heat fluxes at the surface. The present study investigated the snowmelt processes over eastern Mongolia during 2005-2007, using ground-observed temperature, snow depth, albedo, and radiation data. Moreover, we explored atmospheric influences on the snowmelt through the heat budget analysis of reanalysis data provided by the National Centers for Environmental Prediction. The results show that atmospheric heating over eastern Mongolia during the snowmelt season is caused by the combination of vertical adiabatic heating in a traveling anticyclone and one-day-lagging horizontal advection after the passage of the anticyclone. These synoptic processes lead to a drastic increase in temperature and finally to substantial snowmelt under certain atmospheric conditions: the daily-mean surface air temperature exceeding -10°C and downward longwave radiation exceeding 194 W m-2 under clear sky conditions.
This study investigates long-term changes in Baiu rainfall in Eastern and Western Japan using daily precipitation records at 37 stations for the years 1901 through 2009, focusing on its seasonal progress. This period is much longer than various data analyzed in previous observational studies. In the early Baiu season (early to mid June), significant long-term decreasing trends are observed in Eastern and Western Japan, accompanying large inter-decadal variation in the former half of the 20th century. In the late Baiu season (mid to late July), in contrast, significant long-term increasing trends are observed on the Japan Sea side of Eastern and Western Japan. No significant trends are recognized either in the mid Baiu season (late June to early July) or in the entire Baiu season (June to July) over all regions. It is interesting to note that the observed tendency of delayed Baiu withdrawal in the last 109 years, when global warming has been in progress, is similar to its future changes projected by climate models.
The environmental stability for afternoon rain events over the Kanto Plain in summer was investigated. The AMeDAS data were used to extract the hot, sunny days under synoptically undisturbed conditions, and the gridded mesoscale analysis data that cover the southern Kanto Plain were used to examine the difference of the characteristics of environmental stability among no-rain, rain, and strong-rain events in the afternoon by calculating commonly used stability indices and parameters. A statistical analysis by t-test statistic was conducted to determine the significance of the different features of the stability parameters among the events. Among the parameters, K-index indicated the highest significance level. The analyses on the difference of temperature and humidity at each height among the events indicated that the temperatures and moistures at low to middle levels clearly distinguish the stability conditions for the afternoon rain events. It is strongly suggested that colder temperature at middle levels and higher humidity at low to middle levels are favorable conditions for the development of stronger rains in the afternoon.
There is a biennial tendency for the interannual variabilities of the Asian summer monsoon (ASM) referred to as the tropospheric biennial oscillation (TBO). Previous studies have emphasized that land-atmosphere-ocean interactions in association with the El Niño/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) trigger a phase shift of the TBO from one year to another. However, simple lead-lag correlations among the ASM, IOD and ENSO indices reveal that there is a relatively stronger signal of the phase shift of the ASM over two years than one year. Meanwhile, the IOD has been shown to contribute to the TBO by triggering the phase shift of the ENSO by the following winter, although correlations of the ENSO do not identify such a biennial tendency. Through composite analyses, IOD-ENSO coupled events are shown to induce the phase shift of the ENSO within a year, which results in a distinctive TBO pattern of the ASM in the following two years. In contrast, ENSO events without the IOD cannot induce the phase shift, and there is no TBO pattern in the following two years. Thus, IOD-ENSO coupled events and independent ENSO events have contrasting roles in inducing different (two-year and four-year) frequencies of the ASM interannual variability in subsequent years.
The sinking of dense water in the polar oceans plays a key role in global thermohaline circulation, leading to heat and material exchange between the atmosphere and deep ocean. This study provides the first surface heat and salt flux dataset for the Southern Ocean (including a treatment of sea ice growth and melt), based on heat flux calculations and satellite-derived sea ice data. The geographical distribution of annual net heat (salt) flux shows a distinct contrast: significant cooling of (salt release into) the ocean occurs in the coastal region, and net heating of (freshwater release into) the ocean occurs in the offshore region. The work tries a quantitative representation of heat and freshwater transport by sea ice formed in the coastal region to offshore. Since hemispheric-scale heat and salt fluxes associated with sea ice growth and melt have not been estimated from observations to date, the present dataset will provide new information with which to validate coupled ice-ocean models while providing important boundary conditions for the various models.
This paper discusses the accuracy and characteristics of offshore wind speeds simulated by the Weather Research and Forecasting (WRF) model. Firstly, the accuracy of the simulated wind speed is examined using in-situ measurements from the Shirahama offshore research platform for the whole year of 2005. It is found that the WRF surface wind speed has an annual bias of 15.3%, which is much larger than the National Centers for Environmental Prediction Final Analysis data (NCEP FNL) used as input. Secondly, the possibility of accuracy improvement of the simulated wind speed is discussed in terms of the decomposition of RMSE and the estimation error of wind energy density. As a result, it is shown that reducing the large positive bias is a key to increase the accuracy of the WRF wind speed. Without the large bias, the error in the estimation of annual mean wind energy density drastically decreases from 47% to 4%. Finally, the accuracy of wind speeds above the surface layer is examined using wind profiler measurements from Mihama. It is found that the WRF wind speed has large positive biases not only near the sea surface but also in the lower PBL. Thus, the large positive biases are speculated to be mainly due to inaccuracy of the PBL scheme in WRF.
In the winter of 2009/2010, Japan and the East Asian region experienced a frequent occurrence of cold air outbreaks. Although the winter average temperature in the Japan main islands was slightly positive (+0.81°C for DJF average and +0.71°C for NDJFM average), repeated decline in temperature was notable throughout the season. One explanation for this abnormal winter season is the extremely negative condition of the Arctic Oscillation (AO) that persisted from December to mid-January. However, AO alone does not provide sufficient explanation for the cold air outbreak during November or its intraseasonal periodicity. A case study of the cold air outbreak that reached Japan on Dec. 18 reveals an anomalous ridge forming over the Barents-Kara Sea, which leads to the cold air accumulation over western Siberia. The pressure anomaly subsequently shifted westward to mature into a blocking high which created a wave-train pattern downstream, advecting the cold air eastward towards East Asia and Japan. The sequence of events was also apparent in multiple cases throughout the season. This study suggests that there is a strong and systematic linkage in the intraseasonal timescale between the atmospheric condition over the Barents-Kara Sea and the cold air accumulation over the Eurasian continent, leading to the anomalous cold air outbreak over East Asia and Japan. The mechanism may also provide explanation to extreme winter conditions such as those observed during the winter of 2010/2011.
We built a Continuous-Flow Diffusion-Chamber type Ice Nucleus Counter based on the design of Rogers et al. (1988) at Colorado State University, USA and have improved it to operate automatically. The main improvements are 1) automatic procedures to coat the cylinder walls with ice, 2) automatic measurements of temperature and supersaturation spectra of ice nuclei activation, and 3) reducing the heat inertia of inner cylinder to shorten the time for scanning the activation temperatures and supersaturations. Its performance is briefly demonstrated through examples of measurements on activation spectra of ice nuclei for summertime atmospheric aerosol particles in the central part of Japan and for test aerosol particles (Arizona Test Dust and ice nucleating bacteria, Snomax®).
In this letter, we investigate nonlinear properties of the underlying dynamics for the Arctic Oscillation Index (AOI). First, we develop a test statistic for surrogate data analysis so that we can deal with the high-dimensionality of AOI. Second, we validate the test statistic with toy models. Then, by applying the proposed new test statistic with surrogate data to the dataset of AOI for the last 60 years, we show that AOI is nonlinear with determinism beyond pseudo-periodicity. These results mean that the underlying dynamics of AOI is consistent with deterministic chaos, implying that it is predictable in a short-term but not in a long-term.
We have investigated the effects of assimilating sea ice concentration (SIC) data on a simulation of Arctic Ocean climate using an atmosphere-ocean-sea ice coupled model. Our results show that the normal overestimation of summertime SIC in the East Siberian Sea and the Beaufort Sea in simulations without sea-ice data input can be greatly reduced by assimilating sea-ice data and that this improvement is also evident in a following hindcast experiment for 3-4 years after the initialization of the assimilation. In the hindcast experiment, enhanced heat storage in both sea ice and in the ocean surface layer plays a central role in improving the accuracy of the sea ice distribution, particularly in summer. Our detailed investigation suggests that the ice-albedo feedback and the feedback associated with the atmospheric pressure pattern generated by the improved estimation of SIC work more effectively to retain the heat signal after initialization for a coupled atmosphere-ocean-sea ice system prediction. In addition, comparison with field observations confirms that the model fails to produce a realistic feedback loop, which is (presumably) due to inadequacies in both the ice-cloud feedback model and the feedback via the Beaufort Gyre circulation. Further development of coupled models is thus required to better define Arctic Ocean climate processes and to improve the accuracy of their predictions.
We have developed an advanced data assimilation system for a global ozone simulation model with a four-dimensional ensemble Kalman filter (EnKF). Satellite-measured total column ozone data known as EP/TOMS version 8 were assimilated. Two-month data assimilation cycle experiments were performed from January to February 2001 with three different observing density settings: i) full use of EP/TOMS data points, ii) reduced use of the data points at 1/10, and iii) reduced use of the data points at 1/100. For comparison, additional data assimilation experiments were performed with Newtonian relaxation or a nudging technique. In the case of full use of EP/TOMS data points, the EnKF outperformed the nudging technique by approximately 30% in terms of the root mean square difference in the northern hemisphere midlatitudes. Moreover, the reduced use of EP/TOMS data points demonstrated that the EnKF data assimilation was robust and stable, while the nudging technique was vulnerable to the decrease in observations. Overall, the EnKF is a promising data assimilation technique for the total column ozone and is usable for operational applications.
The characteristics of energy spectrum based on 3D normal mode energetics are investigated with the global analysis dataset provided by Japan Meteorological Agency (JMA) with the resolution of TL959L60. The energy spectrum of gravity modes exactly follows the -5/3 power law in the synoptic and mesoscales. In the synoptic scale, the spectral slope of total energy follows the -3 power law because Rossby waves are dominant compared to gravity waves. The energy level of gravity modes becomes larger than that of Rossby modes around the zonal wavenumber k = 80. This scale corresponds to 350 km in 45°circle. The total energy spectrum does not show a clear transition from -3 power slope to -5/3 power slope because the energy level of Rossby and gravity modes become comparable near the transition wavenumbers.
To ascertain the temporal and spatial distribution of mineral dust deposition by wet processes, weekly deposition samples were obtained at Sapporo, Toyama, Nagoya, Tottori, Fukuoka, and Cape Hedo (Okinawa) during October 2008-January 2010 using automatic wet-dry separating samplers. Mineral dust weights in water-insoluble residue mixed with pollen were estimated from Fe contents measured using an X-ray fluorescence analyzer. Highest and lowest annual dust fluxes were found respectively at Toyama (8.7 g m-2 yr-1) and at Cape Hedo (1.1 g m-2 yr-1) in 2009, although their annual levels of precipitation were almost equal (ca. 2300 mm). Wet deposition flux was high in spring and low in summer. Simultaneous wet deposition events were observed five times among the sites, but these events were limited to regional scale. Based on the spatial distribution of dust represented as a mosaic of true color images from MODIS/Terra and vertical distributions of dust concentration from lidar observations, a high wet-dust-deposition event occurring in mid-March at Toyama was attributed to a combination of high dust concentration in the atmosphere during precipitation. Higher wet dust deposition at Toyama and Tottori was ascribed to frequent precipitation during the dusty season.
The modification of the baroclinic instability associated with positive and negative Arctic Oscillation Index (AOI) is theoretically investigated using a linearized 3D spectral primitive equation model. The linear instability analysis shows that the most unstable Charney mode MC changes its structure to intensify (weaken) the polar jet by the eddy momentum flux associated with the positive (negative) AOI. More importantly, the meridionally dipole Charney mode M2 is modified into the monopole Charney mode M1 to transport eddy momentum flux northward under the positive AOI condition. It is found that this modification is essential to intensify the polar jet during the AOI positive phase. Hence, we have theoretically confirmed that there are positive feedbacks between the baroclinic instability waves and the Arctic Oscillation characterized by the intensity of the polar jet.
This study evaluated surface heat fluxes in reanalyses (ERA-Interim, JCDAS, and NCEP/NCAR) at the marginal ice zone during September 2009, a month in which intensive radiosonde soundings were performed during an Arctic cruise by the Japanese R/V Mirai. Two surface conditions are compared: very new ice cover during a period of low temperature and ice-free conditions. ERA-Interim reproduces the observed temperature profiles well because the turbulent heat fluxes are realistic, due to the explicit treatment of the ice concentration. With the relatively simplified treatment in JCDAS, the temperature is significantly underestimated (overestimated) in the quasi-ice-covered (ice-free) area. Although NCEP/NCAR has characteristics between ERA-Interim and JCDAS, the satellite-derived sea surface temperature (SST) distribution near the ice edge efficiently controls the heat fluxes.
In order to evaluate the effectiveness of air pollution control through traffic restriction measure, continuous atmospheric particle samples were collected using step sampler at an urban site in Beijing in August 2007 during traffic restriction and non-restriction periods. Elements concentrations of aerosol samples were analyzed using Particle Induced X-Ray Emission (PIXE). As expected, the concentrations of most elements during the restriction period were considerably lower than those during the non-restriction period. The concentration ratios of most dust elements such as Al, Si, Ca, Ti, and Fe between the restriction and non-restriction periods were less than 0.3, indicating that the traffic restriction measure is an effective way in reducing soil dust trace elements in Beijing. The Enrichment factor (EF) values of elements Al, Si, K, Ca, Ti, Cr, Mn, and Fe were lower than 10 during both the restriction and non-restriction periods while EF values of other elements were in the range of 20-2000 during both periods. The EF values of Cl and Ni were lower during the restriction period compared to the non-restriction periods while the EF of S, Zn, As, and Br were higher during the restriction period. Soil dust concentrations during the traffic restriction period were only about 1/5 of those during the non-restriction period. These results suggested that traffic restriction is more effective on reducing dust elements concentrations, but less effective on reducing anthropogenic elements concentrations.
Climate change can be interpreted from the perspective of fractals. This study uses the wavelet transform to analyze the multifractal behavior of the monthly El Niño/Southern Oscillation index (NINO3.4), the monthly Indian Ocean Dipole Mode Index (DMI), and the monthly North Atlantic Oscillation (NAO) index. Strong multifractality was observed when NINO3.4 and DMI had high coherency. Changes from multifractal to monofractal behavior were observed at the 1976/77 regime shift for NINO3.4 and DMI, and at the 1988/89 regime shift for NAO index. We found that a regime shift occurs when the fluctuation is large and multifractality is strong.
We propose an index of soil and land surface conditions for wind erosion to investigate their effects on dust outbreaks. The index is the normalized dust outbreak frequency (NfDO), which is the ratio of dust outbreak frequency to strong wind frequency. NfDO for April was always low in Mandalgobi, Mongolia, when the accumulated precipitation amount for June to August (PrecJun-Aug), soil moisture averaged for June to August (SMJun-Aug), and above-ground biomass for August (AGBAug) of the previous year exceeded their thresholds (100 mm, 13 mm, and 2.2*10-2 kg m-2, respectively). This suggests that dead leaves of grasses in spring, which are the residues of vegetation from the preceding summer, suppress dust outbreaks. However, when PrecJun-Aug, SMJun-Aug, and AGBAug are lower than the thresholds, NfDO varies over a wide range. This implies that when there are few dead leaves in spring, other possible factors after summer such as liquid precipitation leading to soil freezing, snow cover, melted water, and grazing, affect erodibility in spring. These results suggest that changes in soil and land surface conditions, rather than in wind conditions, chiefly affect the increased frequency of dust outbreaks. This dead-leaf hypothesis can be used as an early warning of dust-storm hazards.
Understanding atmospheric photochemical oxidation capacity is key to the evaluation of human impacts on the atmosphere. In this study, an observation-based box model constrained by in-situ measurements was used to analyze budgets of OH and HO2 (two key oxidants) in polluted conditions during August 2008 in urban Beijing. The results showed that photolysis of HCHO and other VOCs were the dominant factors influencing HOx production. Photolysis of HONO and alkene ozonolysis were also significant. Aerosol had a considerable impact on HOx and Ox production, as revealed by sensitivity simulations, reducing the concentrations of OH and HO2 by 56% and 25%, respectively. Among the reductions, that of OH was mostly through the radiative effect of aerosol, while HO2 reduction was from both radiative and heterogeneous chemical effects of aerosol. This impact of aerosol on atmospheric oxidation capacity led to a 100-200% increase in the lifetimes of NO2, CO and SO2. This implies that aerosols may increase the transport distance of gaseous pollutants.
Mount Kilauea in the Hawaiian Islands experienced an active eruption from March until the end of December 2008 and showed a large-scale impact on aerosol, cloud microphysical properties and atmospheric radiation over the North Pacific. We analyzed the atmospheric impact of this eruption based on the satellite retrievals and 3-D global chemistry-radiation coupled transport model. We showed that approximately 1.8 Tg (±1.2 Tg) release of SO2 was estimated from this eruption, which oxidized into sulfate aerosol during transport to the northwest Pacific Ocean. The volcanic sulfate aerosol layer covering a large area (∼6.5 × 106 km2) of the lower troposphere over the North Pacific for several months was confirmed from both satellite and model results. Sulfate aerosols affected the formation of cumulus water clouds by reducing the typical cloud effective radius by ∼23% and increasing the cloud fractional coverage over the ocean from 9.1% to 13.4% (over the region 170°E-160°W, 10°N-20°N). The affected cumulus clouds appeared whiter than normal and thus reflected more solar radiation. Consequently, satellite observations revealed an approximately 1% increase in albedo at the top of the atmosphere in the area along main volcano plume trajectory, which induced an approximately -5 W m-2 change in the shortwave radiation budget.
We evaluate reproducibility of seasonal evolution of the thermal fields associated with the first transition of Asian summer monsoon (ASM) in atmosphere-ocean coupled general circulation models (CGCMs) of the Coupled Model Intercomparison Project phase 3 (CMIP3). Many CGCMs reproduce seasonal evolution of the thermal fields related to the first transition of ASM well, though the degree of reproducibility differs to some extent. Based on this evaluation, weighted multi-model ensembles are calculated, and the future projections of the ASM onset from the viewpoint of lower-tropospheric westerlies are conducted. The onset dates over the Bay of Bengal, the Indochina Peninsula and the South China Sea are projected to delay by 5 to 10 days in the end of the 21st century under the A1B scenario of the Special Report on Emission Scenarios (SRES), compared to those in the end of the 20th century. This change might be related with delay of the reversal of upper-tropospheric meridional thermal gradient between over the Eurasian Continent and the north Indian Ocean.
Pollutant transport has a significant impact on the air quality of Shanghai. To quantify this impact, we comprehensively analyze the characteristics and formation mechanisms of air pollution in Shanghai, using a data set covering five selected air quality monitoring sites and a meteorological site. We find air pollution events occur ∼10% of the year. These events are classified into three typical categories, namely the local pollution case (LP), the dust storm case (DS) and the regional transport case (RT), based on temporal variations and spatial distributions of PM10 and gaseous pollutants as well as meteorological factors and backward trajectories. The ratios of SO2/PM10, NO2/PM10 and PM2.5/PM10 are used as indicators of long-range transport of mineral dust, while CO proves to be a good tracer of regional transport of anthropogenic pollutants. In 2007, 64% of all pollution events were RT and the mean PM10 concentration reached 220 μg m-3. 17% of all pollution events were DS and the mean PM10 concentration reached 310 μg m-3. We estimate that transport contribution to PM10 abundance was within the range of 34% to 64% by comparing the anomaly of the background concentration with the urban concentration.
In-situ instrument calibration has been assessed at Mt. Waliguan Observatory, China based on continuous CE-318 sunphotometer measurements. The calibration procedure was introduced and calibration results were analyzed. From the analysis results, the measurement data obtained during airmass between 2.0 and 6.0 at AM are suitable for Langley method calibration. The final instrument calibration coefficients were achieved based on more than 10 plot curves with the criteria of relative standard deviation (RSD) less than 0.5% for infrared and visible wavelengths and 1.0% for UV wavelengths, respectively. The in-situ calibration coefficients differed less than 0.5% for infrared and visible wavelengths and 1.0% for UV wavelengths from inter-comparison calibration ones. Based on the analysis results, Mt. Waliguan Observatory is possible for the calibration of sunphotometer, especially in autumn and winter seasons.
The zero-plane displacement height d was evaluated in downtown Tokyo by the two independent methods of temperature variance and scintillometer heat flux. Regardless of the method, d exceeds the area-weighted average building height. This may be because d, which represents the point of effect for wind drag, is elevated by some tall buildings that jut above others. The area-weighted average building height would then be unsuitable as a geometrical index of urban canopies with large height variations. Thus, the height of the upper envelope of the canopy was proposed as the representative canyon height.
Observational studies of the western Pacific Ocean have suggested since the mid-1980s that the barrier layer resulting from the salinity stratification within the mixed layer could influence significantly the ocean-atmosphere interactions. Numerical experiments based on a CGCM are designed and analyzed in such a goal. The formation of the barrier layer is primarily identified as resulting from a tilting/shearing mechanism in which horizontal and vertical gradients of salinity, as well as the dynamical response of the ocean to westerly winds, are tightly coupled. When the contribution of salinity to the computation of the horizontal pressure gradient force in the ocean model is cancelled within the equatorial warm pool, both the mean climatology and the low frequency variability are affected as the result of a complete annihilation of the barrier layer. The decreased sensitivity of the coupling between the SST, winds and atmospheric deep convection is likely due to the deepening of the ocean mixed layer that cools the SST and weakens the amplitude of its variability. These local changes within the western Pacific warm pool also induce a basin scale response that weakens the amplitude of ENSO variability. These results suggest that the formation of the barrier layer is a key element of the whole Pacific ocean-atmosphere coupled system.
The powerful tsunami generated by the massive earthquake that occurred east of Japan on March 11, 2011 caused serious damages of the Fukushima Daiichi Nuclear Power Plant on its cooling facilities for nuclear reactors. Hydrogen and vapor blasts that occurred until March 15 outside of the reactors led to the emission of radioactive materials into the air. Here we show a numerical simulation for the long-range transport from the plant to the U.S. and even Europe with a global aerosol transport model SPRINTARS. Large-scale updraft organized by a low-pressure system traveling across Japan from March 14 to 15 was found effective in lifting the particles from the surface layer to the level of a westerly jet stream that could carry the particles across the Pacific within 3 to 4 days. Their simulated concentration rapidly decreases to the order of 10-8 of its initial level, consistent with the level detected in California on March 18. The simulation also reproduces the subsequent trans-Atlantic transport of those particles by a poleward-deflected jet stream, first toward Iceland and then southward to continental Europe as actually observed.
Observations of carbon dioxide (CO2) flux with the eddy covariance technique were conducted at a burned boreal forest site five years after a wildfire and at a mature forest site in Interior Alaska to investigate the effects of wildfire on CO2 exchange in a boreal forest. Both gross primary productivity and ecosystem respiration were lower at the burned site. The lower amount of vegetation explains the lower gross primary productivity and ecosystem respiration at the burned site. The reduced soil organic layer at the burned site further explains the lower respiration. On an annual basis, the five-year-old burned site was a CO2 sink, which indicated earlier recovery of CO2 exchange compared to other burned boreal forests in North America reported in the literature. The quick recovery of net CO2 exchange is associated with constrained heterotrophic respiration, rather than recovery of vegetation. Burn severity can be a key variable in determining CO2 exchange during the early stage of succession after wildfire.
Wind resource assessment for coastal areas requires accurate wind speed simulation using a mesoscale model. Our previous study found that the annual mean wind speed simulated by the advanced research Weather Research and Forecasting (WRF-ARW) model has a remarkable positive bias in the lower part of the planetary boundary layer (PBL). This result was obtained from a comparison with wind profiler measurements at Mihama, which is an observation station of the WInd profiler Network and Data Acquisition System (WINDAS) operated by the Japan Meteorological Agency (JMA). In this study, we examine whether such a positive bias can be seen at other WINDAS stations from a comparison of the WRF-simulated wind speed profile using the Mellor-Yamada-Janjic (MYJ) PBL scheme with wind profiler measurements at ten WINDAS stations. The results show that the positive bias is found at all stations, and, moreover, that the positive bias is unlikely to be caused by either error in wind profiler measurement or the objective analysis data input into WRF. In addition, we compare the wind speed profiles simulated by WRF with seven different PBL schemes for a month. The result shows that the positive bias cannot be simply reduced by using other PBL schemes.
Parameter dependence of gravity-wave radiation from an unstable jet is investigated using a two-layer, f-plane shallow-water model. The sweeping parameters are the Rossby number Ro and the Froude number Fr. If Ro/Fr ≳ 0.6, the barotropic instability dominates and gravity waves that are almost independent of the longitudinal direction and have latitudinal wavelengths longer than the jet width are radiated. The radiation occurs when the jet develops into cyclonic and anticyclonic vortex trains. The amplitude of the radiated waves increases monotonically with Ro and Fr. If Ro/Fr ≲ 0.6, the baroclinic instability dominates and gravity waves the latitudinal wavelengths of which are shorter than or the same order of length as the jet width are radiated from cyclonic vortices dominating in the jet region. In this regime, the amplitude of the radiated waves increases with Fr, whereas it decreases with Ro. To understand the different dependences of the wave amplitude on Ro, we adopt the Lagrangian Rossby number RLG, and show that the amplitude increases with RLG.
Global weather forecasts do not have sufficient performance to predict the local severe weather events that are accompanied with cyclones and cold fronts due to their coarse horizontal resolution. This study investigated the performance of dynamical downscaling (DD) using mesoscale model to simulate the severe windstorm in eastern Mongolia which occurred on 26-27 May 2008. Our results revealed that the DD experiments were successful in capturing the general features of the windstorm in terms of wind and temperature patterns. The timing and amplitude of drastic changes in the simulated temperature and wind speed were very similar to that observed than that obtained from the global atmospheric data, suggesting that DD is capable of predicting extreme wind storm events in Mongolia. Analyses on the nested domains indicate that the DD has crucial impact on the performance for simulating severe storm even with a moderate resolution (27 km), and further nesting (9 and 3 km) plays a role to improve it. Furthermore, the maximum wind speed approaches the observed value more closely as the horizontal resolution increases, although it still underestimates the observed wind speed even in the 3 km mesh domain. On the other hand, the abrupt temperature change is captured well even in the low-resolution domain, suggesting a difference in necessary horizontal resolution for temperature change and maximum wind speed.
Seasonal characteristics of spherical aerosol distributions in eastern Asia were investigated between July 2006 and December 2008 using data from ground-based and space-borne lidar observations and the Community Multi-scale Air Quality Modeling System (CMAQ) chemical transport model simulation. The seasonal variation of spherical aerosol optical thickness (AOT) observed at four ground lidar stations surrounding the East China Sea (Beijing, Guangzhou, Seoul, and Hedo/Okinawa) was generally consistent with CMAQ simulation results. Detailed analyses confirmed clear regional differences in aerosol compositions. Analyses of aerosol vertical profiles revealed that the spherical aerosol variation is greatest in Beijing, with scale height varying between 720 m and 2100 m. The analyses also revealed that the seasonal variation patterns of spherical AOT are classifiable into ‘summer peak’ and ‘summer trough’ types. Northern sites (Beijing and Seoul) are of the summer peak type; southern sites (Guangzhou and Okinawa) show the summer trough pattern. Our analyses demonstrated that the Asian summer-winter monsoon system plays a major role in regulating such seasonal variation. The CMAQ simulated variation of spherical aerosols is well correlated to the synoptic scale monsoon variation.
Simultaneous observations of cumulonimbi using the 95-GHz FM-CW cloud radar, the X-band radar, and photogrammetry were carried out during the summer of 2010 in the Kanto region, Japan. An isolated cumulonimbus developed above the cloud radar site after 16:00 Japan Standard Time (JST) on 24 July 2010. A continuous generation of turrets was observed and a total of four turrets formed. The growth rates of the turrets were quite different. The first radar echo of the X-band radar was detected at 3 km above ground level (AGL), three minutes after turret 1 reached its maximum height. Turret 2, generated above the cloud radar, reached 10 km AGL and had a growth rate of 8 m s-1. The cloud radar detected echoes approximately two minutes after the generation of turret 2. The intermittent echo pattern observed by the cloud radar denotes fine structures in the cumulonimbus.
Using data from the Japanese long-term Re-Analysis project (JRA-25) and the Japan Meteorological Agency Climate Data Assimilation System (JCDAS), we examined how the East Asian winter monsoon variability regulates the surface cyclogenesis in the vicinity of the East China Sea and the Kuroshio Current in late winter. On a monthly basis, the surface cyclone occurrence has a tendency to concentrate over the East China Sea at the strong phase of the East Asian winter monsoon activity, while it disperses zonally along the Kuroshio Current to the south of Japan in the weak monsoon phase. The scatteredness of the surface cyclogenesis are mainly attributed to the change in the lower-tropospheric baroclinicity between the strong and weak monsoon phases. It is also suggested that, when the monsoon is strong, the enhanced baroclinic zone along the Kuroshio Current south of Japan provides a favorable condition for the rapid development of the surface cyclones that originate over the East China Sea and migrate northeastward.
Deep Blue (DB) is a new MODIS retrieval algorithm to retrieve aerosol properties over bright surfaces such as arid, semiarid and urban areas. It's expected to fill in data gaps over bright surface left by the standard algorithm based on Dark Target (DT) retrieval algorithm. DB has been employed in recent Collection 5.1 AOD product. This study presents a comparison between the DT approach and the more recent DB algorithm using international Aerosol Robotic Network (AERONET) data as reference, analyzes the improvement in DB AOD product over urban surface and discusses the influence of aerosol model variation on MODIS AOD retrieval. Results show that DB products perform better than DT products under clear condition, and a systematic AOD overestimation in DT products doesn't occur in DB products. DT and DB AOD products both appear strong seasonal pattern, with better performance in autumn and winter and worse performance in spring and summer, and the underestimated Aerosol Single Scattering Albedo (SSA) during spring and summer over Beijing in MODIS retrieval probably is the major factor inducing the AOD overestimation in these seasons. In comparison with DB AOD, DT AOD is more likely overestimated, unsuitable surface reflectance over Beijing may be the key factor, and it may play a more important role in MODIS DT AOD retrieval than unsuitable SSA.
Two physical parameters, thermal skin depth and dynamic impedance, are introduced to discuss the vertical structures and dynamic processes of diurnal and annual horizontal convections. These parameters are evaluated for the winds over the Japanese islands. The seasonal and daily wind cycles have almost the same scale of thermal skin depth. The ratio of the dynamic impedance of the annual convection to that of the diurnal convection is about 3.
By applying the Japanese long-term Re-Analysis project (JRA-25) and the Japan Meteorological Agency Climate Data Assimilation System (JCDAS) data to a Rayleigh-type global one-layer isotope circulation model, we performed a long-term simulation and examined how water vapor is remotely transported to the vicinity of Japan from water source regions during the early summer rainy (Baiu) season. We validated the model outputs, comparing them with the stable hydrogen and oxygen isotope ratios (δD and δ18O) of precipitation observed at two in situ sites in southern and central Japan during the 2010 Baiu, and determined that the correlations between the simulation and observation are comparable to those in precipitation in Thailand from August to October when the Asian summer monsoon withdraws. The model results demonstrate that the Baiu is characterized by relatively low values of δD and that the δD values over central Japan are lower than those over southern Japan. When the Baiu commences, Indian Ocean water increases rapidly and then contributes substantially to the total precipitable water until the withdrawal of the Baiu, which is partially responsible for the low values of δD. Once the Baiu withdraws, alternatively, Pacific Ocean water occupies most of the total precipitable water. Another signature of its withdrawal is the decrease in land water of the Eurasian continent. It is also clear that the intrusion of the Indian Ocean water into central Japan remained until the end of August in the extremely cool summers of 1993 and 2003, which is interpreted as an extraordinary persistence of the Baiu period.
Precipitation amounts of three Lena mountain ranges (Verkhoyansk, Dzhugdzhur and Stanovoy) were investigated using numerical simulations for three months. First, we examined the precipitation distribution in eastern Siberia using routine station data. A north-south precipitation gradient was found, with the greatest precipitation in a southern area, and 47% of precipitation occurring in June, July, and August. To isolate the roles of the three Lena mountain ranges in determining precipitation, especially the north-south precipitation gradient, we conducted mountain removal experiments to examine differences in precipitation from the results of a real topography experiment. Results show that the Verkhoyansk (northeast of the Lena Basin) and Dzhugdzhur Mountain Range (southeast) had a weak effect on the north-south precipitation gradient. It was caused by relatively low specific humidity for Verkhoyansk and low elevation for Dzhugdzhur mountain range. The Stanovoy mountain range (south) exhibited a strong effect with higher specific humidity and elevation. Thus Stanovoy mountain range has the most contribution to north-south precipitation gradient among three Lena mountain ranges.
The Japan Meteorological Agency (JMA) started the second Japanese global atmospheric reanalysis project named the Japanese 55-year Reanalysis (JRA-55). It covers 55 years, extending back to 1958, when the global radiosonde observing system was established. Many of the deficiencies found in the first Japanese reanalysis, the Japanese 25-year Reanalysis (JRA-25), have been improved. It aims at providing a comprehensive atmospheric dataset that is suitable for studies of climate change or multi-decadal variability, by producing a more time-consistent dataset for a longer period than JRA-25. Production of JRA-55 started in 2010, and computations for more than 16 years have been completed as of August 2011. The entire JRA-55 production will be completed in early 2013 and thereafter JRA-55 will be continued as a new JCDAS on real time basis. This paper is a brief report to introduce the JRA-55 reanalysis project. The data assimilation and prediction (DA) system used in JRA-55 is introduced and compared to that used in JRA-25. Early results of JRA-55 are presented and discussed, showing general improvements.
This study examines seasonal and regional variations in the amplitudes of convectively coupled equatorial waves (CCEWs), including Madden-Julian oscillation (MJO) and tropical depression (TD) disturbances. The climatological seasonal variations of the n = 1 equatorial Rossby (ERn1) wave are similar to those of the MJO over the Indian Ocean and the central Pacific. The antisymmetric components of CCEWs (mixed Rossby gravity, n = 0 eastward inertial gravity, and n = 2 westward inertial gravity waves; MRG, EIGn0, and WIGn2 waves, respectively) show similar seasonal cycles over the central Pacific with peaks during boreal summer and fall. CCEW amplitudes are also examined as a function of the MJO phase. The activities of all CCEWs except for the MRG wave are significantly enhanced in the convectively mature phase of the MJO over the Indian Ocean, whereas the amplitude peak of the Kelvin and WIGn1 (ERn1 and TD) waves shifts from the mature phase over the Indian Ocean to the convectively developing (decaying) phase of the MJO over the western and central Pacific. The MRG wave has an amplitude maximum in the decaying phase over the Indian Ocean, and the peak shifts to the preceding phase of the MJO over the Pacific Ocean.
Climatological features of the distribution of extreme precipitation in Japan were documented using record values of ten-minute, hourly and daily precipitation for 1994 to 2009. It was found that the distribution of top ten-minute precipitation has a moderate north-south gradient (increasing southward at about 3% per degree in latitude), while ten-minute precipitation of lower ranks (up to 50th to 100th cases at each station) shows a stronger latitude dependence (about 6% per degree) that is comparable to those of hourly and daily precipitations. In addition, extreme precipitation shows local maxima on southern sides of land in western Japan corresponding to orographic enhancement, which is found for all the time scales but is most conspicuous for daily precipitation.
We assessed the utility of global CO2 distributions brought by the Greenhouse gases Observing SATellite (GOSAT) in the estimation of regional CO2 fluxes. We did so by estimating monthly fluxes and their uncertainty over a one-year period between June 2009 and May 2010 from 1) observational data collected in existing networks of surface CO2 measurement sites (GLOBALVIEW-CO2 2010; extrapolated to the year 2010) and 2) both the surface observations and column-averaged dry air mole fractions of CO2 (XCO2) retrieved from GOSAT soundings. Monthly means of the surface observations and GOSAT XCO2 retrievals gridded to 5° × 5° cells were used here. The estimation was performed for 64 subcontinental-scale regions. We compared these two sets of results in terms of change in uncertainty associated with the flux estimates. The rate of reduction in the flux uncertainty, which represents the degree to which the GOSAT XCO2 retrievals contribute to constraining the fluxes, was evaluated. We found that the GOSAT XCO2 retrievals could lower the flux uncertainty by as much as 48% (annual mean). Pronounced uncertainty reduction was found in the fluxes estimated for regions in Africa, South America, and Asia, where the sparsity of the surface monitoring sites is most evident.
A previous Doppler-lidar observation near a sea-breeze front revealed that small-scale vertical vortices, similar to dust devils, have a preferred direction of rotation, which suggests that their rotation was affected by meso-scale vertical vorticity associated with the front. In contrast, planetary vorticity is believed to have a negligible effect on dust devils. This paper investigates the effects of ambient rotation on dust devils by means of a large eddy simulation, which yielded the following findings: when the ambient rotation is as small as the earth's rotation, only a tiny asymmetry is found in the occurrences of vertical vortices of different signs; as the ambient rotation is increased, it significantly affects the rotational direction of the vortices, and the magnitude of their vertical vorticity attains a maximum for an ambient vorticity of about 10-3 s-1; a further increase in the ambient rotation changes the structure of convection in the convective mixed layer and suppresses horizontal convergence, so that vertical vorticity of the vortices is reduced.
The downscaling framework, which is originally proposed by Emanuel et al. (2008), is modified to improve the interannual variation of the tropical cyclone features in Western North Pacific (WNP). The original framework, which utilizes axisymmetric model driven using monthly mean atmosphere and ocean variables, well reproduced interannual variation of tropical cyclone indices over North Atlantic while there is room for further improvement over WNP. The modified treatment of relative vorticity plays an essential role for improving interannual variation of genesis counts and power dissipation index in WNP. In this update, we introduced probability of cyclogenesis special to WNP which is a priori calculated using monthly mean 850-hPa level relative vorticity and best track. The result suggests that the monthly mean relative vorticity represents a potential of characteristic initiating disturbance in WNP such as monsoon trough and intraseasonal variation. Another experiment shows increased influence of vertical wind shear in middle troposphere improves the distribution of tropical cyclogenesis in WNP while it slightly deteriorates the interannual variation of genesis count.
A 20-year-integration of a non-hydrostatic regional climate model (NHRCM) was conducted to reproduce the present climate. The annual mean surface temperature and precipitation of Japan are well simulated in NHRCM. There was one conspicuous outlier in the verification of the model's annual precipitation with observations. An even finer grid spacing than 5 km would be required to reproduce the precipitation at this observation point. The regional model successfully simulated heavy precipitation that a general circulation model could not reproduce. This model reproduced snow depth well in most of Japan, except in the north on the coast of the Sea of Japan, where the depth was underestimated.
Paleovegetational dataset during the mid-Pliocene warm period (MPWP) in lower latitudes are applied to evaluation for annual-mean MPWP climate simulated by a general circulation model (GCM). GCM-derived warmer climate in mid-latitude and wetter condition in subtropics during MPWP are generally confirmed by the terrestrial proxy data. The comparisons of their general patterns reveal an ability of GCM to predict the sustained-warmer climate under the different forcings from present day. In comparison to the proxy data, the enhanced rainfall over semi-arid regions in accordance with the alteration of Hadley/Walker circulation in MPWP is supported but the extent may be partially underestimated. The paleobotanical data in the lower latitudes indicate an importance of interactive response of vegetation to climate under the equilibrium warmer state of the earth system.
The aerosol semi-direct effect is generally explained as follows: aerosols, such as black carbon (BC) and mineral dust, absorb solar radiation, which warms and stabilizes the atmosphere, resulting in reduced cloudiness and cloud formation. However, the present study suggests that BC can intensify atmospheric instability and thus increase cloud water and precipitation if the BC is concentrated near the surface. Simulations using a global aerosol climate model, based on a general circulation model, show decreased cloud water over biomass-burning regions where BC is emitted to the free troposphere through the boundary layer. In contrast, increased cloud water is indicated over East and South Asia where BC from urban and industrial activities is concentrated near the surface. While the global mean change in the radiation budget at the top of the atmosphere due to the semi-direct effect of BC is estimated to be as small as +0.06 W m-2, regional changes in cloud water, precipitation, and shortwave radiation are suggested to be large enough to modify meteorological conditions in urban and biomass-burning regions.
A numerical simulation is carried out to analyze the temperature change of an urban green area for different times of day for a meteorologically calm condition in summer. Several parameters contribute to changes of the surface layer, e.g., irrigation, plants height, water bodies, etc. Grassland cooling effect can lead to substantial decrease of energy consumption, especially in urban areas in warm seasons, for air-conditioning. In this study a numerical model for the simulation of the temperature and humidity changes of an urban grass-covered park was developed. Also the main object in this study was to determine the effect of soil and grass leaf humidity of an urban park with grass cover on air temperature in a humid city in summer. In this approach, air flow over grass was simulated using solutions of two dimensional Reynolds Averaged Navier-Stokes (RANS) equations and a standard k-ε model for turbulence. Also heat and humidity equations with RANS equations were solved using the SIMPLE algorithm with a finite difference method. For the air surface interaction, the Deardorff Soil-Vegetation-Atmosphere Transfer (SVAT) model was used to estimate the evaporation-evaportranspiration of grass which has a key role in controlling the temperature and humidity of air over grass. For a more realistic numerical simulation, instead of well mixed air assumption for the inlet flow, a wind logarithmic profile based on stability conditions of air at the inlet of the computational domain was utilized. The model results, with conditions of an urban grass-covered park in Nagoya, Japan, show similar trends with observation; hence it seems that such model may be used for the study of air temperature and humidity variations, while using an irrigation program over grass, during the diurnal cycle in urban areas.
The marine atmospheric mixed layer (MAML) has an important influence on the diffusion of air pollutants over the East China Sea. We analyzed seasonal and diurnal variations and the vertical structure of the MAML by using continuous Mie-scattering lidar data recorded from March 2008 to February 2010 at Cape Hedo, Okinawa Island, Japan. The height of the MAML showed clear seasonal variation: a summer minimum and a winter maximum, and was significantly correlated with thermal stability (St), which we defined as sea surface temperature minus the temperature at the 850 hPa level. The height of the MAML showed minor diurnal variations in summer: a nighttime minimum and a daytime maximum. We detected the entrainment layer between the MAML and the free atmosphere by analysis of the intensity of fluctuations of aerosol concentrations determined from lidar data. Both the intensity of the fluctuations and the thickness of the entrainment zone increased with increasing St. The altitude of maximum fluctuation was a little above the top of the MAML.
Time-lagged ensemble downscaling experiments with Japan Meteorological Agency-Nonhydrostatic model are performed to study the dependence of heavy precipitation simulated by the model on the horizontal resolution for five days during late January to early February 2007, when the Jakarta Flood event occurred. The model runs with horizontal resolutions of 2, 4, and 5 km downscaled from the model runs with a 20-km resolution demonstrate the ability to reproduce a region of strong convective activity to the north of Java Island during the event. Daily meridional propagation of enhanced precipitation signals is simulated in the model runs with 2- and 4-km resolutions. Cumulative distribution functions of precipitation rate in the model are analyzed for four different regions: ocean, northern coast, mountain, and southern coast. The northern coast region shows the highest contribution of heavy precipitation compared to other regions for all the experiments as well as for satellite-based precipitation estimates. The statistics on the frequency of heavy precipitation show that the diurnal variation of heavy precipitation produced by the model with a 2-km resolution agrees well with that of satellite-based precipitation estimates.
The impact of a Northern Hemisphere stratospheric sudden warming (SSW) event on tropical convection is investigated as an initial value problem using a general circulation model. The amplification of planetary waves that produced the simulated SSW was forced by introducing an anomalous field in the model initial conditions. The impact of this SSW event on the tropics was examined by comparing runs with and without the SSW. An increased stratospheric mean meridional circulation cools the tropical lower stratosphere when planetary waves propagate upward into the stratosphere, leading to enhanced convective activity in the equatorial Southern Hemisphere. This result is consistent with previous observational studies, and indicates that convective activity in the equatorial region is influenced by SSW events.
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